Ligand-dependent effects of methionine-8 oxidation in parathyroid hormone peptide analogs

LA-PTH is a long-acting parathyroid hormone (PTH) peptide analog in pre-clinical development for hypoparathyroidism (HP). Like native PTH, LA-PTH contains a methionine at position 8 that is predicted to be critical for function. We assessed the impact of methionine oxidation on the functional properties of LA-PTH and control PTH ligands. Oxidation of PTH(1-34) resulted in marked (~20-fold) reductions in binding affinity on the PTH receptor-1 (PTHR1) in cell membranes, similarly diminished potency for cAMP signaling in osteoblastic cell lines (SaOS-2 and UMR106), and impaired efficacy for raising blood calcium in mice. Surprisingly, oxidation of LA-PTH resulted in little or no change in these functional responses. The signaling potency of oxidized-LA-PTH was, however, reduced ~40-fold compared to LA-PTH in cells expressing a PTHR1 construct that lacks the N-terminal extracellular domain (ECD). Molecular modeling revealed that while Met8 of both LA-PTH and PTH(1-34) is situated within the...