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Nanomedicine: An unresolved regulatory issue
Regulatory Toxicology and Pharmacology 46 (2006) 218–224 www.elsevier.com/locate/yrtph
Nanomedicine: An unresolved regulatory issue 夽 Vivian S.W. Chan Innovative Therapeutics Group, Centre for Drug Administration, Health Sciences Authority, 11 Biopolis Way #11-03 Helios, Singapore 138667, Singapore Received 15 February 2006 Available online 1 November 2006
Abstract Nanomedicine is a science that uses nanotechnology to maintain and improve human health at the molecular scale. Current and potential applications of nanotechnology in medicine range from research involving diagnostic devices, drug delivery vehicles to enhanced gene therapy and tissue engineering procedures. Its advantage over conventional medicine lies on its size. Particle size has eVect on serum lifetime and pattern of deposition. This allows drugs of nanosize be used in lower concentration and has an earlier onset of therapeutic action. It also provides materials for controlled drug delivery by directing carriers to a speciWc location. Major eVorts are underway, however, very little attention is devoted to assessment of health risks to human or to the ecosystem. Inhaled nanoparticles have already been related to lung injury. It is recognized that physico-chemical properties in conjunction with environmental factors and stability of the nanomaterial all contribute to the overall toxicological responses. Nanotoxicological information, currently insuYcient, will be vital in aiding academia, industry and regulatory bodies in elucidating the mechanisms of action, balancing its risk and beneWt, thus maximizing the utility of these materials in medicine without compromising public health and environmental integrity. © 2006 Elsevier Inc. All rights resrved. Keywords: Nanomedicine; Nanotoxicology; Nanoparticles; Regulatory; Surface properties; Biokinetics; Cytotoxicity; Immune responses
1. Nanomedicine Nanomedicine, which involves the use of nanotechnology in drug development, oVers ever more exciting promises of new diagnoses and cures. It has been deWned as the monitoring, repair, construction and control of human biological systems at the molecular level, using engineered nanodevices and nanostructures (Miller, 2003). A nanometer is one-billionth of a meter. It is at this size scale—about 100 nm or less—that biological molecules and structures inside living cells operate: the diameter of DNA is in the 2.5 nm range, while red blood cells are approximately 7000 nm (Miller, 2003). Our body is constructed from nanoscale building blocks. Drug target to the body’s nanostructures, like the DNA or proteins in the body, has already been developed by pharmaceutical industry long
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before the emergence of nanotechnology (Alivisatos, 2002 in Sherman, 2004). This category of drugs includes aspirin, cisplatin and other anti-cancer agents, as well as much more complex molecules like beta-blockers and antiinXammatory agents (Ratner and Ratner, 2003 in Sherman, 2004). The diVerence between nanomedicine and conventional drugs is that nanomedicine is entirely based on small molecule chemistry. It not only covers the therapeutic agents themselves, but promise to combine the abilities to deliver those agents to speciWc regions or tissues in the body, to speciWc cells, perhaps to a speciWc location within a cell, and also to make release of the therapeutic responsive to a physiological condition and perform speciWc task (Sherman, 2004). It is these enormous potential and promises to reWne medicine and enable it to work more eYciently that encourage most drug companies in the world to engage in nanotechnology research. Applications of nanotechnology in medicine are potentially enormous. It is recognized that as particles get smaller, the surface area increases with a greater proportion of atoms/ molecules found at the surface compared to those inside.
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Nanoparticles, therefore, have a much larger surface area per unit mass compared with larger particles. The increase in the surface-to-volume ratio results in the increase of the particle surface energy (possibly due to quantum eVect) and may render them more biological reactive (Issues in Debate, 2005; Oberdörster et al., 2005b). The increased biological activity can be either positive and desirable (e.g., antioxidant activity, carrier capacity for therapeutics, penetration of cellular barriers for drug delivery) or negative and undesirable (e.g., toxicity, induction of oxidative stress or of cellular dysfunction), or a mix of both (Oberdörster et al., 2005a). Therefore, by presenting them in a diVerent way, compounds that have the same chemical composition and formula but are not therapeutically useful in bulk-sized counterparts may have properties that were not identiWed before. Nanoparticles of silver and gold, for example, are more reactive. Unlike the unreactive properties of their large particles counterparts, they display antimicrobial properties, which help to prevent infection and wound healing (Issues in Debate, 2005). Products, like nanocrystal silver burn cream, nanocrystal shoes and athletic equipment are becoming popular (Sherman, 2004). Sunscreens containing nanoparticles of zinc oxide or titanium dioxide, however, is currently under scrutiny. A theoretical concern on its possible interaction with sunlight to form free radicals in cells, resulting in skin cell damage has been raised (Therapeutic Goods Administration, 2006). 1.1. Nanosized drug Nanotechnology contributes in the development of novel drug especially in the area of cancer therapy or treatment to neurological disorders. It produces nanosized drugs that are only slightly larger in size than proteins and are therefore small enough to move across barriers (e.g., the blood–brain barrier, the stomach wall or tumour pores), dispersed throughout the whole body including entering the central nervous system, escape the vasculature and enter cells directly (The-infoshop.com, 2005). Nanosized drugs, because of their increased surface area and biological activity also allow lower concentrations of drugs to be used; thereby reduce the potential risk of adverse reactions (Sinha et al., 2004 in Sherman, 2005). The increased surface area of nanosized drugs may also help to enhance solubility and increase rate of dissolution, thereby increase oral bioavailability and allow more rapid onset of therapeutic action (Sadrieh, 2006a,; AzoNanotechnology article, 2005). An example of this would be the use of crystal version of naproxen that have been utilized to increase the rise time in the blood of the patients to enable much more rapid release and relief than the conventional presentation of naproxen (Bucolo et al., 2002; Nanotechnology Talk, 2005). 1.2. Nano delivery systems for therapeutics and diagnostics Many approaches have been developed to use nanoparticles in the area of biomedical imaging and drug delivery.
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Applications for these systems will provide materials for controlled drug delivery by directing carriers to a speciWc location with magnetic Welds or Xuorescence biological markers. The carrier will then be activated on demand in a limited region. The predictable and extended duration of action therefore help to reduce inconvenience of frequent redosing, improve patient compliance and avoid the side eVects that so often result from potent medicines (Sinha et al., 2004 in Sherman, 2005). Quantum dots, one of the most well studied, are nanocrystals that Xuoresce in diVerent colours depending on their sizes (e.g., Cadmium selenide) (Hardman, 2005). They typically have a core made of inorganic element, but are generally coated with organic materials such as polyethylene glycol to enhance their biocompatibility or attach them to speciWc target molecules, like proteins or DNA strands (Denison, 2005). If the target molecule is an early indicator of disease, detection of that molecule may indicate a higher propensity for disease. An example is to use nanoparticles to bind to blood clots and to help make clots more visible by ultrasound (Liang and Blomley, 2003). Nanoshells, another nanodelivery system that is composed of copolymers, are used in combination with speciWc wavelengths of lights and heat technology for cancer therapy. These nanodevices can be tuned precisely to absorb or scatter infrared rays. When encased with gold, they can convert these forms of light into heat and either releases the drug into the surrounding tissue or selectively burnt/kill tumor cells that is bind to these nanoshells (NIH News, 2004 in Sherman, 2005). Similarly, magnetic Welds could be used to concentrate drug particles at the tumor site and modulating the Welds would release the drug from the coating to attack tumor cells (Sherman, 2004). Incorporation of cancer-killer genes into nanocapsules is being tried out. One of the gene being investigated is the gene elaborates tumor necrosis factor, a protein that is toxic not only to cancer but also to healthy cells when injected in large doses. To avoid damage to normal tissue, the nanocapsule is coated with sensors that target only on tumor cells. A patient would then be exposed to low-does radiation or drugs that trigger the gene to make necrosis factor (Kotulak, 2004 in Sherman, 2004). Other nanodrug carriers such as gene-gun with gold particles, fullerenes (composed entirely of carbon, in the form of a hollow sphere (C60 Bucky balls) or cylinder (nanotubes)) and dendrimers (spherical polymeric molecules) are also being examined (AzoNanotechnology article, 2005). Another mean of drug delivery is to stabilize drug with nanoparticles like albumin. Example of that would be the FDA approved, nanoparticle albumin bound-paclitaxel (Abraxane™) (Williams, 2005). It uses the natural carrier albumin rather than synthetic solvents to deliver paclitaxel and also for safe administration of high paclitaxel doses without premedication, resulting in signiWcant antitumour activity in patients with metastatic breast cancer (Ibrahim et al., 2005). Delivery of drugs to the brain is always a challenge. The use of nanoparticles to smuggle drug to the brain using the
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PathFinder technology is being investigated. This technology uses nanoparticulate drug carriers in combination with the novel targeting principle of “diVerential protein adsorption” to cross the blood–brain barrier (Müller and Keck, 2004). Another potential beneWt of nanosized drug is its ability to sneak past the immune system. A reduction in the uptake of phagocytic cells of the reticulo-endothelial system in the presence of nanoparticles was demonstrated (Jaulin et al., 2000). As the nanoparticles are not eYciently scavenged by macrophages, the resulting increased in blood circulation time and hence bioavailability is expected to extend the duration of controlled system drug delivery or to improve the prospects for nanoparticles to reach target sites by extravasation (Moghimi et al., 2001).
