UNIVERSITY OF CINCINNATI May 19 03 _____________ , 20 _____
Himanshu Patel I,______________________________________________, hereby submit this as part of the requirements for the degree of:
Doctor of Philosophy ________________________________________________
in: Pharmaceutical Sciences (Industrial Pharmacy) ________________________________________________
It is entitled: THE EFFECT OF FORMULATION AND PROCESSING VARIABLES ________________________________________________
ON THE STABILITY OF LEVOTHYROXINE SODIUM TABLETS ________________________________________________
________________________________________________ ________________________________________________
Approved by: Dr. Adel Sakr, Chairperson ________________________ Dr. Hussein Al-Khalidi ________________________ Dr. Richard Dansereau ________________________ Dr. Pankaj Desai ________________________ Dr. Apryll Stalcup ________________________
THE EFFECT OF FORMULATION AND PROCESSING VARIABLES ON THE STABILITY OF LEVOTHYROXINE SODIUM TABLETS A dissertation submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Industrial Pharmacy Program Division of Pharmaceutical Sciences College of Pharmacy 2003 by Himanshu Patel, B. Pharm. L. M. College of Pharmacy, Gujarat India Committee Chair Adel Sakr, Ph.D.
Abstract These series of experiments were devised to study the effects of various formulation and processing variables on the stability of levothyroxine. The studies were performed with levothyroxine drug substance (powder or solution), excipient slurries with levothyroxine at 50oC and tablet-incorporated levothyroxine under ICH accelerated stability conditions. Additionally, stability of tablets manufactured by wet granulation and / or direct compression at different compression pressures was evaluated under ICH accelerated stability conditions. It was found that the active, levothyroxine, was stable when stored for six months at 40oC/75% Relative Humidity (RH) in open or closed containers; also, it was non-hygroscopic under normal operating conditions (>30%RH). In diluent slurries when stored at 50oC for one month, levothyroxine was more stable at pH 11 than at pH 3. Levothyroxine tablets manufactured with dibasic calcium phosphate or mannitol and stored under accelerated stability conditions met USP assay requirements at three months, but not at six months. Tablets manufactured with lactose anhydrous, starch, and microcrystalline cellulose failed to meet USP requirements at three months.
After six months at accelerated stability
conditions, tablets manufactured with basic pH modifiers had less than 5% loss in potency, and thus, they met USP assay requirements. Therefore, it was concluded that excipients used in the manufacture of levothyroxine tablets affected levothyroxine stability. The use of desiccant for while storing levothyroxine tablets, didn’t appear to affect their stability at ICH accelerated
stability conditions. Furthermore, the use of basic pH modifiers is a technique for improving tablet- incorporated levothyroxine stability. After 3 months storage at ICH accelerated stability conditions the tablets manufactured by wet granulation or direct compression did not show significant difference in stability. Thus, the type of manufacturing method does not influence the stability of levothyroxine tablets, manufactured with / without sodium carbonate and using dibasic calcium phosphate as diluent. Interestingly, it was found that the initial assay value of levothyroxine tablets (compressed at 2000 and 6000 lbs) was lower than that of the uncompressed granules/powder. Thus tablet compression appears to affect the stability of levothyroxine. It was concluded that formulation and processing variables affect the stability of levothyroxine tablets.
Acknowledgments
This work has been possible due to motivation and support of various individuals who gave me sound advice and guidance at numerous occasions throughout my years as a graduate student at the University of Cincinnati.
I express my deepest gratitude to my academic advisor Dr. Adel. Sakr, whose steadfast guidance, encouragement, teaching and support have paved the way for this work as well as contributed to my development as a graduate student. I am also grateful for the financial support and support to various national and international meetings that he has provided during my graduate education. He has been a true guide, a mentor and a friend.
My thanks and appreciation to Dr. Apryll Stalcup, who played a vital role in my education in analytical chemistry and whose analytical expertise is a very critical part of this research project.
I also thank Dr. Richard Dansereau, who was instrumental in guiding me through the initial conception of ideas on the preformulation work and allowing me to carry out part of the experiments in his laboratories at Procter and Gamble.
I am also grateful to Dr. Pankaj Desai, and Dr. Hussein Alkhalidi for being on my dissertation committee and for providing valuable help and guidance.
All of them have shaped my graduate education and life very significantly by their great teaching and thoughtful concerns.
I am very thankful to Dr. Koka Jayasimhulu for his kind assistance with the Mass spectroscopy study, Dr. Jongsung Jin for his kind assistance with the racemization study, and Mr. Mark Bodman for his kind assistance with performing adsorption / desorption study and analyzing isotherms. Additionally I thank Mr. Bill Pickens, for letting me use the flat surface pH meter.
I am grateful to Dr. Ilona Weltrowski who helped me at the beginning of my graduate education in analytical training and for all her support. I also thank Dr. Mahalaxmi Andheria for her friendship and support.
I am most grateful to my colleagues, Elena, Juan, John, Julia, and Susan who made my stay at University of Cincinnati most enjoyable.
I also thank the University of Cincinnati, College of Pharmacy for awarding me a UGS and providing me this education.
And most importantly I would like to acknowledge my family for their unconditional love, encouragement and support, which has allowed me be what I am today. I would also like to thank “The Patel family” in Cincinnati who have been my family away from home.
Contents 1.
INTRODUCTION............................................................................... 12
1.1.
General.............................................................................................. 12
1.1.1.
Stability.............................................................................................. 12
1.1.1.1.
Stability testing of tablets................................................................... 13
1.1.1.2.
Degradation studies .......................................................................... 15
1.1.2.
pH of the microenvironment in the tablet ........................................... 16
1.2.
Background,
regulatory
and
consistency
problems
of
levothyroxine sodium tablets ............................................................. 18 1.2.1.
Reported adverse drug experiences ................................................. 19
1.2.2.
Stability problems .............................................................................. 20
1.2.3.
Formulation change........................................................................... 25
1.2.4.
Regulatory status .............................................................................. 26
1.3.
Levothyroxine sodium ....................................................................... 30
1.3.1.
Description ........................................................................................ 30
1.3.2.
Clinical pharmacology ....................................................................... 33
1.3.3.
Synthetic vs natural levothyroxine products ...................................... 43
1.3.4.
Stability of levothyroxine sodium ....................................................... 44
1.3.5.
Formulations with increased stability ................................................. 48
1.4.
Rationale for the study ...................................................................... 50
2.
OBJECTIVE, HYPOTHESIS AND SPECIFIC AIMS......................... 52
2 2.1.
Objective ........................................................................................... 52
2.2.
Hypothesis......................................................................................... 52
2.3.
Specific aims ..................................................................................... 53
2.3.1.
Formulation variables ........................................................................ 53
2.3.2.
Manufacturing variables .................................................................... 53
3.
EXPERIMENTAL .............................................................................. 54
3.1.
Materials............................................................................................ 54
3.2.
Equipment ......................................................................................... 56
3.3.
Methods............................................................................................. 58
3.3.1.
Stability of levothyroxine sodium pentahydrate ................................. 58
3.3.1.1.
Levothyroxine sodium drug substance .............................................. 58
3.3.1.2.
In solution .......................................................................................... 58
3.3.2.
Hygroscopicity of levothyroxine sodium pentahydrate....................... 61
3.3.3.
Effect of formulation variables on the stability of levothyroxine sodium tablets ................................................................................... 61
3.3.3.1.
Effect of excipients on the stability of levothyroxine sodium tablets................................................................................................ 61
3.3.3.2.
Compatibility with excipients.............................................................. 64
3.3.3.3.
Effect of pH on levothyroxine stability................................................ 65
3.3.4.
Effect of the manufacturing technology on the stability of levothyroxine sodium tablets ............................................................. 70
3.3.4.1.
Levothyroxine
sodium
tablet
manufactured
by
direct
compression ...................................................................................... 70
3 3.3.4.2.
Wet granulation ................................................................................. 71
4.
TABLET MANUFACTURING AND EVALUATION .......................... 75
4.1.
Tablet manufacturing......................................................................... 75
4.2.
Tablet Evaluation............................................................................... 79
4.2.1.
Physical properties ............................................................................ 79
4.2.2.
Content uniformity ............................................................................. 79
4.2.3.
Assay ................................................................................................ 80
4.2.4.
Moisture determination ...................................................................... 81
4.2.5.
Stability.............................................................................................. 81
4.2.6.
Dissolution test .................................................................................. 82
5.
RESULTS AND DISCUSSIONS ....................................................... 83
5.1.
Stability of levothyroxine sodium ....................................................... 83
5.2.
Stability of levothyroxine sodium tablets with various excipients ....... 94
5.2.1.
Stability of levothyroxine sodium commercial tablets ........................ 94
5.2.2.
Stability of levothyroxine sodium tablets with various excipients ....... 94
5.3.
Effect of tablet excipients on the stability of levothyroxine sodium in slurries ............................................................................ 106
5.4.
Effect of pH on stability of levothyroxine sodium in different diluent slurries ................................................................................. 109
5.5.
Effect of pH modifiers on levothyroxine sodium tablets ................... 112
5.5.1.
Stability of levothyroxine sodium tablets manufactured with dibasic calcium phosphate and pH modifiers .................................. 112
4 5.5.2.
Stability of levothyroxine sodium tablets manufactured with microcrystalline cellulose and pH modifiers..................................... 120
5.5.2.1.
Effect of the manufacturing method, compression pressure and pH modifiers on the stability of levothyroxine sodium tablets .......... 122
5.5.2.1.1. Effect of manufacturing method....................................................... 122 5.5.2.1.2. Effect of compression force ............................................................. 123 5.5.2.1.3. Effect of pH modifier ........................................................................ 123 6.
SUMMARY AND CONCLUSIONS.................................................. 137
7.
