Rolipram, a phosphodiesterase 4 inhibitor, prevented cancellous and cortical bone loss by inhibiting endosteal bone resorption and maintaining the elevated periosteal bone formation in adult ovariectomized rats

Cyclic AMP (cAMP) is a continually produced nucleotide inactivated by hydrolysis to 5'AMP via phosphodiesterase (PDE) enzymes. Rolipram is a selective PDE4 inhibitor reported to have anti-inflammatory effects and used in the treatment of asthma and chronic obstructive pulmonary disease (COPD). The current study was designed to determine whether Rolipram could prevent and restore bone loss in ovariectomized (OVX) rats. Six-month-old Sprague Dawley rats underwent either sham-operated or bilateral ovariectomy, and were left untreated for 60 days to develop osteopenia. Then they were treated with vehicle, 6 mg/kg PGE(2), 3 microg/kg Alendronate or 0.1-1.0 mg/kg Rolipram for 60 days. At sacrifice, the right tibiae were processed for quantitative bone histomorphometric measurements. The right femurs were measured by dual energy A-ray absorptiometry and the 5th lumbar vertebrae were subjected to micro-computed tomography to access bone mass and architecture changes. Our results indicated that OVX induced negative bone balance in all five bone sites we tested, with bone resorption exceeding bone formation. Rolipram at 0.1-0.6 mg/kg dose levels prevented while at 1 mg/kg restored ovariectomy-induced cancellous and cortical bone loss in the tibia, femur and lumbar vertebra. Dynamic bone histomorphometry suggested that these beneficial effects were achieved by partially maintaining the elevated bone formation at the trabecular bone surface and increasing bone formation at the periosteal bone surface of the cortex. Furthermore, it reduced bone turnover at the trabecular and the endocortical bone surfaces. The prevention of further bone loss effects were comparable to those of an anti-resorption agent (Alendronate) but were not as great as those of an anabolic agent (PGE(2)). In addition, Rolipram treatment increased body and muscle weights compared to the vehicle-treated OVX rats. In conclusion, our study in an osteopenic rat model suggested that a selective PDE4 inhibitor may be used for the treatment of established osteoporosis.