Estrogen receptor negative (ER(-ve)) and p53 mutant breast tumors are highly aggressive and have fewer treatment options. Previously, we showed that molecular Iodine (I(2)) induces apoptosis in hormone responsive MCF-7 breast cancer cells, and non-apoptotic cell death in ER(-ve)-p53 mutant MDA-MB231 cells (Shrivastava, 2006). Here we show that I(2) (3 mu M) treatment enhanced the features of autophagy in MDA-MB231 cells. Since autophagy is a cell survival response to most anti-cancer therapies, we used both in vitro and in vivo systems to determine whether ER(-ve) mammary tumors could be sensitized to I(2)-induced apoptosis by inhibiting autophagy. Autophagy inhibition with chloroquine (CQ) and inhibitors for PI3K (3MA, LY294002) and H+/ATPase (baflomycin) resulted in enhanced cell death in I(2) treated MDA-MB231 cells. Further, CQ (20 mu M) in combination with I(2), showed apoptotic features such as increased sub-G1 fraction (similar to 5-fold), expression of cleaved caspase-9 and -3 compared to I(2) treatment alone. Flowcytometry of I(2) and CQ co-treated cells revealed increase in mitochondrial membrane permeability (p < 0.01) and translocation of cathepsin D activity to cytosol relative to I(2) treatment. For in vivo studies ICRC mice were transplanted subcutaneously with MMTV-induced mammary tumors. A significant reduction in tumor volumes, as measured by MRI, was found in I(2) and CQ co-treated mice relative to I(2) or vehicle treated mice. These data indicate that inhibition of autophagy renders ER(-ve) breast tumor cells more sensitive to I(2) induced apoptosis. Thus, I(2) together with autophagy inhibitor could have a potential tumorostatic role in ER(-ve) aggressive breast tumors that may be evaluated in future studies. (C) 2011 Elsevier Inc. All rights reserved.