REFERENCES................................................................................ 140
5
List of Figures Figure 1: Ionization of levothyroxine ................................................................. 32 Figure 2: Thyroid gland..................................................................................... 34 Figure 3: Deiodination scheme for levothyroxine.............................................. 45 Figure 4: Example of beam spider compression force and ejection force ........ 74 Figure 5: Process flow chart for tablets manufactured by direct compression ...................................................................................... 75 Figure 6: Process flow chart for tablets manufactured by wet granulation........ 77 Figure 7: Stability of levothyroxine sodium drug substance at 40oC/75 % RH in closed or open containers for six months ................................ 85 Figure 8: Effect of pH on the stability of levothyroxine in solution stored for 7 days at room temperature ......................................................... 86 Figure 9: Effect of pH on the stability of levothyroxine in solution stored for 10 days at room temperature. ...................................................... 87 Figure 10: Total moisture content of levothyroxine sodium pentahydrate stored at 40oC/75% RH for six months .............................................. 90 Figure 11: Absorption / desorption isotherm of levothyroxine sodium pentahydrate ..................................................................................... 91 Figure 12: Absorption / desorption isotherm of levothyroxine sodium pentahydrate after drying at 25oC with nitrogen ................................ 92 Figure 13: Absorption / desorption isotherm (H20 moles) levothyroxine sodium pentahydrate after drying at 25oC with nitrogen .................... 93
6 Figure 14: Stability of levothyroxine sodium 100µg commercial tablets stored at 40oC/75%RH for 6 months ................................................. 98 Figure 15: Effect of filler on the stability of levothyroxine sodium tablets stored without desiccant at 40oC/75%RH for 6 months ................... 100 Figure 16: Moisture content of levothyroxine sodium tablets stored without desiccant at 40oC/75%RH for 6 months .......................................... 101 Figure 17: Effect of filler on the stability (assay) of levothyroxine sodium tablets stored with desiccant at 40oC/75%RH for 6 months ............ 103 Figure 18: Moisture content of levothyroxine sodium tablets stored with desiccant at 40oC/75% RH for 6 months ......................................... 104 Figure 19: Effect of tablet excipients on the stability of levothyroxine sodium in 20% (w/v) slurries stored at 50 oC for 1 month................ 108 Figure 20: Effect of pH on stability of levothyroxine sodium in diluent 20% (w/v) slurries stored at 50oC for 1 month ......................................... 111 Figure 21: Stability (assay) of levothyroxine tablets manufactured with dibasic calcium phosphate and various pH modifying excipients and stored at 40oC/75%RH for 6 months (with no desiccant).......... 116 Figure 22: Stability (assay) of levothyroxine tablets made with dibasic calcium phosphate and various pH modifying excipients and stored at 40oC/75%RH for 6 months (with desiccant)...................... 117 Figure 23: Moisture (%) of levothyroxine tablets manufactured with dibasic calcium phosphate and various pH modifying excipients and stored at 40oC/75%RH for 6 months (without desiccant)................. 118
7 Figure 24: Moisture (%) of levothyroxine tablets manufactured with dibasic calcium phosphate and various pH modifying excipients and stored at 40oC/75%RH for 6 months (with desiccant)...................... 119 Figure 25: Stability (assay) of levothyroxine tablets made with microcrystalline cellulose and various pH modifying excipients and stored at 40oC/75%RH for 3 months ........................................ 121 Figure 26: Stability (assay) of levothyroxine tablets with / without of a pH modifier and manufactured by wet granulation and compressed at various compression forces and stored at 40oC/75%RH for 3 months............................................................................................. 129 Figure 27: Stability (assay) of levothyroxine tablets with / without of a pH modifier and manufactured by direct compression and compressed at various compression forces and stored at 40oC/75%RH for 3 months .............................................................. 130 Figure 28: Stability (assay) of levothyroxine tablets manufactured by direct compression or wet granulation and compressed at various compression forces and stored at 40oC/75%RH for 3 months ........ 131 Figure 29: Stability (assay) of levothyroxine tablets with pH modifier manufactured by direct compression or wet granulation and compressed at various compression forces and stored at 40oC/75%RH for 3 months .............................................................. 132
8 Figure 30: Stability (assay) of levothyroxine uncompressed powder / granules in the presence / absence of pH modifier and stored at 40oC/75%RH for 3 months .............................................................. 133 Figure 31: Moisture content in levothyroxine tablets with / without of a pH modifier and manufactured by direct compression and compressed at various compression forces and stored at 40oC/75%RH for 3 months .............................................................. 134 Figure 32: Moisture content in levothyroxine tablets with / without of a pH modifier and manufactured by wet granulation and compressed at various compression forces and stored at 40oC/75%RH for 3 months............................................................................................. 135 Figure 33: Dissolution of Levothyroxine sodium tablets manufactured by wet granulation or direct compression at 2000 lbs compression pressure .......................................................................................... 136
9
List of Tables Table 1:
Formulations with NDA/ANDA’s submitted / approved Marketed formulations: ...................................................................... 29
Table 2:
Pharmacokinetics of levothyroxine sodium........................................ 41
Table 3:
Degradation of levothyroxine sodium raw material and tablets ......... 47
Table 4:
Buffer solutions used ......................................................................... 60
Table 5:
Formulation of levothyroxine sodium tablets ..................................... 63
Table 6:
Formulation (% w/w) of levothyroxine sodium tablets with pH modifiers............................................................................................ 67
Table 7:
pH of the saturated solutions of pH modifiers.................................... 68
Table 8:
Number of experiments performed to study the effect of formulation variables ......................................................................... 69
Table 9:
Levothyroxine sodium tablets manufactured by wet granulation or direct compression on a rotary tablet press................................... 72
Table 10: Levothyroxine sodium tablets with sodium carbonate manufactured by wet granulation or direct compression on a rotary tablet press.............................................................................. 73 Table 11: Effect of oxidizing agent on the stability of levothyroxine (%) in solution at room temperature............................................................. 88 Table 12: Effect of filler on the stability (assay) of levothyroxine sodium tablets stored without desiccant at 40oC/75%RH for 6 months ......... 99
10 Table 13: Effect of filler on the stability (assay) of levothyroxine sodium tablets stored with desiccant at 40oC/75%RH for 6 months ............ 102 Table 14: The pH of saturated solutions of powdered tablets ......................... 105 Table 15: Effect of different tablet excipients on the stability of levothyroxine sodium in 20% (w/v) slurries stored at 50 oC for 1 month .............................................................................................. 107 Table 16: Effect of pH on stability of levothyroxine sodium in diluent 20% (w/v) slurries stored at 50oC for 1 month ......................................... 110 Table 17: Effect of pH modifiers on the stability (assay) of levothyroxine sodium tablets manufactured with dibasic calcium phosphate and stored with / without desiccant at 40oC/75%RH for 6 months............................................................................................. 115 Table 18: Effect of manufacturing method and compression force on the stability of levothyroxine sodium tablets stored at 40oC/75%RH for 3 months .................................................................................... 125 Table 19: Average weight of levothyroxine sodium tablets manufactured by wet granulation / direct compression with / without pH modifier at different compression forces and stored at 40oC/75%RH for 3 months .............................................................. 126 Table 20: Average thickness of levothyroxine sodium tablets manufactured by wet granulation / direct compression with / without pH modifier at different compression forces and stored at 40oC/75%RH for 3 months .......................................................... 127
11 Table 21: Average hardness of levothyroxine sodium tablets manufactured by wet granulation / direct compression with / without pH modifier at different compression forces and stored at 40oC/75%RH for 3 months .......................................................... 128
12
1. INTRODUCTION
1.1. General 1.1.1.
Stability
Stability is defined as the capacity of a drug substance or a drug product to remain within specifications established to ensure its identity, strength, quality, and purity throughout the retest period or expiration dating period, as appropriate (ICH Q1a, 2000, Q1c, 1996).
In a rational design and evaluation of dosage forms for drugs, the stability of the active components must be the major criterion in determining their suitability. Several forms of instability can occur. First, there may be chemical degradation of the drug, leading to substantial lowering of the quantity of the therapeutic agent in the dosage form. This is even of greater significance in the case of drugs with narrow therapeutic indices, where the patient needs to be carefully titrated so that serum levels are neither so high that they are potentially toxic, nor so low that they are ineffective. Second, although the degradation of the active drug may not be that extensive, a toxic degradant may be formed in the decomposition process. An example of a product of degradation that is significantly more toxic is conversion of tetracycline to epianhydrotetracycline.
13 Third, instability of a drug product can lead to a decrease in its bioavailability, rather than to loss of drug or the formation of toxic degradation products. This reduction in bioavailability can result in a substantial lowering in the therapeutic efficacy of the dosage form. This phenomenon, for example, can be caused by physical and chemical changes in the excipients in the dosage form, independent of whatever changes the active drug may have undergone. Fourth, there may be substantial changes in the physical appearance of the dosage forms (Carstensen, 2000).
Since most drugs are organic molecules, it is important to recognize that many pharmaceutical pathways are, in principle, similar to reactions described for organic compound. The major difference that has to be considered is that most pharmaceutical reactions occur due to or are governed by water, oxygen, or light, rather than other active ingredients. Thus, the most common routes of decomposition
are:
hydrolysis,
oxidation,
photolysis,
racemization,
and
decarboxylation (Carstensen, 2000).
1.1.1.1.
Stability testing of tablets
Because of the concerns for safety, efficacy and quality, the regulatory agencies have found it appropriate to require stability testing for drug products. More recently, the process of globalization and harmonization has stimulated the development of worldwide standards. The International Conference on
14 Harmonization (ICH) has set up draft guidelines for the “ Stability testing of drug substances and drug products”.
According to the general guidance documents, the storage conditions for stability testing at the time of submitting an NDA are the following: •
Long-term testing: 25 oC / 60% RH for at least 12 months
•
Accelerated testing: 40 oC / 75% RH for at least 6 months
A significant change should not occur during this period. A significant change is defined as: •
A 5 percent loss from initial assay value of a batch;
•
Any specified degradant exceeding its limit;
•
Product exceeding its pH limit;
•
Dissolution exceeding limit;
•
Failure to meet specifications of appearance and physical properties.
Tablets should be evaluated for appearance, color, odor, assay, degradation products, dissolution, moisture, and friability (ICH Qa1, 2000).
15 1.1.1.2.
Degradation studies
Forced
degradation
studies
(stress
testing)
are
important
in
preformulation. Such testing is a part of the developmental strategy and is normally carried out under more severe conditions than those used for accelerated tests.
Forced degradation studies are carried out for the following reasons: •
Development and validation of stability indicating methodology;
•
Determination of the intrinsic stability of a drug molecule, and structure elucidation of degradation products;
•
Determination of degradation pathways of drug substances and products;
•
Discernment of drug vs non drug related degradation products in the formulations;
The following are FDA recommended degradation studies for a drug substance (FDA 1998): •
Stressing the drug substance in solution and suspension at acidic and alkaline pH and under high oxygen environment;
•
Stressing the solid drug at temperature and temperature/ humidity conditions in excess to accelerated conditions;
•
Stressing the drug photolytically in the solid state and solution;
16 •
Demonstration of the stability indicating methods with forced degraded / spiked samples;
•
Isolation and/or full characterization of degradation products (by NMR, MS, UV etc);
•
Determination of the mechanism and kinetics of formation of the degradation products if possible.
Thus, for degradation study of a drug substance, it should be exposed to acid / base, oxidative, exposure to light, thermal and humidity.
1.1.2.
pH of the microenvironment in the tablet
The stability of drug substances in solid dosage forms effected by the amount of water in excipients. In the strictest sense, the term “pH” is not defined for solid systems; however, in the tablet microenvironment, some moisture (water) is always present, and the pH of this is considered the pH of the microenvironment of the tablet. For low-dose, highly-potent drug substances, whose stability is sensitive to the pH of the surrounding medium, a potential technique to improve the stability of the drug product is to formulate it at the pH of maximum stability.
To control the pH of the microenvironment in tablets, citric, tartaric, and fumaric acids are the acidic additives of choice, and sodium bicarbonate, sodium carbonate, magnesium and calcium oxides are the basic additives of choice.
17 Calcium pantothenate is not stable at low pH, so frequently it is separately granulated with magnesium oxide and mixed with the other excipients (Carstensen, 2000).
Al-Omari et al. (2001) studied the effect of the pH of the microenvironment in the drug matrix on the stability of enalapril tablets. For enalapril tablets, when the pH of the matrix was basic, the stability of enalapril improved. This suggests that enalapril degradation is pH dependent.
Chen et al. (2000) studied the degradation of a fluoropyridinyl in formulations.
They
found
that
the
drug
was
unstable
in
an
acidic
microenvironment, as well as in the presence of lactose anhydrous. They recommended formulating the drug in a basic microenvironment.
Badaway et al (1999) studied the effect of acids on solid-state stability of an ester prodrug. It was found that in solution, the pH of maximum stability for the drug was 4. They used different acids to adjust the pH of microenvironment of the tablets. They found that the use of various acids reduced hydrolysis of the drug in tablets; however, the use of strong acids (pH of microenvironment lower than that of maximum stability) actually increased the hydrolysis. They concluded that adjusting the pH of the microenvironment to the maximum stability of the drug in solution led to reduced hydrolysis and improved stability.
18
1.2. Background, regulatory and consistency problems of levothyroxine sodium tablets
Orally administered levothyroxine sodium is used as replacement therapy in conditions characterized by diminished or absent thyroid function such as cretinism, myxedema, nontoxic goiter, or hypothyroidism. Hypothyroidism is a common condition; in the United States, 1 in every 4,000 to 5,000 babies is born hypothyroid. Hypothyroidism has a prevalence of 0.5 % to 1.3 % in adults. In people over 60, the prevalence of primary hypothyroidism increases to 2.7 % in men and 7.1 % in women. Because congenital hypothyroidism may result in irreversible mental retardation, which can be avoided with early diagnosis and treatment, newborn screening for this disorder is mandatory in North America, Europe, and Japan (Federal Register, 1997). From the late 1890's until relatively recently, physicians worldwide have treated hypothyroid patients with tablets containing desiccated (dried and powdered) animal thyroid glands. These tablets contained both levothyroxine (T4) and triiodothyronine (T3). In 1958, the first synthetic levothyroxine tablets were marketed in the United States. Because thyroid hormones were on the market before the Food and Drug Administration (FDA) laws were in place, manufacturers of these hormones were not required to meet the extensive testing requirements. In other words, thyroid hormone replacements, such as synthetic levothyroxine, were "grandfathered" into the system.
