The transformation of 3-bromo-1,6-naphthyridin-2(1H)-ones 8 to thiazolo[4,5-b][1,6]naphthyridin-2(1H)-ones 12 resulted in a 2-9-fold increase in cAMP phosphodiesterase (PDE) III inhibitory potency. Unlike the secondary binding sites on the cAMP PDE III isozyme which interact with the methyl group of milrinone (2) and CI-930 (4), the site which interacts with the 5-substituents of 1,6-naphthyridin-2(1H)-ones and the 8-substituents of thiazolo[4,5-b][1,6]naphthyridin-2(1H)-ones 12 is able to accommodate a diverse group of substituents which have different steric and electronic requirements.