CYTO-TOXICITY AND DNA CROSS-LINKING ACTIVITY OF 4-SULFIDOCYCLOPHOSPHAMIDES IN MOUSE LEUKEMIA-CELLS INVITRO

Abstract

Two sulfido derivatives of cyclophosphamide (CP), 4-S-(hexane-6-ol)-sulfidocyclophosphamide and 4-S-(propionic acid)-sulfidocyclophosphamide, were studied for their in vitro cytotoxicity against [mouse leukemia] L1210 cells and for their DNA-damaging effects in these cells. These derivatives spontaneously hydrolyze under physiological conditions to form 4-hydroxycyclophosphamide, the active metabolite of cyclophosphamide. The 2 derivatives were compared with phosphoramide mustard, the presumed alkylated species generated by 4-hydroxycyclophosphamide decomposition, for cytotoxic and DNA cross-linking actions. The 3 compounds yielded colony survival curves that were similar in shape; the sulfido derivatives were 4 or 5 times as potent as was phosphoramide mustard. All 3 compounds produced DNA-protein cross-links and interstrand cross-links as measured by alkaline elution. The time course of cross-link formation and removal for the 3 compounds was similar. The sulfido compounds were 4-5 times as potent as was the phosphoramide mustard in the formation of interstrand cross-links, in agreement with the cytotoxicity findings. The higher potency of the sulfido compounds was not attributable to the generation of acrolein. Sulfido derivatives of CP can act directly (without metabolic activation) on cells, probably through spontaneous stepwise conversion to phosphoramide mustard, the presumed proximal alkylating agent. The cell-killing effect may be mediated by phosphoramide mustard-induced DNA interstrand cross-linking. Sulfidocyclophosphamide CP derivatives appear to be suitable for in vitro studies of the mechanism of action of CP. Sulfidocyclophosphamide CP derivatives may also have therapeutic potential as CP-like drugs that do not require metabolic activation.