Introduction

Stability testing is the primary tool used to assess expiration dating and storage conditions for pharmaceutical products. Many protocols have been used for stability testing, but most in the industry are now standardizing on the recommendations of the International Conference on Harmonization (ICH). These guidelines were developed as a cooperative effort between regulatory agencies and industry officials from Europe, Japan, and the United States.

Stability testing includes long-term studies, where the product is stored at room temperature and humidity conditions, as well as accelerated studies where the product is stored under conditions of high heat and humidity. Proper design, implementation, monitoring and evaluation of the studies are crucial for obtaining useful and accurate stability data. Stability studies are linked to the establishment and assurance of safety, quality and efficacy of the drug product from early phase development through the lifecycle of the drug product. Stability data for the drug substance are used to determine optimal storage and packaging conditions for bulk lots of the material. The stability studies for the drug product are designed to determine the expiration date (or shelf life). In order to assess stability, the appropriate physical, chemical, biological and microbiological testing must be performed. Usually this testing is a subset of the release testing.

Preclinical Studies

Preclinical studies include standard stability studies to assess shelf life, but also utilize special studies to study the drug and its degradation characteristics. The drug substance characterization and stability is usually determined as part of pre-formulation studies. Studies are designed to degrade the solid drug substance and appropriate solutions, allowing the determination of the degradation profile. The drug substance is usually challenged under a variety of accelerated environmental conditions to evaluate its intrinsic stability and degradation profile.

HPLC is the predominant tool used to analyze the drug substance and the impurities, particularly for small molecules. Frequently, the same HPLC method may be used for drug substance and drug product, although different sample preparation methods would normally be required. Often the assay and impurity testing can be performed using a single HPLC method. However, the assay and purity determinations may also be separate methods. At least in the U.S., full validation of the analytical method is not required until the end of Phase 2 clinical trials, but the establishment of specificity, linearity and limit of quantification (for impurities) is important at the earliest stages, since verification of stability hinges on a suitable method for separating impurities from the active ingredient and at least quantifying the impurities relative to the drug substance.

Stress studies at elevated temperature (e.g., 50ÁC, 60ÁC and 70ÁC) for several weeks may be performed to assess thermal stability. Provided the degradation mechanism is the same at the different temperatures used, kinetic or statistical models can be used to determine the rate of degradation at other temperatures (e.g., 25ÁC). The solid stability should also be performed in the presence and absence of water vapor to assess the dependence of stability on humidity.

Degradation studies should also be performed in solution. The solvent used for the solution testing will depend on the solubility of the drug substance and should include water, if the drug substance is water-soluble. Other solutions or solvent systems may be evaluated depending on the anticipated formulation or the synthetic process. A series of buffered solutions in the pH range 2-9 are useful in assessing the impact of solution pH on the degradation. Photostability should also be evaluated. A xenon light source can be used as a stress condition. Alternatively, one can use an accelerated version of either Options 1 or 2 as described in the ICH guideline for determination of photostability. Oxidation of the drug substance under accelerated conditions (e.g., hydrogen peroxide), may also be performed to establish oxidation products that could be formed and sensitivity to oxidative attack.

Early drug product stability studies are designed to help establish a suitable formulation for delivery of the drug substance. Compatibility studies of the drug substance with excipients should be performed to eliminate excipients that are not compatible with the drug substance.

Clinical Studies

Stability testing must be continued throughout clinical trials to support the safety, quality and efficacy of materials released for clinical trials. Stability data must be submitted as part of the IND filing prior to initiating the Phase 1 clinical trial. Prior to the first Phase 1 stability study, the pre-clinical studies should provide information on the appropriate long-term condition and the appropriate container/closure system. ICH Q1A provides the guidance for design of clinical stability studies. Selection of batches, the container closure system, specifications, testing frequency and storage conditions are the most important issues to consider when designing a stability study.

The container closure system must be evaluated for compatibility with the drug substance and drug product to ensure that the container does not contribute to degradation or contamination.

The testing frequency represents the minimum data required for filing. It may be advisable to pull and test a one-month sample for each storage condition to ensure that the study is proceeding as expected.

During Phase 1, it may be necessary to evaluate multiple formulations, dosage strengths and container closure systems. Using bracketing and/or matrixing can frequently reduce the resource allocation for these studies. These two design approaches are discussed in ICH Q1D. Bracketing uses the extremes to provide data for the entire study. For example, if dosage strengths of 10, 25, 50 and 100 mg are to be evaluated, the study may include testing of all strengths at the initial and final time points with only the 10 and 100 mg strengths being tested at the intermediate time points. Matrixing might be used to evaluate the same strength in multiple container/closure system by selecting only certain container closure systems for testing at each time-point. This selection is usually done in a random fashion.

At the end of Phase 1, the process for manufacture of the drug substance, and the drug product should be established (although refinements will typically continue for much longer).

The time period in Table 1 represents the minimum data required for the NDA. The studies must continue until the long-term stability study is completed for the shelf life and retest period proposed in the NDA submission. Temperature cycling studies and in-use stability studies may be needed for certain types of formulations (particularly liquid and semi-solid formulations). In early Phase 3 studies one should expect to be placing the batches on stability (at least three drug substance and drug product lots) that will be used for filing the NDA. These may be the validation batches if process validation is performed early enough. Process validation may be performed near the end of Phase 3 and adequate stability data for these batches may not be available at the time of filing. Shelf life and retest periods may be determined statistically with adequate quantitative data.

The period in Table 1 represents the minimum data required for the NDA. The studies must continue until the longterm stability study is completed for the shelf life and retest period proposed in the NDA submission.

Post-approval (Marketing Phase)

At least one lot of drug substance and one lot of each packaging type for drug product produced each year should be placed on long-term stability. Additional stability testing may be required to support process changes for drug substance and/or drug product. The filing requirements for changes are covered in multiple FDA guidance documents addressing drug product changes (SUPAC) and drug substance changes (BACPAC). Typically, this is an area that requires substantial regulatory understanding and experience to know how to proceed, and is beyond the scope of this article.

Conclusion

Stability testing is interwoven through the entire fabric of the drug product lifecycle. A detailed knowledge of the stability requirements and the impact on other areas (e.g., container closure, process changes) is needed to properly design and evaluate stability studies in order to ensure minimal delays and minimize costs in developing a new drug product.