Monoclonal antibodies targeting cancer: 'magic bullets' or just the trigger?

The first monoclonal antibodies (mAbs) approved for cancer therapy are now in Phase II and III trials, but the critical mechanism(s) determining efficacy and response in patients are still largely undefined. Both the direct antigen-binding (Fab) and constant (Fc) regions of mAbs can contribute to their biological activity. However, Clynes et al (Nat Med 2000, 6:443) recently suggested that the latter (at least in experimental models) might be the dominant component in vivo, triggering host responses to destroy cancer cells. Those workers showed that in mice lacking 'activation' Fc receptors (FcγRI and FcγRIII), anti-tumour effects of certain mAbs were significantly reduced. In contrast, mice deficient in the 'inhibitory' receptor FcγRIIB responded with tumour growth inhibition and enhanced antibody-dependent cellular cytotoxicity (ADCC). These observations suggest that mAbs might be engineered for preferential binding to FcγRIII to maximise therapeutic benefit. However, further work is needed to establish a definitive cause-effect relationship in experimental models that are more clinically relevant, to determine whether human FcγR isoforms behave in a similar fashion, and to confirm that therapeutic mAbs and host cells can adequately access solid tumour deposits to mediate effective ADCC in situ. Finally, the 'cost-benefit' ratio of such modified macromolecules will need to be measured against mini-mAb constructs, antisense oligonucleotides, peptidomimetics and emerging drugs capable of inhibiting key tumour cell signalling pathways.