major challenge for nanotechnology research in the Weld of tissue engineering (Miller, 2003). Some researchers are attempting to construct complex nanorobots that can travel throughout the human body using molecular motors and computers, store and transport molecules, perform operations and communicate with physicians. Respirocytes, a spherical carbon nanorobot, is an example that mimic the action of the natural hemoglobinWlled red blood cells and is being studied (Freitas, 2005a,b). However, despite the pace at which research is progressing and the excitement generated by the prospect of nanorobots, it could be many years before nanorobots are tested in humans.
1.3. Nanodelivery systems for gene therapy
There is currently a lack of information on nanotoxicology. Semiconductor metals, which are commonly used in nanotechnology (Sherman, 2005), are highly toxic, and not even gold has been proven safe in the quantities and formulations employed (Alper, 2005). While the small size of particles is what makes nanotechnology so useful in medicine and industry, it is also one of the main factors that might make them potentially dangerous to human health. Research is now showing that harmless bulk materials tend to become more toxic when they are made into ultra-Wne particles. The general opinion is that the smaller the particles, the more reactive and toxic are their eVects. It is because any intrinsic properties of particles will likely be emphasized with the increase in surface area per unit mass (Nighswonger, 1999; Oberdörster et al., 2005a). Studies have shown that ultraWne particles (e.g., Wne scale carbon graphite), irrespective of their chemical composition, are potent inducers of inXammatory lung injury (Oberdörster, 2006). Potential risks associated with technology are recognized. Of special concern is the unique type of toxicity due to surface modiWcations. Enhanced endocytosis, including a potential for inXammatory and pro-oxidant activity (Klimuk et al., 2000) are showed to be largely dependent on nanoparticles’ surface chemistry (coating) and in vivo surface modiWcations (Oberdörster et al., 2005a). Increased in pulmonary toxicity (e.g., inXammation, granuloma formation) of carbon nanotubes when compared with that of the carbon black and carbonyl iron particles was seen in mice (Lam et al., 2004) and rats (Warheit et al., 2004) respectively. It has also been shown that alteration in the nanoparticle surface charge can aVect its rate of uptake into body. Example of that is the enhanced arterial uptake of the didodecyldimethylammonium surface modiWed nanoparticles observed in an ex vivo dog femoral artery model (Labhasetwar et al., 2000). Induction of cytotoxicity by nanomaterial is another area of concern. Oxidation stress caused by free radical generated by the interaction of particles with cells may result in cell death. Evidence of mitochondrial distribution and oxidative stress response after endocytosis of
Using nanoparticles in place of biological vectors like viruses are one of the ways that nanotechnology may facilitate gene therapy. In theory, nanoparticles are less immunologically reactive than virus vectors and allow delivery of large amount of genetic material with its large surface area to the target cells (Scienceagogo News, 2005). Polymer dendrimers were experimented as vehicles for gene therapy. Animal trials have demonstrated that dendrimers can transfer DNA into the nucleus of cell without triggering a harmful immune reaction (Miller, 2003). 1.4. Nanomedical devices Nanomedical research could result in an array of new medical devices. Interesting research projects include use of nano-electromechanical device or nanowire Weld-eVect transistor to detect insect baculovirus and single inXuenza viruses respectively were conducted (Physicsweb article, 2004). It is hoped that development of these nanodevices can help physician to locate the problem areas in the body more precisely. Other research works involve the use of biochips and microXuidic devices to screen tissues for genetic diVerences and to design genetically target drugs (AzoNanotechnology article, 2005). 1.5. Cell/Tissue engineering and the nanorobot ArtiWcial nanoscale building blocks may one day be used to help repair, maintain, or replacement of the body’s tissue and organs. Using nanotechnology, scientist may be able to make better artiWcial veins, arteries and heart valves; develop a scaVold for growing patches of heart tissue to repair damage from heart attacks; and grow artiWcial lung tissue (NHLBI scientiWc sessions, 2006). The use of simple carbon single-wall nanotube sheets to develop artiWcial muscle is in the earliest stages (Nighswonger, 1999). One of the more important issues is that the foreign materials might be attack by the body’s immune cells, therefore means of reducing the risk of immune reaction remains the
2. Nanotoxicology
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nanoparticles was noted (Oberdörster et al., 2005a). The extent of toxicity is mostly governed by surface properties, in particularly, surface area and charge (Vinardell, 2005). Researchers at Rice University’s Center for Biological and Environmental Nanotechnology (CBEN) found that alteration in surface modiWcations, such as converting insoluble carbon nanotubes to soluble, could dramatically reduce cytotoxicity (USNCIA, 2005). Similarly, in vitro studies have shown mercaptopropionic acid (MPA) and cysteamine coated cadmium–tellurium (CdTe) quantum dots to be more cytotoxic to rat pheochromocytoma cell cultures at concentrations of 10 g/ml than the uncoated CdTe quantum dots (Hardman, 2005). In addition, cytotoxicity was also more pronounced with smaller positively charged quantum dots than with larger equally charged quantum dots, at equal concentrations, using the MTT ([3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] cell proliferation assay (Hardman, 2005). It is not yet known exactly how nanoparticles interact with our immune system and other body tissues. It was suggested that nanoparticles, because of their small sizes, could act like haptens to modify protein structures, either by altering their function or rendering them antigenic, thus raising their potential for autoimmune eVects (Donaldson et al., 2004). The large surface area of nanoparticles may also mean that they can absorb proteins. Internalization via caveolae was shown to become more eYcient when gold was coated with albumin (Oberdörster et al., 2005a). Adsorption of diVerent proteins occur with diVerent nanoparticle surfaces could also modify how nanoparticles are handled by macrophages and other cells and might be potent at activating the complement system (Brown et al., 2001; Kim et al., 2003; Barlow et al., 2005 in Oberdörster et al., 2005b). The long-term toxicity verses acute and subacute toxicity is another major issue. There is a concern that history like the one on asbestos toxicity, might repeat itself: despite the apparent lack of eVect (no deaths have been reported so far) due to acute exposure to asbestos, chronic inhalation exposure can cause death due to asbestosis and cancer (ATSDR, 2006). Dr. Wiesner at Rice University pointed out that carbon nanotubes resemble asbestos Wbers in shape: because they are long and needle-like (Nanoforum Report, 2004 in Nanotech safety-Hazards magazine (2006)). Early warnings on potential long-term side eVect cannot be ignored. Not only may adverse eVects be induced, interactions of nanoparticles with cell and subcellular structures and their distribution/biokinetics are also likely be very diVerent from those of larger-sized particles (Oberdörster et al., 2005a). Risk of bioaccumulation and redistribution from their site of disposition in organs and tissues were suggested. Noteworthy is that there appears to be a natural “passageway” for nanoparticles into and then subsequently around the body. This is through the ‘caveolar’ openings in the membranes that separate body compartments. These openings are between 40 and 100 nm in size and are thought
