CEP-751 inhibits TRK receptor tyrosine kinase activity in vitro exhibits anti-tumor activity

Artificial intelligence to guide precision anticancer therapy with multitargeted kinase inhibitors

BACKGROUND

Vast amounts of rapidly accumulating biological data related to cancer and a remarkable progress in the field of artificial intelligence (AI) have paved the way for precision oncology. Our recent contribution to this area of research is CancerOmicsNet, an AI-based system to predict the therapeutic effects of multitargeted kinase inhibitors across various cancers. This approach was previously demonstrated to outperform other deep learning methods, graph kernel models, molecular docking, and drug binding pocket matching.

METHODS

CancerOmicsNet integrates multiple heterogeneous data by utilizing a deep graph learning model with sophisticated attention propagation mechanisms to extract highly predictive features from cancer-specific networks. The AI-based system was devised to provide more accurate and robust predictions than data-driven therapeutic discovery using gene signature reversion.

RESULTS

Selected CancerOmicsNet predictions obtained for "unseen" data are positively validated against the biomedical literature and by live-cell time course inhibition assays performed against breast, pancreatic, and prostate cancer cell lines. Encouragingly, six molecules exhibited dose-dependent antiproliferative activities, with pan-CDK inhibitor JNJ-7706621 and Src inhibitor PP1 being the most potent against the pancreatic cancer cell line Panc 04.03.

CONCLUSIONS

CancerOmicsNet is a promising AI-based platform to help guide the development of new approaches in precision oncology involving a variety of tumor types and therapeutics.

GraphGR: A graph neural network to predict the effect of pharmacotherapy on the cancer cell growth.. [DOI: 10.1101/2020.05.20.107458]

Genomic profiles of cancer cells provide valuable information on genetic alterations in cancer. Several recent studies employed these data to predict the response of cancer cell lines to treatment with drugs. Nonetheless, due to the multifactorial phenotypes and intricate mechanisms of cancer, the accurate prediction of the effect of pharmacotherapy on a specific cell line based on the genetic information alone is problematic. High prediction accuracies reported in the literature likely result from significant overlaps among training, validation, and testing sets, making many predictors inapplicable to new data. To address these issues, we developed GraphGR, a graph neural network with sophisticated attention propagation mechanisms to predict the therapeutic effects of kinase inhibitors across various tumors. Emphasizing on the system-level complexity of cancer, GraphGR integrates multiple heterogeneous data, such as biological networks, genomics, inhibitor profiling, and genedisease associations, into a unified graph structure. In order to construct diverse and information-rich cancer-specific networks, we devised a novel graph reduction protocol based on not only the topological information, but also the biological knowledge. The performance of GraphGR, properly cross-validated at the tissue level, is 0.83 in terms of the area under the receiver operating characteristics, which is notably higher than those measured for other approaches on the same data. Finally, several new predictions are validated against the biomedical literature demonstrating that GraphGR generalizes well to unseen data, i.e. it can predict therapeutic effects across a variety of cancer cell lines and inhibitors. GraphGR is freely available to the academic community at https://github.com/pulimeng/GraphGR.

Targeting tropomyosin receptor kinase for cancer therapy

NTRKs and their expression product tropomyosin receptor kinases (Trks) are widely distributed in mammals. While neural growth factor (NGF)-induced normal Trk activation plays a key role in nerve growth, NTRK alternations occurring in tumor cells were highly correlated to tumor progression and invasion. Recent clinical data from several pan-Trk inhibitors have demonstrated potential and broad applications in various cancers. This intrigues us to summarize the development of inhibitors targeting Trks with different mechanisms of action and their applications in cancer therapy. We believe that this perspective would be of great help in investigating novel anticancer drugs with better therapeutic index.

Entrectinib and other ALK/TRK inhibitors for the treatment of neuroblastoma

RTK plays important roles in many cellular signaling processes involved in cancer growth and development. ALK, TRKA, TRKB, TRKC, and ROS1 are RTKs involved in several canonical pathways related to oncogenesis. These proteins can be genetically altered in malignancies, leading to receptor activation and constitutive signaling through their respective downstream pathways. Neuroblastoma (NB) is the most common extracranial solid tumor in childhood, and despite intensive therapy, there is a high mortality rate in cases with a high-risk disease. Alterations of ALK and differential expression of TRK proteins are reported in a proportion of NB. Several inhibitors of ALK or TRKA/B/C have been evaluated both preclinically and clinically in the treatment of NB. These agents have had variable success and are not routinely used in the treatment of NB. Entrectinib (RXDX-101) is a pan-ALK, TRKA, TRKB, TRKC, and ROS1 inhibitor with activity against tumors with ALK, NTRK1, NTRK2, NTRK3, and ROS1 alterations in Phase I clinical trials in adults. Entrectinib’s activity against both ALK and TRK proteins suggests a possible role in NB treatment, and it is currently under investigation in both pediatric and adult oncology patients.

TrkAIII signals endoplasmic reticulum stress to the mitochondria in neuroblastoma cells, resulting in glycolytic metabolic adaptation

Alternative TrkAIII splicing characterises advanced stage metastatic disease and post-therapeutic relapse in neuroblastoma (NB), and in NB models TrkAIII exhibits oncogenic activity. In this study, we report a novel role for TrkAIII in signaling ER stress to the mitochondria in SH-SY5Y NB cells that results in glycolytic metabolic adaptation. The ER stress-inducing agents DTT, A23187 and thapsigargin activated the ER stress-response in control pcDNA SH-SY5Y and TrkAIII expressing SH-SY5Y cells and in TrkAIII SH-SY5Y cells increased TrkAIII targeting to mitochondria and internalisation into inner-mitochondrial membranes. Within inner-mitochondrial membranes, TrkAIII was subjected to Omi/HtrA2-dependent cleavage to tyrosine phosphorylated 45–48kDa carboxyl terminal active fragments, localised predominantly in tyrosine kinase-domain mitochondrial matrix orientation. This stress-induced activation of mitochondrial TrkAIII was associated with increased ROS production, prevented by the ROS scavenger Resveratrol and underpinned by changes in Ca2+ movement, implicating ROS/Ca2+ interplay in overcoming the mitochondrial TrkAIII activation threshold. Stress-induced, cleavage-activation of mitochondrial TrkAIII resulted in mitochondrial PDHK1 tyrosine phosphorylation, leading to glycolytic metabolic adaptation. This novel mitochondrial role for TrkAIII provides a potential self-perpetuating, drug reversible way through which tumour microenvironmental stress may maintain the metastasis promoting “Warburg effect” in TrkAIII expressing NBs.

