Fc-based cytokines : prospects for engineering superior therapeutics.

The application of Fc (fragment crystallizable)-based cytokines (the fusion of the constant region of IgG to a cytokine of interest) as biotherapeutic agents to modulate inflammatory and immune responses has become increasingly popular in recent years. This is because in their monomeric form, cytokines are relatively small molecules with short serum half-lives, which necessitates frequent administration and thus limits their clinical utility. To rectify the problem, attempts have been made to improve the stability of these agents in vivo. This has been achieved through diverse strategies such as modification with polyethylene glycol (PEGylation) or by ligating the cytokine to protein moieties such as the constant heavy chain of IgG, known as the Fc fragment. The construction of Fc chimeric proteins has been shown to improve pharmacokinetics. However, since there is an inverse relationship between the size of molecules and the rate at which they diffuse through mucus, Fc fusion constructs potentially have a lower rate of diffusion. Consequently, a compromise is reached whereby Fc constructs are engineered to incorporate ligated cytokines in a monomeric form (one molecule of cytokine fused to a single Fc dimer) rather than in a dimeric form (two molecules of cytokine fused to a single Fc dimer). A recent and novel approach to improve stability in serum is a procedure that involves sheathing cytokines in protective protein covers called latency peptides. The enclosed cytokine is protected from degradation and allowed to act where needed when the outer peptide cover is removed. For some applications, a reduced serum half-life is desirable; for example, where there is a need to reduce IgG levels in antibody-mediated diseases. To achieve this goal, a strategy called AbDeg, which involves enhanced Ig degradation, has been devised. This article provides an overview of the design and construction of Fc-based cytokines, in both dimeric and monomeric forms. Several examples of recent applications of such constructs, which include cytokine antagonism, cytokine traps, gene therapy and drug delivery, are also discussed. Other antibody-engineered constructs such as Fab (fragment, antigen binding) and single chain Fv (fragment, variable) fusions are also briefly covered.