Analog SFF, June 2011 Read online

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  Lipid-based nanoparticles (Figure 2)

  Lipid-based drug carriers include both solid lipid particles and membrane-bounded liposomes, with the latter being more extensively studied. Liposomes resemble tiny cells in that they have an aqueous fluid interior surrounded by a lipid bilayer membrane. As such, they are non-rigid and relatively fragile compared to inorganic nanoparticles or fullerenes. The lipid bilayers are generally composed of phospholipids just as in cell membranes, although the specific phospholipids used to make liposomes may not be naturally occurring. Phospholipid molecules are amphiphilic. That is, the phosphate head at one end of each molecule is hydrophilic whereas the lipid tail at the other end is lipophilic (or hydrophobic). Phospholipids tend naturally to form bilayers in aqueous solution in accordance with the well-known principle that oil and water do not mix. The molecules orient with their lipid tails pointed inward towards each other and the hydrophilic heads on the outside facing the aqueous environment. Liposomes will therefore self-assemble under appropriate conditions, and the most straightforward way to load a drug into them is to form them in solutions that are saturated with the drug, if the drug is water-soluble. There are other ways, involving organic (non-aqueous) solvents and solvent exchange systems, or lipophilic drugs.

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  Liposomes present excellent prospects for drug targeting because it is easy to add additional molecules to the outer surface of the bilayer. A major drawback, however, is that liposomes are comparatively large, about 400 nm in diameter, and, if unprotected, are rapidly cleared from the bloodstream. For this reason all serious efforts to employ liposomes for drug carriers use PEGylated liposomes, and this strategy significantly improves their half-life in the circulation. As already noted, there have been two FDA approvals of liposomes as nanoparticle drug carriers. Despite these successes, however, liposomes have not yet made a major impact in medicine, although they have been used more widely in cosmetics. In a more recent development, researchers are also investigating solid lipid nanospheres, or lipospheres, as alternatives to liposomes. Lipospheres do not so neatly self-assemble, but can be made smaller than liposomes and are less fragile. Solid lipid nanospheres are composed of various types of lipids (triglycerides, fatty acids, or waxes) that are solid at body temperature. These nanospheres are bio-compatible and biodegradable, but have the disadvantage that they can only carry lipid-soluble drugs.

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  Virus-based nanoparticles (Figure 3)

  Viruses are, in effect, naturally occurring nanoparticles that are exquisitely designed to deliver their payload into the interior of a target cell. In the case of natural viruses, of course, the payload is the viral genetic material and the result is to commandeer the biosynthetic machinery of the target/host cell and use it to crank out more copies of the viral nucleic acid and capsid proteins, often resulting in the destruction of the host cell in the process. If a virus could be modified to exclusively target the cells of a tumor, such a result could potentially be beneficial. This approach is indeed being investigated, although care must be taken to make sure the virus is unlikely to mutate in the patient's body into a form that can cause disease or be passed from the patient to other people. Adenovirus, measles virus, and canine parvovirus are among those being studied as “oncolytic” viruses for cancer treatment. The virus’ destructive effects can be targeted to the tumor in various ways, including modifying the virus with surface molecules that make it able to infect only tumor cells, or genetically engineering it to be unable to reproduce in normal cells. In practice, however, it is difficult to achieve complete tumor-specificity, and the best that can be done sometimes is to make the virus more likely to kill tumor cells than normal cells.

