METABOLICALLY CONVERTIBLE LIPOPHILIC DERIVATIVES OF PH-SENSITIVE AMPHIPATHIC PHOTOSENSITIZERS

We propose the use of acetoxymethyl eaters of pH-sensitive amphipathic photosensitizers (PS) for photodynamic therapy (PDT). These compounds may be applicable for PDT involving endocytosis of lipophilic carrier s leading to lysosomal uptake of the esterified PS by target cells. Pa rtial and/or total enzymatic de-esterification may result in the extra lysosomal distribution of the photoactive agents, possibly culminating in a multisite photochemical response. We report here the synthesis a nd properties of chlorin e(6) triacetoxymethyl ester (CAME) and pheoph orbide a acetoxymethyl ester (PAME). Chlorin e(6) and pheophorbide a a re photocytotoxic chlorins that possess free carboxylate groups and ex hibit optimum wavelengths of excitation substantially red shifted rela tive to hematoporphyrin derivative. Acetoxymethyl esterification of ch lorin e(6) and pheophorbide a was accomplished with bromomethyl acetat e. Highperformance liquid chromatography allowed for the purification of FAME, in 87% purity, and CAME, in 63% yield and 94% purity, as well as the detection of the presumed mono- and diesters of chlorin e(6) a s transient intermediates in the synthesis of CAME. The ultraviolet-vi sible absorption, fluorescence excitation and emission, NMR and mass s pectra of the chlorin e(6) triester are consistent with those expected for CAME. The pH-sensitive amphipathicity of pheophorbide a and chlor in e(6) but not CAME was demonstrated using a water/1-octanol partitio n assay. The production of pheophorbide a from FAME and the sequential formation of the di- and monoesters and free chlorin e(6) from CAME, by the action of lysosomal esterases obtained from cancer cells, demon strate the potential of cellular enzymes to convert the lipophilic est ers to pH-sensitive amphipathic PS. It is expected that the product of the esterases' action in the acidic lysosome will be hydrophobic and tend to diffuse into the organelle membrane. Contact with the neutral pH of the adjacent cytosol will result in conversion of the PS to a mo re hydrophilic anionic species, presumably allowing for it to diffuse into that compartment and partition throughout the lipophilic and aque ous compartments of the cell.