Phenotypic discovery and therapeutic evaluation of an ITGA3B1-targeting antibody-drug conjugate for bladder cancer.

Antibody-drug conjugates (ADCs) require antibodies with both high specificity and efficient internalization, features often overlooked by conventional discovery pipelines that rely on preselected antigens and recombinant proteins. Here, we describe an integrated phenotypic platform that combines target-unbiased live-cell biopanning with in situ chemical cross-linking and mass spectrometry to concurrently identify internalizing antibodies and their membrane-bound cognate antigens in a native cellular context. Using this approach, we identified 2E7, an antibody with rapid internalization and specificity for the integrin α3β1 (ITGA3B1) heterodimer. Integrated transcriptomic and proteomic analyses revealed pronounced overexpression of ITGA3B1 across multiple solid tumors, with particularly elevated levels in aggressive bladder cancer subtypes. A 2E7-MMAE (monomethyl auristatin E) ADC exhibited potent, dose-dependent antitumor activity in bladder cancer xenograft models, leading to tumor regression and prolonging survival. This study establishes a generalizable framework for function-first ADC discovery and nominates ITGA3B1 as a promising therapeutic target in bladder cancer.