Chemical Modification of Chlorophylls for Photodynamic Therapy

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It was found that chlorophrylls decompose DNA under light irradiation. To use for photodynamic therapy, chlorophylls must be chemically modified to accumulate into cancer cells. However, chlorophylls are highly sensitive to heat, acid-base, and light, suffering from demetallation or isomerization, and therefore, it is difficult to modify the structure by ordinary chemical reactions. We attempted at hydrolysis and transesterification of chlorophylls by the catalysis of enzymes such as proteases or lipases. However, chlorophylls were only slightly soluble in water, and therefore use of organic solvents was necessary.Organic solvents strongly inhibited the activity of enzymes. The changes in structures of proteases were detected by measurements of CD spectrum, and it was found that inhibition of enzymes by organic solvents was mostly due to modification of secondary structures. Then, the effects of additives on activity and stability of proteases were investigated for transesterification, and it was found that colyophilization of enzymes with cyclodextrins dramatically increased the activity and stability of the enzymes.Photo-sensitizers, such as Photofrin, and eximer laser are currently used for photodynamic therapy. To develop more effective methods of photodynamic therapy, in vitro experiments were made using soditum salt of pheophorbide a as a sensitizer. Accumulation of the pheophorbide into cancer cells was measured using light absorption method. It was found that the pheophorbide was effectively incorporated into cancer cells. Maximum concentration was attained after 4 h. The results suggest that pheophorbides are potential candidate of light-sensitizer that are relatively stable.