Quantitative Translational Analysis of Brain Kynurenic Acid Modulation via Irreversible Kynurenine Aminotransferase II Inhibition.

Kynurenic acid (KYNA) plays a significant role in maintaining normal brain function, and abnormalities in KYNA levels have been associated with various central nervous system disorders. Confirmation of its causality in human diseases requires safe and effective modulation of central KYNA levels in the clinic. The kynurenine aminotransferases (KAT) II enzyme represents an attractive target for pharmacologic modulation of central KYNA levels; however, KAT II and KYNA turnover kinetics, which could contribute to the duration of pharmacologic effect, have not been reported. In this study, the kinetics of central KYNA-lowering effect in rats and nonhuman primates (NHPs, Cynomolgus macaques) was investigated using multiple KAT II irreversible inhibitors as pharmacologic probes. Mechanistic pharmacokinetic-pharmacodynamic analysis of in vivo responses to irreversible inhibition quantitatively revealed that 1) KAT II turnover is relatively slow [16-76 hours' half-life (t1/2)], whereas KYNA is cleared more rapidly from the brain (<1 hour t1/2) in both rats and NHPs, 2) KAT II turnover is slower in NHPs than in rats (76 hours vs. 16 hours t1/2, respectively), and 3) the percent contribution of KAT II to KYNA formation is constant (∼80%) across rats and NHPs. Additionally, modeling results enabled establishment of in vitro-in vivo correlation for both enzyme turnover rates and drug potencies. In summary, quantitative translational analysis confirmed the feasibility of central KYNA modulation in humans. Model-based analysis, where system-specific properties and drug-specific properties are mechanistically separated from in vivo responses, enabled quantitative understanding of the KAT II-KYNA pathway, as well as assisted development of promising candidates to test KYNA hypothesis in humans.

The sulfonylurea glipizide does not inhibit ischemic preconditioning in anesthetized rabbits.

The K(ATP) channel blocker glibenclamide inhibits cardioprotection afforded by ischemic preconditioning (IPC), raising concern about sulfonylurea use by patients with cardiovascular disease. We examined the effects of the widely prescribed sulfonylurea glipizide (Glucotrol XL(R) ) on IPC in anesthetized rabbits. Initially, in parallel studies in pentobarbital-anesthetized rabbits, we identified doses of glipizide (GLIP, 0.17 mg/kg + 0.12 mg/kg/h, IV) and glibenclamide (GLIB, 0.05 mg/kg + 0.03 mg/kg/h, IV) that produced steady-state, clinically relevant plasma levels of both drugs; these doses also significantly increased plasma insulin by 51 +/- 17% (GLIP) and by 57 +/- 17% (GLIB, both p < 0.05 vs. their respective baseline levels). Subsequent parallel studies in ketamine-xylazine-anesthetized rabbits examined the effects of these doses of GLIP and GLIB on IPC. Myocardial injury (30 min coronary occlusion/120 min reperfusion), either with or without IPC (5 min occlusion/10 min reperfusion) was induced midway during a 2 h infusion of vehicle (VEH), GLIP or GLIB (n = 10-11 each). Infarct area (IA) normalized to area-at-risk (%IA/AAR) was 62 +/- 3% in the VEH group, and was significantly reduced to 39 +/- 5% by IPC (p < 0.05 vs. VEH). Neither GLIP nor GLIB treatment had any effect on %IA/AAR in the absence of IPC (p > 0.05). IPC-induced cardioprotection was preserved in the GLIP + IPC treatment group (45 +/- 4%) when compared to VEH alone (p < 0.05), but was attenuated in the presence of GLIB (GLIB+IPC: 53 +/- 4% IA/AAR, p > 0.05 vs. VEH). Thus, at a clinically relevant plasma concentration, glipizide did not limit the cardioprotective effects of IPC, and is unlikely to increase the severity of cardiac ischemic injury.