Reversible covalent linkage of functional molecules

1. A compound of formula (IIIa)

wherein:

R 1 is a peptide or protein containing a first cysteine residue and a second cysteine residue, R 1 is attached at the 2-position in the compound of formula (IIIa) by a thiol bond —S—which is derived from the thiol group in said first cysteine residue, and R 1 is attached at the 3-position in the compound of formula (IIIa) by a thiol bond —S— which is derived from the thiol group in said second cysteine residue;

each R 33a′ is the same or different and represents a hydrogen atom or a group of formula Y, Nu, -L(Z) n , IG, F 2 or -L(F 2 ) m (Z) n-m ;

F 2 is a functional moiety;

each group of formula Y is the same or different and represents an electrophilic leaving group;

each group of formula Nu is the same or different and represents a nucleophile selected from —OH, —SH, —NH 2 and —NH(C 1-6 alkyl);

each group of formula L is the same or different and represents a linker group;

each group of formula Z is the same or different and represents a reactive group attached to a group of formula L which is capable of reacting with a compound containing a functional moiety F 2 such that said functional moiety F 2 becomes linked to said group of formula L;

n is 1, 2 or 3;

each group of formula IG is the same or different and represents a moiety which is a C 1-20 alkyl group, a C 2-20 alkenyl group or a C 2-20 alkynyl group, which is unsubstituted or substituted by one or more substituents selected from halogen atoms and sulfonic acid groups, and in which (a) 0, 1 or 2 carbon atoms are replaced by groups selected from C 6-10 arylene, 5- to 10-membered heteroarylene, C 3-7 carbocyclylene and 5- to 10-membered heterocyclylene groups, and (b) 0, 1 or 2 —CH 2 — groups are replaced by groups selected from —O—, —S—, —S—S—, —C(O)— and —N(C 1-6 alkyl)- groups, wherein:

(i) said arylene, heteroarylene, carbocyclylene and heterocyclylene groups are unsubstituted or substituted by one or more substituents selected from halogen atoms and C 1-6 alkyl, C 1-6 alkoxy, C 1-6 alkylthiol, —N(C 1-6 alkyl)(C 1-6 alkyl), nitro and sulfonic acid groups; and

(ii) 0, 1 or 2 carbon atoms in said carbocyclylene and heterocyclylene groups are replaced by —C(O)— groups; and

m is an integer having a value of from zero to n.

2. A compound according to claim 1 , which comprises at least one group of formula F 2 .

3. A compound according to claim 1 , wherein the first cysteine residue of R 1 and the second cysteine residue of R 1 in the compound of formula (IIIa) are derived from an internal disulfide bridge formed in the peptide or protein.

4. A compound according to claim 1 , wherein R 1 is an antibody or an antibody fragment that is capable of binding to a specific antigen via an epitope on the antigen.

5. A compound according to claim 2 , wherein F 2 is a drug.

6. A compound according to claim 5 , wherein the drug is a cytotoxic agent.

7. A compound according to claim 1 , wherein at least one of the groups R 33a′ , represents a group of formula F 2 or -L(F 2 ).

8. A compound according to claim 7 , wherein one of the groups R 33a′ represents a group of formula F 2 or -L(F 2 ) and the other group R 33a′ represents a hydrogen atom or a group of formula IG.

9. A compound according to claim 5 , wherein at least one of the groups R 33a′ represents a group of formula F 2 or -L(F 2 ).

10. A compound according to claim 9 , wherein one of the groups R 33a′ represents a group of formula F 2 or -L(F 2 ) and the other group R 33a′ represents a hydrogen atom or a group of formula IG.