PF445 INHIBITION OF ACUTE LYMPHOBLASTIC AND MYELOID LEUKEMIAS BY NOVEL CHIMERIC MOUSE‐HUMAN MONOCLONAL ANTIBODIES AGAINST ANTIGEN PRAME

Abstract

Background:A number of the novel immunotherapy approaches revolutionized leukemia treatment. Among most striking ones are CAR‐T and bispecific antibodies targeting CD19 and monoclonal antibodies inhibiting immune checkpoints pathways PD‐1 and CTLA4. Nevertheless, anti‐leukemia immunotherapy is still a challenge due to the lack of suitable leukemia cell surface targets. As a matter of fact, a unique and “table breaking” target molecule CD19 is restricted only by B‐cell lineage making it rather frustrating to find a similar fulcrum in the case of myeloid and T‐cell malignancies. There are a few studies suggesting that frequently activated cancer testis antigen PRAME is not only located within the inner tumor cell compartments but also on the surface of the tumor cells including leukemia cells. We have sought to estimate the value of the tumor cell surface antigen PRAME to be a target of therapeutic monoclonal antibodies.Aims:To evaluate anti‐leukemia activity of the novel chimeric monoclonal mouse‐human antibodies.Methods:recPRAME has been used to develop a number of mouse hybridomas secreting highly specific and affine anti‐PRAME moAbs. 11 of mouse anti‐PRAME moAbs have been tested to evaluate their PRAME‐expressing anti‐leukemia/solid tumor cell lines growth inhibiting activity. Mouse hybridomas producing most efficient anti‐PRAME moAbs to inhibit tumors have been used as a basis to develop chimeric mouse‐human anti‐PRAME moAbs. Gene loci encoding VDJ domains of light and heavy chains of immunoglobulins have been sequenced and corresponding alignments studied. Proteomic MALDI analysis of chimeric mouse‐human anti‐PRAME moAbs has been performed and its result has been compared with data of RNA sequencing of immunoglobulin genes. Leukemic/solid tumor cell lines inhibiting activity of chimeric mouse‐human anti‐PRAME moAbs has been evaluated by means of MMT‐test, flow cytometry and xCELLigence dp. Among cell lines tested were K562, THP1, NOMO1, HT29, WI38, WI38 transfected by PRAME, Raji, Vero, a number of cell lines obtained from melanoma pts, and some others.Results:Distinct ability of chimeric mouse‐human anti‐PRAME moAbs to inhibit growth of PRAME expressing leukemia and solid tumor cells has been revealed. The most sensitive cell lines occured to be K562, NOMO1 and HT29. Negative for PRAME expression line of human fetal fibroblasts WI38 was resistant against anti‐PRAME chimeric moABs. Transfection of PRAME expressing gene construct into WI38 renders it to be sensitive of anti‐PRAME chimeric moABs. The more PRAME was expressed by tumor cells, the more was the observed inhibiting effect. It turned out that one of PRAME epitopes on the surface of leukemic and solid tumor cells is most susceptible as an antitumor target of monoclonal antibodies.Summary/Conclusion:We have developed novel chimeric mouse‐human monoclonal antibodies targeting PRAME antigen to efficiently inhibit growth of lymphoblastic and myeloid leukemia cells lines as well as solid tumor cell lines provided that they express PRAME gene. Our data suggest that PRAME antigen is not only presented on the surface of the PRAME expressing tumor cells but is a very useful target for antitumor drugs as it was shown in the case of PRAME‐binding monoclonal antibodies. We believe that our chimeric mouse‐human anti‐PRAME monoclonal antibodies are a valuable basis for development of humanized monoclonal antibodies and CAR‐T targeting PRAME in leukemias and other PRAME‐positive tumors. This research was supported by Ministry of Science and Higher Education of the Russian Federation (unique project identifier RFMEFI60418X0204)image