We have studied the effect of buthionine sulfoximine (BSO; a γ-glutamylcysteine synthetase inhibitor) administration, either alone or combined with misonidazole (MISO), on five human tumor xenografts (three melanomas: Bell, Mall, and Nall; and two rectocolic adenocarcinomas: HT29 and HRT18) transplanted into mice. Two criteria were used, the nonprotein bound sulfhydryl (NPSH) level (glutathione (GSH) and cysteine (CYS)) and the fraction of surviving tumor cells after γ irradiation. GSH and CYS were estimated by HPLC and cell survival by in vivo-in vitro clonogenic assay. Administration of BSO alone (three injections of 10 μmol/g) prior to irradiation always produced a significant reduction in the GSH level while MISO administration (1 mg/g) did not consistently influence the NPSH level. While BSO had little or no radiosensitizing effect, MISO always induced radiosensitization (enhancement ratio between 1.6 and 1.8). This effect did not depend on the fraction of surviving hypoxic cells. An increase in MISO-induced radiosensitization produced by BSO was cell-line dependent. Results do not seem to support the hypothesis of a relationship between the GSH level at the time of irradiation and the radiosensitization induced by BSO or BSO + MISO. However, BSO treatment may not have been able to reduce endogenous thiols to a low enough level to test the hypothesis.

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