Efficacy and safety of imatinib mesylate in advanced gastrointestinal stromal tumors

The role of receptor tyrosine kinase inhibition in treating gastrointestinal malignancy

Tyrosine kinase receptors are proteins that transduce the signal from many growth factor and cytokine ligands to produce intracellular responses. As such they can activate multiple signalling cascade pathways and influence cell division, migration and survival. Many show upregulation in certain malignancies, including those of the gastrointestinal tract, and are thought to play key roles in carcinogenesis. This makes them attractive targets for drug therapy and in recent years many inhibitors have been developed. This review discusses the current situation regarding the development of inhibitors with particular reference to the erbB family, the insulin-like growth factor receptor, the Met receptor, the receptor for vascular endothelial growth factor and the Kit receptor. The evidence will be related back to cancers of the gut lumen. Clinical effectiveness in this area seems to lie in using a combinatorial approach that inhibits multiple key signalling points, and the reasons for this will be discussed.

Lack of c-kit exon 11 activating mutations in c-KIT/CD117-positive SCLC tumour specimens

Previous studies have shown that STI571, a selective tyrosine kinase inhibitor of c-KIT, is highly effective in c-KIT/CD117-positive gastrointestinal stromal tumours (GIST), especially those that have activating mutations in the c-kit exon 11 that encodes the juxtamembrane (JM) domain of the c-KIT oncoprotein. We examined the prevalence of activating exon 11 c-kit mutations in 26 small-cell lung cancer (SCLC) cases in order to explore whether this disease is also a potential target for treatment with STI571. Expression of c-KIT, estimated by immunohistochemistry, was demonstrated in 14 out of 22 SCLC samples (64%); nine samples showed moderate to strong staining (41%), five samples were weakly positive (23%), whereas eight samples (36%) were negative for CD117. Next, we examined the mutational status of exon 11 of the c-kit gene, by single-stranded conformational polymorphism (SSCP) and sequencing in all of the cKIT/CD117-positive tumours. However, no activating mutations in the c-kit exon 11 were found by either technique. Apparently, c-KIT oncoprotein expression in SCLC was not correlated with activating mutations in c-kit exon 11. In analogy to GISTs, our results could imply that SCLC patients would not benefit from treatment with STI571.

Gastrointestinal stromal tumors (GIST): a model for molecule-based diagnosis and treatment of solid tumors

Gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor of the human gastrointestinal (GI) tract. The c-kit receptor tyrosine kinase (KIT) is expressed by practically all GISTs, and gain-of-function mutations of KIT are present in most GISTs. Interstitial cells of Cajal (ICC) are the pacemaker of the peristaltic movement of the GI tract. Since signals through KIT are essential for development of ICC and since multiple GISTs develop from the hyperplastic lesion of ICCs in familial GIST patients with germline mutations of KIT, GISTs are considered to originate from ICC. Imatinib mesylate, which was developed for treatment of chronic myeloid leukemia (CML), was found to be useful for treatment of GISTs. Imatinib mesylate inhibits BCR-ABL fused tyrosine kinase that causes CML. Imatinib mesylate also inhibits the mutated KIT observed in most GISTs, and this explains the effectiveness of Imatinib mesylate on GISTs. GISTs appear to serve as a model for molecule-based diagnosis and treatment of solid tumors.

Improving the evaluation of new cancer treatments: challenges and opportunities

There are, at present, ten times more anticancer drugs being tested in clinical trials than there were 15 years ago. Many of the new classes of agents, however, are predicted to work in only small subpopulations of patients, target unconventional aspects of tumour development and interact with other agents in an unpredictable manner. How can clinical trials be re-designed to accommodate the new features of targeted anticancer drugs?

