The ability of circulating tumor cells(CTCs) to form clusters has been linked to increased metastatic potential.

After analysing 2,486 FDA-approved compounds, scientists have now discovered that Na+/K+ATPase inhibitors that enable the dissociation of CTC clusters into single cells, leading to metastasis suppression.

https://www.sciencedaily.com/releases/2019/01/190110113005.htm
http://dx.doi.org/10.1016/j.cell.2018.11.046

Interestingly, some of the most effective drugs suppressing metastasis are part of a category comprising some of my favourite anti cancer drugs, i.e. cardiac glycosides. Specifically, the scientists identified Digitoxin and Ouabain as some of the most effective in suppressing metastasis. 

The authors explain this anti metastasis action of cardiac glycosides as being related to the induction of an increase in intracellular Ca2+ levels and CTC cluster disruption through suppression of functional cell-cell junction assembly.

Digoxin, a metabolic product of Digitoxin is accessible as an FDA approved drug used to treat treat various heart conditions.

Ouabain was taken out of the market as it was replaced by newer drugs, but it can still be found at some pharmacies in Europe or as a food supplement in Brazil. 

Other cardiac glycosides that may do the same are Bufalin previously discussed on this website  https://www.cancertreatmentsresearch.com/huachansu-zhusheye-bufalin/ or the well known alternative anti cancer treatment Oleander.