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to be involved in the transport of “macromolecules” such as proteins. There are also indications that size matters as much as or more than the material of which the particle is composed. When the particles fall within a range between about 20 and 50 nm, they encompass the particle sizes that may allow entrance to the central nervous system and cells. In addition, macrophages patrolled on the alveolar surfaces of the lung appear to have diYculty in recognizing particles of less than 70 nm as being “foreign”, thus allowing them to gain access to the pulmonary interstitium, a potentially vulnerable anatomical compartment (ETC Group, 2003; Warheit, 2004; Monastersky, 2004). Inhaled nanoparticles were also shown to be eYciently deposited in all regions of the respiratory tract. They have been reported to travel from nasal nerves to the brain via translocation along exons and dendrites of neurons, a phenomenon seen with some viruses similar in size to nanoparticles (e.g., human meningitis virus), and to gain access to the blood and other organs (Donaldson et al., 2004; Oberdörster et al., 2005a). They can even transcytosis across epithelial and endothelial cells into the blood and lymph circulation to reach potentially sensitive target sites such as bone marrow, lymph nodes, spleen and heart (Oberdörster et al., 2005a). It has been demonstrated that nanoparticles of up to 1 m in diameter can penetrate the skin (ETC Group, 2003) and distribute via uptake into lymphatic channels (Oberdörster et al., 2005a). Access into deeper brain may occur as well, as shown in rats for soluble manganese (Gianutsos et al., 1997). In 2004, British scientist, Vyvyan Howard, published initial Wndings that indicated that gold nanoparticles might move through a mother’s placenta to the fetus (Nanotech safety-Hazards magazine, 2006). EYcient uptake of nanoparticles via the gastrointestinal tract has also been well documented in oral feeding studies and gavage studies using particles ranging from 10 to 500 nm (Jani et al., 1992 and Jani et al., 1994; Powell et al., 2000 in Oberdörster et al., 2005b). Several studies have shown systemic distribution and accumulation of quantum dots in organs and tissues for weeks to months (Hardman, 2005). A study has also shown synthetically produced ultraWne particles, such as bucky balls (spherical fullerenes), to accumulate in the liver (ETC Group, 2003). All these Wndings indicate that nanoparticles may potentially present problems with body burdens. It is not our intention to look into the environmental eVects of nanomaterial. Understanding the risks posed in the environment is complex because nothing is known currently in regard to the stability of nanomaterial in the environment, product lifetimes, or how these materials partition into environment. An experiment conducted by Eva Oberdörster showed that uncoated C60 fullerenes (bucky balls) can induce oxidative stress and depletion of total glutathione levels in the brain of juvenile largemouth bass (Oberdörster, 2004). Concerns on the release of nanomaterial into environment and its impact to other species have been raised by The National Science Foundation (NSF) and the Environmental Protection Agency (EPA). The
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potential adverse eVect has recently been addressed in their research initiatives (Dreher, 2004). It is highly unlikely that all the materials used in construction of nanoparticles will be biologically inert and many quantum dots core metals are already known to be toxic to vertebrate systems at relatively low concentrations (ppm) (Hardman, 2005). 3. Regulatory framework needed for nonclinical toxicity testing of nanomedicine The combination of enhanced surface area to volume ratio and enhanced reactivity, possibly due to quantum eVect, lends far greater complexity to the characterization of nanoparticles, and also precludes easy extrapolation about potential toxicity (Denison, 2005). Several inXuential bodies have conducted risk assessment of nanotechnology in perspective. There are the National Nanotech Initiative (NNI), the European Nanomedicine Technology (ENT), the Innovation Medicines (IM) for Europe Platform and the International Life Sciences Institute (ILSI) Research Foundation/Risk Science Institute Nanomaterial Toxicity Screening Working Group. The NNI is a collaborative work among the NSF, EPA and other US federal agencies, the United Kingdom Royal Society and the United Kingdom Health and Safety Executive Council (Freitas, 2005a). The ENT, launched in September 2005, has the goal of strengthening innovation in nano-biotechnologies for medical use (Technology Platforms, 2005), whereas the IM for European commission has the objective to enhance and accelerate the development process of medicine, in particular to ensure rapid communication among public sector, pharmaceutical industries and regulators (European Commissioner, 2006). The ILSI nanomaterial toxicity screening working group aims at developing a screening strategy for the hazard identiWcation of nanomaterial. Add to these is the International Association of Nanotechnolgy’s Nomenclature and Terminology Subcommittee and the American National Standards Institute Nanotechnology Standard Panel (ANSI-NSP) who are currently developing standard deWnitions for comment terms in nanomaterial science. Regulators, worldwide, are also preparing themselves for the future where nanotechnology is anticipated to integrate into new drug applications. There is currently no speciWc regulatory requirement to test nanoparticles for health, safety and environmental impacts. For example, the micronized titanium dioxide in sunscreen is not considered to be a new ingredient but is considers as a speciWc grade of the titanium dioxide originally reviewed (ETC Group, 2003). Current outlines to nanomaterial are based on regulations developed for standard materials. To facilitate the regulation of nanotechnological products, the US Food and Drug Agency (USFDA) has formed a NanoTechnology Interest Group (NTIG), which is made up of representatives from all the Centers within the agency, to ensure eVective communication and to educate staV and policy makers on scientiWc progress in nanotechnology (USFDA,
2006). Similarly, the European regulators are expected to put in place appropriate actions to address the safety assessment of nanomaterial. The toxicology of nanoparticles is multi-factorial. It ranges from exposure toxicological assessment, eVect on the immune responses (can be due to both the core and surface composition of nanomaterial), to fate transport and deposition of nanomaterial (Earth & Sky Radio Series, 2005). There is evidence that ultraWne particles can gain entry to the body by a number of routes, including inhalation, ingestion and across the skin. It is believe that additional package to include modiWed testing to take into account of the diVerent potential toxicity and burden of nanoparticles at sites distant to the portal of entry (Donaldson et al., 2004; Oberdörster et al., 2005a) is needed to integrate with the standard toxicology package to serve as the basis for regulatory recommendations and decision-making process. In addition, careful considerations on quality control, that is the consistency and reliability on mass production is as important even when toxico/pharmacokinetics/dynamics have proven to be acceptable. It is not unexpected that any slight derivation in size, physico-chemical properties will have great eVect on the toxicity characteristics of nanoparticles. Additional factors such as particle aggregation/disaggregation potential may also be highly variable and diYcult to predict. So far, the three key elements identiWed by the ILSI nanomaterial toxicity screening working group for further development of a detailed testing protocol are in line to those suggested by USFDA. The three elements are: (1) Physicochemical characteristics (both core and composition) which is believe to have a strong link to the biological activity of nanoparticles; (2) Studies in in vitro models, such as those on absorption (oral, dermal, other route), target binging/receptor screening, cellular uptake and cytotoxicity, for allowing speciWc biological and mechanistic pathways to be isolated and tested under controlled conditions; and (3) Studies in in vivo models, such as those on eYcacy/proof of concept, imaging studies, functional studies, mechanisms of tissue uptake and tissue clearance, for better understanding of the biokinetics and toxicity proWle of nanomaterial in animals (Oberdörster et al., 2005b; Sadrieh, 2006a). 