The invention comprises an agent for use in medicine, wherein the agent comprises a compound of Formula (I): B-L-B′ wherein: B and B′ are independently selected from groups of formula (B-I) wherein: Z is selected from —COOH, —CH2COOH, —PO(OH)(OR1), or —CH2PO(OH) (OR1), wherein R1 is H or a phosphate protecting group; W is an alicyclic amine group having from 5 to 12 carbon atoms and at least one amine nitrogen atom; W′ is H, or W′ is linked to W to form said alicyclic amine group; and Y is selected from —NH—, —N(CH3)—, —CH2—, —NHCO—, —CH2CONH—, —CONH—, CH2NHCO—, or —NHCH2—; and L is a linker group.