Nectin-4 expression in primary breast cancer and associated clinical outcomes.

570 Background: Antibody-drug conjugates (ADC) selectively deliver chemotherapy to tumor cells expressing a target antigen. Currently approved ADCs in breast cancer target HER2 or Trop-2. Enfortumab vedotin, an ADC targeting Nectin-4, a cell adhesion molecule, is approved in advanced/metastatic urothelial cancer. This study evaluated the expression landscape of Nectin-4 in breast cancer and its association with clinical outcomes. Methods: Processed mRNAseq data was assessed for expression of the gene encoding Nectin-4 in primary breast cancer tissue and adjacent normal breast tissue from The Cancer Genome Atlas (TCGA), and normal tissue from the Genotype-Tissue Expression (GTEx) Project. Low gene expression was determined as <100 transcripts per million (TPM). TCGA samples had been annotated for putative copy-number alterations using GISTIC 2.0; genes with a score of 2 were marked as amplified. Multivariable Cox regression analysis was used to analyze the association of low vs medium/high gene expression with disease-free survival (DFS) and overall survival (OS), adjusting for age, stage, and subtype. Results: RNA-Seq data was available for 1086 patients with primary breast cancer, including known hormone receptor positive (HR+)/HER2- (n=530), HER2+ (n=112), and triple negative breast cancer (TNBC) (n=120). NECTIN4was amplified in 15% of cases (n=165): 15% (n=78) in HR+, 15% (n=17) in HER2+ and 21% (n=25) in TNBC. NECTIN4 expression was medium/high in 356 (33%) patients: 149 (28%) HR+/HER2- (median 77.8 TPM, IQR 50.1-116.5), 32 (29%) HER2+ (median 73.7 TPM, IQR 51.9-107.8), and 59 (49%) TNBC (median 99.7 TPM, IQR 57.8-148.3). Median expression in normal breast tissue was 47 TPM (TCGA) & 16.74 TPM (GTEx), and highest in normal skin at 205 TPM. In multivariable analysis, medium/high NECTIN4 expression was associated with worse OS (HR 1.96, 95% CI 1.32-2.99, p<0.001). As a continuous variable, increasing NECTIN4 expression levels were associated with worse OS. After correcting for age, stage and subtype, for each 10-unit increase in NECTIN4TPM, the risk of death increased by 5% (HR 1.0050, 95% CI 1.0028 - 1.0072, p < 0.001). Conclusions: NECTIN4 expression was higher in TNBC and associated with worse overall survival. The high expression level in normal skin possibly contributes to the cutaneous toxicity associated with enfortumab vedotin approval and requires careful consideration as ADCs targeting Nectin-4 are developed to target breast cancer.