Genotoxic impurities and how to identify them and control for them have been a concern for several years in the pharmaceutical manufacturing industry. In recent years, FDA has issued draft guidance on the subject, EMA has issued final guidance, and the International Conference on Harmonization (ICH) is putting together a related M7 guideline to complement its Q3A and Q3B guidelines on impurities (1–3). Still, challenges remain. Pharmaceutical Technology spoke with Bo Shen, PhD, principal scientist at Amgen and chair of the American Association of Pharmaceutical Scientists (AAPS) Pharmaceutical Trace Impurities Focus Group, to gain insight on these challenges.

Key challenges and regulatory requirements

PharmTech: What would you say are the key challenges facing drug manufacturers today regarding genotoxic impurities (GTIs)? Is there still concern over FDA and EMA requirements/threshold limits or approaches for controlling for these impurities?

Drug manufacturers have to identify GTIs early in process development, develop analytical methods, and demonstrate a control strategy to ensure patient safety. The assessment/identification phase involves a process by which the synthetic pathway is evaluated and through a combination of chemical reasoning and analytical testing, either known or potential genotoxic impurities (pGTIs) may be identified.

To assess pGTIs, drug manufacturers use a combination of in-silico tools, evaluate scientific literature and public databases, and may conduct an Ames test to determine the mutagenic potential of chemical compounds. The result of an Ames test takes precedent over the predictions made by in-silico programs. Of the in vitro genotoxicity tests, the Ames test is the most reliable predictor of carcinogencity. Therefore, a compound that is positive in the Ames test is classified as genotoxic unless a substantial weight-of- evidence exists to conclude that the compound does not represent a genotoxic risk to patients.

To build such weight-of evidence involves conducting additional in vitro and in vivo gentoxicity tests that require significant resources and can lead to program delays if the GTIs cannot be controlled to the threshold of toxicological concern (TTC) while additional studies are being conducted.

Another challenge is that the Ames test may not be sensitive enough to detect GTIs when the impurity is assessed by testing the drug substance. The detection limit for many mutagens in the Ames test is 250 µg/plate, which represents a level far higher than is typically encountered for impurities in drug substances. Therefore, in many circumstances, the isolated impurity needs to be tested instead. Synthesis of an impurity in amounts and purity sufficient for Ames testing can provide a challenge to drug manufacturers.

After identifying GTIs, to ensure patient safety, drug manufacturers have to establish a robust control strategy to prevent or limit GTIs in the final API. Control strategy approaches will vary by companies. Some companies may choose to alter synthetic routes to avoid using or generating GTIs altogether; whereas, other companies may consider this impractical, especially when multiple GTIs must be handled and making changes might only lead to new ones.

The need of controlling several GTIs at the TTC level during analytical and process development further complicates the control strategy. Structural considerations can be complicated and numerous challenges can exist.

With the regulatory guidance in this area evolving and the need to ensure drug product quality and patient safety, drug manufacturers are faced with the challenge of effectively establishing and implementing robust internal practices which balance time, cost, and risk.