A new highly potent parathyroid hormone antagonist: [D-TRP^l2, TYR³⁴]bPTH-(7--34)NH₂

Based upon N-tertninal parathyroid hormone (PTH) analog structure-activity relationship studies, position 12 was found to possess a wide structural latitude and was chosen as a site for single amino acid substitutions. Replacement of the naturally-occurring Gly with D-Trp at position 12 in the PTH antagonists [Tyr³⁴]bPTH-(7--34)NH₂ and [Nle^8,18, Tyr³⁴]bPTH-(7--34)NH₂ increased in vitro receptor affinity. The D-Trp¹² containing analogs were 12-fold more potent than their unsubstituted counterparts as inhibitors of PTH binding to renal and bone PTH receptors and 13-27-fold more potent as inhibitors of PTHstimulated renal and bone adenylate cydase activity. Based upon Scatchard analyses of saturation binding experiments and Schild analyses of adenylate cyclase experiments, [D-Trp¹², Tyr³⁴]bPTH-(7--34)NH₂ was shown to interact with PTH receptors in a competitive manner. These studies demonstrate, therefore, that D-Trp¹² substitution in PTH antagonists improves inhibitory properties in vitro and is compatible with a helical conformation at this position as a new direction for the design of PTH antagonists.