Assessing the Risk of Formation of Potential Genotoxic Degradation Products in a Small-Molecule Kinase Inhibitor Drug Substance and Drug Product

Galunisertib is a kinase inhibitor designed to selectively inhibit TGF-β signaling. Drug substance stress degradation studies performed during clinical development demonstrated two degradation products via oxidation of the nitrogen(s) of the pyridine moieties in the presence of dilute hydrogen peroxide. These "N-oxide" potential degradation products generated positive alerts for mutagenicity by in silico structure–activity relationship-based genotoxicity assessment, and both tested positive in the Ames bacterial mutagenicity test. These compounds were also identified as potential process impurities that could originate through the use of hydrogen peroxide, and this reagent was subsequently removed from the synthetic route. A toxicology limit of not more than 166 ppm (w/w relative to the drug substance) combined for the two N-oxides was assigned on the basis of clinical dosing. An LC–MS method was developed to test for the N-oxides with a limit of quantitation set at <10% of the toxicology limit in both the drug substance and the drug product tablets. Stability data demonstrated the absence of N-oxide formation under long-term and accelerated storage. On the basis of these results, the oxidative degradation pathway was shown to be inactive and nonrelevant for both the drug substance and the drug product.