Title:  Stabilization of solid thyroid drug formulations

United States Patent:  6,855,333

Issued:  February 15, 2005

Inventors:  Spireas; Spiridon (Newtown, PA)

Assignee:  Mutual Pharmaceutical Co., Inc. (Philadelphia, PA)

Appl. No.:  690973

Filed:  October 18, 2000

Abstract

Pharmaceutical formulations and dosage forms are provided having improved stability to moisture-induced degradation when compared with conventional dosage forms, especially tablets. The invention features low compression forms of thyroid drugs together with excipients, preferably in encapsulated forms. In other embodiments, relatively non-volatile oils can be admixed with the drug and/or the excipients to stabilize the formulation toward moisture-induced degradation. Hydrophobic powders are also optionally added to the formulations.

Description of the Invention

FIELD OF THE INVENTION

The present invention relates to stable formulations of drugs and methods for producing the same. In particular, the present invention relates to stable formulations of thyroid drugs that are susceptible to moisture-induced degradation and methods for producing the same.

BACKGROUND OF THE INVENTION

It is known in the pharmaceutical field that many drugs and classes of drugs exhibit poor or modest shelf stability. For example, a number of solid drug formulations experience such instability that after relatively short periods of time, significant portions of the active materials in the drug have been chemically transformed into other compounds. While such compounds are often relatively benign, occasionally the degradation product or products can either actually comprise an antagonist for the drug or give rise to adverse side effects. In any event, the diminution of desired, active ingredient in such drug formulations is of obvious, deleterious effect, making therapy with such drugs less certain. Accordingly, there is a long-felt need for drug formulations and for unit dosage forms of drugs which experience diminished degradation when compared to typical formulations.

Among the drug classes which are known to be liable to moisture-induced (e.g., hydrolytic) degradation and diminished shelf stability are the thyroid hormones. Thyroid hormones are suitable for therapeutic application in the treatment of hormone disorders. Particularly useful are thyroid hormones of natural or synthetic origin which bear about two to four iodine atoms. Examples of such thyroid hormones are levothyroxine, liothyronin, dextrothyroxine, triiodoacetic acid, thyroglobulin, diiodotyrosine, and analogs and salts thereof.

Levothyroxine is the levo isomer of thyroxine, an active physiological thyroid hormone obtained from the thyroid gland of animals or prepared synthetically. Clinically, levothyroxine is prescribed in thyroid replacement therapy for reduced or absent thyroid function of any etiology, including conditions such as myxedema, cretinism and obesity. Levothyroxine sodium, the sodium salt of levothyroxine, is the preferred form of administration of levothyroxine and- is expressed by the chemical formula C₁₅ H₁₀ I₄ NaO₄.xH₂ O. It is well known that the stability of levothyroxine is poor as it is hygroscopic and degrades rapidly under conditions of high humidity or in the presence of other moisture sources. Levothyroxine also rapidly degrades in the presence of light, under conditions of high temperature, or in the presence of other pharmaceutical excipients, including carbohydrates such as, for example, lactose, sucrose, dextrose and starch, and certain dyes. Accordingly commercially available levothyroxine sodium tablet formulations exhibit a short shelf life.

Thyroid hormones are used therapeutically for thyroid diseases of various origins, including hypothyroidism, hypothyrosis, iodine deficiency and other related secondary diseases. Thyroid hormones are also used prophylactically. Thyroid hormones are used as medicaments and are extremely susceptible to temperature, humidity and oxidation. They are also prone to decomposition by various reaction mechanisms. Additionally, they react with many pharmaceutical excipients which makes it difficult to prepare pharmaceutical formulations containing thyroid hormones which remain effective for a sufficient period of time under regular storage conditions.

Significant efforts have been directed to the development of stable pharmaceutical formulations containing thyroid hormones for use as therapeutic agents. U.S. Pat. No. 5,225,204 (Jul. 6, 1993) is directed to a dosage form containing levothyroxine sodium which includes a stable complex of levothyroxine sodium and a cellulose compound, polyvinylpyrrolidone or a Poloxamer wherein the complex is adsorbed on the surface of a cellulose compound carrier.

