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

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.

Phosphorylation-Driven Epichaperome Assembly: A Critical Regulator of Cellular Adaptability and Proliferation.

The intricate protein-chaperone network is vital for cellular function. Recent discoveries have unveiled the existence of specialized chaperone complexes called epichaperomes, protein assemblies orchestrating the reconfiguration of protein-protein interaction networks, enhancing cellular adaptability and proliferation. This study delves into the structural and regulatory aspects of epichaperomes, with a particular emphasis on the significance of post-translational modifications in shaping their formation and function. A central finding of this investigation is the identification of specific PTMs on HSP90, particularly at residues Ser226 and Ser255 situated within an intrinsically disordered region, as critical determinants in epichaperome assembly. Our data demonstrate that the phosphorylation of these serine residues enhances HSP90's interaction with other chaperones and co-chaperones, creating a microenvironment conducive to epichaperome formation. Furthermore, this study establishes a direct link between epichaperome function and cellular physiology, especially in contexts where robust proliferation and adaptive behavior are essential, such as cancer and stem cell maintenance. These findings not only provide mechanistic insights but also hold promise for the development of novel therapeutic strategies targeting chaperone complexes in diseases characterized by epichaperome dysregulation, bridging the gap between fundamental research and precision medicine.