MON-312 Lewis Y Antigen as a Novel Target for CAR (Chimeric Antigen Receptor) T-cell Therapy in Patients with Neuroendocrine Prostate Cancer

Abstract Disclosure: W.J. Kulakowska: None. R.A. Taylor: None. W. Ranasinghe: None. P.K. Darcy: None. J.A. Trapani: None. G.P. Risbridger: None. Background and aims While CAR (Chimeric Antigen Receptor) T cell therapies targeting PSMA and PSCA receptors are currently in phase 1/2 studies in metastatic castrate-resistant prostate cancer (mCRPC), antigen targets for neuroendocrine prostate cancer (NEPC) remain limited. Lewis Y antigen (LeY) is an oncofetal antigen expressed only in adult tumour cells and is considered a target for CAR T cell therapy in haematological, lung, ovarian and colon cancer. We previously found that LeY is a promising target for CAR T cell therapy in prostate cancer and that preconditioning LeY-directed CAR T cells with carboplatin further enhances tumour responses in mCRPC. We investigated if LeY can be used as a novel CAR T cell target for NEPC. Methods: The expression of the Lewis Y antigen was assessed in our patient-derived prostate cancer xenograft (PDX) collection using immunohistochemistry (IHC). To assess the efficacy of LeY-CAR T cells, we used specimens established from a patient (Patient 508) with NEPC who had incurable metastatic disease and failed standard treatments. We generated CAR T cells from his peripheral blood mononuclear cells (PBMCs) to test their specificity and efficacy in vitro. Results: In our MURAL PDX collection of 59 prostate tumours, 81% (9/11) NEPCs expressed LeY antigen on IHC, including Patient 508. We successfully produced anti-LeY CAR T cells from the patient’s PBMCs using retroviral transduction. Generated CAR T cells presented a nearly 1:1 CD4+:CD8+ T cell ratio (51.7% and 41.4% of total T cells, respectively), which according to existing literature yields better treatment outcomes. Next, we used an immunobead assay to investigate CAR-T specificity in releasing proinflammatory cytokines. Significant cytokine production was observed after a co-culture with LeY+ human ovarian cancer cell line, OVCAR3, vs co-culture with control LeY- MDA-MB435 cells (p=0.0004), demonstrating the specificity of the CAR T cells. Lastly, we showed that patient CAR T cells effectively killed LeY+ cells (OVCAR3), but not control LeY- cells (MDA-MB435; p<0.0001) in a 16-hour chromium release assay. CAR T cells killed LeY+ target cells more effectively than their non-transduced equivalents at a range of effector-to-target ratios, with up to 15-fold increase in cell death. Conclusion: Our preclinical studies show that most NEPC tumors express the LeY antigen and that patient-derived CAR T cells can specifically kill those cells. These findings identified a potential CAR T treatment target for NEPC for a phase 1/2 study. Presentation: Monday, July 14, 2025