Modified antibody with improved half-life

An isolated antibody, immunoglobulin construct or immunoglobulin IgG4 fusion protein with increased in vivo half-life, comprising:

• (i) a human IgG4 Fc region or the FcRn binding domain thereof modified relative to a corresponding unmodified IgG4 Fc region or FcRn binding domain thereof to comprise substitutions at one or more of amino acid residues 251-256 numbered according to the EU index as in Kabat;
• and
• (ii) a human IgG4 core hinge region sequence comprising a substitution of the serine residue within the amino acid sequence CPSCP (SEQ ID NO:1) to proline;
• wherein the in vivo half-life of the modified antibody, immunoglobulin construct or immunoglobulin IgG4 fusion protein is increased compared with the corresponding unmodified antibody, immunoglobulin construct or immunoglobulin IgG4 fusion protein.