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Cell Cycle 8:21, 3480-3487; November 1, 2009; © 2009 Landes Bioscience
Targeting oncogenic signaling pathways by exploiting nanotechnology Sudipta Basu, Padmaparna Chaudhuri and Shiladitya Sengupta* Harvard-MIT Division of Health Science and Technology; Department of Medicine; Brigham and Women’s Hospital; Harvard Medical School; Boston, MA USA
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wo scientific areas have recently emerged that can revolutionize cancer chemotherapy. First, an understanding of the different cellular signaling pathways implicated in the development and progression of cancer resulting in poor prognosis and drug resistance, have identified potential drug targets. Inhibitors of signal transduction pathways are currently in the clinics. Secondly, nanotechnology has emerged as an exciting multidisciplinary field promising to provide breakthrough solutions to the problems of optimizing the efficacy or therapeutic index of anticancer agents. The promise of nanotechnology lies in the ability to engineer customizable nanoscale constructs that can be loaded with one or more payloads such as chemotherapeutics, targeting units, imaging and diagnostic agents. This review addresses the potential integration of these two approaches to engineer nanoparticles that can target various signal transduction pathways in cancer. Introduction
Key words: nanotechnology, cancer, MAPK, signal transduction, nanoparticle Submitted: 04/28/09 Accepted: 08/19/09 Previously published online: www.landesbioscience.com/journals/cc/ article/9851 *Correspondence to: Shiladitya Sengupta; Email:
[email protected]
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Cancer is the second leading cause of mortality in the United States with an estimated 1,479,350 new cases diagnosed in 2009 and an average death toll of 1,500 per day.1 Although there have been significant advances in the fundamental understanding cancer biology over the past 25 years, there have been few successes in translation to the clinics.2 The shortfall of cancer chemotherapy stems from deleterious side effects arising as a result of non-specific toxicities on healthy tissues and the ability of cancer cells to rapidly acquire resistance.3,4
Over the past 10 years, nanotechnology has emerged as an exciting multidisciplinary field promising to provide breakthrough solutions to the problems of optimizing the efficacy or therapeutic index of anticancer agents.5,6 This is achieved by simultaneously diminishing the toxic side effects of therapeutics through the targeted delivery to tumor tissues7 and circumventing the issue of resistance by adopting a multimodal therapeutic regimen.8 The promise of nanotechnology lies in the ability to engineer customizable nanoscale constructs that can be loaded with one or more payloads such as chemotherapeutics,9 targeting units,10 imaging and diagnostic agents etc., (Fig. 1).11 Nanotechnology offers the unique advantage of packaging therapeutic molecules (drug,9,12 DNA,13 protein,14 peptide15 etc.,) into nanoparticles (Fig. 1), which improves their bioavailability, biocompatibility and safety profiles.5,6 While nanotechnology addresses important issues of drug-delivery, there is still a pressing need to overcome the problems of drug-resistance associated with the conventional chemotherapeutics. The development of rational therapeutic approaches for cancer requires the identification of new molecular targets, which in turn requires an understanding of the different cellular signaling pathways that lead to development and progression of cancer and play critical roles in the poor prognosis and drug resistance. This review will cover two aspects of cancer therapy. First, the signal transduction pathways implicated in cancer and the rationale for defining them as drug targets and second, nanotechnology as a convenient platform to combine these pathway inhibitors with conventional
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Figure 1. Schematic representation of a nanoparticle encapsulating multiple payloads (drugs, imaging unit) and surface modified with polyethylene glycol (PEG) and targeting moieties.
chemotherapy and new targeting agents to significantly impact cancer morbidity and mortality in the next decade. Oncogenic Signal Transduction Pathways In multicellular organisms biological signal transduction pathways regulate many important functions including survival, growth, differentiation and metabolism. Cell growth and differentiation controlling signal transduction pathways are almost invariably altered in almost all types of human cancers. Basic cancer research has unveiled the interconnections between different oncogenic signaling labyrinths, but understanding the alterations that lead to cancer and repairing them remains elusive. In last three decades, extensive success in different disciplines of chemistry and biology have led from the discovery of first “oncogene” to the development of new generation of small molecule signaling pathways inhibitors. We discuss below several signal transduction pathways, which regulate key cellular processes, and are additionally implicated in tumorigenesis.
