Improving the efficacy of antibody-interleukin 2 fusion proteins by reducing their interaction with Fc receptors.
Fusion proteins between whole antibodies (Abs) and cytokines (immunocytokines) such as interleukin 2 have shown efficacy in several mouse tumor models despite a circulating half-life that is significantly shorter than that of the original Ab. We have examined the potential mechanisms responsible for clearance and shown that an important factor is enhanced binding to Fc receptor (FcR). Improvements in the half-lives of two different immunocytokines were made by changing the isotype of the human heavy chain C region from IgG1 or IgG3 to those with reduced binding to FcR, e.g., IgG4. The same effect could also be achieved through site-directed mutagenesis of the FcR binding site in the IgG1 H chain. In vitro studies using mouse J774 FcR-expressing cells showed increased binding of interleukin 2-based immunocytokines, relative to their corresponding Abs, and that this was reversed in those fusion proteins made with IgG4 or mutated IgG1 H chains. All of the fusion proteins showing reduced FcR binding also had reduced Ab-dependent cellular cytotoxicity activity, as measured in 4-h chromium release assays. A complete loss of complement-dependent cytotoxicity activity was seen with an IgG4-based immunocytokine derived from an IgG1 Ab with potent activity. Despite these reduced effector functions, the IgG4-based immunocytokines with extended circulating half-lives showed equivalent (in the case of severe combined immunodeficiency mouse xenograft models) or better (in the case of syngeneic models) efficacy in mouse tumor models than the original IgG1-based molecules. These novel immunocytokines may show improved efficacy in therapeutic situations where T cell- rather than natural killer- or complement-mediated antitumor mechanisms are involved.