Enhancement of tumor immunogenicity by the introduction of non- proteinogenic amino acid azetidine-2-carboxylic acid

Despite the clinical success in the treatment of several types of cancers, the immune checkpoint inhibitors (ICIs) show limited response rates in cancers with low tumor mutational burden (TMB) and antigenicity. Here, we aim to enhance tumor antigenicity at the protein translation level by using non-proteinogenic amino acids (NPAs) that cause regional mistranslation and mutated proteins. We utilized proline analogue azetidine-2-carboxylic acid (AZA), which can be discharged into proline tRNA by prolyl-tRNA synthetase, leading to the generation of a proportion of mutated proteins with proline residues substituted with Aze in tumor cells undergoing active protein synthesis. To specifically produce mutated proteins in tumor cells, the anti-Cd44 antibody-coated liposome nanoparticles (NPs) were used to deliver Aze specifically into the breast cancer cells. The Aze delivered by NPs can be incorporated into proteins in the 4T1 tumor allografts in mice, resulting in the activation of cellular immune responses and hence the significant inhibition of the growth of 4T1 allografts and the pulmonary metastasis, eventually prolonging the survival of tumor-bearing mice. Interestingly, Aze increases the response of 4T1 breast cancer allografts to anti-PD1 antibody treatment, suggesting Aze is able to sensitize tumors to the ICIs treatment in the immunotherapy of tumors.