PHOTOSENSITIZATION BY 3,3'-DIHEXYLOXACARBOCYANINE IODIDE - SPECIFIC DISRUPTION OF MICROTUBULES AND INACTIVATION OF ORGANELLE MOTILITY

Photodynamic therapy is a useful new direction for cancer treatment. H owever, relatively little is currently known about the cellular target s and processes underlying the efficacy of these therapies. In this st udy, we report evidence of specific photosensitization of a novel intr acellular target, cytoskeletal microtubules, that has great importance for cancer treatment. Photosensitization destroys microtubules, halts intracellular organelle motility processes, and leads to rapid cell d eath, We have examined the cell biological effects of photosensitizati on with the carbocyanine dye 3,3'-dihexyloxacarbocyanine iodide, which concentrates in mitochondria and the endoplasmic reticulum. Exposure of stained CV-1 kidney epithelial cells to as little as 30-120 s stand ard fluorescence excitation light caused disruption of the interphase microtubule network and complete inhibition of motility of the endopla smic reticulum and all phase-contrast visible organelles, as specific effects of dye photoexcitation. Photoexcitation of rhodamine 123 or Ho echst produced neither of these effects. Furthermore, 3,3'-dihexyloxac arbocyanine iodide-mediated photodamage was specific to microtubules; other elements of the cytoskeleton, including vimentin intermediate fi laments and actin stress fibers, were unaffected, We have reproduced t he photoinactivation of microtubules in vitro with purified microtubul e proteins.