TP53 (tumor protein p53) is the most commonly mutated cancer driver gene, but drugs that target mutant tumor suppressor genes, such as TP53, are not yet available. Here, we describe the identification of an antibody highly specific to the most common TP53 mutation (R175H, in which arginine at position 175 is replaced with histidine) in complex with a common human leukocyte antigen–A (HLA-A) allele on the cell surface. We describe the structural basis of this specificity and its conversion into an immunotherapeutic agent: a bispecific single-chain diabody. Despite the extremely low p53 peptide-HLA complex density on the cancer cell surface, the bispecific antibody effectively activated T cells to lyse cancer cells that presented the neoantigen in vitro and in mice. This approach could in theory be used to target cancers containing mutations that are difficult to target in conventional ways.

Purpose This study tested the hypothesis that topical Toll-Like Receptor (TLR) 7 agonist imiquimod promotes anti-tumor immunity and synergizes with other treatments in a model of skin-involving breast cancer. Experimental design TSA mouse breast carcinoma cells were injected s.c. into syngeneic mice. Imiquimod 5% or placebo cream was applied topically on the shaved skin overlying tumors three times/week. In some experiments, local ionizing radiation therapy (RT) was delivered to the tumor in 3 fractions of 8 Gy, given on consecutive days. Cyclophosphamide (CY) was given i.p. in one dose of 2 mg/mouse. Mice were followed for tumor growth and survival. Results Treatment with imiquimod significantly inhibited tumor growth, an effect that was associated with increased tumor infiltration by CD11c+, CD4+ and CD8+ cells, and abolished by depletion of CD8+ cells. Administration of imiquimod in combination with RT enhanced significantly tumor response compared to either treatment alone (p<0.005), and 11 to 66% of irradiated tumors completely regressed. Importantly, the addition of topical imiquimod also resulted in growth inhibition of a secondary tumor outside of the radiation field. Low dose CY given before start of treatment with imiquimod and RT further improved tumor inhibition and reduced tumor recurrence. Mice that remained tumor-free rejected a tumorigenic inoculum of TSA cells, demonstrating long-term immunological memory. Conclusions Topical imiquimod inhibits tumor growth and synergizes with RT. Addition of CY further increases the therapeutic effect and induces protective immunological memory, suggesting that this combination is a promising strategy for cutaneous breast cancer metastases.