Design, synthesis, and development of cannabinoid type-1 receptor (CB1R) allosteric modulators

Abstract

Listen

Translate article

The 2-phenylindole cannabinoid type-1 receptor (CB1R) positive allosteric modulator (PAM), GAT211, exerts preclinical therapeutic benefits associated with CB1R activation while free of the safety limitations typical of CB1R orthosteric agonists. Despite its therapeutic potential, GAT211 is not a druggable lead because of its limited water solubility and potency. To resolve these setbacks, we executed a broad structural optimization on the GAT211 scaffold by synthesizing a plethora of GAT211 analogs and assessing their in vitro CB1R PAM activity in cAMP and β-arrestin2 assays. We first synthesized and pharmacologically profiled site-deconstructed analogs of GAT211 as well as nitro- and indole- NH-functionalized derivatives. All these analogs were inactive, demonstrating the importance of GAT211's site I, II, and III components and its indole N-H and aliphatic nitro group. We then performed in-depth structure-activity relationship (SAR) studies at the three sites of GAT211 by generating a focused analog library for each site. The resulting data SAR established key SAR trends for the 2-phenylindole scaffold and discovered structurally-modified GAT211 congeners with significantly improved pharmacodynamic and/or pharmacokinetic properties. Bioisosteric replacement of the site-III phenyl ring with a 2-thiazolyl moiety enhanced both potency and kinetic water solubility, the latter a challenging objective. Incorporation of lipophilic groups at the site I C6 position enhanced potency to the greatest degree. Combining a site-I C6 methyl group with a site-III 2-thiazolyl ring or 2-oxazolyl ring resulted in a complementary effect whereby the 6-methyl group imparted potency while the site III substituent compensated water solubility deductions. Installation of electron-withdrawing groups, particularly a cyano group, at GAT211's site-I C6 position substantially prolonged compound half-life in microsomal incubations, demonstrating the significance of this position for tuning metabolic stability. Collectively, the new chemical matter generated through optimized single-site variations of the prototypical 2-phenylindole CB1R PAM, GAT211, illustrates reliable methods for converting GAT211 to more druggable analogs with improved potential as lead compounds.--Author's abstract