Deuterium isotope effects were evaluated as a strategy to optimize the pharmacokinetics of NVS-CRF38, a novel CRF1 receptor antagonist. In an attempt to supress O-demethylation of NVS-CRF38 without losing activity against the CRF1 receptor, the protons at the site of metabolism were replaced with deuterium. For in vitro and in vivo studies intrinsic primary isotope effects (KH/KD) were determined by the ratio of CLint obtained from the nondeuterated and deuterated substrate. In vitro isotope effects were more pronounced when CLint values were calculated based on the rate of formation of the O-desmethyl metabolite (KH/KD ~7), compared to the substrate depletion method (KH/KD ~2). In vivo isotope effects were measured in the rat after intravenous (1 mg/kg) and oral (10 mg/kg) administration. For both administration routes isotope effects calculated from in vivo CLint corresponding to all biotransformation pathways were lower (KH/KD ~2) compared to CLint values calculated from the O-demethylation reaction alone (KH/KD ~7). Comparative metabolite identification studies were undertaken using rat and human microsomes to explore the potential for metabolic switching. As expected a marked reduction of the O-demethylated metabolite was observed for the nondeuterated substrate, however levels of NVS-CRF38’s other metabolites increased, compensating to some extent for the isotope effect.
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