Physical chemistry of porphyrins and their interactions with membranes: the importance of pH

Ultrasound Sonosensitizers for Tumor Sonodynamic Therapy and Imaging: A New Direction with Clinical Translation

With the rapid development of sonodynamic therapy (SDT), sonosensitizers have evolved from traditional treatments to comprehensive diagnostics and therapies. Sonosensitizers play a crucial role in the integration of ultrasound imaging (USI), X-ray computed tomography (CT), and magnetic resonance imaging (MRI) diagnostics while also playing a therapeutic role. This review was based on recent articles on multifunctional sonosensitizers that were used in SDT for the treatment of cancer and have the potential for clinical USI, CT, and MRI applications. Next, some of the shortcomings of the clinical examination and the results of sonosensitizers in animal imaging were described. Finally, this paper attempted to inform the future development of sonosensitizers in the field of integrative diagnostics and therapeutics and to point out current problems and prospects for their application.

The promising interplay between sonodynamic therapy and nanomedicine

Sonodynamic therapy (SDT) is a non-invasive approach for cancer treatment in which chemical compounds, named sonosensitizers, are activated by non-thermal ultrasound (US), able to deeply penetrate into the tissues. Despite increasing interest, the underlying mechanisms by which US triggers the sonosensitizer therapeutic activity are not yet clearly elucidate, slowing down SDT clinical application. In this review we will discuss the main mechanisms involved in SDT with particular attention to the sonosensitizers involved for each described mechanism, in order to highlight how much important are the physicochemical properties of the sonosensitizers and their cellular localization to predict their bioeffects. Moreover, we will also focus our attention on the pivotal role of nanomedicine providing the sonodynamic anticancer approach with the ability to shape US-responsive agents to enhance specific sonodynamic effects as the sonoluminescence-mediated anticancer effects. Indeed, SDT is one of the biomedical fields that has significantly improved in recent years due to the increased knowledge of nanosized materials. The shift of the nanosystem from a delivery system for a therapeutic agent to a therapeutic agent in itself represents a real breakthrough in the development of SDT. In doing so, we have also highlighted potential areas in this field, where substantial improvements may provide a valid SDT implementation as a cancer therapy.

Ultrasound-activated nanomaterials for sonodynamic cancer theranostics

Despite intensive efforts to develop diagnostic and therapeutic tools, the successful treatment of cancer is still hampered by the obscure boundary between cancerous cells and normal cells, recurrence of the cancer, and the development of drug resistance during chemotherapy. In recent years, sonodynamic therapy (SDT), employing therapeutic ultrasound with sonosensitizers, has attracted attention as a potentially promising approach for cancer therapy. This review describes the current understanding of the mechanisms and the preclinical and clinical efficacy of SDT-based applications in tumors, providing an insight into the therapeutic potential offered by SDT. The limitations and future directions of this emerging treatment are also discussed.

Potential-Induced Aggregation of Anionic Porphyrins at Liquid|Liquid Interfaces

The adsorption and self-aggregation of anionic porphyrins were studied at the polarized water|1,2-dichloroethane (DCE) interface by polarization-modulation total internal reflection fluorescence (PM-TIRF) spectroscopy. 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin diacid (H₄TPPS^2-) and protoporphyrin IX (H₂PP^2-) exhibited high surface activities at the interface. The selective excitation of interfacial species in PM-TIRF measurements elucidated the potential-induced aggregation mechanism of the porphyrins. The J-aggregates of H₄TPPS^2- were reversibly formed only at the water|DCE interface by applying appropriate potentials even when the porphyrins exist as monomers in the aqueous and organic solutions. In the H₂PP^2- system, the slow aggregation process was found in the negative potential region. The spectral characteristics and the signal phase of PM-TIRF indicated that the H₂PP^2- monomers were adsorbed with relatively standing orientation and that the long axis of the J-aggregates was nearly in plane of the interface. H₂PP^2- was also investigated at the biomimetic phospholipid-adsorbed water|DCE interface. The competitive adsorption of neutral glycerophospholipids effectively inhibited the potential-dependent adsorption and interfacial aggregation processes of H₂PP^2-. The results demonstrated that the aggregation state of the charged species can reversibly be controlled at liquid|liquid interfaces as a function of externally applied potential.