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Receptor Tyrosine Kinase (RTKs) The receptor tyrosine kinases (RTKs) play key roles in the signaling pathways governing fundamental cellular processes like proliferation, migration, metabolism, differentiation, survival and intercellular communications. Upon binding of growth factors, RTKs typically dimerize, resulting in activation of their intracellular kinase domains, resulting in triggering the downstream signaling cascade.16 Although RTK activity in normal healthy cells is tightly regulated, abnormal activation of RTKs (e.g., EGFR, FGFR, VEGFR and PDGFR) in transformed cells is involved in the development and progression of many human cancers.17 Hence, RTKs and their growth-factor ligands have become rational targets for therapeutic intervention using humanized antibodies and small molecule inhibitors to target the intracellular kinase domains of the RTKs.18 For example, trastuzumab (Herceptin®, Genentech Inc.,), an anti-HER2 monoclonal antibody was approved by United States Food and Drug Administration (FDA) for the treatment of HER2-overxpressing metastatic breast cancer.19,20 Similarly, imatinib (Gleevec®,
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Novartis), a small molecule tyrosine kinase inhibitor of KIT and PDGFR was approved by FDA for treatment of chronic myelogenous leukaemia (CML).21 RTKs have also been implicated in tumor angiogenesis. For example, vascular endothelial growth factor receptors (VEGFR), a RTK and its ligand VEGF have important implications in the regulation of tumor angiogenesis.22,23 Bevacizumab (Avastin, Genentech) is a humanized antibody against VEGF24 that has recently been approved by FDA for the treatment of colorectal cancer in the USA. Moreover, small molecule VEGF antagonist SU5416 (Sugan/Pfizer) and SU6668 entered clinical trials recently. These small molecules competitively block ATP binding to the tyrosine kinase domain of the receptor, thereby inhibiting angiogenesis in vitro and in vivo in different human cancer models.25,26 Multi-Pronged Targeting of RTK Despite the advances in inhibiting the oncogenic kinases, it is increasingly becoming evident that inhibiting a single target will not be enough for optimal
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Figure 2. Oncogenic RTKs, Ras, PI3K, mTOR-signaling labyrinth and inhibitors targeting different facets of the network.
anti-tumor effect due to the development of resistance by activation of other RTKs driven pathways. It is rational to expect agents that target multiple different kinases would have better chance of success than a highly selective kinase inhibitor. Sunitinib (Sutent; SU11248) and sorafenib (Nexavar; BAY 43-9006) are two small molecule inhibitors, which target VEGFR, PDGFR and c-Kit simultaneously and thereby improve the efficacy as next generation targeted cancer chemotherapy. An alternative strategy to exert a global blockage is to target downstream signals. For example, two major intracellular signaling cascades that are activated by receptor tyrosine kinase (RTKs) are the Ras-mitogen activated protein kinase (MAPK) and phosphatylinositol-3-kinase PI3K-AKT-mTOR (mammalian target of rapamycin) pathways (Fig. 2). These signaling pathways constitute an intersecting biochemical network (Fig. 2) and mutations in these signaling pathways drive environmental cue independent unrestricted cell growth. These pathways have been implicated in tumorigenesis, by
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phosphorylation of the proteins directly implicated in protein synthesis, cell cycle progression and metabolism as well as transcription factors involved in gene expression in these processes.27-29 Mitogen Activated Protein Kinase (MAPK) Signaling A vast array of growth factors and cytokines activate the small G protein RAS, which leads to the activation of serine/ threonine kinase RAF followed by the activation of mitogen-activated protein kinase (MAPK) kinase (MEK). MEK then phosphorylates and triggers extracellular signal-regulated kinase (ERK).17 The MAPK pathway comprising of RAS, RAF, MEK and ERK has been implicated in majority of human tumors.30,31 It is now well known that Ras mutations occur in 30% of all cancers, particularly implicated in 90% of pancreatic cancer32 and 50% of colon cancers.33 Raf-1, a serine/threonine kinase was identified as the direct effector of Ras in mammalian cells.26 Activating mutations in one of the Raf isoform, B-raf was found in 60% of human malignant
