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Formation of reactive intermediates by metabolism of xenobiotics represents a potential liability in drug discovery and development. Although it is difficult, if not impossible, to predict toxicities of drug candidates accurately, it is prudent to try to minimize bioactivation liabilities as early as possible in the stage of drug discovery and lead optimization. Measurement of covalent binding to liver microsomal proteins in the presence and the absence of NADPH, as well as the use of trapping agents such as glutathione or cyanide ions to provide structural information on reactive intermediates, have been used routinely to screen drug candidates. These in vitro experiments are often supplemented with in vivo covalent binding data in rats. The resulting data are not only used to eliminate potentially risky compounds, but, more importantly, they provide invaluable information to direct the Medicinal Chemistry group efforts to design analogs with less propensity to undergo bioactivation. Select case histories are presented in which this approach was successfully applied at Merck.

Abstract: A novel class of human β3-adrenergic receptor agonists was designed in effort to improve selectivity and metabolic stability versus previous disclosed β3-AR agonists. As observed, many of the β3-AR agonists seem to need the acyclic ethanolamine core for agonist activity. We have synthesized derivatives that constrained this moiety by introduction of a pyrrolidine. This unique modification maintains human β3 functional potency with improved selectivity versus ancillary targets and also eliminates the possibility of the same oxidative metabolites formed from cleavage of the N–C bond of the ethanolamine. Compound 39 exhibited excellent functional β3 agonist potency across species with good pharmacokinetic properties in rat, dog, and rhesus monkeys. Early de-risking of this novel pyrrolidine core (44) via full AMES study supports further research into various new β3-AR agonists containing the pyrrolidine moiety. [Copyright &y& Elsevier]