Cancer is a complex and heterogeneous disease and continues to be a major health problem worldwide. Nanotechnology - the science and engineering of manipulating matter at the dimensions comparable to the size of biological molecules is finding considerable applications in medicine and carries the potential of improving clinical outcomes. With cancer, nanotechnology offers opportunities for the development of therapies and devices to reduce toxicity, enhance the efficacy and delivery of treatments, and also increase sensitivity in cancer detection. This overview describes recent advances in the field, shows prospects for its future and proposes how engineers and physical scientists can contribute to its further development.
Cancer is arguably the most complex disease known to man and one of the most pressing public health concerns of the 21st century. The statistics are daunting, according to most recent World Health Organization report (1), number of new cancer cases in 2012 stood at 14 million and it is expected to reach 22 million within two decades. Over the same period, cancer deaths are predicted to rise from 8.2 million a year to 13 million. Despite significant efforts, cancer treatment strategy has remained mostly unchanged over the past 30 years – surgical resection of the tumor, followed by cytotoxic chemotherapy and/or radiation (2). There have been improvements to chemotherapies, but appreciable delivery of many drugs to the tumor site is difficult; these drugs are also often and associated with high systemic toxicities and poor pharmacokinetics. Early diagnosis of many malignancies is difficult and in many case achieved only at late, metastatic stages of development, reducing the overall effectiveness of treatment.
In recent years, the field of nanotechnology has emerged as an approach with the potential to produce novel diagnostics and therapeutics (3-8). In its application to cancer, the advantages of nanotechnology are numerous and include enhanced delivery of anticancer agents to tumor tissues, reduced toxic side effects, as well as entirely new modalities of cancer therapy, such as photodynamic and hyperthermia treatments. The research in this area is very active, with handful of drugs already approved and several other nano-formulations participating in Phase I and II clinical trials, suggesting that the field of cancer nanotechnology and its translation are moving forward (8). Similarly, in vitro nanotechnology-based devices and systems are becoming instrumental to early and precise diagnosis of the disease (9-11). They are capable of recognizing disease-specific biomarkers with high sensitivity and specificity and can achieve that for several markers at a time to analyze large panels of genomic or proteomic signatures.
The development of safe and reliable nanomaterials is at the foundation of emerging nanomedicine-based therapies and imaging techniques. A variety of different nanoparticle platforms have been explored as potential delivery vehicles: polymers, liposomes, micelles, emulsions, metal, metal oxide, dendrimers, fullerenes, quantum dots, and carbon nanotubes (5, 12). Not only have the types of nanomaterials in use broadened widely, but researchers have discerned how to functionalize nanoparticles (Figure 1), characterize complex multifunctional conjugates, and understand effects on their biodistribution and toxicity. In vitro and in vivo analysis of these constructs has allowed determination of important trends in nanoparticles interactions with living organisms. Nanoparticle size, surface charge, and hydrophobicity are key factors influencing biocompatibility and biodistribution. For example, a nanoparticle less than about 8 nm will be excreted through the kidneys, and nanoparticles greater than about 200 nm will be taken up by the organs of the mononuclear phagocyte system (MPS), e.g. liver, and spleen (12).
Nanoparticles can be constructed into versatile and complex designs, which facilitate delivery of more than one drug molecule at a time and produce combination therapies, perform multi-modality imaging, or operate in theranostics space through the design of particles simultaneously providing diagnostic and therapeutic capabilities (13). This multi-functionality also demonstrates itself in new and more sophisticated nanosystem designs that are responsive to changes in pH, temperature, and enzymatic environment and can recognize changes in physiology or in the state of the disease. Such nanoparticles become bio-activatable and parameters associated with tumor microenvironment given above can be used to trigger drug release. Similarly, external triggers such as light or applied electromagnetic fields can also be used to activate nanoparticles. Exploitation of external or physiological triggers will allow for more sophisticated nanoparticle designs and programmed drug release (14, 15).
All nanoparticle designs have to be carefully characterized for their interactions with biological systems and living organisms to discern their toxicity, biodistribution, and excretion routes. A complex set of in vitro assays and procedures for animal studies is being developed with the attempt to standardize them to enable uniform characterization of nanoparticles originating from different laboratories. These characterization procedures are further enhanced and refined to evaluate more mature applications which have a potential to enter clinical trials (12, 16).