melanomas and in some colon, thyroid and lung cancer.34 Hence, MAPK signaling has become one of the most well known targets for therapeutic intervention. RAS, RAF and MEK are three proteins that received most attention as targets for pharmacological intervention of the MAPK pathway. RAS, having no transmembrane domain, anchors with plasma membrane through farnesylation post-translational modification. Localization of RAS to the plasma membrane is essential for the biological activity of RAS. Extensive effort has been expended to inhibit RAS activity with selective farnesyltransferase inhibitors (FTIs). Preclinical assessment of several small molecule FTIs, covering chemically diverse structural classes, demonstrated anti-proliferative, pro-apoptotic and anti-angiogenic activities. BMS214662, L-778123, SCH-66336 (Sarasar), R11577 (Zarnestra) and AZD3409 are some examples of FTIs that are in preclinical studies.35 The RAF family consists of three highly regulated ARAF, BRAF and RAF1 proteins involving both positive and negative
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regulatory interactions with a large array of signaling molecules. It was long been speculated that inhibition of RAF might lead to a broader spectrum of antitumor activity in therapeutic community. Although several small molecule RAF inhibitors were reported, only BAY439006 (sorafenib) has reached clinical testing until now.36 MEK play a central role in MAPK signaling and implicated in broad range of human tumors.37 Two MEK homologues, MEK1 and MEK2 are ubiquitously expressed in mammals and subsequently phosphorylate ERK1 and ERK2. The development of pharmacological MEK inhibitor was initiated with the discovery of PD98059. U0126, a more potent MEK inhibitor than PD98059 has also become a widely used small molecule inhibitor for investigating the role of MEK in cellular events.38 The benzimidazole ARRY-142886 has also been reported to be a highly potent MEK inhibitor, with an IC50 = 10 nm for a range of human tumor cells.39 It is now evident that targeting RAS-MAPK signaling might lead to the development of improved cancer therapeutics. PI3 kinase pathway regulates various cellular processes, such as proliferation, growth, survival and apoptosis. Mutations in PI3K signaling pathways components are implicated in 30% of all human tumors, which makes PI3K signaling pathways as one of the most frequently targeted pathways in all sporadic human tumors in last 10 years.27 After activation by RTKs and Ras, PI3K sets off several downstream signaling cascades through the generation of the lipid second messenger phosphatidyl-inositol-3,4,5-triphosphate, particularly the AKT family of serine/threonine kinases. AKT regulates cell survival, cell cycle progression, cell growth and metabolism through the phosphorylation of a diverse set of substrates. Moreover, PI3K signaling has been implicated in tumor angiogenesis downstream of growth factors such as VEGF and HGF.40 Hence, inhibition of PI3K holds the promise of a multi-pronged strategy for tumor inhibition. Inhibitors of PI3 kinase such as LY294002 and wortmannin have been shown to sensitize cancer cells to conventional chemotherapeutics such as taxanes
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in several types of cancers such as ovarian, esophageal, lung etc., both in vitro and in vivo41 although have not been developed for clinical application. The reason for this is not clear, but might reflect unfavorable pharmacokinetics for wortmannin, which has a short half-life and potential toxicities. The mammalian target of rapamycin (mTOR) is serine/threonine kinase downstream of PI3K/Akt pathway (Fig. 2). mTOR complex 1 (mTORC1) plays a cardinal role in integrating signals from growth factors and nutrients to control protein synthesis, cell cycle progression and metabolism.42 mTOR has emerged as one of the critical effectors in cell signaling pathways deregulated in human cancers,43,44 hence become an important target for cancer therapy. Rapamycin binds specifically to the immunophilin FKBP12 and blocks mTORC1 signaling. Rapamycin and its analogues CCI-779 (temsirolimus), RAD001 (everolimus) and AP23573 (deforolimus) are currently undergoing clinical trials as monotherapies or in combination with standard chemotherapies in lymphoma, hepatocellular carcinoma etc., CCI-779 was approved by FDA for renal cell carcinoma in 2007. Because signaling of multiple receptor tyrosine kinases (RTKs) is propagated through Akt, simultaneous inhibition of RTKs such as the erbB family members with pathway components such as Akt or mTOR may circumvent feedback activation seen with either approach alone. Another possible approach is to combine inhibition of the PI3K/Akt/mTOR with inhibition of a parallel pro-survival signaling pathway such as MEK/ERK pathway, which abrogates compensatory activation of pro-survival pathways when PI3K/Akt/ mTOR is inhibited.45 It is now evident that tumor development and progression are initiated and stimulated by a myriad of different aberrant signaling pathways. There is a good reason to believe that many of the signaling pathways and key players involved in tumorigenesis may offer suitable targets for the development of anti-cancer therapies. The key to such success is believed to be addressed by the concept of “targeted therapy”-that is, the development of drugs that perturb (activate or inhibit) the action