U.S. Pat. No. 5,635,209 (Jun. 3, 1997) discloses a medication consisting of the combination of levothyroxine sodium with potassium iodide. This patent further discloses methods for making medication containing levothyroxine sodium comprising combining together levothyroxine mixed with a carrier, potassium iodide mixed with a carrier, a disintegrant, and a lubricant.

U.S. Pat. No. 5,955,105 (Sep. 21, 1999) describes a stable, solid dosage form pharmaceutical preparation, suitable for the treatment of thyroid disorders, comprising a thyroxine drug, a water soluble glucose polymer, and a partially soluble or insoluble cellulose polymer. This patent also discloses a stable pharmaceutical preparation comprising a thyroxine drug, a water soluble polysaccharide and a partially soluble or insoluble cellulose polymer. This patent further describes a stable pharmaceutical preparation comprising sodium levothyroxine, by maltodextrin and microcrystalline cellulose.

U.S. Pat. No. 5,958,979 (Sep. 28, 1999) is directed to stable medicaments containing thyroid hormones wherein the medicament contains sodium thiosulfate as the stabilizing component. This patent also discloses methods for the preparation of stable medicaments comprising adding sodium thiosulfate in a dissolved state to a matrix mixture containing thyroid hormones.

In view of the extreme instability of thyroid hormones, such as levothyroxine, in the presence of moisture, light and heat, there is a long-standing need for stable formulations of thyroid hormones and methods of making such formulations. It is desirable to develop stable thyroid hormone-containing formulations which demonstrate a long enough shelf life for use as therapeutic agents. Also needed are methods for preparing such formulations wherein degradation of the thyroid hormones in the formulations is greatly reduced, thereby providing stable pharmaceutical formulations containing thyroid hormones for use as therapeutic agents in the treatment of disorders associated with reduction or absence of thyroid hormone production.

Accordingly, it is a principal object of the present invention to provide unit dosage forms of solid drug formulations having a reduced tendency to degrade over time when compared with traditional formulations of such drugs. A further object of the invention is to provide methods of therapy comprising administering to a patient in need of a drug dosage form in accordance with the invention having such diminished tendency to degrade. Another object of the invention is to permit the U.S. and international registration and permission to market certain drug formulations which, absent the stabilization of the present invention, would not be registerable due to an unacceptably high rate of degradation. Other objects will become apparent from a review of the present specification and appended claims.

SUMMARY OF THE INVENTION

The present invention relates to a drug dosage form comprising a thyroid drug and at least one pharmaceutically acceptable excipient prepared under conditions of low compression. In one embodiment, the drug is subjected to no compression in excess of about 10,000 psi/g, preferably to no compression in excess of about 5,000 psi/g, and more preferably to no compression in excess of about 2,000 psi/g. The formulation of the drug into dosage forms under low compression conditions preferably gives rise to encapsulated forms, such as hydroxypropyl methylcellulose (HPMC) capsules.

In another of its aspects,; the present invention relates to a drug dosage form for a thyroid drug comprising the drug admixed with a substantially non-volatile, pharmaceutically acceptable oil. Suitable oils include animal or vegetable oils such as olive, corn, peanut, nut, soy, rapeseed, cottonseed, vitamin E, fish, or tallow-derived oils, mineral oils and silicone oils. The drug--oil admixture is optionally present within a capsule, a soft shell capsule, or a specially sealed hard-shell capsule. The drug--oil admixture is also optionally adsorbed on a pharmaceutically acceptable excipient.

In yet another of its aspects, the present invention relates to a drug dosage form for a thyroid drug comprising the drug and a pharmaceutically acceptable excipient admixed with a substantially non-volatile, pharmaceutically acceptable oil. Suitable oils include animal or vegetable oils such as olive, corn, peanut, nut, soy, rapeseed, cottonseed, vitamin E, fish, or tallow-derived oils, mineral oils and silicone oils. The excipient--oil admixture is optionally present within a capsule or a tablet.