Photodamage in a mitochondrial membrane model modulated by the topology of cationic and anionic meso-tetrakis porphyrin free bases

The photodynamic effects of the cationic TMPyP (meso-tetrakis [N-methyl-4-pyridyl]porphyrin) and the anionic TPPS4 (meso-tetrakis[4-sulfonatophenyl]porphyrin) against PC/CL phosphatidylcholine/cardiolipin (85/15%) membranes were probed to address the influence of phorphyrin binding on lipid damage. Electronic absorption spectroscopy and zeta potential measurements demonstrated that only TMPyP binds to PC/CL large unilamellar vesicles (LUVs). The photodamage after irradiation with visible light was analyzed by dosages of lipid peroxides (LOOH) and thiobarbituric reactive substance and by a contrast phase image of the giant unilamellar vesicles (GUVs). Damage to LUVs and GUVs promoted by TMPyP and TPPS4 were qualitatively and quantitatively different. The cationic porphyrin promoted damage more extensive and faster. The increase in LOOH was higher in the presence of D2O, and was impaired by sodium azide and sorbic acid. The effect of D2O was higher for TPPS4 as the photosensitizer. The use of DCFH demonstrated that liposomes prevent the photobleaching of TMPyP. The results are consistent with a more stable TMPyP that generates long-lived singlet oxygen preferentially partitioned in the bilayer. Conversely, TPPS4 generates singlet oxygen in the bulk whose lifetime is increased in D2O. Therefore, the affinity of the porphyrin to the membrane modulates the rate, type and degree of lipid damage.

Sulfonated Phthalocyanines: Photophysical Properties, in vitro Cell Uptake and Structure-activity Relationships

Aluminium phthalocyanines sulfonated to a different degree ( AlPcS n) and consisting of various isomeric species were studied by spectroscopic techniques to determine their tendencies to form dimers and aggregates. These characteristics were compared with the cell-penetrating properties of the species, using the Ehrlich ascites mouse tumor cell line, to arrive at structure-activity relationships. AlPcS n preparations consisting of the least number of isomeric species exhibited the highest tendency to form dimers and aggregates, whereas the more complex preparations, consisting of many isomeric products, showed more consistent monomeric features in aqueous environments. Uptake in cells was shown to correlate well with the overall hydrophobicity of the preparation and inversely with its degree of aggregation in the extracellular environment. Among the purified, single isomeric AlPcS n the amphiphilic disulfonated AlPcS 2a , enriched in positional isomers featuring sulfonate groups on adjacent phthalic subunits, showed the best membrane-penetrating properties. Even higher cell uptake was observed for the AlPcS 2mix reflecting a combination of optimal lipophilicity and a low degree of aggregation. Similarly, in the case of AlPcS 4, the pure isomeric compound showed less cell uptake than the mixed isomeric preparation of similar hydrophobicity, reflecting the higher degree of aggregation invoked by its symmetrical structure. Our data indicate that mixed sulfonated phthalocyanine preparations may exert higher photodynamic efficacy in biological applications as compared to the pure isomeric constituents.

Mechanisms of porphyrinoid localization in tumors

The photosensitizing and pharmacokinetic properties of porphyrin-type compounds have been investigated for nearly a century. In the last decade, two porphyrin derivatives were approved in the U.S.A. and in several other countries for the photodynamic treatment of various lesions. An overview of the different mechanisms for preferential porphyrinoid localization in malignant tumors is presented herein. Several uptake pathways are possible for each photosensitizer, which are determined by its structure, mode of delivery and tumor type. Comparisons of the different mechanisms and correlations with the structure of the sensitizer are presented. Current delivery systems for porphyrin sensitizers are described, as well as recent strategies for enhancing their tumor-specificity, including conjugation to a carrier system that selectively targets a tumor-associated receptor or antigen.

Photobleaching kinetics of Verteporfin and Lemuteporfin in cells and optically trapped multilamellar vesicles using two-photon excitation

Verteporfin and Lemuteporfin are compared to examine the effect of their functional groups and therefore the localization in two-photon excitation (TPE) photodynamic therapy (PDT). We used singlet oxygen-related photobleaching of the sensitizers to assess TPE-induced singlet oxygen generation in multilamellar vesicles (MLVs) and U343 glioma cells under a variety of conditions. It was found that Lemuteporfin photobleached at a faster rate than Verteporfin in the majority of environments. Also, Verteporfin and Lemuteporfin exhibited different behaviors when in hypoxic environments relative to those in oxygenated MLVs. These differences are attributed to the sensitizer location in the membrane and their relative mobilities throughout membranes and cells.

Effect of liposomal confinement on photochemical properties of photosensitizers with varying hydrophilicity

Preferential tumor localization and the aggregation state of photosensitizers (PSs) can depend on the hydrophilic/hydrophobic nature of the molecule and affect their phototoxicity. In this study, three PSs of different hydrophilicity are introduced in liposomes to understand the structure-photochemistry relationship of PSs in this cellular model system. Absorbance and fluorescence spectra of amphiphilic aluminum (III) phthalocyanine disulfonate chloride adjacent isomer (Al-2), hydrophilic aluminum (III) phthalocyanine chloride tetrasulfonic acid (Al-4), and lipophilic 2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide (HPPH) are compared in a liposomal confined state with free PS in bulk solution. For fluorescence measurements, a broad range of concentrations of both bulk and liposomal confined PSs are examined to track the transition from monomers to dimers or higher order aggregates. Epifluorescence microscopy, absorbance, and fluorescence measurements all confirm different localization of the PSs in liposomes, depending on their hydrophilicity. In turn, the localization affects the aggregation of molecules inside the liposome cell model. Data obtained with such cellular models could be useful in optimizing the photochemical properties of photosensitizing drugs based on their structure-dependent interactions with cellular media and subcellular organelles.

