Chlorin-type photosensitizers photochemically derived from vinyl porphyrins

Characterization of the interaction of metal-protoporphyrins photosensitizers with β- lactoglobulin

We investigated the interaction of a series of metal-protoporphyrins (PPIXs) with bovine β- lactoglobulin (BLG) using a combination of optical spectroscopy and computational simulations. Unlike other studies, the simulations were not merely used to rationalize the experimental data but were employed to refine the experimental data itself. The study was carried out at two pH values, 5 and 9, where BLG is known to have different conformation dictated by the so-called Tanford transition which occurs near pH 7.5. The transition is postulated to regulate access to the interior binding cavity of the protein, thus the pH variation was used as a parameter to investigate whether PPIXs access the central cavity of BLG. The results of our study show that indeed binding increases significantly at alkaline pH, however, the increased affinity is not due to the accessibility of the central cavity. Instead, binding appears to be determined by the tendency of PPIXs to form large inhomogeneous aggregates at acidic pH which hinders interactions with proteins. The binding site determined through a combination of experimental and computational methods is located at the interface between two BLG monomers where the long α-helix segment of the protein face each other. This region is rich in positively charged Lys residues that interact with the propionic acid chains of the protoporphyrins. Establishing the modality of binding between protoporphyrins and BLG would have important consequences for the use of BLG:PPIX complexes in applications such as artificial photoreceptors, artificial metallo-enzymes, delivery of photosensitizers for phototherapy and even solar energy conversion.

Preclinical photodynamic therapy in Spain 2: Liposome vectorization of photosensitizers; Different strategies, different outcomes

Photodynamic therapy is an emerging modality of cancer treatment based on the use of photosensitizing drugs, which accumulate selectively in tumor cells. Exposure to visible light induces local cytotoxic effects that lead selectively to tumor cell death in the irradiated region, thereby minimizing the risk and extension of unwanted secondary effects. One of the goals sought in the development of photodynamic therapy drugs is the selective targeting of tumor cells. As a general trend, the indiscriminate delivery of drugs is being increasingly substituted by the selective delivery to pathological tissues which can be achieved by embedding them into transporters that actively recognize differential factors of tumor cells and tissues as compared to healthy ones. Likewise, the chemical modification of the photosensitizers is a valid strategy to change the subcellular localization of the drug. The use of liposomes as transporters for targeted delivery of drugs has attracted particular attention during the last two decades. After a period characterized by the skepticism expressed by certain scientists in the field of drug delivery, interest in liposomes was rejuvenated by the introduction of fresh ideas from membrane biophysics.

Spectroscopic investigations on the binding of the photosensitizer Chlorin p6 with amine-modified silica nanoparticles in aqueous media

Absorption and emission properties of the amphiphilic photosensitizer Chlorin p6 were investigated in aqueous medium in the presence of silica nanoparticles (SiNPs) having positively charged amino groups. The results of these studies reveal that the acid-base ionization equilibrium of Chlorin p6 in aqueous medium is significantly affected as a result of strong electrostatic binding between the negatively charged drug and SiNP. The spectroscopic signature of the drug bound to SiNPs suggests that the tri-anionic form of the drug remains bound to the positively charged SiNPs at pH 8.0. As the pH is progressively decreased the formation of hydrophobic aggregates is disrupted significantly due to the presence of electrostatic binding force, which competes with intermolecular hydrophobic forces. The interplay of hydrophobic and electrostatic forces in the drug-nanoparticle binding process might affect the relative uptake and photodynamic efficacy of the free drug and the drug-nanoparticle complex in cancer cells.

Photosensitization of skin fibroblasts and HeLa cells by three chlorin derivatives: Role of chemical structure and delivery vehicle

The chemical nature of the sensitizer and its selective uptake by malignant cells are decisive to choose an appropriate biocompatible carrier, able to preserve the photosensitizing characteristics of the dye. In this paper we demonstrate the photodynamic properties of three chlorins, derived from chlorophyll a, and the usefulness of liposomal carriers to design pharmaceutical formulations. The chlorins have been quantitatively incorporated into stable liposomes obtained from a mixture of L-alpha-palmitoyloleoylphosphatidylcholine and L-alpha-dioleoylphosphatidylserine in a 13.5:1.5 molar ratio (POPC/OOPS-liposomes). The chlorin uptake by skin fibroblasts increases steadily, reaching in all cases a plateau level dependent on both the chlorin structure and the vehicle employed. The photophysical properties of the three chlorins in THF are nearly identical and fulfill the requirements for a PDT photosensitizer. Incorporation of chlorins into liposomes induces important changes in their photophysics, but does not impair their cellular uptake or their cell photosensitization ability. In fact we observe in the cells the same photophysical behavior as in THF solution. Specifically, we demonstrate, by recording the near-IR phosphorescence of 1O2, that the chlorins are able to photosensitize the production of 1O2 in the cell membrane. The cell-photosensitization efficiency depended on the chlorin and cell line nature, the carrier, and the length of pre-incubation and post-irradiation periods. The high photodynamic activity of chlorin-loaded liposomes and the possibility to design liposomal carriers to achieve a specific target site favors this approach to obtain an eventual pharmaceutical formulation.

The effect of pH and surfactant on the aggregation behavior of chlorin p6: a fluorescence spectroscopic study

Steady state and time-resolved fluorescence properties of chlorin P6, a potential drug for photodynamic therapy, have been investigated as functions of pH. A decrease in pH of the medium has been shown to cause protonation of the ionizable carboxylic acid side chain, leading to an increase in hydrophobicity and consequent aggregation. The aggregates dissociate on further protonation. The dissociation is explained in terms of formation of cations and their mutual repulsion. A synchronous fluorescence spectroscopic study revealed the presence of two anionic forms in equilibrium at physiological pH, with a shift in the equilibrium on slight decrease in the pH. The anionic nature of chlorin P6 in aqueous solutions at physiological pH has been confirmed by complexation with surfactants. The nature of the charge on the headgroups of the surfactants has been found to govern the formation of chlorin-surfactant complexes.