Breast cancer progression depends upon the elaboration of a vasculature sufficient for the nourishment of the developing
tumor.
Breast tumor cells frequently contain a mutant form of p53 (mtp53), a
protein which promotes their survival. The aim of this study was to determine whether combination
therapy targeting mtp53 and anionic
phospholipids (AP) on
tumor blood vessels might be an effective therapeutic strategy for suppressing advanced
breast cancer. We examined the
therapeutic effects, singly, or in combination, of p53 reactivation and induction of massive apoptosis (PRIMA-1), which reactivates mtp53 and induces
tumor cell apoptosis, and 2aG4, a
monoclonal antibody that disrupts
tumor vasculature by targeting AP on the surface of
tumor endothelial cells and causes antibody-dependent destruction of
tumor blood vessels, leading to
ischemia and
tumor cell death. Xenografts from two tumor cell lines containing mtp53, BT-474 and HCC-1428, were grown in nude mice to provide models of advanced
breast tumors.
After treatment with
PRIMA-1 and/or 2aG4, regressing
tumors were analyzed for
vascular endothelial growth factor (
VEGF) expression, blood vessel loss, and apoptotic markers. Individual
drug treatment led to partial suppression of
breast cancer progression. In contrast, combined treatment with
PRIMA-1 and 2aG4 was extremely effective in suppressing
tumor growth in both models and completely eradicated approximately 30% of
tumors in the BT-474 model. Importantly, no toxic effects were observed in any treatment group. Mechanistic studies determined that
PRIMA-1 reactivated mtp53 and also exposed AP on the surface of
tumor cells as determined by enhanced 2aG4 binding. Combination treatment led to significant induction of
tumor cell apoptosis, loss of
VEGF expression, as well as destruction of
tumor blood vessels. Furthermore, combination treatment severely disrupted
tumor blood vessel perfusion in both
tumor models. The observed in vitro PRIMA-1-induced exposure of
tumor epithelial cell AP might provide a target for 2aG4 and contribute to the increased effectiveness of such combination
therapy in vivo. We conclude that the combined targeting of mtp53 and the
tumor vasculature is a novel effective strategy for combating advanced
breast tumors.