Recently, S-(purin-6-yl)-L-cysteine (GC) was shown to be a kidney-selective prodrug of 6-mercaptopurine. In the present study, for further development of kidney-selective chemotherapeutic agents, GC was synthesized, and its metabolism was examined in the rat by cysteine conjugate beta-lyase (beta-lyase) to yield the antitumor and immunosuppressant drug, 6-thioguanine (6-TG). The apparent Km values obtained with renal mitochondrial and cytosolic beta-lyases were similar, but the Vmax value obtained with renal mitochondrial beta-lyase was approximately 45-fold higher than the Vmax value obtained with renal cytosolic beta-lyase. After rats were administered GC (400 mumol/kg), the concentrations of GC in the kidney, liver and plasma at 30 min were higher than the corresponding values at 15 or 60 min. GC concentrations in plasma and kidney were, however, 3- and 5-fold higher than that in liver, respectively. Although GC metabolites were not detected in plasma, they were detectable in liver and kidney; metabolite concentrations at 30 min were higher than those at 15 or 60 min. Renal 6-TG concentration at 30 min was nearly 4-fold higher than hepatic 6-TG concentration; hepatic and renal 6-thioxanthine and 6-thiouric acid concentrations were similar. The amount of GC metabolites excreted in urine within 24 hr was linearly proportional to the administered GC dose. Rats administered GC (400 mumol/kg) excreted nearly 5-fold the amount of metabolites as rats given an equimolar dose of 6-chloroguanine, a GC precursor. These results and the finding that renal 6-TG concentrations after GC treatments were in excess of the ED50 of 6-TG (0.5-1.0 microM) in two human renal carcinoma cell lines (A-498 and CAKI-1) suggest that GC may have clinical usefulness as a prodrug of 6-TG.