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.