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.