Reduction of Dimesna to Mesna by the Isolated Perfused Rat Liver1

Mesna is administered with ifosfamide and cyclophosphamide to re duce the incidence of hemorrhagic cystitis. In the present model of mesna metabolism and disposition, mesna is rapidly and irreversibly oxidized to dimesna in the plasma, passes unchanged through the liver, and is then reduced by the kidney and excreted. Our detection of a high ratio of mesna to dimesna in the plasma of clinical samples led us to reinvestigate the hepatic metabolism of mesna and dimesna. We perfused isolated rat livers from female Sprague Dawley rats with protein-free buffered solu tion containing dimesna at concentrations observed during therapy. In single-pass perfusions, each liver was perfused with up to three dimesna concentrations during consecutive 20-min periods. Recirculating perfu sions were used to study single supratherapeutic concentrations of dimesna or mesna. Mesna and dimesna concentrations were measured by specific Chromatographie procedures. Dimesna reduction, adjusted by the effluent flow rate and liver weight (0.4-58.5 nmol/min/g liver), correlated closely by linear regression (r = 0.98; n = 36) to the perfused dimesna concentration (4.2-249 /UM),indicating a clearance of 0.20 ml/min/g liver. The concentration of dimesna that entered the liver closely matched the summed concentration of mesna and dimesna emerging in the effluent perfusate (single-pass experiments: slope, 0.98; intercept, -0.30; r = 1.00; n = 31 ). Only trace amounts of unidentified thiols were detected in the bile during recirculation of perfusates with 1 HIMmesna or 250 /i\i dimesna. The effluent mesna concentration correlated inversely with the flow rate, which was consistent with a low extraction ratio in the perfusion model. These data suggested that the dimesna reduction rate was limited by hepatic uptake. Dimesna reduction was decreased by agents that deplete glutalhione. Pretreatment of rats with up to 100 mg/kg ifosfamide did not impair hepatic dimesna reduction. In control experiments, dimesna was not reduced during recirculation through the apparatus without a liver. Mesna was oxidized to dimesna during oxygénationof the perfusate in the reservoir, but mesna injected directly into the perfusate just before entry into the liver passed unchanged into the effluent. Extrapolation of the dimesna clearance data from the perfusion model to humans suggests that hepatic dimesna reduction may counterbalance the rapid oxidation of mesna in plasma. The proposed equilibrium is consistent with clinical observations and suggests a new model for mesna metabolism and dispo sition. to mesna, and excreted into the renal tubular lumen (4, 5). Mesna passes into the urine where it protects the surface of the bladder by neutralizing the toxic ifosfamide metabolite acrolein (8, 9). Because of its limited metabolism, mesna protects the urinary tract, but does not impair anticancer therapy (2, 3). In a study to determine the bioavailability of mesna in healthy human subjects, we detected more mesna than dimesna in nearly all plasma samples at all times after both i.v. and oral doses (10). This mesna is unlikely to be the remnant of unchanged drug because of its initial rapid clearance (7, 10-14). One suggestion to explain the persistence of mesna in the blood has been the slow release of sequestered mesna from erythrocytes (7). An alternative explanation might be the organ-mediated uptake and reduction of dimesna, fol lowed by the reintroduction of mesna into the circulation. Earlier studies in rats treated with radiolabeled mesna and dimesna led authors to discount this possibility (4-7). The radioactivity detected in the individual organs of rats could be accounted for by residual blood within the organ, which suggested negligible uptake of the drug. The one exception was the accumulation of radioactivity in rat kidneys, which was consistent with the renal tubular reduction of dimesna and the excretion of mesna into the urine. Perfusion of isolated rat kidneys with dimesna showed that dimesna was reduced and excreted into the urine as mesna, but that only traces of mesna were reintroduced into the effluent perfusate (4, 5). Studies of isolated hepatocytes and of the isolated perfused liver led to the conclusion that the liver was totally inactive in the whole-body handling of mesna and dimesna in the rat (4). Our clinical data (10-12) prompted us to reinvestigate the possi bility of organ-mediated regeneration of mesna from dimesna. We studied the liver because it is a net exporter of glutathione (15), and glutathione seems to be involved in the reduction of dimesna (4). We perfused isolated rat livers with dimesna and looked for mesna in the effluent. Some rats were pretreated with agents that modulate hepatic glutathione (16). Our results show glutathione-dependent hepatic re duction of dimesna to mesna that is proportional to dimesna concen trations over a therapeutically relevant range.