A Phase 1 Study of a Vaccine Targeting Preferentially Expressed Antigen in Melanoma and Prostate-specific Membrane Antigen in Patients With Advanced Solid Tumors

Preferentially expressed antigen in melanoma (PRAME) and prostate-specific membrane antigen (PSMA) are tumor-associated antigens implicated in cellular differentiation, genetic stability, and angiogenesis. MKC1106-PP is an immunotherapeutic regimen cotargeting PRAME and PSMA, comprised of a recombinant plasmid (pPRA-PSM encoding fragments derived from both antigens) and 2 peptides (E-PRA and E-PSM derived from PRAME and PSMA, respectively). This multicenter study evaluated MKC1106-PP with a fixed plasmid dose and 2 different peptide doses, administered by intralymph node injection in a prime-boost sequence in human leukocyte antigen-A*0201 and tumor-antigen-positive patients with progressing metastatic solid tumors who had failed standard therapy. Immune monitoring was done by tetramer and enzymatic-linked immune spot analysis. The treatment was well tolerated, with no significant differences in safety, immune response, and clinical outcome relative to peptide doses. Fifteen of 24 evaluable patients showed an immune response, as defined by the expansion of PRAME-specific or PSMA-specific T cells in the blood. There were no partial or complete responses by the Response Evaluation Criteria in Solid Tumors. Seven patients showed stable disease (SD) for 6 months or longer, or prostate specific antigen decline: 4 of 10 with prostate carcinoma, 2 of 2 with renal clear cell carcinoma, and 1 of 10 with metastatic melanoma. In addition, there was an association between the induction and persistence of antigen-specific T cells in blood above baseline levels and disease control, defined as SD for 6 months or longer. These results support further development of MKC1106-PP in specific clinical indications.