The improved anticancer effects of Bortezomib-loaded hollow mesoporous silica nanospheres on lymphoma development

As the first clinical proteasome inhibitor, Bortezomib (BTZ) has been reported to improve the outcome of lymphoma. However, due to the unstable property, low bioavailability, and hydrophobic properties of BTZ, it is needed to develop effective drug delivery systems to deliver BTZ into targeted cells or organs. Here we developed a bortezomib (BTZ)-loaded HMSNs (BTZ@HMSNs) system, which can sustain the release of BTZ in targeted tissues. In vitro assays showed that BTZ@HMSNs limited cell proliferation and augmented apoptosis of lymphoma SNK-1 cells. Moreover, BTZ@HMSNs significantly diminished migration and invasion of SNK-1 cells as compared with BTZ. In contrast to the upregulation of SHP-1, BTZ@HMSNs decreased the mRNA levels of c-Kit, NF-κB, and JAK1, which elicit oncogenic role in lymphoma development. Importantly, lymphoma mice model showed that BTZ@HMSNs significantly activated p53 signaling and reduced tumor volume and weight compared with free BTZ. Our data thus demonstrate that BTZ@HMSNs manifests improved tumor-suppressing effect in vitro and in vivo compared to free BTZ. We believe that HMSNs is a promising strategy for delivering therapeutic agents for cancer treatment.

Complete response to sunitinib for more than three years in a patient with a jejunum gastrointestinal stromal tumor: A case report

RATIONALE

Gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor of the gastrointestinal tract and is characterized by KIT mutations. Patientsresistant to 1st-line imatinib therapy are usually given sunitinib assecond-line treatment, which provides a median progression-free survival of 8 to 12 months. We report the 1st case of metastatic jejunum GIST with a KIT exon 11 deletion that showed complete response (CR) to sunitinib for more than 3 years.

PATIENT CONCERNS

A 34-year-old man with advanced jejunum GIST was surgically treated upon initial diagnosis, and was histologically found to carry a high recurrence risk. Genetic testing revealed a KIT exon 11 deletion, and adjuvant therapy with imatinib was administered. The imatinib dose was escalated following recurrence in the abdomen, but the mass continued to grow.

DIAGNOSIS

He was diagnosed with abdominal recurrence of GIST based on his medical history and histopathological results.

INTERVENTION

Second-line sunitinib therapy was given.

OUTCOMES

The mass disappeared, and CR was seen following 7 months of sunitinib therapy; this CR was sustained for more than 45 months.

LESSONS

In cases of metastatic jejunum GIST with a KIT exon 11 deletion, sunitinib as second-line therapy can be used to achieve CR for more than 3 years.

Gastrointestinal stromal tumor of unusual phenotype after imatinib treatment: A case report and diagnostic utility of ETV1 mRNA in situ hybridization

RATIONALE

Gastrointestinal stromal tumor (GIST) is the most common tumor of mesenchymal origin in gastrointestinal tract. Immunohistochemical (IHC) staining combined with a typical morphology is used for the diagnosis of GIST. Typically, IHC staining for v-kit Hardy-Zuckerman 4 feline sarcoma viral oncogene (KIT) and discovered on GIST-1(DOG1) is positive in almost all GISTs. However, imatinib mesylate, a specific inhibitor of KIT tyrosine kinase, frequently involves changes in the morphology and IHC staining of GIST, impeding the diagnosis. Recently, in situ hybridization (ISH) for E26 transformation-specific sequence variant 1 (ETV1) mRNA was introduced as a useful marker to diagnose GIST.

PATIENT CONCERNS

We report 2 cases of gastric GIST, which expressed unusual phenotypes after imatinib therapy.

DIAGNOSES

The first patient was found to have a gastric subepithelial tumor in gastroduodenoscopy done for regular checkup. In biopsy of the tumor, it showed homogenous spindle cells that were positive to standard IHC markers for GIST. The second patient visited our hospital because of a palpable mass in the abdomen. In abdominal computed tomography (CT), a tumor arising from the stomach was found. A needle biopsy was done and the patient was diagnosed of gastric GIST because the biopsy showed spindle cells positive to typical IHC markers for GIST. After imatinib treatment, in both patients, the resected tumors were composed of heterogeneous spindle cells negative to KIT, DOG1, and CD34 IHC staining, which was unusual for GIST. However, ISH for ETV1 mRNA done for both biopsied and resected tumors was positive, even after imatinib treatment. A molecular analysis found a mutation in exon 11 of KIT gene before and after imatinib therapy in both patients, confirming the diagnosis of GIST.

INTERVENTIONS

Both patients took neoadjuvant imatinib treatment, and afterwards, underwent a surgical resection.

OUTCOMES

The patients remain on imatinib treatment and no progression or recurrence has been detected to date.

LESSONS

ISH for ETV1 mRNA is a useful technique in diagnosing GIST when IHC with KIT, DOG1, or CD34 fail to stain positive after imatinib therapy.

Discovery of Dual Inhibitors for Wild Type and D816V Mutant of c-KIT Kinase through Virtual and Biochemical Screening of Natural Products

Although stem cell factor receptor (c-KIT) kinase is responsible for various malignant human cancers, the presence of constitutively active gain-of-function mutants has made it difficult to discover new anticancer agents using c-KIT as the target protein. To identify the common inhibitors of wild-type c-KIT and the most abundant gain-of-function mutant (D816V), the virtual screening of natural products was performed for the two target proteins in parallel with the scoring function improved by implementing a sophisticated solvation free energy term. As a result, four common inhibitors of natural origin are found with biochemical potencies ranging from low micromolar to submicromolar levels. The results of extensive docking simulations show that although the natural-product inhibitors establish weaker hydrophobic interactions with the D816V mutant than with the wild type, they exhibit a little higher inhibitory activity for the former than the latter by strengthening the hydrogen-bond interactions to a sufficient extent. Of the four natural-product inhibitors, (Z)-6-hydroxy-2-(4-methoxybenzylidene)benzofuran-3(2H)-one (3) is anticipated to serve as a new molecular core for the structure-activity relationship studies to optimize the biochemical potencies because it exhibits good inhibitory activity against both the wild type and D816V mutant despite its low molecular weight (268.3 amu).

Hotspot mutations in KIT receptor differentially modulate its allosterically coupled conformational dynamics: impact on activation and drug sensitivity

Receptor tyrosine kinase KIT controls many signal transduction pathways and represents a typical allosterically regulated protein. The mutation-induced deregulation of KIT activity impairs cellular physiological functions and causes serious human diseases. The impact of hotspots mutations (D816H/Y/N/V and V560G/D) localized in crucial regulatory segments, the juxtamembrane region (JMR) and the activation (A-) loop, on KIT internal dynamics was systematically studied by molecular dynamics simulations. The mutational outcomes predicted in silico were correlated with in vitro and in vivo activation rates and drug sensitivities of KIT mutants. The allosteric regulation of KIT in the native and mutated forms is described in terms of communication between the two remote segments, JMR and A-loop. A strong correlation between the communication profile and the structural and dynamical features of KIT in the native and mutated forms was established. Our results provide new insight on the determinants of receptor KIT constitutive activation by mutations and resistance of KIT mutants to inhibitors. Depiction of an intra-molecular component of the communication network constitutes a first step towards an integrated description of vast communication pathways established by KIT in physiopathological contexts.

