VARIABLE EFFECTS OF TYROSINE KINASE INHIBITORS ON AVIAN OSTEOCLASTIC ACTIVITY AND REDUCTION OF BONE LOSS IN OVARIECTOMIZED RATS

We compared the effects of the tyrosine kinase inhibitor genistein, a naturally occurring isoflavone, to those of tyrphostin A25, tyrphostin A47, and herbimycin on avian osteoclasts in vitro. Inactive analogs d aidzein and tyrphostin Al were used to control for nonspecific effects . None of the tyrosine kinase inhibitors inhibited bone attachment. Ho wever, bone resorption was inhibited by genistein and herbimycin with ID(50)s of 3 mu M and 0.1 mu M, respectively; tyrphostins and daidzein were inactive at concentrations below 30 mu M, where nonspecific effe cts were noted. Genistein and herbimycin thus inhibit osteoclastic act ivity via a mechanism independent of cellular attachment, and at doses approximating those inhibiting tyrosine kinase autophosphorylation in vitro; the tyrphostins were inactive at meaningful doses. Because tyr osine kinase inhibitors vary widely in activity spectrum, effects of g enistein on cellular metabolic processes were compared to herbimycin. Unlike previously reported osteoclast metabolic inhibitors which achie ve a measure of selectivity by concentrating on bone, neither genistei n nor herbimycin bound significantly to bone. Osteoclastic protein syn thesis, measured as incorporation of H-3-leucine, was significantly in hibited at 10 mu M genistein, a concentration greater than that inhibi ting bone degradation, while herbimycin reduced protein synthesis at 1 0 nM. These data suggested that genistein may reduce osteoclastic acti vity at pharmacologically attainable levels, and that toxic potential was lower than that of herbimycin. To test this hypothesis in a mammal ian system, bone mass was measured in 200 g ovariectomized rats treate d with 44 mu mol/day genistein, relative to untreated controls. During 30 d of treatment, weights of treated and control group animals were indistinguishable, indicating no toxicity, but femoral weight in the t reated group was 12% greater than controls (P < 0.05). Our data indica te that the isoflavone inhibitor genistein suppresses osteoclastic act ivity in vitro and in vivo at concentrations consistent with its ID(50 )s on tyrosine kinases, with a low potential for toxicity. (C) 1996 Wi ley-Liss, Inc.