EFFECTS OF CARBETIMER, A NEW ANTINEOPLASTIC DRUG, ON BONE METABOLISM

Carbetimer is a new antineoplastic agent whose main side effects consist of neurotoxicity and long-term dose-dependent hypercalcemia. We previously showed that Carbetimer is a potent calcium chelator responsible for an acute decrease in ionized Ca levels observed in vivo. However, the mechanism of the progressive increase in serum Ca remains unknown. We have evaluated the bone-resorbing effects of Carbetimer on Ca-45-prelabelled neonatal mouse calvariae. Carbetimer induced a dose-dependent increase in Ca-45 release which started at a concentration of 1 mg/ml and reached a mean of 3.3 times the control values at 10 mg/ml. This marked increase in Ca-45 release was similar on previously killed bones and could not be inhibited by calcitonin. Such concentrations are probably therapeutically relevant given the known affinity of Carbetimer for bone and the large daily doses administered to cancer patients (10-15 g). Since Carbetimer could exert its antineoplastic action through immunomodulation, we also studied its effects on the production of TNF-alpha and IFN-gamma which are also known to affect bone metabolism. Carbetimer did not stimulate TNF-alpha release from isolated normal human monocytes or lymphocytes, but it markedly inhibited T-lymphocyte production of IFN-gamma, which became undetectable at a concentration of 1 mg of Carbetimer/ml. In summary, Carbetimer-induced hypercalcemia appears to be due to a direct stimulation of osteolysis, but possibly also to an inhibition of IFN-gamma production.