Nanoparticle Delivery to Cancer Drug Targets
Curator: Larry H. Bernstein, MD, FCAP
Significance
Nanotechnology is a promising approach for improving cancer diagnosis and treatment with reduced side effects. A key question that has emerged is: What is the ideal nanoparticle size, shape, or surface chemistry for targeting tumors? Here, we show that tumor pathophysiology and volume can significantly impact nanoparticle targeting. This finding presents a paradigm shift in nanomedicine away from identifying and using a universal nanoparticle design for cancer detection and treatment. Rather, our results suggest that future clinicians will be capable of tailoring nanoparticle designs according to the patient’s tumor characteristics. This concept of “personalized nanomedicine” was tested for detection of prostate tumors and was successfully demonstrated to improve nanoparticle targeting by over 50%.
Abstract
Nanoparticles can provide significant improvements in the diagnosis and treatment of cancer. How nanoparticle size, shape, and surface chemistry can affect their accumulation, retention, and penetration in tumors remains heavily investigated, because such findings provide guiding principles for engineering optimal nanosystems for tumor targeting. Currently, the experimental focus has been on particle design and not the biological system. Here, we varied tumor volume to determine whether cancer pathophysiology can influence tumor accumulation and penetration of different sized nanoparticles. Monte Carlo simulations were also used to model the process of nanoparticle accumulation. We discovered that changes in pathophysiology associated with tumor volume can selectively change tumor uptake of nanoparticles of varying size. We further determine that nanoparticle retention within tumors depends on the frequency of interaction of particles with the perivascular extracellular matrix for smaller nanoparticles, whereas transport of larger nanomaterials is dominated by Brownian motion. These results reveal that nanoparticles can potentially be personalized according to a patient’s disease state to achieve optimal diagnostic and therapeutic outcomes.
- PMID: 23621529
Nanoparticle-based targeted drug delivery
Rajesh Singh1 and James W. Lillard Jr.1
Exp Mol Pathol. 2009 June ; 86(3): 215–223. http://dx.doi.org:/10.1016/j.yexmp.2008.12.004
Nanotechnology could be defined as the technology that has allowed for the control, manipulation, study, and manufacture of structures and devices in the “nanometer” size range. These nano-sized objects, e.g., “nanoparticles”, take on novel properties and functions that differ markedly from those seen from items made of identical materials. The small size, customized surface, improved solubility, and multi-functionality of nanoparticles will continue to open many doors and create new biomedical applications. Indeed, the novel properties of nanoparticles offer the ability to interact with complex cellular functions in new ways. This rapidly growing field requires crossdisciplinary research and provides opportunities to design and develop multifunctional devices that can target, diagnose, and treat devastating diseases such as cancer. This article presents an overview of nanotechnology for the biologist and discusses the attributes of our novel XPclad© nanoparticle formulation that has shown efficacy in treating solid tumors, for single dose vaccination, and oral delivery of therapeutic proteins.
The development of a wide spectrum of nanoscale technologies is beginning to change the scientific landscape in terms of disease diagnosis, treatment, and prevention. These technological innovations, referred to as nanomedicines by the National Institutes of Health, have the potential to turn molecular discoveries arising from genomics and proteomics into widespread benefit for patients. Nanoparticles can mimic or alter biological processes (e.g., infection, tissue engineering, de novo synthesis, etc.). These devices include, but are not limited to, functionalized carbon nanotubes, nanomachines (e.g., constructed from interchangeable DNA parts and DNA scaffolds), nanofibers, self-assembling polymeric nanoconstructs, nanomembranes, and nano-sized silicon chips for drug, protein, nucleic acid, or peptide delivery and release, and biosensors and laboratory diagnostics.
Nanotechnology-based Drug Delivery in Cancer
Drug delivery in cancer is important for optimizing the effect of drugs and reducing toxic side effects. Several nanotechnologies, mostly based on nanoparticles, can facilitate drug delivery to tumors.
Hydrogels
Hydrogel-nanoparticles are based on proprietary technology that uses hydrophobic polysaccharides for encapsulation and delivery of drug, therapeutic protein, or vaccine antigen. A novel system using cholesterol pullulan shows great promise. In this regard, four cholesterol molecules gather to form a self-aggregating hydrophobic core with pullulan outside. The resulting cholesterol nanoparticles stabilize entrapped proteins by forming this hybrid complex. These particles stimulate the immune system and are readily taken up by dendritic cells. Alternatively, larger hydrogels can encapsulate and release monoclonal antibodies.