TrkA is a binding partner of NPM-ALK that promotes the survival of ALK+ T-cell lymphoma

Nucleophosmin‐anaplastic lymphoma kinase‐expressing (NPM‐ALK⁺) T‐cell lymphoma is an aggressive neoplasm that is more commonly seen in children and young adults. The pathogenesis of NPM‐ALK⁺ T‐cell lymphoma is not completely understood. Wild‐type ALK is a receptor tyrosine kinase that is physiologically expressed in neural tissues during early stages of human development, which suggests that ALK may interact with neurotrophic factors. The aberrant expression of NPM‐ALK results from a translocation between the ALK gene on chromosome 2p23 and the NPM gene on chromosome 5q35. The nerve growth factor (NGF) is the first neurotrophic factor attributed to non‐neural functions including cancer cell survival, proliferation, and metastasis. These functions are primarily mediated through the tropomyosin receptor kinase A (TrkA). The expression and role of NGF/TrkA in NPM‐ALK⁺ T‐cell lymphoma are not known. In this study, we tested the hypothesis that TrkA signaling is upregulated and sustains the survival of this lymphoma. Our data illustrate that TrkA and NGF are expressed in five NPM‐ALK⁺ T‐cell lymphoma cell lines and TrkA is expressed in 11 of 13 primary lymphoma tumors from patients. In addition, we found evidence to support that NPM‐ALK and TrkA functionally interact. A selective TrkA inhibitor induced apoptosis and decreased cell viability, proliferation, and colony formation of NPM‐ALK⁺ T‐cell lymphoma cell lines. These effects were associated with downregulation of cell survival regulatory proteins. Similar results were also observed using specific knockdown of TrkA in NPM‐ALK⁺ T‐cell lymphoma cells by siRNA. Importantly, the inhibition of TrkA signaling was associated with antitumor effects in vivo, because tumor xenografts in mice regressed and the mice exhibited improved survival. In conclusion, TrkA plays an important role in the pathogenesis of NPM‐ALK⁺ T‐cell lymphoma, and therefore, targeting TrkA signaling may represent a novel approach to eradicate this aggressive neoplasm.

Retrograde TrkAIII transport from ERGIC to ER: a re-localisation mechanism for oncogenic activity

In human SH-SY5Y neuroblastoma (NB) cells, nascent immature N-glycosylated 110kDa TrkA moves rapidly from the endoplasmic reticulum (ER) to the Golgi Network (GN), where it matures into the 140kDa receptor prior to being transported to the cell surface, creating GN and cell surface pools of inactive receptor maintained below the spontaneous activation threshold by a full compliment of inhibitory domains and endogenous PTPases. In contrast, the oncogenic alternative TrkAIII splice variant is not expressed at the cell surface but re-localises to intracellular membranes, within which it exhibits spontaneous ERGIC/COPI-associated activation and oncogenic Akt signalling. In this study, we characterise the mechanism responsible for TrkAIII re-localisation. Spontaneous TrkAIII activation, facilitated by D4 IG-like domain and N-glycosylation site omission, increases spontaneous activation potential by altering intracellular trafficking, inhibiting cell surface expression and eliminating an important inhibitory domain. TrkAIII, spontaneously activated within the permissive ERGIC/COPI compartment, rather than moving in an anterograde direction to the GN exhibits retrograde transport back to the ER, where it is inactivated. This sets-up self-perpetuating TrkAIII re-cycling between the ERGIC and ER, that ensures continual accumulation above the spontaneous activation threshold of the ERGIC/COPI compartment. This is reversed by TrkA tyrosine kinase inhibitors, which promote anterograde transport of inactivated TrkAIII to the GN, resulting in GN-associated TrkAIII maturation to a 120kDa species that is degraded at the proteasome.

NGF sensitizes TrkA SH-SY5Y neuroblastoma cells to TRAIL-induced apoptosis

We report a novel pro-apoptotic function for nerve growth factor (NGF) and its tropomyosin-related kinase A (TrkA) receptor in sensitizing TRAIL (TNF-related apoptotis-inducing ligand)-resistant SH-SY5Y neuroblastoma (NB) cells to TRAIL-induced apoptosis, resulting in the abrogation of anchorage-independent tumourigenic growth in vitro. We show that the TRAIL-resistant SH-SY5Y phenotype is cFLIP (cellular FLICE-like inhibitory protein) dependent and not due to low-level functional TRAIL receptor or caspase expression or an inhibitory equilibrium between functional and decoy TRAIL receptors or B-cell lymphoma 2 (Bcl-2) and BH3-only (Bcl-2 homology domain 3-only) family proteins. NGF sensitization of SH-SY5Y cells to TRAIL-induced apoptosis was dependent upon TrkA expression, activation and subsequent sequestration of cFLIP. This reduces cFLIP recruitment to TRAIL-activated death receptors and increases the recruitment of caspase-8, leading to TRAIL-induced, caspase-dependent, type II apoptosis via the intrinsic mitochondrial pathway. This effect was temporary, inhibited within 6 h by nuclear factor-κ binding (NF-κB)-mediated increase in myeloid cell leukaemia-1 (Mcl-1) expression, abrogated by transient cFLIP or B-cell lymphoma-extra large (Bcl-xL) overexpression and optimized by NF-κB and Mcl-1 inhibitors. This novel mechanism adds an important pro-apoptotic immunological dimension to NGF/TrkA interaction that may not only help to explain the association between TrkA expression, better prognosis and spontaneous remission in NB, but also provides a novel potential pro-apoptotic therapeutic use for NGF, TRAIL and inhibitors of NF-κB and/or Mcl-1 in favourable and unfavourable NBs that express TrkA and exhibit cFLIP-mediated TRAIL resistance.

Cancer metastases: challenges and opportunities

Cancer metastasis is the major cause of cancer morbidity and mortality, and accounts for about 90% of cancer deaths. Although cancer survival rate has been significantly improved over the years, the improvement is primarily due to early diagnosis and cancer growth inhibition. Limited progress has been made in the treatment of cancer metastasis due to various factors. Current treatments for cancer metastasis are mainly chemotherapy and radiotherapy, though the new generation anti-cancer drugs (predominantly neutralizing antibodies for growth factors and small molecule kinase inhibitors) do have the effects on cancer metastasis in addition to their effects on cancer growth. Cancer metastasis begins with detachment of metastatic cells from the primary tumor, travel of the cells to different sites through blood/lymphatic vessels, settlement and growth of the cells at a distal site. During the process, metastatic cells go through detachment, migration, invasion and adhesion. These four essential, metastatic steps are inter-related and affected by multi-biochemical events and parameters. Additionally, it is known that tumor microenvironment (such as extracellular matrix structure, growth factors, chemokines, matrix metalloproteinases) plays a significant role in cancer metastasis. The biochemical events and parameters involved in the metastatic process and tumor microenvironment have been targeted or can be potential targets for metastasis prevention and inhibition. This review provides an overview of these metastasis essential steps, related biochemical factors, and targets for intervention.

Integrated genomic study of quadruple-WT GIST (KIT/PDGFRA/SDH/RAS pathway wild-type GIST)

Background

About 10-15% of adult gastrointestinal stromal tumors (GIST) and the vast majority of pediatric GIST do not harbour KIT or platelet-derived growth factor receptor alpha (PDGFRA) mutations (J Clin Oncol 22:3813–3825, 2004; Hematol Oncol Clin North Am 23:15–34, 2009). The molecular biology of these GIST, originally defined as KIT/PDGFRA wild-type (WT), is complex due to the existence of different subgroups with distinct molecular hallmarks, including defects in the succinate dehydrogenase (SDH) complex and mutations of neurofibromatosis type 1 (NF1), BRAF, or KRAS genes (RAS-pathway or RAS-P).

In this extremely heterogeneous landscape, the clinical profile and molecular abnormalities of the small subgroup of WT GIST suitably referred to as quadruple wild-type GIST (quadruple^WT or KIT^WT/PDGFRA^WT/SDH^WT/RAS-P^WT) remains undefined. The aim of this study is to investigate the genomic profile of KIT^WT/PDGFRA^WT/SDH^WT/RAS-P^WT GIST, by using a massively parallel sequencing and microarray approach, and compare it with the genomic profile of other GIST subtypes.

