Structure-activity relationships of new heterocycle-containing bisphosphonates as inhibitors of bone resorption and as inhibitors of growth of Dictyostelium discoideum amoebae.

The mechanisms by which bisphosphonate drugs inhibit osteoclast-mediated bone resorption are unclear. Effects of bisphosphonates on cellular enzymes, metabolic pathways, and osteoclast morphology have previously been described and could culminate in a generalized cytotoxic effect or a decreased capacity of osteoclasts to resorb bone. Recent studies of the structure-activity relationship for the bisphosphonate side chain indicate, however, that at least the newer generations of nitrogen-containing bisphosphonates probably act by binding to a specific target at a site that is complementary in structure to the bisphosphonate side chain. We have previously proposed that such a target for bisphosphonates is also present in amoebae of the cellular slime mold Dictyostelium discoideum, because growth of this microorganism is inhibited by a wide range of bisphosphonates in a manner that closely reflects the antiresorptive potencies of the bisphosphonates in vivo. We have added support for this view by examining the potency towards Dictyostelium of bisphosphonates in which slight changes in the structure of the side chain or conformational restrictions to the side chain have marked effects on antiresorptive potency. The changes in the side chain that affected the in vivo antiresorptive potency of the bisphosphonates consistently affected in a similar manner the potency of the bisphosphonates as inhibitors of the growth of Dictyostelium amoebae. These observations confirm that bisphosphonate drugs have a molecular target that is common to both Dictyostelium amoebae and osteoclasts.