Curcumin, a substance found in the cooking spice turmeric, has long been known to have anti-cancer properties. Nevertheless, widespread clinical application of this relatively efficacious agent has been limited due to its poor solubility and minimal systemic bioavailability. This problem has been resolved by encapsulating curcumin in a polymeric nanoparticle, creating “nanocurcumin” (Bisht et al., 2007). Further, the mechanism of action of nanocurcumin on pancreatic cancer cells mirrors that of free curcumin, including induction of apoptosis, blockade of nuclear factor kappa B (NFκB) activation, and downregulation of pro-inflammatory cytokines (i.e., IL-6, IL-8 and TNF-α). Nanocurcumin provides an opportunity to expand the clinical repertoire of this efficacious agent by enabling soluble dispersion. Future studies utilizing nanocurcumin are warranted in preclinical in vivo models of cancer and other diseases that might benefit from the effects of curcumin.
Micelles and liposomes
Block-copolymer micelles are spherical super-molecular assemblies of amphiphilic copolymer. The core of micelles can accommodate hydrophobic drugs, and the shell is a hydrophilic brush-like corona that makes the micelle water soluble, thereby allowing delivery of the poorly soluble contents. Camptothecin (CPT) is a topoisomerase I inhibitor that is effective against cancer, but clinical application of CPT is limited by its poor solubility, instability, and toxicity. Biocompatible, targeted sterically stabilized micelles (SSM) have been used as nanocarriers for CPT (CPT-SSM). CPT solubilization in SSM is expensive yet reproducible and is attributed to avoidance of drug aggregate formation. Furthermore, SSM composed of PEGylated phospholipids are attractive nanocarriers for CPT delivery because of their size (14 nm) and ability to extravasate through the leaky microvasculature of tumors and inflamed tissues. This passive targeting results in high drug concentration in tumors and reduced drug toxicity to the normal tissues (Koo et al., 2006).
Stealth micelle formulations have stabilizing PEG coronas to minimize opsonization of the micelles and maximize serum half-life. Currently, SP1049C, NK911, and Genexol-PM have been approved for clinical use (Sutton et al., 2007). SP1049C is formulated as doxorubicin (DOX)-encapsulated pluronic micelles. NK911 is DOX-encapsulated micelles from a copolymer of PEG-DOX-conjugated poly(aspartic acid), and Genexol-PM is a paclitaxelencapsulated PEG-PLA micelle formulation. Polymer micelles have several advantages over other drug delivery systems, including increased drug solubility, prolonged circulation halflife, selective accumulation at tumor sites, and lower toxicity. However, at the present time this technology lacks tumor specificity and the ability to control the release of the entrapped agents. Indeed, the focus of nano-therapy has gradually shifted from passive targeting systems (e.g., micelles) to active targeting.
Super paramagnetic iron oxide particles can be used in conjunction with magnetic resonance imaging (MRI) to localize the tumor as well as for subsequent thermal ablation. This has been used, for example, to target glioblastoma multiforme (GBM), a primary malignant tumor of the brain with few effective therapeutic options. The primary difficulty in treating GBM lies in the difficulty of delivering drugs across the BBB. However, nanoscale liposomal iron oxide preparations were recently shown to improve passage across the BBB (Jain, 2007).
Nanomaterial formulation
Nanomaterials have been successfully manipulated to create a new drug-delivery system that can solve the problem of poor water solubility of most promising currently available anticancer drugs and, thereby, increase their effectiveness. The poorly soluble anticancer drugs require the addition of solvents in order for them to be easily absorbed into cancer cells. Unfortunately, these solvents not only dilute the potency of the drugs but create toxicity. Researchers from the University of California Los Angeles California Nanosystem Institute have devised a novel approach using silica-based nanoparticles to deliver the anticancer drug CPT and other water insoluble drugs to cancer cells (Lu et al., 2007). The method incorporates the hydrophobic anticancer drug CPT into the pores of fluorescent mesoporous silica nanoparticles and delivers the particles into a variety of human cancer cells to induce cell death. The results suggest that the mesoporous silica nanoparticles might be used as a vehicle to overcome the insolubility of many anticancer drugs.
Nanosystems
Novel nanosystems can be pre-programmed to alter their structure and properties during the drug delivery process, allowing for more effective extra- and intra-cellular delivery of encapsulated drug (Wagner, 2007). This is achieved by the incorporation of molecular sensors that respond to physical or biological stimuli, including changes in pH, redox potential, or enzymes. Tumor-targeting principles include systemic passive targeting and active receptor targeting. Physical forces (e.g., electric or magnetic fields, ultrasound, hyperthermia, or light) may contribute to focusing and triggering activation of nano systems. Biological drugs delivered with programmed nanosystems also include plasmid DNA, siRNA, and other therapeutic nucleic acids.
Using a degradable, polyamine ester polymer, polybutanediol diacrylate co amino pentanol (C32), a diptheria toxin suicide gene (DT-A) driven by a prostate-specific promoter was directly injected into normal prostate and prostate tumors in mice (Peng et al., 2007). This C32/DT-A system resulted in significant size reduction, apoptosis in 50% of normal prostate. However, a single injection of C32/DT-A triggered apoptosis in 80% of tumor cells present in the tissue. It is expected that multiple nanoparticle injection would trigger a great percentage of prostate tumor cells to undergo apoptosis. These results suggest that local delivery of polymer/DT-A nanoparticles may have application in the treatment of benign prostatic hypertrophy and prostate cancer.
