Novel Mechanism of Decyanation of GDC-0425 by Cytochrome P450

GDC-0425 [5-((1-ethylpiperidin-4-yl)oxy)-9H-pyrrolo[2,3-b:5,4-c9]dipyridine-6-carbonitrile] is an orally bioavailable small-molecule inhibitor of checkpoint kinase 1 that was investigated as a novel cotherapy to potentiate chemotherapeutic drugs, such as gemcitabine. In a radiolabeled absorption, distribution, metabolism, and excretion study in Sprague-Dawley rats, trace-level but long-lived 14 C-labeled thiocyanate was observed in circulation. This thiocyanate originated from metabolic decyanation of GDC-0425 and rapid conversion of cyanide to thiocyanate. Excretion studies indicated decyanation was a minor metabolic pathway, but placing 14 C at nitrile magnified its observation. Cytochrome P450s catalyzed the oxidative decyanation reaction in vitro when tested with liver microsomes, and in the presence of 18 O 2 , one atom of 18 O was incorporated into the decyanated product. To translate this finding to a clinical risk assessment, the total circulating levels of thiocyanate (endogenous plus drug-derived) were measured following repeated administration of GDC-0425 to rats and cynomolgus monkeys. No overt increases were observed with thiocyanate concentrations of 121–154 µ M in rats and 71–110 µ M in monkeys receiving vehicle and all tested doses of GDC-0425. These findings were consistent with results from the radiolabel rat study where decyanation accounted for conversion of µ M thiocyanate to systemic levels. Further, in vitro studies showed only trace oxidative decyanation for humans. These data indicated that, although cyanide was metabolically released from GDC-0425 and formed low levels of thiocyanate, this pathway was a minor route of metabolism, and GDC-0425–related increases in systemic thiocyanate were unlikely to pose safety concerns for subjects of clinical studies.