Methods

We performed a whole genome analysis using a massively parallel sequencing approach on a total of 16 GIST cases (2 KIT^WT/PDGFRA^WT/SDH^WT and SDHB^IHC+/SDHA^IHC+, 2 KIT^WT/PDGFRA^WT/SDHA^mut and SDHB^IHC-/SDHA^IHC- and 12 cases of KIT^mut or PDGFRA^mut GIST). To confirm and extend the results, whole-genome gene expression analysis by microarray was performed on 9 out 16 patients analyzed by RNAseq and an additional 20 GIST patients (1 KIT^WT/PDGFRA^WTSDHA^mut GIST and 19 KIT^mut or PDGFRA^mut GIST). The most impressive data were validated by quantitave PCR and Western Blot analysis.

Results

We found that both cases of quadruple^WT GIST had a genomic profile profoundly different from both either KIT/PDGFRA mutated or SDHA-mutated GIST. In particular, the quadruple^WT GIST tumors are characterized by the overexpression of molecular markers (CALCRL and COL22A1) and of specific oncogenes including tyrosine and cyclin- dependent kinases (NTRK2 and CDK6) and one member of the ETS-transcription factor family (ERG).

Conclusion

We report for the first time an integrated genomic picture of KIT^WT/PDGFRA^WT/SDH^WT/RAS-P^WT GIST, using massively parallel sequencing and gene expression analyses, and found that quadruple^WT GIST have an expression signature that is distinct from SDH-mutant GIST as well as GIST harbouring mutations in KIT or PDGFRA. Our findings suggest that quadruple^WT GIST represent another unique group within the family of gastrointestintal stromal tumors.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2407-14-685) contains supplementary material, which is available to authorized users.

The TrkAIII oncoprotein inhibits mitochondrial free radical ROS-induced death of SH-SY5Y neuroblastoma cells by augmenting SOD2 expression and activity at the mitochondria, within the context of a tumour stem cell-like phenotype

The developmental and stress-regulated alternative TrkAIII splice variant of the NGF receptor TrkA is expressed by advanced stage human neuroblastomas (NBs), correlates with worse outcome in high TrkA expressing unfavourable tumours and exhibits oncogenic activity in NB models. In the present study, we report that constitutive TrkAIII expression in human SH-SY5Y NB cells inhibits Rotenone, Paraquat and LY83583-induced mitochondrial free radical reactive oxygen species (ROS)-mediated death by stimulating SOD2 expression, increasing mitochondrial SOD2 activity and attenuating mitochondrial free radical ROS production, in association with increased mitochondrial capacity to produce H2O2, within the context of a more tumour stem cell-like phenotype. This effect can be reversed by the specific TrkA tyrosine kinase inhibitor GW441756, by the multi-kinase TrkA inhibitors K252a, CEP-701 and Gö6976, which inhibit SOD2 expression, and by siRNA knockdown of SOD2 expression, which restores the sensitivity of TrkAIII expressing SH-SY5Y cells to Rotenone, Paraquat and LY83583-induced mitochondrial free radical ROS production and ROS-mediated death. The data implicate the novel TrkAIII/SOD2 axis in promoting NB resistance to mitochondrial free radical-mediated death and staminality, and suggest that the combined use of TrkAIII and/or SOD2 inhibitors together with agents that induce mitochondrial free radical ROS-mediated death could provide a therapeutic advantage that may also target the stem cell niche in high TrkA expressing unfavourable NB.

Identification of novel candidate compounds targeting TrkB to induce apoptosis in neuroblastoma

Neuroblastoma (NB) is one of the most frequent solid tumors in children and its prognosis is still poor. The neurotrophin receptor TrkB and its ligand brain-derived neurotrophic factor (BDNF) are expressed at high levels in high-risk NBs and are involved in defining the poor prognosis of the patients. However, the TrkB targeting therapy has never been realized in the clinic. We performed an in silico screening procedure utilizing an AutoDock/grid computing technology in order to identify novel small chemical compounds targeting the BDNF-binding domain of TrkB. For the first screening, a library of three million synthetic compounds was screened in silico and was ranked according to the Docking energy. The top-ranked 37 compounds were further functionally screened for cytotoxicity by using NB cell lines. We have finally identified seven compounds that kill NB cells with the IC₅₀ values of 0.07–4.6 μmol/L. The terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay showed that these molecules induce apoptosis accompanied by p53 activation in NB cell lines. The candidate compounds and BDNF demonstrated an antagonistic effect on cell growth, invasion, and colony formation, possibly suggesting competition at the BDNF-binding site of TrkB. The candidate compounds had tumor-suppressive activity in xenograft and in vivo toxicity tests (oral and intravenous administrations) using mice, and did not show any abnormal signs. Using in silico Docking screening we have found new candidate TrkB inhibitors against high-risk NBs, which could lead to new anti-cancer drugs.

TrkAIII promotes microtubule nucleation and assembly at the centrosome in SH-SY5Y neuroblastoma cells, contributing to an undifferentiated anaplastic phenotype

The alternative TrkAIII splice variant is expressed by advanced stage human neuroblastomas (NBs) and exhibits oncogenic activity in NB models. In the present study, employing stable transfected cell lines and assays of indirect immunofluorescence, immunoprecipitation, Western blotting, microtubule regrowth, tubulin kinase, and tubulin polymerisation, we report that TrkAIII binds α -tubulin and promotes MT nucleation and assembly at the centrosome. This effect depends upon spontaneous TrkAIII activity, TrkAIII localisation to the centrosome and pericentrosomal area, and the capacity of TrkAIII to bind, phosphorylate, and polymerise tubulin. We propose that this novel role for TrkAIII contributes to MT involvement in the promotion and maintenance of an undifferentiated anaplastic NB cell morphology by restricting and augmenting MT nucleation and assembly at the centrosomal MTOC.

Towards a turning point of neuroblastoma therapy

In 1983, the MYCN oncogene amplification was discovered in neuroblastoma cells and few years later its prognostic role was clearly demonstrated. The first European study, in which MYCN gene status is taken as prognostic factor for therapeutic decision, was released by SIOPEN and GPOH in 1995. Afterward, other trials were released by SIOPEN, GPOH, COG, and JNBSG in which MYCN gene amplification was employed as prognostic risk factor. However, since MYCN is abnormal in about 20% of tumors and is a reliable prognostic marker for only some subgroups of patients, additional chromosomal abnormalities have been introduced for clinical decisions: 1p deletions/imbalances (GPOH), 11q deletions/imbalances (COG), and structural copy number aberrations (SIOPEN). MYCN gene status and chromosome aberrations improved patients' risk evaluation and helped to develop tailored therapy for diverse subgroups of patients. Unfortunately, high risk patients still have an unfavorable prognosis and are the major challenge for oncologists. In the last decade, the advent of genome-wide analysis and the next generation sequencing technique have given the opportunity to deeply investigate the genome of neuroblastoma to identify both candidate genes associated with tumor progression and druggable target genes. ALK gene is an actual candidate for the therapy with small molecule inhibitors, but others as mTOR, Aurora, and TRK, are suitable targets for subgroups of high risk patients. The huge amount of clinical and biological information collected in the last 20 years suggests that it is time to turn on for new and more personalized therapies of neuroblastoma.