Multidrug resistance (MDR) of tumor cells is known to develop through a variety of molecular mechanisms. Glucosylceramide synthase (GCS) is responsible for the activation of the pro-apoptotic mediator, ceramide, to a nonfunctional moiety, glucosylceramide. This molecule is over-expressed by many MDR tumor types and has been implicated in cell survival in the presence of chemotherapy. A study has investigated the therapeutic strategy of co-administering ceramide with paclitaxel in an attempt to restore apoptotic signaling and overcome MDR in a human ovarian cancer cell line using modified poly(epsiloncaprolactone) (PEO-PCL) nanoparticles to encapsulate and deliver the therapeutic agents for enhanced efficacy (van Vlerken and Amiji, 2006). Results show that MDR cancer cells can be completely eradicated by this approach. Using this approach, MDR cells can be resensitized to a dose of paclitaxel near the IC50 of non-MDR cells. Molecular analysis of activity verified the hypothesis that the efficacy of this therapeutic approach is due to a restoration in apoptotic signaling, showing the promising potential for clinical use of this therapeutic strategy to overcome MDR.
Nanocells
Indiscriminate drug distribution and severe toxicity of systemic administration of chemotherapeutic agents can be overcome through encapsulation and cancer cell targeting of chemotherapeutics in 400 nm nanocells, which can be packaged with significant concentrations of chemotherapeutics of different charge, hydrophobicity, and solubility (MacDiarmid et al., 2007). Targeting of nanocells via bispecific antibodies to receptors on cancer cell membranes results in endocytosis, intracellular degradation, and drug release. Doses of drugs delivered via nanocells are ∼1,000 times less than the dose of the free drug required for equivalent tumor regression. It produces significant tumor growth inhibition and regression in mouse xenografts and lymphoma in dogs, despite administration of minute amounts of drug and antibody. Indeed, reduced dosage is a critical factor for limiting systemic toxicity. Clinical trials are planned for testing this method of drug delivery.
Dendrimers
In early studies, dendrimer-based drug delivery systems focused on encapsulating drugs. However, it was difficult to control the release of drugs associated with dendrimers. Recent developments in polymer and dendrimer chemistry have provided a new class of molecules called dendronized polymers, which are linear polymers that bear dendrons at each repeat unit. Their behavior differs from that of linear polymers and provides drug delivery advantages because of their enhanced circulation time. Another approach is to synthesize or conjugate the drug to the dendrimers so that incorporating a degradable link can be further used to control the release of the drug.
DOX was conjugated to a biodegradable dendrimer with optimized blood circulation time through the careful design of size and molecular architecture (Lee et al., 2006). Specifically, the DOX-dendrimer controlled drug-loading through multiple attachment sites, solubility through PEGylation, and drug release through the use of pH-sensitive hydrazone dendrimer linkages. In culture, DOX-dendrimers were >10 times less toxic than free DOX toward colon carcinoma cells. Upon intravenous administration to tumor bearing mice, tumor uptake of DOX-dendrimers were nine-fold higher than intravenous free DOX and caused complete tumor regression and 100% survival of the mice after 60 days.
Nanotubes Even though it was previously possible to attach drug molecules directly to antibodies, attaching more than a handful of drug molecules to an antibody significantly limits its targeting ability because the chemical bonds that are used tend to impede antibody activity. A number of nanoparticles have been investigated to overcome this limitation. Tumor targeting single-walled carbon nano-tube (SWCNT) have been synthesized by covalently attaching multiple copies of tumor-specific monoclonal antibodies (MAbs), radiation ion chelates and fluorescent probes (McDevitt et al., 2007). A new class of anticancer compound was created that contains both tumor-targeting antibodies and nanoparticles called fullerenes (C60). This delivery system can be loaded with several molecules of an anticancer drug, e.g., Taxol® (Ashcroft et al., 2006). It is possible to load as many as 40 fullerenes onto a single skin cancer antibody called ZME-108, which can be used to deliver drugs directly into melanomas. Certain binding sites on the antibody are hydrophobic (water repelling) and attract the hydrophobic fullerenes in large numbers so multiple drugs can be loaded into a single antibody in a spontaneous manner. No covalent bonds are required, so the increased payload does not significantly change the targeting ability of the antibody. The real advantage of fullerene-based therapies vs. other targeted therapeutic agents is likely to be fullerene’s potential to carry multiple drug payloads, such as taxol plus other chemotherapeutic drugs. Cancer cells can become drug resistant, and one can cut down on the possibility of their escaping treatment by attacking them with more than one kind of drug at a time. The first fullerene immuno-conjugates have been prepared and characterized as an initial step toward the development of fullerene immunotherapy.
