p53MVA Therapy in Patients with Refractory Gastrointestinal Malignancies Elevates p53-Specific CD8+ T-cell Responses

To conduct a phase I trial of a modified vaccinia Ankara (MVA) vaccine delivering wild-type human p53 (p53MVA) in patients with refractory gastrointestinal cancers. Three patients were vaccinated with 1.0×10(8) plaque-forming unit (pfu) p53MVA followed by nine patients at 5.6×10(8) pfu. Toxicity was classified using the NCI Common Toxicity Criteria and clinical responses were assessed by CT scan. Peripheral blood samples were collected pre- and post-immunization for immunophenotyping, monitoring of p53MVA-induced immune response, and examination of PD1 checkpoint inhibition in vitro. p53MVA immunization was well tolerated at both doses, with no adverse events above grade 2. CD4+ and CD8+ T cells showing enhanced recognition of a p53 overlapping peptide library were detectable after the first immunization, particularly in the CD8+ T-cell compartment (P=0.03). However, in most patients, this did not expand further with the second and third immunization. The frequency of PD1+ T cells detectable in patients' peripheral blood mononuclear cells (PBMC) was significantly higher than in healthy controls. Furthermore, the frequency of PD1+ CD8+ T cells showed an inverse correlation with the peak CD8+ p53 response (P=0.02) and antibody blockade of PD1 in vitro increased the p53 immune responses detected after the second or third immunizations. Induction of strong T-cell and antibody responses to the MVA backbone were also apparent. p53MVA was well tolerated and induced robust CD8+ T-cell responses. Combination of p53MVA with immune checkpoint inhibition could help sustain immune responses and lead to enhanced clinical benefit.