Bisphosphonates mimic the endogenous inorganic pyrophosphate where the oxygen backbone is replaced with carbon (P-C-P for P-O-P). The two additional groups or side chains on the carbon backbone are usually referred to as R and R. R is usually a hydroxyl group which enhances the affinity for the calcium by forming a tridentate ligand along with the phosphate groups. The compound can be made more potent by optimizing the structure of the R group to best inhibit bone resorption.