Multispecific antibodies

1. A complex comprising

(A) a multispecific antibody comprising at least three antigen binding sites,

wherein two antigen binding sites are formed by a first Fab fragment and a second Fab fragment, and

wherein:

(a) a third antigen binding site is formed by a variable heavy chain domain (VH3) and a variable light chain domain (VL3),

wherein the N-terminus of the VH3 domain is connected to the C-terminus of the constant heavy chain domain (CH1) or the constant light chain domain (CL) of the first Fab fragment via a first peptide connector, and

wherein the N-terminus of the VL3 domain is connected to the C-terminus of the constant heavy chain domain (CH1) or the constant light chain domain (CL) of the second Fab fragment via a second peptide connector

wherein the third binding site is disulfide stabilized by introduction of cysteine residues at the following positions to form a disulfide bond between the VH 3 and VL 3 domains (numbering according to Kabat):

(i) VH 3 at position 44, and VL 3 at position 100;

(ii) VH 3 at position 105, and VL 3 at position 43; or

(iii) VH 3 at position 101, and VL 3 at position 100;

(b) the multispecific antibody comprises a first and a second constant heavy chain domains 3 (CH3), which are altered to promote heterodimerization by:

(i) generation of a protuberance in the first CH3 domain by substituting at least one original amino acid residue by an amino acid residue having a larger side chain volume than the original amino acid residue, and generation of a cavity in the second CH3 domain by substituting at least one original amino acid residue by an amino acid residue having a smaller side chain volume than the original amino acid residue, such that the protuberance generated in the first CH3 domain is positionable in the cavity generated in the second CH3 domain; or

substituting at least one original amino acid residue in the first CH3 domain by a positively charged amino acid, and substituting at least one original amino acid residue in the second CH3 domain by a negatively charged amino acid;

(ii) introduction of at least one cysteine residue in each CH3 domain such that a disulfide bond is formed between the CH3 domains; or

(iii) both modifications of (i) and (ii);

(c) the C-terminus of the VH13 domain of the third antigen binding site is connected to the first CH3 domain, and the C-terminus of the VL3 domain of the third antigen binding site is connected to the second CH3 domain; and

(d) the multispecific antibody is devoid of constant heavy chain domains 2 (CH2) and wherein the multispecific antibody specifically binds to a hapten and a target protein, and

(B) the hapten, wherein the hapten is conjugated to a therapeutic or diagnostic agent.

2. The complex according to claim 1 , wherein the first and second peptide connectors are peptides of at least 15 amino acids.

3. The complex according to claim 1 , wherein no interchain disulfide bond is formed between the first and the second peptide connectors.

4. The complex according to claim 1 , wherein the C-terminus of the VH 3 domain is directly connected to the first CH3 domain, and the C-terminus of the VL 3 domain is directly connected to the second CH3 domain.

5. The complex according to claim 1 , wherein the multispecific antibody is trivalent.

6. The complex according to claim 1 , wherein the multispecific antibody is bispecific or trispecific.

7. A pharmaceutical composition comprising the complex of claim 1 , in combination with at least one pharmaceutically acceptable carrier.