19 Now, more than 40 years after their introduction into the market, the U.S. Food and Drug Administration (FDA) issued notice (Federal Register, August 14, 1997) stating “The Food and Drug Administration (FDA) is announcing that orally administered drug products containing levothyroxine sodium are new drugs. There is new information showing significant stability and potency problems with orally administered levothyroxine sodium products. Also, these products fail to maintain potency through the expiration date, and tablets of the same dosage strength from the same manufacturer often vary from lot to lot in the amount of active ingredient present. This lack of stability and consistent potency has the potential to cause serious health consequences to the public. Manufacturers who wish to continue to market orally administered levothyroxine sodium products must submit new drug applications (NDA's). The FDA has determined that orally administered levothyroxine sodium products are medically necessary, and accordingly the agency is allowing current manufacturers 3 years to obtain approved NDA's” (Federal Register, 1997).
1.2.1.
Reported adverse drug experiences
Between 1987 and 1994, the FDA received fifty-eight adverse drug experience reports associated with the potency of orally administered levothyroxine sodium products (Federal Register, 1997). Forty-seven of the reports suggested that the products were subpotent, while nine suggested superpotency. Two of the reports involved inconsistency in thyroid hormone
20 blood levels. Four hospitalizations were included in the reports; two were attributed to product subpotency and two were attributed to product superpotency. More than half of the 58 reports were supported by thyroid function blood tests. Some of the reported problems were the result of switching brands. However, other adverse events occurred when patients received a refill of a product on which they had previously been stable, indicating a lack of consistency in stability, potency, and bioavailability between different lots of tablets from the same manufacturer. Because levothyroxine sodium products were prescription drugs marketed without approved NDA's, manufacturers are expressly required, under 21 CFR 310.305, to report adverse drug experiences that are unexpected and serious; however they are not required, as for the products with approved applications (21 CFR 314.80) to periodically report all adverse drug experiences, including expected or less serious events. Thus, some adverse drug experiences related to inconsistencies in potency of orally administered levothyroxine sodium products may not be regarded as serious or unexpected and, as a result, may go unreported. Reports received by the FDA, therefore, may not reflect the total number of adverse events associated with inconsistencies in product potency (Federal Register, 1997).
1.2.2.
Stability problems
The FDA, in conjunction with the United States Pharmacopeial Convention, took the initiative in organizing a workshop in 1982 to set the
21 standard
for
the
use
of
a
stability-indicating
high-performance
liquid
chromatographic (HPLC) assay for the quality control of thyroid hormone drug products (Garnick, 1982). The former assay method was based on iodine content and was not stability- indicating. Using the developed HPLC method, there have been numerous reports indicating problems with the stability of orally administered levothyroxine sodium products during the past several years. Almost every manufacturer of orally administered levothyroxine sodium products, including the market leader, has reported recalls that were the result of potency or stability problems.
Since 1991, there have been more than 10 firm-initiated recalls of levothyroxine sodium tablets involving 150 lots and more than 100 million tablets. In all but one case, the recalls were initiated because tablets were found to be subpotent or potency could not be assured through the expiration date. The remaining recall was initiated for a product that was found to be superpotent. During this period, the FDA also issued two warning letters to manufacturers citing stability problems with orally administered levothyroxine sodium products (Federal Register, 1997).
At one firm, potency problems with levothyroxine sodium tablets resulted in destruction of products and repeated recalls. From 1990 to 1992, the firm destroyed 46 lots of levothyroxine sodium tablets that failed to meet potency or content uniformity specifications during finished product testing. In August 1989,
22 this firm recalled 21 lots due to subpotency. In 1991, the firm recalled 26 lots in February and 15 lots in June because of subpotency (Federal Register, 1997).
An
FDA
inspection
report
concerning
another
manufacturer
of
levothyroxine sodium showed that 14 % of all lots manufactured from 1991 through 1993 were rejected and destroyed due to failure to meet the in-house assay specifications of 103 to 110 %, established by the firm.
In March 1993, the FDA sent a warning letter to a firm stating that its levothyroxine tablets were adulterated because the expiration date was not supported by adequate stability studies. Five lots of the firm's levothyroxine sodium tablets, labeled for storage within controlled room temperature range, had failed stability testing when stored at the higher end of the range. The warning letter also objected to the labeled storage conditions specifying a nonstandard storage range of 15 to 22 oC. The FDA objected to this labeling because it did not conform to any storage conditions defined in United States Pharmacopeia (USP). In response, the firm changed the labeling instruction to store the product at 8 - 15oC. The firm informed the FDA that it would reformulate its levothyroxine sodium tablets to be stable at room temperature (Federal Register, 1997).
The five failing lots named in the FDA's warning letter were recalled in April 1994. Previously, in December 1993, a lot of levothyroxine sodium tablets
23 was recalled by the same firm because potency was not assured through the expiration date. In November 1994, the renamed successor firm recalled one lot of levothyroxine sodium tablets due to superpotency.
Another firm recalled six lots of levothyroxine sodium tablets in 1993 because of a potential for subpotency to occur before the expiration date. The USP specifies a potency range for levothyroxine sodium from 90 % to 110 %. Analysis of the recalled tablets showed potencies ranging from 74.7 % to 90.4 %. Six months later, this firm recalled another lot of levothyroxine sodium tablets when it fell below labeled potency during routine stability testing. Content analysis found the potency of the failed lot to be 85.5 % to 86.2 %. Subsequently, an FDA inspection at the firm led to the issuance of a warning letter regarding the firm's levothyroxine sodium products. One of the deviations from good manufacturing practice regulations cited in that letter was failure to determine, by appropriate stability testing, the expiration date of some strengths of levothyroxine sodium. Another deviation concerned failure to establish adequate procedures for monitoring and controlling the temperature and humidity during the manufacturing process (Federal Register, 1997).
In April 1994, one manufacturer recalled seven lots of levothyroxine sodium products because potency could not be assured through the expiration date. In February 1995, the same manufacturer initiated a major recall of levothyroxine sodium affecting 60 lots and 50,436,000 tablets. The recall was
24 initiated when the product was found to be below potency at 18-month stability testing.
In December 1995, a manufacturer recalled 22 lots of levothyroxine sodium products because potency could not be assured through the expiration date (Federal Register, 1997).
In addition to raising concerns about the consistent potency of orally administered levothyroxine sodium products, this pattern of stability problems suggests that the customary 2-year shelf life may not be appropriate for these products because they are prone to experience accelerated degradation in response to a variety of factors.
Levothyroxine sodium is unstable in the presence of light, temperature, air, and humidity. One study found that some excipients used with levothyroxine sodium act as catalysts to hasten its degradation (Gupta, 1990). In addition, the kinetics of levothyroxine sodium degradation are complex. Stability studies show that levothyroxine sodium exhibits a biphasic first order degradation profile, with an initial fast degradation rate followed by a slower rate. The initial fast rate varies depending on temperature. To compensate for the initial accelerated degradation, some manufacturers use an overage of active ingredient in their formulation, which can lead to occasional instances of superpotency (Rhodes, 2000, Federal Register, 1997).
25
1.2.3.
Formulation change
Because orally administered levothyroxine sodium products were marketed without approved applications, manufacturers have not sought FDA approval each time they reformulate their products. In 1982, for example, one manufacturer reformulated its levothyroxine sodium product by removing two inactive ingredients and changing the physical form of coloring agents (Hannessey, 1985). The reformulated product has significantly increased in potency. One study found that the reformulated product contained 100 percent of the label claim, compared to 78 percent before the reformulation (Stoffer, 1984). Another study estimated that the levothyroxine content of the old formulation was approximately 70 percent of the stated value (Fish, 1987). This increase in product potency resulted in serious clinical problems. On January 17, 1984, a physician reported to FDA: “I have noticed a recent significant problem with the use of [this levothyroxine sodium product]. People who have been on it for years are suddenly becoming toxic on the same dose. Also, people starting on the medication become toxic on 0.1 mg [milligram] which is unheard of”. On May 25, 1984, another physician reported that 15 to 20 percent of his patients using the product had become hyperthyroid although they had been completely controlled up until that time. Another physician reported in May 1984 that three patients, previously well-controlled on the product, had developed thyroid toxicity. One of these patients experienced atrial fibrillation (Federal Register, 1997).
26
In a 1990 study (Gupta, 1990), one manufacturer's levothyroxine sodium tablets selected from different batches showed chromatographic variations suggesting the presence of different excipients.
1.2.4.
Regulatory status
Levothyroxine sodium is used as replacement therapy when endogenous thyroid hormone production is deficient. The maintenance dosage must be determined on a patient-by-patient basis. Levothyroxine sodium products are marketed in multiple dosage strengths, that may vary by only 12 µg, thus permitting careful titration of dose. Because of levothyroxine sodium's narrow therapeutic index, it is particularly important that the amount of available active drug be consistent for a given tablet strength. Variations in the amount of available active drug can affect both safety and effectiveness. Subpotent tablets will not be effective in controlling hypothyroid symptoms or sequelae. The drug substance, levothyroxine sodium, is unstable in the presence of light, temperature, air, and humidity. Unless the manufacturing process can be carefully and consistently controlled, orally administered levothyroxine sodium products may not be fully potent through the labeled expiration date, or be of consistent potency from lot to lot. There is evidence from recalls, adverse drug experience reports, and inspection reports that even when a physician consistently prescribes the same brand of orally administered levothyroxine
27 sodium, patients may receive products of variable potency at a given dose. Such variations in product potency present actual safety and effectiveness concerns (Federal Register, 1997).
According to the FDA, levothyroxine sodium is safe and effective in treating hypothyroidism, when it is carefully and consistently manufactured and stored, and it is prescribed in the correct amount. However as of 1997, none of the commercially available oral levothyroxine sodium products had demonstrated consistent potency and stability and, thus, were not generally recognized as safe and effective (Federal Register, 1997).
Levothyroxine sodium products are medically necessary because they are used to treat hypothyroidism and no alternative drug is relied upon by the medical community as an adequate substitute. Accordingly, FDA permitted orally administered levothyroxine sodium products to be marketed without approved NDA's until August 14, 2000, in order to give manufacturers time to conduct the required studies and to prepare and submit applications, and to allow time for review of and action on these applications. This date was extended to August 14, 2001 at which time only two (Unithyroid and Levoxyl) products were approved, with an expiration date of just 18 months. Currently, there are 8 applications filed with the FDA as listed in Table 1 (page 29). All the remaining manufacturers have until December 2003 to gradually withdraw their product
28 from the market. The FDA has issued a guideline by which this withdrawal will occur.
29
Table 1:
Formulations with NDA/ANDA’s submitted / approved
marketed formulations:
Product Name / strengths Unithroid Levothyroxine sodium 25, 50, 75, 88, 100, 112, 125, 150, 175, 200 & 300 mcg, tab. Levo-T Levothyroxine sodium 25, 50, 75, 88, 100, 112, 125, 150, 175, 200 & 300 mcg, tab.
Manufacturer Jerome Stevens Pharmaceuticals
Approval date Approved: 8/21/2000 Approval Type: Original (NDA)
Mova Pharmaceutical
Approved: 3/1/2002 Approval Type: Original (NDA)
Levothroid Levothyroxine Sodium 0.025, 0.05, 0.075, 0.088, 0.1, 0.112, 0.125, 0.15, 0.175, 0.2 & 0.3 mg, tab. Levoxyl Levothyroxine 25, 50, 88, 100, 125, 137, 150, 200 & 300 mcg, tab. Novothyrox 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 and 300 mcg Synthroid levothyroxine sodium 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 & 300 mcg, tab.
Forest Laboratories Mylan
Application Submitted
Jones Pharma (Parent: King Pharmaceuticals)
Approved: 5/25/2001 Approval Type: Original (NDA) Approved: 5/31/2002 Approval Type: Original (NDA) Approved: 7/24/2002 Approval Type: Original (NDA)
Thyro-Tabs Levothyroxine sodium USP 25, 50, 75, 88, 100, 112, 125, 150, 175, 200 & 300 mcg, tablets
Lloyd Inc.
(NDApipeline.com 4/17/03)
GenPharm International (Parent: Medarex)
Knoll Pharmaceutical (Parent: Abbott Laboratories)
Approved: 6/5/2002 ApprovalType: Original (ANDA)
Approved: 10/24/2002 Approval Type: Original (NDA)
30
1.3. Levothyroxine sodium 1.3.1.
Description
Levothyroxine sodium is synthetic crystalline L-3, 3’, 5, 5’-tetraiodothyronine sodium salt. Synthetic levothyroxine sodium is identical to that produced in the human thyroid gland (Clarke’s 1986).
