Modifications of position 12 in a parathyroid hormone and parathyroid hormone-related protein: toward the design of highly potent antagonists

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Abstract
Truncated N-terminal fragments of parathyroid hormone (PTH), [Tyr34]bovine PTH(7-34)NH2, and parathyroid hormone related protein (PTHrP), PTHrP(7-34)NH2, inhibit [Nle8,18[125I]iodo-Tyr34]-bPTH(1-34)NH2 binding and PTH-stimulated adenylate cyclase in bone and kidney assays. However, the receptor interactions of these peptides are 2-3 orders of magnitude weaker than those of their agonist counterparts. To produce an antagonist with increased receptor-binding affinity but lacking agonist-like properties, structure-function studies were undertaken. Glycine at position 12 (present in all homologues of PTH and in PTHrP), which is predicted in both hormones to participate in a .beta.-turn, was examined by substituting conformational reporters, such as D- or L-Ala, Pro, and .alpha.-aminoisobutyric acid (Aib), in both agonist and antagonist analogues. Except for N-substituted amino acids, which substantially diminished potency, substitutions were well tolerated, indicating that this site can accept a wide latitude of modifications. To augment receptor avidity, hydrophobic residues compatible with helical secondary structure were introduced. Incorporation of the nonnatural amino acids D-Trp, D-.alpha.-naphthylalanine (D-.alpha.-Nal), or D-.beta.-Nal into either [Tyr34]bPTH(7-34)NH2 or [Nle8,18, Tyr34]bPTH(7-34)NH2 resulted in antagonists that were about 10-fold more active than their respective 7-34 parent compound. Similarly, [D-Trp12]PTHrP(7-34)NH2 was 6 times more potent than the unsubstituted peptide but retained partial agonistic properties, although markedly reduced, similar to PTHrP(7-34)NH2. The antagonistic potentiating effect was configurationally specific. This study provides the basis for a rational approach toward the design of more potent antagonists of both PTH and PTHrP based on the introduction of hydrophobic residues (to increase receptor avidity) into sites within the antagonist sequence which have been established to be tolerant of structural manipulation.