Visualization of intra-tumor pharmacokinetics of [fam-] trastuzumab deruxtecan (DS-8201a) in HER2 heterogeneous model using phosphor-integrated dots imaging analysis.

Purpose: We assessed the intra-tumor pharmacokinetics of [fam-] trastuzumab deruxtecan, T-DXd (known as DS-8201a), a novel human epidermal growth factor receptor 2 (HER2)-targeted antibody-drug conjugate (ADC), using phosphor-integrated dots (PID) imaging analysis to elucidate its pharmacologic mechanism. Experimental design: We used two mouse xenograft models administered T-DXd at the concentration of 4 mg/kg: (i) a heterogeneous model in which HER2-positive and -negative cell lines were mixed, and (ii) a homogeneous model in which both cell types were transplanted separately into the same mouse. PID-imaging involved immunostaining using novel high-intensity fluorescent nanoparticles. The distribution of T-DXd was assessed by PID-imaging targeting the parent antibody, trastuzumab, and the payload, DXd, in serial frozen sections, respectively. Results: After T-DXd administration in the heterogeneous model, HER2 expression tended to decrease in a time-dependent manner. The distribution of trastuzumab and DXd was observed by PID-imaging along the HER2-positive area throughout the observation period. A detailed comparison of the PID distribution between trastuzumab and DXd showed that trastuzumab matched almost perfectly with the HER2-positive area. In contrast, DXd exhibited widespread distribution in the surrounding HER2-negative area as well. In the HER2-negative tumor of the homogeneous model, the PID distribution of trastuzumab and DXd remained extremely low throughout the observation period. Conclusions: Our results suggest that T-DXd is distributed to tumor tissues via trastuzumab in a HER2-dependent manner and then to adjacent HER2-negative areas. We successfully visualized the intra-tumor distribution of T-DXd and its mechanism of action, the so-called "bystander effect".