Erlotinib

 
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What is/are Erlotinib?

Erlotinib hydrochloride (trade name Tarceva) is a drug used to treat non-small cell lung cancer, pancreatic cancer and several other types of cancer. It is a reversible tyrosine kinase inhibitor, which acts on the epidermal growth factor receptor (EGFR). It is marketed in the United States by Genentech and OSI Pharmaceuticals and elsewhere by Roche. In lung cancer, it extends life by an average of 3.3 months.

Mechanism

Erlotinib is an EGFR inhibitor. The drug follows Iressa (gefitinib), which was the first drug of this type. Erlotinib specifically targets the epidermal growth factor receptor (EGFR) tyrosine kinase, which is highly expressed and occasionally mutated in various forms of cancer. It binds in a reversible fashion to the adenosine triphosphate (ATP) binding site of the receptor. For the signal to be transmitted, two EGFR molecules need to come together to form a homodimer. These then use the molecule of ATP to trans-phosphorylate each other on tyrosine residues, which generates phosphotyrosine residues, recruiting the phosphotyrosine-binding proteins to EGFR to assemble protein complexes that transduce signal cascades to the nucleus or activate other cellular biochemical processes. By inhibiting the ATP, formation of phosphotyrosine residues in EGFR is not possible and the signal cascades are not initiated.

Clinical applications

Erlotinib has shown a survival benefit in the treatment of lung cancer in phase III trials. The SATURN (Sequential Tarceva in Unresectable NSCLC) study found that erlotinib added to chemotherapy improved overall survival by 19%, and improved progression-free survival (PFS) by 29%, when compared to chemotherapy alone. The manufacturer estimated that erlotinib can extend life by approximately 3.3 months. Some researchers call the drug "marginally" cost effective. In a draft guidance, NICE has recommened Tarceva for lung cancer.

The U.S. Food and Drug Administration (FDA) has 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 November 2005, the FDA approved erlotinib in combination with gemcitabine for treatment of locally advanced, unresectable, or metastatic pancreatic cancer.

In lung cancer, erlotinib has been shown to be effective in patients with or without EGFR mutations, but appears to be more effective in the group of patients with EGFR mutations. A test for the EGFR mutation in cancer patients has been developed by Genzyme. The response rate among EGFR mutation positive patients is approximately 60%. 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.

EGFR positive patients are generally KRAS negative.

Erlotinib has recently been shown to be a potent inhibitor of JAK2V617F activity. JAK2V617F is a mutant of tyrosine kinase JAK2, is found in most patients with polycythemia vera (PV) and a substantial proportion of patients with idiopathic myelofibrosis or essential thrombocythemia. The study suggests that erlotinib may be used for treatment of JAK2V617F-positive PV and other myeloproliferative disorders.

The drug's US patent will expire in 2020. In May 2012, the US District Court of Delaware passed an order in favour of OSI Pharmaceutical LLC against Mylan Pharmaceuticals upholding the validity of the patent for Erlotinib. In India, generic pharmaceutical firm Cipla is battling with Roche against the Indian patent for this drug.

Interactions

Erlotinib is mainly metabolised by the liver enzyme CYP3A4. Compounds which induce this enzyme (i.e. stimulate its production), such as St John's wort, can lower erlotinib concentrations, while inhibitors can increase concentrations.

Resistance to treatment

As with other ATP competitive small molecule tyrosine kinase inhibitors, such as imatinib (Gleevec) 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). While proponents of the 'gatekeeper' mutation hypothesis suggest this mutation prevents the binding of erlotinib through steric hindrance, research suggests that T790M confers an increase in ATP binding affinity reducing the inhibitory effect of erlotinib.

Approximately 20% of drug resistance is caused by amplification of the hepatocyte growth factor receptor, which drives ERBB3 dependent activation of PI3K.

Other cases of resistance can involve numerous mutations, including recruitment of a mutated IGF-1 receptor to homodimerise with EGFR so forming a heterodimer. This allows activation of the downstream effectors of EGFR even in the presence of an EGFR inhibitor. Some IGR-1R inhibitors are in various stages of development (based either around TKIs such as AG1024 or AG538 or pyrrolo[2,3-d]-pyrimidine derivatives such as NVP-AEW541). The monoclonal antibody figitumumab which targets the IGF-1R is currently undergoing clinical trials.

Another cause of resistance can be inactivating mutations of the PTEN tumour suppressor which allow increased activation of Akt independent of stimulation by EGFR.

The most promising approach to combating resistance is likely to be combination therapy. Commencing treatment with a number of different therapeutic agents with differing modes of action is thought to provide the best defence against development of T790M and other resistance conferring mutations.

This article uses material from the Wikipedia article Erlotinib, which is released under the Creative Commons Attribution-Share-Alike License 3.0.

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