Effects on bone of oral hormone replacement therapy initiated 2 years after ovariectomy in young adult monkeys

The purpose of this study was to determine the effects of oral estrogen replacement therapy with conjugated equine estrogens (CEE), alone or in combination with continuous medroxyprogesterone acetate (MPA), on lumbar spine bone mineral content (BMC) and density (BMD) and on serum chemistries in ovariectomized cynomolgus monkeys when therapy is initiated following a 2 year hypoestrogenic period. Study design was done in the form of a randomized, placebo-controlled, nonhuman primate paraclinical trial. Monkeys were subjects in an experiment designed to study the effects of a lipid-lowering diet combined with hormone replacement therapy on atherosclerosis. Initially, they were ovariectomized and fed a high-fat diet for 24 months. They were then were allocated to three treatment groups by stratified randomization and were fed a diet containing reduced dietary fat for an additional 28 months. Treatment groups consisted of: (1) an untreated group (ovx, n = 24); (2) a CEE-treated group (CEE, n = 19); and (3) a CEE plus continuous MPA group (CEE + MPA, n = 20). Lumbar spine BMC and BMD values were measured by dual-energy x-ray absorptiometry at baseline and 4, 10, 16, 22, and 28 months of treatment. Serum chemistries were relevant to bone metabolism at 22 and 28 months. Rates of gain in BMC and BMD were greater (p< 0.05) in hormone-supplemented animals (groups 2 and 3) than in untreated ovx animals during the first 16 months of treatment, resulting in increased BMC and BMD measurements in these groups. Serum markers of bone metabolism were significantly lower (p< 0.05) in the hormone-treated groups (groups 2 and 3) compared with ovx animals after 22 and 28 months of treatment, indicating reductions in bone turnover rate. Oral estrogen replacement with CEE at doses similar to those taken by women leads to significantly increased BMC and BMD in monkeys, even when therapy is begun 2 years after ovariectomy. Most of the increase occurred during the first 16 months of treatment. The addition of MPA to the CEE regimen provided no additional benefit.