Discovery of 3-methyl-N-(1-oxy-3',4',5',6'-tetrahydro-2'H-[2,4'-bipyridine]-1'-ylmethyl)benzamide (ABT-670), an orally bioavailable dopamine D4 agonist for the treatment of erectile dysfunction.

The goal of this study was to identify a structurally distinct D(4)-selective agonist with superior oral bioavailability to our first-generation clinical candidate 1a (ABT-724) for the potential treatment of erectile dysfunction. Arylpiperazines such as (heteroarylmethyl)piperazine 1a, benzamide 2, and acetamides such as 3a,b exhibit poor oral bioavailability. Structure-activity relationship (SAR) studies with the arylpiperidine template provided potent partial agonists such as 4d and 5k that demonstrated no improvement in oral bioavailability. Further optimization with the (N-oxy-2-pyridinyl)piperidine template led to the discovery of compound 6b (ABT-670), which exhibited excellent oral bioavailability in rat, dog, and monkey (68%, 85%, and 91%, respectively) with comparable efficacy, safety, and tolerability to 1a. The N-oxy-2-pyridinyl moiety not only provided the structural motif required for agonist function but also reduced metabolism rates. The SAR study leading to the discovery of 6b is described herein.