HER3 upregulation reduces DS-8201 sensitivity in HER2-positive tumor cells by ATR/CHK1/FoxO1 signaling cascade.

The anti-HER2 antibody‒drug conjugate (ADC) DS-8201 presents new hope for patients with advanced HER2-positive tumors. Its clinical application, however, is hindered by serious adverse reactions and reduced efficacy following long-term treatment. In this study, we investigated the factors influencing the sensitivity of DS-8201 and developed effective combination regimens to optimize its therapeutic efficacy. We showed that HER3 upregulation diminished the sensitivity of HER2-positive tumor cells to DS-8201. We found that DS-8201 treatment activated DNA damage repair responses in BT-474 cells, in which the ATR kinase pathway induced the expression of the HER3 transcription factor FoxO1, leading to increased HER3 levels. This process was triggered by the payload component of DS-8201, the topoisomerase I inhibitor DXd, rather than the antibody. Based on this finding, we showed that combining DS-8201 with either a HER3-targeting antibody (SIBP-03) or an ATR inhibitor (BAY1895344) resulted in significant synergistic antitumor efficacy without substantial toxicity in vitro or in vivo. Overall, this study revealed that the ATR/FoxO1/HER3 pathway plays a critical role in modulating the efficacy of DS-8201, suggesting that combining DS-8201 with ATR or HER3 inhibition represents a promising therapeutic strategy for HER2-positive cancers.