Despite a variety of therapeutic strategies for breast cancers treatment, the survival rate of breast cancer patients has not been improved due to its increasing resistance to radio- and chemo-therapy. Since drug repurposing for breast cancer treatment can lead to significant cost and time savings compared to new drug development, I have investigated the anticancer effects and action mechanisms of Adefovir dipivoxil (ADV), an FDA-approved antiviral nucleoside analogue used in the therapy of human hepatitis B virus, in two human breast cancer cell lines: MCF7 (wild-type p53, estrogen receptor (ER), and progesterone receptor (PR)-positive) and MDA-MB-231 (mutated p53 and triple hormone receptors-negative). ADV exhibited anti-proliferative effects on both breast cancer cell lines, suggesting that ADV may have anticancer effects in a p53- and hormone receptors-independent manner. ADV induced DNA replication stress, leading to stalling of cell cycle progression at S-phase and activation of surrogate DNA damage signaling ATM-CHK2 and ATR-CHK1 pathways in both cell lines. Interestingly, the apoptosis hallmarks, cleavages of caspases and poly(ADP-ribose) polymerase (PARP-1) were observed in both ADV-treated MCF7 and MDA-MB-231 cells, but DNA laddering and Annexin V-positive cell number was detected only in ADV-treated MCF7 cells, suggesting that ADV-mediated cell death pathways in two cell lines might be differential. In addition, reactive oxygen species (ROS) level was highly enhanced in MDA-MB-231 cells but it was marginally increased in MCF7 cells by ADV. The increased ROS-activated mitogen-activated protein kinases (MAPKs), p38, JNK1/2, and ERK1/2 could lead to necrosis, suggesting that ADV promotes apoptotic and necrotic induction in MDA-MB-231 cells. In conclusion, I herein demonstrate the potential of ADV as a novel therapeutic agent for breast cancer treatment including triple-negative breast cancer, which tends to grow and spread quickly but have, has fewer treatment options. Keywords: Adefovir dipivoxil, breast cancer, replication stress, reactive oxygen species, apoptosis
