Described are compounds of formula (I) for use in combination treatment with a second active agent, such as rifampicin, for example for treatment of a microbial infection. The compound of formula (I) is a polymyxin compound is:where the groups -A-, —R1, —R2, —R3, —R4, —R5, —R6, —R7, —R8, and —X— are described in detail within the description.
