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Erlotinib, launched as once daily oral treatment for patients with non-small-cell lung cancer (NSCLC), is an inhibitor of the epidermal growth factor receptor (EGFR) tyrosine kinase, and it is the second small-molecule drug to be marketed with this mechanism of action. Both erlotinib and its predecessor, gefitinib, are members of the anilinoquinazoline class of tyrosine kinase inhibitors. They compete with the binding of ATP to the intracellular tyrosine kinase domain of EGFR, thereby inhibiting receptor autophosphorylation and blocking downstream signal transduction. Erlotinib is prepared by the condensation of 3-ethynylaniline with 4-chloro-6,7-bis(2-methoxyethoxy)quinazoline, which is a key intermediate obtained in five synthetic steps starting from ethyl 3,4- dihydroxybenzoate. In vitro, Erlotinib inhibits purified human EGFR tyrosine kinase with an IC₅₀ of 2 nM and blocks EGFR autophosphorylation in cellular assays with an IC₅₀ of 20nM. Treatment of human colon cancer cells with erlotinib was associated with growth inhibition, G1 cell cycle arrest, and apoptosis. Oral administration of erlotinib in athymic mice produced potent antitumor effects with an ED₅₀ of 9.2 mg/kg/day for HN5 head and neck xenografts and 14 mg/ kg/day for A431 epidermoid xenografts. The absorption of Erlotinib following oral dosing is approximately 60%. Food greatly enhances the absorption allowing for almost 100% bioavailability of the dose. The time to reach peak plasma levels of the drug is about 4 hours, and the half-life is approximately 36 hours. Steady-state drug levels are reached in 7 to 8 days. Erlotinib has high protein binding (93%) and has an apparent volume of distribution of 232 L. It is metabolized primarily by CYP3A4 and to a lesser extent by CYP1A2 and CYP1A1. The drug is mainly excreted in the feces with less than 9% of the dose found in the urine. Erlotinib is labeled for the treatment of patients with locally advanced or metastatic NSCLC who have failed one or more previous chemotherapy regimens. The recommended dosage is 150 mg daily until disease progression is detected. In a randomized, double blind, placebo-controlled trial involving 731 patients, 150 mg/day oral dose of erlotinib resulted in a median overall survival of 6.7 months compared with 4.7 months in the placebo group (p＜0.001). Progression-free survival was 9.9 weeks and 7.9 weeks in the erlotinib and placebo groups, respectively (p＜0.001). Survival at one year was 31.2% in the erlotinib group versus 21.5% in the placebo group. The use of erlotinib showed greater benefit in patients with EGFR positive tumors and in those who never smoked. The most common adverse events reported in clinical trials were rash (9%) and diarrhea (6%). Elevations in liver function tests were also seen; however, these effects were mainly transient or associated with liver metastases. As previously noted for gefitinib, erlotinib is also shown to lack any clinical benefit in concurrent administration with platinum-based chemotherapy.