Imatinib: a targeted clinical drug development

Imatinib (Gleevec) (formerly STI571) is an orally bioavailable rationally developed inhibitor of the tyrosine kinases Bcr-Abl, Kit, and platelet-derived growth factor receptor (PDGFR). In 4 years of clinical development, more than 12,000 patients have been treated in the clinical development program. Imatinib was first shown to be highly effective in the treatment of all stages of chronic myelogenous leukemia (CML). Moreover, preliminary results of a randomized study have demonstrated superior efficacy and safety of first-line imatinib therapy compared with a combination of interferon and cytarabine. Imatinib has also been shown to be the only effective drug therapy in the treatment of patients with metastatic gastrointestinal stromal tumors expressing the stem cell factor (SCF) receptor Kit. This review outlines the successive steps in the clinical development of this new, targeted anticancer agent.

A comparison of EUS features between CD-117 positive GI stromal tumors and CD-117 negative GI spindle cell tumors

BACKGROUND

GI stromal tumors are spindle cell tumors that stain positive for immunohistochemical CD-117 (c-kit). Prognostic factors for malignancy include size (> or =4 cm), mitotic index (5 mitotic figures/50 high-powered fields), and ulcerated, cystic, or necrotic areas within the tumor. The purpose of this study was to compare these features in c-kit positive vs. c-kit negative tumors.

METHODS

All patients referred for EUS of submucosal lesions were identified, and histopathology, including immunohistochemical staining, was reviewed to determine all diagnoses of GI stromal tumors. Size, echo pattern, and presence of cystic spaces and ulceration were recorded as diagnosed by EUS. Histopathologic diagnoses were made by FNA or endoscopic submucosal-mucosal resection. If surgical resection followed, the surgical diagnosis, staining pattern, mitotic index, and presence of ulceration, necrosis, and nuclear atypia were recorded.

RESULTS

Forty patients (21 men, 19 women; 38 white, 2 African American; mean age 58 +/- 2.6 years) had 46 EUS procedures performed for evaluation of spindle cell tumors. Seventeen stained positive for c-kit (mean age, 59 +/- 3.6 years; range 19 to 80 years) and 12 negative (mean age, 57 +/- 3.8 years; range 31 to 76 years); 11 were not stained for c-kit (excluded from analysis). On EUS, 7 were ulcerated, 3 cystic, and 6 were larger than 4 cm. This group of findings was observed in 12 patients, 11 of whom had c-kit positive tumors (11/17 vs. 1/12; p = 0.006). Tumors positive for c-kit were larger (42.4 +/- 5.5 mm vs. 19.0 +/- 5.9 mm; p = 0.005). There were 13 c-kit positive tumors in the stomach, 2 in the duodenum, and 1 each in the esophagus and at the gastroesophageal junction. Of the 12 c-kit negative tumors, 8 were located in the esophagus and 1 at the gastroesophageal junction (9/12 vs. 2/17; p < 0.01). Surgical resection was performed on 13 patients, 12 of whom had c-kit positive tumors, and 3 of these 12 tumors had greater than 5 mitoses per 50 high-powered field.

CONCLUSIONS

If a GI stromal tumor is suspected, EUS-FNA with immunohistochemical staining should be performed for CD-117 (c-kit). C-kit tumors are more likely to have malignant features and should be resected or subjected to close clinical follow-up.

A tyrosine kinase created by fusion of the PDGFRA and FIP1L1 genes as a therapeutic target of imatinib in idiopathic hypereosinophilic syndrome

BACKGROUND

Idiopathic hypereosinophilic syndrome involves a prolonged state of eosinophilia associated with organ dysfunction. It is of unknown cause. Recent reports of responses to imatinib in patients with the syndrome suggested that an activated kinase such as ABL, platelet-derived growth factor receptor (PDGFR), or KIT, all of which are inhibited by imatinib, might be the cause.

METHODS

We treated 11 patients with the hypereosinophilic syndrome with imatinib and identified the molecular basis for the response.

