Therapeutic inhibition of breast cancer bone metastasis progression and lung colonization: breaking the vicious cycle by targeting α5β1 integrin

At diagnosis, 10 % of breast cancer patients already have locally advanced or metastatic disease; moreover, metastasis eventually develops in at least 40 % of early breast cancer patients. Osteolytic bone colonization occurs in 80-85 % of metastatic breast cancer patients and is thought to be an early step in metastatic progression. Thus, breast cancer displays a strong preference for metastasis to bone, and most metastatic breast cancer patients will experience its complications. Our prior research has shown that the alpha 5 beta 1 integrin fibronectin receptor mediates both metastatic and angiogenic invasion. We invented a targeted peptide inhibitor of activated alpha 5 beta 1, Ac-PHSCN-NH2 (PHSCN), as a validated lead compound to impede both metastatic invasion and neovascularization. Systemic PHSCN monotherapy prevented disease progression for up to 14 months in Phase I clinical trial. Here, we report that the next-generation construct, Ac-PhScN-NH2 (PhScN), which contains D-isomers of histidine (h) and cysteine (c), is greater than 100,000-fold more potent than PHSCN at blocking basement membrane invasion. Moreover, PhScN is also up to 10,000-fold more potent than PHSCN at inhibiting lung extravasation and colonization in athymic mice for both MDA-MB-231 metastatic and SUM149PT inflammatory breast cancer cells. Furthermore, we show that systemic treatment with 50 mg/kg PhScN monotherapy reduces established intratibial MDA-MB-231 bone colony progression by 80 %. Thus, PhScN is a highly potent, well-tolerated inhibitor of both lung colonization and bone colony progression.