Photochemical Internalization (PCI): A New Modality for Light Activation of Endocytosed Therapeuticals

Photochemical internalization (PCI) in cancer therapy: from bench towards bedside medicine

PDT in cancer therapy has been reviewed several times recently and many published reports have been showing promising results. The clinical approvals for PDT include curative treatment of early or superficial cancers and palliative treatment of more advanced disease. Still PDT has yet to become a widely used cancer treatment. This may partly be due to limitations in current PDT regimens and partly due to effective alternative treatment modalities. If the specificity and selectivity of PDT could be improved, PDT would probably make substantial progress and comprise an even more competitive alternative in cancer treatment. The PCI technology is based on the same principles as PDT, the activation of a photosensitizer by light and subsequently followed by formation of reactive oxygen species. Unlike PDT, the photosensitizer used in PCI has to be located in the endocytic vesicles of the targeted cells and will, upon activation of light, induce a release of endocytosed therapeutic agents after a photochemically induced rupture of the endocytic vesicles. The endocytosed therapeutic agent will then be released and may reach their intracellular target of action before being degraded in lysosomes. This site-specific drug delivery induced by PCI will take place in addition to the well described cytotoxic, vascular and immunostimulatory effects of PDT. PCI has been shown to facilitate intracellular delivery of a large variety of macromolecules that do not otherwise readily penetrate the plasma membrane, including type I ribosome-inactivating proteins (RIPs), RIP-based immunotoxins, genes and some chemotherapeutic agents. Several animal models have been used for in vivo documentation of the PCI principle and more animal models of clinical relevance have recently been utilized for addressing clinical issues. This review will focus on the possibilities and limitations offered by PCI to overcome some of the challenges recognized in current PDT regimens in cancer treatment.

Tetrapyrrole-photosensitizers vectorization and plasma LDL: A physico-chemical approach

A photosensitizer is defined as a chemical entity able to induce, under light-irradiation effect, a chemical or physical alteration of another chemical entity. Thanks to their preferential retention in proliferating tissues, some photosensitizers are therapeutically used such as in photodynamic therapy (PDT). Besides, this method has already been approved for several indications. The selectivity of photosenzitizers for cells in proliferation involves both their association with low density lipoproteins (LDLs) and their ability to cross membranes under various pH conditions. The photosensitizers used are in most cases based on the porphyrin structure, but other compounds, of which far-red-light absorption properties are most compatible with biological tissues irradiation, have been developed, such as phthalocyanines. This paper presents physico-chemical studies of the interaction of a disulfonated aluminium phthalocyanine (AlPcS2) with human LDLs. The data obtained are compared with the parameters of the interaction of these lipoproteins with deuteroporphyrin (DP) and chlorin e6 (Ce6). A close attention is paid to the dynamic aspects of these phenomena. The data obtained on these simple systems then allowed us to interpret the sub-cellular localization of the photosensitizers on a human line of fibroblasts, and to evaluate the influence of LDLs on the intracellular distribution of the compounds. This last point is of major importance because the localization of such photosensitizers (in particular AlPcS2) in endocytic vesicles and their subsequent ability to induce a release of the contents of these vesicles - including externally added macromolecules - into the cytosol is the basis for a recent method for macromolecule activation, named photochemical internalization (PCI). PCI has been shown to potentiate the biological activity of a large variety of macromolecules. The comprehension of the mechanisms governing this particular sub-cellular localization could allow the design of better candidates for PCI.