RESULTS

Nine of the 11 patients treated with imatinib had responses lasting more than three months in which the eosinophil count returned to normal. One such patient had a complex chromosomal abnormality, leading to the identification of a fusion of the Fip1-like 1 (FIP1L1) gene to the PDGFRalpha (PDGFRA) gene generated by an interstitial deletion on chromosome 4q12. FIP1L1-PDGFRalpha is a constitutively activated tyrosine kinase that transforms hematopoietic cells and is inhibited by imatinib (50 percent inhibitory concentration, 3.2 nM). The FIP1L1-PDGFRA fusion gene was subsequently detected in 9 of 16 patients with the syndrome and in 5 of the 9 patients with responses to imatinib that lasted more than three months. Relapse in one patient correlated with the appearance of a T674I mutation in PDGFRA that confers resistance to imatinib.

CONCLUSIONS

The hypereosinophilic syndrome may result from a novel fusion tyrosine kinase - FIP1L1-PDGFRalpha - that is a consequence of an interstitial chromosomal deletion. The acquisition of a T674I resistance mutation at the time of relapse demonstrates that FIP1L1-PDGFRalpha is the target of imatinib. Our data indicate that the deletion of genetic material may result in gain-of-function fusion proteins.

Response to imatinib mesylate of a gastrointestinal stromal tumor with very low expression of KIT

More than 90% of gastrointestinal stromal tumors (GISTs) express the receptor tyrosine kinase KIT, and activating mutations of the KIT gene are detectable in the vast majority of these tumors. Imatinib mesylate (formerly STI571) is a potent inhibitor of KIT kinase activity and has been proven to be highly active in patients with unresectable or metastatic GIST expressing immunohistochemically detectable KIT protein. Here we report a patient with metastatic GIST who responded well to imatinib mesylate treatment despite the near absence of KIT expression in two different samples of his tumor. The tumor was morphologically typical for a GIST, stained positively for CD34, and harbored an in-frame deletion (WK 557-558) in KIT exon 11 that is common in GISTs. Our experience with this patient suggests that even GISTs with very low levels of KIT expression may respond to imatinib mesylate therapy.

Inhibition of FLT3 in MLL. Validation of a therapeutic target identified by gene expression based classification

We recently found that MLL-rearranged acute lymphoblastic leukemias (MLL) have a unique gene expression profile including high level expression of the receptor tyrosine kinase FLT3. We hypothesized that FLT3 might be a therapeutic target in MLL and found that 5 of 30 MLLs contain mutations in the activation loop of FLT3 that result in constitutive activation. Three are a newly described deletion of I836 and the others are D835 mutations. The recently described FLT3 inhibitor PKC412 proved cytotoxic to Ba/F3 cells dependent upon activated FLT3 containing either mutation. PKC412 is also differentially cytotoxic to leukemia cells with MLL translocations and FLT3 that is activated by either overexpression of the wild-type receptor or mutation. Finally, we developed a mouse model of MLL and used bioluminescent imaging to determine that PKC412 is active against MLL in vivo.

Molecular mechanisms of resistance to imatinib in Philadelphia-chromosome-positive leukaemias

Imatinib (STI571 or CGP57148B) is an innovative treatment for tumours with a constitutively activated form of c-ABL, c-KIT, or PDGFR. Such tumours include Philadelphia-chromosome-positive (Ph-positive) leukaemias, gastrointestinal stromal tumours, and PDGFR-positive leukaemias. Diseases such as primary hypereosinophilia and dermatofibrosarcoma protuberans also seem to respond to imatinib. Clinical trials assessing the therapeutic effects of imatinib have shown that the drug is highly effective with few associated side-effects, achieving durable cytogenetic responses in many patients with chronic-phase BCR-ABL-positive leukaemias. However, the emergence of resistance, particularly in patients with acute leukaemias, has prompted intense research, and many are concerned about the future prospects for imatinib. The resistance has been found in patients with acute-phase disease, but may also occur in patients with chronic-phase disease. Two cellular mechanisms for resistance to imatinib have been identified: amplification of BCR-ABL gene and mutations in the catalytic domain of the protein. In addition, suboptimum inhibition of BCR-ABL in vivo could contribute to the selection of resistant cells. We have summarised all currently available data on resistance to imatinib, both published and unpublished, including the mechanisms of resistance identified so far, and their clinical relevance to the different forms of Ph-positive leukaemias is discussed. Furthermore, we discuss strategies to overcome or prevent the development of resistance.

