Analog SFF, June 2011 (14 page)

Fullerenes (Figure 1)

Fullerenes are molecules composed entirely of carbon atoms covalently linked in lattices of six- and five-membered rings. They are structurally related to graphite, which consists of flat sheets of six-membered carbon rings. A carbon nanotube is formed by rolling such a sheet into a tube and connecting the edges to form a continuous cylinder. Typically these tubes are several mm long and 0.7-1.5 nm in diameter. The other fullerenes are spherical or spheroidal molecules that are formed when some of the six-membered rings are replaced by five-membered rings. The smallest spherical fullerene is C60, the classic buckminsterfullerene, consisting of twenty six-membered and twelve five-membered rings arranged in a very specific pattern that resembles a soccer ball or the geodesic domes created by Richard Buckminster Fuller. While fullerenes can be synthesized with difficulty in the laboratory, they are easily produced by burning simple hydrocarbons in a fuel-rich flame. The space inside open-ended carbon nanotubes is generally too small to accommodate drug molecules, and the cavities inside the completely closed fullerenes are not accessible. Therefore, drug molecules would have to be bound to the outside of these structures. The carbon atoms of which they are composed readily accept such chemical modification, which is fortunate considering that fullerenes are completely insoluble in all solvents and are only made water-soluble through surface modification. Toxicity is a potential problem for fullerenes, although the data are ambiguous. Some studies have shown inflammatory effects or oxidative stress in cultured cells exposed to some types of fullerenes. Platelet aggregation has also been seen, which could lead to blood clots if it occurred inside a human body.

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.

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.

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.

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.

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.

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 orally 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.