Parathyroid hormone is more effective than estrogen or bisphosphonates for restoration of lost bone mass in ovariectomized rats.

The study was designed to compare the therapeutic efficacy of estrogen, the bisphosphonate risedronate (NE-58095), and PTH for restoration of lost bone mass in osteopenic, ovariectomized (OVX) rats. In addition, the skeletal effects of these single treatments were compared to those of concurrent treatments with PTH + estrogen or PTH + NE-58095. OVX rats were untreated for the first 4 weeks postovariectomy to allow for the development of moderate tibial osteopenia. These animals were then subjected to the various treatments for periods of 5, 10, and 15 weeks. Their proximal tibiae were processed undecalcified for quantitative bone histomorphometry. Treatment of osteopenic OVX rats with estrogen or NE-58095 alone depressed bone turnover and prevented additional cancellous bone loss from occurring during the treatment period. However, these therapeutic agents failed to restore lost bone in OVX rats to control levels. In contrast, OVX rats treated with PTH alone exhibited a marked stimulation of bone formation which resulted in augmentation of cancellous bone mass to a level 2-fold greater than that of vehicle-treated control rats. Concurrent treatment of OVX rats with PTH + estrogen as well as PTH + NE-58095 also effectively reversed cancellous osteopenia in OVX rats, but did not appear to be more beneficial to the estrogen-deplete skeleton than treatment with PTH alone. The results indicate that PTH is a powerful stimulator of bone formation and completely restores lost cancellous bone in osteopenic OVX rats. Furthermore, the bone anabolic effects of PTH are much more pronounced than those of estrogen or bisphosphonates. These findings in an animal model of estrogen depletion provide support for PTH as a potentially effective treatment for oophorectomized and postmenopausal women with established osteoporosis.