Nanotechnology-based Therapeutic Platforms
By either encapsulating or conjugating existing chemotherapeutics to nanoparticle surfaces, one can convert established drugs into nanoformulations. There are several immediate benefits of using nanoparticles as such drug carriers (4, 5, 6). Nanoparticles’ unique physical properties (size, charge, biocompatibility, solubility) can be manipulated to increase circulation half-life, which in turn can lead to increased accumulation of particles and associated drug cargo at the tumor site (Figure 2). It is believed that Enhanced Permeability and Retention (EPR) effect is responsible for this accumulation (4, 17, 18). Tumor vasculature develops rapidly to support its growth and as such it is more porous than the vasculature in a healthy tissue. The vasculature porosity in the tumor surroundings allows macromolecules and nanoparticles to enter the tumor interstitial space. The strength of EPR, however varies among different tumor types and most likely among the individual patients as well (18). If the nanoparticle delivery relies solely on EPR mechanism, such nanoparticle delivery vehicles are called ‘passively’ targeted. ‘Active’ targeting occurs when particle carriers are combined with targeting ligands which can further enhance drug delivery to tumors (5, 8). Drug payloads can be quite large, due to large surface to volume ratios at the nanoscale. Furthermore, nanoparticle encapsulation techniques can improve the solubility of hydrophobic drugs, thereby eliminating harmful organic solvents from drug formulations, prevent drug degradation in vivo and shield the patient organism from toxic drug properties prior to drug release at the tumor site. Thus nanoparticle formulation can modify pharmacokinetic (PK) and pharmacodynamic (PD) characteristics of the construct and tailor drug delivery profiles (5). Nanoparticles can be further designed into multi-functional delivery systems with a tumor specific targeting moiety, therapeutic payload, and diagnostic tool (imaging or biochemical sensor) that enables monitoring of therapeutic efficacy (5,13).
The benefits described above have been utilized in several early demonstrations of nanoparticle-based drug delivery. In most of these cases, well established chemotherapeutic drug molecules (paclitaxel, doxorubicin, docetaxel, methotrexate) have been combined with liposomal or polymeric nanoparticle platforms. Few of these formulations progressed all the way to the Food and Drug Administration (FDA) approval in the United States, with DOXIL® and Abraxane® being the most known. DOXIL®, a liposomal formulation of doxorubicin, was approved by the FDA in the mid-1990s for treatment of Kaposi’s sarcoma and is now also indicated for the treatment of refractory breast and ovarian cancer (19). Abraxane®, an albumin-bound formulation of paclitaxel for the treatment of metastatic breast cancer, was approved by the FDA in 2005 (20). The albumin-based formulation allows for elimination of cremophor and reduces hypersensitivity reactions which are typical for free paclitaxel treatment. More recently, Marqibo (liposomal vincristine sulfate) was also approved in the US for acute lymphoblastic leukemia (21) and NanoTherm (superparamagnetic iron oxide nanoparticles) was approved in Europe for local ablation in brain tumor - glioblastoma multiforme (22). Overall, the progression of new nanodrugs to the approval is fairly slow, although it is encouraging that several clinical trials using nanoparticle delivery platforms are being pursued (23).
Nanoparticle-based therapies are not limited to delivery of drug molecules. In some cases, nanoparticles are creating new cancer treatments such as hyperthermia (15) and photothermal therapies (14). Iron oxide nanoparticles can be heated in an applied alternating magnetic field, while gold nanoshells respond to near infrared light exposure by releasing energy in the form of heat. In both cases, elevated temperature can eradicate cancer cells. Importantly, thermal ablation can be combined with therapies such as radiation and chemotherapeutic drug release from nanoparticles to establish platforms for combined therapies.
Devices and Nanoparticles for Diagnosis of Cancer
In addition to their role in drug delivery, nanomaterials can be used for detection and diagnosis of early stage cancers as well as for monitoring of the effectiveness of their treatment. This can be accomplished through two different modalities: the development of new constructs for in vivo imaging (24, 25) and design of devices for in vitro assays (9-11). Iron oxide nanoparticles provide a sensitive, low toxicity alternative to standard magnetic resonance imaging (MRI) contrast agents such as injected gadolinium , nanoparticles has been also used in ultrasound.
More sophisticated, layered nanoparticle designs led to constructs capable of multi-modal imaging (26). The tumor specificity of targeted nanomaterials can be used to delineate the margins of cancerous from healthy tissues, providing guidance to assure complete removal of tumor tissue during surgery. In one of the recent demonstrations, a gold core was coated with a Raman molecular tag and gadolinium to construct triple modality agent (26). This platform enabled to precisely visualize tumors and extending metastases in the brain using photoacoustic, Raman, and MRI imaging prior, during, and after the surgery.
Nanotechnology-enabled in vitro diagnostics (Figure 3) offers high sensitivity and selectivity, and capability to perform simultaneous measurements of multiple targets. Well-established fabrication techniques (e.g., lithography) can be used for the manufacture of integrated, portable devices, enhancing the probability of commercial use as point-of-care devices. The transduction mechanisms to report the data can rely on light (9), magnetic (10, 11), or electronic effects (27). Several devices have been designed for protein capture and detection, either to measure proteins as serum or tissue biomarkers or to use proteins as tags to capture or label cells or vesicles. These devices are capable of analyzing large panels of proteomic signatures at the same time providing for high level of multiplexing. The data analysis can establish correlations among different biomarker levels and map correlations of network signaling and thus provide tools for patient stratification based on their response to different treatments and ultimately improve therapeutic efficacy of the one selected. The current designs of in vitro detection platforms can be readily adapted to new biomarker classes, such as miRNA for example (28).