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of a specific or multiple signaling pathways. Despite an incredible potential of small molecule signaling pathway inhibitors in targeted molecular cancer therapeutics, the “Achilles’ Heel” is the drug related toxicity, which causes the attrition for a blockbuster drug. One of the emerging strategies for targeted chemotherapeutics is to harness nanotechnology based platforms for preferential delivery of small molecule signaling inhibitors or drugs to the tumor. Nanotechnology in Cancer Nanotechnology is a paradigm-changing opportunity with potential to make unprecedented breakthrough in disease diagnosis and therapy. Nanoparticles, usually in the size range of 60–200 nm, are capable of carrying multiple payloads for targeted transport, immune evasion and promote favorable drug release kinetics at the target site (Fig. 1). Because the size of nanoparticles is similar to that of naturally occurring components of cells, including surface receptors, cellular organelles, proteins, etc., they can easily interfere with biological molecules, hence have significant impact on the biological fate of the cargo they deliver. Nanotechnology to Improve Bioavailability and Pharmacokinetics of Drugs Nanotechnology drug delivery systems have been reported in literature to offer useful strategies to overcome some of the problems associated with drug therapy like insufficient drug concentration in the blood due to poor absorption, rapid metabolism, elimination, poor solubility and bioavailability. It has been mentioned that one in ten marketed drugs has solubility problems and close to a third fail to reach profitability due to poor bioavailability and pharmacokinetics.46 Nanoparticles, due to their small size, exhibit very high surface area to volume ratio and as a result their dissolution rate is increased according to Noyes Whitney and Kevin equation.46,47 Hence using the platform of nanotechnology, drug products can be reformulated to increase solubility and bioavailability leading to
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decreased drug dose, cost and side effects. Nanotechnology has been used to increase solubility of potent but poorly soluble drugs such as paclitaxel,48 cyclosporine49 and amphotericin B.50 The platform of nanotechnology has often been used to achieve improved tissue selectivity by exploiting the physical and chemical properties of nanomaterials such as size, charge and surface chemistry. These parameters can be custom-tailored in a nanoparticle to our advantage, to get a better control over the important pharmacokinetic parameters including maximum concentration, half-life, clearance and mean resident time of the particle in the body.51 For example polyethylene glycol has been tagged on nanoparticles to improve circulation time by stabilizing and protecting micelles and liposomes from opsonization.52 By optimizing the drug formulation it is possible to achieve improved drug delivery to the target tissue thereby increasing the therapeutic activity with minimal side effect. Nanotechnology often opens new delivery options for conventional therapeutics. Using the platform of nanotechnology, it has been possible to achieve oral administration of water insoluble drugs such as rapamycin53 or doxorubicin54 whose oral bioavailability is poor; oral chemotherapy offering several advantages over intravenous administration, including better patient compliance and cost reduction. A significant application of nanotechnology has been to device vehicles to cross the blood brain barrier (BBB) for the delivery of therapeutics and diagnostics.55 Various compounds including chemotherapeutic drug doxorubicin have been attached to the surface of poly(butylcyanoacrylate) nanoparticles coated with polysorbate 80,56 and doxorubicin-nanoparticles formulations demonstrated significant remission in a rat glioblastoma model with minimal toxicity setting the stage for potential clinical trials.57 Also in a very recent report, gold nanorod-siRNA nanoplex was utilized to target the dopaminergic signaling pathway in the brain.58 The advent of nanotechnology has reignited interest in the lungs as a potential, non-invasive route of drug delivery
