Epidermal Growth Factor Receptor Tyrosine Kinase Inhibitors: Similar but Different?

Two small-molecule epidermal growth factor receptor tyrosine kinase inhibitors, gefitinib and erlotinib, have been approved for the treatment of non-small-cell lung cancer. Here, we compare the pharmacology and pharmacokinetics of these agents, and reflect on how these properties may affect important clinical questions including the clinical efficacy, optimum dose, and whether there is a relationship between skin rash and clinical outcome for each of these agents. Gefitinib and erlotinib have similar mechanisms of action and pharmacological profiles; however, different molecular structures confer pharmacokinetic differences that may have important clinical implications. Although gefitinib 250 mg/day produces lower mean plasma concentrations and area under the plasma concentration versus time curve compared with erlotinib 150 mg/day, published data suggest that gefitinib significantly accumulates in tumour tissue. This difference may partly explain why it seems possible to achieve maximum clinical efficacy with gefitinib at doses significantly lower than its maximum tolerated dose and, hence, use of an optimal biological dose approach with this agent. We hypothesize that gefitinib is used and is effective at a dose below the maximum tolerated dose as it accumulates in tumour tissue, thus providing the concentration needed at its target to achieve effective epidermal growth factor receptor inhibition in the tumour while causing less skin toxicity than erlotinib; therefore, skin rash is not a useful predictive factor for efficacy with gefitinib.