Identifying the reactive metabolites of tyrosine kinase inhibitors in a comprehensive approach: Implications for drug-drug interactions and hepatotoxicity

Tyrosine kinase inhibitors (TKI) are small heterocyclic molecules targeting transmembrane and cytoplasmic tyrosine kinases that have met with considerable success in clinical oncology. TKI are associated with toxicities including liver injury that may be serious and even life-threatening. Many of them require warnings in drug labeling against liver injury, and five of them have Black Box Warning (BBW) labels. Although drug-induced liver injury is a matter of clinical and industrial concern, little is known about the underlying mechanisms that likely involve reactive metabolites (RM). RM are electrophiles or radicals originating from the metabolic activation of particular functional groups, known as structural alerts or toxicophores. RM are able to covalently bind to proteins and macromolecules, causing cellular damage and even cell death. If the adducted protein is the enzyme involved in RM formation, time-dependent inhibition of the enzyme-also called mechanism-based inhibition (MBI) or inactivation-can occur and lead to pharmacokinetic drug-drug interactions. To mitigate RM liabilities, common practice in drug development includes avoiding structural alerts and assessing RM formation via RM trapping screens with soft and hard nucleophiles (glutathione, potassium cyanide, and methoxylamine) in liver microsomes. RM-positive derivatives are further optimized to afford drug candidates with blocked or minimized bioactivation potential. However, different structural alerts are still commonly used scaffolds in drug design, including in TKI structures. This review focuses on the current state of knowledge of the relations among TKI structures, bioactivation pathways, RM characterization, and hepatotoxicity and cytochrome P450 MBI in vitro.