The development of a pharmaceutical product requires a broad spectrum of scientific expertise to lead it through the complex pathway from discovery through characterization of quality, efficacy and safety, which are the hallmarks of a successful drug product. A company must be highly proactive in setting targets for appraising and selecting a compound that has the highest probability of success. In addition, the compound and its therapeutic use must be consistent with the research and marketing goals of the company in order to leverage existing resources and experience. To ensure scientific and commercial success, it is critical to understand the drug development process (Figure 1) and the myriad tasks and milestones that are vital to a comprehensive development plan.
Although the primary purpose of a well-designed pathway is to assure an efficient process for providing new, high quality and effective drugs for patients, it is also essential to effectively maximize the return on investment. In this context, some primary drivers contributing to maximizing return on investment include the cost of development, market price, product life cycle and competition (Table 1). Each step along the path from discovery to commercialization is important. However, if material cannot be manufactured, the drug development process cannot proceed. As a result, an effective chemistry, manufacturing and controls (CMC) process plays an integral role in the success of a therapeutic compound.
The ability to assure, over time, the physical and chemical properties of an active pharmaceutical ingredient, drug product or nutraceutical is critical for regulatory approval and therapeutic success. The CMC process is necessary for an efficient and comprehensive development strategy. The major challenges for the manufacturing and control component of drug development is to assure the chemical and physical properties of the compound and product are monitored at all critical phases of the pathway. This process matches the scientific and analytical tasks to the manufacturing and commercialization strategy (Table 2).
In recent years, the International Conference on Harmon-isation of Technical Requirements for Registration of Pharma-ceuticals for Human Use (ICH) has adopted scientific standards for quality control monitoring. These standards are the basis of most regulatory guidelines, including those published by the FDA. Key steps on the path include pharmaceutical analysis and stability studies that are required to determine and assure the identity, potency and purity of ingredients, as well as those formulated products. Stability testing facilitates the establishment of recommended storage conditions, determination of retest periods and definition of acceptable shelf life. These data play a key role in determining labeling requirements, as well as in the development and monitoring process.
A Continuous Process
Stability testing is performed on drug substances and products at various stages of product development (Table 3). In early stages, accelerated stability testing (at relatively high temperatures and/or humidities) is used as a "worst case" evaluation to determine what types of degradation products may be found after long-term storage. In preformulation studies, interactions between excipients and the drug substance are studied under stress conditions to access compatibility.
The design of a complete stability testing program for a drug or nutraceutical product is based upon an understanding of the behavior, properties and stability of the drug substance or active ingredient and the experience gained from preformulation studies and early clinical formulations. Products are analyzed at specific intervals to evaluate a variety of parameters, such as the identity of the active ingredient, potency, measurement of degradation products, dissolution time, physical properties and appearance. Samples from production lots of approved products are retained for stability testing and for comparison testing in the case of product failure. Testing of retained samples alongside returned samples is key to ascertaining whether the product failure was manufacturing or storage related.
The objective of analytical testing during preclinical evaluation and Phase I clinical development is to evaluate the stability of the investigational formulations used in initial clinical trials, to obtain information needed to develop a final formulation, and to select the most appropriate container and closure (e.g., compatibility studies of potential interactive effects between a drug substance and other components). Information from the experiments listed in Table 3 under Discovery to Phase I is summarized in the Investigational New Drug application (IND) with the initiation of Phase I clinical trials. When the delivery mechanism of the drug is an integral part of creating the therapeutic effect and must be used in the Phase I trials, formulation data, container closure data and corresponding short-term accelerated stability data should be included in the IND prior to Phase I trials.
Analysis studies on formulations should be underway by the end of Phase II and the stability protocol for study of both the drug substance and drug product should be defined. This will help assure that analytical chemistry data generated during Phase III are appropriate for submission. Prior to Phase I, stability of the drug substance and the formulation to be used must be evaluated. Impurities from the manufacturing and degradants that form are quantitated and tracked to ensure safety prior to moving into the Phase I clinical trials and continuity of material used for laboratory safety testing and clinical trials.
Stability testing done during Phase III studies focuses on testing final formulations in the proposed packaging produced at the manufacturing site. It is recommended that the stability protocol is defined prior to the initiation of Phase III studies. In this regard, consideration should be given to establish appropriate linkage between the non-clinical and clinical batches of the drug substance and drug product and those of the primary stability batches in support of the proposed expiration dating period. Factors to be considered include the source, quality and purity of various components of the drug product, manufacturing process and production facility for the drug substance and the drug product, as well as use of same containers. Data obtained on tests done under controlled conditions replicating conditions recommended for long-term storage and slightly elevated temperatures are used to determine a product's shelf life and expiration dates. In some cases, comprehensive stability testing must also be conducted after approval (Table 4).
