Short Cyclic Regimen With Parathyroid Hormone (PTH) Results in Prolonged Anabolic Effect Relative to Continuous Treatment Followed by Discontinuation in Ovariectomized Rats.

Despite the potent effect of intermittent parathyroid hormone (PTH) treatment on promoting new bone formation, bone mineral density (BMD) rapidly decreases upon discontinuation of PTH administration. To uncover the mechanisms behind this adverse phenomenon, we investigated the immediate responses in bone microstructure and bone cell activities to PTH treatment withdrawal and the associated long-term consequences. Unexpectedly, intact female and estrogen-deficient female rats had distinct responses to the discontinuation of PTH treatment. Significant tibial bone loss and bone microarchitecture deterioration occurred in estrogen-deficient rats, with the treatment benefits of PTH completely lost 9 weeks after discontinuation. In contrast, no adverse effect was observed in intact rats, with sustained treatment benefit 9 weeks after discontinuation. Intriguingly, there is an extended anabolic period during the first week of treatment withdrawal in estrogen-deficient rats, during which no significant change occurred in the number of osteoclasts, whereas the number of osteoblasts remained elevated compared with vehicle-treated rats. However, increases in number of osteoclasts and decreases in number of osteoblasts occurred 2 weeks after discontinuation of PTH treatment, leading to significant reduction in bone mass and bone microarchitecture. To leverage the extended anabolic period upon early withdrawal from PTH, a cyclic administration regimen with repeated cycles of on and off PTH treatment was explored. We demonstrated that the cyclic treatment regimen efficiently alleviated the PTH withdrawal-induced bone loss, improved bone mass, bone microarchitecture, and whole-bone mechanical properties, and extended the treatment duration.