INTRODUCTION:

Buckminsterfullerene (C₆₀) was named after Richard Buckminster Fuller, a noted architectural modeler who popularized the geodesic dome. C60 is a molecule that consists of 60 carbon atoms, arranged as 12 pentagons and 20 hexagons. Fullerene molecules are composed entirely of carbon, in form of a hollow sphere, ellipsoid or tube. Spherical fullerenes are also referred to as bucky balls. An important property of C60 molecule is its high symmetry. There are 120 symmetrical operations, like rotation around the axis and reflection in a plane, which map the molecule onto itself. This makes C60 the most symmetrical molecule (Taylor et al1990). The C60 fullerene surface contains 20 hexagons and 12 pentagons. All the rings are fused and all double bonds are conjugated. In spite of their extreme conjugation, they behave chemically and physically as electron-deficient alkenes rather than electron rich aromatic systems (Fowler and Ceulemans 1995).

The unique physical and chemical properties of these new forms of carbon led many scientists to predict several technological applications. However, the difficult processibility of fullerenes has presented a major problem in hectic search for medicinal applications. C60 areinsoluble in aqueous media and aggregate very easily (Prato 1997).

Types of fullerance

Since the discovery of fullerenes in 1985, structural variations on fullerene have evolved well beyond the individual clusters themselves. Examples include

§ Buckyball clusters: smallest member is C₂₀ (unsaturated version of dodecahedrane) and the most common is C₆₀.

§ Nanotubes: hollow tubes of very small dimensions, having single or multiple walls; potential applications in electronics industry.

§ Megatubes: larger in diameter than nanotubes and prepared with walls of different thickness; potentially used for the transport of a variety of molecules of different sizes.

§ Polymers: chain, two-dimensional and three-dimensional polymers are formed under high-pressure high-temperature conditions; single-strand polymers are formed using the Atom Transfer Radical Addition Polymerization (ATRAP) route.

§ Nano"onions": spherical particles based on multiple carbon layers surrounding a buckyball core; proposed for lubricants.

§ Linked "ball-and-chain" dimers: two buckyballs linked by a carbon chain.

§ Fullerene rings.

The discovery of C60 has stimulated a large activity in chemistry. It opened up the new branch of Fullerene-Chemistry which studies the new families of molecules that are based on Fullerenes. By 1997 about 9000 Fullerene compounds were known.C60 molecules condense to form a solid of weakly bound molecules. This crystalline state is a new form of solid carbon, besides the long known diamond and graphite. It is called Fullerite. Much of the work in physics is centered on the solid phases of C60.

Application of C₆₀

Antiviral activity

Compounds with antiviral activity are generally of great medical interest and different modes of pharmaceutical actions have been described. Replication of the human immunodeficiency virus (HIV) can be suppressed by several antiviral compounds, which are effective in preventing or delaying the onset of acquired immunodeficiency syndrome(AIDS). Fullerenes (C60) and their derivatives have potential antiviral activity, which has strong implications on the treatment of HIV-infection. The antiviral activity of fullerene derivatives is based on several biological properties including their unique molecular architecture and antioxidant activity .It has been shown that fullerenes derivatives can inhibit and make complex with HIV protease (HIV-P) (Friedman et al1993; Sijbesma et al 1993). Dendrofullerene 1 has shown the highest anti-protease activity (Brettreich and Hirsch 1998; Schuster et al 2000).

Fulleropyrrolidines with two ammonium groups have been found active against HIV-1 and HIV-2 (Marchesan et al 2005).The relative positions of side chains on fullerenes have a strong influence on antiviral activity. A series of fullerene derivatives have been synthesized to elucidate the structural parameters that affect antiviral activity of fullerenes. The results reveal that trans fullerenes derivatives are more active than cis- counter parts whereas the equatorial one is totally inactive. Fullerenes C60 derivatized with two or more solubilizing side chains have also been active, when tested in lymphocyte CEM cell cultures infected with HIV-1 and HIV-2 (Bosiet al 2003).

Amino acid derivatives of fullerene C60 (ADF) are found to inhibit HIV and human cytomegalovirus replication (Kotelnikova et al 2003). The mechanism is based on penetration of ADF carrying bivalent metal ions through lipid bilayer of liposomes, insertion to the hydrophobic domains of proteins and changing their functions of membrane bound enzymes. The observation that fullerenes and C60 derivatives are not immunogenetic further supports their potential as pharmaceutical compounds. On the other hand, water-insoluble fullerene (C60) derivatives have antiviral activity against enveloped viruses.

Fullerenes as photosensitizers

Another potential medical application of C60 is related to the photo excitation of fullerenes. In fact, fullerene can be excited from ground state by photo irradiation. In presence of molecular oxygen, the fullerene can decay from its triplet to ground state, transferringits energy to O2, generating singlet oxygen, known to be highly cytotoxic species. In addition, the high-energy species1C60 and 3C60 are excellent acceptors and in the presence of a donor, can undergo a different process, being easily reduced toC60 by electron transfer. Again, in the presence of oxygen, the fullerene radical anion can transfer one electron, producing a superoxide anion radical O2•- and hydroxyl radical •OH (Yamakoshi et al 2003). The excited fullerene can be reducedunder biological conditions in the presence of biological reducing agents eg, guanosin. On the other hand, singlet oxygen and superoxide radical anions are well known reactive species towards DNA (Da Ros et al 2001). This property offullerenes renders them potential photo sensitizers for their use in photodynamic therapy (PDT).

