Aryl substituted polymyxin derivatives

A compound of formula (I):

• wherein:
• —X— represents —C(O)—;
• —R1 together with the carbonyl group and nitrogen alpha to the carbon to which it is attached, is phenylalanine, leucine, or valine;
• —R2 is C1-4 alkyl optionally substituted with one hydroxyl group;
• —R3 together with the carbonyl group and nitrogen alpha to the carbon to which it is attached, is a threonine residue;
• —R4 together with the carbonyl group and nitrogen alpha to the carbon to which it is attached, is Dap;
• —R8 is hydrogen or methyl;
• and
• —R15 is a group:
• where:
• —R16 is hydrogen;
• —R17 is hydrogen;
• -L- is a covalent bond or methylene;
• —Ar is optionally substituted aryl,
• and salts, solvates and protected forms thereof.