Abstract 4632: Lineage plasticity as a determinant of antibody-drug conjugate target expression in urothelial bladder cancer

Abstract Bladder cancers display a wide spectrum of morphologies that frequently co-exist within individual tumors. Several histologic variants, including plasmacytoid, neuroendocrine, and micropapillary subtypes, are associated with an increased bladder cancer recurrence risk and cancer-specific mortality. The molecular basis and therapeutic implications of this phenotypical plasticity remain poorly understood. Enfortumab vedotin (EV), an antibody-drug conjugate targeting Nectin-4, has also emerged as a new standard-of-care for metastatic bladder cancer patients, but the association between lineage plasticity as manifested by morphologic heterogeneity in bladder cancer, Nectin-4 expression and ADC response remains poorly defined. Previous DNA-based molecular profiling studies of histologic variants revealed that CDH1 mutations were the pathognomonic molecular alteration in plasmacytoid urothelial carcinoma, but no recurrent driver mutations have been found to be unique to the aberrant histology regions of other mixed histology subtypes. Thus, to explore the molecular basis for the lineage plasticity of bladder cancers with aberrant histology and to define the association between lineage plasticity manifested as morphologic heterogeneity and the expression of Nectin-4, HER2 and other potential ADC targets, we performed RNA-seq of pure urothelial carcinomas (n=118, not otherwise specified, UC-NOS) and bladder tumors with divergent differentiation or histologic subtypes (n=199). Our cohort included a subset of tumors in which RNA-seq was performed on microdissected UC-NOS and variant histology regions of the same tumor. Tumors with neuroendocrine, sarcomatoid and squamous histology had the lowest Nectin-4 expression, whereas histologic subtypes that retained a luminal transcriptional profile such as micropapillary, plasmacytoid and nested histology had higher levels of Nectin-4 expression. In mixed histology tumors, Nectin-4 and/or HER2 expression was often restricted to only one of the two histologic components. Gene Set Enrichment Analysis noted enrichment of distinct oncogenic pathway gene sets in different histologic variants. Preliminary investigation into the mechanism(s) of Nectin-4 regulation using bladder cancer cell lines and patient-derived organoids (PDOs) has identified targetable pathways which when inhibited revert lineage plasticity and induce Nectin-4 expression. In sum, lineage plasticity manifested as heterogenous histomorphologies was associated with heterogeneous Nectin-4 expression in some but not all histologic variants of bladder cancer. Ongoing studies using PDOs from bladder cancer patients with mixed histology will seek to identify molecular pathways whose inhibition leads to increased expression of Nectin-4 or other ADC targets such as HER2 and thus increased sensitivity of ADCs targeting these cell surface proteins. Citation Format: Jiaqian Luo, Sizhi P. Gao, Fengshen Kuo, Gamze Gokturk Ozcan, Merve Basar, Florestan Koll, Jacob E. Tallman, Syed M. Alam, Carissa E. Chu, Ziyu Chen, Eugene J. Pietzak, Gopakumar Iyer, Jonathan E. Rosenberg, David B. Solit, Hikmat Al-Ahmadie. Lineage plasticity as a determinant of antibody-drug conjugate target expression in urothelial bladder cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4632.