Tyrosine kinase inhibitors in the treatment of unresectable or metastatic gastrointestinal stromal tumors

Introduction: Gastrointestinal stromal tumor (GIST) is the most common sarcoma of the gastrointestinal tract. Proliferation of GIST is driven by activating mutations in the KIT or PDGFRA genes that found in most sporadic GISTs. Surgery is the main remedial measure for primary GIST, and imatinib is the principal therapeutic of choice for unresectable or metastatic GIST. Imatinib revolutionized treatment for unresectable or metastatic GISTs; however, resistance to imatinib has inevitably developed for most GIST patients.

Areas covered: PubMed was searched to find biological studies of GIST and clinical trials of molecularly targeted agents on unresectable or metastatic GISTs, and the key papers found have been reviewed. In this paper, the standard therapy which includes imatinib, sunitinib and regorafenib for unresectable or metastatic GIST has been reviewed and molecular mechanisms of resistance for tyrosine kinase inhibitors (TKIs) have been postulated and discussed. Treatment measures for resistant GIST and therapeutic choices after the standard therapy have also been described.

Expert opinion: The standard therapy for unresectable or metastatic GISTs is first-line imatinib, second-line sunitinib and third-line regorafenib. After standard therapy, best supportive care or clinical trials is recommended in the guidelines. However, patients may benefit from continuation of TKIs beyond disease progression and from rechallenge of TKIs used previously.     

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.     

Molecular targeted therapy of gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GIST) are mesenchymal tumors that occur predominantly in the stomach and the small bowel. Their pathogenesis is generally based on primary activating mutations in the KIT or PDGFRα genes that result in constitutive activation of receptor tyrosine kinase activity. Imatinib, first designed to competitively inhibit the ATP-binding pocket of the BCR-ABL tyrosin kinase exhibits inhibition also in the KIT and PDGFRα tyrosine kinases, which revolutionized the therapy of gastrointestinal stromal tumors, a disease without any systemic treatment options prior to imatinib. Clinical benefit is achieved in approximately 85% of patients with unresectable or metastatic disease with a median progression-free survival of 19 to 26 months and an overall survival approaching 5 years. Disease progression results from different mechanisms of resistance most frequently involving the emergence of secondary mutations in KIT exons 13, 14, or 17. Several newer drugs have been studied in patients failing or being intolerant to imatinib, including the multitargeted agent sunitinib as well as other KIT targeting tyrosine kinase inhibitors like nilotinib or agents targeting alternative pathways like anti-angiogenic agents, mTOR-, RAF kinase- and chaperone inhibitors.     

Beyond standard therapy: drugs under investigation for the treatment of gastrointestinal stromal tumor

Introduction: Gastrointestinal stromal tumor (GIST) is the most common nonepithelial malignancy of the GI tract. With the discovery of KIT and later platelet-derived growth factor α (PDGFRA) gain-of-function mutations as factors in the pathogenesis of the disease, GIST was the quintessential model for targeted therapy. Despite the successful clinical use of imatinib mesylate, a selective receptor tyrosine kinase (RTK) inhibitor that targets KIT, PDGFRA and BCR-ABL, we still do not have treatment for the long-term control of advanced GIST.

Areas covered: This review summarizes the drugs that are under investigation or have been assessed in trials for GIST treatment. The article focuses on their mechanisms of actions, the preclinical evidence of efficacy, and the clinical trials concerning safety and efficacy in humans.

Expert opinion: It is known that KIT and PDGFRA mutations in GIST patients influence the response to treatment. This observation should be taken into consideration when investigating new drugs. RECIST was developed to help uniformly report efficacy trials in oncology. Despite the usefulness of this system, many questions are being addressed about its validity in evaluating the true efficacy of drugs knowing that new targeted therapies do not affect the tumor size as much as they halt progression and prolong survival.     

Resistance to imatinib (Glivec): update on clinical mechanisms.