EGFR/HER-targeted therapeutics in ovarian cancer

Despite decades of research and evolving treatment modalities, survival among patients with epithelial ovarian cancer has improved only incrementally. During this same period, the development of biologically targeted therapeutics has improved survival for patients with diverse malignancies. Many of these new drugs target the human epidermal growth factor receptor (EGFR/HER/ErbB) family of tyrosine kinases, which play a major role in the etiology and progression of many carcinomas, including epithelial ovarian cancer. While several HER-targeted therapeutics are US FDA approved for the treatment of various malignancies, none have gained approval for the treatment of ovarian cancer. Here, we review the published literature on HER-targeted therapeutics for the treatment of ovarian cancer, including novel HER-targeted therapeutics in various stages of clinical development, as well as the challenges that have limited the use of these inhibitors in clinical settings.

FLT3 inhibition as therapy in acute myeloid leukemia: a record of trials and tribulations

Acute myeloid leukemia (AML) is a hematologic malignancy with a poor prognosis. Approximately one quarter of the patients with AML also carry an internal tandem duplication (ITD) mutation in the gene encoding FMS-like tyrosine kinase 3 (FLT3), which has a significantly deleterious impact on prognosis. The ITD mutation renders FLT3 constitutively active and leads to uncontrolled proliferation of the leukemic blast. Over the course of the last decade, a variety of compounds have been developed in preclinical and clinical studies as potent inhibitors of FLT3. Many of the earlier agents under investigation, such as lestaurtinib, midostaurin, and sunitinib, were initially developed as inhibitors of other tyrosine kinases and as targeted therapies in a variety of malignancies. These compounds have been demonstrated to have some efficacy in clinical trials of AML, mainly manifesting as transient decreases in circulating blasts correlating with effective in vivo suppression of the FLT3 target. Nevertheless, the cumbersome pharmacokinetics of some compounds and the suboptimal specificity and potency of others have limited their therapeutic efficacy. In the last few years, newer, more potent and specific agents have been under investigation, with the leading example being AC220. This agent has shown significant promise in early phases of clinical investigation, and is currently in more advanced clinical trials. Hope remains that FLT3 inhibition will be become an effective therapeutic adjunct to our current treatment approach to AML.

FLT3 inhibitors: a story of the old and the new

PURPOSE OF REVIEW

Ever since the recognition that FMS-like tyrosine kinase 3 (FLT3) mutations exert a profound negative prognostic impact on the clinical outcome of patients with acute myeloid leukemia (AML), researchers have sought to find effective small-molecule inhibitors of this receptor tyrosine kinase. This review will attempt to provide a survey of the FLT3 inhibitors currently under investigation and provide a discussion on their current status in clinical trials.

RECENT FINDINGS

Over the past 10 years, a number of different compounds have been studied in vitro and clinically as FLT3 inhibitors. The first inhibitors studied were hampered by cumbersome pharmacokinetics and a general lack of potency. However, some agents have shown promise in clinical trials with transient responses in AML. Newer compounds, such as AC220, have demonstrated profound selectivity and potency against the FLT3 target, and are currently being investigated in clinical trials.

SUMMARY

Clinical trials have so far demonstrated that inhibitors of FLT3 do have clinical activity in patients with FLT3-mutant AML, although this activity is often transient and correlates with effective in-vivo suppression of the FLT3 target. As newer, more potent agents are now entering advanced clinical trials, opportunities will emerge for real progress against this grim disease.

Comparison of the specificity of Trk inhibitors in recombinant and neuronal assays

Neurotrophins are important mediators of neuronal development, survival and plasticity. They act via binding to Trk receptors, which results in the stimulation of the intracellular tyrosine kinase domain of the receptor leading to autophosphorylation of this domain. This in turn creates a scaffold that recruits various adapter proteins allowing the activation of intracellular signaling cascades including the PLCγ, MAPK and PI3K pathways. Compounds that specifically block the activity of the tyrosine kinase domain of Trk receptors would provide a powerful tool to study the role of these receptors in cells. K252a has previously been used for this purpose, however we show here that it can inhibit many tyrosine and serine/threonine kinases in vitro. Profiling of 3 newer inhibitors, referred to here as SHN-753, SHN-722 and GSK-Trk, demonstrate that they have significantly improved specificity for the kinase activity of TrkA in vitro compared to K252a. In addition these compounds were found to block the TrkB mediated activation of ERK1/2 by BDNF, but did not affect NMDA induced ERK1/2 activation. These compounds, while still not completely specific for Trk receptor kinase activity, do represent a considerable improvement over K252a and should prove valuable in the study of neurotrophin-mediated actions in the nervous system.

Theraputic targeting of Trk supresses tumor proliferation and enhances cisplatin activity in HNSCC

Head and neck squamous cell carcinoma (HNSCC) is a biologically aggressive disease that has been modestly impacted by improvements in therapeutic strategies. Several lines of evidence support the role of TrkB for invasion and metastasis in various solid tumor models, and we have shown an important function of this receptor in HNSCC tumor biology. Therapeutic modulation of TrkB function has been supported in the literature by the development of small molecule inhibitors (SMI) with minimal success. To assess the validity of targeting TrkB in HNSCC, we tested a novel agent, AZ64 and show significant dose and time-dependent inhibition of cellular proliferation in cell lines. Genetic studies revealed the specificity of this compound for the TrkB receptor, as exposure of cells that had genetic suppression of TrkB did not demonstrate abrogated oncogenic signaling. We next assessed the impact of AZ64 as a chemotherapy-sensitizer and identified an enhancement of cisplatin-mediated anti-proliferation across all cell lines. We then demonstrated that AZ64 can overcome chemotherapy resistance in a novel model of cisplatin resistance in HNSCC. Modulation of the pro-oncogenic STAT3 and Src pathways was identified, suggesting molecular mechanisms of action for AZ64. In this study, we demonstrate the feasibility of targeting TrkB and suggest a novel approach for the treatment of some chemotherapy-resistant HNSCC.

Therapeutic potential of Janus-activated kinase-2 inhibitors for the management of myelofibrosis

Myelofibrosis (either primary or postpolycythemia vera/essential thrombocythemia) is a chronic and debilitating myeloproliferative neoplasm for which there is no well-accepted standard of care. Clinical manifestations of this disease (e.g., cytopenias, splenomegaly, bone marrow fibrosis) and constitutional symptoms (e.g., hypercatabolic state, fatigue, night sweats, fever) create significant treatment challenges. For example, progressive splenomegaly increases the risk for more serious clinical sequelae (e.g., portal hypertension, splenic infarction). Myelofibrosis arises from hematopoietic stem cells or early progenitor cells. However, the molecular mechanisms underlying its pathogenesis and clinical presentation are poorly understood, delaying the development of effective and targeted treatments. Recent studies have implicated mutations that directly or indirectly lead to the deregulated activation of Janus-activated kinase 2 (JAK2). Appreciation for the activation of JAK2 and the importance of increased levels of circulating proinflammatory cytokines in the pathogenesis and clinical manifestations of myelofibrosis has led to novel therapeutic agents targeting JAKs. This review will briefly discuss the origins of the JAK2 hypothesis, the clinical relevance of JAK2 mutations in myelofibrosis, and recent clinical progress in targeting JAKs as a therapeutic intervention for patients with this chronic and debilitating disease.

