Strategies toward optimization of the metabolism of a series of serotonin-4 partial agonists: investigation of azetidines as piperidine isosteres.

1.The first generation 5HT-4 partial agonist, 4-{4-[4-Tetrahydrofuran-3-yloxy)-benzo[d]isoxazol-3-yloxymethyl]-piperidin-1-ylmethyl}-tetrahydropyran-4-ol, PF-4995274 (TBPT), was metabolized to N-dealkylated (M1) and an unusual, cyclized oxazolidine (M2) metabolites. M1 and M2 demonstrated pharmacological activity at 5HT receptor subtypes warranting further investigation into their dispositional properties in humans; M2 was a minor component in vitro but was the pre-dominant metabolite identified in human plasma. 2.To shift metabolism away from the piperidine ring of TBPT, a series of heterocyclic replacements were designed, synthesized, and profiled. Groups including azetidines, pyrrolidines, as well as functionalized piperidines were evaluated with the goal of identifying an alternative group that maintained the desired potency, functional activity, and reduced turnover in human hepatocytes. 3.Activities of 4-substituted piperidines or pyrrolidine analogs at the pharmacological target were not significantly altered, but the same metabolic pathways of N-dealkylation and oxazolidine formation were still observed. Altering these to bridged ring systems lowered oxazolidine metabolite formation, but not N-dealkylation. 4.The effort concluded with identification of azetidines as second-generation 5HT4 partial agonists. These were neither metabolized via N-dealkylation nor converted to cyclized oxazolidine metabolites rather oxidized on the isoxazole ring. The use of azetidine as a replacement for aliphatic aza-heterocyclic rings in drug design to alter drug metabolism and pharmacology is discussed.

Strategies for a comprehensive understanding of metabolism by aldehyde oxidase.

INTRODUCTION: Aldehyde oxidase (AO) is a drug-metabolizing molybdo-flavoenzyme with profound species differences in expression and activity toward various substrates. The contribution of this enzyme to the metabolism and clearance of heterocyclic-containing xenobiotics appears to have increased in recent years, but has not always been identified prior to clinical studies. As a result, drug candidates have been negatively impacted in development. AREAS COVERED: This review provides the most recent in vitro and in vivo strategies for the drug metabolism-pharmacokinetic (DMPK) scientist. The review details approaches for confirmation of AO as an operable metabolic pathway, estimating clearance and fraction of total metabolism, and identification of an appropriate surrogate species for human AO activity for evaluating safety of clinically relevant metabolites. EXPERT OPINION: As the role of AO in metabolism of new drug molecules continues to emerge, it is critical that DMPK scientists have the most updated methodologies to enable formulation of a thorough experimental plan to understand the potential implications of this metabolic pathway. Whether it is higher-than-expected clearance, contributing to an unfavorable half-life, or the formation of an AO-derived disproportionate human metabolite (DHM), such a plan would serve to minimize complications or attrition of drug candidates due to unforeseen issues in the clinic.