Imatinib resistance in gastrointestinal stromal tumors

GI Stromal Tumors: 15 Years of Lessons From a Rare Cancer

A confluence of factors, most prominently the recognition of GI stromal tumor (GIST) as a specific sarcoma subtype and the availability of imatinib, led to the “Big Bang” of GIST therapy (ie, the successful treatment of the first patient with GIST with imatinib in 2000). The trail blazed by imatinib for chronic myelogenous leukemia and GIST has become a desired route to regulatory approval of an increasing number of oral kinase inhibitors and other novel therapeutics. In this review, the status of GIST management before and after GIST's “Big Bang” and new steps being taken to further improve on therapy are reviewed.

Sorafenib cardiotoxicity increases mortality after myocardial infarction

RATIONALE

Sorafenib is an effective treatment for renal cell carcinoma, but recent clinical reports have documented its cardiotoxicity through an unknown mechanism.

OBJECTIVE

Determining the mechanism of sorafenib-mediated cardiotoxicity.

METHODS AND RESULTS

Mice treated with sorafenib or vehicle for 3 weeks underwent induced myocardial infarction (MI) after 1 week of treatment. Sorafenib markedly decreased 2-week survival relative to vehicle-treated controls, but echocardiography at 1 and 2 weeks post MI detected no differences in cardiac function. Sorafenib-treated hearts had significantly smaller diastolic and systolic volumes and reduced heart weights. High doses of sorafenib induced necrotic death of isolated myocytes in vitro, but lower doses did not induce myocyte death or affect inotropy. Histological analysis documented increased myocyte cross-sectional area despite smaller heart sizes after sorafenib treatment, further suggesting myocyte loss. Sorafenib caused apoptotic cell death of cardiac- and bone-derived c-kit+ stem cells in vitro and decreased the number of BrdU+ (5-bromo-2'-deoxyuridine+) myocytes detected at the infarct border zone in fixed tissues. Sorafenib had no effect on infarct size, fibrosis, or post-MI neovascularization. When sorafenib-treated animals received metoprolol treatment post MI, the sorafenib-induced increase in post-MI mortality was eliminated, cardiac function was improved, and myocyte loss was ameliorated.

CONCLUSIONS

Sorafenib cardiotoxicity results from myocyte necrosis rather than from any direct effect on myocyte function. Surviving myocytes undergo pathological hypertrophy. Inhibition of c-kit+ stem cell proliferation by inducing apoptosis exacerbates damage by decreasing endogenous cardiac repair. In the setting of MI, which also causes large-scale cell loss, sorafenib cardiotoxicity dramatically increases mortality.

Rectum sarcoma: challenging diagnostic and therapeutic modalities

INTRODUCTION

Sarcomas are malignant tumors that arise from mesenchymal tissue at any of the body sites. They incorporate the wide category of GISTs and are classified in various histological types. Histological grading is another indicator of the degree of malignancy, the probability of distant metastases, and survival but remains a poor definition of local recurrence.

DISCUSSION

The size and depth of invasion are the most important prognostic factors. Since they grow within the intestinal wall, the symptoms are usually few or late, leading to delays in diagnosis. Most common signs are rectal bleeding, abdominal or anal pain, diarrhea, tenesmus and weight loss. The diagnostic and staging protocol of stromal tumors of the rectum includes mainly endoscopic surveillance, computed tomography, and magnetic resonance imaging. Therefore, rectum sarcoma (RS) consists one of the most biologically virulent cancers and is difficult to cure by conventional procedures. The treatment is primarily surgical, where possible, and should guarantee complete clearance of the tumor, which often requires an aggressive approach. Unfortunately, the minority of patients is eligible to undergo surgical intervention. In addition, surgical removal of RS does not necessarily indicate a patient's long-term recovery. Alternative therapies, such as radio- and chemotherapy, proved insufficient. Elucidation of its molecular basis may prove useful in developing and identifying prognostic biomarkers.

