Medicinal applications of fullerenes.
Fullerenes have attracted considerable attention in different fields of science since their discovery in 1985. Investigations of physical, chemical and biological properties of fullerenes have yielded promising information. It is inferred that size, hydrophobicity, three-dimensionality and electronic configurations make them an appealing subject in medicinal chemistry. Their unique carbon cage structure coupled with immense scope for derivatization make them a potential therapeutic agent. The study of biological applications has attracted increasing attention despite the low solubility of carbon spheres in physiological media.
The fullerene family, and especially C60, has appealing photo, electrochemical and physical properties, which can be exploited in various medical fields. Fullerene is able to fit inside the hydrophobic cavity of HIV proteases, inhibiting the access of substrates to the catalytic site of enzyme. It can be used as radical scavenger and antioxidant. At the same time, if exposed to light, fullerene can produce singlet oxygen in high quantum yields. This action, together with direct electron transfer from excited state of fullerene and DNA bases, can be used to cleave DNA. In addition, fullerenes have been used as a carrier for gene and drug delivery systems. Also they are used for serum protein profiling as MELDI material for biomarker discovery. In this review we report the aspects of medicinal applications of fullerenes.
An important area of research in modern material nanoscience concerns carbon-based materials, among which fullerenes take one of the first places. Since their first detection and bulk production, they have gained a prime role on scientific scene, reaching the climax when 1996 Nobel Prize for Chemistry was awarded to Kroto, Curl and Smalley for their seminal discovery (Kroto et al 1985). Fullerene, the most abundant representative of the fullerene family was produced for the first time on a preparative scale in 1990, by resistive heating of graphite (Kraetschmer et al 1990). Fullerene molecules are composed entirely of carbon, in form of a hollow sphere, ellipsoid or tube. Spherical fullerenes are also referred to as buckyballs. 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 al 1990). The C60 fullerene surface contains 20 hexagons and 12 pentagons. All the rings are fused; all the 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 are insoluble in aqueous media and aggregate very easily (Prato 1997). There have been several attempts to overcome the natural repulsion of fullerenes for water. The most widely used methodologies are:
(a) Encapsulation or micro-encapsulation in special carriers like cyclodextrins (Youle and Karbowski 2005), calixarenes (Shinkai and Ikeda 1998), polyvinylpyrrolidone (Yamakoshi et al 1994), micelles and liposomes (Bensasson et al 1994). In addition, the combination of fullerenes and lipid membranes has led to very interesting results. Lipid bilayers are dynamically mobile structures, partially ordered and of biopharmaceutical interest for covering biocompatible surfaces or for the controlled release of drugs (Hetzer et al 1997);
(b) Suspension with the help of co-solvents by saturating fullerenes in benzene solutions poured into THF. The resulting mixture is added dropwise to acetone, and then water is slowly added. A yellow suspension is formed and solvents are evaporated to a final known volume of water (Scrivens et al 1994);
(c) Chemical functionalization to increase the hydrophilicity eg, with amino acid, carboxylic acid, polyhydroxyl group, amphiphilic polymers etc (Hirsch et al 1994; Bianco et al 1996; Brettreich and Hirsch 1998; Chen et al 2001).
The current list of fullerene derivatives covers practically all known classes of chemical compounds, demonstrating both high chemical activity and a broad versatility of chemical reactions. This outstanding chemical appearance generates great interest for their practical applications in creating novel materials for medical use. In this review we are reporting the medical applications of fullerenes, including, antiviral activity, antioxidant activity and their use in drug delivery. In addition, the powerful photoinduced biological activities as a potential scaffold for photodynamic therapy and diagnostic applications are highlighted.
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 al 1993; Sijbesma et al 1993). Dendrofullerene 1 (Figure 1) has shown the highest anti-protease activity (Brettreich and Hirsch 1998; Schuster et al 2000). Derivative 2, the trans-2 isomer (Figure 1), is a strong inhibitor of HIV-1 replication. The study suggests that relative position (trans-2) of substituents on fullerenes and positive charges near to fullerenes cage provide an antiviral structural activity.
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