4. Conclusions While nanomedicine provides great promises, it also imparts great challenge to public health. Not all nanomaterial is toxic, it is important to screen oV those with undesirable properties to aid industry in producing nanomaterial of minimum risk. Results of older kinetic studies together with newer epidemiologic and toxicological studies of nanoparticles can be viewed as the basis for expanding the Weld of nanotoxicology (Donaldson et al., 2004; Oberdörster et al., 2005a). Recent toxicity and exposure data, like studies on magnetic nanoparticles and copper nanoparticles, which provide in vivo information on tissue
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distribution, morphological changes, pathological examinations and blood biochemical (Kim et al., 2006; Chen et al., 2006), combined with therapeutic and other related literature, are beginning to shape risk assessments that will be used to regulate the use of nanomaterial in consumer products (ETC Group, 2003). It is however, in view of the inXuence of the physico-chemical properties (size, charge, concentration, outer coating bioactivity and stability) of nanomaterial to their toxicity proWle, it is believed that an open publicly accessible robust nano database serving as a foundation for gathering the data and experience from world-wide experts is needed to understand the mechanism of toxic action and how does the reactive surface of ultraWne particles react with the body, the relative contribution of particle size versus particle composition on the deposition, translocation, and overall toxicity of small particles, is needed to provide the essential bridging information for assessing the safety and potential hazards of nanoparticles. The general is for government to play a leading role, together with academia and commercial organizations, in developing the enabling infrastructure for identifying and assessing nanomaterial’ potential risks, including (1) identiWcation of “representative or model’ nanomaterial, in relation to the structure, size, property and activity; (2) developing toxicity testing protocol, covering inhalation, dermal, ingestion, and injection exposure routes, for identiWcation of full character of the nanomaterial in relation to material property, the proposed use by the potentially predisposed, susceptible patient and the ultimate clinical applications; (3) strengthening the manufacturing processes under Good manufacturing Practice (GMP) for nanomedicine in particularly in the area of fabrication, shipping, handling and storage to ensure that stability and quality of the Wnal product is maintained; (4) detecting and monitoring the exposure level in workplace, air/ waterborne releases, humans and other organisms and environmental media; and (5) assessing the impact of nanotechnology products on the environment as a proactive risk management. In sum, the experience on the safety use of nanomedicine is still very limited seen from the regulator’s viewpoint. It is considered premature to make any general regulatory recommendations on how the regulators would conduct the risk-based assessment. It is recommended that every nanomedicinal application be evaluated based on a case-by-case approach until speciWc guidances can be Wnalized. Acknowledgments The author thank the CEO and directors in the Centre for Drug Administration, Health Sciences Authority, Singapore for their encouragement and support. References Agency for Toxic Substances and Disease Registry (ATSDR), Department of Health and Human Services. Asbestos toxicity: Physiologic
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Jaulin, N., Appel, M., Passirani, C., Barratt, G., Labarre, D., 2000. Reduction of the uptake by a macrophagic cell line of nanoparticles bearing heparin or dextran covalently bound to poly(methyl methacrylate). J. Drug Target 8, 165–172. Kim, H., Liu, X., Kobayashi, T., Kohyama, T., Wen, F.Q., Romberger, D.J., Conner, H., Gilmour, P.S., Donaldson, K., MacNee, W., Rennard, S.I., 2003. UltraWne carbon black particles inhibit human lung Wbroblast-mediated collagen gel contraction. Am. J. Respir. Cell Mol. Biol. 28, 111–121. Kim, J.S., Yoon, T.J., Yu, K.N., Kim, B.G., Park, S.J., Kim, H.W., Lee, K.H., Park, S.B., Lee, J.K., Cho, M.H., 2006. Toxicity and tissue distribution of magnetic nanoparticles in mice. Toxicol. Sci. 89, 338–347. Klimuk, S.K., Semple, S.C., Nahirney, P.N., Mullen, M.C., Bennett, C.F., Scherrer, P., Hope, M.J., 2000. Enhanced anti-inXammatory activity of a liposomal intercellular adhesion molecule-1 antisense oligodeoxynucleotide in an acute model of contact hypersensitivity. J. Pharmacol. Exp. Ther. 292, 480–488. Kotulak, R., 2004/09/14. Tiny battleWeld in the war on disease. Chicago Tribune. Labhasetwar, V., Song, C., Humphrey, W., Shebuski, R., Levy, R.J., 2000. Arterial uptake of biodegradable nanoparticles: eVect of surface modiWcations. J. Pharmaceut. Sci. 87, 1229–1234. Lam, C.W., James, J.T., McCluskey, R., Hunter, R.L., 2004. Pulmonary toxicity of single-wall carbon nanotubes in mice 7 and 90 days after intratracheal instillation. Toxicol. Sci. 77, 3–5. Liang, H.D., Blomley, M.D., 2003. The role of ultrasound in molecular imaging. Br. J. Radiol. 76, 140–150. Miller, J., 2003. The Columbia Science and technology law review 2003: Beyond biotechnology: FDA regulation of nanomedicine 23 [on line]. Available from URL: http://www.stlr.org/html/volume4/miller.txt [Assessed 2006 Jan 25]. Moghimi, S.M., Hunter, A.C., Murray, J.C., 2001. Long-circulating and target-speciWc nanoparticles: Theory to Practice. Pharmacol. Rev. 53, 283–318. Monastersky, R., 2004. The dark side of small: As nanotechnology takes oV, researchers scramble to assess its risks [on line]. Available from URL http://www.missouri.edu/~chemrg/current_news/Article_Chron _NanoTech.pdf [Assessed 2006 Jan 25]. Müller, R.H., Keck, C.M., 2004. Drug delivery to the brain—realization by novel drug carriers. J. Nanosci. Nanotech. 4, 471–483. 4th Nanoforum Report., 2004/6. BeneWts, risks, ethical, legal and social aspects of nanotechnology. Nanotech safety-Hazards magazine issue 87: Dangers come in small particles [on line]. Available from URL: http://www.hazards.org/nanotech/ safety.htm [Assessed 2006 April 4]. Nanotechnology Talk: Nanotechnology & Medicine., 2005/01/12 [on line]. Available from URL: http://www.eurekalert.org/nanotalk/20050112/ talk.php [Assessed 2006 Jan 25]. National Heart, lung, and blood Institute (NHLBI): Summary of the 6th Annual public interest organization meeting: scientiWc sessions [on line]. Available from URL: http://www.nhlbi.nih.gov/public/pio2005/ #sessions [Assessed 2006 Jan 25]. Nighswonger, G., 1999/8. A Medical Device Link MD& DI column: New polymers and nanotubes add muscle to prosthetic limbs [on line]. Available from URL: http://www.devicelink.com/mddi/archive/99/08/ 004.html [Assessed 2006 Jan 25]. NIH News, National Institutes of Health, 2004/09/13. Oberdörster, E., 2004. Manufactured nanomaterial (Fullerenes, C60) induce oxidative stress in the brain of juvenile largemouth bass. Environ. Health Perspect. 112, 1058–1062. Oberdörster, G. Toxicology of airborne environmental and occupational particles [on line]. Available from URL: http://www2.envmed.rochester. edu/envmed/tox/faculty/oberdoerster.html [Assessed 2006 Jan 25].