PDGFRA activating mutations in gastrointestinal stromal tumors

Most gastrointestinal stromal tumors (GISTs) have activating mutations in the KIT receptor tyrosine kinase, and most patients with GISTs respond well to Gleevec, which inhibits KIT kinase activity. Here we show that approximately 35% (14 of 40) of GISTs lacking KIT mutations have intragenic activation mutations in the related receptor tyrosine kinase, platelet-derived growth factor receptor alpha (PDGFRA). Tumors expressing KIT or PDGFRA oncoproteins were indistinguishable with respect to activation of downstream signaling intermediates and cytogenetic changes associated with tumor progression. Thus, KIT and PDGFRA mutations appear to be alternative and mutually exclusive oncogenic mechanisms in GISTs.

Gleevec (STI-571) inhibits lung cancer cell growth (A549) and potentiates the cisplatin effect in vitro

BACKGROUND

Gleevec (aka STI571, Imatinib) is a recently FDA approved anti-tumor drug for chronic myelogenous leukemia. Gleevec binds specifically to BCR-ABL tyrosine kinase and inhibit the tyrosine kinase activity. It cross-reacts with another two important membrane tyrosine kinase receptors, c-kit and PDGF receptors. We sought to investigate if Gleevec has a potential role in treatment of non-small cell lung cancer.

RESULTS

We have shown that Gleevec alone can inhibit the A549 lung cancer cell growth in dose-dependent manner, and the optimal concentration of Gleevec inhibition of A549 cell growth is at the range of 2-3 microM (IC50). We have also shown that A549 cells are resistant to cisplatin treatment (IC50 64 microM). Addition of Gleevec to the A549 cells treated with cisplatin resulted in a synergistic cell killing effect, suggesting that Gleevec can potentiate the effect of cisplatin on A549 cells. We also showed that the A549 lung cancer cells expresses the platelet derived growth factor receptor alpha, and the inhibitory effects of Gleevec on A549 cells is likely mediated through inhibition of PDGFR alpha phosphorylation. We further tested 33 lung cancer patients' tumor specimens to see the frequency of PDGFR-alpha expression by tissue micro-arrays and immunohistochemistry. We found that 16 of the 18 squamous carcinomas (89%), 11 of the 11 adenocarcinomas (100%), and 4 of the 4 small cell lung cancers (100%) expressed PDGFR-alpha.

CONCLUSION

These results suggest a potential role of Gleevec as adjuvant therapeutic agent for treatment of non-small cell lung cancer.

Applications of Positron Emission Tomography in the Development of Molecular Targeted Cancer Therapeutics

For molecular targeted cancer therapies to fulfill their promise in cancer treatment, innovative approaches are required to overcome significant obstacles that exist in the clinical development of these agents. Positron emission tomography (PET) is a functional imaging technology that allows rapid, repeated, noninvasive, in vivo assessment and quantification of many biological processes and in some cases molecular pathways targeted by these therapies. It is highly sensitive, with the capacity to detect subnanomolar concentrations of radiotracer and provides superior image resolution to conventional nuclear medicine imaging with gamma cameras. Novel PET radiotracers have been developed that allow visualisation of a variety of processes including tumour metabolism, cell proliferation, apoptosis, hypoxia and blood flow. Furthermore, specific molecular targets including cellular receptors can be identified using radiolabelled receptor ligands or specific monoclonal antibodies. Improvements in imaging technology leading to the development of small-animal PET scanners, with resolution capable of imaging commonly used mouse models of cancer, will enable PET to play an important role in preclinical proof-of-principle drug studies. Such improvements will also facilitate the validation of imaging protocols that can be readily translated to studies in humans. The greatest utility of PET in the development of molecular targeted therapeutics, however, lies in clinical studies, where PET may play a valuable role in a number of situations. These include selection of patients for therapy through noninvasive identification of the presence of specific molecular targets, pharmacokinetic studies with labelled drugs and pharmacodynamic evaluations of biological parameters to select the optimal biological dose, and assessment of response to therapies.

