This invention relates to compounds that are useful as cancer therapies. The compounds comprise azithromycin derivatives having a phosphonium cation tethered to the azithromycin macrocycle. The invention also relates to methods of using said compounds and to pharmaceutical formulations comprising said compounds. The compounds comprise an ion of formula (I): wherein either Z1 is and Z2 is R4b; or Z2 is and Z1 is R2b.