Oberdörster, G., Oberdörster, E., Oberdörster, J., 2005a. Nanotoxicology: an emerging discipline evolving from studies of ultraWne particles. Environ. Health Perspect. 113, 823–839. Oberdörster, G., Maynard, A., Donaldson, K., Carter, J., Karn, B., Kreyling, W., Lai, D., Olin, S., Monteiro-Riviere, N., Warheit, D., Yang, H., 2005b. Principles for characterizing the potential human health eVects from exposure to nanomaterial: elements of a screening strategy. A report from the ILSI Research Foundation/Risk Science Institute Nanomaterial Toxicity Screening Working Group. Particle Fibre Toxicol. 2, 1–35. Physicsweb article: Nanodevices target viruses., 2004/10 [on line]. Available from URL: http://www.physicsweb.org/articles/news/8/10/6/1 [Assessed 2006 Jan 6]. Powell, J.J., Harvey, R.S., Ashwood, P., Wolstencroft, R., Gershwin, M.E., Thompson, R.P., 2000. Immune potentiation of ultraWne dietary particles in normal subjects and patients with inXammatory bowel disease. J. Autoimmune 14, 99–105. Ratner, M., Ratner, D., 2003. Nanotechnology: A Gentle Introduction to the Next Big Idea. Prentice Hall, Upper Saddle River, NJ. Sadrieh, N. Nanotechnology: Regulatory Perspective for drug development [on line]. Available from URL: http://www.fda.gov/nanotechnology/ powerpoint_conversions/regulatory_perspective_Wles/outline/ index.html [Assessed 2006 April 4]. Sadrieh, N. Nanotechnology: Regulatory considerations for drug development [on line]. Available from URL: http://www.fda.gov/nanotechnology/ ChBSA-nanotech-presentation06-04.ppt [Assessed 2006a Jan 25]. Scienceagogo News: Nanoparticles used to deliver gene therapy., 2005/07/ 27 [on line]. Available from URL: http://www.scienceagogo.com/news/ 20050627015104data_trunc_sys.shtml [Assessed 2006b April 5b]. Sherman, M., 2004. The world of nanotechnology. US Pharm. 12: HS-3HS-4. Sherman, M., 2005. Regulatory AVairs focus: Nanotechnology [on line]. Available from URL: http://www.raps.org/s_raps/rafocus_article.asp? TRACKID D &CID D 61&DID D 25797 [Assessed 2005 Aug 23]. Sinha, P.M. et al., 2004. Nanoengineered device for drug delivery application. Nanotechnology 15, S585–S589. Technology Platforms: News and Events [on line]. Available from URL: http://www.cordis.lu/technology-platforms/newev.htm [Assessed 2005 Nov 8]. The-infoshop.com: Nanotechnology in healthcare to 2009.,2005/05 [on line]. Available from URL: http://www.the-infoshop.com/study/ fd29054_nanotechnology.html [Assessed 2006 Jan 27]. Therapeutic Goods Administration: OTC medicines: Sunscreens: Safety of sunscreens containing nanoparticles of zinc oxide or titanium dioxide [on line]. Available from URL: http://www.tga.gov.au/npmeds/ sunscreen-zotd.htm [Assessed 2006 Jan 25]. U.S. Food and Drug Administration: Nanotechnology [on line]. Available from URL: http://www.fda.gov/nanotechnology/ [Assessed 2006 Jan 25]. U.S National Cancer Institute Alliance (USNCIA) for Nanotechnology in Cancer, ModiWcations render carbon nanotubes nontoxic., 2005/11/07 [on line]. Available from URL: http://nano.cancer.gov/news_center/ nanotech_news_2005-11-07b.asp. [Assessed on 4 Apr 2006]. Vinardell, M.P., 2005. Toxicological highlight: In vitro cytotoxicity of nanoparticles in mammalian germ-line stem cell. Toxicol. Sci. 88, 285–286. Warheit, D.B., 2004. Nanoparticles: Health impacts? Materials Today 7, 32–35. Warheit, D.B., Laurence, B.R., Reed, K.L., Roach, D.H., Reynolds, G.A.M., Webb, T.R., 2004. Comparative pulmonary toxicity assessment of single-wall carbon nanotubes in rats. Toxicol. Sci. 77, 117–125. Williams, N., 2005/01/09. Medical News: FDA approves Abraxane(™) for metastatic breast cancer [on line]. Available from URL: http://www. medicalnewstoday.com/medicalnews.php?newsid D 18716 [Assessed 2006 April 4].
Nanomedicine: An unresolved regulatory issue 夽 Vivian S.W. Chan Innovative Therapeutics Group, Centre for Drug Administration, Health Sciences Authority, 11 Biopolis Way #11-03 Helios, Singapore 138667, Singapore Received 15 February 2006 Available online 1 November 2006
Abstract Nanomedicine is a science that uses nanotechnology to maintain and improve human health at the molecular scale. Current and potential applications of nanotechnology in medicine range from research involving diagnostic devices, drug delivery vehicles to enhanced gene therapy and tissue engineering procedures. Its advantage over conventional medicine lies on its size. Particle size has eVect on serum lifetime and pattern of deposition. This allows drugs of nanosize be used in lower concentration and has an earlier onset of therapeutic action. It also provides materials for controlled drug delivery by directing carriers to a speciWc location. Major eVorts are underway, however, very little attention is devoted to assessment of health risks to human or to the ecosystem. Inhaled nanoparticles have already been related to lung injury. It is recognized that physico-chemical properties in conjunction with environmental factors and stability of the nanomaterial all contribute to the overall toxicological responses. Nanotoxicological information, currently insuYcient, will be vital in aiding academia, industry and regulatory bodies in elucidating the mechanisms of action, balancing its risk and beneWt, thus maximizing the utility of these materials in medicine without compromising public health and environmental integrity. © 2006 Elsevier Inc. All rights resrved. Keywords: Nanomedicine; Nanotoxicology; Nanoparticles; Regulatory; Surface properties; Biokinetics; Cytotoxicity; Immune responses
1. Nanomedicine Nanomedicine, which involves the use of nanotechnology in drug development, oVers ever more exciting promises of new diagnoses and cures. It has been deWned as the monitoring, repair, construction and control of human biological systems at the molecular level, using engineered nanodevices and nanostructures (Miller, 2003). A nanometer is one-billionth of a meter. It is at this size scale—about 100 nm or less—that biological molecules and structures inside living cells operate: the diameter of DNA is in the 2.5 nm range, while red blood cells are approximately 7000 nm (Miller, 2003). Our body is constructed from nanoscale building blocks. Drug target to the body’s nanostructures, like the DNA or proteins in the body, has already been developed by pharmaceutical industry long
夽 Disclaimer: The views expressed in this article are personal and do not necessarily represent those of the Health Sciences Authority, Singapore. E-mail address: [email protected]
0273-2300/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.yrtph.2006.04.009
before the emergence of nanotechnology (Alivisatos, 2002 in Sherman, 2004). This category of drugs includes aspirin, cisplatin and other anti-cancer agents, as well as much more complex molecules like beta-blockers and antiinXammatory agents (Ratner and Ratner, 2003 in Sherman, 2004). The diVerence between nanomedicine and conventional drugs is that nanomedicine is entirely based on small molecule chemistry. It not only covers the therapeutic agents themselves, but promise to combine the abilities to deliver those agents to speciWc regions or tissues in the body, to speciWc cells, perhaps to a speciWc location within a cell, and also to make release of the therapeutic responsive to a physiological condition and perform speciWc task (Sherman, 2004). It is these enormous potential and promises to reWne medicine and enable it to work more eYciently that encourage most drug companies in the world to engage in nanotechnology research. Applications of nanotechnology in medicine are potentially enormous. It is recognized that as particles get smaller, the surface area increases with a greater proportion of atoms/ molecules found at the surface compared to those inside.
