Canarypox virus expressing wild type p53 for gene therapy in murine tumors mutated in p53

The antitumor activity of a recombinant canarypox virus expressing wild type murine p53 (ALVAC-p53) was investigated in two murine syngeneic tumors harboring an endogenous p53 mutation (CMS4 and TS/A). Direct intratumor injections of ALVAC-p53 in CMS4 pre-established subcutaneous tumors induced total tumor regression in 66% of mice. Furthermore, 100% of the cured mice was protected against a contralateral subsequent challenge with the parental tumor cells. The intravenous treatment of experimental lung metastasis by ALVAC-p53 also induced significant tumor growth inhibition in both models. The antitumor effect of ALVAC-p53 was only observed in immunocompetent animals and was associated with the generation of a specific antitumor immune response. ALVAC-p53 induced the expression of a functional p53 wild type protein as demonstrated by up-regulation of p21waf1 and induction of apoptosis. A vaccine strategy using intravenous or subcutaneous ALVAC-p53/NYVAC-p53 prime boost protocol failed to induce CTL against p53 wild type used as target tumor antigen, and failed to protect mice against challenge with the mutated tumor cells. The mechanism of the curative and protective effects observed after direct intratumor injections results from the induction of a specific antitumor response directed against other antigens than p53. Our results suggest that the local induction of tumor apoptosis, combined with the adjuvant effect of ALVAC vector, enhances the immunogenicity of the intratumor environment and allows induction of specific antitumor immune response.