Enzymatic and non-enzymatic mechanisms of dimesna metabolism

The chemical reduction of the disulfide homodimer dimesna to its constituent mesna moieties is essential for its mitigation of nephrotoxicity associated with cisplatin and ifosfamide anticancer therapies and enhancement of dialytic clearance of the cardiovascular risk factor homocysteine. The objective of this study was to investigate potential enzymatic and non-enzymatic mechanisms of intracellular dimesna reduction. Similar to endogenous intracellular disulfides, dimesna undergoes thiol-disulfide exchange with thiolate anion-forming sulfhydryl groups via the two-step SN2 reaction. Determination of equilibrium constants of dimesna reduction when mixed with cysteine or glutathione provided a mechanistic explanation for dramatic cysteine and homocysteine depletion, but sparing of the endogenous antioxidant glutathione, previously observed during mesna therapy. Dimesna was reduced by recombinant enzymes of the thioredoxin system; however, oxidation of NADPH by the glutaredoxin system was only observed in the presence of combined dimesna and reduced glutathione, suggesting formation of oxidized glutathione following an initial non-enzymatic reduction of dimesna. Production of mesna by enzymatic and non-enzymatic mechanisms in HeLa cell lysate following dimesna incubation was demonstrated by a loss in mesna production following protein denaturation and prediction of residual non-enzymatic mesna production by mathematical modeling of thiol-disulfide exchange reactions. Reaction modeling also revealed that mixed disulfides make up a significant proportion of intracellular thiols, supporting their role in providing additional nephroprotection, independent of direct platinum conjugation.