Imatinib mesylate, an orally administered 2-phenylaminopyrimidine derivative that inhibits BCR/ABL tyrosine kinase activity, has shown great promise in the treatment of chronic myelogenous leukemia (CML). This small molecule, tyrosine kinase inhibitor, has also been shown to be effective against metastatic gastrointestinal stromal tumors (GISTs) expressing the stem cell factor (SCF) receptor kit. However, the threat of resistance in patients has prompted investigators to uncover the mechanisms whereby malignant cells develop resistance to imatinib, and has also led to the establishment of strategies designed to over-ride imatinib resistance. Here, we provide a comprehensive overview of the effectiveness of imatinib in the treatment of chronic, accelerated and blast crisis-phase CML, Philadelphia chromosome-positive (Ph+) acute lymphoid leukemia (ALL) and metastatic GIST. Established mechanisms of resistance to imatinib are discussed, as are novel therapeutic approaches to improving drug responsiveness by reversing development of imatinib resistance in patients.     

Future options for imatinib mesilate-resistant tumors

The outcome of patients with gastrointestinal stromal tumors has been dramatically improved by therapy with imatinib mesilate (imatinib mesylate), a KIT and platelet-derived growth factor (PDGFR) tyrosine kinase inhibitor. Unfortunately, the majority of patients eventually experience disease progression due to drug resistance. Recent elucidation of the mechanisms of resistance to imatinib, particularly the acquisition of secondary mutations of the KIT and PDGF receptors, has provided significant insight and potential for the development of novel therapies. This review discusses the efficacy of sunitinib, which is approved for the treatment of patients with imatinib-resistant tumors, and highlights a number of emerging second-generation receptor tyrosine kinase inhibitors that show therapeutic potential in imatinib-resistant patients. Also considered are several promising agents targeting pathways downstream of the constitutionally activated KIT and PDGF receptors. Strategies to overcome imatinib resistance by optimizing combination therapy and selecting specific kinase inhibitors based on the secondary mutations identified in tumors of individual patients are presented.     

Impressive long-term disease stabilization by nilotinib in two pretreated patients with KIT/PDGFRA wild-type metastatic gastrointestinal stromal tumours

KIT/PDGFRA wild-type (WT) gastrointestinal stromal tumours (GISTs) showed a response rate to imatinib ranging from 0 to 25%. Nilotinib is a new-generation tyrosine kinase inhibitor that has demonstrated clinical activity in pretreated GIST patients. At present, no correlation between nilotinib activity and clinical/pathological/molecular features is available. We report on two WT GIST patients resistant to imatinib and sunitinib, and enrolled in the CAMN107A2201 study who achieved an impressive disease control by nilotinib. Both patients have germ-line mutations in the SDHA gene. In April 2004, a 39-year-old woman presented gastric GIST with multiple liver metastases and was treated with imatinib 400 mg/day, followed by imatinib 800 mg/day and then sunitinib. In August 2007, because of disease progression, she was enrolled in the CAMN107A2201 study and assigned to the nilotinib 800 mg/day arm. In March 2005, a 27-year-old woman started imatinib 600 mg/day and then sunitinib for gastric GIST with multiple liver and lung metastases. In October 2007, because of disease progression, she was enrolled in the CAMN107A2201 study and assigned to the nilotinib 800 mg/day arm. One patient still showed stable disease after 46 months of treatment according to the Response Evaluation Criteria In Solid Tumors, and a partial response after 9 months according to Choi's criteria. The other patient still showed stable disease after 42 months according to Response Evaluation Criteria In Solid Tumors. At present, they continue to receive nilotinib. We report very long-term disease stabilization under nilotinib treatment in two pretreated WT GIST patients. In-vitro studies and clinical analyses are warranted to evaluate a potential correlation between nilotinib activity and WT genotype or other clinical/pathological features.     

