(a) Effect of 6-thioguanine and 6-thioguanosine on survival. MDA-MB-231 tumor cells were inoculated on day 0. Treatment with 6-thioguanine or 6-thioguanosine (1.0 mg/kg/day subcutaneously) was begun on the following day. (b) Effect of 6-thioguanine and 6-thioguanosine on survival. MDA-MB-231 tumor cells were inoculated on day 0. Treatment with 6-thioguanine or 6-thioguanosine (1.0 mg/kg/day subcutaneously) was begun on day 15. For both panels, the log-rank test (Wilcoxon survival) was used to analyze survival data. *P < 0.05 was considered significant (n = 5 per group).