Doxycycline and therapeutic targeting of the DNA damage response in cancer cells: old drug, new purpose.

There is a small proportion of cells within a tumour with self-renewing properties, which is resistant to conventional therapy, and is responsible for tumour initiation, maintenance and metastasis. These cells are known as cancer stem cells (CSCs) or tumour-initiating cells (TICs) [1]. Recent publications identify several antibiotics, such as salinomycin or doxycycline, as selective CSCs inhibitors [2-4]. However, the mechanisms of action of these antibiotics on CSCs are not fully understood.