Structure 1: Levothyroxine sodium pentahydrate
Empirical Formula, Molecular Weight: It available in one of the three states. a) hydrated sodium salt, b) anhydrous sodium salt and c) as an acid. a) C15H10I4NNaO4 . 5 H2O –
888.96
b) C15H10I4NNaO4 –
798.86
c) C15H11I4NO4 –
776.93
It is a pale brownish yellow powder, is odorless, tasteless; and it is found in amorphous or crystalline forms;
31
Solubility: Very slightly soluble in water (1 in 700), slightly soluble in alcohol (1 in 300), and dissolves in aqueous solutions of alkaline hydroxides (Moffat, 1986). The aqueous solubility of levothyroxine is dependent on the pH of the surrounding media. The solubility reduces from pH 1 to 3, remains constant from 3 – 7, and significantly increases above pH ~7 (Won, 1992, Post and Warren, 1976).
Levothyroxine sodium has three ionizable moieties and it can exist as cation, zwitterion, anion, or diion depending on the pH of the solution. For the carboxyl group, pka1 = 2.40; for the phenolic group, pka2 =6.87 and for the amino group, pka3 = 10.1 ( Won, 1992. Post, 1976) (Figure 1 - page 32).
32 Figure 1:
Ionization of levothyroxine I O
I
O
I
HO
OH
+
NH3
I
pKa1 I O
I
O
I
HO
O
-
+
NH3
I
pKa2 I
O
O
O
I
-
I
O
-
+
NH3
I
pKa3 I
O
O
O
I
-
I I
O
-
NH2
33
1.3.2.
Clinical pharmacology
The thyroid is a small gland, measuring about 2 inches across, that lies just under the skin below the Adam's apple in the neck. The two halves (lobes) of the gland are connected in the middle (called the isthmus), so the thyroid gland resembles the letter H or a bow tie (Williams, 1989., Merck manual).
Inorganic iodide is actively concentrated in the thyroid epithelial cells to a level approximately 30 times its concentration in plasma. Within minutes after entering the thyroid, inorganic iodide is oxidized, in a peroxidase-dependent reaction, to an organic form. This oxidation is hardly distinguishable from the next reaction, in which organic iodine is incorporated into tyrosine residues within thyroglobulin, a large glycoprotein molecule. The resultant monoiodotyrosine (MIT) and diiodotyrosine (DIT) are brought into proximity and are coupled through an ether linkage to form thyroxine (also called tetraiodothyronine, or T4) and triiodothyronine (T3), the principal thyroid hormones. Apparently, only iodinated tyrosines are coupled because noniodinated thyronine is not found within the thyroid gland. Both coupling and iodination require oxidative conditions, both may involve the same peroxidase, and both are inhibited by thiourea derivatives.
34
Figure 2:
Thyroid gland
(www.mercksource.com)
35 The thyroid is unique among endocrine glands in its large storage capacity and relatively slow release of hormone: the gland normally contains about 8,000 µg of iodine, a reserve sufficient for at least 100 days. T4 and T3 are principally stored in the colloid of the thyroid gland lumen as part of thyroglobulin. The thyroid also contains a much smaller amount of another iodoprotein, thyralbumin, which is very similar to albumin. This substance is increased in many hyperfunctioning thyroids and in some neoplasms.
Release of hormone from the thyroid involves reentry of thyroglobulin by endocytosis from the colloid into the apical portions of the thyroid follicular cell. The engulfed droplets fuse with lysosomes and are hydrolyzed, releasing T4 and T3 into the circulation. Hydrolysis of thyroglobulin also produces some iodotyrosines within the follicular cells; these compounds are deiodinated in a reaction catalyzed by iodotyrosine deiodinase, restoring iodide to an intracellular pool from which reincorporation into hormone can occur. The intracellular pool is an important source of iodine; hypothyroidism and goiter develop in patients deficient in iodotyrosine deiodinase. The proteolytic step is stimulated by thyroidstimulating hormone (also called thyrotropin, or TSH) and inhibited by iodine; this action of iodine is probably its principal antithyroid effect. The proteolytic step is also inhibited by lithium but perhaps at a site different from that of inhibition by iodine; the antithyroid effects of iodine and lithium are sometimes additive.
36 The active circulating thyroid hormones are T4 and T3. Circulating T3 is produced within the thyroid from the coupling of MIT and DIT and from the monodeiodination of T4. The latter process also operates within the tissues to yield additional T3. Approximately 15 to 20 percent of circulating T3 arises from thyroid secretion, and the rest arises peripherally. In both hyperthyroidism and hypothyroidism, however, a significantly larger fraction of the circulating T3 than usual is derived from the thyroid.
Approximately 30µg of T3 is produced daily by peripheral deiodination of T4. This process is important in overall regulation of thyroid hormone. There are two deiodinases. Type I deiodinase, found in the liver and the kidney, acts on circulating T4 to produce T3 for peripheral tissues. Hypothyroidism can occur as a result of under activity of type I deiodinase induced by insufficiency of selenium, one of its components. Type II deiodinase is found principally in the pituitary, the brain, and the placenta. It is responsible for the monodeiodination of T4 in the pituitary; the resulting T3 has direct regulatory effects on TSH synthesis. The physiologic effects of thyroid hormones are produced primarily by T3, a large portion of which is derived from the deiodination of T4 in peripheral tissues. About 70 to 90% of peripheral T3 is produced by monodeoidination of T4 at the 5' position (outer ring). Peripheral monodeiodination of T4 at the 5 position (inner ring) results in the formation of reverse triiodothyronine (rT3), which is calorigenically inactive.
37 The synthesis and secretion of the major thyroid hormones, levothyroxine (T4) and l-triiodothyronine (T3), from the normally functioning thyroid gland are regulated by complex feedback mechanisms of the hypothalamic-pituitary-thyroid axis. The thyroid gland is stimulated to secrete thyroid hormones by the action of thyrotropin (thyroid stimulating hormone, TSH) which is produced in the anterior pituitary gland. TSH secretion is in turn controlled by thyrotropin-releasing hormone (TRH) produced in the hypothalamus, circulating thyroid hormones, and possibly other mechanisms. Thyroid hormones circulating in the blood act as feedback inhibitors of both TSH and TRH secretion. Thus, when serum concentrations of T3 and T4 are increased, secretion of TSH and TRH is decreased. Administration of exogenous thyroid hormones to euthyroid individuals results in suppression of endogenous thyroid hormone secretion.
The mechanisms by which thyroid hormones exert their physiologic actions have not been completely elucidated. T4 and T3 are transported into cells by passive and active mechanisms. T3 in cell cytoplasm and T3 generated from T4 within the cell diffuse into the nucleus and bind to thyroid receptor proteins, which appear to be primarily attached to DNA. Receptor binding leads to activation or repression of DNA transcription, thereby altering the amounts of mRNA and resultant proteins. Changes in protein concentrations are responsible for the metabolic changes observed in organs and tissues. Thyroid hormones enhance oxygen consumption of most body tissues and increase the basal metabolic rate and metabolism of carbohydrates, lipids, and proteins. Thus, they
38 exert a profound influence on every organ system and are of particular importance in the development of the central nervous system. Thyroid hormones also appear to have direct effects on tissues, such as increased myocardial contractility and decreased systemic vascular resistance (Mercksource.com, Jameson, 2003., Gilman, 2001).
Indications 1. As replacement or supplemental therapy in patients of any age or state (including pregnancy) with hypothyroidism of any etiology except transient hypothyroidism during the recovery phase of subacute thyroiditis: primary hypothyroidism resulting from thyroid dysfunction, primary atrophy, or partial or total absence of the thyroid gland, or from the effects of surgery, radiation or drugs, with or without the presence of goiter, including subclinical hypothyroidism; secondary (pituitary) hypothyroidism; and tertiary (hypothalamic) hypothyroidism. Levothyroxine sodium injection can be used intravenously when rapid repletion is required, and either intravenously or intramuscularly when the oral route is precluded.
2. As a pituitary TSH suppressant in the treatment or prevention of various types of euthyroid goiters, including thyroid nodules, subacute or chronic lymphocytic thydroiditis (Hashimoto's), multinodular goiter, and in conjunction
with
surgery
and
radioactive
iodine
therapy
in
the
39 management of thyrotropin-dependent well-differentiated papillary or follicular carcinoma of the thyroid.
The dosage and rate of administration of levothyroxine sodium is determined by the indication, and must in every case be individualized according to patient response and laboratory findings (www.levoxyl.com/insert, PDR, 2002).
Pharmacokinetics
Absorption – Absorption of orally administered T4 from the gastrointestinal (GI) tract ranges from 40% to 80%. The majority of the levothyroxine dose is absorbed from the jejunum and upper ileum. The relative bioavailability of levothyroxine tablets, compared to an equal nominal dose of oral levothyroxine sodium solution, is approximately 98%. T4 absorption is increased by fasting, and decreased in malabsorption syndromes and by certain foods such as soybean infant formula. Dietary fiber decreases bioavailability of T4. Absorption may also decrease with age. In addition, many drugs and foods affect T4 absorption.
Distribution – Circulating thyroid hormones are greater than 99% bound to plasma proteins, including thyroxine-binding globulin (TBG), thyroxine-binding prealbumin (TBPA), and albumin (TBA), whose capacities and affinities vary for each hormone. The higher affinity of both TBG and TBPA for T4 partially explains
40 the higher serum levels, slower metabolic clearance, and longer half-life of T4 compared to T3. Protein-bound thyroid hormones exist in reverse equilibrium with small amounts of free hormone. Only unbound hormone is metabolically active. Many drugs and physiologic conditions affect the binding of thyroid hormones to serum proteins. Thyroid hormones do not readily cross the placental barrier.
Metabolism –T4 is slowly eliminated (Table 2 – page 41). The major pathway of thyroid hormone metabolism is through sequential deiodination. Approximately eighty-percent
of
circulating
T3
is
derived
from
peripheral
T4
by
monodeiodination. The liver is the major site of degradation for both T4 and T3, with T4 deiodination also occurring at a number of additional sites, including the kidney and other tissues. Approximately 80% of the daily dose of T4 is deiodinated to yield equal amounts of T3 and reverse T3 (rT3). T3 and rT3 are further deiodinated to diiodothyronine. Thyroid hormones are also metabolized via conjugation with glucuronides and sulfates and excreted directly into the bile and gut where they undergo enterohepatic recirculation.
Elimination – Thyroid hormones are primarily eliminated by the kidneys. A portion of the conjugated hormone reaches the colon unchanged and is eliminated in the feces. Approximately 20% of T4 is eliminated in the stool. Urinary excretion of T4 decreases with age (www.levoxyl.com/insert).
41
Table 2:
Pharmacokinetics of levothyroxine sodium
Pharmacokinetic parameters of thyroid hormones patients Hormone Levothyroxine (T4) Liothyronine (T3)
Ratio in Thyroglobulin 10 –20
Biologic Potency 1
1
4
(www.levoxyl.com/insert)
t1/2 (days) 6–7 2
Protein Binding (%) 99.96 99.5
42
Adverse Reactions
Adverse reactions associated with levothyroxine therapy are primarily those of hyperthyroidism due to therapeutic overdosage. They include the following (www.levoxyl.com/insert, PDR 2002):
General: fatigue, increased appetite, weight loss, heat intolerance, fever, excessive sweating; Central nervous system: headache, hyperactivity, nervousness, anxiety, irritability, emotional lability, insomnia; Musculoskeletal: tremors, muscle weakness; Cardiac: palpitations, tachycardia, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest; Pulmonary: dyspnea; GI: diarrhea, vomiting, abdominal cramps; Dermatologic: hair loss, flushing; Reproductive: menstrual irregularities, impaired fertility.
43
1.3.3.
Synthetic vs natural levothyroxine products
Currently both natural and synthetic thyroid preparations are commercially available. Patients, thinking that "natural" means better, prefer natural thyroid hormones, which are made of desiccated animal thyroid glands. Nearly everyone can take synthetic levothyroxine; it is identical to the body's own levothyroxine. Thyroid hormones derived from animals (natural) contain liothyronine and levothyroxine and thus is a mixture.
