Ketoconazole-induced reduction in serum 1,25-dihydroxyvitamin D and total serum calcium in hypercalcemic patients

Administration of the antifungal drug ketoconazole reduces serum 1,25-dihydroxyvitamin D (1,25-D) levels in normal subjects. To determine whether a similar effect occurs in hypercalcemic patients, ketoconazole (200 mg every 8 h for 7 days) was given to nine patients with confirmed primary hyperparathyroidism, three patients with probable primary hyperparathyroidism who were awaiting surgery, and three patients with mild hypercalcemia of uncertain etiology who were being followed. Ketoconazole administration led to a significant reduction in mean serum 1,25-D levels in the hypercalcemic patients [basal, 64 +/- 7 (+/- SEM) pg/mL (154 +/- 17 pmol/L) vs. 36 +/- 5 pg/mL (86 +/- 12 pmol/L) after ketoconazole; P less than 0.001]. Serum total calcium fell slightly but significantly [basal, 11.05 +/- 0.17 mg/dL (2.76 +/- 0.04 mmol/L) vs. 10.77 +/- 0.16 (2.69 +/- 0.04 mmol/L) after ketoconazole; P less than 0.02], but the falls in total serum calcium and serum 1,25-D after ketoconazole treatment were not correlated with one another. Ketoconazole administration did not alter serum ionized calcium, 25-hydroxyvitamin D, phosphate, alkaline phosphatase, or PTH concentrations or urinary cAMP excretion. The responses to ketoconazole were similar in all three patient subgroups. We conclude that short term administration of ketoconazole to hypercalcemic patients causes a substantial fall in serum 1,25-D and a small fall in total serum calcium. These effects render ketoconazole a potentially useful agent for investigation of the importance of 1,25-D in patients with hypercalcemic disorders and for their treatment.