Effects of raloxifene on bone density,biomarkers, and histomorphometric and biomechanical measures in ovariectomized cynomolgus monkeys

OBJECTIVE: The purpose of this study was to determine the effect of raloxifene on bone density, strength, metabolism, and histomorphometric characteristics in ovariectomized cynomolgus monkeys. DESIGN: A prospective, longitudinal study was designed to examine the effects of conjugated equine estrogens (0.04 mg/kg, CEE) and raloxifene (1 or 5 mg/kg, R1 and R5, respectively) on bone density, biomarkers, histomorphometry, and strength. Control groups included ovariectomized and sham-operated monkeys. Treatment was initiated the day after ovariectomy and continued for 24 months. Bone biomarker data were collected at baseline and every 3 months after surgery. Bone mass was determined at baseline and every 6 months after ovariectomy. Iliac biopsies were collected at baseline and 16 months postovariectomy, and the second lumbar vertebra and left midshaft femur collected at necropsy were examined histomorphometrically. Bone biomechanical properties were determined for the right femur and vertebrae. RESULTS: Compared with the placebo-treated ovariectomized monkeys, the high-dose raloxifene group had lower levels of alkaline phosphatase, tartrate-resistant acid phosphatase, urinary CrossLaps (collagen degradation products), and greater bone mass in the lumbar vertebrae. In the endocortical compartment, the high-dose raloxifene group had significantly lower mineralizing surface, mineral apposition rate, and bone formation rate in the iliac biopsy collected at 16 months and lower bone formation rate in the second lumbar vertebra. Within the midshaft femur, low-dose raloxifene significantly decreased the osteonal and total bone formation rates and also prevented the decrease in Young's modulus induced by ovariectomy in the midshaft femur. CONCLUSIONS: High-dose raloxifene prevented the development of osteopenia in the ovariectomized monkey by reducing bone turnover, albeit to a lesser extent than CEE. Histomorphometric and biomarker data suggest that mechanisms underlying the effect of raloxifene differ somewhat from that of CEE.