Trastuzumab deruxtecan (DS-8201a), a HER2-targeting antibody-drug conjugate with a topoisomerase I inhibitor exatecan derivative (DX-8951 derivative, DXd), has been reported to exert potent antitumor effects in xenograft mouse models and clinical trials. In this study, the immune system-activating ability of DS-8201a was assessed. DS-8201a significantly suppressed tumor growth in an immunocompetent mouse model with human HER2-expressing CT26.WT (CT26.WT-hHER2) cells. Cured immunocompetent mice rejected not only rechallenged CT26.WT-hHER2 cells, but also CT26.WT-mock cells. Splenocytes from the cured mice responded to both CT26.WT-hHER2 and CT26.WT-mock cells. Further analyses revealed that DXd upregulated CD86 expression on bone marrow-derived dendritic cells (DC) and that DS-8201a increased tumor-infiltrating DCs and upregulated their CD86 expression DS-8201a also increased tumor-infiltrating CD8 T cells and enhanced PD-L1 and MHC class I expression on tumor cells. Furthermore, combination therapy with DS-8201a and anti-PD-1 antibody was more effective than either monotherapy. In conclusion, DS-8201a enhanced antitumor immunity, as evidenced by the increased expression of DC markers, augmented expression of MHC class I in tumor cells, and rejection of rechallenged tumor cells by adaptive immune cells, suggesting that DS-8201a enhanced tumor recognition by T cells. Furthermore, DS-8201a treatment benefited from combination with anti-PD-1 antibody, possibly due to increased T-cell activity and upregulated PD-L1 expression induced by DS-8201a. .

[Fam-] trastuzumab deruxtecan (DS-8201a) is a HER2 (ERBB2)-targeting antibody-drug conjugate, composed of a HER2-targeting antibody and a topoisomerase I inhibitor, exatecan derivative, that has antitumor effects in preclinical xenograft models and clinical trials. Recently, [fam-] trastuzumab deruxtecan was reported to enhance antitumor immunity and was beneficial in combination with an anti–PD-1 antibody in a mouse model. In this study, the antitumor effect of [fam-] trastuzumab deruxtecan in combination with an anti–CTLA-4 antibody was evaluated. [Fam-] trastuzumab deruxtecan monotherapy had antitumor activity in an immunocompetent mouse model with EMT6 human HER2-expressing mouse breast cancer cells (EMT6-hHER2). [Fam-] trastuzumab deruxtecan in combination with the anti–CTLA-4 antibody induced more potent antitumor activity than that by monotherapy with either agent. The combination therapy increased tumor-infiltrating CD4+ and CD8+ T cells in vivo. Mechanistically, cured mice with treatment of [fam-] trastuzumab deruxtecan and an anti–CTLA-4 antibody completely rejected EMT6-mock cells similar to EMT6-hHER2 cells, and splenocytes from the cured mice responded to both EMT6-hHER2 and EMT6-mock cells as measured by interferon-gamma release. Taken together, these results indicate that antitumor immunity is induced by [fam-] trastuzumab deruxtecan and is facilitated in combination with anti–CTLA-4 antibody.

1031 Background: DS-8201a, a HER2-targeting antibody–drug conjugate (ADC), with a topoisomerase I inhibitor, exatecan drivative (DX-8951 derivative, DXd) has been shown to have antitumor effects in preclinical xenograft models and clinical trials, but the involvement of the immune system in the antitumor efficacy of DS-8201a has not been elucidated yet. Methods: The antitumor efficacy of DS-8201a individually and in combination with an anti-PD-1 antibody was determined in a syngeneic mouse model with human HER2-expressing CT26.WT (CT26.WT-hHER2) cells. Mice whose tumors had been cured by DS-8201a treatment were rechallenged with CT26.WT-hHER2 cells; their splenocytes were co-cultured with CT26.WT-hHER2 or CT26.WT-mock cells, and IFN-g secretion by these cells was determined. To investigate effects of DXd and DS-8201a on dendritic cells (DCs), the expression of DC markers on bone marrow derived DCs (BMDCs) and intratumoral DCs was analyzed by flow cytometry. Furthermore, MHC class I and PD-L1 expression on tumor cells was analyzed. Results: At a weekly dosage of 10 mg/kg, DS-8201a showed significant antitumor effects in the mouse model. Mice whose tumors had been cured by DS-8201a treatment rejected the rechallenge with CT26.WT-hHER2 cells, and splenocytes from these mice were activated by both CT26.WT-hHER2 and CT26.WT-mock cells. In the mouse model, DS-8201a treatment raised a population of intratumoral DCs (CD45+CD11c+MHC class II+) and increased DC expression of CD86, a DC activation marker; DXd also up-regulated CD86 expression on BMDCs in vitro. Furthermore, DS-8201a up-regulated PD-L1 and MHC class I expression on tumor cells. Notably, antitumor effects of the combination of DS-8201a with an anti-PD-1 antibody were better than those of monotherapy. Conclusions: DS-8201a elicits immune responses via mechanisms other than cytotoxic effects on tumor cells. This finding suggests additional benefits of combining DS-8201a with an immune checkpoint inhibitor (ICI). The combination of DS-8201a and an anti-PD-1 antibody was effective in tumor suppression, indicating that DS-8201a may be successfully combined with an ICI in human clinical applications.

<p>Supplementary materials and methods</p>

<p>Supplementary materials and methods</p>