Abstract 495: Impact of lineage plasticity on regulating target expression for antibody-drug conjugate therapy in urothelial bladder cancer

Abstract Bladder cancers display a wide spectrum of histologic phenotypes, the biologic basis and clinical implications of which remain poorly defined. Common histologic subtypes include squamous, glandular, micropapillary, plasmacytoid, small cell/neuroendocrine, sarcomatoid, and nested urothelial carcinoma (UC). Aside from the presence of CDH1 mutations as the pathognomonic alteration in plasmacytoid UC, DNA-based profiling studies failed to identify driver mutations specific to other histologic subtypes. To elucidate the molecular changes associated with phenotypic plasticity in mixed histology bladder cancer, we performed bulk RNA-seq of a large cohort of pure urothelial carcinomas (n=118, not otherwise specified, UC-NOS) and bladder tumors with divergent differentiation or histologic subtypes (n=199). This cohort included a subset of tumors in which RNA-seq was performed on macrodissected UC-NOS and variant histology regions of the same tumor. Our results showed that tumors with micropapillary, plasmacytoid and nested histology, which retain a urothelial-like transcriptional profile, also retain the expression of Nectin-4, Trop-2 and HER2, whereas other subtypes with histologic features of neuroendocrine, sarcomatoid, and squamous morphology exhibit lineage plasticity associated with lower or null expression of Nectin-4, Trop2, and HER2. Spatial transcriptomic sequencing of mixed histology tumors further confirmed that the expression of ADC targets including Nectin-4 and/or HER2 are often discordant between urothelial and variant regions of mixed histology bladder tumors. Furthermore, tumors with variant histology demonstrated a shorter duration of response to enfortumab vedotin (EV), an ADC targeting Nectin-4. Preliminary mechanistic investigation of Nectin-4 regulation using bladder cancer cell lines and patient-derived organoids (PDOs) has identified candidate driver genes which when inhibited revert lineage plasticity and increase the surface expression of Nectin-4, enhancing sensitivity to EV. Future studies with patient-derived models including organoids and xenografts will focus on identifying molecular targets 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. The long-term translational goals of these studies will be to identify rational combination strategies for enhancing the sensitivity to enfortumab vedotin in bladder cancer patients. Citation Format: Jiaqian Luo, Sizhi P. Gao, Fengshen Kuo, Jacob E. Tallman, Gamze Gokturk Ozcan, Merve Basar, Florestan Koll, Syed M. Alam, Carissa E. Chu, Ziyu Chen, Hui Jiang, Xinran Tang, Eugene J. Pietzak, Gopa Iyer, Jonathan E. Rosenberg, David B. Solit, Hikmat Al-Ahmadie. Impact of lineage plasticity on regulating target expression for antibody-drug conjugate therapy in urothelial bladder cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 495.