Cellular uptake of the tyrosine kinase inhibitors imatinib and AMN107 in gastrointestinal stromal tumor cell lines

Imatinib and AMN107 are protein tyrosine kinase inhibitors which reduce KIT autophosphorylation with similar potency. This report describes the cellular uptake of these compounds in two human gastrointestinal stromal tumor (GIST)-derived cell lines (GIST882 and GIST GDG1), which both express constitutively activated KIT. In GIST882 and GIST GDG1 cell lines, HPLC analysis revealed AMN107 intracellular concentrations to be 7- and 10-fold greater than those of imatinib. These data indicate either increased cellular uptake or decreased cellular efflux of AMN107 when compared to imatinib in GIST cell lines. The finding suggests that AMN107 might be less susceptible to transport-driven imatinib resistance. The stable and increased exposure of GIST cells to a highly active AMN107 agent could be important in the treatment of imatinib-resistant GIST patients in whom resistance has developed as a result of changes in cellular transport mechanisms for which AMN107 is not a substrate.     

Imatinib: as adjuvant therapy for gastrointestinal stromal tumour

Imatinib is a protein-tyrosine kinase inhibitor with antitumour effects in patients with gastrointestinal stromal tumour (GIST) that is indicated for the treatment of unresectable and/or metastatic GIST and as adjuvant therapy in patients with KIT-positive GIST. Imatinib binds to and inhibits KIT and platelet-derived growth factor receptor (PDGFR)-α tyrosine kinases, interfering with their downstream tumourogenic processes. Cell lines with KIT mutations that are common in patients with GIST were sensitive to imatinib at low in vitro concentrations. Patients with exon 11 KIT mutations were significantly more likely to have partial tumour responses and longer overall survival (OS) and were significantly less likely to have progression of disease than patients with exon 9 KIT mutations or no detectable KIT or PDGFR mutations. In a large (n?>?700) randomized, double-blind, placebo-controlled, multinational trial (ACOSOG [American College of Surgeons Oncology Group] Z9001), patients who received 1 year of adjuvant treatment with oral imatinib 400?mg/day after surgical resection of GIST had significantly longer recurrence-free survival than placebo recipients, with an overall hazard ratio of 0.35 (95% CI 0.22, 0.53) [primary endpoint]. At the time of reporting, there was no significant between-group difference in OS. In patients with GIST who received adjuvant imatinib in this trial, adverse events were mostly of mild or moderate severity; the imatinib treatment group had an almost 2-fold higher rate of US National Cancer Institute Common Toxicity Criteria grade 3 or 4 adverse events than the placebo group.     

A potential role for nilotinib in KIT-mutated melanoma

INTRODUCTION: The advent of effective immunotherapy and targeted therapy, such as ipilimumab (anti-CTLA-4 monoclonal antibody) and vemurafenib (BRAF inhibitor), are changing the treatment paradigm for metastatic melanoma. One of the most readily recognized targets in metastatic melanoma is the oncogenic 'driver' mutations KIT, which is thought to be an important driver mutation in up to 3% of melanomas. AREAS COVERED: We review the current state of development of KIT-targeted agents in melanoma with KIT mutations. The pharmacokinetic and pharmacodynamic profiles of nilotinib are presented, as well its safety clinical activity data. Finally, we present the knowledge learned from the experience of nilotinib in chronic myeloid leukemia (CML) and gastrointestinal stromal tumor (GIST) to guide its development for melanoma. EXPERT OPINION: Given its favorable safety and efficacy profile in CML and imatinib-resistant GISTs, nilotinib, a second-generation tyrosine kinase inhibitor with greater potency than imatinib, emerges as a promising agent in the treatment of metastatic melanoma harboring the KIT mutation and warrants clinical investigation in this setting.     