Lestaurtinib enhances the antitumor efficacy of chemotherapy in murine xenograft models of neuroblastoma

PURPOSE

Neuroblastoma, a common pediatric tumor of the sympathetic nervous system, is characterized by clinical heterogeneity. The Trk family neurotrophin receptors play an important role in this behavior. Expression of TrkA is associated with favorable clinical features and outcome, whereas TrkB expression is associated with an unfavorable prognosis. We wanted to determine if the Trk-selective inhibitor lestaurtinib had therapeutic efficacy in a preclinical neuroblastoma model.

EXPERIMENTAL DESIGN

We performed intervention trials of lestaurtinib alone or in combination with other agents in TrkB-overexpressing neuroblastoma xenograft models.

RESULTS

Lestaurtinib alone significantly inhibited tumor growth compared to vehicle-treated animals [P = 0.0004 for tumor size and P = 0.011 for event-free survival (EFS)]. Lestaurtinib also enhanced the antitumor efficacy of the combinations of topotecan plus cyclophosphamide (P < 0.0001 for size and P < 0.0001 for EFS) or irinotecan plus temozolomide (P = 0.011 for size and P = 0.012 for EFS). There was no additive benefit of combining either 13-cis-retinoic acid or fenretinide with lestaurtinib compared to lestaurtinib alone. There was dramatic growth inhibition combining lestaurtinib with bevacizumab (P < 0.0001), but this combination had substantial systemic toxicity.

CONCLUSIONS

We show that lestaurtinib can inhibit the growth of neuroblastoma both in vitro and in vivo and can substantially enhance the efficacy of conventional chemotherapy, presumably by inhibition of the Trk/brain-derived neurotrophic factor autocrine survival pathway. It may also enhance the efficacy of selected biological agents, but further testing is required to rule out unanticipated toxicities. Our data support the incorporation of Trk inhibitors, such as lestaurtinib, in clinical trials of neuroblastoma or other tumors relying on Trk signaling pathways for survival.

On Trk--the TrkB signal transduction pathway is an increasingly important target in cancer biology

In the beginning, Trk was an oncogene. Yet Neurotrophin-Trk signaling came to preeminence in the field of neurobiology. Now it is appreciated that Trks regulate important processes in nonneuronal cells and, in addition to their impact on tumors of neural origin, may contribute to the pathogenesis of carcinomas, myelomas, and prostate and lymphoid tumors. Although mutations and rearrangements of Trk are seen only sporadically in human cancers, such as medullary thyroid carcinoma, a number of recent studies indicate that expression of TrkB contributes to tumor pathology. In neuroblastoma, TrkA expression marks good prognosis which TrkB and Brain-derived neurotrophic factor (BDNF) expression marks poor prognosis. Activation of the BDNF/TrkB signal transduction pathway also stimulates tumor cell survival and angiogenesis and contributes to resistance to cytotoxic drugs and anoikis, enabling cells to acquire many of the characteristic features required for tumorigenesis. Small molecule inhibitors, such as Cephalon's CEP-701, are in phase 1 and 2 clinical trials, and a series of AstraZeneca Trk inhibitors are poised to enter the clinic. As monotherapy, inhibitors may be effective only in tumors with activating Trk mutations. Important clinical follow-up will be the assessment of Trk inhibitors in combination with standard chemo- or radiotherapy or other signal transduction pathway inhibitors.

The alternative TrkAIII splice variant targets the centrosome and promotes genetic instability

The hypoxia-regulated alternative TrkAIII splice variant expressed by human neuroblastomas exhibits oncogenic potential, driven by in-frame exon 6 and 7 alternative splicing, leading to omission of the receptor extracellular immunoglobulin C(1) domain and several N-glycosylation sites. Here, we show that the TrkAIII oncogene promotes genetic instability by interacting with and exhibiting catalytic activity at the centrosome. This function depends upon intracellular TrkAIII accumulation and spontaneous interphase-restricted activation, in cytoplasmic tyrosine kinase (tk) domain orientation, predominantly within structures that closely associate with the fully assembled endoplasmic reticulum intermediate compartment and Golgi network. This facilitates TrkAIII tk-mediated binding of gamma-tubulin, which is regulated by endogenous protein tyrosine phosphatases and geldanamycin-sensitive interaction with Hsp90, paving the way for TrkAIII recruitment to the centrosome. At the centrosome, TrkAIII differentially phosphorylates several centrosome-associated components, increases centrosome interaction with polo kinase 4, and decreases centrosome interaction with separase, the net results of which are centrosome amplification and increased genetic instability. The data characterize TrkAIII as a novel internal membrane-associated centrosome kinase, unveiling an important alternative mechanism to "classical" cell surface oncogenic receptor tk signaling through which stress-regulated alternative TrkAIII splicing influences the oncogenic process.

Identification and preclinical characterization of AZ-23, a novel, selective, and orally bioavailable inhibitor of the Trk kinase pathway

Tropomyosin-related kinases (TrkA, TrkB, and TrkC) are receptor tyrosine kinases that, along with their ligands, the neurotrophins, are involved in neuronal cell growth, development, and survival. The Trk-neurotrophin pathway may also play a role in tumorigenesis through oncogenic fusions, mutations, and autocrine signaling, prompting the development of novel Trk inhibitors as agents for cancer therapy. This report describes the identification of AZ-23, a novel, potent, and selective Trk kinase inhibitor. In vitro studies with AZ-23 showed improved selectivity over previous compounds and inhibition of Trk kinase activity in cells at low nanomolar concentrations. AZ-23 showed in vivo TrkA kinase inhibition and efficacy in mice following oral administration in a TrkA-driven allograft model and significant tumor growth inhibition in a Trk-expressing xenograft model of neuroblastoma. AZ-23 represents a potent and selective Trk kinase inhibitor from a novel series with the potential for use as a treatment for cancer.

Frequent mutations in the neurotrophic tyrosine receptor kinase gene family in large cell neuroendocrine carcinoma of the lung

The neurotrophic tyrosine receptor kinase (NTRK) family is potentially implicated in tumorigenesis and progression of several neoplastic diseases, including lung cancer. We investigated a large number of pulmonary neuroendocrine tumors (PNETs) and non-small cell lung carcinomas (NSCLCs) without morphological evidence of neuroendocrine differentiation for mutations in the NTRK gene family. A total of 538 primary lung carcinomas, including 17 typical carcinoids (TCs), 10 atypical carcinoids (ACs), 39 small cell lung carcinomas (SCLCs), 29 large cell neuroendocrine carcinomas (LCNECs), and 443 NSCLCs were evaluated by single-strand conformation polymorphism (SSCP) and sequencing of the tyrosine kinase domain (TKD) of NTRK1, NTRK2, and NTRK3. The NTRK1 gene was never found to be mutated. A total of 10 somatic mutations were detected in NTRK2 and NTRK3, mostly located in the activating and catalytic loops. NTRK mutations were seen in 9 (10%) out of 95 PNETs but in 0 out of 443 NSCLCs investigated. No mutations were observed in TCs, ACs, and SCLCs. Interestingly, all the mutations were restricted to the LCNEC histotype, in which they accounted for 31% of cases. A mutational analysis, performed after microdissection of LCNECs combined with adenocarcinoma (ADC), showed that only neuroendocrine areas were positive, suggesting that NTRK mutations are involved in the genesis of the neuroendocrine component of combined LCNECs. Our data indicate that somatic mutations in the TKD of NTRK genes are frequent in LCNECs. Such mutational events could represent an important step in the cancerogenesis of these tumors and may have potential implications for the selection of patients for targeted therapy.

