IN-VIVO THERAPEUTIC POTENTIAL OF COMBINATION THIOL DEPLETION AND ALKYLATING CHEMOTHERAPY

The effect of administering the thiol modulating agent buthionine sulf oximine (BSO) in conjunction with alkylating chemotherapy was investig ated in vivo in the mouse KHT sarcomas and bone marrow stem cells. Tum our response to treatment was assessed by an in vivo to in vitro excis ion assay and bone marrow survival was determined in vitro by CFU-G(M) . Glutathione (GSH) depletion and recovery kinetics were determined at various times after treatment using high performance liquid chromatog raphy (HPLC) techniques. Following a single 2.5 mmol kg-1 dose of BSO, tumour GSH reached a nadir of approximately 40% of control 12-16 h af ter treatment. Bone marrow GSH was depeleted to approximately 45% of c ontrol 4 - 8 h after treatment but recovered to normal by 16 h. When a range of doses of CCNU, mitomycin C, cyclophosphamide or melphalan (M EL) were given 16 h after mice were exposed to a 2.5 mmol kg-1 dose of BSO, only the antitumour efficacy of MEL was effectively enhanced (by a factor of approximately 1.4). This BSO-MEL combination appeared to be selective for the tumour as the bone marrow toxicity was not increa sed beyond that seen for MEL alone. Since increasing the administered dose of BSO neither increased the extent of thiol depletion in the tum our nor enhanced the antitumour efficacy of MEL, three other protocols for delivering the thiol depletor were explored. BSO was given either as multiple 2.5 mmol kg-1 doses administered at 6 or 16 h intervals o r continuously at a concentration of 30 mm supplied in the animals' dr inking water. Both multi-dose BSO pretreatments were found to increase both the antitumour efficacy and normal tissue toxicity of MEL such t hat no advantage compared to the single dose combination was achieved. In contrast, maintaining the thiol depletor in the drinking water led to an approximately 1.7-fold increase in the antitumour efficacy of M EL without any corresponding increase in bone marrow stem cell toxicit y. For the various pretreatment strategies it was possible, in all cas es, to account for the presence or absence of a net therapeutic benefi t on the basis of the tumour and bone marrow GSH depletion and recover y kinetics.