Effects of hydrophobic substitutions at position 18 on the potency of parathyroid hormone antagonists

Position 18 in a parathyroid hormone (PTH) antagonist, [Nle^8,18,Tyr³⁴]bPTH(7‐34)NH₂ (ii), was shown to tolerate substitutions by a range of amino acids with retention of inhibitory activity. The effects of hydrophobic substitutions at this position as a means of enhancing binding interactions with the receptor were evaluated. Substitution of Nle at position 18 with either d‐Ala, d‐Trp, or l‐Trp in analog ii or with Trp (d or l) in the recently reported, highly potent antagonist, [Nle^8,18,d‐Trp¹²,Tyr³⁴]bPTH(7‐34)NH₂ (in vitro activities; K_b= 15 nM and K_i= 125 nM), was performed. In terms of activity on renal receptors, one antagonist, [Nle⁸,d‐Trp^12,18,Tyr³⁴]bPTH(7‐34)NH₂, is the most active in vitro PTH antagonist yet reported (K_b= 4nm; K_i= 30 nM). The rationale for design of this antagonist and the conclusions regarding PTH‐receptor interactions are discussed.