In still another of its aspects, the present invention relates to a drug dosage form comprising a thyroid drug admixed with a first pharmaceutically acceptable oil together with a pharmaceutically acceptable excipient admixed with a second pharmaceutically acceptable oil. Suitable first and second pharmaceutically acceptable oils are, independently, an animal or vegetable oil such as olive, corn, peanut, nut, soy, rapeseed, cottonseed, vitamin E, fish, or tallow-derived oil, a mineral oil or a silicone oil.

In a further of its aspects, the present invention relates to a drug dosage form comprising a thyroid drug and at least one pharmaceutically acceptable hydrophobic powder. In one particular embodiment, the hydrophobic powder is magnesium stearate. Preferably, the hydrophobic powder is triturated directly with the drug.

In still a further of its aspects, the present invention relates to a method for administering a thyroid drug to a patient comprising providing a unit dose of the drug which has not been processed employing high compression. Alternatively or additionally, the drug and/or the excipient are pretreated with a non-volatile, pharmaceutically acceptable oil.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention employs, unless otherwise indicated, conventional methods of chemistry, drug synthesis and formulation, all within the knowledge of those skilled in the art. Such techniques are explained fully in the literature. See, e.g., Remington's Pharmaceutical Sciences, 18th Edition (Easton, Pa.: Mack Publishing Company, 1990), incorporated herein by reference.

In accordance with the present invention, it is alternatively possible either to pretreat a drug, powder excipients, or both with one or more non-volatile, water-immiscible liquids such as oils, and each method of formulation has been found to be beneficial in improving the stability of drugs such as levothyroxine sodium and to form new and useful, solid dosage forms. Without wishing to be bound by theory, the highly desirable stability performance of these new formulations is believed to be based on the fact that the oils are acting as a protective device against extensive contact of the drug with environmental moisture or the equilibrium moisture inherent in mixed powder excipients. In this way, hydrolysis and degradation of the drug in such formulations is significantly reduced.

The present invention may be applied to any of the solid drugs which are known to be subject to moisture-induced degradation in tablet form. Thus, the thyroid hormones such as levothyroxine, ACE-inhibitors such as quinapril, cyclic amino acids such as gabapentin, cholesterol lowering agents such as statins (e.g., lovastatin), non-steroidal anti-inflammatory agents such as aspirin, peptides and proteins such as insulin, anticancer and oncology drugs such as methotrexate, steroids and steroidal esters such as methylprednisone sodium succinate, antibiotics such as mitomycin C, nystatin, Rifampin, and others, other cardiac drugs such as nitroglycerin and dioxin, and other drug classes may benefit from the present invention, and its application to all such classes is contemplated hereby. Additionally, in one particular embodiment, the drug is optionally purified prior to use.

The phrase "moisture-induced degradation" as used herein includes, but is not limited to, drug degradation due to hydrolysis only. Instead, the phrase "moisture-induced degradation" also includes any other type of degradation pathway such as oxidation, photodegradation, cyclization, and even dehydration due to ionic attractions, which may be induced or catalyzed by moisture. Thermal degradation initiated by compression and facilitated by moisture is also included in the above phrase.

The term "excipient" as used herein includes, but is not limited to, the family of modified celluloses (e.g., carboxymethyl and ethyl cellulose, hydroxymethyl and ethyl cellulose, microcrystalline cellulose and others), amorphous silicon dioxide, magnesium stearate, starch, sodium starch glycolate, or a combination thereof. In one embodiment, the excipient is at least one of microcrystalline cellulose, starch, and sodium starch glycolate.

Microcrystalline cellulose is known per se and a variety of such are commercially available. Exemplary among these is the family of products sold by the FMC Corporation under the trademark Avicel.RTM.. Any of the members of this family may be used in connection with the practice of one or more embodiments of the present invention and all are contemplated hereby. Other cellulose products which are similar in nature to microcrystalline cellulose may find utility herein, such a parenchymal cell cellulose.

