649. Developing FGFR4 Chimeric Antigen Receptor CAR T Cell Therapy Against Rhabdomyosarcoma

Abstract

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in pediatrics with an annual incidence of 4.5 cases per 1 million. Patients with high-risk metastatic disease have dismal prognosis and newer treatments are needed. We identified, fibroblast growth factor receptor 4 (FGFR4) as an overexpressed cell surface protein in RMS by mRNA expression analysis. Furthermore, activating mutations in FGFR4 are associated with metastatic disease. FGFR4 protein overexpression in RMS provides a specific target for immune-based therapy of RMS. We are developing T cells genetically modified to express chimeric antigen receptors (CARs) that target FGFR4.To verify FGFR4 RNA expression at the protein level we performed both immunohistochemistry (IHC) and electrochemillumescence (ECL) ELISA assays. Using IHC analysis we measured FGFR4 protein levels on tissue microarrays (TMA) of normal tissue and primary tumor from RMS patients, increased staining for FGFR4 protein on RMS primary tumors, compared to normal tissues was demonstrated. FGFR4 expression measured using ECL ELISA assay, shows a range of 300 - 800pg FGFR4 per 1mg of total lysate in RMS cell lines. The range for normal tissues was 30 - 40 pg/mg for all tissues with the exception of liver, which expressed 70 pg/mg. A single-chain variable fragment (scFv) cDNA library derived from a human B cells was screened, and clones that showed binding to recombinant FGFR4 extracellular domain (ECD) selected. We identified ten specific human anti-FGFR4 scFv binders. The scFvs were cloned into prokaryotic expression vector containing the human IgG1 Fc region. Anti-FGFR4 scFv-Fc were expressed in 293FT cells by transient transfection and purified using Protein A affinity chromatography. The binding of scFv-Fcs to recombinant FGFR4 ECD was verified by ELISA. scFv-Fc binders were then assayed for binding to cell surface FGFR4 on RMS cell lines using flow cytometry. Anti-FGFR4 scFv-Fc bound to 293T cells transfected to express FGFR4 but not 293T control cells. Anti-FGFR4 scFv-Fc also bound FGFR4 on three RMS cell lines. The first two anti-FGFR4 scFv binder sequences evaluated, M410 and M412, were used to make short (S, extracellular scFv only) and long format (L, scFv with a CH2CH3 domain of IgG1) CAR constructs. Activated T cells were transduced with lentiviral CAR expression vector (LV) encoding M410-L, M412-S and M412-L CAR constructs and cell surface expression of FGFR4 CAR on transduced T cells was measured using flow cytometry. The M410 and M412 FGFR4 CARs, both short and long constructs, were tested for cell-mediated cytotoxicity against RMS cell lines. M410-L showed higher cytotoxic activity compared to M412-L. M412-S showed greater cytotoxic activity compared to M412-L CAR. Thus, overall CAR structure format may be important for its functional activity. The remaining scFv will be further analyzed in various CAR formats for its functional activity including cytotoxicity and interferon-gamma production. In summary, the overexpression of FGFR4 protein in RMS versus normal cell lines demonstrates that FGFR4 may be a suitable target for immune-based therapy. FGFR4 CAR-T cell therapy offers the potential of a novel therapeutic intervention for high-risk, refractory and relapsed RMS.