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  In the case of oncolytic viruses, the virus is both the vehicle and the therapeutic agent. Researchers have also been actively investigating viruses as vehicles for other kinds of genetic messages besides the viral genes. Primarily the purpose has been gene therapy, which is beyond the scope of this article. There is also interest in using modified viruses, or viral-like particles, to deliver more conventional drug payloads. Bacteriophage viruses (ones that naturally infect only bacteria) are being studied for this purpose. These viruses are not infectious in humans and have advantages over some other types of nanoparticles in that they are biodegradable, come in very uniform sizes, are fully self-assembling, and can be grown and harvested from large cultures of laboratory strains of the common intestinal bacterium E. coli. A major obstacle to the use of any type of virus particles for human therapy, however, is the fact that they evoke an immune response. At best this response can mean that the virus particles are destroyed by the body's defenses before they can be effective, and at worst it can threaten the life of the patient. The surfaces of viral particles can be modified to reduce their tendency to trigger the immune system, but this may interfere with infectivity in the case of oncolytic viruses, for example, and it may be necessary for patients receiving some viral therapies to simultaneously be given immunosuppressive drugs.

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  Polymer-based nanoparticles (Figure 4)

  Polymer nanoparticles vary widely in composition and structure and are a very active and promising area of research. What they have in common is that they are composed of molecular subunits, or monomers, that are covalently linked together to form larger molecules. In some cases these larger molecules function as nanoparticles, in others they undergo further assembly. The advantage of polymers is that constituent monomers can be chosen that are biocompatible and biodegradable, and that offer versatile opportunities for surface modification of the resulting particles. Naturally occurring polymers such as albumin (the major serum protein) and chitosan (chemically modified chitin) have been used, as well as synthetic polymers including PEG, polylactic acid (PLA), poly(lactic-co-glycolic acid) copolymer (PLGA), and [N-(2-hydroxypropyl) methacrylamide] copolymer (HPMA). Synthetic polymers are created through standard methods of chemical synthesis. A drug can be either covalently bound to the polymer or physically trapped within the structure of the particles, and the particles can be in the form of capsules, core-shell micelles, or hyper-branched “dendrimers.”

  Core-shell micelles are formed by self- assembly of amphiphilic block copolymers, each consisting of a stretch or “block” of hydrophilic polymer joined to a block of hydrophobic polymer. Spontaneous assembly of micelles occurs in aqueous solutions at higher concentrations of block copolymers as the molecules coalesce into clusters, aligned with their hydrophobic ends inside and their hydrophilic ends outside. Dendrimers are so named because of their treelike structure. They are synthesized starting from a core that can accept two branches at each end. Successive generations of subunit branches, each of which can in turn accept two branches, are then added so that there are four branches in the first generation (G1), eight in the second (G2), and so on. By G3, the resulting macromolecule approaches the form of a spherical particle. Drug molecules could be sequestered in the spaces between the lower-level branches, but are more commonly attached to the branch tips at the surface, where targeting molecules can be attached as well. Dendrimer-based drug carrier systems have been the subject of clinical trials, and an albumin nanoparticle-based formulation of the anti-tumor drug paclitaxel has been approved by the FDA for treatment of metastatic breast cancer.

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  Targeting disease with nanoparticles

  In some cases it may be possible to inject a drug/carrier directly into the diseased target tissue or organ. In this case it would be desirable for the nanoparticles to remain close to the injection site and not enter the bloodstream until after they have delivered their payload or have undergone degradation. Other methods of drug administration are via injection into a vein or artery, through oral administration, or by inhalation. A drug that is swallowed has direct access to the cells lining the digestive tract before it may cross into the blood, and orall
y delivered nanoparticle drug carriers could be used to treat disorders affecting the intestinal mucosa, such as irritable bowel disease. In addition, substances like nutrients and drugs that cross the intestinal mucosa into the bloodstream are routed first to the liver, offering the potential to target this organ. The tendency of the liver to filter out small particles could make nanoparticles good candidates for treatment of liver diseases. Similarly, an inhaled medication potentially has contact with the cells lining the respiratory tract all the way from the mouth or nose to the alveoli, the tiny air sacks in the lungs where gas exchange takes place. Obviously, inhaled nanoparticles could be used to treat the cells lining the nasal cavity and sinuses as well as the trachea, bronchi, or the tissue of the lungs. It may be less obvious that they could enter the blood, but there is clear evidence that nanoparticles are able to reach the bloodstream by this route.

