Dimeric ^R25C PTH(1–34) Activates the Parathyroid Hormone-1 Receptor in vitro and Stimulates Bone Formation in Osteoporotic Female Mice

Osteoporosis, characterized by reduced bone density and strength, increases fracture risk, pain, and limits mobility. Established therapies of Parathyroid hormone (PTH) analogs effectively promote bone formation and reduce fractures in severe osteoporosis, their use is limited by potential adverse effects. In the pursuit of safer osteoporosis treatments, we investigated ^R25C PTH, a PTH variant wherein the native arginine at position 25 is substituted by cysteine. These studies were prompted by our finding of high bone mineral density in a hypoparathyroidism patient with the R25C homozygous mutation, we explored its effects on PTH type-1 receptor (PTH1R) signaling in cells and bone metabolism in mice. Our findings indicate that ^R25C PTH(1–84) forms dimers both intracellularly and extracellularly, and the synthetic dimeric peptide, ^R25C PTH(1–34), exhibiting altered activity in PTH1R-mediated cAMP response. Upon a single injection in mice, dimeric ^R25C PTH(1–34) induced acute calcemic and phosphaturic responses comparable to PTH(1–34). Furthermore, repeated daily injections increased calvarial bone thickness in intact mice and improved trabecular and cortical bone parameters in ovariectomized (OVX) mice, akin to PTH(1–34). The overall results reveal a surprising capacity of a dimeric PTH peptide ligand to activate the PTH1R in vitro and in vivo , suggesting a potential new path of therapeutic PTH analog development.