Pertuzumab Enhances the Antitumor Activity of T-DXd in HER2-Positive Gastric Cancer Cells

For HER2-positive advanced gastric cancer (GC), a recent major therapeutic advancement is the development of trastuzumab deruxtecan (T-DXd), a HER2-directed antibody-drug conjugate. In this disease, simultaneously targeting HER2 and HER3 pathways has the potential to be a promising therapeutic strategy. However, the therapeutic approach of combining T-DXd with pertuzumab, which disrupts HER2-HER3 heterodimerization, has not yet been explored in GC, making this study a pioneering effort. In vitro, T-DXd efficacy correlated with high levels of membrane HER2 expression. Among the 12 cell lines tested, two cell lines (NCI-N87 and OE19) confirmed as HER2 3+ by immunohistochemistry showed the most effective proliferation inhibition by T-DXd. When comparing NCI-N87 and OE19, HER2-HER3 dimerization was found to be more abundant in NCI-N87, and combination treatment with pertuzumab and T-DXd showed synergy in cell growth inhibition in NCI-N87, but not in OE19. NRG1 stimulation attenuated the antiproliferative effect of T-DXd. This attenuation of T-DXd activity by NRG1 was partially reversed by the addition of pertuzumab in NCI-N87, but not in OE19. Notably, the combination of T-DXd and pertuzumab enhanced membrane HER2 internalization more effectively in NCI-N87 than in OE19. In vivo mouse experiments using NCI-N87 cells showed the combination treatment significantly suppressed tumor growth compared to either monotherapy. Taken together, our findings suggest that dual targeting of HER2 and HER3 with T-DXd and pertuzumab may improve therapeutic outcomes in HER2-positive GC, particularly in tumors enriched with HER2-HER3 heterodimers. These preclinical data provide strong rationale for clinical trials evaluating this combination strategy in HER2-positive GC.