In addition to the preferred microcrystalline celluloses and similar materials, other cellulosic materials may also be employed in connection with one or more embodiments of the present invention. Thus, modified celluloses such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, carboxymethyl cellulose salts and esters, (e.g., sodium, potassium etc. salts), and other cellulose derivatives may be so employed. It will be appreciated by persons of ordinary skill in the art that such cellulosic materials should, be consistent with the overall spirit of the invention. Thus, such materials may be employed which do not adversely effect the processing set forth herein and which do not interfere with the stability of the resulting products.

Particularly useful non-volatile, water-immiscible liquids include oils such as olive, corn, peanut, nut, soy, rapeseed, cottonseed, vitamin E, fish, tallow-derived or other oils. The oil may also be a mineral oil, silicone oil or the like and may be present as a mixture of oils. By "non-volatile" it is meant that the oils have a relatively low vapor pressure at conventional temperatures and other conditions. Oils such as the essential oils which have relatively high vapor pressures (e.g., aromatic constitutives) are not preferred for this application, although some of such species may find some utility herein.

In accordance with these aspects of the invention, drug and oil are combined in any ratio which gives rise to the beneficial properties desired. Conveniently, drug and oil can be combined in ratios of about 1:1 to about 1:5000 or more, weight to weight. Ratios of from about 1:40 to about 1:400 are more preferred. When the oil is admixed with excipient, ratios of excipient to oil of from about 1 1 to about 1:5000 or more are also conveniently used with ratios of from about 1:40 to about 1:400 being preferred. Other ratios may also be used, especially when both drug and excipient are admixed with oils.

The drug and/or excipients are mixed with the non-volatile, water-immiscible liquids using any of a variety of conventional techniques. For example, when the excipients are to be pretreated, the liquids can be dissolved in a solvent (e.g., acetone) and the resulting solution used to granulate the excipients. When the drug is to be pretreated, the drug can be suspended directly in the liquids and homogenized to form a drug/liquid suspension.

A further way has also been found to greatly reduce unwanted moisture-induced degradation in drugs. For example, with respect to levothyroxine, it has now been surprisingly discovered that an important factor which can be used to achieve stable powder formulations of levothyroxine and other solid drugs which can experience undesired rates of moisture-induced degradation, is the avoidance of high degrees of compression. Without being bound by theory, it has now been found that when a powder formulation comprising levothyroxine mixed with pharmaceutically acceptable powder excipients is compressed into tablets, some of the equilibrium moisture inherently possessed by such inert powder excipients is squeezed out of the interior bulk of the powder particles to the exterior surfaces of the powder particles; those surfaces being in contact with the drug particles. Such drug-moisture contact is believed to result in initializing at a relatively high rate, the hydrolysis of levothyroxine to its degradation products.

It has actually been reported that the degradation of levothyroxine from highly compressed systems, i.e., tablets, is biphasic wherein, at a first stage immediately after compression into tablets, the drug degrades at a much higher rate as compared to a second, slower degradation stage. It has now been found that the drug degradation stage occurring immediately after compression is associated with the aforementioned contact of the drug particles with the equilibrium inherent moisture squeezed onto the particle surfaces of the inert powder excipients included in the tableting mixture. Several commonly used and pharmaceutically acceptable powder excipients may possess inherent moisture levels equivalent of up to 10% of their weight. It is desired to continue to use such excipients, however, especially those with moisture contents of 5% and even 10% by weight. Thus, the compression of such materials can force very significant amounts of moisture onto the surfaces of such particles with the attendant, rapid moisture-induced degradation of drugs in contact with such compressed particles.

Accordingly, it has now been found that provision of levothyroxine in dosage forms which are highly compressed, such as tablets, should be avoided when neither the drug nor the excipients are pretreated as described above. Rather, in accordance with the invention, the drug is preferably provided in unit dosage forms in which the drug has not been compressed in the presence of moisture-containing excipients to a degree such that moisture is exuded onto the surfaces of the excipient particles. For example, a hard-shell capsule of levothyroxine, in which the powder mixture is not strongly compressed exhibits very greatly improved stability to moisture-induced degradation as compared to previously available, highly-compressed tablets of the drug.