Imatinib mesilate for the treatment of gastrointestinal stromal tumour

Background: The molecular hallmark of gastrointestinal stromal tumours (GISTs), the mutation of the KIT gene, was discovered 10 years ago. GISTs have since been recognized as separate pathological entities among sarcomas, and have become a model for targeted treatment of solid tumours. Imatinib mesilate, which was approved in 2002 for the treatment of patients with advanced GIST, has dramatically changed the course of the disease. Objective: This article will focus on the development of imatinib mesilate in the treatment of patients with GIST. Methods: A Pubmed search was performed using the keywords ‘imatinib’, ‘gastrointestinal stromal’, ‘GIST’, ‘KIT’ and ‘PDGFR’. Websites of the American Society of Clinical Oncology and the European Society of Medical Oncology were searched for data reported in abstract form at recent symposiums. Personal communications from opinion leaders were sought for additional information that might be relevant. Results: Imatinib has changed the clinical course of patients with advanced GISTs and further development in the adjuvant setting as well as prospective assessment of predictive factors are the current focus of ongoing research.     

Imatinib mesylate in the treatment of gastrointestinal stromal tumour

Imatinib mesylate is a selective and potent small-molecule inhibitor of tyrosine kinases, including Kit, platelet-derived growth factor receptor, and the BCR-Abl fusion protein. Kit plays an important role in gastrointestinal stromal tumours (GISTs) and is one of the most exciting therapeutic targets discovered so far. Clinical trials have consistently shown the dramatic efficacy of imatinib mesylate in patients with GIST. This article will review the development and pharmacology of this small-molecule inhibitor and summarise the clinical trials of imatinib mesylate for the treatment of GIST. Although imatinib mesylate has significantly improved the outcomes of most patients with advanced GIST, unanswered questions remain: what is the role of imatinib mesylate in the pre- and postoperative settings? What is the mechanism of the antitumour activity of imatinib? How do you manage patients whose tumours are refractory to imatinib mesylate?     

Current status of unoprostone for the management of glaucoma and the future of its use in the treatment of retinal disease

Imatinib mesylate is a selective and potent small-molecule inhibitor of tyrosine kinases, including Kit, platelet-derived growth factor receptor, and the BCR–Abl fusion protein. Kit plays an important role in gastrointestinal stromal tumours (GISTs) and is one of the most exciting therapeutic targets discovered so far. Clinical trials have consistently shown the dramatic efficacy of imatinib mesylate in patients with GIST. This article will review the development and pharmacology of this small-molecule inhibitor and summarise the clinical trials of imatinib mesylate for the treatment of GIST. Although imatinib mesylate has significantly improved the outcomes of most patients with advanced GIST, unanswered questions remain: what is the role of imatinib mesylate in the pre- and postoperative settings? What is the mechanism of the antitumour activity of imatinib? How do you manage patients whose tumours are refractory to imatinib mesylate?     

Anacetrapib,a cholesteryl ester transfer protein inhibitor

Introduction: Mutated forms of the receptor tyrosine kinase c-KIT are “drivers” in several cancers and are attractive targets for therapy. While benefits have been obtained from use of inhibitors of KIT kinase activity such as imatinib, especially in gastrointestinal stromal tumours (GIST), primary resistance occurs with certain oncogenic mutations. Furthermore, resistance frequently develops due to secondary mutations. Approaches to addressing both of these issues as well as combination therapies to optimise use of KIT kinase inhibitors are discussed.

Areas covered: This review covers the occurrence of oncogenic KIT mutations in different cancers and the molecular basis of their action. The action of KIT kinase inhibitors, especially imatinib, sunitinib, dasatinib and PKC412, on different primary and secondary mutants is discussed. Outcomes of clinical trials in GIST, acute myeloid leukaemia (AML), systemic mastocytosis and melanoma and their implications for future directions are considered.

Expert opinion: Analysis of KIT mutations in individual patients is an essential prerequisite to the use of kinase inhibitors for therapy, and monitoring for development of secondary mutations that confer drug resistance is necessary. However, it is unlikely that KIT inhibitors alone can lead to cure. KIT mutations alone do not seem to be sufficient for transformation; thus identification and co-targeting of synergistic oncogenic pathways should lead to improved outcomes.     

