MELANIN CONTENT AND DOWN-REGULATION 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 increas ed capacity to deplete glutathione (GSH), and to inhibit the prolifera tion of tumor cell lines and fresh human tumor samples. Increased acti vity was observed for the enriched preparation of L-S-BSO in direct pr oportion to its increased L-S-diastereomeric percentage. Significant a ntitumor activity towards melanoma, breast and ovarian carcinoma speci mens was noted, with the greatest activity directed against malignant melanoma. The activity of BSO on melanoma specimens was found to be co rrelated with their melanin content, suggesting that free radicals gen erated during melanin synthesis may become cytotoxic after GSH-depende nt scavenging has been eliminated by BSO treatment. The antimelanoma a ctivity 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 melano ma cell lines resulted in decreased GSH levels and glutathione S-trans ferase (GST) activity. Western and Northern blot analyses indicated th at GST-mu was the predominant isozyme downregulated after L-S-BSO trea tment. Both M14 and ZAZ cell lines selected for resistance to L-S-BSO also showed decreased levels of GST-mu expression. However, in drug fr ee 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 GS H and down-regulation of GST expression. Purified L-S-BSO should be ex plored clinically as an active agent for the treatment of melanoma. (C ) 1998 Elsevier Science Ireland Ltd. All rights reserved.