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for both systemic and local treatments. For example nanoparticle agglomerates of paclitaxel in combination with cisplatin was developed as effective chemotherapeutic dry powder aerosols to enable regional treatment of certain lung cancers, minimizing systemic side effects.59 Alternately, systemic nanoparticle-mediated drug delivery to the lung would enable avoidance of first-pass metabolism and reduction of drug degradation in the GI tract. PLGA nanoparticles prepared with insulin (dosed in guinea pig lung) was shown to cause significant reduction in blood glucose level with prolonged effect over 48 h (compared to insulin solution).60 Nanotechnology for Tumor Targeting Using the platform of nanotechnology, it has been possible to achieve passive and active targeted delivery of therapeutics to tumors. Passive targeting exploits the inherent size of nanoparticles (60–200 nm) and characteristic features of tumor biology that allows nanoparticles to accumulate in the tumor by the enhanced permeability and retention (EPR) effect.5 Whereas free drugs may diffuse non-specifically to normal healthy tissues, a nanocarrier can escape into tumor via the leaky blood vessels and accumulate in the tumor vicinity owing to dysfunctional lymphatic drainage in tumors. Different nanoparticle formulations of chemotherapeutics (of optimum size to make use of the EPR effect) have been devised to ameliorate the toxic side effects of the drugs to normal tissues. For example nanocomposites of doxorubicin with lipids,61 polymers,62 carbon nanoparticles63 etc., have been evaluated to reduce the drug’s cardiotoxic and myelosuppressive effects, thereby improving the therapeutic potential. The second modality that has been used for tumor specific delivery is active targeting, which involves conjugating tumor specific ligands to nanoparticles. Targeting units include small-molecule ligands such as folate,64 peptides (such as RGD) 65 and sequences identified by phage display, proteins (such as hormones),66 transferrin,67 antibodies and antibody fragments68 and oligosaccharides.69
Nanotechnology in Controlled Drug Release A major challenge faced by all current drug delivery techniques is the control of drug release rates according to the therapeutic needs of the patient. Nanotechnologybased ‘smart’ devices,70 which respond to external signals such as pH,71 temperature,72 light,73 electric74 or magnetic75 field or triggers drug release in a pre-determined mode, offer potential benefits of increased therapeutic efficacy and reduced side-effects. Hydrogels responding to specific environmental changes are being considered for remote controlled drug release. For example, temperature sensitive hydrogels incorporating gold nanoshells or iron oxide76 have been remotely heated by exposure to near IR light or application of alternating magnetic field respectively to release therapeutics. Recently, echogenic liposomes for drug and gene delivery have found application in cardiac diseases and tumor therapy for their several properties, including targeting, ultrasound-controlled drug release, enhanced cell and tissue permeability, and ability to be imaged by ultra-sound.77 Nanotechnology in Combination Therapy Because of their multifunctional capabilities, nanoscale devices can combine multiple therapeutic agents or therapeutic and diagnostic agents in the same package, thereby improving the therapeutic potential of the individual drugs by synergistic effect. For example polymer-based nanoshells with thick hydrophobic membrane and an aqueous lumen have been used to deliver an anticancer drug cocktail of hydrophobic paclitaxel and hydrophilic doxorubicin.78 In another report a nanocell containing a nuclear nanoparticle within an extranuclear pegylated lipid envelope was designed for the temporal release of two drugs, an antiangiogenesis agent, trapped in the outer envelope causing vascular shut down, followed by a chemotherapy agent in the inner nanoparticle.79 Alternately epidermal growth factor (EGF)-micelles has been pursued as versatile nanotechnology platforms for the targeted delivery of a wide range of
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having surface decorated with Herceptin were engineered to target ERBB2 receptors in human breast cancer SK-BR-3 cell lines. A significant reduction in Akt and MAPK activation was observed, followed by repressed expression of cyclin D1. Future Direction
Figure 3. Transmission electron micrographs of MAPK-targeting nanoparticles. The image on the left shows the uniformity of nanoparticle size of around 100 nm, which facilitates homing into the tumors through the EPR effect. The nanoparticles were engineered from a mixture of PD98059-hexadentate PLGA and a PEG-PLGA polymers. The image on the right shows a higher resolution image of the cross section of a single nanoparticle, where the PEG was end-derivatized with biotin. The nanoparticles were probed with 5 nm streptavidin-gold nanoparticles. The distribution of the gold nanoparticles on the surface of the polymeric nanoparticles confirmed the pegylation of the PD98059 nanoparticles, which can confer long circulating time by escaping from the RES.
