(a) Effect of 6-thioguanine and 6-thioguanosine on tumor weight. Tumor weight was assessed after intramuscular injection of MDA-MB-231 tumor cells on day 0. Treatment with 6-thioguanine or 6-thioguanosine (1.0 mg/kg/day subcutaneously) was begun on the following day and and treated daily for the duration of the experiment. (b) Effect of 6-thioguanine and 6-thioguanosine on tumor burden in bone. Tumor burden in bone was assessed following intracardiac injection of MDA-MB-231 F9AD/Luc tumor cells on day 0. Treatment with 6-thioguanine or 6-thioguanosine (1.0 mg/kg/day subcutaneously) was begun on the following day and treated daily for the duration of the experiment. Values represent means ± SEM (n = 5 per group). Ctrl, control; 6-TG, 6-thioguanine; 6-TGu, 6-thioguanosine. *P < 0.05.