Receptor tyrosine kinase KIT plays a crucial role in the regulation of cell signaling. This allosterically controlled activity may be affected by gain-of-function mutations that promote the development of several cancers. Identification of the molecular basis of KIT constitutive activation and allosteric regulation has inspired computational study of KIT hotspot mutations. In the present contribution, we investigated the mutation-induced effects on KIT conformational dynamics and intra-protein communication conditionally on the mutation location and the nature of the substituting amino acid. Our data elucidate that all studied mutations stabilize an inactive non-autoinhibited state of KIT over the inactive auto-inhibited state prevalent for the native protein. This shift in the protein conformational landscape promotes KIT constitutive activation. Our in silico analysis established correlations between the structural and dynamical effects induced by oncogenic mutations and the mutants auto-activation rates and drug sensitivities measured in vitro and in vivo. Particularly, the A-loop mutations stabilize the drug-resistant forms, while the JMR mutations may facilitate inhibitors binding to the active site. Cross-correlations established between local and long-range structural and dynamical effects demonstrate the allosteric character of the gain-of-function mutations mode of action.

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.

Identification of distant drug off-targets by direct superposition of binding pocket surfaces

Correctly predicting off-targets for a given molecular structure, which would have the ability to bind a large range of ligands, is both particularly difficult and important if they share no significant sequence or fold similarity with the respective molecular target ("distant off-targets"). A novel approach for identification of off-targets by direct superposition of protein binding pocket surfaces is presented and applied to a set of well-studied and highly relevant drug targets, including representative kinases and nuclear hormone receptors. The entire Protein Data Bank is searched for similar binding pockets and convincing distant off-target candidates were identified that share no significant sequence or fold similarity with the respective target structure. These putative target off-target pairs are further supported by the existence of compounds that bind strongly to both with high topological similarity, and in some cases, literature examples of individual compounds that bind to both. Also, our results clearly show that it is possible for binding pockets to exhibit a striking surface similarity, while the respective off-target shares neither significant sequence nor significant fold similarity with the respective molecular target ("distant off-target").

The effect of estrogen supplementation on cell proliferation and expression of c-kit positive cells in the rat prostate

BACKGROUND

Benign prostatic hyperplasia (BPH) is associated with the proliferation of prostate tissue and an increase in smooth muscle tone. However, the way in which the hormonal environment affects cell proliferation and prostatic interstitial cells (PIC) responsible for the maintenance of the smooth muscle tone is not clear. The present study investigated the effect of estrogen supplementation on cell proliferation, androgen/estrogen ratio, and expression and/or distribution of PIC.

METHODS

Male Sprague-Dawley rats were anesthetized with isoflurane/oxygen breathing mixture and subcutaneously implanted with silastic capsules. These capsules were either empty or filled with a 10 or 20 mg of crystalline estrogen.

RESULTS

Estrogen exerted a potent effect on ventral prostate weight, which was manifested as a significant decrease between controls and the E(10)- and E(20)-treated rats. Active cell proliferation detected as Ki67-positive nuclei was observed in the stromal and epithelial cells of the ventral prostatic lobes from estrogen-treated rats and controls. Estrogen supplementation caused a significant and dose-dependent increase in prostatic estradiol and 5alpha-dihydrotestosterone (DHT) concentration but the ratios of either DHT/E(2) or E(2)/DHT were not significantly affected. PIC were observed in the region between the fibromuscular stroma and the glandular epithelium in all three experimental groups. E(20)-treated rats showed a higher expression of PIC than controls and E(10)-treated rats.

CONCLUSIONS

The present study provides novel information regarding the synergistic role of estrogens and androgens in the prostate: estrogen may prevent prostatic hyperplasia via mechanisms other than affecting cell proliferation or DHT/estrogen ratio.

[Expression of c-kit and PDGFR and possibility of tyrosine kinase inhibitor employment in treatment of neuroblastoma in children]

Because of unsatisfactory treatment results in high risk neuroblastoma, it is necessary to find new, effective and safe, treatment options. Tyrosine kinase inhibitors, including imatinib, are one of the most profoundly examinated drugs. Its employment in neuroblastoma treatment is potentially possible because of expression of c-kit and PDGFR, which are cellular targets of imatinib. It was shown that this drug inhibits growth of neuroblastoma cell lines both in vivo and in vitro. The prognostic meaning of receptors expression is still not clear and depends on the method used for evaluation. The only found phase II study did not reveal the satisfactory response for imanitinib in treatment of resistant neuroblastoma, but the presence of receptors expression was not inclusion criteria. However, it seems that assuming the correct patient's eligibility criteria and proper dosage schedule, possible in combination with chemotherapy, imatinib may become safe and effective treatment modality.

Phase II study of safety and efficacy of motesanib in patients with progressive or symptomatic, advanced or metastatic medullary thyroid cancer

PURPOSE

This phase II study investigated the efficacy and tolerability of motesanib, an investigational, highly selective inhibitor of vascular endothelial growth factor receptors 1, 2, and 3; platelet-derived growth factor receptor; and Kit in advanced medullary thyroid cancer (MTC).

PATIENTS AND METHODS

Patients with locally advanced or metastatic, progressive or symptomatic MTC received motesanib 125 mg/d orally for up to 48 weeks or until unacceptable toxicity or disease progression. The primary end point was objective response by independent review. Other end points included duration of response, progression-free survival, safety, pharmacokinetics, and changes in tumor markers.

RESULTS

Of 91 enrolled patients who received motesanib, two (2%) achieved objective response (95% CI, 0.3% to 7.7%); their duration of response was 32 weeks (censored) and 21 weeks (disease progressed). Eighty-one percent of patients had stable disease (48% had durable stable disease > or = 24 weeks), 8% had disease progression as best response, and 9% were not evaluated; 76% experienced a decrease from baseline in target lesion measurement. Median progression-free survival was 48 weeks (95% CI, 43 to 56 weeks). Among patients with tumor marker analysis, 69 (83%) of 83 and 63 (75%) of 84 had decreased serum calcitonin and carcinoembryonic antigen during treatment, respectively, compared with baseline. The most common treatment-related adverse events were diarrhea (41%), fatigue (41%), hypothyroidism (29%), hypertension (27%), and anorexia (27%). In pharmacokinetic analyses, motesanib trough concentrations were lower compared with differentiated thyroid cancer patients from the same study.

CONCLUSION

Although the objective response rate was low, a significant proportion of MTC patients (81%) achieved stable disease while receiving motesanib.

A phase II trial of imatinib in patients with refractory/relapsed myeloma

Although imatinib was designed to specifically inhibit the bcr-abl gene product, it inhibits other receptor tyrosine kinases including c-kit. As pre-clinical data, 126 patients with plasma cell disorders and 19 controls were evaluated for c-kit expression. Patients were eligible for the treatment trial if they had relapsed/refractory myeloma. The primary end-point of the study was response. Of the 145 studied before the trial, c-kit expression was present on the bone marrow plasma cells of control (11%), AL amyloid (53%), MGUS (47%), SMM (67%) and MM (42%) patients. Twenty-three MM patients were enrolled on the therapeutic trial (imatinib 400 mg daily) and 52% had positive c-kit staining. There were no responses. The median duration of treatment was 48 days (range: 12-349). Patients ended treatment due to progressive disease (18 patients), death (3) and other (2). The data suggest that imatinib is not an active agent in patients with relapsed or refractory multiple myeloma.

Beyond chemotherapy: targeted therapies in ovarian cancer

Ovarian cancer is the leading cause of death from gynaecological malignancies in the Western world. Despite the evolution of surgical techniques and meticulously designed chemotherapy regimens, relapse remains almost inevitable in patients with advanced disease. In an age when great advances have been made in understanding the genetics and molecular biology of this heterogeneous disease, it is likely that the introduction of novel targeted therapies will have a major impact on the management of ovarian cancer. Importantly, such strategies might allow selection of treatments based on the molecular characteristics of tumours and bring us closer to an era of personalized medicine.

