Amplification of HER2 leads to development of HER2-positive (HER2+) cancers with high rates of metastasis compared to other cancer subtypes. The goal of this study was to probe the vulnerability of HER2+ cancer cells to a filamentous actin (F-actin) severing and capping toxin. The growth and viability of human HER2+ breast cancer (HCC1954) and ovarian cancer (SKOV3) cell lines were significantly impaired upon treatment with the marine macrolide mycalolide B (Myc B) at doses above 100 nanomolar. Further testing of Myc B in combination with the antibody-drug conjugate Trastuzumab-emtansine (T-DM1) led to improved killing of SKOV3 cells compared to either treatment alone. At sub-lethal doses, treatment of HER2+ cancer cells with Myc B resulted in rapid loss of leading edge protrusions and formation of aggresomes containing F-actin and the actin regulatory protein Cortactin. This correlated with robust inhibition of HER2+ cancer cell motility and invasion with Myc B treatment. In SKOV3 tumor xenograft assays, intratumoral injections of Myc B impaired HER2+ tumor growth and metastasis, with maximal effects observed in combination with systemic delivery of Trastuzumab. Metastasis of SKOV3 cells to the lungs following tail vein injection was also reduced by Myc B. Together, these findings provide rationale for targeting F-actin in combination with existing therapies for HER2+ cancers to reduce metastasis.

HER2 is a driver and clinical target in cancers afflicting women. HER2-positive (HER2+) cancers have high rates of metastasis and lower overall survival rates compared to other cancer subtypes. The goal of this study is to test the vulnerability of HER2+ cancer cells and tumors to disruption of their actin cytoskeleton using the marine macrolide toxin mycalolide B (Myc B), as these cells are highly dependent on rapid actin polymerization and remodeling for migration and invasion into new sites of tumor growth. The effects of Myc B treatment on HER2+ breast (HCC1954) and ovarian (SKOV3) cancer cell lines were profiled in assays of cell viability, motility, and invasion. Treatments of Myc B alone or in combination with Trastuzumab were also performed in HER2+ tumor xenograft assays. Myc B showed potent growth suppressive and cytotoxic effects on HER2+ cancer cells at doses in the 70-100 nM range. At sub-lethal doses, Myc B caused a rapid loss of leading edge protrusions, and sustained defects in HER2+ cancer cell motility and invasion. HER2 internalization and killing of HER2+ cancer cells by Trastuzumab-emtansine was not compromised with Myc B treatment. In a HER2+ tumor xenograft model, Myc B treatment alone, or in combination with Trastuzumab, led to significant reductions in tumor growth and metastasis. Together, these findings identify a major vulnerability in metastasis-initiating HER2+ cancer cells to the actin toxin Myc B, and provide a rationale to exploit this vulnerability with the development of new therapeutics targeting actin. Citation Format: Sarah Nersesian, Rodette Williams, Dr. Andrew Evans, Dr. John Allingham, Dr. Andrew Craig. The macrolide toxin mycalolide B disrupts actin-driven invasion and metastasis of HER2-positive cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3161.

Breast and ovarian cancers are among the leading causes of cancer related deaths in women worldwide. Human epidermal growth factor receptor 2 (HER2) is highly expressed in a subset of these cancers that show high rates of progression to metastatic disease. Metastasis is driven by filamentous actin (F-actin) based protrusions that penetrate and degrade extracellular matrix (ECM) to facilitate tumor invasion. In this study, we tested an actin-depolymerizing macrolide toxin Mycalolide B, as a potential suppressor of highly metastatic HER2+ breast and ovarian cancer cell models. Dose responses with Mycalolide B in SKBR3 breast cancer and SKOV3 ovarian cancer cells resulted in similar cytotoxicity (∼65 nM IC50) profiles. In addition, Mycalolide B doses well below the IC50 showed potent suppression of leading edge protrusions, and motility in SKBR3 and SKOV3 cancer cells. This also correlated with reduced ECM degradation and Transwell invasion at low nanomolar doses of Mycalolide B. In contrast, other F-actin based processes such as endocytosis of HER2 and EGFR, were less sensitive to Mycalolide B treatment. However, Mycalolide B treatment did skew the size of endocytic vesicles, which may reflect defects in F-actin based vesicle motility or maturation. Given that HER2 cancers have been effectively targeted by Trastuzumab and Trastuzumab-based antibody-drug conjugates, we are currently testing the compatibility of combined treatments with Mycalolide B and Trastuzumab, for their effects on F-actin based invasion of HER2+ cancers. This strategy may yield a novel class of antibody-drug conjugate, and lead to new HER2 targeted therapies that suppress tumor metastasis by disrupting early steps that are dependent on efficient F-actin polymerization. Citation Format: Rodette N. Williams, Andrew W. Craig, John S. Allingham. Macrolide toxin Mycalolide B is a potent inhibitor of HER2 cancer cell invasion and is the basis of actin targeted therapy for metastatic cancers. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2179.