Interstitial Cells in the Musculature of the Gastrointestinal Tract: Cajal and Beyond

Expression of the receptor tyrosine kinase KIT on cells referred to as interstitial cells of Cajal (ICC) has been instrumental during the past decade in the tremendous interest in cells in the interstitium of the smooth muscle layers of the digestive tract. ICC generate the pacemaker component (electrical slow waves of depolarization) of the smooth musculature and are involved in neurotransmission. By integration of ICC functions, substantial progress has been made in our understanding of the neuromuscular control of gastrointestinal motility, opening novel therapeutic perspectives. In this article, the ultrastructure and light microscopic morphology, as well as the functions and the development of ICC and of neighboring fibroblast-like cells (FLC), are critically reviewed. Directions for future research are considered and a unifying concept of mesenchymal cells, either KIT positive (the "ICC") or KIT negative "non-Cajal" (including the FLC and possibly also other cell types) cell types in the interstitium of the smooth musculature of the gastrointestinal tract, is proposed. Furthermore, evidence is accumulating to suggest that, as postulated by Santiago Ramon y Cajal, the concept of interstitial cells is not likely to be restricted to the gastrointestinal musculature.

Management of gastrointestinal stromal tumors in the era of tyrosine kinase inhibitors

Gastrointestinal stromal tumors (GIST) represent the most common type of mesenchymal malignancy in the gastrointestinal tract. With the discovery of uncontrolled KIT tyrosine kinase signaling as a critical component in the pathogenesis of this disease, the diagnostic and treatment options for patients with GIST have evolved rapidly. Pathology review by an experienced pathologist is critical to the classification of this disease. Expert and definitive surgery remains the mainstay of treatment in patients with localized, resectable disease. Imatinib mesylate has been shown to be the first successful systemic therapy for patients with metastatic or unresectable disease and has revolutionized the treatment of this often rapidly progressive and fatal disease. Ongoing studies are evaluating the role of imatinib in the preoperative and postsurgical adjuvant settings. Although resistance to imatinib will appear over time, there is reason for optimism that the mechanisms of resistance will be identified and eventually overcome. The application of molecular understanding of GIST to the development of a selective, scientifically rational therapy is a classic example of multidisciplinary translational oncology research.

Response of extraabdominal desmoid tumors to therapy with imatinib mesylate

BACKGROUND

Desmoid tumor represents a rare monoclonal neoplasm arising from deep musculoaponeurotic structures and may occur sporadically or in association with the familial adenomatous polyposis and Gardner syndromes. Desmoid tumors do not appear to demonstrate metastatic potential; however, local infiltrative growth results in significant morbidity and potential mortality. Although the delineation of optimal therapy for desmoid tumors has been confounded by several factors, surgical resection with adjuvant radiotherapy for a positive surgical margin remains the standard approach. Responses have been demonstrated to nonsteroidal antiinflammatory agents, antiestrogen compounds, and a variety of other agents in small series. Imatinib mesylate appears to demonstrate inhibitory activity against multiple class 3 receptor tyrosine kinases, including platelet-derived growth factor receptor (PDGFR)-alpha and PDGFR-beta, as well as c-kit.

METHODS

The authors performed immunohistochemical and qualitative real-time polymerase chain reaction analysis on nine desmoid tumor specimens that demonstrated consistent positivity for c-kit as well as PDGFR-alpha and PDGFR-beta. At the time of last follow-up, 2 patients had received therapy with imatinib mesylate at a dose of 400 mg twice daily.

RESULTS

Both patients demonstrated ongoing radiographic and clinical responses with a duration of 9 months and 11 months, respectively.

