A COMPARISON OF THE RELATIVE CHEMOSENSITIVITY OF HUMAN GLIOMAS TO TAMOXIFEN AND N-DESMETHYLTAMOXIFEN IN-VITRO

Tamoxifen has been shown to inhibit the proliferation of human gliomas in vitro. This inhibition is independent of tamoxifen's known anti-es trogenic properties. Tamoxifen is an inhibitor of protein kinase C (PK C), a calcium- and phospholipid-dependent serine kinase which plays a critical role in the proliferation of certain cell lines. Gliomas over express PKC, and their growth rate is coupled to the level of this key enzyme. As such, the effect of tamoxifen may be mediated by its inhib itory effect on PKC. To further investigate this possibility, we compa red the chemosensitivity of cultured glioma lines to both tamoxifen an d N-desmethyltamoxifen (DMT). DMT is the major metabolite of tamoxifen in humans and is a ten-fold more potent inhibitor of PKC. Seven lines were tested using the standard MTT assay, which quantitates metabolic ally active cells colorimetrically using a tetrazolium dye. Four of th e seven lines were also tested using a tritiated thymidine uptake assa y. In the MTT assay, all seven lines showed significantly greater sens itivity to DMT, while three of the four lines tested in the thymidine uptake assay were more sensitive to DMT Correlation between the two as says was good. The dose of tamoxifen required to produce a 50% inhibit ion of optical absorbance or thymidine uptake (ID50) was typically fiv e- to ten-fold greater than the ID50 for DMT approximating the relativ e strength of the two compounds as PKC inhibitors. In addition to prov iding some support for the ypothesis that triphenylethylenes inhibit g liomas via PKC inhibition, these findings have clinical significance. Levels of DMT in serum, brain and brain metastases are about twice the comparable levels of tamoxifen during chronic therapy. DMT achieved c omplete inhibition of all seven glioma lines at concentrations of 200 ng/ml. Levels as high as 2000 ng/g can be achieved in tumor tissue, su ggesting that chronic tamoxifen therapy may have some role in the ther apy of these aggressive tumors.