Clinical implications of the ErbB/epidermal growth factor (EGF) receptor family and its ligands in ovarian cancer

The ERBB or EGF receptor (EGFR) proto-oncogene family, which consists of four structurally-related transmembrane receptors (i.e., EGFR, ErbB2, ErbB3, and ErbB4), plays an etiological role in the molecular pathogenesis of cancer and is a key therapeutic target in many types of cancer, including ovarian cancer. These ErbB/EGF receptor tyrosine kinases play important physiologic roles in cell proliferation, survival, adhesion, motility, invasion, and angiogenesis. It is, therefore, not surprising that gene amplification, genetic mutation, and altered transcription/translation result in aberrant ErbB/EGF receptor expression and/or signal transduction, contributing to the development of malignant transformation. Clinically, the diagnostic, prognostic, and theragnostic significance of any single ErbB receptor and/or ErbB ligand is controversial, but generally, ErbB receptor overexpression has been correlated with poor prognosis and decreased therapeutic responsiveness in ovarian cancer patients. Thus, anticancer agents targeting ErbB/EGF receptors hold great promise for personalized cancer treatment. Yet, challenges remain in designing prospective clinical trials to assess the clinical utility of ErbB receptors and their ligands to diagnose cancer; to predict progression-free and overall survival, therapeutic responsiveness, and disease recurrence; and to monitor treatment responsiveness. Here, we review the tissue expression and serum biomarker studies that have evaluated the diagnostic, prognostic, and theragnostic utility of ErbB/EGF receptors, their circulating soluble isoforms (sEGFR/sErbBs), and their cognate ligands in ovarian cancer patients.

Genetic organization of the biosynthetic gene cluster for the indolocarbazole K-252a in Nonomuraea longicatena JCM 11136

Indolocarbazole metabolite K-252a is a natural product that was previously reported as a potent protein kinase C inhibitor with in vitro and in vivo potency. From a biosynthetic viewpoint, this compound possesses structurally interesting features such as an unusual furanosyl sugar moiety, which are absent in the well-studied staurosporine and rebeccamycin. A cosmid library from genomic DNA of Nonomuraea longicatena JCM 11136 was constructed and screened for the presence of genes to be involved in the biosynthesis of indolocarbazole K-252a. Using as a probe an internal fragment of vioB, a Chromobacterium violaceum gene encoding a multifunctional enzyme that catalyzes tryptophan decarboxylation and condensation reaction in violacein biosynthesis, we isolated a DNA region that directed the biosynthesis of K-252a when introduced into the heterologous expression host Streptomyces albus. Sequence analysis of 45 kb revealed genes for indolocarbazole core formation, glycosylation, and sugar methylation, as well as a regulatory gene and two resistance/secretion genes. The cloned genes should help to elucidate the molecular basis for indolocarbazole biosynthesis and generate new indolocarbazole analogues by genetic engineering.

Indolocarbazole natural products: occurrence, biosynthesis, and biological activity

The indolocarbazole family of natural products, including the biosynthetically related bisindolylmaleimides, is reviewed (with 316 references cited). The isolation of indolocarbazoles from natural sources and the biosynthesis of this class of compounds are thoroughly reviewed, including recent developments in molecular genetics, enzymology and metabolic engineering. The biological activities and underlying modes of action displayed by natural and synthetic indolocarbazoles is also presented, with an emphasis on the development of analogs that have entered clinical trials for its future use against cancer or other diseases.

Identification of 2-acylaminothiophene-3-carboxamides as potent inhibitors of FLT3

A series of 2-acylaminothiophene-3-carboxamides has been identified which exhibit potent inhibitory activity against the FLT3 tyrosine kinase. Compound 44 inhibits the isolated enzyme (IC50 = 0.027 microM) and blocks the proliferation of MV4-11 cells (IC50 = 0.41 microM). Structure-activity relationship studies within this series are described in the context of a proposed binding model within the ATP binding site of the enzyme.

Activity of irinotecan and the tyrosine kinase inhibitor CEP-751 in medullary thyroid cancer

CONTEXT

Medullary thyroid cancer (MTC) is a cancer of the parafollicular C cells that commonly presents with an inherited or acquired RET gene mutation. There is currently no effective systemic treatment for MTC.

OBJECTIVE

The objective of this study was to investigate a systemic therapeutic approach to treat MTC. We studied the sensitivity of an MTC cell line and xenograft to irinotecan, alone and in combination with the tyrosine kinase inhibitor, CEP-751.

RESULTS

In TT cell culture and xenografts, irinotecan treatment was highly effective. This effect was augmented by treatment with CEP-751. Treatment of TT cell xenografts resulted in durable complete remission in 100% of the mice, with median time to recurrence of 70 d for irinotecan alone and more than 130 d for irinotecan plus CEP-751. Although irinotecan induced an S phase checkpoint arrest in TT cells, CEP-751 in combination with irinotecan resulted in a loss of this arrest. CEP-751 induced a loss in the induction of the DNA repair program marked by phospho-H2AX and the checkpoint pathway marked by the activated Chk1 pathway.

CONCLUSIONS

Irinotecan treatment was highly effective in a preclinical model of human MTC, resulting in complete remission in 100% of the xenografts treated. The duration of remission was further enhanced by combination with the kinase inhibitor, CEP-751. These results suggest that irinotecan, alone or in combination, may be useful for the treatment of MTC.

Phase I clinical trial of CEP-2563 dihydrochloride, a receptor tyrosine kinase inhibitor, in patients with refractory solid tumors

CEP-2563 dihydrochloride (CEP-2563) is a soluble lysinyl-beta-alanyl ester of CEP-751, a potent inhibitor of the trk family of receptor tyrosine kinases and the platelet-derived growth factor (PDGF) receptor tyrosine kinase. CEP-2563 was developed because of the limited aqueous solubility of CEP-751. Preclinical models have demonstrated that both CEP-751 and CEP-2563 have antitumor activity in a variety of tumors. A Phase I clinical trial involving 18 patients was conducted to determine the toxicity profile, maximum tolerated dose (MTD), toxicity profile, and pharmacokinetics of CEP-2563 in patients with advanced solid tumors refractory to standard therapy. CEP-2563 was administered over 1 hour via a central venous catheter once daily for five consecutive days every three weeks. A rapid dose titration strategy with initial single patient cohorts and 100% dose escalations was used. With the appearance of drug-related toxicity, escalations were decreased to 50% or 25% and cohorts were expanded to 3 or 6 patients until establishment of the MTD. Dose escalation rapidly proceeded to 320 mg/m(2)/d. The dose limiting toxicities (DLTs) observed were grade 3 hypotension and grade 2 allergic reaction. Other toxicities included anemia, thrombocytopenia, anorexia, asthenia, diarrhea, fatigue, headache, nausea, vomiting, and rash. Pharmacokinetic analysis showed that CEP-2563 is reliably converted to CEP-751. This study demonstrated that single agent CEP-2563 therapy is feasible with acceptable toxicities. The recommended phase II dose is 256 mg/m(2)/d. Rapid dose escalation with single patient cohorts was a safe and efficient method of conducting this phase I trial.

