Review
Development of validated stability-indicating assay methods—critical review

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Abstract

This write-up provides a review on the development of validated stability-indicating assay methods (SIAMs) for drug substances and products. The shortcomings of reported methods with respect to regulatory requirements are highlighted. A systematic approach for the development of stability-indicating methods is discussed. Critical issues related to development of SIAMs, such as separation of all degradation products, establishment of mass balance, stress testing of formulations, development of SIAMs for combination products, etc. are also addressed. The applicability of pharmacopoeial methods for the analysis of stability samples is discussed. The requirements of SIAMs for stability study of biotechnological substances and products are also touched upon.

Introduction

The stability-indicating assay is a method that is employed for the analysis of stability samples in pharmaceutical industry. With the advent of International Conference on Harmonisation (ICH) guidelines, the requirement of establishment of stability-indicating assay method (SIAM) has become more clearly mandated. The guidelines explicitly require conduct of forced decomposition studies under a variety of conditions, like pH, light, oxidation, dry heat, etc. and separation of drug from degradation products. The method is expected to allow analysis of individual degradation products.

A review of literature reveals a large number of methods reported over the period of last 3–4 decades under the nomenclature ‘stability-indicating’. However, most of the reported methods fall short in meeting the current regulatory requirements.

Accordingly, the purpose of this write-up is to suggest a systematic approach for the development of validated SIAMs that should meet the current ICH and regulatory requirements. The discussion also touches upon various critical issues, such as the extent of separation of degradation products, establishment of mass balance, etc., which are important with respect to the development of stability-indicating assays, but are not yet fully resolved. Some other aspects like suitability of pharmacopoeial methods for the purpose and the role of SIAMs in stability evaluation of biological/biotechnological substances and products are also delved upon.

Section snippets

Regulatory status of stability-indicating assays

The ICH guidelines have been incorporated as law in the EU, Japan and in the US, but in reality, besides these other countries are also using them. As these guidelines reflect the current inspectional tendencies, they carry the de facto force of regulation. The ICH guideline Q1A on Stability Testing of New Drug Substances and Products [1] emphasizes that the testing of those features which are susceptible to change during storage and are likely to influence quality, safety and/or efficacy must

Review of the literature on stability-indicating assays

In absence of any guidance from regulatory agencies on practical steps to be followed for establishment of stability-indicating assays, a search was done on the available information in literature. The literature was found to be replete with publications on development of stability-indicating assays of specific drugs. A general review was published as early as 1971, and it gave general principles and discussed the methods developed till that period [12]. Kumar and Sunder also discussed the

An assessment of the extent to which the reported methods meet current regulatory requirements

A review of various literature reports shows that very few methods that are titled or claimed to be stability-indicating fit into the current definition of a stability-indicating assay in true sense. While the current requirement is of subjecting the drug substance to variety of stress conditions and then separation of drug from all degradation products, many studies have just shown the separation of drug from known synthetic impurities and/or potential degradation products without subjecting

Techniques employed in literature reports

If one critically evaluates the literature reports, titrimetric, spectrophotometric and chromatographic techniques have been commonly employed in analysis of stability samples. There are also sporadic reports of the use of miscellaneous techniques.

Development of validated SIAMs that are likely to meet regulatory requirements

Though the requirements with respect to SIAM have been spelt out in regulatory documents, information on the basic steps to be followed for the development and validation of stability-indicating methods is neither provided in the regulatory guidelines nor in the pharmacopoeias. Therefore, the practical steps involved in the development of SIAMs are discussed below. It is expected that by following the steps, one should be in a position to develop a SIAM that would meet the regulatory

Some critical issues concerning development of SIAMs and their validation

There are several other issues concerning development of SIAMs on which routinely the questions are asked and clarifications are sought. The important ones are discussed below.

Commercial availability of standards of degradation products

The success of establishment of validated SIAM and also the mass balance depends much on the availability of standards of degradation products. Therefore, a brief discussion on the commercial sources from where one can acquire them would be pertinent here. The standards for old and established degradation products controlled by pharmacopoeial monographs can be procured from the respective pharmacopoeial authorities (www.pheur.org; www.usp.org; www.promochem.com). Also, there are other national

The emerging techniques for analysis of stability samples

As discussed under the instrumental methods employed in literature reports and elsewhere in the text, there is an increasing trend in recent times on involvement of hyphenated techniques (GC-MS, LC-MS or LC-MS-MS, CE-MS, LC-NMR, etc.) at various stages in development of SIAMs. Their use is picking up due to easy availability of bench-top instrumentation and their distinct advantages, like versatility; sensitivity; possibility of profiling, substructural analysis and rapid selective quantitative

Making use of computer simulation in development and optimization of SIAMs

As must be realized from the above discussion, the process of development of SIAMs by HPLC is a time consuming and difficult exercise. In general also, there are a large number of interdependent parameters, which exist in the practice of HPLC and the consequent requirement to study these parameters during method development through multiple chromatographic runs makes the situation very difficult overall [191].

A good strategy for development of a SIAM, like any other HPLC method, should require

The SIAM requirements for stability study of biotechnological products

Biotechnological products also undergo degradation during storage. A variety of degradation products arise resulting from deamidation, oxidation, sulfoxidation, aggregation or fragmentation. No single stability-indicating assay or parameter is available that profiles the stability characteristics of biotechnological products, unlike those of chemical drugs. Hence it is a requirement in the ICH guideline Q5C on Stability Testing of Biotechnological Products that the manufacturer should propose a

Conclusions

As can be seen from the plenty of the examples given in the tables in the text above, the stability-indicating assays have been developed for a large number of drugs for last several decades, starting almost from 1960s. But most of them unfortunately fail to meet the current regulatory requirements of separation and analysis of individual degradation products. Furthermore, there is little guidance provided in the literature on how to establish true ‘Selective’ stability-indicating methods. In

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