Advantages of synthetic levothyroxine sodium therapy: 1) low-cost 2) stability: levothyroxine is more stabile then liothyroxine 3) non-allergenic (no foreign protein) 4) long half-life (seven days) -- supports once-daily dosing 5) more consistent manufacturing process/ source possible. Thus, synthetic levothyroxine sodium is preferred over natural thyroid preparations in the treatment of hypothyroidism.
44
1.3.4.
Stability of levothyroxine sodium
Won (1992) studied the kinetics of degradation of levothyroxine sodium in aqueous solutions. Levothyroxine sodium solutions were stored for prolonged periods at various temperatures. TLC, EI-MS and NMR were used to identify the degradation products. It was concluded that levothyroxine sodium in solution degraded by deiodination. The degradation was pH dependent and followed first order kinetics. The log k – pH profile of deiodination of levothyroxine sodium showed a plateau in the acidic pH region, dropped off sigmoidally in the neutral pH region and showed another plateau in the alkaline region. The author concluded that the kinetics of deiodination include proton attack on the anion and dianion in acidic solution and water attack on the anion and dianion in basic solution (Figure 3 – page 45).
45 Figure 3:
Deiodination scheme for levothyroxine
I O
I
O
-
O
I
OH
NH2
I
I O
I
O
I
O I
OH
NH2
-
+H+ I O
I
O
I
O I
OH
NH2
H
-I+ I I
O
-
O
O
I
OH
NH2
46
Won (1992) studied the solid state stability of levothyroxine sodium at elevated temperatures (open vial at 60oC for 7 weeks). The degraded levothyroxine sodium was separated using TLC and then an EI-MS was done on the major degradation product. In solid state, the degradation of levothyroxine sodium indicated a deamination reaction following bi-phasic degradation pattern.
Won (1992) further studied the effect of temperature (50-80oC) on the degradation of levothyroxine sodium. Levothyroxine sodium degraded at a faster rate as the temperature increased and showed bi-phasic degradation kinetics. There was little or no degradation observed at 50 oC, above which temperature, levothyroxine sodium degradation was observed.
Wortsman et al. (1989) studied the thermal inactivation (HPLC and DSC techniques) of levothyroxine sodium from 50–250oC. They observed that the degradation of levothyroxine sodium started at 80oC or higher.
Kazemiford et al. (2001) studied the photo-degradation of levothyroxine sodium tablets from three manufacturers. The extracted levothyroxine sodium solution was irradiated with a 500W Xenon lamp at 320 nm for 2 hours. Assay was performed using an HPLC method with electrochemical detection. Also LC/MS/MS was done in order to elucidate the structure of the degradation products.
The
observed
degradation
products
were
Liothyroxine,
47 Diiodothyronine, Iodothyronine, Diiodotyrosine, Iodotyrosine and Tyrosine. It was concluded that levothyroxine sodium is photosensitive.
Garnick et al. (1984) reported the degradation of levothyroxine sodium raw material (drug substance) and tablets under various conditions. The results are shown in Table 3. These results show that levothyroxine sodium is sensitive to temperature at 80oC (in tablets), which is consistent to previous findings (Won, 1992). They also found levothyroxine showed enhanced degradation in the presence of an oxidizing agent. They observed levothyroxine was not photosensitive which is contradicting to the results of Kazamefard et. al. (2001). Table 3:
Degradation of levothyroxine sodium raw material and tablets
Degradation
Reagents
Time (h)
Assay (%)
/ Conditions o
Raw material
Thermal
80 C
168
100.8
Hydrolytic
0.1 M HCl
87.5
90.6
0.1 M NaOH
44
92.8
Tert-butyl
1
37.5
UV (shortwave)
144
98.9
UV (longwave)
48
101.4
Intact Tablets UV (longwave) 168
91.9
Oxidative
hydroperoxide Photochemical
Photochemical Thermal
80oC
168
50.0
48
Gupta et al. (1990) studied levothyroxine sodium tablets from two different manufacturers and showed that one set of tablets had some excipients (not reported) that hasten the decomposition after extraction of levothyroxine sodium in the presence of light. They concluded that the presence of certain excipients affected the stability of levothyroxine sodium tablets.
Rhodes (1998) reviewed the regulatory aspects of the formulation and evaluation of levothyroxine tablets. He reported that levothyroxine tablets stability is a complex problem and a scientific study of the degradation process would result in the use of appropriate formulation and processing methods, which would effectively remove current problems. The above statement might shed light to some of the factors that have been associated with the current problems associated with commercial levothyroxine sodium tablets.
1.3.5.
Formulations with increased stability
There have been numerous attempts to produce formulations with improved stability. US Patent 5,225,204 (Chen et. al. 1993) described a way of producing a stabilized complex of levothyroxine sodium in tablets by dissolving poloxamer or povidone and levothyroxine sodium in a polar solvent and adding a cellulose carrier in the liquid and drying this. It was also claimed that a dry geometric
49 mixture of levothyroxine sodium and a cellulose carrier (microcrystalline cellulose, hydroxymethyl propylcellulose, etc) forms a stable complex.
US Patent 6,056,975 (Mitra, 1998) argued the claims of the previous patent and showed that such a production procedure resulted in a product with worse stability than the commercially available products. It claimed another way to stabilize levothyroxine sodium products was by formulating with: 1) a carbohydrate with MW between 500 and 1500000. 2) an inorganic salt 3) glycine.
US
Patent
6,190,696
(Groenewoud,
1999)
claimed
to
stabilize
levothyroxine sodium tablets using an iodine salt or an iodine donor compound as a stabilizing agent.
50
1.4. Rationale for the study
According to the Federal Register Notice, 1997 “no currently marketed orally administered Levothyroxine sodium product has been shown to demonstrate consistent potency and stability and, thus, no currently marketed orally administered Levothyroxine sodium product is generally recognized as safe and effective.”
The FDA required all manufactures to file new NDA’s. Current commercial formulations are approved with an 18-month expiration date. It is known that levothyroxine sodium is unstable to light, moisture, pH (acidic), temperature and oxidation. Also, the type of excipient influences levothyroxine sodium stability in solution. According to Rhodes, 1998, levothyroxine tablets stability is a complex problem and a scientific study of the degradation process would result in the use of appropriate formulation and processing methods, which would effectively remove current problems.
Thus, a study of the effect of formulation and processing variables on the stability of levothyroxine sodium tablets, will help bring insight on some of the
51 problems which lead to inconsistencies, and thereby assist in the production of stable and consistent levothyroxine sodium tablets.
52
2. OBJECTIVE, HYPOTHESIS AND SPECIFIC AIMS
2.1. Objective
To study the influence of the formulation and processing variables on the quality and stability of levothyroxine sodium tablets.
2.2. Hypothesis
The stability of levothyroxine sodium is influenced by the type of formulation and processing variables used.
53
2.3. Specific aims
To study the effect of the following variables on the stability of levothyroxine sodium tablets.
2.3.1.
Formulation variables
•
Type of excipient o Diluents o Lubricants o Disintegrants o Binders o pH adjusters (acidic, basic).
2.3.2.
Manufacturing variables
•
Method of manufacture o Direct compression vs. wet granulation
•
Compression force.
54
3. EXPERIMENTAL
3.1. Materials
Acetonitrile - HPLC (Fisher Scientific, Fair Lawn NJ, USA) Aluminum lake blue # 2 (Colorcon, West Point, PA, USA) ATLAS® mannitol (ICI Americas, Inc., Wilmington, DE, USA) Citric acid (Fisher Scientific, Fair Lawn NJ, USA) Croscarmellose sodium (AcDiSol®, FMC Corporation, Newark, DE, USA) Crospovidone (BASF, Ludwigshafen, Germany) D-Thyroxine (ACROS Organics, Fair Lawn NJ, USA) Dibasic calcium phosphate (Emcompress®, Penwest, Patterson NY, USA) Diiodo L thyronine (Sigma Chemical Co., St. Louis, MO, USA) Diiodothyroacetic acid (Sigma Chemical Co., St. Louis, MO, USA) Fumed silica (Cab-O-Sil® M-5P, Cabot Corporation, Tuscola, IL, USA) HYDRANAL® Composite 2 & HYDRANAL® Methanol (Aldrich Chemical Company, Milwaukee, WI, USA) High-density polyethylene bottles (HDPE) (SETCO Inc., Anaheim, CA, USA) Hydrochloric acid (Fisher Scientific, Fair Lawn NJ, USA) Lactose anhydrous (Quest International, Norwich, NY, USA) Levothyroxine sodium pentahydrate (ACROS Organics, Fair Lawn NJ, USA)
55 Magnesium oxide (Fisher Scientific, Fair Lawn NJ, USA) Magnesium stearate (Mallinckrodt Chemical Inc, St. Louis MO, USA) Microcrystalline cellulose (MCC) (Emcocel® 90M, Penwest, Patterson NY, USA) Starch® 1500 (Colorcon, West Point, PA, USA) Phosphoric acid (Fisher Scientific, Fair Lawn NJ, USA) Potassium chloride (Fisher Scientific, Fair Lawn NJ, USA) Potassium hydrogen phthalate (Fisher Scientific, Fair Lawn NJ, USA) Potassium Phosphate monobasic (Fisher Scientific, Fair Lawn NJ, USA) Povidone (PVP, BASF, Ludwigshafen, Germany) Hydroxypropyl methycellulose (HPMC, Methocel® K100LV, Dow Chemical Co. Midland, MI, USA) Sodium bicarbonate (Fisher Scientific, Fair Lawn NJ, USA) Sodium carbonate (Fisher Scientific, Fair Lawn NJ, USA) Sodium hydroxide (Fisher Scientific, Fair Lawn NJ, USA) Sodium starch glycolate (Explotab®, Penwest, Patterson NY, USA) Stearic acid (Mallinckrodt Chemical Inc, St. Louis MO, USA) Tartaric acid (Fisher Scientific, Fair Lawn NJ, USA) Tetraiodothyroacetic acid (Sigma Chemical Co. St. Louis, MO, USA) Trifluoroacetic acid (Fisher Scientific, Fair Lawn NJ, USA) Triiodo L thyronine (Fisher Scientific, Fair Lawn NJ, USA) Triiodothyroacetic acid (Sigma Chemical Co., St. Louis, MO, USA) Water – HPLC (Fisher Scientific, Fair Lawn NJ, USA)
56
3.2. Equipment
Accumet 1002 pH meter (Fisher Scientific, Fair Lawn NJ, USA) Balances PB1502, AB104 (Mettler Toledo, Switzerland) Beckman System Gold HPLC system(Beckman Coulter, Fullerton CA, USA), equipped with a Beckman 168 UV detector Carver Laboratory Press (Fred S. Carver Inc. Menomonee Falls, WI, USA) Cyano - spherisorb (5µm, 25cm x 2mm I.D.) HPLC Column (Water Corp., Milford, Massachusettes, USA) Computrac Moisture Analyzer MAX 50 (Arizona Instrum., Phoenix AZ, USA) Dissolution Tester VK7000 (VanKel Technology Group, Carry, NC, USA) Espec Humidity Cabinet LHL112 (Tabai Espec Corp, Osaka, Japan) Hardness Tester (Key International Inc., Englishtown NJ, USA) Isotemp Incubator 655D (Fisher Scientific, Pittsburgh PA, USA) Karl Fisher Moisture Analyzer (Metrohm, Switzerland) Micromass mass spectrophotometer with electro-spray ionization (Micromass LTD, UK) Moisture sorption balance (VTI Corporation, Hialeah, Fl, USA) pH meter equipped with flat surface electrode (PH 900®, Courage-Khazaka, Köln, Germany) Planetary Mixer (Kitchen Aid, St. Joseph MI, USA)
57 Rotary tablet press Manesty D3B (Manesty Machines Ltd., Liverpool, UK) Spectrophotometer DU 640 (Beckman Coulter, Fullerton CA, USA) Starrett Micrometer (Starrett Athol MA, USA) Stirrer/Hot plate PC 620 (Corning Inc., New York NY, USA) Turbula Mixer T2G (Glen Mills Inc., Maywood NJ, USA) Ultrasonic Cleaner FS30 (Fisher Scientific, Pittsburg PA, USA)
58
3.3. Methods 3.3.1.
Stability of levothyroxine sodium pentahydrate
The stability of levothyroxine sodium was tested in: a) Powder: ICH accelerated stability conditions b) Solution: Effect of pH and oxidizing agent
3.3.1.1.
