Multiple low-dose treatments with S-NPAA significantly reduced tumorigenic cell viability. A) Non-tumorigenic (RWPE-1) and tumorigenic prostate cells (LNCaP, DU145 and PC-3) were treated with the indicated doses of S-NPAA at 0 hours and given additional doses at 6, 12, 24, and 36 hours. Cell viability was measured as a percent of control at 48 hours using a MTS viability assay. Data points marked with letters that are not the same are significantly different at p < 0.05. B) Non-tumorigenic and tumorigenic prostate cells were treated with a 15 μM dose of S-NPAA at 0 hours and given additional 15 μM doses at 6, 12, 24, and 36 hours. Cell viability was measured at the indicated time point as a percent of control using a MTS viability assay; *indicates a significant difference between tumorigenic cells and RWPE-1 at p < 0.05.