THIOL AND DISULFIDE METABOLITES OF THE RADIATION PROTECTOR AND POTENTIAL CHEMOPREVENTIVE AGENT WR-2721 ARE LINKED TO BOTH ITS ANTI-CYTOTOXIC AND ANTIMUTAGENIC MECHANISMS OF ACTION

The ability of the potential chemopreventive agent S-2-(3-aminopropyla mino)ethylphosphorothioic acid (WR-2721) to protect against radiation- induced mutagenesis at the hprt locus and cell killing was studied usi ng CHO-AAS cells incubated for 30 mim at 37 degrees C in growth medium containing its active thiol 2-[(aminopropyl)amino]ethanethiol (WR-106 5). In parallel experiments, the thiol and disulfide forms of the drug present in cells and incubation medium were determined in order to id entify which, if either, of the components were associated with the ob served protective effects, Treatment with 4 mM WR-1065 produced signif icant intracellular levels of the thiol (WRSH) and disulfide (WRSS) fo rms of the drug, but also caused dramatic elevation of cellular glutat hione (GSH) and cysteine-levels, accompanied by marked protection agai nst Co-60 gamma-photon- and neutron-induced cell killing and mutagenes is, When drug-treated cells were transferred to drug-free medium and i ncubated for 4 h at 37 degrees C, levels of WRSH and WRSS and protecti on against cell killing decreased markedly, whereas levels of GSH and cysteine and protection against mutagenesis showed little change, GSH and cysteine levels were not associated with protection against radiat ion-induced mutagenesis, as established by experiments performed with buthionine sulfoximine to block GSH synthesis, These data do not suppo rt the hypothesis that modulation of GSH or cysteine levels by WR-1065 is a major mechanism accounting for protection, Protection against mu tagenesis was seen for cells incubated in medium with concentrations o f added WR-1065 as low as 10 mu M, where cellular levels of WRSH and W RSS became difficult to measure (less than or equal to 5 mu M) and no protection against cell killing was found, An unexpected observation w as that cells incubated in 40 mu M WR-1065 incorporated the drug much more rapidly than expected for uptake by passive diffusion and concent rated the drug to a marked degree; this indicates that a cell-mediated transport system is involved in the uptake of WR-1065 at low drug con centrations.