SELECTIVE AND SYNERGISTIC ACTIVITY OF L-S,R-BUTHIONINE SULFOXIMINE ONMALIGNANT-MELANOMA IS ACCOMPANIED BY DECREASED EXPRESSION OF GLUTATHIONE-S-TRANSFERASE

L-buthionine-S,R-sulfoximine (ESO) selectively inhibits glutathione (G SH) synthesis. Malignant melanoma may be uniquely dependent on GSH and its linked enzymes, glutathione S-transferase (GST) and GSH-peroxidas e, for metabolism of reactive orthoquinones and peroxides produced dur ing melanin synthesis. We compared the in vitro effects of BSO on mela noma cell lines and fresh melanoma specimens (n = 118) with breast and ovarian cell lines and solid tumors (n = 244). IC50 values (mu M) for BSO on melanoma, breast and ovarian tumor specimens were 1.9, 8.6, an d 29, respectively. The IC50 for melanoma was 25.5 mu M, a level 20-fo ld lower than steady state levels achieved clinically The sensitivity of individual specimens of melanoma correlated with their melanin cont ent (r = 0.63), BSO synergistically enhanced BCNU activity against mel anoma cell lines and human tumors. We followed GSH levels, GST enzyme activity, GST isoenzyme profiles and mRNA levels after BSO, BSO (50 mu M) treatment for 48 hr resulted in a 95% decrease in ZAZ and M14 mela noma cell line GSH levels, and a 60% decrease in GST enzyme activity G ST-mu protein and mRNA levels were significantly reduced in both cell lines. GST-pi expression was unaffected. These data suggest that BSO a ction on melanoma may be related to GSH depletion, diminishing the cap acity to scavenge toxic metabolites produced during melanin synthesis, We report here for the first time that. ESO enhancement of alkylator action may be related in part to down regulation of GST. BSO may be a clinically useful adjunct in the treatment of malignant melanoma.