Polymersomes
Polymersomes, hollow shell nanoparticles, have unique properties that allow delivery of distinct drugs. Loading, delivery and cytosolic uptake of drug mixtures from degradable polymersomes were shown to exploit the thick membrane of these block copolymer vesicles, their aqueous lumen, and pH-triggered release within endolysosomes. Polymersomes break down in the acidic environments for targeted release of these drugs within tumor cell endosomes. While cell membranes and liposomes are created from a double layer of phospholipids, a polymersome is comprised of two layers of synthetic polymers. The individual polymers are considerably larger than individual phospholipids but have many of the same chemical features.
Polymersomes have been used to encapsulate paclitaxel and DOX for passive delivery to tumor-bearing mice (Ahmed et al., 2006). The large polymers making up the polymersome allows paclitaxel, which is water insoluble, to embed within the shell. DOX is water-soluble and stays within the interior of the polymersome until it degrades. The polymersome and drug combination spontaneously self-assembles when mixed together. Recently, studies have shown that cocktails of paclitaxel and DOX lead to better tumor regression that either drug alone, but previously there was no carrier system that could carry both drugs as efficiently to a tumor. Hence, this approach shows great promise.
Quantum dots
Single-particle quantum dots conjugated to tumor-targeting anti-human epidermal growth factor receptor 2 (HER2) MAb have been used to locate tumors using high-speed confocal microscopy (Tada et al., 2007). Following injection of quantum dot-MAb conjugate, six distinct stop-and-go steps were identified in the process as the particles traveled from the injection site to the tumor where they bound HER2. These blood-borne conjugates extravasated into the tumor, bound HER2 on cell membranes, entered the tumor cells and migrated to the perinuclear region. The image analysis of the delivery processes of single particles in vivo provided valuable information on MAb-conjugated therapeutic particles, which will be useful in increasing their anticancer therapeutic efficacy. However, the therapeutic utility of quantum dots remains undetermined.
XPclad® nanoparticles
The poor aqueous solubility of many drug candidates presents a significant problem in drug delivery and related requirements such as bioavailability and absorption. Recently, our laboratory has developed XPclad® nanoparticles that represent a novel formulation method that uses planetary ball milling to generate particles of uniform size (Figure 1), 100% loading efficiency of hydrophobic or hydrophilic drugs, subsequent coating for targeted delivery, and control of LogP for systemic, cutaneous, or oral administration of cancer drugs, vaccines, or therapeutic proteins (Figure 2).
The method for making XPclad® nanoparticles uses a novel and relatively inexpensive preparation technique (i.e., planetary ball milling), which allows for controlling the size of the particles (100 nm to 50 μm; ± 10% of mean size) with >99% loading efficiency, polymer- or ligand-coating for controlled-, protected-, and targeted-release and delivery of their contents. The nanoparticles produced thereby contain the desired biologically active agent(s) in a biopolymer excipient such as alginate, cellulose, starch or collagen and biologically active agents. Generally, there are two types of mills that have been employed for making particles: vibratory or planetary ball mills. The vibratory ball milling grinds powders by high velocity impact while planetary ball milling employs a grinding motion. Typically, planetary ball milling has been used only to generate micron-sized particles, while vibratory milling can yield nano-particles. However, the high impact resulting from the vibratory milling technique makes incorporating biologicals difficult. Planetary ball mills pulverize and mix materials ranging from soft and medium to extremely hard, brittle and fibrous materials. Both wet and dry grinding can be carried out. Minerals, ores, alloys, chemicals, glass, ceramics, plant materials, soil samples, sewage sludge, household and industrial waste and many other substances can be reduced in size simply, quickly and without loss. Planetary ball mills have been successfully used in many industrial and research sectors, particularly wherever there is high demand for purity, speed, fineness and reproducibility. The planetary ball mills produce extremely high centrifugal forces with very high pulverization energies and short grinding times. Because of the extreme forces exerted, the use of vibratory and planetary ball mills to formulate therapeutics has not been practiced until now. In general, XPclad® particle size can be engineered to range from 5 to 30 nm up to 10 to 60 μm by controlling the size and number of planetary balls, grinding speed, milling cycles, and centrifugal force by varying the revolutions per second and planetary jar velocity.
Nano delivery systems hold great potential to overcome some of the obstacles to efficiently target a number of diverse cell types. This represents an exciting possibility to overcome problems of drug resistance in target cells and to facilitate the movement of drugs across barriers (e.g., BBB). The challenge, however, remains the precise characterization of molecular targets and ensuring that these molecules only affect targeted organs. Furthermore, it is important to understand the fate of the drugs once delivered to the nucleus and other sensitive cells organelles.
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