Evidence that geniposide abrogates norepinephrine-induced hypopigmentation by the activation of GLP-1R-dependent c-kit receptor signaling in melanocyte

Geniposide (GP) as an agonist of glucagon-like peptide-1 receptor (GLP-1R) is an iridoid glycoside from the fruit of Gardenia jasminoides Ellis used as a Chinese traditional medicine for treatment of vitiligo vulgaris. Interaction of c-kit receptor with its ligand-SCF potent enhances the melanocytic melanogenesis, which can be repressed by norepinephrine (NE). To discover economic and efficient drug against vitiligo vulgaris, this paper addresses the action and mechanism of GP abrogating the NE-induced hypopigmentation in melanocyte. Flow cytometry exhibited the up-regulation effect of GP on NE-suppressed production of c-kit by normal human epidermal melanocyte (HEMn) in a concentration-dependent manner, and exendin-(9-39) (selective GLP-1R antagonist) appeared to alleviate the GP-stimulated expression of c-kit. However, neither NE nor GP affected the production of SCF by normal human epidermal keratinocyte (HEKn) assessed by cellular enzyme-linked immunosorbent assay. Spectrophotometry documented that GP abrogated the repression effect of NE on tyrosinase activity and melanin production in HEMn in the presence of recombination SCF significantly. The response of melanocytic melanogenesis to GP was blocked by exendin-(9-39) or K44.2 antibody (c-kit inhibitory antibody). Data from this paper provide the evidence that GP abrogates the NE-induced hypopigmentation by the activation of GLP-1R-dependent c-kit receptor signaling in which c-kit expression is augmented in HEMn.

Initial testing of the VEGFR inhibitor AZD2171 by the pediatric preclinical testing program

BACKGROUND

Inhibition of vascular endothelial growth factor mediated signaling shows promise as an antiangiogenic strategy for solid tumors. AZD2171 is a potent and relatively selective inhibitor of the vascular endothelial growth factor (VEGF) receptor family that is orally bioavailable. This study was designed to screen for antitumor activity of AZD2171 against the in vitro and in vivo childhood cancer preclinical models of the Pediatric Preclinical Testing Program (PPTP).

PROCEDURES

AZD2171 was tested at concentrations from 0.1 nM to 1.0 microM against the in vitro panel and was tested against the in vivo tumor panels using a 6-week exposure to daily gavage administration of AZD2171 (3 or 6 mg/kg) or vehicle.

RESULTS

One of 22 cell lines evaluated was sensitive to AZD2171 in vitro (maximum concentration 1 microM). Evidence of in vivo antitumor activity (primarily tumor growth delay) was observed in 78% of solid tumor xenografts (3/3 rhabdoid, 2/3 Wilms', 3/3 Ewing's, 5/5 rhabdomyosarcoma, 1/3 medulloblastoma, 2/4 glioblastoma, 5/6 neuroblastoma, 4/5 osteosarcoma). Objective responses (both complete responses) were observed in two of 32 (6%) solid tumor xenografts (a rhabdoid xenograft and an osteosarcoma xenograft). No activity was observed against 7 acute lymphoblastic leukemia models.

CONCLUSIONS

AZD2171 demonstrated broad tumor growth inhibition against the PPTP's solid tumor xenografts and much less commonly induced tumor regression. This pattern of in vivo activity, combined with the disassociation of in vitro and in vivo efficacy, are consistent with AZD2171 inhibiting growth of the PPTP's solid tumor xenografts primarily through an anti-angiogenesis mechanism of action.

AMG 706, an oral, multikinase inhibitor that selectively targets vascular endothelial growth factor, platelet-derived growth factor, and kit receptors, potently inhibits angiogenesis and induces regression in tumor xenografts

The growth of solid tumors is dependent on the continued stimulation of endothelial cell proliferation and migration resulting in angiogenesis. The angiogenic process is controlled by a variety of factors of which the vascular endothelial growth factor (VEGF) pathway and its receptors play a pivotal role. Small-molecule inhibitors of VEGF receptors (VEGFR) have been shown to inhibit angiogenesis and tumor growth in preclinical models and in clinical trials. A novel nicotinamide, AMG 706, was identified as a potent, orally bioavailable inhibitor of the VEGFR1/Flt1, VEGFR2/kinase domain receptor/Flk-1, VEGFR3/Flt4, platelet-derived growth factor receptor, and Kit receptors in preclinical models. AMG 706 inhibited human endothelial cell proliferation induced by VEGF, but not by basic fibroblast growth factor in vitro, as well as vascular permeability induced by VEGF in mice. Oral administration of AMG 706 potently inhibited VEGF-induced angiogenesis in the rat corneal model and induced regression of established A431 xenografts. AMG 706 was well tolerated and had no significant effects on body weight or on the general health of the animals. Histologic analysis of tumor xenografts from AMG 706-treated animals revealed an increase in endothelial apoptosis and a reduction in blood vessel area that preceded an increase in tumor cell apoptosis. In summary, AMG 706 is an orally bioavailable, well-tolerated multikinase inhibitor that is presently under clinical investigation for the treatment of human malignancies.

[Is positron emission tomography (PET) scan useful for studying the extent of gastrointestinal stromal tumors]

Gastrointestinal tumor (GIST) is the most frequent mesenchymal malignancy of the gastrointestinal tract. Liver and peritoneum are the most frequent metastatic sites. Surgery is the standard treatment in patients with localized disease and imatinib mesylate has been shown to be an effective treatment in metastatic or unresectable disease. We present a case of ileal GIST that was diagnosed as metastatic just with the findings of the fluorodeoxyglucose positron emission tomography (PET) and showed a total response with Imatinib on PET.

Molecular mechanisms of signal transduction: epidermal growth factor receptor family, vascular endothelial growth factor family, Kit, platelet-derived growth factor receptor, Ras

One of the major mechanisms of communication between cells is the binding of polypeptide ligands to cell surface receptors possessing tyrosine kinase (TK) activity. Though many actions of these receptors involve physiological processes, perturbation of TK signaling can result in malignant transformation. During the last few decades, these signaling networks have been studied in detail and finally pharmacological agents targeted at key molecules could be produced. Though many agents have failed, some drugs like bevacizumab, trastuzumab, cetuximab, and erlotinib have already become part of the standard care for common tumors like breast, colorectal, lung, head and neck cancers. Furthermore, imatinib has shown unsurpassed efficacy in the less common, but not less problematic chronic myeloid leukemia and gastrointestinal stromal tumors. Multi-target agents like sunitinib, sorafenib, and lapatinib have already yielded promising results and new agents are being developed. The clinical successes of these agents are unquestionably based on the expanding body of knowledge on the molecular mechanisms that underlie the development and progression of a malignant phenotype. This review will focus on some of the most extensively studied signaling networks like Erb family of receptors, vascular endothelial growth factor (VEGF) and its receptor (VEGFR), Kit, platelet-derived growth factor receptor (PDGFR), and Ras.

Phase II, open-label study of PTK787/ZK222584 for the treatment of metastatic gastrointestinal stromal tumors resistant to imatinib mesylate

BACKGROUND

We evaluated safety and efficacy of PTK787/ZK222584 (PTK/ZK), a novel tyrosine kinase inhibitor of KIT, platelet-derived growth factor receptors and vascular endothelial cell growth factor receptors (VEGFRs), in patients with imatinib-resistant gastrointestinal stromal tumor (GIST). This is the first study of PTK/ZK in this population.