Long-surviving patients with recurrent GIST after receiving cytoreductive surgery with imatinib therapy

In the treatment of recurrent or metastatic gastrointestinal stromal tumors (GIST), good prognoses may not be expected by surgery alone. Recently, imatinib has been applied for the treatment of GISTs, resulting in improved patient survival. However, long-term success is limited due to the development of resistance. Herein, we report two cases of long-surviving patients with recurrent GIST after receiving cytoreductive surgery with imatinib therapy. A 49 year-old man was diagnosed to a duodenal GIST with single hepatic metastasis, and an antrectomy including the duodenal lesion with intraoperative radiofrequency ablation were performed in April, 2002. After four months, a new metastatic hepatic lesion was identified. Percutaneous radiofrequency ablation was done, and imatinib therapy was started. A 56 year-old man underwent laparoscopic segmental resection of the distal ileum and partial excision of parietal peritoneum in March, 2001 to treat a malignant GIST of the distal ileum that was attached to parietal peritoneum. After six months, recurrence of GIST with peritoneal seeding and hepatic metastasis was found, and he underwent cytoreductive surgery including right hemicolectomy and wedge resection of liver. After surgery, there was no residual tumor grossly and imatinib therapy was started. In both cases, they were doing well with no evidence of recurrence for 5 years with imatinib therapy. Therefore, in patients with a recurrent GIST, improved survival can be expected with imatinib therapy after cytoreductive surgery.

New Strategies for Treating GIST When Imatinib Fails

Gastrointestinal stromal tumors (GISTs) are soft tissue sarcomas arising in the GI tract. Most GISTs have an activating mutation in KIT or PDGFR-alpha and respond to treatment with imatinib mesylate (Gleevec, Novartis), a small molecule tyrosine kinase inhibitor that blocks downstream signaling of the mutated kinase. Imatinib has dramatically improved survival in patients with unresectable or metastatic GIST; however, approximately 15 percent of patients do not respond to imatinib, and many others progress after an initial period of response or disease stabilization. New agents that target multiple kinases in GIST as well as downstream effector molecules are being developed and tested in clinical trials.

Surgical outcomes of patients with gastrointestinal stromal tumors in the era of targeted drug therapy

BACKGROUND

The discovery of the c-KIT mutation and the advent of targeted drug therapy with imatinib mesylate have revolutionized the management of gastrointestinal stromal tumors (GISTs). The outcome of patients with surgically treated GISTs treated in the era of targeted drug therapy was assessed and factors associated with adverse outcomes determined.

MATERIALS AND METHODS

Patients with GISTs requiring surgery at a tertiary care center from 2002 to 2007 were reviewed and prognostic factors determined.

RESULTS

Forty patients were surgically treated for GISTs. The median age at presentation was 59 years. The stomach (55%) was the main site of primary disease. The median tumor size was 7 cm. Eleven (28%) patients had metastatic disease at presentation. Surgery was undertaken in all patients with curative intent. Multi-organ resection was required in 10 (25%) patients. Imatinib mesylate was administered postoperatively in 68% of cases. Median follow-up was 24 months. There was a 40% recurrence rate with 63% undergoing repeat surgical resection. The peritoneum and liver were the main sites of recurrent disease. The 5-year disease-specific survival and disease-free survival (DFS) were 65% and 35%, respectively. High mitotic rate (P = 0.017) and tumor size greater than 10 cm (P = 0.009) were the only prognostically significant adverse factors of DFS on multivariate analysis, independent of imatinib mesylate treatment.

CONCLUSION

Aggressive surgical treatment and follow-up of GISTs, combined with targeted drug therapy, leads to long-term DFS survival. Tumor recurrence is independently associated with a high tumor mitotic rate and size greater than 10 cm, despite the use of adjuvant targeted drug therapy.

