Analogues of an in vitro parathyroid hormone inhibitor: Modifications at the amino terminus

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Abstract
Four analogues of parathyroid hormone were synthesized, based on a sequence previously shown to yield an in vitro inhibitor of PTH action. Binding properties of the analogues to presumed parathyroid hormone receptors were evaluated. Each of these analogues contains a structural alteration of the amino terminus of the compound [Nle-8,Nle-18,Tyr-34]bPTH-(3-34)amide. Each of the analogues—[desamino-Ser-3, Nle-8, Nle-18, Tyr-34]bPTH-(3-34)amide, [acetyl-Glu-4,Nle-8,Nle-18,Tyr-34]bPTH-(3-34)amide, [d-Ser-3,Nle-8,Nle-18,Tyr-34]bPTH-(3-34)amide, and [pyroGlu-4,Nle-8,Nle-18,Tyr-34]bPTH-(3-34)amide—contains modifications selected to diminish or eliminate the weak PTH-like agonist properties detected in the parent compound in vivo. To permit valid comparison of receptor-binding properties, all the analogues were derived from a common solid-phase synthesis. When assayed in the renal adenylate cyclase assay, each of the compounds inhibited completely PTH-stimulated adenylate cyclase activity and was devoid of PTH-like agonist activity. To compare the binding affinity of the analogues for parathyroid hormone binding sites, the peptides were evaluated in a parathyroid hormone renal radioreceptor assay. Each peptide demonstrated a binding affinity comparable with one another and only slightly weaker than that of the unmodified inhibitory analogue, [Nle-8,Nle-18,Tyr-34]bPTH-(3-34)amide. These studies demonstrate that it is possible to alter the amino terminus of parathyroid hormone analogues without causing a dramatic decline in affinity for the parathyroid hormone receptor. Preservation of high receptor-binding affinity in these analogues indicates that synthesis of one or several of these peptides on a scale sufficiently large to permit in vivo evaluation of both agonist and antagonist properties is warranted.