We report here on an MDR modulator that is a small molecule inhibitor of P-glycoprotein, but is not a pump substrate for P-gp and we show for the first time that extended exposure of an MDR prostate cancer cell line to the inhibitor following treatment with chemotherapeutics and inhibitor resulted in trapping of the chemotherapeutics within the cancerous cells. This evidence concerns the gene ABCB1 and Familial prostate cancer.