Mechanisms of cross-resistance between radiation and antineoplastic drugs.

The mechanisms responsible for the cross-resistance between radiation and certain antineoplastic agents have been examined in human ovarian cancer cell lines. Cell lines established from patients at a time when they were resistant to combination chemotherapy regimens, which included cisplatin and an alkylating agent as well as cell lines with resistance induced in vitro to melphalan and cisplatin, all have increased cellular levels of glutathione (GSH) compared with drug-sensitive cell lines from untreated patients. In addition, cell lines with acquired resistance to melphalan and cisplatin, but not to doxorubicin, were cross-resistant to radiation. L-Buthionine sulfoximine (BSO), an irreversible inhibitor of gamma-glutamylcysteine synthetase, lowered GSH levels in all the resistant cell lines studied. Lowering of GSH levels to less than 10% of control values potentiated the in vitro cytotoxicity of melphalan and cisplatin. Furthermore, BSO was also shown to potentiate the cytotoxicity of melphalan in a nude mouse model system of ovarian cancer in which mice die of disseminated intra-abdominal carcinomatosis. The BSO administered in the drinking water decreased GSH levels by 96%. A single melphalan treatment of 5 mg/kg following GSH depletion produced a 72% increase in median survival time compared with treatment with melphalan alone. In addition, depletion of GSH levels in cell lines with acquired resistance to melphalan led to a marked sensitization of these cells to irradiation.(ABSTRACT TRUNCATED AT 250 WORDS)