Genetic aberrations of gastrointestinal stromal tumors

Gastrointestinal stromal tumor (GIST) is the most common mesenchymal neoplasm in the gastrointestinal tract and is associated with mutations of the KIT or PDGFRA gene. In addition, other genetic events are believed to be involved in GIST tumorigenesis. Cytogenetic aberrations associated with these tumors thus far described include loss of 1p, 13q, 14q, or 15q, loss of heterozygosity of 22q, numeric chromosomal imbalances, and nuclear/mitochondrial microsatellite instability. Molecular genetic aberrations include loss of heterozygosity of p16(INK4A) and p14(ARF), methylation of p15(INK4B), homozygous loss of the Hox11L1 gene, and amplification of C-MYC, MDM2, EGFR1, and CCND1. GISTs in patients with neurofibromatosis type 1 appear to lack the KIT and PDGFRA mutations characteristic of GISTs and may have a different pathogenetic mechanism. Gene mutations of KIT or PDGFRA are critical in GISTs, because the aberrant versions not only are correlated with the specific cell morphology, histologic phenotype, metastasis, and prognosis, but also are the targets of therapy with imatinib and other agents. Furthermore, specific mutations in KIT and PDGFR appear to lead to differential drug sensitivity and may in the future guide selection of tyrosine kinase inhibitors. Activation of the receptor tyrosine kinases involves a signal transduction pathway whose components (mitogen-activated protein kinase, AKT, phosphoinositide 3-kinase, mammalian target of rapamycin, and RAS) are also possible targets of inhibition. A new paradigm of classification, integrating the standard clinical and pathological criteria with molecular aberrations, may permit personalized prognosis and treatment.

Evolution from heterozygous to homozygous KIT mutation in gastrointestinal stromal tumor correlates with the mechanism of mitotic nondisjunction and significant tumor progression

Activating mutation in KIT or platelet-derived growth factor-alpha can lead to gastrointestinal stromal tumors (GISTs). Eighty-four cases from two institutes were analyzed. Of them, 62 (74%) harbored KIT mutations, 7 of which are previously unreported. One exhibited duplication from both intron 11 and exon 11, which has not been reported in KIT in human cancer. A homozygous/hemizygous KIT-activating mutation was found in 9 of the 62 cases (15%). We identified three GIST patients with heterozygous KIT-activating mutations at initial presentation, who later recurred with highly aggressive clinical courses. Molecular analysis at recurrence showed total dominance of homozygous (diploid) KIT-activating mutation within a short period of 6-13 months, suggesting an important role of oncogene homozygosity in tumor progression. Topoisomerase II is active in the S- and G(2) phases of cell cycle and is a direct and accurate proliferative indicator. Cellular and molecular analysis of serial tumor specimens obtained from consecutive surgeries or biopsy within the same patient revealed that these clones that acquired the homozygous KIT mutation exhibited an increased mitotic count and a striking fourfold increase in topoisomerase II proliferative index (percentage cells show positive topoisomerase II nuclear staining compared to the heterozygous counterpart within the same patient. KIT forms a homodimer as the initial step in signal transduction and this may account for the quadruple increase in proliferation. Using SNPs for allelotyping on the serial tumor specimens, we demonstrate that the mechanism of the second hit resulting in homozygous KIT-activating mutation and loss of heterozygosity is achieved by mitotic nondisjunction, contrary to the commonly reported mechanism of mitotic recombination.

EXEL-7647 inhibits mutant forms of ErbB2 associated with lapatinib resistance and neoplastic transformation

PURPOSE

Mutations associated with resistance to kinase inhibition are an important mechanism of intrinsic or acquired loss of clinical efficacy for kinase-targeted therapeutics. We report the prospective discovery of ErbB2 mutations that confer resistance to the small-molecule inhibitor lapatinib.

EXPERIMENTAL DESIGN

We did in vitro screening using a randomly mutagenized ErbB2 expression library in Ba/F3 cells, which were dependent on ErbB2 activity for survival and growth.