A Focus on Stability
The stability of a product may be defined as the extent to which a product retains, within specified limits, throughout its period of storage and use, the same properties and characteristics possessed at the time of its packaging. Stability testing provides evidence on how the quality of a drug substance or drug product varies with time under the influence of a variety of environmental factors such as temperature, humidity and light. These studies are designed to determine if a drug substance will remain within specifications during its shelf life if stored under recommended storage conditions.
Stability testing focuses on the chemical (i.e., integrity, potency, degradation) and physical properties (e.g., appearance, hardness, particle size, solubility) of active pharmaceutical ingredients (API) and products (Table 5). In addition, microbiological testing is done to ensure the substance and product maintain their resistance to microbial and bacterial growth. Assuring the physical/chemical properties and effectiveness properties of a pharmaceutical is critical for labeling and marketing purposes. A wide range of testing is used to evaluate and verify the identity, potency and availability of the API in the product (Table 6). Stability testing is done at all phases of the development, production and marketing process for quality control and monitoring purposes. A wide scope of analytical methodologies is used, including high-performance liquid, gas and thin-layer chromatography (HPLC, GC, TLC) as well as IR and LC/mass spectrometry.
Stability testing requires the use of specialized environmental chambers that can simulate long-term storage conditions. The stress conditions in the chambers include heat, humidity and light. These chambers enable researchers to evaluate product stability based on real-time, accelerated and long-term protocols and are available in both walk-in and reach-in styles. Chambers are engineered and qualified to ensure uniform exposure of the stress conditions to all material in the chamber. Early in the development of the drug product, purposeful degradation studies are done as a means to predict possible degradation pathways of an API. This information is used in the validation of stability indicating analytical methods and in pre-formulation studies. Degradation studies include stress conditions such as heat, oxidative, light, acidic conditions, basic conditions and heat/humidity.
The physical-chemical properties of the substance are analyzed to verify the identity/structure of the drug substance or product API. Many of these tests require specialized instrumentation and laboratory expertise. In addition, the organoleptic properties—including appearance, hardness and moisture—are evaluated. For quality control purposes, the potency, availability and microbial quality are monitored. All of these factors are key ingredients in stability evaluations.
Testing to assure that products meet specifications for the presence of degradation and impurities are usually intensive chromatographic separations with detection down to the 0.01% levels. Typically, impurities and degradation products that are 0.1% and above need to be evaluated for identity and chemical structure. The level of the impurities allowed depends on the toxicity of the impurity and the daily dose levels of the drug.
Identity information on the stability of a drug substance under defined storage conditions is an integral part of the systematic approach to stability evaluation. Stress testing helps to determine the intrinsic stability characteristics of a molecule by establishing degradation pathways to identify the likely degradation products and to validate the stability indicating power of the analytical procedures used.
Along with chemical and physical testing, a number of microbiological tests must be performed based on the dosage form. For sterile products, the microbiological tests performed include sterility, bioburden and bacterial endotoxins. These tests must be validated to show that the compendial tests are suitable. For example, to validate sterility, the test for bacteriostasis and fungistasis (BF) is performed at time of set down. The BF test ensures that any BF activity does not adversely affect the reliability of the sterility test. Bioburden requires validation to show that the test article will not adversely affect the growth of positive controls.
Non-sterile products have different testing requirements depending on if preservatives are used. Orally administered suspensions or liquids with a preservative are evaluated for microbial limits, total yeasts and molds and antimicrobial preservative effectiveness test. The variations of container closure systems will determine the frequency of testing during the stability study. For example, the sterility of a formulation in a sealed glass ampule need not be tested after sterility is established. For most container closure systems, microbiological testing is performed initially, at 12 months and annually thereafter. For accelerated conditions, testing is minimally performed at end of the storage time.
The severe conditions encountered during distribution are covered by stress testing of definitive batches of the drug substance. Stress testing provides data on forced decomposition products and mechanisms. These studies establish the inherent stability characteristics of the molecule (e.g., degradation pathways) and lead to identification of degradation products and support the suitability of the proposed analytical procedures. The detailed nature of the studies will depend on the individual drug substance and type of drug product.
Testing is carried out on a single batch of a drug substance and includes the effects of temperatures in 10°C increments above the accelerated temperature test condition and humidity, where appropriate (e.g., 75 % or greater). In addition, one must evaluate oxidation and photolysis on the drug substance, plus its susceptibility to hydrolysis across a wide range of pH values when in solution or suspension.
Photostability (i.e., light) testing is an integral part of stress testing. Some degradation pathways can be complex and, under forced conditions, decomposition products may be observed that are unlikely to be formed under accelerated or long-term testing. This information is useful in developing and validating suitable analytical methods, but may not be necessary to examine specifically for all degradation products if it has been demonstrated that in practice these are not formed. Information obtained from photostability is key in choosing appropriate container/closure systems.