PATIENTS AND METHODS

Patients with metastatic GIST that had progressed after >/= 4-week treatment with imatinib mesylate were eligible. Prior VEGFR-2 inhibitor therapy was not permitted. PTK/ZK 1250 mg orally once-daily was administered to 15 patients (accrued as a two-stage procedure), most of whom (n = 11) had been unsuccessfully treated with imatinib 800 mg daily, until treatment failure. Patients were monitored at 4- to 8-week intervals.

RESULTS

All 15 patients enrolled were eligible; two (13%) achieved partial response (PR), eight (53%) had stable disease (SD) >/=3 months, and five (33%) progressed. The clinical benefit rate (PR + SD) was 67% (95% CI 38% to 86%). Median time to progression was 8.5 months (range 0.9-24.8+ months). Three patients had not progressed at the time of analysis, including one PR at 24.8 months and two SDs at 16.6 and 18.6 months on treatment. PTK/ZK was generally well tolerated.

CONCLUSION

PTK/ZK 1250 mg p.o. once daily is active and well tolerated in patients with imatinib-resistant GIST.

[Clinical and molecular characterization of gastrointestinal stromal tumors (GIST)]

Gastrointestinal stromal tumors (GIST) are rare mesenchymal tumors of the gastrointestinal tract, the diagnosis of which is difficult. The presence of membrane receptor c-KIT on GIST cells implies their origin from a precursor of interstitial cells of Cajal. Mutations in the c-kit gene result in the constitutive ligand-independent activation of the c-KIT receptor and, consequently, aberrant cell division and tumor growth. Imatinib (STI571), used in the treatment of unresectable GIST, selectively inhibits the enzymatic activity of the tyrosine kinase c-KIT. However, it is known that some mutations in the c-kit gene lead to resistance to imatinib. Therefore, the search for new effective agents is being continued. In this paper the clinical and molecular characterization of the GIST is presented as well as data related to imatinib in the treatment of GIST. Moreover, novel agents for the treatment of patients with advanced GIST are described.

Beyond imatinib: second generation c-KIT inhibitors for the management of gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal cancer of teh gastrointestinal tract. They are characterized by the expression of KIT. Therapeutically, metastatic GISTs are effectively treated by imatinib, a tyrosine kinase inhibitor (TKI) with activity against KIT and platelet-derived growth factor receptor. Gastrointestinal stromal tumors refractory to standard therapy with imatinib are a clinical challenge. This has lead to the clinical testing of a variety of agents used alone or in combination with other TKIs. Sunitinib, a multitargeted TKI, is the first drug available fort eh treatment of these patients. Additional trials are ongoing, evaluating the efficacy of the novel KIT TKIs AMG 706 and AMN 107 (nilotinib). RAD001, PKC412, and bavacizumab are being tested in conjunction with imatinib. Lastly, the heat-shock protein-90 inhibitor IPI-540 is also in phase I evaluation in imatinib-refractory patients with GIST. The future management of GIST is likely to be altered by the availability of more agents and by better biologic understanding of the patient populations each agent best treats.

An in vitro cytologic assay for evaluation of the KIT signaling pathway in gastrointestinal stromal tumors

Specific inhibitors can be designed to inactivate the molecular pathways involved in tumor growth. A compelling example is the use of small molecule drugs, such as imatinib (Gleevec), which inhibit the KIT tyrosine kinase in gastrointestinal stromal tumors (GIST). Assays are needed to determine which inhibitor is most effective at silencing the KIT kinase in each GIST patient. The aim of this study was to develop a robust, cytology-based assay to measure tumor susceptibility to target-specific small molecule inhibitors. We created an immortal GIST cell line (GIST882) that was treated in vitro with several inhibitors of the KIT --> AKT --> mTOR --> S6 signaling pathway. KIT was inhibited with imatinib, and mTOR with RAD001. Treatment response was assessed in cytologic preparations by immunocytochemical staining with antibodies to KIT, phospho-KIT, phospho-AKT, and phospho-S6. Optimization was performed to maximize staining in the absence of inhibitor, and minimize staining in the presence of inhibitor. GIST882 cells demonstrated strong, robust phospho-S6 expression in the absence of inhibitor. This expression was completely inhibited by treatment with upstream signaling pathway inhibitors (imatinib and RAD001). Other phospho-specific antibodies had weaker baseline reactivity in the absence of inhibitor. The accuracy of the immunocytochemical results on the cytologic preparations was validated by immunoblotting studies. Our study demonstrates the feasibility of cytologic methods to monitor labile biochemical responses in tumor cells during drug therapy. Such approaches will be enhanced by the development of additional activation state-specific antibodies, particularly those optimized for use in cytologic preparations.

KIT oncoprotein interactions in gastrointestinal stromal tumors: therapeutic relevance

Most gastrointestinal stromal tumors (GISTs) express oncogenic and constitutively active forms of the KIT or platelet-derived growth factor receptor alpha (PDGFRA) receptor tyrosine kinase proteins, and these kinase oncoproteins serve as targets for effective therapies. Given that mutant KIT oncoproteins serve crucial transforming roles in GISTs, we evaluated interactions with the KIT oncoproteins and determined signaling pathways that are dependent on KIT oncogenic activation in GISTs. Tyrosine-phosphorylated KIT oncoproteins interacted with PDGFRA, PDGFRB, phosphatidylinositol 3-kinase (PI3-K) and PKCtheta in GIST cells, and these interactions were abolished by KIT inhibition with imatinib or PKC412 or KIT RNAi. Notably, tyrosine-phosphorylated PDGFRA was prominent in frozen GIST tumors expressing KIT oncoproteins, suggesting that KIT-mediated PDGFRA phosphorylation is an efficient and biologically consequential mechanism in GISTs. Activated signaling intermediates were identified by immunoaffinity purification of tyrosine-phosphorylated proteins in GIST cells before and after treatment with KIT inhibitors, and these analyses show that GRB2, SHC, CBL and MAPK activation are largely KIT dependent in GISTs, whereas PI3-K, STAT1 and STAT3 activation are partially KIT dependent. In addition, we found that phosphorylation of several tyrosine kinase proteins - including JAK1 and EPHA4 - did not depend on KIT activation. Likewise, paxillin activation was independent of the KIT oncogenic signal. These studies identify signaling pathways that can provide both KIT-dependent and KIT-independent therapeutic synergies in GIST, and thereby highlight clinical strategies that might consolidate GIST therapeutic response to KIT/PDGFRA inhibition.

C-kit, GIST, and imatinib

A gain-of-function mutation of c-kit is the crucial step in tumorigenesis of gastrointestinal stromal tumors (GIST). Imatinib can block the activated receptor tyrosine kinase activity of c-kit. These findings made GIST an ideal candidate for evaluation of new targeted therapeutic approaches. Clinical studies demonstrated that, with imatinib, objective responses can be reached in more than 50% of patients with advanced GIST. Furthermore, even in those patients with stable disease, long-term tumor control could be achieved. Therefore imatinib at a dose of 400 mg/day is now the standard treatment of advanced GIST in which RO-resection cannot be reached. As imatinib resistance in GIST occurs at a median of 18 to 26 months, further targeted therapies have been explored. Sunitinib, another tyrosine kinase inhibitor, seems to be useful especially in patients with exon 9 mutations of c-kit, who usually have a worse response to imatinib. This might indicate that more exactly targeted therapies in GIST might improve clinical outcomes in the future.

