The invention provides a polymyxin compound of formula (I) and salts, solvates and protected forms thereof, pharmaceutical compositions comprising the compounds of formula (I), and the use of the compounds and compositions in methods of treatment, such as methods for the treatment of microbial infections. The compounds of formula (I) are represented thus:wherein -R15is a group:and -R16is hydrogen； -R17is hydrogen； -L- is a covalent bond or methylene； and -Ar is optionally substituted aryl. The groups -X-, -R1, -R2, -R3, -R4, and -R8are as defined herein.本發明提供式(I)之多黏菌素化合物及其鹽、溶劑合物及經保護形式、包含式(I)化合物之醫藥組合物，以及該等化合物及組合物在治療方法，諸如治療微生物感染之方法中的用途。式(I)化合物表示如下：，其中-R15為以下基團：，且-R16為氫；-R17為氫；-L-為共價鍵或亞甲基；且-Ar為視情況經取代之芳基。基團-X-、-R1、-R2、-R3、-R4及-R8如本文中所定義。
