Imatinib mesylate: in the treatment of gastrointestinal stromal tumours

Imatinib mesylate (imatinib) is an orally administered competitive inhibitor of the tyrosine kinases associated with the KIT protein (stem cell factor receptor), ABL protein and platelet-derived growth factor receptors. The KIT tyrosine kinase is abnormally expressed in gastrointestinal stromal tumour (GIST), a rare neoplasm for which there has been no effective systemic therapy. In a randomised, nonblind, multicentre study that evaluated imatinib 400 or 600mg once daily in 147 patients with advanced GIST, confirmed partial responses were achieved in 54% of patients overall (median duration of follow-up was 288 days). Stable disease was experienced by 28% of patients and the estimated 1-year survival rate was 88%. Similar response rates were reported in a smaller, dose-escalation study, in which objective tumour response was a secondary endpoint. Although nearly all patients with GIST treated with imatinib experienced adverse events, most events were mild or moderate in nature. Severe or serious adverse events occurred in 21% of patients in the larger study, and included gastrointestinal or tumour haemorrhage. The control of cellular processes, such as cell growth, division and death, involves signal transduction, which commonly involves the transfer of phosphate from adenosine triphosphate (ATP) to tyrosine residues on substrate proteins, by tyrosine kinase enzymes. Activation of oncogenes coding for kinase proteins can lead to the production of kinases that are continually active in the absence of a normal stimulus,leading to increased cell proliferation and/or decreased apoptosis. A major focus of cancer research in recent years has been to identify oncogenic molecules and the signal transduction pathways in which they are involved, in order to develop specifically targeted drugs. One such drug is imatinib mesylate (imatinib, Glivic/Gleevec), an orally administered 2-phenylaminopyrimidine derivative that is a competitive inhibitor of the tyrosine kinases associated with platelet-derived growth factor (PDGF) receptors, the Abelson (ABL) protein and the KIT protein (also known as stem cell factor [SCF] receptor). Imatinib was initially evaluated for the treatment of chronic myeloid leukaemia (CML) [reviewed previously in Drugs]. More recently, imatinib has been approved for the treatment of patients with advanced gastrointestinal stromal tumour (GIST), in which KIT, a tyrosine kinase receptor, is abnormally expressed. GISTs are soft tissue gastrointestinal sarcomas probably arising from mesenchymal cells. They are rare neoplasms, with between 5000 and 10 000 new cases being diagnosed each year in the US. GISTs occur throughout the gastrointestinal tract but the stomach and small intestine are the most common sites. Symptoms depend on the site and size of the tumour, and may include abdominal pain, gastrointestinal bleeding or signs of obstruction; small tumours may be asymptomatic. The diagnosis of GIST is made by immunohistochemical staining for CD117, a cell surface antigen on the extracellular domain of KIT, in conjunction with pathological examination of tissue with light microscopy. All GISTs may have some degree of malignant potential. They are unresponsive to standard chemotherapy and to radiotherapy, and the mainstay of treatment in the past has been surgery. However, recurrence rates are high, and there has been no effective systemic treatment for unresectable GIST or metastatic disease. For patients in whom complete resection is not possible, or in patients with metastatic or recurrent disease, the median duration of survival is 9-12 months, and 10-19 months, respectively. Gain-of-function mutations of the KIT proto-oncogene occur in up to 90% of GISTs, allowing constitutive activation of tyrosine kinase (i.e. auto-phosphorylation of tyrosine residues independent of ligand-receptor binding), leading to aberrant cell division and tumour growth. Imatinib selectively inhibits the tyrosine kinase activity associated with KIT, which forms the rationale for evaluating its effects in GIST. Subsequent to initial evidence of the clinical efficacy of imatinib in a single patient with progressive, metastatic, CD117-positive GIST, formal studies of imatinib in this new indication were initiated. This article summarises the pharmacology, efficacy and tolerability profile of imatinib in the treatment of patients with advanced GIST.