Melanin content and downregulation of glutathione S-transferase contribute to the action of L-buthionine-S-sulfoximine on human melanoma.

L-buthionine-S,R-sulfoximine (L-S,R-BSO) was enriched for the active L-buthionine-S-sulfoximine (L-S-BSO) diastereomer. Comparative analysis was performed to determine if this enriched form possessed an increased capacity to deplete glutathione (GSH), and to inhibit the proliferation of tumor cell lines and fresh human tumor samples. Increased activity was observed for the enriched preparation of L-S-BSO in direct proportion to its increased L-S-diastereomeric percentage. Significant antitumor activity towards melanoma, breast and ovarian carcinoma specimens was noted, with the greatest activity directed against malignant melanoma. The activity of BSO on melanoma specimens was found to be correlated with their melanin content, suggesting that free radicals generated during melanin synthesis may become cytotoxic after GSH-dependent scavenging has been eliminated by BSO treatment. The antimelanoma activity of melphalan and BCNU were found to be significantly enhanced in combination with L-S-BSO. With respect to the mechanism of L-S-BSO synergy with alkylators, L-S-BSO treatment of M14 and ZAZ human melanoma cell lines resulted in decreased GSH levels and glutathione S-transferase (GST) activity. Western and Northern blot analyses indicated that GST-mu was the predominant isozyme downregulated after L-S-BSO treatment. Both M14 and ZAZ cell lines selected for resistance to L-S-BSO also showed decreased levels of GST-mu expression. However, in drug free media GST enzyme activity returned to pre-treatment levels without altering the BSO-resistance status of the cell lines. We conclude that L-S-BSO may be an active agent in the treatment of melanoma, and that it may enhance alkylator activity on melanoma through depletion of GSH and down-regulation of GST expression. Purified L-S-BSO should be explored clinically as an active agent for the treatment of melanoma.