Which physical and structural factors of liposome carriers control their drug-loading efficiency?

The correlation between structural and physical properties of lipid membrane and its drug-loading efficiency were studied. The properties of bilayer were altered by incorporation of several lipidic modifiers: cholesterol, oleic acid, methyl oleate, and pegylated lipid. By using the molecular probe technique it was demonstrated that the membrane properties, such as micropolarity, microviscosity and free volume were considerably changed by incorporation of the modifiers. The partitioning of two different porphyrins between the bulk aqueous phase and the modified liposomes was studied using the fluorescence methods, and liposome-binding constants were determined. It was found that cholesterol reduced the partitioning of both porphyrins into liposomal bilayer. On the contrary, the incorporation of methyl oleate and pegylated lipid causes a pronounced increase in the value of the binding constants of both porphyrins. It was concluded that the free volume rather than hydrophobicity of bilayer is a governing factor in the solute partitioning into lipid bilayers.

Effect of Sensitizer Protonation on Singlet Oxygen Production in Aqueous and Nonaqueous Media

The yield of singlet molecular oxygen, O2(a(1)Delta(g)), produced in a photosensitized process can be very susceptible to environmental perturbations. In the present study, protonation of photosensitizers whose chromophores contain amine functional groups is shown to adversely affect the singlet oxygen yield. Specifically, for bis(amino) phenylene vinylenes dissolved both in water and in toluene, addition of a protic acid to the solution alters properties of the system that, in turn, result in a decrease in the efficiency of singlet oxygen production. In light of previous studies on other molecules where protonation-dependent changes in the yield of photosensitized singlet oxygen production have been ascribed to changes in the quantum yield of the sensitizer triplet state, Phi(T), and to possible changes in the triplet state energy, E(T), our results demonstrate that this photosystem can respond to protonation in other ways. Although protonation-dependent changes in the amount of charge-transfer character in the sensitizer-oxygen complex may influence the singlet oxygen yield, it is likely that other processes also play a role. These include (a) protonation-dependent changes in sensitizer aggregation and (b) nonradiative channels for sensitizer deactivation that are enhanced as a consequence of the reversible protonation/deprotonation of the chromophore. The data obtained, although complicated, are relevant for understanding and ultimately controlling the behavior of photosensitizers in systems with microheterogeneous domains that have appreciable pH gradients. These data are particularly important given the use of such bi-basic chromophores as two-photon singlet oxygen sensitizers, with applications in spatially resolved singlet oxygen experiments (e.g., imaging experiments).

Pegylated tetraarylporphyrin entrapped in liposomal membranes

A system of poly(ethylene glycol) bound tetraarylporphyrin entrapped in liposomal membranes was investigated. The interactions between the 5-(4-hydroxymethylphenyl)-10,15,20-tritolylporphyrin (Po) covalently attached to the poly(ethylene glycol) chain (PEG-Po), and phosphatidylcholine liposomes in the aqueous solution were studied. The adsorption of the investigated polymer to lipid vesicles was confirmed by measurements of dynamic light scattering and zeta potential. Experimental results demonstrate that the diameter of liposomes increased and the absolute value of the zeta potential decreased after addition of PEG-Po. The binding constants (K(b)) of Po chromophores to liposome in pH range from 5.2 to 9.0 were determined using fluorescence spectroscopy. The degree of binding was found to be pH-independent and the average value was 24.6 +/- 0.9 mg ml(-1). The acid-base properties of the porphyrin chromophores and their aggregation in an aqueous solution were also studied. pK values associated with imine-N protonation of the porphyrin core were found to be 2.59 and 0.68 at the ionic strength of 0.1 M. The equilibrium constant for dimerization, K(D), was found to be 5 x 10(3) M(-1).