Imatinib mesylate,the first molecularly targeted gene suppressor

OBJECTIVE: To review the pharmacology, pharmacokinetics, efficacy, safety, and drug-drug and drug-food interactions of imatinib and the economic considerations of imatinib in the treatment of chronic myeloid leukemia (CML). DATA SOURCES: Literature accessed through MEDLINE (January 1970-January 2002), abstracts from the 2001 annual meetings of the American Society of Clinical Oncology and the American Society of Hematology, imatinib product labeling, and additional studies or abstracts identified from the bibliographies of the reviewed literature were used to compile data. Key search terms were allogeneic bone marrow transplant and stem cell transplant, chronic myeloid leukemia, imatinib, interferon, Gleevec, leukemia, gastrointestinal stromal tumors, STI-571, and tyrosine kinase inhibitors. FINDINGS: Imatinib is a distinctively characteristic drug targeted toward inhibition of tyrosine kinase activity. Imatinib is indicated for the treatment of patients with CML who failed interferon (IFN)-alpha therapy and for the treatment of patients with gastrointestinal stromal tumors (GISTs) expressing the tyrosine kinase receptor c-kit. Imatinib produces positive short-term hematologic and cytogenetic responses in patients with CML; short-term positive objective responses have been shown for patients with GISTs. To our knowledge, there are no controlled trials demonstrating long-term safety, improvement in disease-related symptoms, or increased survival with imatinib. Serious adverse effects requiring dosage decreases and/or therapy termination are edema, hepatotoxicity, and hematologic toxicity. Imatinib also has been found to inhibit tyrosine kinases involved in the growth of other malignancies. The role of imatinib in tumors that express a tyrosine kinase is constantly evolving with new research results. CONCLUSIONS: Imatinib therapy should be limited to patients whose tumor growth is related to a genetically defective tyrosine kinase. In cases of CML, imatinib should be further limited to patients who have tried and failed IFN-alpha therapy or who are not candidates for an allogeneic stem cell transplant.     

The development and application of imatinib

The hallmark characteristics of cancer include an unrestrained proliferation involving activation of growth signals, loss of negative regulation and dysfunctional apoptotic pathways. Targeting abnormal cell signalling pathways should provide a more selective approach to cancer treatment than conventional cytotoxic chemotherapy. Tyrosine kinases play an essential role in the signalling pathways involved in the control of cellular proliferation and growth. Imatinib is a small-molecule tyrosine kinase inhibitor of the ABL fusion gene, platelet derived growth factor receptors (PDGFR) and KIT. This agent has demonstrated considerable activity in chronic myeloid leukaemia (CML) by inhibiting the BCR-ABL fusion protein and gastrointestinal stromal tumours (GISTs), which are predominantly driven by activating mutations in KIT. A number of other rare conditions are also responsive, for example, dermatofibrosarcoma protuberans, which is driven by a chromosomal translocation involving PDGF-B and Col1A1, resulting in overexpression of PDGF-B, and hypereosinophillic syndrome, which can be caused by activating PDGFR mutations. The pivotal registration study for newly diagnosed CML was a large randomised trial comparing 400 mg/day of imatinib to a combination of IFN-alpha and cytarabine, which demonstrated a significantly higher complete haematological and cytogenetic response rate in the imatinib arm. In the case of GIST a randomised study in patients with inoperable or metastatic disease explored doses of 400 - 600mg and reported a response rate of > 50% in each arm plus disease stabilisation and an improvement in performance status. Large randomised trials have subsequently been performed, comparing 400 with 800mg/day. The first to report indicates that the larger dose is associated with improved progression-free survival, although it is not yet known whether or not this will translate into a difference in overall survival. The most common KIT mutation involves exon 11 and is associated with a statistically significant better response and prognosis compared with other mutations or no detectable mutations. Mutational analysis is likely to become increasingly important in the selection of patients for neoadjuvant and adjuvant treatment and in helping to understand the nature of acquired resistance.     