3D QSAR studies on a series of potent and high selective inhibitors for three kinases of RTK family

For targets belonging to the same family of receptors, inhibitors often act at more than one biological target and produce a synergistic effect. Separate CoMFA and CoMSIA models were developed from our data set for the KDR, cKit and Tie-2 inhibitors. These models showed excellent internal predictability and consistency, and validation using test-set compounds yielded a good predictive power for the pIC(50) value. The field contour maps (CoMFA and CoMSIA) corresponding to the KDR, cKit and Tie-2 kinase subtypes reflected the characteristic similarities and differences between these types. These maps provided valuable information to facilitate structural modifications of the inhibitor to increase selectivity for the KDR over cKit and Tie-2.

Novel approaches in the treatment of systemic mastocytosis

In the absence of curative options, therapy for aggressive forms of systemic mastocytosis (SM) has relied in the use of cytoreductive agents, mainly interferon-alpha (IFN-alpha) and cladribine. However, responses are transient and only occur in a subset of patients. Gain-of-function mutations at codon 816 of the KIT protooncogene lead to constitutively active Kit receptor molecules, which are central to the pathogenesis of SM. Recent advances in the understanding of the molecular underpinnings of SM have led to the development of small molecules targeting mutant Kit tyrosine kinase isoforms that significantly have widened the range of therapeutic options for patients with SM. Some of these promising agents, such as dasatinib, AMN107, and PKC412, currently are under investigation in clinical trials whereas, others are at different stages of preclinical development. In addition, monoclonal antibodies directed to neoplastic mast cell-restricted surface antigens constitute a viable option for the treatment of SM that warrants further investigation.

Predicting the antitumor effects of STI571 by analysis of c-kit gene mutations in gastrointestinal stromal tumors of the stomach: Report of a case

We report a case of a large gastrointestinal stromal tumor (GIST), greater than 5 cm in diameter, in the stomach. Microscopically, high levels of mitosis were observed, indicative of a high-grade malignancy. We analyzed the c-kit gene mutations by a replication competent retrovirus assay and DNA sequencing, which revealed a c-kit mutation in exon 11. Liver metastases were detected 7 months after surgery. Patients with an exon 11 mutation of the c-kit gene are reported to have a high response to STI571 (imatinib mesylate, Glivec). Accordingly, a 1-month course of STI571 treatment clearly changed the characterization of the metastatic tumors radiographically. Thus, it may be important to analyze c-kit gene mutations in patients presenting with GISTs to predict the effectiveness of STI571 in suppressing GISTs, especially tumors thought to have malignant potential.

Effect of SU11248 on gastrointestinal stromal tumor-T1 cells: enhancement of growth inhibition via inhibition of 3-kinase/Akt/mammalian target of rapamycin signaling

SU11248 is an orally available type III and V receptor tyrosine kinase inhibitor. Clinical studies have shown the efficacy of SU11248 in individuals with gastrointestinal stromal tumors (GIST); however, the molecular mechanisms by which SU11248 inhibits the proliferation of these tumor cells remains to be fully elucidated. Taking advantage of GIST-T1 cells, which possess an activating mutation in exon 11 of the c-KIT gene, we examined the medicinal action of SU11248 in GIST cells. Clonogenic and MTT assays showed that SU11248 potently inhibited the proliferation of GIST-T1 cells with IC50 of approximately 1 nM and 40 nM, respectively. SU11248 (10 or 20 nM, 48 h) activated caspase-3 and induced apoptosis of GIST-T1 cells as measured by caspase assay, annexin V staining and cleavage of poly (ADP-ribose) polymerase. Western blot analyses found that SU11248 blocked autophosphorylation of c-KIT in association with inhibition of its downstream effectors, including Akt and extracellular signal-regulated kinase, but not signal transducers and activators of transcription. Interestingly, when phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin signaling was blocked simultaneously by either LY294002 or rapamycin, growth inhibition mediated by SU11248 was potentiated. Taken together, this study supports clinical studies of SU11248 for individuals with GIST, and the combination of SU11248 and inhibitors of 3-kinase/Akt/mammalian target of rapamycin signaling represents a promising novel treatment strategy.

The stem cell factor/c-kit receptor pathway enhances proliferation and invasion of pancreatic cancer cells

BACKGROUND

The transmembrane protein c-kit is a receptor tyrosine kinase (KIT) and KIT is expressed in solid tumors and hematological malignancies such as gastrointestinal stromal tumor (GIST), small-cell lung cancer and chronic myelogenous leukemia (CML). KIT plays a critical role in cell proliferation and differentiation and represents a logical therapeutic target in GIST and CML. In pancreatic cancer, c-kit expression has been observed by immunohistochemical techniques. In this study, we examined the influence of c-kit expression on proliferation and invasion using five pancreatic cancer cell lines. In addition, the inhibitory effect of imatinib mesylate on stem cell factor (SCF)-induced proliferation and invasion was evaluated. Finally, we also analyzed KIT and SCF expression in pancreatic cancer tissues using immunohistochemistry and correlated the results with clinical features.

RESULTS

RT-PCR revealed that two pancreatic cancer cell lines, PANC-1 and SW1990, expressed c-kit mRNA. By Western blot analysis, c-kit protein was also present in those lines. In KIT-positive pancreatic cancer cell lines, proliferation and invasion were significantly enhanced by addition of SCF. In contrast, SCF did not enhance proliferation and invasion in the three KIT-negative lines (BxPC-3, Capan-2 and MIA PaCa-2). 5 muM imatinib mesylate significantly inhibited SCF-enhanced proliferation to the same extent compared with the control. Similarly, SCF-enhanced invasive ability was significantly inhibited by 5 muM imatinib mesylate. KIT was expressed in 16 of 42 clinical specimens by immunohistochemistry, and KIT expression was significantly related to venous system invasion. Furthermore, patients expressing both KIT and SCF had a somewhat lower survival.

CONCLUSION

Our results demonstrated that the SCF-KIT pathway enhanced the proliferation and invasiveness in KIT-positive pancreatic cancer cell lines and that the enhanced proliferation and invasion were inhibited by imatinib mesylate. We propose that inhibitors of c-kit tyrosine kinase receptor have the potential to slow the progression of KIT-positive pancreatic cancers.

Imatinib inhibits c-Kit-induced hypoxia-inducible factor-1alpha activity and vascular endothelial growth factor expression in small cell lung cancer cells

Vascular endothelial growth factor (VEGF) is one of the most important mediators of tumor angiogenesis. In addition to hypoxia, peptide growth factors are known to regulate VEGF expression but the effect of stem cell factor (SCF), the ligand for c-Kit, on VEGF expression has not been characterized. We therefore studied the effect of SCF-mediated c-Kit activation on VEGF expression by the H526 small cell lung cancer (SCLC) cell line. SCF treatment doubled VEGF mRNA expression and VEGF secretion in the absence of other exogenous growth factors, an effect efficiently blocked by imatinib. The increase in VEGF mRNA occurred within the first 2 hours of treatment and was not caused by alterations in mRNA stability. The phosphatidylinositol 3-kinase inhibitor LY294002 blocked the increase in VEGF mRNA, implicating c-Kit-mediated activation of phosphatidylinositol 3-kinase in the phenomenon. VEGF promoter-reporter transfections indicated that a SCF-mediated increase in VEGF promoter activity paralleled the increase in VEGF mRNA, documenting that SCF mediated its effects through enhanced VEGF transcription. Mutation of the core hypoxia-inducible factor (HIF)-1 binding element in the VEGF promoter significantly blunted SCF-responsiveness. SCF increased nuclear levels of the HIF-1alpha transcription factor, which correlated well with increased HIF-1alpha binding to a consensus hypoxia-responsive element. SCF-mediated effects on HIF-1alpha expression were additive with those produced by CoCl2, a hypoxia-mimetic agent. These data indicate that activation of c-Kit by SCF leads to a predominantly HIF-1alpha-mediated enhancement of VEGF expression and that inhibition of c-Kit signaling with imatinib could result in inhibition of tumor angiogenesis.