  In most cases, the circulating blood is the route by which a drug is expected to reach its target. Nanoparticles in the blood are carried throughout the body, and for some diseases such systemic distribution would be sufficient. For example, if the aim is to supply insulin to a diabetic, no further targeting would be necessary because all the body's cells need insulin. In other cases, however, targeting would require that nanoparticles come to lodge preferentially in the location where the disease process is active, or that they be able to selectively release active drug close to, or inside, the appropriate cells. If you were trying to treat an infection, like tuberculosis, you would want the nanoparticles to preferentially come to rest at the foci of infection, or to selectively be taken up by infected cells or by the disease-causing bacteria themselves.

  The problem of targeting nanoparticles to cancer cells provides a good illustration of the issues involved in treating disease with nanoparticles. Such targeting can be passive, in which the drug delivery system takes advantage of natural characteristics of the tissue or of the tumor. Using the particle-trapping characteristics of the liver to passively target liver cancer would be an example. Nanoparticles can actually pass through the cells that line capillaries, or they can pass between the cells to enter the surrounding tissue. It turns out that tumors (and also inflamed tissues) have leakier blood vessel walls than normal healthy tissue, causing circulating nanoparticles to preferentially accumulate inside tumors in what is termed the “enhanced permeability and retention effect.” Tumors also have a higher metabolic rate due to increased cell division and may be poorly oxygenated as a result, so that the tumor becomes more acidic than the surrounding tissue. This fact can be used to enhance release of an anti-cancer drug at the site of the tumor by designing nanoparticles that release the drug in an acidic environment.

  Often the objective is to have the nanoparticle internalized by the target cell. Binding to cell surface receptors often results in internalization as a result of the receptor's normal function. Cells will also engulf particles by the process of endocytosis without the involvement of a specific receptor. Active targeting can improve specificity but requires that the tumor cells express some molecule on their surface that is either unique to the tumor cells or is expressed more highly on tumor than on normal cells. If the molecule in question happens to be a receptor, then that receptor's natural ligand can be attached to nanoparticles to produce active targeting. Receptors for the vitamin, folate, and the iron-carrier transferrin have both been studied for tumor targeting. Alternatively, antibodies can be generated and attached to nanoparticles to target other types of cell-surface molecules. The technology has long existed to produce large quantities of antibodies that bind to a molecule of choice.

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  Nanoparticle toxicity and clearance from the body

  Nanoparticle drug delivery systems pose a number of toxicity issues. There may be residual toxicity of the drug being delivered, which may not be completely masked by the nanoparticles. In this context, the rate of release of the drug and the conditions under which it is released may be relevant to nanoparticle design. There is also the matter of the body's response to the particles themselves, and finally there is the question of what happens to the nanoparticle drug carriers after they have served their function.

  Obviously it is undesirable to make nanoparticles for drug delivery out of materials that are themselves toxic, but even materials that seem relatively inert may pose a health risk if present in the blood in the form of nanosized particles. This concern is raised by research on the health risks of particulate air pollution. Ultra-fine particles that are inhaled don't just cause problems in the lungs; they also cross into the bloodstream, and there is substantial evidence linking exposure to nano-sized particles in the air with atherosclerosis or “hardening of the arteries.” Presumably the particles can become lodged in atherosclerotic plaques that are forming on the walls of blood vessels and exacerbate the inflammation that is part of the disease process. This problem may be avoided by choosing materials for nanoparticle construction that are more “familiar” to the body—biologically derived polypeptides, lipids, and polysaccharides—or by altering the surface characteristics of the particles. Particles with negatively charged surfaces cause fewer problems than positively charged particles, for example.

