1-Methylxanthine- 13 C,d 3 is the 13 C- and deuterium labeled 1-Methylxanthine. 1-Methylxanthine, a caffeine derivative, is an essential human urinary metabolite of caffeine and theophylline (1,3-dimethylxanthine, TP). 1-Methylxanthine enhances the radiosensitivity of tumor cells.