Clinical usefulness of bone alkaline phosphatase in osteoporosis

We have evaluated two commercial assays for serum bone specific alkaline phosphatase (bALP) as a marker of bone turnover in a group of 35 postmenopausal women with osteoporosis during their first year of treatment with the anti-resorptive drug, alendronate. The immunoradiometric assay (bALP-I) measured protein mass, whereas the immunocapture assay (bALP-E) measured activity. Before treatment, bALP values with both methods were within the postmenopausal reference ranges. Treatment with alendronate produced a decrease in bALP after 3 months, reaching a plateau after 6 months: for bALP-I a mean change of -34%, (P < 0.0001), bALP-E, -19% (P < 0.001), and total ALP, -19% (P < 0.0001). The decrease was significant in 53% (bALP-I) and 31% (bALP-E) of patients. The ratio of serum bALP-E/bALP-I in patients was not constant but rose after therapy with alendronate. Neither baseline bALP, nor the per cent change in bALP after 6 months, correlated with bone mineral density (BMD) change after 1 year at either lumbar spine or femoral neck. We conclude that bALP-I appeared to be a more sensitive marker of the suppression of bone turnover by alendronate than did bALP-E, but that neither was useful in the detection of osteoporosis, nor the prediction of individual BMD response to alendronate therapy.