The Role of Stability-Indicating Methods to Determine the Potency of an API

The Key to Quality Control Processes for Drug Substances and Drug Products

Choosing a stability indicating method can be an effective route for detecting and quantifying the assay or potency of a chemical substance and its impurities profile. The ultimate goal of any method is to prove that, based on the profile of the drug and establishment of the Limit of Quantitation (LOQ), the final drug substance or drug product is safe and effective and its specifications are met.


According to FDA guidelines, a Stability Indicating Method (SIM) is a validated analytical testing procedure that measures active ingredients to determine the level of process impurities, and degradation products (based on established product specification). The FDA recommends that all assay procedures for stability should be stability indicating. A SIM provides a very solid indication when investigating out-of-trend (OOT) or out-of-specification (OOS) results in quality control processes.

As part of the process of conducting a forced degradation study for a drug substance or drug product. A scientist will degrade the molecule beyond its specification limits. If the study is performed based on an area percent concept, then there is a good chance that the method is not stability-indicating. For example, an acid hydrolysis study of a drug substance could indicate an assay of 97 percent, and the related substance method could show a purity of 98 percent by area.  At first glance, one could think that the acid hydrolysis study was successful, and the study met the acceptance criteria, but that’s not necessarily so.

It’s Time to Reconsider an Area-Percentage Approach to Forced Degradation

When an area-percentage approach for calculating the degree of degradation of a drug substance by the related substance method is used, the results can be misleading due to the method linearity, and impurity peak’s response factors.  In addition, inaccuracy increases considerably when the study uses a non UV-VIS type detector, such as CAD or ELSD or other types of detectors that have different linearity ranges and different response factors than would be achieved with UV-VIS detectors. Scientists should instead consider calculating forced degradation peaks based on weight/weight percent for both assay and related substance methods, examine impurity peak response in respect to the API, and fully evaluate all data to ensure the integrity of the results.

Forced Degradation Studies Require a Comprehensive View

The goal of a stability-indicating method is to have a better understanding of the stability of an API or drug product under different environmental and chemical conditions, its overall impurity profile based on the raw materials/reagents used, and the process-related impurities that might exist. In order to do this, the testing method must be developed so that all of the impurities are specific, identified (if possible) and quantifiable.

Weeding Out the Impurities Through Key Studies

The ultimate goal of any stability indicating method is to determine if some of the impurities can be removed or minimized by the manufacturing process, or if the process can be reinvented to prevent their formation in the first place Being able to accomplish this would significantly increase the quality of the API or drug product. For this reason, a reliable, robust and accurate method, conducted by a strong organic/process team and a strong analytical team, is a must.

Understanding the fate of each impurity requires the performance of key studies, including process mapping, spiking and purge studies, which are the elements of a robust process. They not only ensure a purer API or drug product, but also  enable a clearer path for effective monitoring and quantifying of the impurities that might exist in the product. Although many stability-indicating methods do not include these types of studies, they should always be part of the stability-indicating method design, and are the only effective way to detect and monitor impurities, including genotoxic compounds.

When developing a method, or conducting a forced degradation study , it pays to look at the molecule very comprehensively and study the entire process with a group of knowledgeable and experienced organic chemists, process chemists, and analytical chemists. Aside from process mapping, identifying unknown impurities, their synthesis, and qualifying them as reference standards, conducting spike and purging studies allow us to assess the impurity and its degradation pathway, and ultimately how to control and monitor it and improve the product quality. Therefore, the meaningful design of an experimental study for a molecule is very crucial and should be conducted under cGMP and defined protocols.

Here at Seqens, we’ve mastered these requirements. We recognize the importance of adhering to the highest standards for quality, accomplished through sound analytical procedures in raw materials, intermediates and final products; analytical support for all manufacturing operations; and in-house stability programs.

Stability-indicating methods can hold the key to optimized processes, and high product quality that ensures safe and effective drug products with very few impurities. Selecting the right CDMO for your drug substance  requires strong analytical expertise along with the flexibility and agility to adapt to changing requirements as continued advances in method testing add to the analytical tool-kit.

For additional best practices in API development, check out: The Key to Optimizing Process Research; API Manufacturing Technology Transfer; or The Art of API Method Development.