Lactam Formation Increases Receptor Binding, Adenylyl Cyclase Stimulation and Bone Growth Stimulation by Human Parathyroid Hormone (hPTH)(1–28)NH2

Human parathyroid hormone (1–28)NH2 [hPTH(1–28)NH2] is the smallest of the PTH fragments that can fully stimulate adenylyl cyclase in ROS 17/2 rat osteoblast‐like osteosarcoma cells. This fragment has an IC50 of 110 nM for displacing 125I‐[Nle8,18, Tyr34]bovine PTH(1–34)NH2 from HKRK B7 porcine kidney cells, which stably express 950,000 human type 1 PTH/PTH‐related protein (PTHrP) receptors (PTH1Rs) per cell. It also has an EC50 of 23.9 nM for stimulating adenylyl cyclase in ROS 17/2 cells. Increasing the amphiphilicity of the α‐helix in the residue 17–28 region by replacing Lys27 with Leu and stabilizing the helix by forming a lactam between Glu22 and Lys26 to produce the [Leu27]cyclo(Glu22‐Lys26)hPTH(1–28)NH2 analog dramatically reduced the IC50 for displacing 125I‐[Nle8,18, Tyr34]bPTH(1–34)NH2 from hPTHIRs from 110 to 6 nM and dropped the EC50 for adenylyl cyclase stimulation in ROS 17/2 cells from 23.9 to 9.6 nM. These modifications also increased the osteogenic potency of hPTH(1–28)NH2. Thus, hPTH(1–28)NH2 did not significantly stimulate either femoral or vertebral trabecular bone growth in rats when injected daily at a dose of 5 nmol/100 g body weight for 6 weeks, beginning 2 weeks after ovariectomy (OVX), but it strongly stimulated the growth of trabeculae in the cancellous bone of the distal femurs and L5 vertebrae when injected at 25 nmol/100 g body weight. By contrast [Leu27]cyclo(Glu22‐Lys26)hPTH(1–28)NH2 significantly stimulated trabecular bone growth when injected at 5 nmol/100 g of body weight. Thus, these modifications have brought the bone anabolic potency of hPTH(1–28)NH2 considerably closer to the potencies of the larger PTH peptides and analogs. (J Bone Miner Res 2000;15:964–970)