A compound of formula, wherein Ar is an aryl group, wherein the aryl group is a C 6 to C 14 monovalent carbocyclic aromatic group derived from a single aromatic ring or multiple condensed aromatic rings, optionally further substituted with one or more groups R 3; A is a lipophilic, hydrophobic moiety; R 1 is: (a) a phosphodiester, wherein the phosphodiester is a group of the formula O=P (OR')OH, wherein R and R' are carbon containing groups bonded to oxygen through a carbon atom; (b) a phosphotriester, wherein the phosphotriester is a group of the formula O=P (OR')OR", wherein R, R' and R" are carbon containing groups bonded to oxygen through a carbon atom; (c) a thioether; or (d) a amide group, wherein the amide group is a group of the formula RC(=O)NR'R'' wherein R, R' and R" are either hydrogen or carbon containing groups bonded to carbon or nitrogen through a carbon atom; X is an unsubstituted or substituted C 6 to C 24 alkylene or alkenylene group, which is optionally interrupted by one or more -NR 9 -, -O- or -S- linkages, wherein R 9 is a C 1 to C 6 alkyl group; R 2 is -YC(R 4)(R 5)CO 2 R 6; Y is a covalent bond or -(CH 2) m - wherein m is 1, 2 or 3; R 4 and R 5 are the same or different and each is a substituted or unsubstituted C 1 to C 6 alkyl, C 2 to C 6 alkenyl or C 2 to C 6 alkynyl group; R 6 is a hydrogen atom or an unsubstituted or substituted C 1 to C 6 alkyl group; each R 3 moiety is the same or different and each is selected from a halogen atom, a C 1 to C 6 alkyl group, a C 1 to C 6 alkenyl group, a C 1 to C 6 alkynl group, a C 1 to C 6 alkoxy group, and -YC(R 4)(R 5)CO 2 R 6, and wherein when an R 3 moiety is -YC(R 4)(R 5)CO 2 R 6 it may be the same or different to R 2; wherein R 4, R 5 and R 6 are unsubstituted or substituted with 1, 2 or 3 substituents selected from a halogen atom, a hydroxyl group, C 1 to C 6 alkoxy and -NR 7 R 8 wherein R 7 and R 8 are the same or different and each is a C 1 to C 6 alkyl group; or a pharmaceutically acceptable salt or solvate thereof. Ar is an aryl group, wherein the aryl group is a C 6 to C 14 monovalent carbocyclic aromatic group derived from a single aromatic ring or multiple condensed aromatic rings, optionally further substituted with one or more groups R 3; A is a lipophilic, hydrophobic moiety; R 1 is: (a) a phosphodiester, wherein the phosphodiester is a group of the formula O=P (OR')OH, wherein R and R' are carbon containing groups bonded to oxygen through a carbon atom; (b) a phosphotriester, wherein the phosphotriester is a group of the formula O=P (OR')OR", wherein R, R' and R" are carbon containing groups bonded to oxygen through a carbon atom; (c) a thioether; or (d) a amide group, wherein the amide group is a group of the formula RC(=O)NR'R'' wherein R, R' and R" are either hydrogen or carbon containing groups bonded to carbon or nitrogen through a carbon atom; (a) a phosphodiester, wherein the phosphodiester is a group of the formula O=P (OR')OH, wherein R and R' are carbon containing groups bonded to oxygen through a carbon atom; (b) a phosphotriester, wherein the phosphotriester is a group of the formula O=P (OR')OR", wherein R, R' and R" are carbon containing groups bonded to oxygen through a carbon atom; (c) a thioether; or (d) a amide group, wherein the amide group is a group of the formula RC(=O)NR'R'' wherein R, R' and R" are either hydrogen or carbon containing groups bonded to carbon or nitrogen through a carbon atom; X is an unsubstituted or substituted C 6 to C 24 alkylene or alkenylene group, which is optionally interrupted by one or more -NR 9 -, -O- or -S- linkages, wherein R 9 is a C 1 to C 6 alkyl group; R 2 is -YC(R 4)(R 5)CO 2 R 6; Y is a covalent bond or -(CH 2) m - wherein m is 1, 2 or 3; R 4 and R 5 are the same or different and each is a substituted or unsubstituted C 1 to C 6 alkyl, C 2 to C 6 alkenyl or C 2 to C 6 alkynyl group; R 6 is a hydrogen atom or an unsubstituted or substituted C 1 to C 6 alkyl group; each R 3 moiety is the same or different and each is selected from a halogen atom, a C 1 to C 6 alkyl group, a C 1 to C 6 alkenyl group, a C 1 to C 6 alkynl group, a C 1 to C 6 alkoxy group, and -YC(R 4)(R 5)CO 2 R 6, and wherein when an R 3 moiety is -YC(R 4)(R 5)CO 2 R 6 it may be the same or different to R 2;