Use of a novel rapid and resource-efficient cassette dosing approach to determine the pharmacokinetics and CNS distribution of small molecule 7-transmembrane receptor allosteric modulators in rat

Approaches to efficiently and accurately define the pharmacokinetics (PK) of large sets of small molecules in rodents have been previously described. Likewise, a variety of methods for determining brain tissue distribution (BTD) have been reported for use in the discovery of therapeutics targeting the central nervous system (CNS). Herein we describe a novel cassette approach to efficiently obtain concurrent PK and BTD data from a dose of up to five compounds in one rat over 24 h. In conjunction with fraction unbound (fu) data obtained in plasma and brain homogenate, this approach serves as an efficient means to determine compound unbound brain:unbound plasma partition coefficients (K p,uu), thereby providing insight to compounds bearing poor permeability and/or active transporter activity impacting their permeation of the blood-brain barrier (BBB). This integrated approach was utilized in a lead optimization effort towards the discovery of CNS-penetrant allosteric modulators of a seven-transmembrane (7TM) receptor target. Rat PK and brain distribution was rapidly obtained for 70 compounds and correlated to data obtained from in vitro assessments. Two compounds that were evaluated in cassette and discrete studies, displayed agreement in PK (compound 1: cassette CLp = 1.6 mL min(-1) kg(-1), discrete CLp = 1.6 mL min(-1) kg(-1); compound 2: cassette CLp = 11 mL min(-1) kg(-1), discrete CLp = 8.1 mL min(-1) kg(-1)) and BTD (compound 1: cassette K p = 0.11, discrete K p = 0.09; compound 2: cassette K p < 0.05, discrete K p = 0.04). The resulting data were used to guide medicinal chemistry efforts and to enable the progression of optimized compounds to in vivo pharmacodynamic assessments.