Levothyroxine sodium drug substance
Levothyroxine sodium drug substance was stored in open and closed vials at 40oC and 75% RH (ICH Q1A(R), 2000., FDA, 1998) for a total of 6 months. Assay (3 replicates) and moisture determination was performed at 0, 3 and 6 months.
3.3.1.2.
In solution
The effect of pH and oxidizing agent on the stability of levothyroxine in solution was evaluated.
Levothyroxine solutions at a final concentration of 20µg / ml were prepared at the following pH: 2, 4, 6, 8, 10 and 12 using buffers (Table 4, page 60). One milliliter of the test solutions was diluted to ten milliliters using mobile phase to prepare
59 assay samples. Assay (3 replicates) was performed at time 0 and after 7 days storage at room temperature. Additionally, levothyroxine aqueous solutions (50 µg / ml) at pH 2 and pH 10 (Table 4, page 60) were prepared and stored at 25oC. One milliliter of the test solutions was diluted to twenty-five milliliters using mobile phase to prepare assay samples. Samples were withdrawn at 0, 1, 2, 3, 4 and 10 days, and assayed (3 replicates).
Levothyroxine aqueous solution (50 µg/ml) samples with 1% hydrogen peroxide were prepared at pH 2 and pH 10 (Table 4, page 60). One milliliter of the test solutions was diluted to twenty-five milliliters using mobile phase to prepare assay samples. Samples were withdrawn after 0, 1, 7 and 24 hours, and then assayed (3 replicates).
60
Table 4:
Buffer solutions used
pH
Buffer used (final volume 100ml)
2
50ml 0.2M KCl + 13ml 0.2M HCl
4
50ml 0.2M potassium hydrogen phthalate + 0.1ml 0.1M HCl
6
50ml 0.2M Potassium Phosphate monobasic + 5.6ml 0.1M NaOH
8
50ml 0.2M Potassium Phosphate monobasic + 46.1ml 0.1M NaOH
10
50ml 0.2M Sodium bicarbonate + 10.7ml 0.1M NaOH
12
25ml 0.2M KCl + 66ml 0.2M NaOH
61
3.3.2.
Hygroscopicity of levothyroxine sodium pentahydrate
The hygroscopicity of levothyroxine sodium was determined with a moisture sorption balance. Levothyroxine sodium was placed on the weighing tray and the relative humidity in the environment was increased gradually from 30 to 100 % R H (5 % RH increments), after allowing the necessary time for the sample stabilization, up to 2 hours, at each period. Subsequently, the humidity was gradually reduced to 10% RH in a similar manner. Additionally, levothyroxine sodium was placed on a tray and then dried using nitrogen at 25 oC and the above mentioned procedure was performed.
3.3.3.
Effect
of
formulation
variables
on
the
stability
of
levothyroxine sodium tablets
3.3.3.1.
Effect of excipients on the stability of levothyroxine sodium tablets
The effect of diluents on the stability of levothyroxine sodium tablets was studied. The diluents and storage conditions used were: Diluent type – Five commonly used diluents with different hygroscopicities (lactose anhydrous, microcrystalline cellulose, dibasic calcium phosphate, starch, mannitol).
62 Storage - with or without desiccant at ICH accelerated stability conditions.
Different batches of 0.05 % levothyroxine sodium tablets were manufactured using one of the above mentioned diluents. All tablets contained magnesium stearate and aluminum lake blue # 2. The ingredients, according to the ratios listed in Table 5 (page - 63) were mixed by geometric dilution in a mortar and pestle.Tablets were directly compressed at 1000 lbs using a Carver Press. The tablets were stored in HDPE bottles with or without desiccant.
The stability of the tablet batches was evaluated for a total of six months under ICH accelerated stability conditions (40 oC and 75 % R H). After storage, assay (3 replicates) and moisture determination were performed at 0, 3 and 6 months. For the 6-month samples, which exhibited degradation, mass spectroscopy was performed to identify degradation products.
The 3 month
samples were analyzed for racemization by the same assay procedure using a chiral crown ether column (prepared in-house)(Jin, 2001).
63
Table 5:
1
Formulation of levothyroxine sodium tablets
Ingredients
Percentage W/W
Levothyroxine sodium
0.05
Aluminum lake blue # 2
0.05
Magnesium stearate
1.00
Diluent
98.9
lactose anhydrous, microcrystalline cellulose, mannitol, starch or dibasic calcium
phosphate ∗
Tablets contain 50µg levothyroxine sodium/ 100mg, and compressed using
Carver Press
64
3.3.3.2.
Compatibility with excipients
Levothyroxine sodium compatibility with additives (binder, disintegrant, glidant / lubricant) was tested in the presence of dibasic calcium phosphate in 20% w/w aqueous slurries at 50oC for 30 days. The following additives were tested:
Binders; hydroxypropyl methylcellulose, polyvinylpyrollidone at 5% of tablet weight Disintegrants; sodium starch glycolate, croscarmellose sodium, crospovidone at 5% of tablet weight Lubricants / glidants; fumed silica, stearic acid, magnesium stearate at 5% of tablet weight
The stability of levothyroxine sodium in slurries was tested in the presence of the excipients. Since the above-mentioned excipients are generally used in low concentrations, for this study, the excipients were added at 5% of the solid content. Dibasic calcium phosphate was chosen, as it was found to be one of the most inert diluents during the preliminary studies.
Five % of each of the above excipients were individually mixed with ninety five % dibasic calcium phosphate. The above powder blends were mixed with
65 water to make 20 % (w/v) aqueous slurries. Levothyroxine solution was added to all the slurries to obtain a final drug concentration of 4µg/ml. Assay ( 2 replicates) was performed at time 0 and after storage at 50oC for 1 month. The study was performed with levothyroxine in solution to maximize its availability for reaction. The temperature was set at 50oC, since above this temperature thermal degradation is reported.
3.3.3.3.
Effect of pH on levothyroxine stability
In slurries
Levothyroxine sodium stability at different pH (3, 5, 7, 9, 11) in the presence of microcrystalline cellulose, dibasic calcium phosphate, starch or mannitol was evaluated in aqueous slurries at 50oC for 30 days.
Twenty percent (w/v) aqueous slurries were prepared using each of the above-mentioned diluents. For each diluent, the pH of the slurries was adjusted to 3, 5, 7, 9 and 11 with 0.1N HCl and/or 0.1N NaOH. Additional slurries were prepared for each diluent without pH adjustment. Levothyroxine solution was added to all the slurries to obtain a final drug concentration of 4µg/ml and the pH of all the slurries was verified using a pH meter outfitted with a flat-surface electrode. Assay (2 replicates) was performed at times 0 and 1 month after storage at 50 oC.
66
In tablets
The stability of levothyroxine sodium tablets manufacture with pH modifiers was studied. •
Additives used o Acidic: citric or tartaric acid o Basic: sodium bicarbonate, sodium carbonate or magnesium oxide
•
Diluents: dibasic calcium phosphate or microcrystalline cellulose
Different batches of levothyroxine sodium tablets were formulated using two different diluents and 5 different pH modifiers as mentioned above. The pH’s of these substances in solution are listed in Table 7, page 68. The ingredients in the ratios listed in Table 6 page 67, were mixed by geometric dilution and directly compressed at a 1000lb of applied force, using a Carver press. All tablets were stored in HDPE bottles and their stability was evaluated for a total of 6 months under ICH accelerated stability conditions (40oC and 75% RH). The assay (3 replicates) and moisture content were determined at 0, 3 and 6 months.
67
Table 6:
Formulation (% w/w) of levothyroxine sodium tablets with pH
modifiers
Ingredients
Percentage % W/W
1
Levothyroxine sodium
0.05
Aluminum lake blue # 2
0.05
Magnesium stearate
1.00
pH modifiers1
10.00
Diluent2
88.90
sodium carbonate, sodium bicarbonate, magnesium oxide, tartaric acid or citric
acid 2 ∗
dibasic calcium phosphate or microcrystalline cellulose
Tablets contain 50µg levothyroxine sodium /100mg, and compressed using Carver Press
68 Table 7:
*
pH of the saturated solutions of pH modifiers.
pH Modifier
pH
Tartaric acid
1.7
Citric acid*
0.4
Sodium bicarbonate
8.1
Sodium carbonate
11.9
Magnesium oxide
10.3
Obtained from Badaway et al. (1999).
69
Table 8:
Number of experiments performed to study the effect of formulation
variables
Slurry Factor studied (Number of levels)
Diluent
Tablets
mixtures
Total
(Selected
(stored at
20% w/w
Experiments
after
40oC/75%RH)
(50oC-30 days)
screening) Diluents (5)
-
5
5
5
Binders (2)
1
-
2
2
Disintegrants (3)
1
-
3
3
Glidant/Lubricants
1
-
3
3
24
24
-
10
(3) pH adjuster(6)
4
pH modifiers (5)
2
Total
10 -
47
70
3.3.4.
Effect of the manufacturing technology on the stability of levothyroxine sodium tablets
The stability of levothyroxine sodium (50µg) tablets with and without a pH modifier and manufactured by either direct compression or wet granulation, and compressed at different compression forces (0, 2000 or 6000 lbs) was evaluated. The pH modifier used was sodium carbonate.
3.3.4.1.
Levothyroxine
sodium
tablet
manufactured
by
direct
compression
Levothyroxine sodium, and part of the dibasic calcium phospahte (q.s. 100 gm) was premixed by geometric dilution in a mortar and pestle, transferred to a turbula jar and mixed with the remaining ingredients, except magnesium stearate, for 20 minutes (Table 9 – page 72 and Table 10 – page 73). Magnesium stearate was added and mixed for 3 minutes. The powder blend was compressed (Figure 5 – page 75) using a rotary tablet press at 2000 and 6000 lbs of applied force which was monitored using beam spider software (example Figure 4 – page 74). The target tablet weight was set at 150mg. All tablets were stored in HDPE bottles.
The stability of the powder blend and the corresponding tablets was evaluated for a total of six months under ICH accelerated stability conditions (40
71 o
C and 75% R H). The hardness, thickness, weight variation, assay, moisture
content, and dissolution were determined at 0, 3 and 6 months.
3.3.4.2.
Wet granulation
Levothyroxine sodium, and part of the dibasic calcium phosphate were premixed by geometric dilution in a mortar and pestle. The premixed powder, the sodium carbonate (only for batches made with pH modifiers), half of the croscarmellose sodium and the remaining dibasic calcium phosphate were mixed in a turbula mixer for 20 minutes. A 10% polyvinylpyrrolidone binder solution was used to granulate the powder in a planetary mixer. The granules were dried at 40oC for 12 hours and the moisture content was determined using a computrac moisture analyzer. The granules and the remaining half of the croscarmellose sodium were mixed using the turbula mixer for 5 minutes. Magnesium stearate was mixed for 3 minutes. The mixture was compressed (Figure 6– page 77) at 2000 and 6000 lbs of applied force which was monitered using beam spider software (Example Figure 4 - page 74), using a rotary tablet press. All tablets were stored in HDPE bottles. The stability of the granule blend and the corresponding tablets was evaluated for a total of 6 months under ICH accelerated stability conditions (40 oC and 75% RH). The assay, moisture content, dissolution, hardness, thickness and weight variation were determined at 0, 3 and 6 months.
72 Table 9:
Levothyroxine sodium tablets manufactured by wet granulation or
direct compression on a rotary tablet press
Ingredients
Percentage
Weight (500 gm total)
Levothyroxine sodium
0.033
0.1667 gm
Polyvinylpyrollidone
5
25.0
Croscarmellose sodium
2
10.0
Magnesium stearate
1.00
5
Dibasic calcium phosphate
91.967
459.833
∗
Tablets contain 50µg levothyroxine sodium/ 150mg, and compressed using
instrumented rotary tablet press
73
Table 10:
Levothyroxine sodium tablets with sodium carbonate manufactured
by wet granulation or direct compression on a rotary tablet press
Ingredients
Percentage
Weight (500 gm total)
Levothyroxine sodium
0.033
0.1667 gm
Polyvinylpyrollidone
5
25.0
Croscarmellose sodium
2
10.0
Magnesium stearate
1.00
5
Sodium carbonate
10
50
Dibasic calcium phosphate
81.967
409.833
∗
Tablets contain 50µg levothyroxine sodium/ 150mg, and compressed using
instrumented rotary tablet press
74 Figure 4:
Example of beam spider compression force and ejection force
75
4. TABLET MANUFACTURING AND EVALUATION
4.1. Tablet manufacturing Tablets were manufactured either by wet granulation or direct compression. The general manufacturing procedures are described in Figures 5. & 6.