It is also known that cyclic amino acids are subject to degradation during storage. The degradation is believed to be due, at least in part, to conversion of the cyclic amino acid to its lactam form.

Cyclization of the amino acid to form the lactam impurity results in the loss of water. The cyclization is believed to be catalyzed by highly ionized, electronegative anion impurities (e.g., Cl^-). Pretreatment of the amino acid and/or the excipients with which it is compounded in accordance with the present invention is expected to serve to isolate the amino acid from the anion impurities, thereby stabilizing the amino acid relative to its lactam form.

In addition, ACE inhibitors, or inhibitors of Angiotensin Converting Enzymes, are drugs useful in the treatment of cardiovascular disorders, especially hypertension. ACE inhibitors include, but are not limited to, enalapril maleate and similar salts; quinapril hydrochloride and similar salts; benazepril hydrochloride and similar salts; moexipril hydrochloride and similar salts; lisonopril hydrochloride and similar salts; ramipril hydrochloride and similar salts; and indopril hydrochloride and similar salts. Typical breakdown products of ACE inhibitors include, but are not limited to, enalaprilat and/or enalapril-diketopiperazine (DKP) for enalapril species, quinaprilat and/or quinapril-DKP for quinapril drugs, and other breakdown products well-known to those of skill in the art.

However, it has been widely observed that ACE inhibitors are susceptible to breakdown, especially due to degradation and/or cyclization between the time of manufacture and the time of desired usage. Breakdown of ACE inhibitors has been found to occur both in solid and in liquid states. Such breakdown is due, at least in part, to hydrolysis of the drug by water. As breakdown of ACE inhibitor increases, the concentration of available, functional ACE inhibitor decreases. Also, at least some of the degradation products of such breakdown are believed to be deleterious. Accordingly, such breakdown is to be avoided. It is expected that the moisture-induced hydrolysis of ACE inhibitors can be reduced by pretreating the drug and/or the excipients with which it is compounded in accordance with the present invention.

Persons of ordinary skill in the pharmaceutical formulation art will recognize that the present invention distinguishes between highly compressed drug forms and unit dosages and such forms and dosages which have not been highly compressed. It is believed that the relationship between the compression of solid forms of drugs susceptible to moisture-induced degradation and the rate of or tendency toward such degradation for such drugs has not been appreciated heretofore. For purposes of this invention, the degree of compression can be defined functionally. Thus, a drug form or unit dosage has been "highly compressed" if the amount of pressure applied to the form or dosage is such as to exacerbate the moisture-induced degradation of the drug comprising the form or dosage. This amount of compression can vary with the identity of drug, excipient and other factors. However, it is believed that no more than ordinary skill and routine evaluation is needed to determine whether the foregoing conditions have been met with respect to any particular drug composition, form, formulation or unit dose. Without departing from the generality of the foregoing, it is believed that compression in excess of about 10,000 pounds per square inch per gram of compacted mass (psi/g), preferably about 5,000 psi/g and still more preferred about 2,000 psi/g should be avoided in such processing. For the purposes of the present invention, formulation of drug forms, dosage forms and the like with the avoidance of compression in excess of about 10,000 psi/g or which otherwise avoids the above-described exacerbation of moisture-induced degradation of drugs which are susceptible to such degradation is referred to as "conditions of low compression. "

As used herein, the terms "dosage form ," "pharmaceutical dosage form," "pharmaceutical formulation," and "pharmaceutical preparation" refer to the final solid pharmaceutical product. These terms include, but are not limited to, tablets included molded tablets, caplets, beads, wafers, and capsules (including both hard shell capsules and soft gelatin capsules). These terms also refer to liquisolid systems which are flowing and compressible powdered forms of liquid medications. The processes of preparing pharmaceutical preparations and dosage forms are well known to those of skill in the art. See, e.g., Theory & Practice of Industrial Pharmacy, 3^rd Edition, Liberman, Lachman, and Kanig, eds. (Philadelphia, Pa. : Lea & Febiger), incorporated herein by reference.