Future Nanoparticle Designs
As more basic capabilities are mastered in nanoparticle design, it is likely that combined functionalities will become more prevalent. Theranostics, the joining of therapeutics, diagnostics, and often post-therapy monitoring, is a new area of interest in nanotechnology (13, 29). By design, metallic and magnetic nanomaterials, such as gold and iron oxide, respectively have proven to be ideal for these types of applications - both display imaging properties and can be designed to serve as nanocarriers.
With the development of more complex nanosystems, it seems clear that there is room to further evolve cancer nanotherapeutics away from reformulations and to utilize their properties in the development of smarter, biologically responsive nanosystems. For example, nanoparticles able to communicate with one another in response to the activation of a biological cascade were recently developed to signal for increased accumulation at tumor and metastatic sites (30). However, not all future nanosystems need to be so far reaching. There are advantages of mimicking the naturally occurring cells and molecules in the body (e.g., coating of nanoparticles with cell membranes and self peptides) as a way to outwit the immune system, providing another avenue for smarter nanoplatform design with increased utility in the body (31).
How Engineers and Physical Scientists Can Contribute to Further Progress in the Field?
Cancer nanotechnology is a multi-disciplinary endeavor which leverages knowledge and innovation from several disciplines ranging from materials science and physics to cancer biology and clinical practice (32). It allows researchers from disparate communities to contribute their diverse knowledge, experience, and creativity into a final goal of designing a better drug or better diagnostic tool. With the evidence of benefits emerging from several fields working together, the concept of science convergence was established (33). It is a new approach to science, collaboration, and cross-cutting interaction with the argument that new innovations are more likely to occur at the boundaries of intertwining fields as compared to individual fields progressing on their own when frequently evolutionary, rather than revolutionary change occurs.
Nanotechnologies in cancer are expected to significantly improve cancer treatment and diagnosis (34). Many of the improvements will be important, but incremental - reduced side effects of the treatment, ability to modify drug dosing, and capability of tracking the delivery of therapeutics with imaging in theranostics modalities. But, some applications are expected to make a major difference – an ability of crossing biological barriers may enable more effective treatments of brain and pancreatic cancers, for example. The same is true with nanoparticle delivery of siRNA for genetic therapies allowing for avoidance of siRNA degradation in contact with blood or nanoparticle-based reformulation of potent drugs which can not be delivered in free form due to high toxicity. Multi-modal imaging constructs will bring new opportunities to real time surgery monitoring, while in vitro diagnostic devices have the potential to replace existing tests due to their improved sensitivity and potential for multi-stage analysis integration.
Overall, further progress in the field is expected to move along two parallel tracks. First one will be associated with on-going translation to the clinical environment; while the second with the development of new tools and techniques in research arena, where continuous innovation in the area of nanomaterials will lead to more sophisticated and multifunctional nanoparticle and nano-devices designs.
Author: Piotr Grodzinski, Ph.D.
Office of Cancer Nanotechnology Research
National Cancer Institute, NIH
31 Center Drive, Room 10A52
Bethesda, Maryland 20892, USA
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Dr. Piotr Grodzinski is a Director of NCI Alliance for Nanotechnology in Cancer at the National Cancer Institute in Bethesda, Maryland. He coordinates program and research activities of the Alliance which dedicates around $150M over funding period of 5 years to form interdisciplinary centers as well as fund individual research and training programs targeting nanotechnology solutions for improved prevention, detection, and therapy of cancer.
Dr. Grodzinski graduated from the University of Science and Technology (AGH) in Krakow, Poland and continued his studies at the University of Southern California in Los Angeles, where he researched novel semiconductor materials used in low threshold lasers. In mid-nineties, Dr. Grodzinski left the world of semiconductor research and got interested in biotechnology. He built a large microfluidics program at Motorola Corporate R&D in Arizona. The group made important contributions to the development of integrated microfluidics for genetic sample preparation with its work being featured in Highlights of Chemical Engineering News and Nature reviews. After his tenure at Motorola, Dr. Grodzinski was with Bioscience Division of Los Alamos National Laboratory where he served as a Group Leader and an interim Chief Scientist for DOE Center for Integrated Nanotechnologies (CINT). At the National Institutes of Health (NIH), in addition to his programmatic responsibilities, he co-chaired Trans-NIH Nanotechnology Task Force, which is coordinating the nanotechnology efforts across 27 institutes of the agency with the budget over $300M/year.
Dr. Grodzinski received Ph.D. in Materials Science from the University of Southern California, Los Angeles in 1992. He is an inventor on 17 patents and published 58 peer-reviewed papers and 10 book chapters. Dr. Grodzinski has been recently elected a Fellow of the American Institute for Medical and Biological Engineering (AIMBE).
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