but differ significantly from those for theobromine and theophylline, the other major caffeine-derived methylxanthine metabolites in humans (Table 1). Uses Paraxanthine is a phosphodiesterase type 9 (PDE9) inhibitor and it is sold as a research molecule for this same purpose. Toxicity Paraxanthine is believed to exhibit a lower toxicity than caffeine and the caffeine metabolite, theophylline. In a mouse model, intraperitoneal paraxanthine doses of 175 mg/kg/day did not result in animal death or overt signs of stress; by comparison, the intraperitoneal LD50 for caffeine in mice is reported at 168 mg/kg. In in vitro cell culture studies, paraxanthine is reported to be less harmful than caffeine and the least harmful of the caffeine-derived metabolites in terms of hepatocyte toxicity. As with other methylxanthines, paraxanthine is reported to be teratogenic when administered in high doses; but it is a less potent teratogen as compared to caffeine and theophylline. A mouse study on the potentiating effects of methylxanthines coadministered with mitoycin C on teratogenicity reported the incidence of birth defects for caffeine, theophylline, and paraxanthine to be 94.2%, 80.0%, and 16.9%, respectively; additionally, average birth weight decreased significantly in mice exposed to caffeine or theophylline when coadministered with mitomycin C, but not for paraxanthine coadministered with mitomycin C. Paraxanthine was reported to be significantly less clastogenic compared to caffeine or theophylline in an in vitro study using human lymphocytes. References External links Adenosine receptor antagonists Animal metabolites Phosphodiesterase inhibitors Stimulants Xanthines