Gefitinib is the first selective inhibitor of epidermal growth factor receptor's tyrosine kinase domain. Thus gefitinib is an EGFR inhibitor. The target protein is a member of a family of receptors which includes Her1, Her2, Her3 and Her4. EGFR is overexpressed in the cells of certain types of human carcinomas - for example in lung and breast cancers. This leads to inappropriate activation of the anti-apoptotic Ras signalling cascade, eventually leading to uncontrolled cell proliferation. Research on gefitinib-sensitive non-small cell lung cancers has shown that a mutation in the EGFR tyrosine kinase domain is responsible for activating anti-apoptotic pathways. These mutations tend to confer increased sensitivity to tyrosine kinase inhibitors such as gefitinib and erlotinib. Of the types of non-small cell lung cancer histologies, adenocarcinoma is the type that most often harbors these mutations. These mutations are more commonly seen in Asians, women, and non-smokers. Gefitinib inhibits EGFR tyrosine kinase by binding to the adenosine triphosphate -binding site of the enzyme. Thus the function of the EGFR tyrosine kinase in activating the anti-apoptotic Ras signal transduction cascade is inhibited, and malignant cells are inhibited.
Clinical uses
Gefitinib is currently marketed in over 64 countries. Iressa was approved and marketed from July 2002 in Japan, making it the first country to import the drug. The FDA approved Gefitinib in May 2003 for non-small cell lung cancer. It was approved as monotherapy for the treatment of patients with locally advanced or metastatic NSCLC after failure of both platinum-based and docetaxel chemotherapies. i.e. as a third-line therapy. In June 2005 the FDA withdrew approval for use in new patients due to lack of evidence that it extended life. In Europe gefitinib is indicated since 2009 in advanced NSCLC in all lines of treatment for patients harbouring EGFR mutations. This label was granted after gefitinib demonstrated as a first-line treatment to significantly improve progression-free survival vs. a platinum doublet regime in patients harbouring such mutations. IPASS has been the first of four phase III trials to have confirmed gefitinib superiority in this patient population. In most of the other countries where gefitinib is currently marketed it is approved for patients with advanced NSCLC who had received at least one previous chemotherapy regime. However, applications to expand its label as a first-line treatment in patients harbouring EGFR mutations is currently in process based on the latest scientific evidence. As at August 2012 New Zealand has approved gefitinib as first-line treatment for patients with EGFR mutation for naive locally advanced or metastatic, unresectable NSCLC. This is publicly funded for an initial 4-month term and renewal if no progression. On July 13, 2015, the FDA approved gefitinib as a first-line treatment for NSCLC.
IPASS was a randomized, large-scale, double-blinded study which compared gefitinib vs. carboplatin/ paclitaxel as a first-line treatment in advanced NSCLC. IPASS studied 1,217 patients with confirmed adenocarcinoma histology who were former or never smokers. A pre-planned sub-group analyses showed that progression-free survival was significantly longer for gefitinib than chemotherapy in patients with EGFR mutation positive tumours, and significantly longer for chemotherapy than gefitinib in patients with EGFR mutation negative tumours. This, in 2009, was the first time a targeted monotherapy has demonstrated significantly longer PFS than doublet chemotherapy.
EGFR diagnostic tests
Roche Diagnostics, Genzyme, QIAGEN, Argenomics S.A. & other companies make tests to detect EGFR mutations, designed to help predict which lung cancer patients may respond best to some therapies, including gefitinib and erlotinib. The tests examine the genetics of tumors removed for biopsy for mutations that make them susceptible to treatment. The EGFR mutation test may also help AstraZeneca win regulatory approval for use of their drugs as initial therapies. Currently the TK inhibitors are approved for use only after other drugs fail. In the case of gefitinib, the drug works only in about 10% of patients with advanced non-small cell lung cancer, the most common type of lung cancer.