chemotherapeutic agents as a combination therapy for the treatment of EGFRoverexpressing cancers.80 Nanotechnology in Targeting Signaling Pathways Although there are few reports on the application of nanotechnology in targeting signaling pathways, it is the next logical extension to combine signaling pathway targeting therapeutics with nanoparticlebased tumor targeting. Kinase is one of the most well studied oncogenic signaling pathways with approximately 30 distinct kinase targets being developed at the level of Phase I clinical trial.81 In last three decades success in cancer therapeutics is exemplified by discovering kinases inhibitors such as Sprycel (dasatinib), Tarceva (erlotinib) and Gleevec (imatinib) as high-profile drugs. However, only 5% of kinase signaling inhibitors entering clinical trials reach marketing approval,82 the major stumbling block being drug related toxicity, which causes the shelving of an otherwise potent drug. Harnessing nanotechnology based approaches to deliver signaling pathway inhibitors selectively to the tumor leaving healthy cells untouched could provide a solution for this unmet need and usher a new direction in targeted cancer therapeutics. In a very recent report by Basu et al. a nanoparticle-mediated targeting of MAPK
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signaling pathway (Fig. 3) was shown to enhance the antitumor efficacy of cisplatin chemotherapy. Specifically PD98059, a selective MAPK inhibitor, was chemically conjugated to an engineered hexadentatePLGA polymer and was shown to inhibit the proliferation of melanoma and lung cancer cells in vitro and sensitize tumor cells to cisplatin chemotherapy in melanoma tumor bearing mice.83 In another study, Harfouche et al. targeted PI3 kinase signaling by encapsulating LY294002, a potent PI3 kinase inhibitor into biodegradable PLGA nanoparticle, which successfully results in the inhibition of downstream Akt phosphorylation, leading to inhibition of proliferation and induction of apoptosis of B16/F10 melanoma cells in vitro. Moreover, the nanoparticleenabled targeting of the PI3-kinase pathways resulted inhibition of B16/F10 and MDA-MB-231 induced angiogenesis in a zebrafish tumor xenograft model.84 ERBB2, one of the members of the epidermal growth factor family (Erbb family), amplification and overexpression have been implicated in a number of human tumor including breast cancer, ovarian cancer, gastric carcinoma and salivary gland tumors. Herceptin (trastuzumab), a monoclonal antibody, which binds to ERBB2 and inhibits downstream signaling, was approved by FDA to treat breast and gastric cancer.85 Gold and silver nanoparticles in the 2–100 nm size range,
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Having the path laid by the above mentioned examples of targeting oncogenic signaling pathways exploiting nanotechnology based platforms, we are now in the dawn of a new era of targeted molecular cancer therapeutics. Multiple signaling pathways including different kinases, TGFβ, Wnt/β-catenin, Hedgehog (HH), Notch, NFκB and many more have critical role in malignant tumor progression and development. Despite the initial success of different signaling inhibitors as high profile drugs, a small fraction of them enter clinical trials and reach marketing approval. Solubility problems associated with many of the very potent inhibitors, coupled to non-specific toxicities, severely limit the clinical application of many signaling inhibitors. Different strategies can be envisaged for the covalent or non-covalent encapsulation of drugs into a variety of nanoparticles thereby conferring water solubility to the final formulation. Therapeutic molecules containing functional groups that are amenable to conjugation such as carboxyl, hydroxyl, thiol, amine, phosphate/phosphonate and carbonyl groups can be covalently coupled to a variety of organic and inorganic carriers. The resulting linkages typically produced via the modification of these groups include esters, carbonates, carbamates, dithiols, amides, phosphates and oximes.86 These groups are easily bioconverted by hydrolysis or enzymatic cleavage to release the active free drugs inside the cells. Also, controlled release of different drugs can be achieved by optimizing the mode of encapsulation or the linkages. For example, Sengupta et al. demonstrated the application of a nanocell comprising of a nuclear nanoparticle within a pegylated lipid envelope for the temporal release of two different drugs, one non-covalently encapsulated and the other covalently conjugated to the polymer backbone.79 This
