The response to calcitriol therapy in postmenopausal osteoporotic women is a function of initial calcium absorptive status

Abstract

Calcitriol is used in the treatment of osteoporosis but the indications for its use have not been clearly defined. Because it stimulates calcium absorption, we have tended to select osteoporotic patients with low calcium absorption for this therapy and now report the results. We measured the hourly fractional rate of calcium absorption (alpha) with 45Ca and fasting urinary calcium/creatinine (Ca/Cr) and hydroxyproline/creatinine (OHPr/Cr) in 103 postmenopausal women aged 68 (0.67SE) years with vertebral compression fractures (77) or forearm or vertebral bone density below the young normal range (26). They were given 0.25 microg daily of calcitriol (Rocaltrol, Roche, Basle, Switzerland) with a 1 g calcium supplement daily for 6-12 weeks, when the biochemical tests were repeated. Initial OHPr/Cr was inversely related to initial alpha (P = 0.001) and positively to initial Ca/Cr (P < 0.001). alpha rose on therapy from 0.47 (0.018) to 0.59 (0.018) per hour (P < 0. 001) and OHPr/Cr fell in the whole group from 19.1 (0.83) to 13.8 (0. 58) (P < 0.001). The change in alpha on therapy (corrected for the "regression to the mean effect") was inversely related to initial alpha (P < 0.001) as was the change in OHPr/Cr (P = 0.001). There was no relationship, however, between initial Ca/Cr and either the rise in alpha or the fall in OHPr/Cr on therapy. The data support the concept that low calcium absorption is a cause of negative calcium balance in postmenopausal osteoporosis and that the effectiveness of calcitriol therapy is inversely related to the initial rate of calcium absorption.