Object: A combined tamoxifen and radiation therapy is being used in clinical trials to treat glioblastoma multiforme (GBM). The rationale behind this therapy is that tamoxifen is a radiosensitizer. However, the evidence for this is weak. The authors, therefore, examined the effect of combined radiation-tamoxifen therapy in three GBM cell lines of human origin.
Methods: The GBM cell lines were exposed to different concentrations (0.3-5 microg/ml) of tamoxifen and subsequently irradiated at varying doses (0.8-5 Gy). Tumor growth inhibition was measured using a proliferation assay. The interaction of tamoxifen and radiation therapies was quantified using the combination index method, which distinguishes whether a combined antitumor effect is synergistic, additive, or antagonistic. At high doses of tamoxifen or radiation there was significant inhibition of tumor cell proliferation. At low doses of either therapeutic agent, there was little effect. In one cell line, synergism occurred at high doses of tamoxifen and radiation. In the other two cell lines, an additive effect was observed. In only one of the three cell lines was there synergy between tamoxifen and radiation at doses that significantly inhibited tumor proliferation.
Conclusions: Because synergy could not be demonstrated in all three cell lines at active dosages, the clinical combination of tamoxifen and radiation therapies may not be of benefit to all patients.