Figure 5:
Levothyroxine Diluents (sieve #35)
Process flow chart for tablets manufactured by direct compression
sodium
Mixing 20 Minutes (Turbula)
Pre mixing by geometric dilution
Lubricant
Further mixing by geometric dilution (Mortar and pestle)
Lubricant Final Mixing 3 Minutes
Compression(*) (Manesty D3B Press)
(*)
in process control of tablets’ weight and hardness recording of the compression and ejection
Compression (Carver Press)
forces
(Beam
Spider
Software)
76
Direct compression - Process flow: o The drug and excipients were accurately weighed. o The powders were screened using screen #35. o The drug and diluent were premixed by geometric dilution. o For tablet manufactured using Carver press (20gm batch size), mixing was performed using a mortar and pestle by geometric dilution and the tablets compressed at 1000 lbs using the Carver press. o For tablets manufactured by rotary press, the above premixed powder and the remaining excipients (except lubricant) were transferred to a turbula jar and mixed for 20 minutes using the turbula mixer. o Magnesium stearate was accurately weighted and mixed with the powder in the turbula jar for an additional 3 minutes. o The powder was compressed into tablets using an instrumented rotary tablet press.
77 Figure 6:
Process flow chart for tablets manufactured by wet granulation
Levothyroxine sodium Dibasic calcium phosphate (sieve #35)
½ Disintegrant and remaining excipients (sieve #35) Binder dispersion
Pre mixing by geometric dilution
Mixing 20 Minutes
(Turbula Mixer)
Granulation
(Planetary Mixer)
Wet screening
(# 12 mesh)
Drying
(Oven 40°C)
Dry screening
½ Disintegrant
(# 18 mesh)
Mixing 5 Minutes (Turbula Mixer)
Lubricant
Final Mixing 3 Minutes
Compression(*)
(*)
(Manesty D3B Press)
in process control of tablets’ weight and hardness recording of the compression and ejection forces (Beam Spider Software)
78 Wet granulation - Process flow: o The drug and excipients were accurately weighed. o The powders were screened using screen #35. o The drug and diluent were premixed by geometric dilution. o Half the disintegrant and the remaining excipients (except lubricant) were transferred to the turbula jar and mixed for 20 minutes. o The powder mixture was transferred to the planetary mixer and granulated with the binder dispersion. o The wet mass was passed through a # 12 sieve and the resulting granules were placed on trays and dried in an oven at 40°C for 12 hours. o The dried granules were passed through a # 18 sieve. o Remaining disintegrant and the dried granules were accurately weighted and mixed in the turbula jar for an additional 5 minutes. o Magnesium stearate was accurately weighted and mixed in the turbula jar for additional 3 minutes. o The powder was compressed into tablets using an instrumented rotary tablet press and tablets collected during compression for in-process testing (weight and hardness).
79
4.2. Tablet Evaluation 4.2.1.
Physical properties
Tablet weight variation - ten tablets from each batch were individually weighed and the average weight, standard deviation and relative standard deviation were reported.
Thickness - was determined for 10 pre-weighed tablets of each batch using a micrometer, and the average thickness, standard deviation and relative standard variation are reported.
Hardness - was determined for 10 tablets (with known weight and thickness) of each batch; the average hardness, standard deviation and relative standard variation are reported.
4.2.2.
Content uniformity
This was assessed according to the USP requirements under “Uniformity of Dosage Units” , using the assay method described in section # 4.2.3. The batch meets the USP requirements if the amount of the active ingredient in each of the 10 tablets tested/batch lies within the range of 85% to 115% of the label claim and relative standard deviation is less than or equal to 6%. If one of
80 these conditions is not met, an additional 20 tablets will be tested. Not more than 1 unit of the 30 tested tablets should be outside the range of 85% and 115% of the label claim and no unit outside the range of 75% to 125% of label claim; also relative standard deviation should not exceed 7.8%. The content uniformity of all batches was tested using these specifications and the assay procedure described below.
4.2.3.
Assay
An HPLC system equipped with an auto sampler and a UV detector set at 225 nm was used for the analysis of all samples (Garnick et al, 1984). The reversed phase HPLC assay method used a Waters® Spherisorb Cyano, 25 cm x 2 mm column (particle size 5 µm) and an acetonitrile : water mixture (40 : 60) with 0.5 ml/l phosphoric acid as mobile phase at a flow rate of 0.3 ml/min. For the analysis of samples of tablets manufactured with pH modifiers, the phosphoric acid in the mobile phase was replaced with trifluoroacetic acid (0.3 ml/l) as this enhanced the recovery of levothyroxine sodium. Spiked levothyroxine sodium samples with various degradation products, namely, triiodo L thyronine, diiodo L thyronine, tetraiodothyroacetic acid, triiodothyroacetic acid and diiodothyroacetic acid were injected to test the methods (Garnick et al, 1984).
81
4.2.4.
Moisture determination
Moisture content in samples (approximately 100 mg powdered sample) was determined by Karl Fisher titration with Hydranal® Composite 2, according to the USP titrimetric method for water determination. Three determinations were performed for the water titer value and relative standard deviation.
4.2.5.
Stability
Tablets
The tablets were evaluated for stability using ICH accelerated stability conditions. The tablets were packed in a HDPE bottle with / without desiccant and stored at 40oC / 75% relative humidity for 6 months. A stability-indicating assay was performed at 0, 3, 6 months. The tablets were tested for: •
% loss in potency (Assay value of levothyroxine);
•
% moisture content;
•
identification of possible tablet degradation products.
82
Slurries
The aqueous slurries (20% w/v) were evaluated for stability at 50 oC for one month. The assay (2 replicates) was performed at times 0 and 1 month. The pH of all the slurries was verified using a pH meter outfitted with a flat-surface electrode.
4.2.6.
Dissolution test
Dissolution of levothyroxine sodium was performed as follows: Apparatus 2 (paddle): 50 RPM Medium: 1000 ml 0.1 N HCl. Samples were drawn at 0, 45, 60, 120, 180 and 240 minutes. 100µl of the above samples were assayed for levothyroxne sodium by assay method in section 4.2.3.
83
5. RESULTS AND DISCUSSIONS
5.1. Stability of levothyroxine sodium
Drug substance
Levothyroxine sodium drug substance was found to be stable when stored for 6 months at accelerated stability conditions (40oC / 75% RH) in open or closed containers, with assay values of 97.5% and 97.1%, respectively (Figure 7 – page 85).
In solution
The pH of the surrounding medium affected the stability of levothyroxine sodium confirming results by Won (1992). As the pH of the surrounding aqueous medium was increased, the stability of levothyroxine in solution improved or less degradation was observed over time (Figure 8 - page 86). This was further confirmed by testing the stability of levothyroxine sodium at pH 2 and 10, for 10 days at room temperature with sampling at different time intervals in which case similar phenomena were observed (Figure 9 – page 87).
84 In the presence of H2O2, more oxidation was observed at pH 10 (complete loss of drug in 1 hour) than at pH 2 (6.7% degradation in 24 hours)(Table 11 page 88). Thus, it was concluded that levothyroxine sodium was more sensitive to degradation by oxidation at basic pH.
85
Stability of levothyroxine sodium drug substance at 40oC/75 % RH
Figure 7:
in closed or open containers for six months
120
100
Assay (%)
80
0month 3month 6month
60
40
20
0 Closed Vial
*Average of 3 determinations each
Open Vial
86 Figure 8:
Effect of pH on the stability of levothyroxine in solution stored for 7
days at room temperature
110 105 100 95
Assay(%)
90 85 80 75 70 65 60 0
2
4
6 pH
*Average of 2 determinations each
8
10
12
87 Figure 9:
Effect of pH on the stability of levothyroxine in solution stored for 10
days at room temperature.
120
100
Assay (%)
80
60
40
pH2 pH10
20
0 0
2
4
6 Days
*Average of 3 determinations each
8
10
12
88
Table 11:
Effect of oxidizing agent on the stability of levothyroxine (%) in
solution at room temperature.
Time
2µg/ml levothyroxine
2µg/ml levothyroxine
(hrs)
and 1% H2O2 at pH 2
and 1% H2O2 at pH 10
1
97.6
0
7
97.4
-
24
93.6
-
89
Hygroscopicity and moisture content of levothyroxine sodium pentahydrate
The moisture / water content of levothyroxine sodium pentahydrate was measured by Karl Fisher moisture determination (USP titrimetric method) and found to be 9.6%. This is considered to be the total moisture (bound and unbound), as the drug is completely soluble in the Hydranal methanol titration medium. This is consistent with the findings of Post and Warren (1976). The moisture did not increase when stored at ICH accelerated stability conditions for 6 months in an open or closed vial (Figure 13 – page 93).
Levothyroxine sodium showed little moisture gain (adsorption) (~1%) when exposed to relative humidity between 30 to 90 %, both with or without prior drying with nitrogen in the moisture adsorption-desorption study (Figure 11 – page 91 and Figure 13 – page 93). It lost a significant amount of moisture (3.5 % weight change) rapidly from 30 to 10 % RH, which corresponds to nearly 2 moles of water as seen in Figure 12 - page 92. This indicates levothyroxine sodium pentahydrate is non-hygroscopic under normal processing conditions (RH > 30%), and below this RH, it loses moisture rapidly.
90 Figure 10:
Total moisture content of levothyroxine sodium pentahydrate stored
at 40oC/75% RH for six months
12 10
% moisture
8 6 4 2 0 standard (Initial)
stored in a closed vial Pure Levothyroxine sodium
stored in an open vial
91 Figure 11:
Absorption / desorption isotherm of levothyroxine sodium pentahydrate
1.0 0.5 0.0 0
20
40
60
80
Weight (% change)
-0.5 -1.0 Adsorption Desorption
-1.5 -2.0 -2.5 -3.0 -3.5 -4.0 %RH
100
92 Figure 12:
Absorption / desorption isotherm of levothyroxine sodium
pentahydrate after drying at 25oC with nitrogen
9.0 8.0 7.0
Weight (% change)
6.0 Adsorption Desorption
5.0 4.0 3.0 2.0 1.0 0.0 0
20
40
60 %RH
80
100
93 Figure 13:
Absorption / desorption isotherm (H20 moles) levothyroxine sodium
pentahydrate after drying at 25oC with nitrogen
4.0
3.5
H2O/sample (moles)
3.0
2.5 Adsorption Desorption
2.0
1.5
1.0
0.5
0.0 0
20
40
60 %RH
80
100
94
5.2. Stability of levothyroxine sodium tablets with various excipients
5.2.1.
Stability of levothyroxine sodium commercial tablets
A commercially available levothyroxine product (100µm/tablet) which had multiple recalls was obtained and stored under ICH accelerated stability conditions for 6 months. The tablets were formulated with microcrystalline cellulose, magnesium stearate and AcDiSol®. These tablets were manufactured prior to the filing of NDA’s for levothyroxine as recommended by FDA. The initial levothyroxine assay value for these tablets was 100.05 % and they met USP Uniformity of Dosage units criteria (RSD = 3.18 %). About 20 % loss in potency was observed after six months (Figure 14 – page 98).
5.2.2.
Stability of levothyroxine sodium tablets with various excipients
It was found that, to varying extents, different diluents influenced the stability of levothyroxine sodium tablets at accelerated stability conditions. The 6 month assay values of levothyroxine sodium in tablets manufactured with lactose
95 anhydrous, microcrystalline cellulose, starch, mannitol or dibasic calcium phosphate and stored without a dessicant were 68.7, 70.7, 73.3, 86.9 and 85.3 %, respectively. Tablets manufactured with mannitol or dibasic calcium phosphate were stable (met USP assay requirements) for 3 months (Table 12 – page 99), but not for 6 months at accelerated stability conditions. Tablets manufactured using lactose anhydrous, microcrystalline cellulose or starch as diluents did not meet USP assay requirements (90 to 110 %) after only 3 months of storage, at accelerated stability conditions. Thus, dibasic calcium phosphate and mannitol proved to be the most suitable diluents, while starch, microcrystalline cellulose and lactose resulted in the highest loss in potency.
