Chemosensitization of L-phenylalanine mustard by the thiol-modulating agent buthionine sulfoximine

Glutathione (GSH) plays a crucial role in the protection of normal and tumor tissue against the toxic effects of numerous chemotherapeutic drugs. Therefore, the possible therapeutic benefit of thiol depletion in cancer treatment is dependent upon the relative degree to which tumor or normal tissue is sensitized to the toxic effects of subsequent chemotherapy. To address this issue, the following studies on the chemosensitization of melphalan (L-PAM) by the thiol-depleting agent buthionine sulfoximine (BSO) were conducted in vivo in BDF mice inoculated with L-PAM-resistant murine L1210 leukemia. Different dosing regimens of BSO were found to potentiate L-PAM toxicity in a manner that depended upon the degree of GSH depletion. Multiple i.p. injections of BSO (450 mg/kg every 6 h X 5) were found to reduce GSH concentrations in most tissues by 70-80%, and to decrease the LD50 for L-PAM from 22 to 14 mg/kg. No two organs were found to behave entirely the same with respect to the rate of depletion or recovery of GSH, or to the maximum depletion that could be obtained by BSO. In this regard, the bone marrow was found to be the most resistant tissue to thiol depletion by BSO and was found to tolerate the combination of BSO and therapeutic doses of L-PAM. However, BSO pretreatment markedly inhibited the recovery of the peripheral WBC population at the LD10 dose of L-PAM. Differences also were found in the in vivo metabolism of GSH by L-PAM-sensitive and -resistant murine L1210 leukemia cells. The intracellular concentration of GSH in the resistant cell line was 1.6-fold higher than in the sensitive tumor. Moreover, GSH levels were depleted more rapidly in the resistant tumor relative to the sensitive cell line. A single injection of BSO decreased GSH concentrations in both tumors to equivalent levels (20 nmol/10(7) cells) within 24 h. However, multiple i.p. injections of BSO failed to produce a significant increase in the life-span of L-PAM-treated animals despite a 90% reduction in tumor GSH concentrations (5.5 nmol/10(7) cells). In contrast to the median day survival data, BSO was found to enhance the antitumor activity of L-PAM as determined by an in vivo/in vitro clonogenic assay or by in vivo thymidine incorporation. Using decreased thymidine incorporation as an index of antitumor activity, BSO was found to increase the therapeutic index (LD10/ED50) of L-PAM from 3.6 to 6.5.(ABSTRACT TRUNCATED AT 400 WORDS)