Transcriptomic and Protein Analysis of Small-cell Bladder Cancer (SCBC) Identifies Prognostic Biomarkers and DLL3 as a Relevant Therapeutic Target.

Abstract

Transcriptomic profiling can shed light on the biology of small-cell bladder cancer (SCBC), nominating biomarkers, and novel therapeutic targets. Sixty-three patients with SCBC had small-cell histology confirmed and quantified by a genitourinary pathologist. Gene expression profiling was performed for 39 primary tumor samples, 1 metastatic sample, and 6 adjacent normal urothelium samples (46 total) from the same cohort. Protein levels of differentially expressed therapeutic targets, DLL3 and PDL1, and also CD56 and ASCL1, were confirmed by IHC. A SCBC PDX model was utilized to assess <i>in vivo</i> efficacy of DLL3-targeting antibody-drug conjugate (ADC). Unsupervised hierarchical clustering of 46 samples produced 4 clusters that correlated with clinical phenotypes. Patients whose tumors had the most "normal-like" pattern of gene expression had longer overall survival (OS) compared with the other 3 clusters while patients with the most "metastasis-like" pattern had the shortest OS (<i>P</i> = 0.047). Expression of DLL3, PDL1, ASCL1, and CD56 was confirmed by IHC in 68%, 30%, 52%, and 81% of tissue samples, respectively. In a multivariate analysis, DLL3 protein expression on >10% and CD56 expression on >30% of tumor cells were both prognostic of shorter OS (<i>P</i> = 0.03 each). A DLL3-targeting ADC showed durable antitumor efficacy in a SCBC PDX model. Gene expression patterns in SCBC are associated with distinct clinical phenotypes ranging from more indolent to aggressive disease. Overexpression of <i>DLL3</i> mRNA and protein is common in SCBC and correlates with shorter OS. A DLL3-targeted ADC demonstrated <i>in vivo</i> efficacy superior to chemotherapy in a PDX model of SCBC.