RESULTS

Lapatinib resistance screens identified mutations at 16 different ErbB2 amino acid residues, with 12 mutated amino acids mapping to the kinase domain. Mutations conferring the greatest lapatinib resistance cluster in the NH2-terminal kinase lobe and hinge region. Structural computer modeling studies suggest that lapatinib resistance is caused by multiple mechanisms; including direct steric interference and restriction of conformational flexibility (the inactive state required for lapatinib binding is energetically unfavorable). ErbB2 T798I imparts the strongest lapatinib resistance effect and is analogous to the epidermal growth factor receptor T790M, ABL T315I, and cKIT T670I gatekeeper mutations that are associated with clinical drug resistance. ErbB2 mutants associated with lapatinib resistance transformed NIH-3T3 cells, including L755S and T733I mutations known to occur in human breast and gastric carcinomas, supporting a direct mechanism for lapatinib resistance in ErbB2-driven human cancers. The epidermal growth factor receptor/ErbB2/vascular endothelial growth factor receptor inhibitor EXEL-7647 was found to inhibit almost all lapatinib resistance-associated mutations. Furthermore, no ErbB2 mutations were found to be associated with EXEL-7647 resistance and lapatinib sensitivity.

CONCLUSIONS

Taken together, these data suggest potential target-based mechanisms of resistance to lapatinib and suggest that EXEL-7647 may be able to circumvent these effects.

Sorafenib inhibits imatinib-resistant KIT and platelet-derived growth factor receptor beta gatekeeper mutants

PURPOSE

Targeting of KIT and platelet-derived growth factor receptor (PDGFR) tyrosine kinases by imatinib is an effective anticancer strategy. However, mutations of the gatekeeper residue (T670 in KIT and T681 in PDGFRbeta) render the two kinases resistant to imatinib. The aim of this study was to evaluate whether sorafenib (BAY 43-9006), a multitargeted ATP-competitive inhibitor of KIT and PDGFR, was active against imatinib-resistant KIT and PDGFRbeta kinases.

EXPERIMENTAL DESIGN

We used in vitro kinase assays and immunoblot with phosphospecific antibodies to determine the activity of sorafenib on KIT and PDGFRbeta kinases. We also exploited reporter luciferase assays to measure the effects of sorafenib on KIT and PDGFRbeta downstream signaling events. The activity of sorafenib on interleukin-3-independent proliferation of Ba/F3 cells expressing oncogenic KIT or its imatinib-resistant T670I mutant was also tested.

RESULTS

Sorafenib efficiently inhibited gatekeeper mutants of KIT and PDGFRbeta (IC(50) for KIT T670I, 60 nmol/L; IC(50) for PDGFRbeta T681I, 110 nmol/L). Instead, it was less active against activation loop mutants of the two receptors (IC(50) for KIT D816V, 3.8 micromol/L; IC(50) for PDGFRbeta D850V, 1.17 micromol/L) that are also imatinib-resistant. Sorafenib blocked receptor autophosphorylation and signaling of KIT and PDGFRbeta gatekeeper mutants in intact cells as well as activation of AP1-responsive and cyclin D1 gene promoters, respectively. Finally, the compound inhibited KIT-dependent proliferation of Ba/F3 cells expressing the oncogenic KIT mutant carrying the T670I mutation.

CONCLUSIONS

Sorafenib might be a promising anticancer agent for patients carrying KIT and PDGFRbeta gatekeeper mutations.

Surgical interventions for focal progression of advanced gastrointestinal stromal tumors during imatinib therapy

BACKGROUND

Although imatinib has shown high activity in the majority of patients with advanced gastrointestinal stromal tumors (GIST), it has become clear that secondary resistance appears during chronic therapy. The aim of this study was to retrospectively analyze the safety and prognostic effects of surgical interventions for focal progression during imatinib treatment.

METHODS

Between January 2002 and May 2005, 16 patients who had focal lesions of secondary-resistant GIST to imatinib treatment (male/female, 12:4; median age, 62 years) underwent surgical interventions such as resection, radiofrequency ablation, and their combination.

RESULTS

Postoperative complications, including liver abscess, bile leak, wound infection, and ileus were mostly mild, and the patients recovered with conservative therapy. There was no hospital death. The median time to progression (TTP) of all patients was 5.5 months, and only one patient died of the disease; the others are alive after a median follow up of 12.4 months. Patients with complete resections of resistant lesions (n = 7) showed significantly better median TTP than those with incomplete resections (n = 9; P = 0.014). The impact of curability on focal lesions with secondary resistance was mainly significant in patients with tumors of stomach origin (P = 0.013), and a smaller number (P = 0.014) and smaller size (P = 0.018) of resistant lesions. Overall survival was 100% at 1 year and 75% at 2 years.

