Short-term effects of Carbetimer on calcium and bone metabolism in man.

Carbetimer is a new antineoplastic agent whose limiting toxicity consists of dose- and treatment duration-dependent hypercalcemia. We examined the short-term effects of Carbetimer on calcium metabolism on days, 1, 3 and 5 during 11 5-day courses (6.5-8.2 g/m2/day given over daily 2-h infusions, q 3-4 weeks). Blood parameters were measured before and after Carbetimer, whereas urinary parameters were studied in three consecutive 2-h collections before, during and after Carbetimer infusions. Carbetimer effects were similar regardless of the infusion day. We found a consistent decrease of plasma ionized Ca (Ca2+) levels from 4.56 +/- 0.05 mg/dl before infusion to 4.28 +/- 0.06 mg/dl after infusion (P less than 0.001) whereas total serum Ca (corrected for protein levels) did not change. The fall of Ca2+ stimulated parathyroid function, as suggested by the increased plasma PTH levels, the decreased serum phosphorus and TmP/GFR index, or the increased urinary phosphate and cyclic AMP excretion. Carbetimer infusions also induced a marked increase in urinary Ca excretion (expressed as mg Ca/mg creatinine) from 0.093 +/- 0.011 before to 0.359 +/- 0.042 during and 0.177 +/- 0.031 after infusion (P less than 0.011). These changes were best explained by Carbetimer-induced Ca chelation that we confirmed in vitro by incubating Carbetimer at various concentrations in whole blood for 2 h at 37 degrees C, e.g. 2 mg of Carbetimer/ml lowered Ca2+ from 4.82 to 3.20 mg/dl without changing total Ca levels. On the other hand, a direct effect of Carbetimer on bone cannot be excluded since we observed an increase of serum osteocalcin levels from 2.0 +/- 0.3 to 2.5 +/- 0.4 ng/ml after infusion (P less than 0.001). In summary, the short-term effects of Carbetimer on calcium metabolism markedly differ from the long-term effects. They mainly consist of a dose-related calcium chelation leading to a decrease in Ca2+ levels, an increase in urinary Ca excretion and a stimulation of parathyroid function.