New contrast agents for optical imaging: acid-cleavable conjugates of cyanine dyes with biomolecules

The investigation of cyanine dyes as contrast agents in optical tumor imagin has been a focus of our recent work. We have shown that i.v. injected hydrophilic indotricarbocyanine derivatives enable tumor detection by fluorescence imagin and by frequency-domain absorption spectroscopy. Our current objective is to extend this approach by conjugating these dyes with specific biomolecules in order to enhance targetability and to introduce acid-cleavable links that enable dye release in acidic cell compartments. Accordingly, we have synthesized cyanine dyes which contain different acid-cleavable hydrazone links and which were coupled to peptides, proteins and antibodies. We have studied the release of the dyes under various pH conditions. Our results show that dye release from transferrin increased under acidic conditions, while at neutral pH the stability was higher. Additionally, we observed pH-dependent fluorescence enhancement during cleavage. Cellular fluorescence microscopy experiments indicated that intracellular trapping is possible. In conclusion, cyanine dyes bound to biomolecules by acid- cleavable bonds could act as promising optical contrast agents. Further work will include optimization of release rates by chemical modification and in vivo imaging studies.