CONCLUSION

Our study indicates that surgical interventions in patients with GIST resistant to imatinib therapy are efficacious when complete resections are performed, when the lesions are of gastric origin, when the number of lesions is lower, and when the lesions are a smaller size.

Prospective multicentric randomized phase III study of imatinib in patients with advanced gastrointestinal stromal tumors comparing interruption versus continuation of treatment beyond 1 year: the French Sarcoma Group

PURPOSE

Imatinib is the standard treatment of advanced GI stromal tumors (GISTs). It is not known whether imatinib may be stopped in patients in whom disease is controlled.

METHODS

This prospective, randomized, multicentric phase III study was designed to compare continuous (CONT) compared with interrupted (INT) imatinib beyond 1 year of treatment in patients with advanced GIST. The primary end point was progression-free survival. Secondary end points included overall survival, response rate after reinitiation of imatinib, and quality of life. Early stopping rules in cases of rapid progression of disease were defined, with preplanned interim analyses.

RESULTS

Between May 2002 and April 2004, 182 patients with advanced GIST were enrolled. Between May 2003 and April 2004, 98 patients in response or stable disease under imatinib reached more than 1 year of follow-up. Forty were not eligible for randomization, and 58 patients were randomly assigned, 32 and 26 patients in the INT and CONT arms, respectively. As of October 15, 2005, eight of 26 patients in the CONT group and 26 of 32 patients in the INT group had documented disease progression (P < .0001). Twenty-four of 26 patients with documented progression in the INT arm responded to imatinib reintroduction. No differences in overall survival or imatinib resistance were observed between the two arms. Quality of life evaluated 6 months after random assignment using the 30-item Quality of Life Questionnaire was not significantly different between the two groups of randomly assigned patients.

CONCLUSION

Imatinib interruption results in rapid progression in most patients with advanced GIST, and cannot be recommended in routine practice unless patient experience significant toxicity

KIT and PDGFRA mutations in gastrointestinal stromal tumors (GISTs)

Mutually exclusive KIT and PDGFRA mutations are central events in GIST pathogenesis, and their understanding is becoming increasingly important, because specific treatment targeting oncogenic KIT and PDGFRA activation (especially imatinib mesylate) has become available. KIT mutations in GIST are clustered in four exons. Most common are exon 11 (juxtamembrane domain) mutations that include deletions, point mutations (affecting a few codons), and duplications (mostly in the 3' region). The latter mutations most often occur in gastric GISTs. Among gastric GISTs, tumors with deletions are more aggressive than those with point mutations; this does not seem to hold true in small intestinal GISTs. Exon 9 mutations (5-10%) usually are 2-codon 502-503 duplications, and these occur predominantly in intestinal versus gastric GISTs. Lesser imatinib sensitivity of these tumors has been noted. Kinase domain mutations are very rare; GISTs with such mutations are variably sensitive to imatinib. PDGFRA mutations usually occur in gastric GISTs, especially in the epithelioid variants; their overall frequency is approximately 30% to 40% of KIT mutation negative GISTs. Most common is exon 18 mutation leading Asp842Val at the protein level. This mutation causes imatinib resistance. Exon 12 and 14 mutations are rare. Most mutations are somatic (in tumor tissue only), but patients with familial GIST syndrome have consitutitonal KIT/PDGFRA mutations; >10 families have been reported worldwide with mutations generally similar to those in sporadic GISTs. GISTs in neurofibromatosis 1 patients, children, and Carney triad seem to lack GIST-specific KIT and PDGFRA mutations and may have a different disease mechanism. Secondary mutations usually occur in KIT kinase domains in patients after imatinib treatment resulting in resistance to this drug. Mutation genotyping is a tool in GIST diagnosis and in assessment of sensitivity to kinase inhibitors. This is a US government work. There are no restrictions on its use.

