Research

　Cyclodextrins (CDs) have attracted attention as functional materials and are used in a variety of applications, including solubilization of poorly water-soluble drugs, improvement of stability, controlled release, improvement of bioavailability, and reduction of bitter taste, bad odor, and local irritation. The complexation with CDs generally enhances the solubility of drugs in water, slows down the diffusion rate of drug molecules due to an increase in molecular mass, and inhibits association of drugs due to a masking of the site that is associated with intermolecular interaction.

　Based on the host-guest interaction, we have focused on the use of CDs for additives which can modify the drug crystallization in solution and in solid state. Selective crystallization of a metastable form or a new polymorph was achieved by using the solution containing CDs. Modification of crystal growth was also induced by adding CDs in drug solutions. We are currently working to stabilize amorphous states and supersaturated solutions of drugs by combining different types of CDs or by using CDs in combination with additives.

　Hydrogels are composed of three dimensional networks of hydrophilic polymers in which the polymer chains are chemically or physically inter- or intramolecularly cross-linked. Physically linked hydrogels that can be prepared by inter- or intramolecular interaction such as hydrophobic or related interactions, hydrogen bonding and host–guest interactions, show sol-gel transitions in response to various external stimuli (pH, light, temperature and oxidizing/reducing agent, etc.). Hydrogels have been frequently used as drug or cell delivery vehicles.

　We recently developed a new strategy for preparing the thermoresponsive materials based on interactions between CD and hydrophobically modified polymers. The hydrogels are typically form sols at room temperature that have a low viscosity, but form gels when they are warmed to body temperature. The thermal response of the HM-HPMC/CD material can be tuned by selecting an appropriate CD and the concentrations of the components. A thermoresponsive injectable hydrogel, in which interactions between the CD and hydrophobically modified polymers determine its properties, promises to be useful as an advanced drug carrier for drugs including proteins.

　Nanocarbons such as fullerene, carbon nanotube and graphene have attracted much attention for practical applications that take advantage of their unique structure, physical properties, and biological activities. For example, fullerene C₆₀ is well known to function as a photosensitizer or an antioxidant. When carbon nanotubes or graphene is excited by near-infrared irradiation, they rise to the temperature high enough to kill cancer cells, suggesting that these nanocarbons could be used as photothermal agents for cancer therapy. Furthermore, these nanocarbons are explored for use in areas such as gene or drug delivery, bioimaging and other fields. However, the practical use of these nanocarbons for medical applications have been hampered by the fact that they are only sparingly soluble in water.

　We developed the hydrophilic C₆₀ nanoparticles, the surface of which were covered by CD, and evaluated their biological activities. C₆₀/CD nanoparticles were stable under physiological conditions and even in much harsher conditions. C₆₀ based nanoparticles represent a potentially promising material for use in the treatment of cancer and oxidative stress-related disease as well as having extensive biological applications.