Xanthine crystals induced by topiroxostat, a xanthine oxidoreductase inhibitor, in rats, cause transitional cell tumors.

The present study was performed to elucidate the underlying mechanism of transitional cell tumors found in the carcinogenicity testing of topiroxostat, a xanthine oxidoreductase inhibitor, in which topiroxostat was orally given to F344 rats at 0.3, 1, and 3 mg/kg for 2 years. In the urinary bladder, transitional cell papillomas and/or carcinomas were seen in males receiving 0.3, 1, and 3 mg/kg (1/49, 3/49, and 10/50, respectively). In the kidney, transitional cell papillomas and/or carcinomas in the pelvis were seen in 2/50 males and 1/50 females receiving 3 mg/kg. In the mechanistic study by 52-week oral treatment with topiroxostat at 3 mg/kg to F344 male rats, with and without citrate, simple and papillary transitional cell hyperplasias of the urinary bladder epithelium were observed in 5/17 in the topiroxostat-alone treatment group, along with xanthine-induced nephropathy, in contrast to neither xanthine crystals nor lesions in urinary organs by co-treatment group with citrate. As for sex differences of urinary bladder tumors, the BrdU labeling index for epithelial cells of the urinary bladder by 5-week oral treatment with topiroxostat at 10 mg/kg to F344 rats was increased in males only, showing consistency with histopathological findings. Therefore, the present study indicates that transitional cell tumors induced by topiroxostat in rats were due to physical stimulation to transitional cells of xanthine crystals/calculi and provides that other factors were not implicated in this tumorigenesis. Furthermore, the present study suggests that such tumors do not predict for humans since topiroxostat-induced xanthine deposition is a rodent-specific event.

Establishment of simultaneous treatment model with citrate for preventing nephropathy induced by FYX-051, a xanthine oxidoreductase inhibitor, in rats.

As a precedent study for elucidating the mechanism of possible urinary bladder carcinogenesis due to xanthine crystals induced by FYX-051, a xanthine oxidoreductase inhibitor, we have determined the experimental conditions suitable for the 52-week simultaneous treatment with citrate in F344 rats. Simultaneous treatment with citrate and FYX-051 produced both increased urinary citrate excretion and suppression of urinary xanthine deposition at around 4 hours after a single dosing, but these effects disappeared 2 hours later, indicating a lack of the durability of citrate effects. Next, we carried out a 7-day simultaneous treatment study by two daily treatments, that is, FYX-051 (6 mg/kg) and citrate (2,000 mg/kg), followed by citrate-alone treatment, under the conditions of selected dosing intervals, the second dose of citrate, and dosing volume. As a result, the dosing interval of citrate was found to be optimal at 4 hours, but not at 3 or 5 hours, because this treatment completely inhibited intrarenal xanthine deposition. The dose of citrate for the second treatment and the dosing volume were found to be sufficient at 1,500 mg/kg and 10 mL/kg, respectively. Subsequently, a 4-week study by simultaneous treatment at 3 mg/kg of FYX-051 and citrate (2,000 mg/kg) + citrate (1,500 mg/kg), under the improved conditions, revealed that renal lesions could be drastically inhibited. Thus, the present study demonstrated that the interval of two citrate treatments is pivotal and indicated that the improved model would be useful for the mechanistic study of FYX-051-induced urinary bladder carcinogenesis because of an easier treatment method than our previous model.

Strain differences in the responsiveness between Sprague-Dawley and Fischer rats to nephropathy induced by FYX-051, a xanthine oxidoreductase inhibitor.

To determine a rat strain appropriate for carcinogenicity testing of FYX-051, a xanthine oxidoreductase inhibitor, we performed a 4-week oral toxicity study by administering 0.3, 1 and 3 mg/kg, and 1, 3 and 10 mg/kg of FYX-051 to male Sprague-Dawley (SD) and Fischer (F344) rats, respectively. Histopathology revealed that the degree of FYX-051-induced nephropathy was 3-fold stronger in SD rats than in F344 rats. Our previous study demonstrated that the key factor of species differences in FYX-051-induced nephropathy is purine metabolism. This observation led us to examine the involvement of purine metabolism in differences among two strains of rats. However, purine metabolism was proven not to be implicated as an important factor. Subsequently, other factors responsible for the strain differences were examined. FYX-051-induced increases in plasma xanthine concentrations were higher in SD rats than in F344 rats, suggesting more remarkable effects on pharmacodynamics in the former than the latter. Urinary volume was greater in F344 rats administered 10 mg/kg of FYX-051 (6.8 ml/h/kg) than in SD rats administered 3 mg/kg of FYX-051 (5.0 ml/h/kg), implying easier xanthine excretion in the former. Urinary xanthine solubility was 55 mg/dl in F344 rats aged 6 weeks, in contrast to 38 mg/dl in SD rats of the same age. Also, there were no significant differences in exposure levels at the same dose between SD and F344 rats. The outcomes of exposure levels and renal histopathology in both rats suggest the possibility that F344 rats could be exposed to a 3-fold higher amount of drug than SD rats in a carcinogenicity bioassay. The present study, therefore, suggested that strain differences of nephrotoxicity were caused by the combined effects of pharmacodynamics, xanthine excretion capacity, and urinary xanthine solubility. Furthermore, these results indicate that F344 rats would be a suitable strain for the carcinogenicity study of FYX-051.