CONCLUSIONS

Imatinib mesylate has been reported to have activity against desmoid tumor, most likely because of c-kit and PDGFR receptor tyrosine kinase activity inhibition, and warrants further study. The relative novelty of this agent and the lack of long-term toxicity data should prompt its use only in the salvage setting in which established local and systemic approaches fail to control disease. In addition, the use of imatinib mesylate in the treatment of this neoplasm preferably should be in the context of a formal prospective clinical trial.

A paradigm for therapy-induced microenvironmental changes in solid tumors leading to drug resistance

Intrinsic alterations in the tumor microenvironment are known to contribute to various forms of drug resistance. For example, tumor hypoxia, due to abnormal or sluggish blood flow within areas of solid tumors, can result in both microenvironment-mediated radiation and chemotherapeutic drug resistance. In contrast, acquired resistance to chemotherapy is generally considered to be the result of the gradual selection of mutant subpopulations having genetic mutations and biochemical alterations responsible for the resistant phenotype. Here we present a paradigm for therapyinduced microenvironment-mediated acquired drug resistance. It is based on the results showing that tumor cells appear to be heterogeneous in their relative dependence on adjacent tumor-associated vasculature for survival. Some tumor cells are highly vessel dependent, whereas some are significantly less so, and thus can survive in more hypoxic regions of tumors, distal from such tumor vessels. Hence, it is possible that variant tumor cells that are less vessel dependent may therefore be selected for over time by successful antiangiogenic drug therapies. This results in loss of response or attenuated responses to the therapy. Preliminary evidence is summarized in support of this hypothesis, using paired human colon cancer (HCT116) cell lines that contain two copies of either the wild-type or the disrupted p53 tumor suppressor gene. The mutant cells were found to be less responsive to antiangiogenic therapy, compared to the wild-type cells, and could be progressively selected for in mixed cell populations. Because p53 inactivation can lead to resistance to hypoxia-mediated apoptosis, the results suggest that a protracted and successful antiangiogenic therapy may create more hypoxic tumor microenvironments, thereby creating the necessary conditions to accelerate the selection of mutant tumor cells that are more adept in surviving and growing in such environments. As such, consideration might be given to the combined use of bioreductive hypoxic cell cytotoxic drugs and angiogenesis inhibitors to prolong the efficacy of antiangiogenic therapeutics.

Management of malignant gastrointestinal stromal tumours

Gastrointestinal stromal tumours (GISTs) are the most common form of mesenchymal tumour of the gastrointestinal tract. Clinically, they range from small indolent tumours curable with surgery alone to aggressive cancers. Making a distinction between an indolent and a malignant GIST is unreliable with conventional histopathological techniques. The presence of metastases at the time of diagnosis confirms malignancy, but all GISTs should be regarded as having malignant potential. GISTs characteristically express the KIT protein, a transmembrane tyrosine kinase receptor for stem-cell factor. Most GISTs have a mutation in the KIT proto-oncogene that translates into a gain-of-function constitutive activation of the KIT kinase. KIT activation seems to be an early tumour-promoting event in pathogenesis. Commonly, malignant GISTs show high-level primary resistance to conventional chemotherapy. Imatinib mesylate is an orally administered selective inhibitor of certain tyrosine kinases including KIT. Most patients with advanced malignant GISTs achieve clinical benefit and significant antitumour responses with imatinib mesylate. Responses have been durable, and most patients tolerate the drug well at clinically effective doses. Imatinib mesylate is the first effective systemic therapy for advanced GIST.

Targeting the molecular pathophysiology of gastrointestinal stromal tumors with imatinib. Mechanisms, successes, and challenges to rational drug development

