The pseudotetrapeptide FR-139317 is a potent and highly selective antagonist of the endothelin-A (ETA) receptor; however, its peptidic nature leads to poor oral absorption characteristics which make it an unlikely drug candidate. In an attempt to improve these properties, we have replaced a portion of the amide bond framework of FR-139317 with a heterocyclic surrogate. The resultant analogs are also ETA-selective antagonists, but ...