Phase I trial of orally administered CEP-701, a novel neurotrophin receptor-linked tyrosine kinase inhibitor

PURPOSE

A phase I clinical trial in patients with advanced carcinomas was conducted using the orally available neurotrophin receptor-linked tyrosine kinase receptor inhibitor, CEP-701. The objectives were to determine the maximum tolerated dose (MTD), dose-limiting toxicities, and pharmacokinetic profile of this orally administered agent.

PATIENTS AND METHODS

A total of 30 patients were accrued to receive escalating BID doses of CEP-701 in cycles lasting 28 days. Between 3 and 6 patients were enrolled at each dose level. Once the MTD was determined, nine de novo patients were recruited to receive that level of drug. Pharmacokinetic studies were performed after the first dose, with additional sampling to assess intraindividual variability.

RESULTS

The dosages ranged from 5 mg BID to 160 mg BID. While the criteria for MTD were not met at the dose levels administered, DLTs were observed at 80 and 120 mg BID. Treatment related adverse events, especially of the gastrointestinal system, made CEP-701 poorly tolerated at dosages above 40 mg BID. While CEP-701 did not produce an objective tumor response in any patient, 7 of the 30 patients received treatment for 3 months or more, including 3 who were on study with stable disease for more than 6 months. Orally administered CEP-701 was rapidly absorbed, with a mean t(max) between 1 and 3 hours. At higher dose levels, serum drug levels showed greater than dose-proportional increases by Day 28 versus Day 1.

CONCLUSION

CEP-701 40 mg BID was well tolerated by patients with advanced malignancy and is the recommended dose level for planned phase II trials. Further study is necessary to determine the clinical efficacy of this novel new chemotherapeutic agent.

Synthesis, Modeling, and In Vitro Activity of (3‘S)-epi-K-252a Analogues. Elucidating the Stereochemical Requirements of the 3‘-Sugar Alcohol on trkA Tyrosine Kinase Activity

Utilizing our recently published semisynthetic approach to the (3'S)-K-252a diastereomer, we report the first synthesis of the (3'R)-10 diastereomer and a set of related epimers, with the goal of defining the stereochemical role of the 3'-sugar hydroxyl group on trkA tyrosine kinase activity and selectivity. (3'R)-10 displayed potent trkA inhibitory activity with an IC50 value of 4 nM. The corresponding deshydroxy epimer (3'S)-14 was 7-fold more potent than its 3'R counterpart (natural stereochemistry) with a trkA IC50 value of 3 nM and demonstrated >280-fold selectivity over PKC (IC50 = 850 nM). In cells, (3'S)-14 displayed potent inhibition of trkA autophosphorylation with an IC50 < 10 nM. Molecular modeling studies revealed that the 3'-OH, due to the inverted geometry, forms significant H-bonding interactions with Glu27 and Arg195, an interaction that is not attainable with the natural isomers.

Synthesis and kinase inhibitory activity of 3'-(S)-epi-K-252a

The 3'-epi diastereomer of K-252a was synthesized with the goal of evaluating the stereochemical requirements of the 3'-sugar alcohol on kinase inhibitory activity. Inverting the 3'-alcohol resulted in a 20 nM inhibitor of VEGFR2 and a 1 nM inhibitor of TrkA tyrosine kinase.

Targeted inhibition of FLT3 overcomes the block to myeloid differentiation in 32Dcl3 cells caused by expression of FLT3/ITD mutations

Internal tandem duplication (ITD) mutations of the juxtamembrane domain-coding sequence of the FLT3 gene are found in up to 34% of patients with acute myeloid leukemia (AML) and are associated with a poor prognosis. FLT3/ITDs result in constitutive activation of the tyrosine kinase domain and transform growth factor-dependent cell lines. FLT3 activation leads to antiapoptotic and proliferative signals, but little is known about the impact of FLT3/ITDs on differentiation. This study was designed to investigate the effect of FLT3/ITD expression on the differentiation of the 32Dcl3 (32D) myeloblastic cell line to neutrophils in response to granulocyte colony-stimulating factor (G-CSF). Expression of FLT3/ITD completely blocked morphologic differentiation and induction of myeloperoxidase (MPO), lysozyme, and CCAAT/enhancer-binding protein epsilon (C/EBPepsilon) in response to G-CSF. Wild-type FLT3 and vector-transfected 32D cells were able to differentiate, although the maturation of FLT3-transfected cells was delayed by FLT3 ligand (FL) stimulation. CEP-701, a potent FLT3 tyrosine kinase inhibitor, overcame the morphologic block in differentiation caused by FLT3/ITD expression and allowed G-CSF induction of myeloid maturation markers. These findings suggest that blocking differentiation may be one of the mechanisms by which FLT3/ITDs contribute to leukemogenesis. CEP-701 and other FLT3 inhibitors may be useful for overcoming the block to differentiation (as well as the block to apoptosis) in the leukemic cells of patients with AML.

K252a inhibits the oncogenic properties of Met, the HGF receptor

The ATP analog K252a is a potent inhibitor for receptor tyrosine kinases of the Trk family. Here we show that nanomolar concentrations of K252a prevent HGF-mediated scattering in MLP-29 cells (30 nM), reduce Met-driven proliferation in GTL-16 gastric carcinoma cells (100 nM), and cause reversion in NIH3T3 fibroblasts transformed by the oncogenic form of the receptor, Tpr-Met (75 nM). K252a inhibits Met autophosphorylation in cultured cells and in immunoprecipitates and prevents activation of its downstream effectors MAPKinase and Akt. Interestingly, K252a seems to be more effective at inhibiting the mutated form of Met (M1268T) found in papillary carcinoma of the kidney than the wild type receptor. Pretreatment of both Tpr-Met-transformed NIH3T3 fibroblasts and of GTL-16 gastric carcinoma cells with K252a results in loss of their ability to form lung metastases in nude mice upon injection into the caudal vein. These observations suggest that K252a derivatives, which are active in vivo as anti-cancer drugs in models of Trk-driven malignancies, should also be effective for treatment of Met-mediated tumors.

The K252a derivatives, inhibitors for the PAK/MLK kinase family selectively block the growth of RAS transformants

BACKGROUND

Oncogenic RAS mutants such as v-Ha-RAS activate members of Rac/CDC42-dependent kinases (PAKs) and appear to contribute to the development of more than 30% of all human cancers. PAK1 activation is essential for oncogenic RAS transformation, and several chemical compounds that inhibit Tyr kinases essential for the RAS-induced activation of PAK1 strongly suppress RAS transformation either in cell culture or in vivo (nude mice). Although we have developed a cell-permeable PAK-specific peptide inhibitor called WR-PA18, so far no chemical (metabolically stable) compound has been developed that directly inhibits PAK1 in a highly selective manner. Thus, we have explored such a PAK1 inhibitor(s) among synthetic derivatives of an adenosine triphosphate antagonist.

RESULTS

From the naturally occurring adenosine triphosphate antagonist K252a, we have developed two bulky derivatives, called CEP-1347 and KT D606 (a K252a dimer), which selectively inhibit PAKs or mixed-lineage kinases both in vitro and in cell culture and convert v-Ha-RAS-transformed NIH 3T3 cells to flat fibroblasts similar to the parental normal cells. Furthermore, these two K252a analogues suppress the proliferation of v-Ha-RAS transformants, but not the normal cells.

