Cancer therapeutic antibodies come of age: Targeting minimal residual disease

Timeline of discoveries leading to the burgeoning clinical application of cancer therapeutic antibodies.

Mechanisms of action of therapeutic antibodies. Binding of mAbs (orange) to antigens (green) on target cells can induce complement binding (via the C1 component) and activate ADCC, which requires interaction between the Fc portion of the antibody and FcγR molecules on effector cells, such as natural killer (NK) lymphocytes. Angiogenesis and cell growth are inhibited through downregulation of the vascular endothelial growth factor (VEGF) and mAbs interfering with growth factor (GF) binding, respectively. Intracellular degradation of surface antigens is preceded by endocytosis (internalization) of antigen-bound mAbs (e.g., Trastuzumab). In the case of Rituximab, apoptosis occurs upon tyrosine kinase signaling and mobilization of intracellular calcium.

Generation of therapeutic monoclonal antibodies (mAbs). Despite their early promise, murine antibodies (red) produced by hybridomas are largely unsuccessful as therapeutic reagents, due to both insufficient activation of human effector functions and immune reactions against proteins of rodent origin. Chimeric antibodies are generated by fusion of mouse-derived variable (V) regions to human constant (C) regions (human antibodies are shown in gray). The chimeric antibody molecule retains binding affinity for an antigen, and acquires the function of the substituted human C regions, allowing more efficient interaction with human CDC and ADCC, as well as less immunogenic reactions compared to murine C regions. Humanization of murine antibodies offers an alternative strategy. Humanized antibodies are generated by grafting the mouse complementarity-determining regions (CDRs; dark red) into the human V-framework regions and expressed with human C regions. This procedure decreases immunogenicity of the mAb. Unlike chimeric and humanized antibodies, the production of fully human antibodies does not rely on a parental murine antibody. Strategies for the production of fully human antibodies include the use of phage display antibody libraries or transgenic animals, both utilizing human V region repertories. Recombinant fragments of murine or human antibodies offer other advantages. They include a variable fragment (Fv) and single chain Fv (scFv) molecules. The small scFvs are particularly interesting for clinical application, since they are relatively small, thus have lower retention times in non-target tissues, and they better penetrate into tumors. Bivalent scFv fragments may be produced by adding a carboxyl-terminal cysteine.