Fig. 4. Mechanisms of action of T-DXd.

a, Illustration of the correlation between T-DXd distribution and HER2 expression. T-DXd was determined by IHC using an Ac anti-DXd (H-score) and HER2 by an enhanced protocol of IHC (H-score) in seven paired samples at baseline and during treatment. The staining was performed in one sample per case. The correlation was calculated by Pearson correlation coefficient, which showed a moderate correlation (r = 0.75, P = 0.053). P value was calculated using a two-sided Pearson correlation test. On the bottom, a pathology slide that shows HER2 staining (red arrows) on the left and T-DXd staining (red arrows) on the right. b, Illustration of the immune microenvironment modulation by T-DXd. Tumor biopsies at baseline and days 22–43 after cycle 1 of T-DXd were assessed by multiplex immunofluorescence (n = 31). No quantitative modulation of the immune microenvironment by T-DXd in the overall population (n = 31) was observed. There was a significant decrease in PD-L1 expression presumably due to the cytotoxic effect of T-DXd on tumor cells (CK⁺/PD-L1⁺) in patients with HER2-overexpressing mBC (n = 18, P = 0.002). Immune cells, represented by CD3⁺/PD-L1⁺ or CD68⁺/PD-L1⁺, did not show a decrease during treatment in cohort 1 (n = 18, P = 0.42). No significant decrease of PD-L1⁺ tumor (P = 0.17) or immune cells (P = 0.65) was observed in patients with HER2-low and HER2-non-expressing mBC (n = 13) during treatment. Blue bullets and red bullets represent at-baseline and on-treatment samples, respectively. P values were calculated using the Wilcoxon matched-pairs signed-rank test. All statistical tests were two-sided.Mφ, macrophage; T_reg, regulatory T cell.