This month's installment of "Validation Viewpoint"outlines how forced degradation is used to develop a stability-indicating method (SIM) and examines some of the method development and validation parameters critical to its success.
The Stability-Indicating Method
A SIM is a quantitative analytical procedure used to detect a decrease in the amount of the active pharmaceutical ingredient (API) present due to degradation. According to FDA guidelines, a SIM is defined as a validated analytical procedure that accurately and precisely measures active ingredients (drug substance or drug product) free from potential interferences like degradation products, process impurities, excipients, or other potential impurities, and the FDA recommends that all assay procedures for stability studies be stability indicating (3). During stability studies, liquid chromatography (LC) is used routinely to separate and quantitate the analytes of interest. There are three components necessary for implementing a SIM: sample generation, method development, and method validation.
Step One: Generate the Sample: SIMs are developed routinely by stressing the API under conditions exceeding those normally used for accelerated stability testing. In addition to demonstrating specificity in SIMs, stress testing, also referred to as forced degradation, also can be used to provide information about degradation pathways and products that could form during storage and help facilitate formulation development, manufacturing, and packaging. Stressing the API in both solutions and in solid-state form generates the sample that contains the products most likely to form under most realistic storage conditions, which is in turn used to develop the SIM. In simplest terms, the goal of the SIM is to obtain baseline resolution of all the resulting products (the API and all the degradation products) with no coelutions. Table I lists some common conditions used in conducting forced degradation studies for drug substances (4).
For most samples, storage for as long as two weeks at 80 °C, or six weeks at 60 °C, is used. Samples should be stored in appropriate vessels that allow sampling at timed intervals and that protect and preserve the integrity of the sample. Thermostated and humidity-controlled ovens also should be employed. Generally, the goal of these studies is to degrade the API 5–10 %. Any more than this and relevant compounds can be destroyed, or irrelevant degradation products produced (for example, degradation products of the degradation products). Any less, and important products might be missed. Experience and data obtained from studies performed previously on related compounds also should be used when developing new protocols.
Table I: Common conditions used in forced degradation studies. Acid and base solutions should be neutralized before analysis. Initial sample concentrations in the range of 1-10 mg/mL normally are used.