Lactam Formation Increases Receptor Binding, Adenylyl Cyclase Stimulation and Bone Growth Stimulation by Human Parathyroid Hormone (hPTH)(1–28)NH 2

Human parathyroid hormone (1–28)NH 2 [hPTH(1–28)NH 2 ] 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 IC 50 of 110 nM for displacing 125 I‐[Nle 8,18 , Tyr 34 ]bovine PTH(1–34)NH 2 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 EC 50 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 Glu 22 and Lys 26 to produce the [Leu 27 ]cyclo(Glu 22 ‐Lys 26 )hPTH(1–28)NH 2 analog dramatically reduced the IC 50 for displacing 125 I‐[Nle 8,18 , Tyr 34 ]bPTH(1–34)NH 2 from hPTHIRs from 110 to 6 nM and dropped the EC 50 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)NH 2 . Thus, hPTH(1–28)NH 2 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 [Leu 27 ]cyclo(Glu 22 ‐Lys 26 )hPTH(1–28)NH 2 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)NH 2 considerably closer to the potencies of the larger PTH peptides and analogs. (J Bone Miner Res 2000;15:964–970)