PPD can induce apoptosis (programmed cell death) of a variety of cancer cells (e.g., lung cancer, colorectal cancer, breast cancer, prostate cancer, gastric cancer, hepatocarcinoma/liver cancer, pancreatic cancer, melanoma, glioma, lymphoma, leukemia, etc.). In in-vitro studies, PPD can enhance the chemo efficacy by 1) elimination of the multi-drug resistance of cancer cells and 2) synergism with a number of chemo drugs (cytotoxic drugs), including 5-FU, cyclophosphamide, paclitaxel, tamoxifen, gemzar, etc.