HER2 antibody-drug conjugates are active against desmoplastic small round cell tumor

Abstract Purpose: Desmoplastic small round cell tumor (DSRCT) is a rare but highly aggressive soft tissue sarcoma that arises in the abdominopelvic cavity of young males. Since the discovery of EWSR1::WT1 fusion as the driver of DSRCT, no actionable genomic alterations have been identified, limiting disease management to a combination of surgery, chemotherapy, and radiation with very poor outcomes. Herein, we leveraged ERBB2/HER2 expression in DSRCT as a therapeutic target. Experimental Design: ERBB2/HER2 expression was evaluated in clinical samples and patient-derived xenografts (PDX) using RNA-seq, RT-qPCR, and a newly developed HER2 IHC assay (Clone 29D8). Responses to HER2 antibody-drug conjugates (ADC) –trastuzumab deruxtecan (DS-8201, T-DXd) and trastuzumab emtansine (T-DM1)– were evaluated in DSRCT-PDX, cell line, and organoid models. Drug internalization was demonstrated by live microscopy. Apoptosis was evaluated by Western blotting and caspase activity assays. Results: ERBB2/HER2 was detectable in DSRCT samples from patients and PDXs, with higher sensitivity of RNA assays and improved IHC detectability using Clone 29D8. Treatment of ERBB2/HER2-expressing DSRCT PDX, cell line, and organoid models with T-DXd or T-DM1 resulted in tumor regression. This therapeutic response was long-lasting in T-DXd-treated xenografts and was mediated by rapid HER2-ADC complex internalization and cytotoxicity, triggering p53-mediated apoptosis and growth arrest. Xenograft regression was associated with bystander payload effects triggering global tumor niche responses proportional to HER2 status. Conclusions ERBB2/HER2 is a therapeutic target for DSRCT. HER2-ADCs are novel options for managing this exceptionally aggressive sarcoma and may fulfill its urgent and historically unmet need for more effective clinical therapy.