A novel tyrosine kinase switch is a mechanism of imatinib resistance in gastrointestinal stromal tumors

KIT or alpha-platelet-derived growth factor receptor (alpha-PDGFR) activating mutations are the pathogenic mechanisms that characterize gastrointestinal stromal tumors (GIST). Despite excellent responses to imatinib mesylate (IM), patients are relapsing. We developed an IM-resistant GIST cell line (GIST-R) from the IM-sensitive GIST882 cell line (GIST-S) by growing these cells in IM. Gene expression profiling (GEP) of GIST-S, GIST-R cells and two IM resistant GIST patients demonstrated that KIT is downregulated implying a major role in IM resistance. Instead, GIST-R cells have acquired IM resistance by overexpressing the oncogenic receptor tyrosine kinase - AXL - in a 'kinase switch'. Further, the two IM resistant GIST patients express AXL and not c-Kit, seen by immunohistochemistry (IHC). Real time reverse transcriptase-polymerase chain reaction and Western blotting of the GIST-S and GIST-R cells confirmed the switch from Kit to AXL. In GIST-R, AXL is tyrosine phosphorylated and its ligand growth-arrest-specific gene 6 is overexpressed implying autocrine activation. The kinase switch is associated with a morphological change from spindle to epithelioid. Molecular modeling of the kinase domain of mutant c-Kit (V654A) and AXL showed no binding to IM but efficient binding to MP470, a novel c-Kit/AXL kinase inhibitor. MP470 synergizes with docetaxel (taxotere) and is cytotoxic to GIST cells.

Gastrointestinal stromal tumors: review on morphology, molecular pathology, prognosis, and differential diagnosis

CONTEXT

Gastrointestinal stromal tumors (GISTs) are specific, generally Kit (CD117)-positive, mesenchymal tumors of the gastrointestinal tract encompassing a majority of tumors previously considered gastrointestinal smooth muscle tumors. They are believed to originate from interstitial cells of Cajal or related stem cells.

OBJECTIVE

To review current clinicopathologically relevant information on GIST.

DATA SOURCES

Literature in Medline and authors' own experience.

CONCLUSIONS

GISTs usually occur in older adults (median age 55-60 years) and rarely in children in the second decade (<1%) throughout the gastrointestinal tract: 60% in stomach, 35% in small intestine, and less than 5% in rectum, esophagus, omentum, and mesentery; most GISTs in the latter 2 sites are metastatic. Five percent of GISTs occur in patients with neurofibromatosis type 1 syndrome (multiple small intestinal tumors) and in Carney triad (gastric epithelioid GISTs in young females). Familial GISTs occur in patients with inheritable germline Kit or platelet-derived growth factor receptor alpha (PDGFRA) mutations. Histologically GISTs vary from spindle cell tumors to epithelioid and pleomorphic tumors. Most GISTs (95%) express Kit (CD117), CD34 (70%), and heavy caldesmon (80%), whereas 25% are positive for smooth muscle actin and less than 5% for desmin. Tumor size and mitotic activity are best predictive prognostic features; small intestinal tumors behave more aggressively than gastric tumors with similar parameters. Mutually exclusive gain-of-function Kit or PDGFRA mutations occur in a majority of GISTs representing in-frame deletions, point mutations, duplications and insertions. Mutations in Kit juxtamembrane domain (exon 11) are the most common in GISTs of all sites, whereas rare Kit extracellular domain (exon 9) Ala502-Tyr503 duplication is specific for intestinal GISTs. Mutations in PDGFRA have been identified in juxtamembrane (exon 12) and tyrosine kinase domains (exons 14 and 18), nearly exclusively in gastric GISTs, mostly in epithelioid variants. Some Kit and PDGFRA mutations have a prognostic value. Kit/PDGFRA tyrosine kinase inhibitor imatinib has been successfully used in the treatment of metastatic GISTs for more than 5 years. However, primary and acquired secondary resistance linked to certain types of Kit and PDGFRA mutations is limiting long-term success necessitating the use of alternative treatments.