Mechanisms of resistance to small molecule kinase inhibition in the treatment of solid tumors

A growing number of tumors are characterized by simple genetic changes that activate important biochemical pathways, which are involved in their pathogenesis. These findings have led to the concept of targeted small molecule inhibitor treatment. The prototype for this type of therapy has been treatment of chronic myelogenous leukemia with imatinib mesylate (Gleevec), which targets BCR-ABL kinase. More recently, imatinib has been used to inhibit KIT in gastrointestinal (GI) stromal tumor, a mesenchymal tumor that arises in the GI tract. Furthermore, it has been possible to target EGFR in non-small-cell lung cancer with gefitinib and erlotinib. While initial results have been encouraging, resistance to small molecule kinase inhibitors is a substantial drawback. This paper focuses on what is known about mechanisms of resistance in the treatment of solid tumors by small molecule kinase inhibitors.

Natural killer cell-dendritic cell crosstalk in the initiation of immune responses

Dendritic cells (DCs) and natural killer (NK) cells play a critical role in early defences against cancer and infections. They specialise in complementary functions, including IL-12 or IFN-alpha/beta secretion and antigen presentation for the former, and IFN-gamma secretion and killing of infected or tumour cells for the latter. Both DCs and NK cells are also sensors of the immune system that have developed different, but partially overlapping, systems to identify pathology associated danger signals. Evidence of NK-DC interaction has accumulated recently. This interaction may lead to NK cell activation, DC activation, or apoptosis depending on the activation status of both cell types. Thus, the outcome of NK-DC crosstalk is likely to influence both innate and adaptive immune responses. This review addresses the molecular mechanisms under-lying the different NK-DC interactions, and their in vivo significance in anti-tumour or antimicrobial immunity. Finally, we discuss the potential clinical implications of this new field.

EXEL-0862, a novel tyrosine kinase inhibitor, induces apoptosis in vitro and ex vivo in human mast cells expressing the KIT D816V mutation

Gain-of-function mutations of the receptor tyrosine kinase KIT play a key role in the pathogenesis of systemic mastocytosis (SM), gastrointestinal stromal tumors (GISTs), and some cases of acute myeloid leukemia (AML). Whereas KIT juxtamembrane domain mutations seen in most patients with GIST are highly sensitive to imatinib, the kinase activation loop mutant D816V, frequently encountered in SM, hampers the binding ability of imatinib. We investigated the inhibitory activity of the novel tyrosine kinase inhibitor EXEL-0862 against 2 subclones of human mast cell line-1 (HMC-1)-HMC-1.1, harboring the juxtamembrane domain mutation V560G, and HMC-1.2, carrying V560G and the activation loop mutation D816V, found in more than 80% of patients with SM. EXEL-0862 inhibited the phosphorylation of KIT in a dose-dependent manner and decreased cell proliferation in both mast cell lines with higher activity against HMC-1.2 cells. The phosphorylation of KIT-dependent signal transducer and activator of transcription-3 (STAT3) and STAT5 was abrogated upon exposure to nanomolar concentrations of EXEL-0862. In addition, EXEL-0862 induced a time- and dose-dependent proapoptotic effect in both mast cell lines and caused a significant reduction in mast-cell content in bone marrow samples from patients with SM harboring D816V and from those without the D816V mutation. We conclude that EXEL-0862 is active against KIT activation loop mutants and is a promising candidate for the treatment of patients with SM and other KIT-driven malignancies harboring active site mutations.

The anti-angiogenesis agent, AG-013736, has minimal activity in elderly patients with poor prognosis acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS)

AG-013736 is an oral anti-angiogenesis agent with activity against a variety of receptor tyrosine kinases, including VEGFR-1, VEGFR-2, VEGFR-3, c-kit, and PDGFR-beta. A phase 2 study was conducted in patients with poor prognosis AML or MDS. Twelve patients (six AML; six MDS) were treated with AG-013736 at a dose of 10mg orally daily for a median of 56 days (range, 1-248 days). Median age was 80 years (range, 58-88 years). Grade 3 or 4 drug-related toxicities included hypertension (42%), mucositis (8%) and deep venous thrombosis (8%). No objective responses occurred; two patients with MDS had stable disease for 8.3 and 6.2 months, respectively. Bone marrow expression of VEGFR-1 and VEGFR-2 was observed in 11% and 0% of patients, respectively. Sustained decreases in soluble VEGFR-2 plasma levels with concomitant elevation in plasma VEGF and placental growth factor levels were obtained during the course of therapy with AG-013736. AG-01736 had minimal biologic or clinical activity in this elderly patient population.

Dasatinib (BMS-354825) inhibits KITD816V, an imatinib-resistant activating mutation that triggers neoplastic growth in most patients with systemic mastocytosis

Mastocytosis is associated with an activating mutation in the KIT oncoprotein (KITD816V) that results in autophosphorylation of the KIT receptor in a ligand-independent manner. This mutation is inherently resistant to imatinib and, to date, there remains no effective curative therapy for systemic mastocytosis associated with KITD816V. Dasatinib (BMS-354825) is a novel orally bioavailable SRC/ABL inhibitor that has activity against multiple imatinib-resistant BCR-ABL isoforms in vitro that is presently showing considerable promise in early-phase clinical trials of chronic myeloid leukemia (CML). Pharmacokinetic analysis suggests that high nanomolar concentrations of dasatinib can be achieved safely in humans. In this study, we demonstrate significant inhibitory activity of dasatinib against both wild-type KIT and the KITD816V mutation in the nanomolar range in in vitro and cell-based kinase assays. Additionally, dasatinib leads to growth inhibition of a KITD816V-harboring human masto-cytosis cell line. Significantly, dasatinib selectively kills primary neoplastic bone marrow mast cells from patients with systemic mastocytosis while sparing other hematopoietic cells. Computer modeling suggests that the KITD816V mutation destabilizes the inactive conformation of the KIT activation loop to which imatinib binds, but it is not predicted to impair binding of KIT by dasatinib. Based upon our results, further evaluation of dasatinib for the treatment of systemic masto-cytosis in clinical trials is warranted. Moreover, dasatinib may be of clinical utility in other disease settings driven by activating KIT mutations. (Blood. 2006;108:286-291)

Potent inhibition of small-cell lung cancer cell growth by simvastatin reveals selective functions of Ras isoforms in growth factor signalling

The impact of the 3-hydroxy-3methylglutaryl CoA reductase inhibitor simvastatin on human small-cell lung cancer (SCLC) cell growth and survival was investigated. Simvastatin profoundly impaired basal and growth factor-stimulated SCLC cell growth in vitro and induced apoptosis. SCLC cells treated with simvastatin were sensitized to the effects of the chemotherapeutic agent etoposide. Moreover, SCLC tumour growth in vivo was inhibited by simvastatin. These responses correlated with the inhibition of stem cell factor (SCF)-stimulated activation of extracellular signal-regulated kinase (Erk), protein kinase B (PKB) and ribosomal S6 kinase by simvastatin. Constitutive activation of the Erk pathway was sufficient to rescue SCLC cell from the effects of simvastatin. The drug did not directly affect activation of c-Kit or its localization to lipid rafts, but in addition to its ability to block Ras membrane localization, it selectively downregulated H-Ras protein levels at the post-translational level. Downregulation of either H- or K-Ras by RNA interference (RNAi) did not impair Erk activation by growth factors, whereas an RNAi specific for N-Ras inhibited activation of Erk, PKB and SCLC cell growth. Together our data demonstrate that inhibiting Ras signalling with simvastatin potently disrupts growth and survival in human SCLC cells.

