Compounds effective as IP receptor modulators, particularly IP receptor antagonists, that are of the formula I: ##STR00001## wherein A, R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are as defined in the specification; and individual isomers, racemic or non-racemic mixtures of isomers, and pharmaceutically acceptable salts or solvates thereof. Also disclosed are pharmaceutical compositions containing such compounds and methods for their use as therapeutic agents.