Discovery and development of sorafenib: a multikinase inhibitor for treating cancer

Since the molecular revolution of the 1980s, knowledge of the aetiology of cancer has increased considerably, which has led to the discovery and development of targeted therapies tailored to inhibit cancer-specific pathways. The introduction and refinement of rapid, high-throughput screening technologies over the past decade has greatly facilitated this targeted discovery and development process. Here, we describe the discovery and continuing development of sorafenib (previously known as BAY 43-9006), the first oral multikinase inhibitor that targets Raf and affects tumour signalling and the tumour vasculature. The discovery cycle of sorafenib (Nexavar; Bayer Pharmaceuticals) - from initial screening for a lead compound to FDA approval for the treatment of advanced renal cell carcinoma in December 2005 - was completed in just 11 years, with approval being received approximately 5 years after the initiation of the first Phase I trial.

Gastrointestinal stromal tumor (GIST) and imatinib

Imatinib mesylate is the first and only effective drug for the treatment of gastrointestinal stromal tumor at present. Mutated exon 11 of the KIT receptor is essential for the pathogenesis and response to imatinib mesylate of gastrointestinal stromal tumor; the efficacy rate (complete response+partial response) of imatinib mesylate is 53.8%, and the disease-control rate (complete response+partial response+stable disease) is 84%. Almost 90% of patients experienced non-hematological and hematological adverse effects, which were tolerable, in particular at a daily dose of 400 mg imatinib mesylate, which warranted response induction for half of the patients, and is the dose approved by Japanese medical insurance. Clinical trials suggest that an increased dose of imatinib mesylate would be beneficial, and that the interruption of imatinib treatment might result in disease progression even after a partial response. Tentative Japanese guidelines for the diagnosis and therapy of gastrointestinal stromal tumors are being prepared by the Gastrointestinal Stromal Tumor Committee of the Japan Society of Clinical Oncology, and are presented here for critical comments by colleagues.

Sleeping beauty: a novel cancer gene discovery tool

The National Cancer Institute and the National Human Genome Research Institute recently announced a 3-year 100-million-dollar pilot study to use large-scale resequencing of genes in human tumors to identify new cancer genes. The hope is that some of these genes can be used as drug targets for developing better therapeutics for treating cancer. Although this effort will identify new cancer genes, it could be made more efficient by preferentially resequencing genes identified as novel candidate cancer genes in animal models of cancer. Although retroviral insertional mutagenesis has proven to be an effective tool for identifying novel cancer genes in the mouse, these studies are limited by the fact that retroviral mutagenesis primarily induces hematopoietic and mammary cancer, but little else, while the majority of cancers affecting humans are solid tumors. Recently, two groups have shown that sleeping beauty (SB) transposon-based insertional mutagenesis can also identify novel candidate cancer genes in the mouse. Unlike retroviral infection, SB transposition can be controlled to mutagenize any target tissue and thus potentially induce many different kinds of cancer, including solid tumors. SB transposition in animal models of cancer could therefore greatly facilitate the identification of novel human cancer genes and the development of better cancer therapies.

[Gastrointestinal stromal tumors: definition, histological, immunohistochemical, and molecular features, and diagnostic strategy]

Gastrointestinal stromal tumors (GISTs) are the most frequent mesenchymal tumors of the gastrointestinal tract. Major advances in their definition and classification and the understanding of their molecular mechanisms have recently been made. These advances have resulted in the delineation of a treatment that has become a model of targeted therapy in oncology. GISTs are defined as tumors of the gastrointestinal tract, but also of the mesentery and peritoneum, constituted by a proliferation of usually spindle-shaped, rarely epithelioid cells, usually, but not consistently expressing the KIT protein. Most GISTs are associated with molecular abnormalities in two target genes: KIT (which encodes the KIT protein) and PDGFRA (which encodes the A chain of the PDGF receptor). The diagnosis of GIST relies on histological arguments (proliferation of spindle-shaped cells in 70% of cases, of epithelioid cells in 20%; histological variants are rare and sometimes misleading) and on immunohistochemical arguments (expression of KIT in 95%, usually associated with CD34 expression in 60%-70% of cases). The demonstration of mutations in target genes is required only in cases that are histologically suggestive but KIT-negative; beyond this indication, this is only undertaken in research protocols. The differential diagnosis of GIST includes the other mesenchymal tumors of the gastrointestinal tract, such as leiomyomas and leiomyosarcomas, and the digestive locations of some sarcomas; it relies on both histological and immunohistochemical arguments. The evaluation of the prognosis is essential. According to the current concept, every GIST carries a risk of malignancy, which may vary from very low to very high. Prognosis is based on a simple algorithm using two histoprognostic parameters, i.e., tumor size and mitotic index. The treatment of localized GIST is surgical resection, which must be complete; that of advanced or unresectable GIST is based on the use of a targeted therapy, imatinib, which is a pharmacological antagonist of the KIT protein. Proper understanding and utilisation of the diagnostic criteria and classification of GIST by pathologists are essential for good patient management.