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Nanoparticles, therefore, have a much larger surface area per unit mass compared with larger particles. The increase in the surface-to-volume ratio results in the increase of the particle surface energy (possibly due to quantum eVect) and may render them more biological reactive (Issues in Debate, 2005; Oberdörster et al., 2005b). The increased biological activity can be either positive and desirable (e.g., antioxidant activity, carrier capacity for therapeutics, penetration of cellular barriers for drug delivery) or negative and undesirable (e.g., toxicity, induction of oxidative stress or of cellular dysfunction), or a mix of both (Oberdörster et al., 2005a). Therefore, by presenting them in a diVerent way, compounds that have the same chemical composition and formula but are not therapeutically useful in bulk-sized counterparts may have properties that were not identiWed before. Nanoparticles of silver and gold, for example, are more reactive. Unlike the unreactive properties of their large particles counterparts, they display antimicrobial properties, which help to prevent infection and wound healing (Issues in Debate, 2005). Products, like nanocrystal silver burn cream, nanocrystal shoes and athletic equipment are becoming popular (Sherman, 2004). Sunscreens containing nanoparticles of zinc oxide or titanium dioxide, however, is currently under scrutiny. A theoretical concern on its possible interaction with sunlight to form free radicals in cells, resulting in skin cell damage has been raised (Therapeutic Goods Administration, 2006). 1.1. Nanosized drug Nanotechnology contributes in the development of novel drug especially in the area of cancer therapy or treatment to neurological disorders. It produces nanosized drugs that are only slightly larger in size than proteins and are therefore small enough to move across barriers (e.g., the blood–brain barrier, the stomach wall or tumour pores), dispersed throughout the whole body including entering the central nervous system, escape the vasculature and enter cells directly (The-infoshop.com, 2005). Nanosized drugs, because of their increased surface area and biological activity also allow lower concentrations of drugs to be used; thereby reduce the potential risk of adverse reactions (Sinha et al., 2004 in Sherman, 2005). The increased surface area of nanosized drugs may also help to enhance solubility and increase rate of dissolution, thereby increase oral bioavailability and allow more rapid onset of therapeutic action (Sadrieh, 2006a,; AzoNanotechnology article, 2005). An example of this would be the use of crystal version of naproxen that have been utilized to increase the rise time in the blood of the patients to enable much more rapid release and relief than the conventional presentation of naproxen (Bucolo et al., 2002; Nanotechnology Talk, 2005). 1.2. Nano delivery systems for therapeutics and diagnostics Many approaches have been developed to use nanoparticles in the area of biomedical imaging and drug delivery.
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Applications for these systems will provide materials for controlled drug delivery by directing carriers to a speciWc location with magnetic Welds or Xuorescence biological markers. The carrier will then be activated on demand in a limited region. The predictable and extended duration of action therefore help to reduce inconvenience of frequent redosing, improve patient compliance and avoid the side eVects that so often result from potent medicines (Sinha et al., 2004 in Sherman, 2005). Quantum dots, one of the most well studied, are nanocrystals that Xuoresce in diVerent colours depending on their sizes (e.g., Cadmium selenide) (Hardman, 2005). They typically have a core made of inorganic element, but are generally coated with organic materials such as polyethylene glycol to enhance their biocompatibility or attach them to speciWc target molecules, like proteins or DNA strands (Denison, 2005). If the target molecule is an early indicator of disease, detection of that molecule may indicate a higher propensity for disease. An example is to use nanoparticles to bind to blood clots and to help make clots more visible by ultrasound (Liang and Blomley, 2003). Nanoshells, another nanodelivery system that is composed of copolymers, are used in combination with speciWc wavelengths of lights and heat technology for cancer therapy. These nanodevices can be tuned precisely to absorb or scatter infrared rays. When encased with gold, they can convert these forms of light into heat and either releases the drug into the surrounding tissue or selectively burnt/kill tumor cells that is bind to these nanoshells (NIH News, 2004 in Sherman, 2005). Similarly, magnetic Welds could be used to concentrate drug particles at the tumor site and modulating the Welds would release the drug from the coating to attack tumor cells (Sherman, 2004). Incorporation of cancer-killer genes into nanocapsules is being tried out. One of the gene being investigated is the gene elaborates tumor necrosis factor, a protein that is toxic not only to cancer but also to healthy cells when injected in large doses. To avoid damage to normal tissue, the nanocapsule is coated with sensors that target only on tumor cells. A patient would then be exposed to low-does radiation or drugs that trigger the gene to make necrosis factor (Kotulak, 2004 in Sherman, 2004). Other nanodrug carriers such as gene-gun with gold particles, fullerenes (composed entirely of carbon, in the form of a hollow sphere (C60 Bucky balls) or cylinder (nanotubes)) and dendrimers (spherical polymeric molecules) are also being examined (AzoNanotechnology article, 2005). Another mean of drug delivery is to stabilize drug with nanoparticles like albumin. Example of that would be the FDA approved, nanoparticle albumin bound-paclitaxel (Abraxane™) (Williams, 2005). It uses the natural carrier albumin rather than synthetic solvents to deliver paclitaxel and also for safe administration of high paclitaxel doses without premedication, resulting in signiWcant antitumour activity in patients with metastatic breast cancer (Ibrahim et al., 2005). Delivery of drugs to the brain is always a challenge. The use of nanoparticles to smuggle drug to the brain using the
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PathFinder technology is being investigated. This technology uses nanoparticulate drug carriers in combination with the novel targeting principle of “diVerential protein adsorption” to cross the blood–brain barrier (Müller and Keck, 2004). Another potential beneWt of nanosized drug is its ability to sneak past the immune system. A reduction in the uptake of phagocytic cells of the reticulo-endothelial system in the presence of nanoparticles was demonstrated (Jaulin et al., 2000). As the nanoparticles are not eYciently scavenged by macrophages, the resulting increased in blood circulation time and hence bioavailability is expected to extend the duration of controlled system drug delivery or to improve the prospects for nanoparticles to reach target sites by extravasation (Moghimi et al., 2001).
major challenge for nanotechnology research in the Weld of tissue engineering (Miller, 2003). Some researchers are attempting to construct complex nanorobots that can travel throughout the human body using molecular motors and computers, store and transport molecules, perform operations and communicate with physicians. Respirocytes, a spherical carbon nanorobot, is an example that mimic the action of the natural hemoglobinWlled red blood cells and is being studied (Freitas, 2005a,b). However, despite the pace at which research is progressing and the excitement generated by the prospect of nanorobots, it could be many years before nanorobots are tested in humans.
