Most human epidermal growth factor-2 (HER2)-positive breast and gastric cancers eventually become resistant to HER2-targeting antibody-drug conjugates (ADCs), such as trastuzumab deruxtecan (T-DXd) and trastuzumab emtansine (T-DM1), which are widely used for their treatment. We hypothesized that combination therapy with an HER2-targeting ADC and enfortumab vedotin (EV), an anti-nectin-4 ADC approved for the treatment of advanced urothelial cancer, could be more effective than an HER2-targeting ADC alone, as HER2-positive breast and gastric cancers frequently express nectin-4.

HER2 and nectin-4 protein expression levels were assessed with flow cytometry. The efficacy of T-DM1 and EV, both as single agents and in combination, was first assessed in breast and gastric cancer cell lines using the AlamarBlue cell proliferation assay. The antitumor activity of T-DM1, EV, and their combination was next evaluated in breast cancer and gastric cancer SCID mouse xenograft models, including a model resistant to T-DM1. Xenograft tumor samples were analyzed by immunohistochemistry. Comparisons between groups were performed using one-way analysis of variance (ANOVA), and two-way repeated measures ANOVA. Survival differences between groups were assessed using the log-rank test.

All studied HER2-positive breast cancer cell lines (SKBR-3, UACC-812, MDA-453, and EFM-192A), the gastric cancer cell line (N87), and their corresponding xenograft tumors expressed nectin-4 protein. The combination of T-DM1 and EV demonstrated greater efficacy than either agent alone in SKBR-3, UACC-812, EFM-192A, and N87 cell lines. Similarly, the combination was more effective at reducing tumor size in xenograft models derived from MDA-453, N87, and RN87 cells, and it significantly prolonged survival in treated mice. Histologically, the combination treatment induced widespread apoptosis and tumor necrosis.

These findings indicate that co-administration of an anti-HER2 ADC and EV may substantially enhance anticancer efficacy compared to either agent alone in HER2-positive breast and gastric cancer cell lines and xenograft models. The results support further evaluation of the T-DM1 and EV combination in clinical trials.