Tablets
with
higher
moisture
content
(those
manufactured
with
microcrystalline cellulose, starch and lactose as diluents) exhibited higher degradation (Table 12 – page 99, Figure 15 - page 100 and Figure 16 – page 101). Thus, it was concluded that the type of diluent used for manufacturing levothyroxine tablets significantly influenced its stability. When levothyroxine sodium tablets manufactured with various diluents, were stored in the presence of desiccant, the results were similar (Table 13 - page 102). The type of excipient used influenced the stability of levothyroxine tablets. The presence or absence of desiccant did not significantly (p=0.66) influence the stability of levothyroxine sodium, even though there was a difference in the amount of moisture in the tablets when stored at accelerated conditions with or without a desiccant (Figure 16 – page 101 and Figure 18 - page 104).
96
The pH of saturated solutions of the powdered levothyroxine sodium tablets ranged from 5.5 – 7.3, while the pH of saturated solution of levothyroxine sodium was 8.0. Thus, the inherent pH of the microenvironment found in the compressed tablets did not coincide with the pH of maximum stability of levothyroxine sodium. This also explains why levothyroxine sodium drug substance alone was more stable than levothyroxine sodium in tablets at ICH accelerated stability conditions.
The primary degradation pathways identified by mass spectroscopy were deiodination and deamination. These findings were consistent with the previous findings of Andre et al. (1996), and Kazemifard et al. (2001). Decarboxylation products were also observed. Racemization was not detected in any of the tablet formulations.
It was concluded that the type of diluent used in the manufacture of levothyroxine sodium tablets affects its stability. Thus, the study and proper choice of excipients in the manufacturing of levothyroxine sodium tablets are critical. Although lactose and microcrystalline cellulose are the most commonly used diluents in currently marketed levothyroxine products, they did not prove to be the most suitable diluents in this study.
97 This is in agreement with Gupta et al. (1990) who studied levothyroxine sodium tablets from two different manufacturers. They reported that tablets from the same manufacturer might have lot-to-lot variation of the excipients. They also reported that tablets from a particular manufacturer contained excipient(s) that act as catalyst(s) to hasten decomposition.
98 Figure 14:
Stability of levothyroxine sodium 100µg commercial tablets stored
at 40oC/75%RH for 6 months
110
100
Assay (%)
90
80
70
60 0
3 Months
*Average of 3 determinations each
6
99 Table 12:
Effect of filler on the stability (assay) of levothyroxine sodium
tablets stored without desiccant at 40oC/75%RH for 6 months
Assay (%) of levothyroxine stored without desiccant Months
0
1
2
3
6
100.0
93.9
77.7
79.4
70.7
94.4
79.3
81.3
73.3
93.1
92.5
91.9
85.3
96.5
91.3
86.9
68.7
94.4
90.3
90.3
86.9
Filler Microcrystalline cellulose
(2.40) Starch
100.0 (5.40)
Dibasic calcium phosphate
100.0 (1.24)
Lactose anhydrous
100.0 (1.84)
Mannitol
100.0 (2.50)
( ) relative standard deviation for content uniformity * All batches meets USP uniformity of dosage units criteria
100 Figure 15:
Effect of filler on the stability of levothyroxine sodium tablets stored
without desiccant at 40oC/75%RH for 6 months
Microcrystalline cellulose
110
Starch Dibasic calcium phosphate Lactose
100
Mannitol
Assay (%)
90
80
70
60 0
3 months
*Average of 3 determinations each
6
101
Figure 16:
Moisture content of levothyroxine sodium tablets stored without
desiccant at 40oC/75%RH for 6 months
12
10 Dibasic calcium phosphate Lactose
8 % moisture
Mannitol Microcrystalline celluose
6
Starch
4
2
0 0 month
3 month
6 month
102 Table 13:
Effect of filler on the stability (assay) of levothyroxine sodium
tablets stored with desiccant at 40oC/75%RH for 6 months
Assay (%) of levothyroxine stored with desiccant Months
0
3
6
100.0
75.8
57.0
73.7
66.5
91.9
86.3
77.5
69.9
93.0
89.1
Filler Microcrystalline cellulose
(2.4) Starch
100.0 (5.4)
Dibasic calcium phosphate
100.0 (1.24)
Lactose anhydrous
100.0 (1.84)
Mannitol
100.0 (2.54)
( ) relative standard deviation for content uniformity * All batches meets USP uniformity of dosage units criteria
103 Figure 17:
Effect of filler on the stability (assay) of levothyroxine sodium
tablets stored with desiccant at 40oC/75%RH for 6 months
Dibasic calcium phosphate
110
Mannitol Starch 1500 Microcrystalline cellulose Lactose
100
Assay (%)
90
80
70
60
50 0
1
2
3 months
*Average of 3 determinations each
4
5
6
104
Figure 18:
Moisture content of levothyroxine sodium tablets stored with
desiccant at 40oC/75% RH for 6 months
9
Dibasic calcium phosphate Lactose
8
Mannitol Microcrystalline cellulose
7
Starch
% moisture
6 5 4 3 2 1 0 0 month
3 month
6 month
105 Table 14:
The pH of saturated solutions of powdered tablets
Levothyroxine sodium tablets
pH
manufactured with Lactose anhydrous
6.6
Microcrystalline cellulose
5.5
Starch
6.2
Dibasic calcium phosphate
7.3
Mannitol
5.3
* samples prepared by mixing powder : water in a 5:1 ratio and filtering
106
5.3. Effect of tablet excipients on the stability of levothyroxine sodium in slurries
Different tablet excipients influenced the stability of levothyroxine sodium in slurries to varying extents. Levothyroxine sodium assay of the various additive slurries, after storage at 50oC for 1 month was as follows: with crospovidone 37.2 %, with stearic acid - 62.3 %, with sodium starch glycolate - 77.9 %, with povidone - 77.9 %, with croscarmellose sodium - 83.1 %, with HPMC - 84.9 %, with fumed silica - 89.9 %, with magnesium stearate - 95.7 % and with no additive - 86.7 % (Table 15 – page 107, Figure 19 - page 108). It was observed that by varying the additive at just a 5% solid content level, the assay values of levothyroxine after one month, varied from ~40 to 90 % (compared to levothyroxine in solution under similar conditions) after one month. This indicates that the stability of levothyroxine was influenced by the type of additive used in the formulation. Thus screening and proper selection of additives for the formulation of levothyroxine sodium tablets is very important. Drugexcipient slurries provided a good and quick experimental method for screening excipients.
107 Table 15:
Effect of different tablet excipients on the stability of levothyroxine
sodium in 20% (w/v) slurries stored at 50 oC for 1 month
Sample / solid content of slurry
Assay (%)
pH
Standard*
100
7.8
Dibasic calcium phosphate
86.7
7.3
& 5 % Croscarmellose sodium
83.1
6.2
& 5 % Sodium starch glycolate
77.9
6.2
& 5 % Crospovidone
37.2
5.9
& 5 % Povidone
81.6
5.8
& 5 % HPMC
84.7
5.9
& 5 % Fumed silica
89.9
5.8
& 5 % Magnesium stearate
95.8
7.1
& 5 % Stearic acid
62.3
5.5
* levothyroxine solution under similar storage conditions after one month
108 Figure 19:
Effect of tablet excipients on the stability of levothyroxine sodium in
20% (w/v) slurries stored at 50 oC for 1 month
100 90 80
Assay (%)
70 60 50 40 30
di ba si c & ca 5 lc % iu cr m o ph & sc os 5 ar % m ph el so at l os e di um e so st di ar um ch & gy 5 lc % ol cr at os e po v & id on 5 % e po vi do & ne 5 % & & H 5 5 PM % % f C u m m ag ed ne si si lic um a st & ea 5 % ra te st ea ric ac id
20
* Assay value of levothyroxine solution under similar storage conditions after one month considered standard value
109
5.4. Effect of pH on stability of levothyroxine sodium in different diluent slurries
Levothyroxine sodium showed reduced degradation at basic pH as compared to its degradation at acidic pH. To test if the stability of levothyroxine sodium was improved in the presence of additives at basic pH, levothyroxine excipient slurries were prepared at various pH and stored at 50oC for one month.
It was found that pH influenced the stability of levothyroxine sodium in diluent slurries. In all the diluent slurries, levothyroxine sodium showed more degradation at pH 3 than at pH 11. Levothyroxine sodium assay value for the various slurries at pH 11 ranged from 89–96% vs. 70-87% for slurries with no pH modification and 49-68% for slurries at pH 3 (after storage at 50oC for 1 month) (Table 16 – page 110, Figure 20 – page 111). For each diluent, levothyroxine sodium stability in slurries improved, as the pH increased from acidic to basic with a good linear correlation. This demonstrated that the addition of a basic pH modifier improved the stability of levothyroxine sodium in the presence of various excipients. Thus, using pH modifiers is a potential technique to improve the stability of levothyroxine sodium in tablets.
110
Table 16:
Effect of pH on stability of levothyroxine sodium in diluent 20%
(w/v) slurries stored at 50oC for 1 month
Assay (%) of Levothyroxine stored without desiccant mannitol pH
microcrystalline
starch
cellulose
dibasic calcium phosphate
Blank
75.6 ±0.6
72.6 ±2.0
70.4 ±2.2
87.1 ±0.8
3
72.9 ±0.9
49.1 ±0.7
61.0 ±4.4
48.4 ±0.7
5
76.6 ±1.5
66.1 ±2.9
63.7 ±3.3
74.3 ±0.8
7
82.9 ±1.2
67.8 ±4.3
71.3 ±3.9
81.8 ±1.6
9
84.2 ±3.2
85.5 ±2.3
80.8 ±0.4
93.0 ±1.3
11
90.2 ±3.8
89.3 ±3.7
89.1 ±2.6
96.4 ± 0.6
Average of 2 determinations ± Standard deviation
111
Figure 20:
Effect of pH on stability of levothyroxine sodium in diluent 20%
(w/v) slurries stored at 50oC for 1 month
100
90
Assay (%)
80
70
60
50
Mannitol
R2 = 0.9733
Microcrystalline cellulose
R2 = 0.9413
Starch
R2 = 0.9704
Dibasic calcium phosphate
R2 = 0.8985
40 1
3
5
7 pH
9
11
13
112
5.5. Effect of pH modifiers on levothyroxine sodium tablets
Two sets of diluents were chosen and tablets were manufactured without the addition of a pH modifier, with basic pH modifiers and with acidic pH modifiers as a negative control. The two excipients chosen were dibasic calcium phosphate and microcrystalline cellulose, the first one proved to be the most inert diluent and the other resulted in the most degradation, respectively.
5.5.1.
Stability of levothyroxine sodium tablets manufactured with dibasic calcium phosphate and pH modifiers
In the presence of dibasic calcium phosphate and either of the basic additives used, the stability of the levothyroxine sodium tablets was improved significantly for tablets stored without a desiccant at accelerated ICH stability conditions. The six month assay values of levothyroxine sodium in tablets manufactured with dibasic calcium phosphate and sodium carbonate, sodium bicarbonate, magnesium oxide, tartaric acid or citric acid were 95.2, 94.7, 96.8, 78.3 and 74.4 %, respectively. Levothyroxine sodium tablets containing basic pH modifiers, namely sodium carbonate, sodium bicarbonate or magnesium oxide met USP assay requirements (90-110 %) after 0, 3 and 6 months storage at 40oC / 75%RH. After 6 months at accelerated ICH stability conditions, the tablets showed less than 5% loss in assay value from initial value. Levothyroxine sodium
113 tablets manufactured with dibasic calcium phosphate and acidic additives, namely tartaric acid and citric acid did not meet USP assay requirements after only 3 months at 40oC / 75%RH. Levothyroxine sodium tablets manufactured with dibasic calcium phosphate and no pH modifier assayed 91.1% after 3 months and met USP assay requirements. However, the same tablets assayed 87.2% after 6 months, and thus, they did not meet USP requirements at the extended time (Table 17 – page 115, Figure 21 – page 116). Thus, the use of pH modifier affected levothyroxine sodium stability significantly (p
