Chemosensitization: do thiols matter?

It is well known that endogenous sulfhydryls are radioprotective in mammalian cells. Their comparable role in chemotherapeutic drug toxicity has been known for almost as long but less well defined. Thiol depletion as a mechanism responsible for enhanced cytotoxicity of melphalan was assayed by pretreatment of cells in vitro with misonidazole and buthionine sulfoximine (BSO). Hypoxic cell sensitizers, such as MISO, deplete endogenous thiols by metabolic activation under hypoxic conditions to thiol reactive intermediates, whereas BSO specifically inhibits a key enzyme in the synthesis of glutathione. For a given level of thiol reduction, sensitization to melphalan was far greater by preincubation with MISO than it was for BSO. This indicated that thiol reduction itself was not the sole factor involved in chemosensitization by MISO. As evidence that the method of thiol depletion predisposes to the expression of biological damage, it was shown that cells preincubated with MISO were appreciably more vulnerable to oxidative stress than those exposed to BSO. BSO was shown to totally inhibit the repair of damage from a preincubation treatment with MISO, demonstrating that recovery is dependent upon thiol regeneration. Thiol depletion "per se" is a good qualitative but not necessarily a quantitative indicator of chemosensitization--the biological and biochemical function of the thiol depleting agents used influences further drug interactions. The results of the study with these two agents suggest that thiols may play a potentially more critical role in the repair rather than the initiation of drug-induced damage.