1.3. Nanodelivery systems for gene therapy
There is currently a lack of information on nanotoxicology. Semiconductor metals, which are commonly used in nanotechnology (Sherman, 2005), are highly toxic, and not even gold has been proven safe in the quantities and formulations employed (Alper, 2005). While the small size of particles is what makes nanotechnology so useful in medicine and industry, it is also one of the main factors that might make them potentially dangerous to human health. Research is now showing that harmless bulk materials tend to become more toxic when they are made into ultra-Wne particles. The general opinion is that the smaller the particles, the more reactive and toxic are their eVects. It is because any intrinsic properties of particles will likely be emphasized with the increase in surface area per unit mass (Nighswonger, 1999; Oberdörster et al., 2005a). Studies have shown that ultraWne particles (e.g., Wne scale carbon graphite), irrespective of their chemical composition, are potent inducers of inXammatory lung injury (Oberdörster, 2006). Potential risks associated with technology are recognized. Of special concern is the unique type of toxicity due to surface modiWcations. Enhanced endocytosis, including a potential for inXammatory and pro-oxidant activity (Klimuk et al., 2000) are showed to be largely dependent on nanoparticles’ surface chemistry (coating) and in vivo surface modiWcations (Oberdörster et al., 2005a). Increased in pulmonary toxicity (e.g., inXammation, granuloma formation) of carbon nanotubes when compared with that of the carbon black and carbonyl iron particles was seen in mice (Lam et al., 2004) and rats (Warheit et al., 2004) respectively. It has also been shown that alteration in the nanoparticle surface charge can aVect its rate of uptake into body. Example of that is the enhanced arterial uptake of the didodecyldimethylammonium surface modiWed nanoparticles observed in an ex vivo dog femoral artery model (Labhasetwar et al., 2000). Induction of cytotoxicity by nanomaterial is another area of concern. Oxidation stress caused by free radical generated by the interaction of particles with cells may result in cell death. Evidence of mitochondrial distribution and oxidative stress response after endocytosis of
Using nanoparticles in place of biological vectors like viruses are one of the ways that nanotechnology may facilitate gene therapy. In theory, nanoparticles are less immunologically reactive than virus vectors and allow delivery of large amount of genetic material with its large surface area to the target cells (Scienceagogo News, 2005). Polymer dendrimers were experimented as vehicles for gene therapy. Animal trials have demonstrated that dendrimers can transfer DNA into the nucleus of cell without triggering a harmful immune reaction (Miller, 2003). 1.4. Nanomedical devices Nanomedical research could result in an array of new medical devices. Interesting research projects include use of nano-electromechanical device or nanowire Weld-eVect transistor to detect insect baculovirus and single inXuenza viruses respectively were conducted (Physicsweb article, 2004). It is hoped that development of these nanodevices can help physician to locate the problem areas in the body more precisely. Other research works involve the use of biochips and microXuidic devices to screen tissues for genetic diVerences and to design genetically target drugs (AzoNanotechnology article, 2005). 1.5. Cell/Tissue engineering and the nanorobot ArtiWcial nanoscale building blocks may one day be used to help repair, maintain, or replacement of the body’s tissue and organs. Using nanotechnology, scientist may be able to make better artiWcial veins, arteries and heart valves; develop a scaVold for growing patches of heart tissue to repair damage from heart attacks; and grow artiWcial lung tissue (NHLBI scientiWc sessions, 2006). The use of simple carbon single-wall nanotube sheets to develop artiWcial muscle is in the earliest stages (Nighswonger, 1999). One of the more important issues is that the foreign materials might be attack by the body’s immune cells, therefore means of reducing the risk of immune reaction remains the
2. Nanotoxicology
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nanoparticles was noted (Oberdörster et al., 2005a). The extent of toxicity is mostly governed by surface properties, in particularly, surface area and charge (Vinardell, 2005). Researchers at Rice University’s Center for Biological and Environmental Nanotechnology (CBEN) found that alteration in surface modiWcations, such as converting insoluble carbon nanotubes to soluble, could dramatically reduce cytotoxicity (USNCIA, 2005). Similarly, in vitro studies have shown mercaptopropionic acid (MPA) and cysteamine coated cadmium–tellurium (CdTe) quantum dots to be more cytotoxic to rat pheochromocytoma cell cultures at concentrations of 10 g/ml than the uncoated CdTe quantum dots (Hardman, 2005). In addition, cytotoxicity was also more pronounced with smaller positively charged quantum dots than with larger equally charged quantum dots, at equal concentrations, using the MTT ([3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] cell proliferation assay (Hardman, 2005). It is not yet known exactly how nanoparticles interact with our immune system and other body tissues. It was suggested that nanoparticles, because of their small sizes, could act like haptens to modify protein structures, either by altering their function or rendering them antigenic, thus raising their potential for autoimmune eVects (Donaldson et al., 2004). The large surface area of nanoparticles may also mean that they can absorb proteins. Internalization via caveolae was shown to become more eYcient when gold was coated with albumin (Oberdörster et al., 2005a). Adsorption of diVerent proteins occur with diVerent nanoparticle surfaces could also modify how nanoparticles are handled by macrophages and other cells and might be potent at activating the complement system (Brown et al., 2001; Kim et al., 2003; Barlow et al., 2005 in Oberdörster et al., 2005b). The long-term toxicity verses acute and subacute toxicity is another major issue. There is a concern that history like the one on asbestos toxicity, might repeat itself: despite the apparent lack of eVect (no deaths have been reported so far) due to acute exposure to asbestos, chronic inhalation exposure can cause death due to asbestosis and cancer (ATSDR, 2006). Dr. Wiesner at Rice University pointed out that carbon nanotubes resemble asbestos Wbers in shape: because they are long and needle-like (Nanoforum Report, 2004 in Nanotech safety-Hazards magazine (2006)). Early warnings on potential long-term side eVect cannot be ignored. Not only may adverse eVects be induced, interactions of nanoparticles with cell and subcellular structures and their distribution/biokinetics are also likely be very diVerent from those of larger-sized particles (Oberdörster et al., 2005a). Risk of bioaccumulation and redistribution from their site of disposition in organs and tissues were suggested. Noteworthy is that there appears to be a natural “passageway” for nanoparticles into and then subsequently around the body. This is through the ‘caveolar’ openings in the membranes that separate body compartments. These openings are between 40 and 100 nm in size and are thought
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to be involved in the transport of “macromolecules” such as proteins. There are also indications that size matters as much as or more than the material of which the particle is composed. When the particles fall within a range between about 20 and 50 nm, they encompass the particle sizes that may allow entrance to the central nervous system and cells. In addition, macrophages patrolled on the alveolar surfaces of the lung appear to have diYculty in recognizing particles of less than 70 nm as being “foreign”, thus allowing them to gain access to the pulmonary interstitium, a potentially vulnerable anatomical compartment (ETC Group, 2003; Warheit, 2004; Monastersky, 2004). Inhaled nanoparticles were also shown to be eYciently deposited in all regions of the respiratory tract. They have been reported to travel from nasal nerves to the brain via translocation along exons and dendrites of neurons, a phenomenon seen with some viruses similar in size to nanoparticles (e.g., human meningitis virus), and to gain access to the blood and other organs (Donaldson et al., 2004; Oberdörster et al., 2005a). They can even transcytosis across epithelial and endothelial cells into the blood and lymph circulation to reach potentially sensitive target sites such as bone marrow, lymph nodes, spleen and heart (Oberdörster et al., 2005a). It has been demonstrated that nanoparticles of up to 1 m in diameter can penetrate the skin (ETC Group, 2003) and distribute via uptake into lymphatic channels (Oberdörster et al., 2005a). Access into deeper brain may occur as well, as shown in rats for soluble manganese (Gianutsos et al., 1997). In 2004, British scientist, Vyvyan Howard, published initial Wndings that indicated that gold nanoparticles might move through a mother’s placenta to the fetus (Nanotech safety-Hazards magazine, 2006). EYcient uptake of nanoparticles via the gastrointestinal tract has also been well documented in oral feeding studies and gavage studies using particles ranging from 10 to 500 nm (Jani et al., 1992 and Jani et al., 1994; Powell et al., 2000 in Oberdörster et al., 2005b). Several studies have shown systemic distribution and accumulation of quantum dots in organs and tissues for weeks to months (Hardman, 2005). A study has also shown synthetically produced ultraWne particles, such as bucky balls (spherical fullerenes), to accumulate in the liver (ETC Group, 2003). All these Wndings indicate that nanoparticles may potentially present problems with body burdens. It is not our intention to look into the environmental eVects of nanomaterial. Understanding the risks posed in the environment is complex because nothing is known currently in regard to the stability of nanomaterial in the environment, product lifetimes, or how these materials partition into environment. An experiment conducted by Eva Oberdörster showed that uncoated C60 fullerenes (bucky balls) can induce oxidative stress and depletion of total glutathione levels in the brain of juvenile largemouth bass (Oberdörster, 2004). Concerns on the release of nanomaterial into environment and its impact to other species have been raised by The National Science Foundation (NSF) and the Environmental Protection Agency (EPA). The