The development of imatinib as molecularly targeted therapy for GIST represents an important case study of rational drug development. It is a paradigm of how the molecular understanding of a cancer has resulted in a new effective therapy that targets the critical pathway upon which the GIST cells were dependent: the uncontrollably active KIT signaling pathway. Unresectable or metastatic GISTs have traditionally exhibited a rapid and fatal clinical course, with no evidence of benefit from any standard cytotoxic chemotherapy. The identification of KIT activation as a critical factor in the pathogenesis of GIST led to the search for a new type of therapeutic compound to serve as a KIT inhibitor and to interfere with the constitutive phosphorylation of the KIT kinase in GIST cells. Clarification of the molecular genetic pathophysiology of GIST and the role of KIT activation in this disease, therefore, has not only enabled improved diagnosis and differentiation of GIST from other mesenchymal neoplasms but has also been key in identifying new targeted strategies for therapeutic intervention. The improved understanding of the molecular pathophysiology of GIST, a disease that was previously untreatable with any available systemic therapy, has led to the development of imatinib, a well-tolerated agent that can inhibit the dysregulated KIT signaling pathways in GIST. Imatinib represents the first (and currently the only) effective systemic therapy for patients with unresectable GIST. Imatinib therapy can induce objective responses and stabilization of disease and can provide clinical benefit in the majority of GIST patients treated with the drug. Other strategies are beginning to be explored, such as the use of imatinib earlier the in course of GIST (e.g., as adjuvant therapy after definitive surgical resection of early-stage disease). Integration of signal transduction inhibitors into the armamentarium of cancer therapeutics will undoubtedly continue based on this important paradigm of GIST.

Identification and treatment of chemoresistant inoperable or metastatic GIST: experience with the selective tyrosine kinase inhibitor imatinib mesylate (STI571)

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms of the gastrointestinal tract. The concept of GIST and the definition of GIST pathology have evolved greatly over the past 5 years. GIST has been shown to share immunohistochemical, ultrastructural and histogenic similarities with the interstitial cells of Cajal. Both GIST and the interstitial cells of Cajal express KIT, the receptor tyrosine kinase that is the protein product of the c-kit proto-oncogene. KIT is universally phosphorylated in GISTs. Sequencing of c-kit complementary DNA from human GIST cells has demonstrated a high frequency of mutations that lead to constitutive activation of the KIT tyrosine kinase in the absence of stimulation by its physiologic ligand (stem cell factor). This, in turn, causes uncontrolled stimulation of downstream signaling cascades with aberrant cellular proliferation and resistance to apoptosis. Historically, malignant GIST has been highly refractory to conventional cytotoxic therapy. Signal transduction inhibition as cancer therapy was first tested successfully with imatinib mesylate (formerly known as STI571), a selective small-molecule tyrosine kinase inhibitor, with the initial target being blockade of Bcr-Abl, the oncogene with tyrosine kinase activity responsible for the pathogenesis of chronic myelogenous leukemia (CML). Imatinib was subsequently shown to block activity of the KIT tyrosine kinase as well, and in laboratory studies this led to apoptotic death of GIST cells. The first GIST patient to receive imatinib exhibited dramatic benefit despite far-advanced metastatic disease that was previously refractory to all chemotherapy. Subsequently, multicenter clinical trials have been performed to assess the safety, efficacy and biologic activity of imatinib in patients with advanced GIST. The results from these studies have established imatinib as an effective new therapeutic alternative for the majority of patients with advanced GIST, a solid tumor for which no prior chemotherapy has ever shown antitumor efficacy. This work provides proof of concept to the hypothesis that selective inhibition of aberrant signal transduction can provide important anticancer activity, if the proper signaling pathways are identified and blocked.

Overview of issues related to imatinib therapy of advanced gastrointestinal stromal tumors: a discussion among the experts

Information regarding the activity of imatinib in patients with metastatic gastrointestinal stromal tumors (GISTs) has accumulated rapidly. Nevertheless, several important issues about imatinib therapy as well as these tumors themselves remain to be answered. Importantly, the optimal dose and duration of imatinib therapy are unknown, with daily doses of 400 mg and 600 mg producing comparable response rates in a phase II study. Moreover, the role of surgery following maximal responses to imatinib and those of functional imaging and use of biopsies in monitoring treatment responses need to be investigated. Further understanding of the molecular and pathologic characteristics of GISTs that are responsive or resistant to imatinib is also needed. This paper summarizes a symposium that was held in Helsinki, Finland, in September 2001.