Oxidative metabolism of the trifluoromethoxy moiety of OSI-930.

Cytochrome P450 can catalyze a wide array of remarkable oxidations, including O-dealkylations, which are performed via oxidation of the alpha-carbon of the ether. When C-H bonds are replaced with C-F bonds, however, the bond strength is much greater, and it significantly deters oxidation at the carbon. Another recently elucidated reaction catalyzed by P450, ipso substitution, results in displacement of aromatic ring substituents such as an alkoxy group via hydroxyl substitution. Through LC/MS/MS, we show the CYP-mediated oxidative displacement of the trifluoromethoxy group from the phenyl constituent in OSI-930, a novel small molecule c-Kit/VEGF-r inhibitor in clinical studies to treat cancer. Based on C-F bond strength, reported phenacetin studies, and alpha-quaternary alkylphenol studies, we propose an ipso-substitution mechanism for this oxidative biotransformation. In vivo, this hydroxylated metabolite goes on to form the ether conjugate with glucuronide.

Safety assessment of 4'-thio-beta-D-arabinofuranosylcytosine in the beagle dog suggests a drug-induced centrally mediated effect on the hypothalamic-pituitary-adrenal axis.

4'-Thio-beta-D-arabinofuranosylcytosine (OSI-7836) is a nucleoside analogue with structural similarity to gemcitabine and cytarabine (ara-C). Myelosuppression, reversible transaminase elevations, and flu-like symptoms are common side effects associated with human use of gemcitabine and ara-C. Fatigue is also associated with the use of gemcitabine and OSI-7836 in humans. To better understand the toxicity of OSI-7836, subchronic studies were conducted in dogs. OSI-7836 was administered on days 1 and 8 or on days 1, 2, and 3 of a 21-day dose regimen. These schedules attempted to match clinical trial dosing regimens. Routine toxicity study end points demonstrated that OSI-7836 was primarily cytotoxic to the gastrointestinal tract, bone marrow, and testes; the myelotoxicity was mild and reversible. Plasma pharmacokinetics were dose-linear with an elimination half-life of 2.2 h. Follow-up single dose experiments in dogs assessed drug effects on lymphocyte subpopulations and on adrenal and thyroid function. Populations of T and B cells were equally reduced following OSI-7836 administration. There were no adverse effects on thyroid function, but there were marked reductions in circulating cortisol and adrenocorticotropic hormone concentrations suggesting a centrally mediated impairment of the hypothalamic-pituitary-adrenal axis. These findings show a toxicological profile with OSI-7836 similar to other nucleoside analogues and suggest that the beagle is a model for studying one possible cause of OSI-7836-related fatigue, impaired function of the hypothalamic-pituitary-adrenal axis.