Preclinical modeling of trastuzumab deruxtecan in a mouse model of brain metastatic triple-negative breast cancer identifies two contributors to efficacy.

Abstract

Trastuzumab deruxtecan (T-DXd) has demonstrated clinical efficacy in advanced HER2-low to -ultralow breast cancer. We employed a mouse model of triple-negative breast cancer in the difficult-to-treat brain metastasis setting to determine T-DXd efficacy and investigate mechanism(s). T-DXd (10 and 3 mg/kg) was compared to an identical ADC except HER2-nonbinding (control) in a hematogenous model of HER2-ultralow brain-tropic triple-negative breast cancer, MDA-MB-231-BR7 (231-BR). The roles of HER2 and the neonatal Fc receptor (FcRn) were determined by CRISPR-Cas9 deletion and shRNA knockdown, respectively. 231-BR cells exhibited HER2 0+ staining, with occasional membrane expression. T-DXd at 10 mg/kg reduced 231-BR brain metastases by 49% (P=0.002), while a dose reduction showed a 30% (ns) metastasis reduction, each compared to the control-ADC. HER2-specific and -nonspecific ADC uptake into metastases was accompanied by a 23-47% tumor cell Ki-67 decline, and 5-fold increase in tumor apoptosis. A HER2 CRISPR-Cas9 knockout 231-BR formed multiple brain metastases but was insensitive to T-DXd in vivo. 231-BR tumor cells expressed FcRn, potentially enabling nonspecific ADC uptake. Analysis of human tumors showed TNBC tumors expressed FcRn mRNA. T-DXd failed to inhibit brain metastases in two 231-BR FcRn knockdown clones compared to a 231-BR clone transduced with a scrambled shRNA where T-DXd inhibited brain metastases (67% reduction, P=0.01). Despite ultralow HER2 expression, the HER2 binding capacity of T-DXd is a significant contributor to its efficacy. Breast cancers variably express FcRn, suggesting that nonspecific tumor cell uptake of T-DXd via FcRn is also a potential mechanism of T-DXd action.