Pyrrolidine-3-carboxylic acids as endothelin antagonists.: 3.: Discovery of a potent, 2-nonaryl, highly selective ETA antagonist (A-216546)

Previously we have reported the discovery of ABT-627 (1, A-147627, active enantiomer of A-127722), a 2,4-diaryl substituted pyrrolidine-3-carboxylic acid based endothelin receptor-A antagonist. This compound binds to the ETA receptor with an affinity (K-i) of 0.034 nM and with a 2000-fold selectivity for the ETA receptor versus the ETB receptor. We have expanded our structure-activity studies in this series, in an attempt to further increase the ETA selectivity. When the p-anisyl group of 1 was replaced by an n-pentyl group, the resultant antagonist 3 exhibited substantially increased ETB/ETA activity ratio, but a decreased ETA affinity. Structure-activity studies revealed that substitution and geometry of this alkyl group, and substitution on the benzodioxolyl ring, are important in optimizing this series of highly ETA selective antagonists. In particular, the combination of a (E)-2,2-dimethyl-3-pentenyl group and a 7-methoxy-1,3-benzodioxol-5-yl group provided hydrophobic compound 10b with subnanomolar affinity for human ETA receptor subtype and with an ETB/ETA activity ratio of over 130000. Meanwhile, synthetic efforts en route to olefinic compounds led to the discovery that 2-pyridylethyl (9o) and 2-(2-oxopyrrolidinyl)ethyl (9u) replacement of the p-anisyl group of 1 yielded very hydrophilic ETA antagonists with potency and selectivity equal to those of 10b. On the basis of overall superior affinity, high selectivity for the ETA receptor (K-i, 0.46 nM for ETA and 13000 nM for ETB), and good oral bioavailability (48% in rats), A-216546 (10a) was selected as a potential clinical backup for 1.