In the field of cancer management, small molecule and antibody drugs have been developed. The EGFR and HER2 receptors are good examples of target systems.

Small drug molecules (e.g. lapatinib) inhibit kinase activity. As such, downstream signalling is blocked and this prevents gene transcription and cell-cycle progression. The drug molecules called gefitinib (Iressa®), erlotinib and lapatinib inhibit kinase activity. The latter agent is a HER2 targeting kinase inhibitor with good selectivity for this protein-based receptor.

With the larger antibody molecules in mind, we have cetuximab for the EGFR system and trastuzumab (Herceptin®) for the HER2 system. The binding of the antibodies can naturally inhibit the binding of growth factors or change the overall appearance at the cell surface to prevent the association of the molecules altogether. The antibody activity is extracellular, as opposed to intracellular like the small molecules (e.g. lapatinib). These therapeutic agents do not enter the cell as they are too big. As such, they can only bind to the cell surface to inhibit neoplasm development.