The invention provides a compound which is a polyunsaturated fatty acid (PUFA) derivative of formula (I),or a pharmaceutically acceptable salt, or solvate thereof, wherein- -Alk- is -CH(OR<;sub>;2<;/sub>;)-[trans]CH=CH-[cis]CH=CH-CH<;sub>;2<;/sub>;-[cis]CH=CH-C<;sub>;3<;/sub>;H<;sub>;6<;/sub>;-,-(CH<;sub>;2<;/sub>;)<;sub>;3<;/sub>;-CH(OR<;sub>;2<;/sub>;)-[trans]CH=CH-[cis]CH=CH-CH<;sub>;2<;/sub>;-[cis]CH=CH-, -(CH<;sub>;2<;/sub>;)<;sub>;3<;/sub>;-[cis]CH=CH-CH<;sub>;2<;/sub>;-[cis]CH=CH-[trans]CH=CH-CH(OR<;sub>;2<;/sub>;)- or -CH<;sub>;2<;/sub>;-[cis]CH=CH-CH<;sub>;2<;/sub>;-[cis]CH=CH-[trans]CH=CH-CH(OR<;sub>;2<;/sub>;)-;- R<;sub>;1<;/sub>;is a hydrogen atom; orR<;sub>;1<;/sub>;is a C<;sub>;1<;/sub>;-C<;sub>;6<;/sub>;alkyl, C<;sub>;2<;/sub>;-C<;sub>;6<;/sub>;alkenyl, C<;sub>;2<;/sub>;-C<;sub>;6<;/sub>;alkynyl, C<;sub>;6<;/sub>;-C<;sub>;10<;/sub>;aryl, 5- to 10-membered heteroaryl, C<;sub>;3<;/sub>;-C<;sub>;7<;/sub>;carbocyclyl or 5- to 10-membered heterocyclyl group; or R<;sub>;1<;/sub>;is a group of formula -CH<;sub>;2<;/sub>;-CH(OR<;sub>;3<;/sub>;)-CH<;sub>;2<;/sub>;-(OR<;sub>;4<;/sub>;), wherein R<;sub>;3<;/sub>;and R<;sub>;4<;/sub>;are each independently hydrogen atoms or -(C=O)-R<;sub>;6<;/sub>;, wherein R<;sub>;6<;/sub>;is an aliphatic group having from 3 to 29 carbon atoms; orR<;sub>;1<;/sub>;is a group of formula -(CH<;sub>;2<;/sub>;OCH<;sub>;2<;/sub>;)<;sub>;m<;/sub>;OH, wherein m is an integer of from 1 to 200;- R<;sub>;2<;/sub>;is a hydrogen atom; orR<;sub>;2<;/sub>;is a group -(C=O)-R<;sub>;5<;/sub>;, wherein R<;sub>;5<;/sub>;is a C<;sub>;1<;/sub>;-C<;sub>;6<;/sub>;alkyl, C<;sub>;2<;/su