Many fullerene conjugates with different functional groups possessing biological affinity to nucleic acids or proteins, are being investigated for anticancer activity. In particular, conjugates of C60 andacridine or complementary oligo nucleotide, which interact with nucleic acids, have been synthesized with the objective of increasing cytotoxicity (An et al 1996; Yamakoshi et al1996).

Antioxidant activity

Results published in 1999 have shown that fullerenes have a potential as biological antioxidants. The antioxidant property is based on the fact that fullerenes possess large amount of conjugated double bonds and low lying lowest unoccupied molecular orbital (LUMO) which can easily take up an electron, making an attack of radical species highly possible. Fullerene can react with many superoxides without being consumed. Due to this feature fullerenes are considered to be the world’s most efficient radical scavenger and are described as radical sponges (Krusic et al 1991). The major advantage of using fullerenes as medical antioxidant is their ability to localize within the cell to mitochondria and other cell compartment sites, where in diseased states, the production of free radical stakes place.

When fullerene is derivatized with polar groups, as in case of polyhydroxylated fullerenes (fullerenol) and C₆₀tris (malonic) acid, they become water soluble enabling them to cross the cell membrane and localize preferentially to mitochondria (Foley et al 2002; Youle and Karbowski 2005),which generate great masses of cellular oxygen free radicals. This phenomenon makes them useful for a variety of medical applications (Tsai et al 1997; Lotharius et al 1999; Bisagliaet al 2000). These radical scavengers have shown to protect cell growth from various toxins that can induce apoptotic injuries in vitro (Lin et al 1999; Lin et al 2002; Chen et al2004) in different cell types such as neuronal cells (Duganet al 1997; Bisaglia et al 2000), hepatoma cells (Huang et al1998), or epithelial cells (Straface et al 1999).Apoptosis is of critical importance for variety of physiological and pathological phenomenon which led numerous scientists to design experiments in this regard. Fullerenes are also used for cytoprotective action against UVA irradiation (Xiao et al 2006). The ultraviolet A radiation (320–400nm) generates reactive oxygen species, which have a biological effect on human skin cells, leading to cell damage or cell death.

Fullerenes in drug and gene delivery

The direct delivery of drugs and biomolecules through cell membrane into cells has attained increasing attention and has put a main focus on the development of efficient and safe carriers to transport genes or drugs. Transport of any compound into the nucleus of an intact cell is a major challenge, as transfer is limited by at least three membrane barriers which are the cell membrane, the endosomal membrane and the nuclear membrane. Hence it is important to fully underst and the mechanism through which carriers enter cells. There are four major groups of drug and gene carriers which are organic cationic compounds, viral carriers, recombinant proteins and inorganic nanoparticles (Azzam and Domb 2004; Xu et al 2005). A large number of nanoparticles can be potentially used as carriers for the cellular delivery because of their versatile properties, including good biocompatibility, selective targeted delivery and controlled release of carried drugs. Fullerenes belong to the class of inorganic nanoparticles and show wide availability due to their small size (~ 1 nm) and biological activity. The activities of this allotropic form of carbon rest upon the properties of both, the fullerene core and its chemical modification. The fullerene core is very hydrophobic, while the functional groups attached to the core add further complexity to the behaviour of fullerene molecule. By attaching hydrophilic moieties, fullerenes become water-soluble and are capable of carrying drugs and genes for the cellular delivery. Derivatized fullerene can cross the cell membrane and bind to themitochondria as demonstrated by Foley et al (2002).

A lipophilic slow-release drug delivery system which employs fullerene derivatives to enhance therapeutic efficacy in tissue culture was designed by Zakharian et al (2005). Modified fullerenes have the potential to provide such a lipophilic slow-release system and is comprised of significant anticancer activity in cell culture as demonstrated withC60-paclitaxel conjugate. Furthermore the ability of fullerenes to penetrate through intact skin is widening their application in cellular drug and gene delivery (Ryman-Rasmussen et al2006). A fullerene-based peptide was synthesized by Rouseet al and its ability to penetrate through flexed and unflexed skin was observed (Rouse et al 2007).

Diagnostic application

Several studies have already shown that fullerene cages are relatively non-toxic and resistant to body metabolism (Moussaet al 1997; Chen et al 1998). Bio distribution studies with water soluble derivatives of C60 demonstrate that these compounds are primarily localised in the liver and their clearance is very slow (Moussa et al 1997). Endofullerenes can be applied as magnetic resonance imaging contrast agent MRI, X-ray imaging agent and radiopharmaceuticals. Fullerol, which is highly water soluble, was chosen for radio labeling. It was found that holmium metallofullerol molecules could significantly accumulate in liver; moreover, they could be detected in the bone. The localization of the metallofullerolin bone can bring an important conclusion that these species are selectively targeted to tissues rich in macrophages and might be useful chemotherapeutic agent for treatment of leukemia and bone cancer (Thrash et al 1999).

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Received on 22.12.2011 Accepted on 10.03.2012

Asian J. Pharm. Res. 2(2): April-June 2012; Page 47-50