Conjugates And Conjugating Reagents

A conjugate of a protein or peptide with a therapeutic, diagnostic or labelling agent, said conjugate containing a protein or peptide bonding portion and a polyethylene glycol portion;

• in which said protein or peptide bonding portion has the general formula:
• in which Pr represents said protein or peptide, each Nu represents a nucleophile present in or attached to the protein or peptide, each of A and B independently represents a C1-4alkylene or alkenylene chain, and W′ represents an electron withdrawing group or a group obtained by reduction of an electron withdrawing group;
• wherein the conjugate comprises a linker between the therapeutic, diagnostic or labelling agent and the group of formula (I), which linker includes a portion comprising an optionally substituted aryl or heteroaryl group immediately adjacent the group of formula (I), and also a —NRa.C(O)— or —C(O).NRa— group adjacent said aryl or heteroaryl group, said portion having the formula
• wherein Ra represents C1-4 alkyl or hydrogen;
• and in which said polyethylene glycol portion is or includes a pendant polyethylene glycol chain which has a terminal end group of formula —CH2CH2OR in which R represents a hydrogen atom, an alkyl group, or an optionally substituted aryl group.