Dosage Form/Delivery System Requirements
The route of administration and delivery system used are key components to the successful development of new drugs and therapies. In addition, these choices have a significant impact on the scientific and regulatory aspects of a stability protocol. The diversity of testing needed for all dosage forms and delivery systems requires a broad range of expertise and methodologies.
In general, all dosage forms are evaluated for appearance, assay and degradation products. Additional tests (i.e., potency) are needed for specific dosage forms. For example, sterility is needed for sterile products but not for tablets or capsules. In addition, not every test will be performed at each time point.
The evaluation of inhalation powders includes aerodynamic particle size distribution of the emitted dose, microscopic evaluation, microbial limit, moisture content, foreign particulates, content uniformity of the emitted dose and number of medication doses per device that meets content uniformity of the emitted dose (metered dose products). The unique characteristics of metered-dose and dry-powder inhalers can affect the product's efficacy as well as the ability of the product to deliver reproducible doses. These factors must be considered during development with respect to formulation, stability, manufacturing, container and closure system and quality control (Table 7).
Stability data for products supplied in closed-end tubes should support the maximum anticipated use period after the tube seal is punctured, allowing product contact with the cap. Ointments, pastes, gels and creams in large containers, including tubes, should be assayed by sampling at the surface, top, middle and bottom of the container. In addition, tubes should be sampled near the crimp.
Evaluation of ophthalmic or optic products (e.g., creams, ointments, solutions and suspensions) includes sterility, particulate matter and extractables. Evaluation of nonmetered topical aerosols includes appearance, assay, degradation products, pressure, weight loss, net weight dispensed, delivery rate, microbial limits, spray pattern, water content and particle size distribution (for suspensions).
Studies of drugs for injection (i.e., parenterals) include monitoring for appearance, clarity, color, reconstitution time and residual moisture content. The stability of drug for injection products must also be evaluated after reconstitution, according to the recommended labeling. Small volume parenterals (SVPs) are a wide range of injection products (e.g., drugs for injection, drugs for injectable suspension and drugs for injectable emulsion). Large volume parenterals (LVPs) studies include evaluation of product stability following exposure to at least the maximum specified process lethality. Interaction of administration sets and dispensing devices are considered to ensure that absorption and adsorption during dwell time do not occur. In veterinary applications, some LVPs are designed for multiple use. These products are evaluated for stability after opening with part of the content removed. The "in-use" studies last from seven days to four weeks.
The functionality and integrity of parenterals in prefilled syringe delivery systems needs to be evaluated throughout the expiration dating period with regard to factors, such as the applied extrusion force, syringeability, pressure rating and leakage. Continued assurance of sterility for products is by a variety of means, including evaluation of the container and closure integrity.
Specific parameters to be examined for reconstituted drug products include appearance, clarity, color, pH, assay (i.e., potency), preservative, degradation products/aggregates, sterility, pyrogenicity and particulate matter. Studies for drug injectable suspension and drug for injectable suspension also include particle size distribution, redispersibility and rheological properties. The studies for drug injectable emulsion products also include phase separation, viscosity and mean size and distribution of dispersed phase globules.
Package Extraction and Migration
A widely overlooked factor in pharmaceutical analysis testing is the determination of potential impurities resulting from migration from packaging components. This includes testing for nitrosamine residue testing as well as both quantitative and qualitative techniques for nitrosamines and olefin polymers used in packages and closures. The 1998 draft stability guidance recommends performing extractable studies on the container/closure (C/C) system using sensitive and quantitative methods even if the C/C system meets compendial suitability tests. Concern over extractables/leachables from the C/C system depends on the route of administration and the likelihood of a packaging component-dosage form interaction. For example, routes of adminstration such as inhalation aerosals and injectables are of highest concern, whereas orally administered solid dosage forms are of lower priority.
An Integral Component
Although stability is an integral component of a CMC program, a comprehensive testing regimen includes a broad scope of analytical evaluations. The importance of assuring the physical and chemical properties throughout the development and commercialization of a compound is key to effectively managing resources and costs. The inclusion of a well-designed chemistry, manufacturing and controls process in the development pathway can help alleviate devastating pitfalls and facilitate a cost-effective process.
1. U.S. Department of Health and Human Services, "Guidance for Industry: Q1A Stability Testing of New Drug Substances and Products." Food and Drug Administration, August 2001.
2. International Conference on Harmonisation of Technical Requirements for Registration of Pharamceuticals for Human Use (ICH), "Stability Testing of New Drug Substances and Products (ICH Q1A)." ICH, September 1993.
3. Gallanger, Maxine M.; A Comparative Analysis of International Regulations and Guidances presented at PDA Scientific Forum: The Extractables Puzzle, November 2001