The formulation of solid drugs into dosage forms under low compression conditions may conveniently and preferably give rise to encapsulated forms. Hard shell capsules filled with low compression powdered drug composition are most convenient; their manufacture and processing is well-known and routine. A particularly preferred capsule shell type is one comprising hydroxypropyl methylcellulose (HPMC), although all capsule forms may beneficially be used in conjunction with this invention. Other capsule shells, such as those consisting of polyethylene glycols or other cellulosic derivatives may also be advantageously used herein.

Other examples of solid dosage forms which do not require high-compression conditions during their preparation are pellets, beads, liquisolid systems, soft gelatin capsules containing liquid, specially-sealed hard-shell capsules containing liquid, molded tablets, wafers, etc.

A "liquisolid system" refers to formulations formed by conversion of liquid drugs, drug suspensions or drug solutions in non-volatile solvents into dry, nonadherent, free-flowing and compressible powder admixtures by blending the suspension or solution with selected carriers and coating materials. Based upon the type of liquid medication contained, liquisolid systems are classified into three categories: (I) powdered drug solution (containing a drug solution); (ii) powdered drug suspension (containing a drug suspension); and (iii) powdered liquid drug (containing a liquid drug). Liquisolid systems are described in U.S. Pat. Nos. 5,968,550 and 5,800,834, each of which is incorporated herein by reference in its entirety. It will be appreciated that such liquisolid systems may be prepared in accordance with the present invention and that the same is within the spirit hereof. Although not generally preferred, when the drug and/or excipients are pretreated with a water-immiscible liquid in accordance with the present invention, liquisolid powder systems can be also compressed into tablets. Such tablets possess stability properties superior to those of commercial products.

Significant stability enhancement benefits of premixing the drug with some traditionally hydrophobic powders have also been discovered as a way to reduce unwanted moisture-induced degradation in drugs. The hydrophobic powders are preferably triturated directly with the drug. In one particular embodiment, the hydrophobic powders are triturated directly with the drug prior to blending the drug with other powder excipients, including powder excipients previously admixed with a non-volatile oil. Suitable hydrophobic powders include, but are not limited to, lubricants such as magnesium stearate, antioxidants, other solid waterproofing agents, and combinations thereof. It would be appreciated by those skilled in the art that this method can be used in various intensities and in combination with one or more of the other methods described herein. However, it is well known that extensive use of such hydrophobic powders (i.e., magnesium stearate) may deleteriously affect the dissolution of drugs in aqueous media by waterproofing the drugs to irreversible levels. Accordingly, the use of hydrophobic powders should be optimized to also maintain acceptable drug dissolution properties.

As used herein, "substantially free" refers to compositions that have significantly reduced levels of detectable breakdown products or degradation products. The terms "breakdown products" and "degradation products" refer to undesired contaminants formed by the decomposition or degradation of the thyroid hormone. Decomposition or degradation of thyroid hormones may be caused by exposure of the thyroid hormone to moisture, heat or light.

By "pharmaceutically acceptable" or "pharmacologically acceptable" is meant a material which is not biologically or otherwise undesirable, i.e., the material can be administered to an individual along with the stabilized drug formulation without causing any undesirable biological effects or interacting in a deleterious manner with any of the components of the formulation in which it is contained.

Claim 1 of 28 Claims

What is claimed is:

1. A stable drug dosage form prepared be compression techniques comprising:

a thyroid hormone susceptible to moisture induced degradations, and

particles of at least one pharmaceutically acceptable excipient, each particle having an exterior surface, an interior, equilibrium moisture disposed within the interior of the particles, the thyroid hormone being in contact with the exterior surface of the particles of the at least one pharmaceutically acceptable excipient; the dosage form prepared by:

admixing the thyroid hormone and the at least one pharmaceutically acceptable excipient; and

compacting the thyroid hormone and the at least one pharmaceutically acceptable excipient into unit dosage forms using compression pressures of less than about 5000 psi/g;

wherein the compression pressure limits the amount of equilibrium moisture available to react with the thyroid hormone at the exterior surface of the particles of the at least one pharmaceutically acceptable excipient.

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