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potential adverse eVect has recently been addressed in their research initiatives (Dreher, 2004). It is highly unlikely that all the materials used in construction of nanoparticles will be biologically inert and many quantum dots core metals are already known to be toxic to vertebrate systems at relatively low concentrations (ppm) (Hardman, 2005). 3. Regulatory framework needed for nonclinical toxicity testing of nanomedicine The combination of enhanced surface area to volume ratio and enhanced reactivity, possibly due to quantum eVect, lends far greater complexity to the characterization of nanoparticles, and also precludes easy extrapolation about potential toxicity (Denison, 2005). Several inXuential bodies have conducted risk assessment of nanotechnology in perspective. There are the National Nanotech Initiative (NNI), the European Nanomedicine Technology (ENT), the Innovation Medicines (IM) for Europe Platform and the International Life Sciences Institute (ILSI) Research Foundation/Risk Science Institute Nanomaterial Toxicity Screening Working Group. The NNI is a collaborative work among the NSF, EPA and other US federal agencies, the United Kingdom Royal Society and the United Kingdom Health and Safety Executive Council (Freitas, 2005a). The ENT, launched in September 2005, has the goal of strengthening innovation in nano-biotechnologies for medical use (Technology Platforms, 2005), whereas the IM for European commission has the objective to enhance and accelerate the development process of medicine, in particular to ensure rapid communication among public sector, pharmaceutical industries and regulators (European Commissioner, 2006). The ILSI nanomaterial toxicity screening working group aims at developing a screening strategy for the hazard identiWcation of nanomaterial. Add to these is the International Association of Nanotechnolgy’s Nomenclature and Terminology Subcommittee and the American National Standards Institute Nanotechnology Standard Panel (ANSI-NSP) who are currently developing standard deWnitions for comment terms in nanomaterial science. Regulators, worldwide, are also preparing themselves for the future where nanotechnology is anticipated to integrate into new drug applications. There is currently no speciWc regulatory requirement to test nanoparticles for health, safety and environmental impacts. For example, the micronized titanium dioxide in sunscreen is not considered to be a new ingredient but is considers as a speciWc grade of the titanium dioxide originally reviewed (ETC Group, 2003). Current outlines to nanomaterial are based on regulations developed for standard materials. To facilitate the regulation of nanotechnological products, the US Food and Drug Agency (USFDA) has formed a NanoTechnology Interest Group (NTIG), which is made up of representatives from all the Centers within the agency, to ensure eVective communication and to educate staV and policy makers on scientiWc progress in nanotechnology (USFDA,
2006). Similarly, the European regulators are expected to put in place appropriate actions to address the safety assessment of nanomaterial. The toxicology of nanoparticles is multi-factorial. It ranges from exposure toxicological assessment, eVect on the immune responses (can be due to both the core and surface composition of nanomaterial), to fate transport and deposition of nanomaterial (Earth & Sky Radio Series, 2005). There is evidence that ultraWne particles can gain entry to the body by a number of routes, including inhalation, ingestion and across the skin. It is believe that additional package to include modiWed testing to take into account of the diVerent potential toxicity and burden of nanoparticles at sites distant to the portal of entry (Donaldson et al., 2004; Oberdörster et al., 2005a) is needed to integrate with the standard toxicology package to serve as the basis for regulatory recommendations and decision-making process. In addition, careful considerations on quality control, that is the consistency and reliability on mass production is as important even when toxico/pharmacokinetics/dynamics have proven to be acceptable. It is not unexpected that any slight derivation in size, physico-chemical properties will have great eVect on the toxicity characteristics of nanoparticles. Additional factors such as particle aggregation/disaggregation potential may also be highly variable and diYcult to predict. So far, the three key elements identiWed by the ILSI nanomaterial toxicity screening working group for further development of a detailed testing protocol are in line to those suggested by USFDA. The three elements are: (1) Physicochemical characteristics (both core and composition) which is believe to have a strong link to the biological activity of nanoparticles; (2) Studies in in vitro models, such as those on absorption (oral, dermal, other route), target binging/receptor screening, cellular uptake and cytotoxicity, for allowing speciWc biological and mechanistic pathways to be isolated and tested under controlled conditions; and (3) Studies in in vivo models, such as those on eYcacy/proof of concept, imaging studies, functional studies, mechanisms of tissue uptake and tissue clearance, for better understanding of the biokinetics and toxicity proWle of nanomaterial in animals (Oberdörster et al., 2005b; Sadrieh, 2006a). 4. Conclusions While nanomedicine provides great promises, it also imparts great challenge to public health. Not all nanomaterial is toxic, it is important to screen oV those with undesirable properties to aid industry in producing nanomaterial of minimum risk. Results of older kinetic studies together with newer epidemiologic and toxicological studies of nanoparticles can be viewed as the basis for expanding the Weld of nanotoxicology (Donaldson et al., 2004; Oberdörster et al., 2005a). Recent toxicity and exposure data, like studies on magnetic nanoparticles and copper nanoparticles, which provide in vivo information on tissue
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distribution, morphological changes, pathological examinations and blood biochemical (Kim et al., 2006; Chen et al., 2006), combined with therapeutic and other related literature, are beginning to shape risk assessments that will be used to regulate the use of nanomaterial in consumer products (ETC Group, 2003). It is however, in view of the inXuence of the physico-chemical properties (size, charge, concentration, outer coating bioactivity and stability) of nanomaterial to their toxicity proWle, it is believed that an open publicly accessible robust nano database serving as a foundation for gathering the data and experience from world-wide experts is needed to understand the mechanism of toxic action and how does the reactive surface of ultraWne particles react with the body, the relative contribution of particle size versus particle composition on the deposition, translocation, and overall toxicity of small particles, is needed to provide the essential bridging information for assessing the safety and potential hazards of nanoparticles. The general is for government to play a leading role, together with academia and commercial organizations, in developing the enabling infrastructure for identifying and assessing nanomaterial’ potential risks, including (1) identiWcation of “representative or model’ nanomaterial, in relation to the structure, size, property and activity; (2) developing toxicity testing protocol, covering inhalation, dermal, ingestion, and injection exposure routes, for identiWcation of full character of the nanomaterial in relation to material property, the proposed use by the potentially predisposed, susceptible patient and the ultimate clinical applications; (3) strengthening the manufacturing processes under Good manufacturing Practice (GMP) for nanomedicine in particularly in the area of fabrication, shipping, handling and storage to ensure that stability and quality of the Wnal product is maintained; (4) detecting and monitoring the exposure level in workplace, air/ waterborne releases, humans and other organisms and environmental media; and (5) assessing the impact of nanotechnology products on the environment as a proactive risk management. In sum, the experience on the safety use of nanomedicine is still very limited seen from the regulator’s viewpoint. It is considered premature to make any general regulatory recommendations on how the regulators would conduct the risk-based assessment. It is recommended that every nanomedicinal application be evaluated based on a case-by-case approach until speciWc guidances can be Wnalized. Acknowledgments The author thank the CEO and directors in the Centre for Drug Administration, Health Sciences Authority, Singapore for their encouragement and support. References Agency for Toxic Substances and Disease Registry (ATSDR), Department of Health and Human Services. Asbestos toxicity: Physiologic
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