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.本發明描述式(I)之化合物,該等化合物與第二活性劑(諸如利福平)一起用於組合治療(例如,用於治療微生物感染)。式(I)之化合物為多黏菌素化合物:其中基團-A-、-R1、-R2、-R3、-R4、-R5、-R6、-R7、-R8及-X-在描述中詳細描述。