CONCLUSION

These bulky adenosine triphosphate antagonists derived from K252a or related indolocarbazole compounds such as staurosporine would be potentially useful for the treatment of RAS/ PAK1-induced cancers, once their anti-PAK1 activity is significantly potentiated by a few additional chemical modifications at the sugar ring suggested in this paper.

Effect of CEP-751 (KT-6587) on neuroblastoma xenografts expressing TrkB

BACKGROUND

The compound CEP-751 (KT-6587), a potent and selective inhibitor of the Trk family of tyrosine kinases, has been shown to inhibit the growth of human neuroblastoma (NB) xenografts in nude mice [1].

PROCEDURE

To address its mechanism of action, we studied SY5Y, a human NB cell line with no detectable Trk expression, and two subclones transfected with TrkB. The transfected clones, SY5Y (G8) and SY5Y (G12), expressed moderate and high levels, respectively, of TrkB mRNA and protein. These TrkB-expressing subclones and the parental line were then grown as xenografts in nude mice, and CEP-751 was used to inhibit TrkB tyrosine kinase activity in these xenografts. Animals were treated twice a day with CEP-751 (21 mg/kg), or with the carrier vehicle as a control. TrkB expression in the resultant tumors was examined by quantitative RT-PCR. The effect of CEP-751 on TrkB activation by BDNF was examined in G12 cells in culture by immunoprecipitation with antipan Trk antiserum, followed by Western blot analysis using antiphosphotyrosine antibodies. To determine if CEP-751 was causing apoptosis, the TUNEL assay was used.

RESULTS

CEP-751 had little effect on the growth of SY5Y tumors, but did slow the growth rate of the C8 and G12 tumors. The daily growth rate of the treated tumors was 0.16, 0.13, and 0.10 cm3, respectively, for the SY5Y, G8, and G12 tumors. RT PCR analysis confirmed the expression of TrkB in G8 and G12, but not in SY5Y tumors. Activation of TrkB by BDNF in G12 cells was inhibited by CEP-751 in a dose dependent fashion. The treated tumors showed marked evidence of apoptosis.

CONCLUSIONS

These data suggest that the effect of CEP-751 is due, at least in part, to its inhibition of TrkB kinase, and that CEP-751 may become a useful therapeutic tool for the treatment of aggressive neuroblastomas, which often express TrkB.

Rational basis for Trk inhibition therapy for prostate cancer

BACKGROUND

Prostatic cancer cells are lethal because they acquire the ability to activate survival pathways that do not require androgenic stimulation. As a rational approach to developing effective therapy for these devastating cells, specific signal transduction pathways uniquely required for the survival of these nonandrogen-dependent prostate cancer cells must be identified. Previous studies suggested that the neurotrophin/trk signal transduction axis may regulate such unique survival pathways. In the present study, the changes in expression of the neurotrophins (NGF, BDNF, and NT-3) and their cognate receptors (i.e., trk and p75NTR) during the progression of normal prostatic epithelial cells to malignancy were documented. Additionally, the consequences of inhibiting these trk signaling pathways on the in vitro survival of prostate cancer cells was tested.

METHODS

Immmunocytochemistry, RT-PCR, and ELISA assays were used to characterize the changes in the neurotrophin ligands (i.e., NGF, BDNF, and NT-3) and their cognate high-affinity (i.e., trk A, B, and C) and low-affinity neurotrophin (i.e., p75 NTR) receptors in normal vs. malignant human prostatic tissues. CEP-751 is an indolocarbazole compound specifically designed to inhibit the initiation of these neurotrophin/trk signal transductions. The consequence of CEP-751 inhibition of trk signaling for in vitro clonogenic survival of a series of human prostatic cancer lines was also tested.

RESULTS

These studies demonstrated that normal prostatic tissue from patients without prostate cancer contains substantial levels of nerve growth factor (NGF), which is produced in a paracrine manner by stromal cells. These stromal cells lack both trk and p75NTR receptors. In contrast, normal prostatic epithelial cells from patients without prostate cancer do not secrete detectable levels of neurotrophins, but do express trk A and p75 NTR. While the NGF/trkA/p75 NTR axis is present in the normal prostate, normal prostatic epithelial cells do not depend on this axis for their survival. In contrast, malignant prostate epithelial cells directly secrete a series of neurotrophins (i.e., NGF, BDNF, and/or NT-3) and express at least one if not more of the trk receptor proteins (i.e., trk A, B, and/or C), while no longer expressing the p75NTR receptors. In addition, inhibition of autocrine trk signaling via CEP-751 treatment induces the apoptotic death of these malignant cells.

CONCLUSIONS

Prostate carcinogenesis involves molecular changes leading to the paracrine and/or autocrine production of a series of neurotrophins. This is coupled to the ectopic expression of trk B and trk C, as well as to the continued expression of trk A, and the loss of expression of p75NTR receptors. These changes result in the acquisition by malignant prostate cells of a unique requirement for trk signaling pathways for survival. Based on these findings, trk inhibition is a novel, rational approach for prostate cancer therapy.

Molecular events that regulate cell proliferation: an approach for the development of new anticancer drugs

Cancer chemotherapy is the object of many fundamental and clinical researches. The development in molecular techniques and structural studies at the molecular level have led to the discovery of key proteins involved in the regulation of cell proliferation. This opened perspectives to characterize new anticancer drugs in order to reduce the limitations found with conventional drugs such as the lack of selectivity for cancer cells and resistance phenomena. This review presents the anticancer drugs in clinical investigations that target molecules involved in the signal transduction impairment, the cell cycle deregulation and the differentiation with comments on their mechanisms of action.

Quantitative, high-throughput cell-based assays for inhibitors of trkA receptor

Two quantitative, high-throughput cell-based assays for evaluating inhibitors of NGF-stimulated trkA phosphorylation in trkA-transfected NIH3T3 cells have been established. Both assays involve capture of the trkA receptor from cell lysates in microtiter plates coated with an anti-trk antibody. The amount of trkA phosphorylation is then measured using either an anti-phosphotyrosine antibody with a colorimetric readout or a lanthanide (europium)-labeled anti-phosphotyrosine antibody with a fluorometric detection. The two assay formats exhibited at least a fivefold increase in phosphorylated trkA signal in trkA-transfected cells compared to vector control. Inhibition plots generated for trkA kinase inhibitors using the two detection systems yielded comparable IC(50) values. Overall, the two assays represent a marked improvement over the standard gel-based/western blot method in terms of throughput, quantitation, and amenability to automation.

ATP site-directed competitive and irreversible inhibitors of protein kinases

Several tyrosine and serine/threonine protein kinases have emerged in the last few years as attractive targets in the search for new therapeutic agents being applicable in many different disease indications. Initially, inhibition of these protein kinases by ATP site-directed inhibitors was considered less prone to success, but medicinal chemists from both academia and industry have been able to impart potency and selectivity to a limited number of scaffolds by modulating and fine-tuning the interactions of the modified template with the ATP binding site of the selected kinase. The chemical templates that have been used in the synthesis of ATP site-directed protein kinase inhibitors are reviewed with emphasis on the kinase inhibitors that have entered or are about to enter clinical trials. Examples have been selected to illustrate how structure-based design approaches and new methods to increase compound diversity have had an impact on this area of research.