  Little is known about potential long-term health effects of using nanoparticle drug carriers. For a person receiving a single small dose, the risk may be minimal or may be outweighed by the benefits. For large doses and long-term treatment of chronic conditions, however, it becomes increasingly important to know how the nanoparticles will be cleared from the body. Particles that are less than 30 nm in diameter can be cleared by the kidneys, passing from the blood into the urine. The issue then remains of what impact the excreted nanoparticles might have in the environment. Larger particles will remain in the body unless they are broken down. Not surprisingly, current research runs heavily toward biodegradable nanoparticles. In many cases the particles are designed to be digested by the cells that take them up, and this may be planned as part of the mechanism of drug release. The trade-off for drug carriers that are too biologically “fragile” is that they may break down at inappropriate times and places in the body, causing toxicity or reducing their effectiveness. Despite these lingering issues, the great potential of nanoparticle drug delivery systems to improve drug effectiveness relative to drug toxicity means that research on these systems will surely continue, and in the future we are likely to see more use of nanoparticles in clinical applications.

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  Bibliography

  Amir H Faraji and Peter Wipf. “Nanoparticles in cellular drug delivery.” (review) Bioorganic and Medicinal Chemistry 17: 2950-2962, 2009

  Wim H De Jong and Paul JA Borm. “Drug delivery and nanoparticles: Applications and hazards.” (review) International Journal of Nanomedicine 3 (2): 133-149, 2008

  Kwongjae Cho, Xu Wang, Shuming Nie, Zhou (Georgia) Chen, and Dong M. Shin. “Therapeutic Nanoparticles for Drug Delivery in Cancer.” (review) Clinical Cancer Research 14 (5): 1310-1316, 2008

  Yogeshkumar Malam, Marilena Loizidou, and Alexander M Seifalian. “Liposomes and nanoparticles: nanosized vehicles for drug delivery in cancer.” (review) Trends in Pharmacological Sciences 30 (11): 592-599, 2009

  Stephen J. Russell and Kah-Whye Peng. “Viruses as anticancer drugs.” (Review) Trends in Pharmacological Sciences 28 (7): 326-333, 2007

  Andrew K. Udit, Christian P.R. Hackenberger, and Mary K. O'Reilly. “Chemically Tailored Multivalent Virus Platforms: From Drug Delivery to Catalysis.” (highlight) ChemBioChem 11: 481-484, 2010

  Anupa R. Menjoge, Rangaramanujam M. Kannan, and Donald A. Tomalia. “Dendrimer-based drug and imaging conjugates: design considerations for nanomedical applications.” (review) Drug Discovery Today 15(5/6):171- 185, 2010

  M. Talelli, C.J.F. Rijcken, C.F. van Nostrum, G. Storm, and W.E. Hennink. “Micelles based on HPMA copolymers.” Advanced Drug Delivery Reviews 62: 231-239, 2010

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  About the Author:

  Carol Wuenschell o
riginally set out to be a biological illustrator with a B.A. degree in biology and art (double major) from Occidental College and an M.A. in biological illustration from Cal. State University, Long Beach. Before even finishing the Master's, however, she had decided she wanted to be a scientist and went on to earn a Ph.D. in biology from UCLA. After postdoctoral work at Caltech and USC, she joined the faculty of the USC School of Dentistry. There she maintained an independent laboratory research program for seven years, working on nicotine receptors and the effects of nicotine exposure on fetal lung development in mice. While at USCSD, she also helped teach basic biomedical sciences to dental students by serving as a “facilitator” of small-group learning in the Dental School's problem-based learning program. Currently Dr. Wuenschell is employed as a scientific writer at the City of Hope National Medical Center in Duarte, California.

  Copyright © 2011 Carol Wuenschell

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  Novelette: CITIZEN-ASTRONAUT

  by David D. Levine

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  Illustrated by Laurie Harden

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  Is honesty always the best policy? Think twice before you answer. . . .

  I was trying to fix my kitchen garbage disposal when my phone trilled. I put it on speaker. “Gary Shu,” I said, wiping my hands on a rag.