Imatinib

Imatinib is a drug used to treat certain types of cancer. It is currently marketed by Novartis as Gleevec® (USA) or Glivec® (Europe/Australia) as its mesylate salt, imatinib mesilate (INN). It is occasionally still referred to as CGP57148B or STI571 (especially in older publications). It is used in treating chronic myelogenous leukaemia (CML), gastrointestinal stromal tumours (GISTs) and a number of other malignancies.

It is the first member of a new class of agents that act by inhibiting particular tyrosine kinase enzymes, instead of non-specifically inhibiting rapidly dividing cells.

Molecular biology

Imatinib is a 2-phenylaminopyrimidine derivative that functions as a specific inhibitor of a number of tyrosine kinase enzymes. It occupies the TK active site, leading to a decrease in activity.

There are a large number of TK enzymes in the body, including the insulin receptor. Imatinib is specific for the TK domain in abl (the Abelson proto-oncogene), c-kit and PDGF-R (platelet-derived growth factor receptor).

In chronic myelogenous leukaemia, the Philadelphia chromosome leads to a fusion protein of abl with bcr (breakpoint cluster region), termed bcr-abl. As this is now a continuously active tyrosine kinase, imatinib is used to decrease bcr-abl activity.
Mechanism of action of imatinib

The active sites of tyrosine kinases each have a binding site for ATP. The enzymatic activity catalyzed by a tyrosine kinase is the transfer of the terminal phosphate from ATP to tyrosine residues on its substrates, a process known as protein tyrosine phosphorylation. Imatinib works by binding to the ATP binding site of bcr-abl and inhibiting the enzyme activity of the protein competitively. Imatinib is quite selective for bcr-abl – it does also inhibit other targets mentioned above (c-kit and PDGF-R), but no other known tyrosine kinases. Imatinib also inhibits the abl protein of non-cancer cells but cells normally have additional redundant tyrosine kinases which allow them to continue to function even if abl tyrosine kinase is inhibited. Some tumour cells, however, have a dependence on bcr-abl (Deininger and Druker, 2003). Inhibition of the bcr-abl tyrosine kinase also stimulates its entry in to the nucleus, where it is unable to perform any of its normal anti-apoptopic functions (Vigneri et al 2001).

Uses

Imatinib is used in chronic myelogenous leukaemia (CML), gastrointestinal stromal tumours (GISTs) and a number of other malignancies. One study demonstrated that Imatinib mesylate was effective in patients with systemic mastocytosis, including those who had the D816V mutation (Droogendijk et al, 2006). Early clinical trials also show its potential for treatment of hypereosinophilic syndrome and dermatofibrosarcoma protuberans.

In laboratory settings, imatinib is being used increasingly as an experimental agent to suppress platelet-derived growth factor (PDGF) by inhibiting its receptor (PDGF-Rβ). One of its effects is delaying atherosclerosis in mice with diabetes (Lassila 2004).

Recent mouse animal studies at Emory University in Atlanta have suggested that imatinib and related drugs may be useful in treating smallpox, should an outbreak ever occur (Reeves et al 2005).

Gleevec is also being used in the treatment of certain brain tumours to include high grade glioblastoma.

Tolerability and side effects

In the United States, the Food and Drug Administration has approved imatinib as first-line treatment for CML (Deininger and Druker 2003). Imatinib has passed through Phase III trials for CML, and has been shown to be more effective than the previous standard treatment of α-interferon and cytarabine. Although the long-term side effects of imatinib have not yet been ascertained, research suggests that it is generally very well tolerated (eg. liver toxicity was much less than predicted). Broadly, side effects such as oedema, nausea, rash and musculoskeletal pain are common but mild.

Severe congestive cardiac failure is an uncommon but recognised side effect of imatinib and mice treated with large doses of imatinib show toxic damage to their myocardium.

Metabolism

Metabolism of imatinib occurs in the liver and the main metabolite, N-demethylated piperazine derivative, is also active. The major route of elimination is in the bile, only a small portion is excreted in the urine. Most of imatinib is eliminated as metabolites, only 25% is eliminated unchanged. The half-lives of imatinib and its main metabolite are 18 and 40 hours, respectively.

History

Imatinib was identified in the late 1990s by Dr Brian J. Druker for the pharmaceutical company Novartis as a potential agent for the use in CML. Its development is the template for rational drug design. Soon after identification of the bcr-abl target, the search for an inhibitor began. Chemists used a high-throughput screen of chemical libraries to identify the molecule 2-phenylaminopyrimidine. This lead compound was then tested and modified by the introduction of methyl and benzamide groups to give it enhanced binding properties, resulting in imatinib (Druker and Lydon 2000).

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