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technology can be extended to additional agents, so as to target multiple signaling pathways. Owing to the multiple interconnected signaling pathways implicated in cancer development and progression, inhibition of one pathway can lead to feedback activation of others. Hence the solution often lies in combining multiple pathway inhibitors or an inhibitor with a conventional chemotherapeutic. For example investigations with different cancers suggest that mTOR blockade can paradoxically induce activation of prosurvival, protumorigenic signaling molecules, especially downstream Akt.87 Hence simultaneous inhibition of (PI3K)/Akt-mTOR with mTOR inhibitor rapamycin and LY294002, an upstream inhibitor of PI3K, often result in synergistic effects.88 Similarly synergism and reduced drug resistance can be achieved by concurrent inhibition of different pathways89 or combining pathway inhibitor with inhibitors of receptor tyrosine kinases90 or conventional chemotherapy.91 Such multiple therapeutic molecules can be introduced for example to the same polymeric backbone, or encapsulated into liposomes and formulated into nanoparticles for targeted (passive or active) delivery into cancer cells. A wide range of materials, including liposomes, micelles, polymeric nanoparticles, silicon and gold nanoshells, dendrimers and carbon-based nanomaterials can be suitably optimized to our advantage to function as nanovectors for the delivery of drugs. Basu et al. in a recent report functionalized PLGA polymer to generate hexa-carboxylic-PLGA in order to achieve increased drug loading.83 Alternately fullerene, a carbon nanoparticle, was shown to be surface-modified with hydroxyl and/ or carboxyl groups thereby offering dual modalities for the possible attachment of different drug molecules.63 Nanotechnology offers the unique platform that makes it possible to reduce toxic side-effects of drugs through targeted delivery, improve their bioavailability through reformulation, and combine multiple therapeutic agents in the same package to overcome resistance and achieve synergistic effect. Nanotechnology-based approaches to deliver the inhibitors selectively to the tumor could thus lead this
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drug discovery mission from “bench to bedside”. Acknowledgements
This work was supported by a Department of Defense BCRP Era of Hope Scholar award [W81xWH-07-1-0482] and a Mary Kay Ash Charitable Foundation grant to S.S. References 1. http://www.cancer.org (Statistics for 2009). 2. Heath JR, Davis ME. Nanotechnology and cancer. Annu Rev Med 2008; 59:251-65. 3. Chari RV. Targeted cancer therapy: conferring specificity to cytotoxic drugs. Acc Chem Res 2008; 41:98101. 4. Katsman A, Umezawa K, Bonavida B. Chemosensitization and immunosensitization of resistant cancer cells to apoptosis and inhibition of metastasis by the specific NFkappaB inhibitor DHMEQ. Curr Pharm Des 2009; 15:792-808. 5. Peer D, Karp JM, Hong S, Farokhzad OC, Margalit R, Langer R. Nanocarriers as an Emerging Platform for Cancer Therapy. Nature Nanotech 2007; 2:75160. 6. Ferrari M. Cancer Nanotechnology: opportunities and challenges. Nat Rev Cancer 2005; 5:161-71. 7. Gu F, Langer R, Farokhzad OC. Formulation/ preparation of functionalized nanoparticles for in vivo targeted drug delivery. Methods Mol Biol 2009; 544:589-98. 8. Lee AL, Wang Y, Cheng HY, Pervaiz S, Yang YY. The co-delivery of paclitaxel and Herceptin using cationic micellar nanoparticles. Biomaterials 2009; 30:91927. 9. Liu Z, Chen K, Davis C, Sherlock S, Cao Q, Chen X, et al. Drug delivery with carbon nanotube for in vivo cancer treatment. Cancer Res 2008; 68:6652-60. 10. Sapra P, Allen TM. Internalizing antibodies are necessary for improved therapeutic efficacy of antibody-targeted liposomal drugs. Cancer Res 2002; 62:7190-4. 11. Bailey VJ, Easwaran H, Zhang Y, Griffiths E, Belinsky SA, Herman JG, et al. MS-qFRET: A quantum dot-based method for analysis of DNA methylation. Genome Res 2009; Epub ahead of print. 12. Saravanakumar G, Min KH, Min DS, Kim AY, Lee CM, Cho YW, et al. Hydrotropic oligomerconjugated glycol chitosan as a carrier of paxclitaxel: Synthesis, characterization and in vivo biodistribution. J Controlled Release 2009; Epub ahead of print. 13. Jin S, Leach JC, Ye K. Nanoparticle-mediated gene delivery. Methods Mol. Biol 2009; 544:547-57. 14. Zhang S, Doschak MR, Uludaq H. Pharmacokinetics and bone formation by BMP-2 entrapped in polyethylenimine-coated albumin nanoparticle. Biomaterials 2009; Epub ahead of print. 15. Chittasupho C, Xie SX, Baoum A, Yakovleva T, Siahaan TJ, Berkland CJ. ICAM-1 targeting of doxorubicin-loaded PLGA nanoparticles to lung epithelial cells. Eur J Pharm 2009; 37:141-50. 16. Schlessinger J. Cell signaling by receptor tyrosine kinase. Cell 2000; 103:211-25. 17. Gschwind A, Fischer OM, Ullrich A. The discovery of receptor tyrosine kinase: targets for cancer therapy. Nature Rev Cancer 2004; 4:361-70. 18. Sebolt-Leopold JS, English JM. Mechanism of drug inhibition of signaling molecules. Nature 2006; 441:457-62.
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