Combined treatment with p53-activating drug APR-246 and a phosphatidylserine-targeting antibody, 2aG4, inhibits growth of human triple-negative breast cancer xenografts

Around 15-20% of human breast cancers are classified as triple-negative (TNBC) because they are devoid of the three common chemotherapy targets. Since current protocols to treat TNBC are largely ineffective, new therapies for TNBC need to be identified. Almost 80% of TNBCs express a functionally defective form of the p53 tumor suppressor protein (mutant p53; mtp53). In mtp53-expressing cancers, most p53 mutations occur in the DNA-binding domain, blocking normal regulation of p53 target genes that are involved in apoptosis, cell-cycle arrest, and angiogenesis and resulting in resistance to chemotherapy and metastasis. Restoring p53 function is therefore a potentially effective strategy for combating TNBC. APR-246 is a small-molecule drug that has been shown to reactivate mtp53 and restore p53 function. We examined whether APR-246 could inhibit TNBC growth in vitro and in vivo . TNBC growth in vitro was measured using cell viability assays, and TNBC growth in vivo was assessed using a tumor xenograft mouse model. Nuclear extracts of APR-246-treated MDA-MB-231 TNBC cells exhibited significantly increased DNA binding compared with untreated cells, indicating that APR-246 converts mtp53 to a wild-type p53 form (wtp53) in these cells. APR-246 significantly reduced viability of MDA-MB-231 and MDA-MB-468 TNBC cells in vitro , but had no effect on normal mammary cells (AG11132A) or breast cancer cells that express wtp53 (MCF-7). In our tumor xenograft mouse model, administration of APR-246 alone or in combination with 2aG4, an antibody that disrupts tumor vasculature, significantly reduced TNBC tumor growth (MDA-MB-231), as well as two markers of angiogenesis (tumor vascular endothelial growth factor expression and blood-vessel density). APR-246 in combination with 2aG4 completely eradicated almost 20% of the TNBC tumors present prior to treatment. Thus, in conclusion, APR-246 alone and in combination with 2aG4 inhibits TNBC tumor growth, and could represent an innovative therapy for TNBC, for which few effective treatment options are currently available. These suggest novel