Erlotinib, sold under the brand name Tarceva among others, is a medication used to treat non-small cell lung cancer (NSCLC) and pancreatic cancer. Specifically it is used for NSCLC with mutations in the epidermal growth factor receptor (EGFR) — either an exon 19 deletion (del19) or exon 21 (L858R) substitution mutation — which has spread to other parts of the body. It is taken by mouth. Common side effects include rash, diarrhea, muscle pain, joint pain, and cough. Serious side effects may include lung problems, kidney problems, liver failure, gastrointestinal perforation, stroke, and corneal ulceration. Use in pregnancy may harm the baby. It is a receptor tyrosine kinase inhibitor, which acts on the epidermal growth factor receptor (EGFR). Erlotinib was approved for medical use in the United States in 2004. It is on the World Health Organization's List of Essential Medicines. Medical uses Lung cancer Erlotinib in unresectable non-small cell lung cancer when added to chemotherapy improves overall survival by 19%, and improved progression-free survival (PFS) by 29%, when compared to chemotherapy alone. The U.S. Food and Drug Administration (FDA) approved erlotinib for the treatment of locally advanced or metastatic non-small cell lung cancer that has failed at least one prior chemotherapy regimen. In lung cancer, erlotinib has been shown to be effective in patients with or without EGFR mutations, but appears to be more effective in patients with EGFR mutations. Overall survival, progression-free survival and one-year survival are similar to standard second-line therapy (docetaxel or pemetrexed). Overall response rate is about 50% better than standard second-line chemotherapy. Patients who are non-smokers, and light former smokers, with adenocarcinoma or subtypes like BAC are more likely to have EGFR mutations, but mutations can occur in all types of patients. A test for the EGFR mutation has been developed by Genzyme. Pancreatic cancer In November 2005, the FDA approved erlotinib in combination with gemcitabine for treatment of locally advanced, unresectable, or metastatic pancreatic cancer. Resistance to treatment As with other ATP competitive small molecule tyrosine kinase inhibitors, such as imatinib in CML, patients rapidly develop resistance. In the case of erlotinib this typically occurs 8–12 months from the start of treatment. Over 50% of resistance is caused by a mutation in the ATP binding pocket of the EGFR kinase domain involving substitution of a small polar threonine residue with a large nonpolar methionine residue (T790M). Approximately 20% of drug resistance is caused