The stem cell factor receptor/c-Kit as a drug target in cancer

Tyrosine phosphorylation has a key role in intracellular signaling. Inappropriate proliferation and survival cues in tumor cells often occur as a consequence of unregulated tyrosine kinase activity. Much of the current development of anti-cancer therapies tries to target causative proteins in a specific manner to minimize side-effects. One attractive group of target proteins is the kinases. c-Kit is a receptor tyrosine kinase that normally controls the function of primitive hematopoietic cells, melanocytes and germ cells. It has become clear that uncontrolled activity of c-Kit contributes to formation of an array of human tumors. The unregulated activity of c-Kit may be due to overexpression, autocrine loops or mutational activation. This makes c-Kit an excellent target for cancer therapies in these tumors. In this review we will highlight the current knowledge on the signal transduction molecules and pathways activated by c-Kit under normal conditions and in cancer cells, and the role of aberrant c-Kit signaling in cancer progression. Recent advances in the development of specific inhibitors interfering with these signal transduction pathways will be discussed.

Stem cell factor and its receptor c-Kit as targets for inflammatory diseases

Stem cell factor (SCF), the ligand of the c-Kit receptor, is expressed by various structural and inflammatory cells in the airways. Binding of SCF to c-Kit leads to activation of multiple pathways, including phosphatidyl-inositol-3 (PI3)-kinase, phospholipase C (PLC)-gamma, Src kinase, Janus kinase (JAK)/Signal Transducers and Activators of Transcription (STAT) and mitogen activated protein (MAP) kinase pathways. SCF is an important growth factor for mast cells, promoting their generation from CD34+ progenitor cells. In vitro, SCF induces mast cells survival, adhesion to extracellular matrix and degranulation, leading to expression and release of histamine, pro-inflammatory cytokines and chemokines. SCF also induces eosinophil adhesion and activation. SCF is upregulated in inflammatory conditions both in vitro and in vivo, in human and mice. Inhibition of the SCF/c-Kit pathway leads to significant decrease of histamine levels, mast cells and eosinophil infiltration, interleukin (IL)-4 production and airway hyperresponsiveness in vivo. Taken together, these data suggest that SCF/c-Kit may be a potential therapeutic target for the control of mast cell and eosinophil number and activation in inflammatory diseases.

[STI571: a summary of targeted therapy]

STI571 (imatinib; Gleevec) was developed as the first molecularly targeted therapy. It was the result of an extensive search for molecules to block the aberrant activity of Abl kinase in the fusion protein Bcr-Abl. In addition, it can specifically inhibit the activity of c-Kit and PDGF receptors. This orally bioavailable drug has a low toxicity profile. It is approved to treat the patients with chronic myelogenous leukemia (CML) or gastrointestinal stromal tumor (GIST). It produces hematological, cytogenetic, and molecular remission with significant efficacy, particularly in patients with chronic-phase CML. However, there is well-documented proof of primary and secondary resistance to STI571 with progression of leukemia. More evidence indicates that this single drug may not be sufficient to completely eradicate BCR-ABL-positive stem cells. A variety of strategies has already been developed to improve the effectiveness of CML treatment, including targeting the expression or stability of the Bcr-Abl kinase itself, targeting signaling pathways activated by this kinase, as well as designing novel Abl inhibitors. In this review the molecular mechanisms of STI571 action, its effectivenes against CML, GIST, and melanoma, as well as new approaches to improve its efficacy, mainly by overcoming STI571 resistance, are discussed.

Kit-activating mutations cooperate with Spi-1/PU.1 overexpression to promote tumorigenic progression during erythroleukemia in mice

The erythroleukemia developed by spi-1/PU.1 transgenic mice is a multistage process characterized by an early arrest of the proerythroblast differentiation followed later on by malignant transformation. Herein, we report the presence of acquired mutations in the SCF receptor gene (Kit) in 86% of tumors isolated during the late stage of the disease. Kit mutations affect codon 814 or 818. Ectopic expression of Kit mutants in nonmalignant proerythroblasts confers erythropoietin independence and tumorigenicity to cells. Using PP1, PP2, and imatinib mesylate, we show that Kit mutants are responsible for the autonomous expansion of malignant cells via Erk1/2 and PI3K/Akt activations. These findings represent a proof of principle for oncogenic cooperativity between one proliferative and one differentiation blocking event for the development of an overt leukemia.

Temporal quantitation of mutant Kit tyrosine kinase signaling attenuated by a novel thiophene kinase inhibitor OSI-930

OSI-930, a potent thiophene inhibitor of the Kit, KDR, and platelet-derived growth factor receptor tyrosine kinases, was used to selectively inhibit tyrosine phosphorylation downstream of juxtamembrane mutant Kit in the mast cell leukemia line HMC-1. Inhibition of Kit kinase activity resulted in a rapid dephosphorylation of Kit and inhibition of the downstream signaling pathways. Attenuation of Ras-Raf-Erk (phospho-Erk, phospho-p38), phosphatidyl inositol-3' kinase (phospho-p85, phospho-Akt, phospho-S6), and signal transducers and activators of transcription signaling pathways (phospho-STAT3/5/6) were measured by affinity liquid chromatography tandem mass spectrometry, by immunoblot, and by tissue microarrays of fixed cell pellets. To more globally define additional components of Kit signaling temporally altered by kinase inhibition, a novel multiplex quantitative isobaric peptide labeling approach was used. This approach allowed clustering of proteins by temporal expression patterns. Kit kinase, which dephosphorylates rapidly upon kinase inhibition, was shown to regulate both Shp-1 and BDP-1 tyrosine phosphatases and the phosphatase-interacting protein PSTPIP2. Interactions with SH2 domain adapters [growth factor receptor binding protein 2 (Grb2), Cbl, Slp-76] and SH3 domain adapters (HS1, cortactin, CD2BP3) were attenuated by inhibition of Kit kinase activity. Functional crosstalk between Kit and the non-receptor tyrosine kinases Fes/Fps, Fer, Btk, and Syk was observed. Inhibition of Kit modulated phosphorylation-dependent interactions with pathways controlling focal adhesion (paxillin, leupaxin, p130CAS, FAK1, the Src family kinase Lyn, Wasp, Fhl-3, G25K, Ack-1, Nap1, SH3P12/ponsin) and septin-actin complexes (NEDD5, cdc11, actin). The combined use of isobaric protein quantitation and expression clustering, immunoblot, and tissue microarray strategies allowed temporal measurement signaling pathways modulated by mutant Kit inhibition in a model of mast cell leukemia.