Kinase mutations in cancer: chinks in the enemyʼs armour?

PURPOSE OF REVIEW

Over the past few years, a revolution has transformed the oncology field. This revolution is characterized by two main features. The first is the introduction of the concept of individualized cancer therapy. The second is the development of drugs targeting molecules selectively altered in tumours. This review analyses these aspects by looking at the role that altered kinases and their inhibitors have played in this historical process.

RECENT FINDINGS

Tumour progression is the result of the sequential accumulation of mutations in genes monitoring the rates of cell birth and cell death. The molecular profiling of cancers has shown that protein and lipid kinases are frequently altered in tumour cells. In most cases, these alterations translate in constitutively active proteins, which are amenable of therapeutic targeting. Intriguingly, even 'established' cancer cells remain somewhat 'addicted' to the deregulated activity of mutated kinases. This feature appears to be the basis for the ability of kinase inhibitors in controlling the development of a number of cancers. The therapeutic efficacy of kinase inhibitors is impaired by the emergence of tumour cells carrying 'resistance' mutations.

SUMMARY

Many oncogenes are mutated kinase genes. In most cases, the mutations result in the constitutive activation of the affected kinase that can be pharmacologically inhibited. Unfortunately, upon treatment with kinase inhibitors, resistant clones develop rapidly, impairing their therapeutic effect. Strategies to overcome resistance are discussed as well as the possibility to target kinases regulating cancer stem cells.

Structure and regulation of Kit protein-tyrosine kinase--the stem cell factor receptor

Signaling by stem cell factor and Kit, its receptor, play important roles in gametogenesis, hematopoiesis, mast cell development and function, and melanogenesis. Moreover, human and mouse embryonic stem cells express Kit transcripts. Stem cell factor exists as both a soluble and a membrane-bound glycoprotein while Kit is a glycoprotein receptor protein-tyrosine kinase. The complete absence of stem cell factor or Kit is lethal. Gain-of-function mutations of Kit are associated with several human neoplasms including acute myelogenous leukemia, gastrointestinal stromal tumors, mastocytomas, and nasal T-cell lymphomas. Binding of stem cell factor to Kit results in receptor dimerization and activation of protein kinase activity. The activated receptor becomes autophosphorylated at tyrosine residues that serve as docking sites for signal transduction molecules containing SH2 domains. Kit activates Akt, Src family kinases, phosphatidylinositol 3-kinase, phospholipase Cgamma, and Ras/mitogen-activated protein kinases. Kit exists in active and inactive conformations as determined by X-ray crystallography. Kit consists of an extracellular domain, a transmembrane segment, a juxtamembrane domain, and a protein kinase domain that contains an insert of about 80 amino acid residues. The juxtamembrane domain inhibits enzyme activity in cis by maintaining the control alphaC-helix and the activation loop in their inactive conformations. The juxtamembrane domain also inhibits receptor dimerization. STI-571, a clinically effective targeted protein-tyrosine kinase inhibitor, binds to an inactive conformation of Kit. The majority of human gastrointestinal stromal tumors have Kit gain-of-function mutations in the juxtamembrane domain, and most people with these tumors respond to STI-571. STI-571 binds to Kit and Bcr-Abl (the oncoprotein of chronic myelogenous leukemia) at their ATP-binding sites.