Tuberoinfundibular peptide (7-39) [TIP(7-39)], a novel, selective, high-affinity antagonist for the parathyroid hormone-1 receptor with no detectable agonist activity.

The parathyroid hormone (PTH)-1 receptor mediates the pathophysiological effects of PTH in hyperparathyroidism and PTH-related protein (PTHrP) in humoral hypercalcemia of malignancy. A PTH1 receptor antagonist may be of therapeutic utility in these disorders. We recently identified a novel antagonist, tuberoinfundibular peptide (7-39) [TIP(7-39)], derived from the likely endogenous ligand for the PTH2 receptor TIP39. In this study its in vitro profile is evaluated and compared with that of [D-Trp(12),Tyr(34)]bPTH(7-34) and PTHrP(7-34), representing the two previously known structural classes of PTH1 receptor antagonists. TIP(7-39) binds with higher affinity (6.2 nM) to the PTH1 receptor than [D-Trp(12),Tyr(34)]bPTH(7-34) (45 nM) and PTHrP(7-34) (65 nM) and displays a 5.5-fold greater PTH1/PTH2 receptor selectivity. TIP(7-39) does not stimulate cAMP accumulation via the PTH1 receptor [in a sensitive assay that detects the activity of the weak partial agonist [Nle(8,18),Tyr(34)]bPTH(3-34)] and does not increase intracellular calcium. Schild analysis for TIP(7-39) was consistent with purely competitive antagonism of PTH(1-34)'s stimulation of cAMP accumulation (slope = 0.99 +/- 0.24). The pK(B) for TIP(7-39) (7.1 +/- 0.3) was higher than that for [D-Trp(12),Tyr(34)]bPTH(7-34) (6.5 +/- 0.0) and PTHrP(7-34) (6.0 +/- 0.1). Binding of (125)I-TIP(7-39) to the PTH1 receptor could be measured (K(D) = 1.3 +/- 0.1 nM, B(max) = 1.3 +/- 0.1 pmol/mg), whereas binding of (125)I-[Nle(8,18),D-Trp(12),Tyr(34)]bPTH(7-34) could not be detected. Kinetic analysis indicated that (125)I-TIP(7-39) dissociates much more slowly (t(1/2) = 14 min) than [D-Trp(12),Tyr(34)]bPTH(7-34) (13 s) and PTHrP(7-34) (9 s). The novel antagonist TIP(7-39) therefore displays a more favorable in vitro pharmacological profile than antagonists derived from PTH and PTHrP and may be useful for demonstrating the utility of PTH1 receptor antagonism in the treatment of hypercalcemia.