Mutations in KIT and RAS are frequent events in pediatric core-binding factor acute myeloid leukemia

Activating mutations in RAS and receptor tyrosine kinases such as KIT and FLT3 are hypothesized to cooperate with chimeric transcription factors in the pathogenesis of acute myeloid leukemia (AML). To test this hypothesis, we genotyped 150 pediatric AML samples for mutations in KIT (exons 8, 17), NRAS and KRAS (exons 1, 2) and FLT3/ITD. This is the largest cohort of pediatric AML patients reported thus far screened for all four mutations. Of the children with AML, 40% had a mutation in KIT (11.3%), RAS (18%) or FLT3/ITD (11.1%), and 70% of cases of core-binding factor (CBF) leukemia were associated with a mutation of KIT or RAS. Mutations in RAS or FLT3/ITD were frequently found in association with a normal karyotype. Patients with a FLT3/ITD mutation had a significantly worse clinical outcome. However, the presence of a KIT or RAS mutation did not significantly influence clinical outcome. We demonstrate that KIT exon 8 mutations result in constitutive ligand-independent kinase activation that can be inhibited by clinically relevant concentrations of imatinib. Our results demonstrate that abnormalities of signal transduction pathways are frequent in pediatric AML. Future clinical studies are needed to determine whether selective targeting of these abnormalities will improve treatment results.

[GIST: a clinical case as a model of surgical and pharmacological therapy of solid tumours]

Gastrointestinal stromal tumour: report of a case as a model of surgical and pharmacological therapy of solid tumours. mutation of this protein is present in most of these tumours. Gastrointestinal stromal tumours are notoriously unresponsive to chemotherapy and radiotherapy and prior to the recent introduction of the kit inhibitor imatinib, there was no effective therapy for advanced, metastatic disease. We report a case of metastatic gastrointestinal stromal tumour located primarily in the ileum and examine it in detail in order to contribute both to identifying factors capable of predicting its clinical course and evaluating the efficacy of imatinib as adjuvant therapy for this rare type of tumour. We share the opinion that, given the present state of the art, surgical resection remains the gold standard of treatment for these tumours and imatinib is a valid systemic therapy for metastastic and locally unresectable gastrointestinal stromal tumours.

Molecularly targeted therapy for gastrointestinal cancer

Receptor and non-receptor tyrosine kinases (TKs) have emerged as clinically useful drug target molecules for treating gastrointestinal cancer. Imatinib mesilate (STI-571, Gleevec(TM)), an inhibitior of bcr-abl TK, which was primarily designed to treat chronic myeloid leukemia is also an inhibitor of c-kit receptor TK, and is currently the drug of choice for the therapy of metastatic gastrointestinal stromal tumors (GISTs), which frequently express constitutively activated forms of the c-kit-receptor. The epidermal growth factor receptor (EGFR), which is involved in cell proliferation, metastasis and angiogenesis, is another important target. The two main classes of EGFR inhibitors are the TK inhibitors and monoclonal antibodies. Gefitinib (ZD1839, Iressa(TM)) has been on trial for esophageal and colorectal cancer (CRC) and erlotinib (OSI-774, Tarceva(TM)) on trial for esophageal, colorectal, hepatocellular, and biliary carcinoma. In addition, erlotinib has been evaluated in a Phase III study for the treatment of pancreatic cancer. Cetuximab (IMC-C225, Erbitux(TM)), a monoclonal EGFR antibody, has been FDA approved for the therapy of irinotecan resistant colorectal cancer and has been tested for pancreatic cancer. Vascular endothelial growth factor (VEGF) and its receptor (VEGFR) are critical regulators of tumor angiogenesis. Bevacizumab (Avastin(TM)), a monoclonal antibody against VEGF, was efficient in two randomized clinical trials investigating the treatment of metastatic colorectal cancer. It is also currently investigated for the therapy of pancreatic cancer in combination with gemcitabine. Other promising new drugs currently under preclinical and clinical evaluation, are VEGFR2 inhibitor PTK787/ZK 222584, thalidomide, farnesyl transferase inhibitor R115777 (tipifarnib, Zarnestra(TM)), matrix metalloproteinase inhibitors, proteasome inhibitor bortezomib (Velcade(TM)), mammalian target of rapamycin (mTOR) inhibitors, cyclooxygenase-2 (COX-2) inhibitors, platelet derived growth factor receptor (PDGF-R) inhibitors, protein kinase C (PKC) inhibitors, mitogen-activated protein kinase kinase (MEK) 1/2 inhibitors, Rous sarcoma virus transforming oncogene (SRC) kinase inhibitors, histondeacetylase (HDAC) inhibitors, small hypoxia-inducible factor (HIF) inhibitors, aurora kinase inhibitors, hedgehog inhibitors, and TGF-beta signalling inhibitors.

Lack of clinical efficacy of imatinib in metastatic melanoma

This two-centre phase-II trial aimed at investigating the efficacy of imatinib in metastasised melanoma patients in correlation to the tumour expression profile of the imatinib targets c-kit and platelet-derived growth factor receptor (PDGF-R). The primary study end point was objective response according to RECIST, secondary end points were safety, overall and progression-free survival. In all, 18 patients with treatment-refractory advanced melanoma received imatinib 800 mg day(-1). In 16 evaluable patients no objective responses could be observed. The median overall survival was 3.9 months, the median time to progression was 1.9 months. Tumour biopsy specimens were obtained from 12 patients prior to imatinib therapy and analysed for c-kit, PDGF-Ralpha and -Rbeta expression by immunohistochemistry. In four cases, cell lines established from these tumour specimens were tested for the antiproliferative effects of imatinib and for functional mutations of genes encoding the imatinib target molecules. The tumour specimens stained positive for CD117/c-kit in nine out of 12 cases (75%), for PDGF-Ralpha in seven out of 12 cases (58%) and for PDGF-Rbeta in eight out of 12 cases (67%). The melanoma cell lines showed a heterogenous expression of the imatinib target molecules without functional mutations in the corresponding amino-acid sequences. In vitro imatinib treatment of the cell lines showed no antiproliferative effect. In conclusion, this study did not reveal an efficacy of imatinib in advanced metastatic melanoma, regardless of the expression pattern of the imatinib target molecules c-kit and PDGF-R.

Establishment of a Kit-negative cell line of melanocyte precursors from mouse neural crest cells

We previously established a mouse neural crest cell line named NCCmelb4, which is positive for Kit and negative for tyrosinase. NCCmelb4 cells were useful to study the effects of extrinsic factors such as retinoic acids and vitamin D(3) on melanocyte differentiation, but in order to study the development of melanocytes from multipotent neural crest cells, cell lines of melanocyte progenitors in earlier developmental stages are needed. In the present study, we established an immortal cell line named NCCmelb4M5 that was derived from NCCmelb4 cells. NCCmelb4M5 cells do not express Kit and are immortal and stable in the absence of Kit ligand. They are positive for melanocyte markers such as tyrosinase-related protein 1 and DOPAchrome tautomerase and they contain stage I melanosomes. Interestingly, glial fibrillary acidic protein, which is a marker for glial cells, is also positive in NCCmelb4M5 cells, while NCCmelb4 cells are negative for this protein. Immunostaining and a cell ELISA assay revealed that 12-O-tetradecanoylphorbol 13-acetate (TPA) and cholera toxin (CT) induce Kit expression in NCCmelb4M5 cells. Real-time polymerase chain reaction analysis also demonstrated the induction of Kit mRNA by TPA and CT. Microphthalmia-associated transcription factor mRNA is simultaneously enhanced by the same treatment. Kit induced by TPA/CT in NCCmelb4M5 cells disappeared after the cells were subcultured and incubated without TPA/CT. These findings show that NCCmelb4M5 cells have the potential to differentiate into Kit-positive melanocyte precursors and may be useful to study mechanisms of development and differentiation of melanocytes in mouse neural crest cells.