Imatinib: a review of its use in the management of gastrointestinal stromal tumours

Imatinib (Gleevec, Glivec is a small molecule inhibitor of tyrosine kinase that has been evaluated for efficacy in patients with gastrointestinal stromal tumours (GIST). The drug is approved for the treatment of unresectable and/or metastatic, KIT-positive GIST in the US, Europe and many other countries. Imatinib has had a significant impact on the management of advanced GIST, which has traditionally had a poor prognosis, and has quickly become the first choice of treatment in the medical therapy of unresectable and/or metastatic, KIT-positive GIST. In randomised, nonblind trials, imatinib 400-800 mg/day produced complete or partial responses in up to two-thirds of patients, with long-term efficacy, and substantially prolonged progression-free and overall survival. The drug was generally well tolerated in GIST patients, including during long-term treatment. Imatinib dosages higher than 400 mg/day (up to 800 mg/day) may improve progression-free survival, with an increase in dosage benefiting some patients who show disease progression at the lower dosage, particularly in those with exon 9 mutation; however, there is also a dose-related increase in imatinib toxicity. Mutational genotype and other, non-biomolecular factors may aid in guiding imatinib therapy and predicting prognosis in GIST patients. Further data are required to evaluate the use of imatinib in adjuvant and neoadjuvant settings. Nevertheless, imatinib currently provides the most effective treatment option in the management of advanced GIST.     

Targeted therapies of gastrointestinal stromal tumors (GIST)—The next frontiers

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal (GI) tract and are caused by activating KIT or PDGFRA mutations. GISTs can be successfully treated with the small molecule kinase inhibitor imatinib mesylate (Gleevec^®, Novartis) with response rates of up to 85%. However, complete responses are rare, and most patients will develop imatinib resistance over time. Recent results have shown that although imatinib effectively stimulates apoptotic cell death in sensitive GIST cells, a considerable proportion of cells does not undergo apoptosis, but instead enters a state of quiescence. Quiescence is characterized by a reversible withdrawal from the cell division cycle, during which the cells remain alive and metabolically active. It is conceivable that quiescence not only plays a pivotal role in the emergence of residual disease but also in creating a pool of tumor cells that survive continuous small molecule therapy and may hence represent the “seeds” for the outgrowth of resistant clones. This review will summarize the current knowledge about GIST biology and treatment response to imatinib including the induction of cellular quiescence in GIST. In addition, we will highlight future strategies to design more effective treatment options to overcome these problems with an aim towards cure of this hitherto untreatable tumor entity.     

Pigmentary changes in a patient treated with imatinib

Imatinib mesylate (STI 571; Gleevec; Novartis Pharmaceuticals, Basel, Switzerland) is an orally available tyrosine kinase inhibitor that targets a constitutively activated BCR-ABL tyrosine kinase with additional inhibitory effects on platelet derived growth factor (PDGF) receptors alpha and beta, and KIT. It has revolutionized the treatment of adult and pediatric patients with Philadelphia chromosome positive chronic myelogenous leukemia (CML) and is also FDA-approved for KIT-positive advanced gastrointestinal tumor (GIST) and dermatofibrosarcoma protuberans. A wide spectrum of dermatologic toxicities has been associated with this agent, among which a maculopapular rash is the most common event. In addition, a variety of pigmentary abnormalities of the skin and mucosal surfaces have been reported. Hypopigmentation is a well-recognized adverse effect. In contrast, paradoxical hyperpigmentation has only rarely been documented. In this case report we describe imatinib-induced cutaneous hyperpigmentation and graying of hair occurring in the same patient with dermatofibrosarcoma protuberans treated with imatinib.