The synthesis, biological evaluation, and structure-activity relationships of a series of N-phenyl heteroaryl-fused isothiazolones are described. These isothiazolones have been shown to exhibit potent, dose-dependent inhibition of IL-1 beta-induced breakdown of proteoglycan in a cartilage organ culture assay. This effect is likely due to inhibition of MMP activation and a consequent reduction in MMP activity following IL-1 beta stimulation. Thus these compounds potentially represent simple, non-peptidic disease-modifying agents for the treatment of arthritic diseases. To examine the effects of structure on in vitro activity, three general features of the molecules were varied, substituents on the pendant N-phenyl group, the position of ring fusion to the isothiazolone, and substituents on the fused ring peri to the isothiazolone sulfur.