The GABAB receptor positive allosteric modulator COR659: in vitro metabolism, in vivo pharmacokinetics in rats, synthesis and pharmacological characterization of metabolically protected derivatives.
We report an in vitro phase I metabolism study on COR659 (1), a 2-acylaminothiophene derivative able to suppress alcohol and chocolate self-administration in rats, likely via positive allosteric modulation of the GABAB receptor and antagonism/inverse agonism at the cannabinoid CB1 receptor. Given the identification of the methyl ester group at C-3 of the thiophene ring as a metabolic soft spot, we also report the chemical optimization project aimed to balance metabolic stability with in vitro and in vivo potency on a set of 3-substituted COR659 analogues. High performance liquid chromatography coupled to tandem and high resolution mass spectrometry was employed for the characterization of in vitro metabolism and in vivo pharmacokinetics of COR659 in rats. In vitro [35S]GTPγS binding assays on stimulated GABAB and CB1 receptors, in combination with alcohol and chocolate self-administration experiments in rats, were employed to assess the pharmacological profile of this novel set of analogues, using COR659 as reference compound. Eight metabolites of COR659 were discovered in liver microsomal incubates; two of them (M1, M2) were identified by comparison with synthetic reference standards. M2, oxidation product of methyl group at C-5 of the thiophene ring, was a major metabolite in vitro, but showed a low systemic exposure in vivo. M1, cleavage product of the methyl ester group at C-3, revealed in vitro an unusual mechanism of metabolism by a NADPH-dependent route and, in vivo, it maintained high and persistent levels in plasma, which could represent a potential pharmacokinetic and toxicological issue. In the novel set of COR659 analogues, those bearing branched alkyl substituents on the ester group, showed an improved in vitro metabolic stability (2-4), had an in vitro GABAB PAM (2-4) and/or CB1 partial agonist/antagonist profile (2-3) and maintained the ability to reduce alcohol (2-4) and/or chocolate (4) self-administration in rats. Both PK and PD data ruled out any involvement of metabolite M1 in the in vivo potency of COR659 and 4. The present results, therefore, highlight the importance to design and synthesize novel compounds endowed with the dual activity profile and devoid of metabolic liabilities.