Enhanced Acidity, Photophysical Properties and Liposome Binding of Perfluoroalkylated Phthalocyanines Lacking C-H Bonds

The acid-base, spectroscopic, photophysical and liposome-binding properties of the recently synthesized free base, 29H,31H,1,4,8,11,15,18,22,25-octafluoro-2,3,9,10,16,17,23, 24-octakisperfluoro(isopropyl) phthalocyanine, F64PcH2, are reported. The perfluoroalkylation of the phthalocyanine core renders the hydrogen atoms acidic, with a pK(a) = 6. The F64Pc(-2) dianion is detected already at pH 3, by singular-value decomposition analysis of electronic spectra. F64Pc(-2) generates 1O2 with quantum yields phi(delta) = 0.252 (in MeOH) and 0.019 in liposomes. Metallation of the Pc macrocycle to yield F64PcZn increases phi(delta) to 0.606 and 0.126 in MeOH and liposomes, respectively. Surprisingly, F64Pc(-2) (but not F64PcH2 or F64PcZn) binds strongly to liposomes, with a binding constant K(b) = 25 (mg/mL)(-1). The fully protonated F64PcH2, but not the zwitterionic F64Pc(-2), might favor hydrogen bonding, thus reducing its lipophilicity. Similarly, the Lewis acidity of Zn in F64PcZn, and thus its ability to bind water within a hydrophobic perfluoroalkyl pocket, is significantly enhanced by the fluorinated substituents.

Pharmacokinetics and phototoxicity of purpurin-18 in human colon carcinoma cells using liposomes as delivery vehicles

Pharmacokinetics and phototoxicity of purpurin-18 (Pp18) in human colon carcinoma cells (Colo-205) was studied using liposomes as delivery vehicles. Cytotoxicity was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and neutral red uptake assay, and mode of cell death was assessed by the study of cell morphology and nuclear staining with Hoechst 33342-propidium iodide. Pp18 solubilized in dimethyl sulfoxide saline solution was observed to aggregate (Q-band absorption 740 nm), resulting in very poor cellular uptake. Pp18 incorporated in liposome remained in monomeric form (Q-band absorption 695 nm), but due to the presence of an anhydride ring in the molecule it readily yielded another photosensitizer, chlorin p6 (Q-band absorption 662 nm). Measurements at various pH showed that Pp18 in liposome was stable at acidic pH (6.5). Incubation of cells with 6.0 microM Pp18 in liposome at pH 6.5 showed a rapid cellular uptake. Spectrofluorometric measurements showed the presence of both Pp18 and chlorin p6, indicating conversion of some amount of Pp18 into chlorin p6 in the cells. Fluorescence microscopy revealed that the fluorescence was localized mainly in the cytoplasm, sparing the nucleus. Illumination of cells to white light after 4-h incubation with Pp18 liposome preparation was observed to lead to dose-dependent decrease in cell viability. At low irradiation time, cells displayed formation of plasma membrane blebs and micronuclei typical of apoptotic cell death. In contrast, at higher irradiation time, cell swelling and vacuolization in nucleus was observed, suggesting cell death due to necrosis. Irradiation with narrow bandwidth light showed that at low pH, the relative phototoxicity due to pp18 was higher than that due to chlorin p6. It is suggested that the pH-dependent conversion of pp18 to chlorin p6 can be exploited to increase PDT selectivity.

Effects of photodynamic therapy on tumor stroma

Photodynamic therapy (PDT) is a treatment that combines a photosensitizer with light to generate oxygen-dependent photochemical destruction of diseased tissue. This modality has been approved worldwide since 1993 for the treatment of several oncological and nononcological disorders. PDT continues to be interested in both preclinical and clinical research, with more than 500 publications each year during the past 5 years. This minireview focuses on the effects of PDT on tumor stroma. A tumor consists of two fundamental elements: parenchyma (neoplastic cells) and stroma. The stroma is composed of vasculature, cellular components, and intercellular matrix and is necessary for tumor growth. All the stromal components can be targeted by PDT. Although the exact mechanism of PDT is unknown, emerging evidence has indicated that effective PDT of tumor requires destruction of both parenchyma and stroma. Further, damage to subendothelial zone of vasculature, in addition to endothelium, also appears to be a crucial factor. The PDT-generated immune response as a way of vaccination for treatment and prevention of metastatic tumors remains to be exploited.

Interactions of Porphyrin Covalently Attached to Poly(methacrylic acid) with Liposomal Membranes

The interactions between the 5-(4-acryloyloxyphenyl)-10,15,20-tritolylporphyrin covalently attached to poly(methacrylic acid) chain (PMA-Po) and phosphatidylcholine liposomes in aqueous solution at different pH values were studied. The binding constants (K(b)) for the liposome- PMA-Po in solutions in the pH range from 6.5 to 9.2 were determined using fluorescence spectroscopy. The binding was found to be efficient. The acid-base properties of the porphyrin chromophores were also studied. Both pK values associated with imine-N protonation of the porphyrin core were found to be 6.4. The quantum yield (Phi(Delta)) of singlet oxygen production by Po in the lipid-PMA-Po system was found to be high (0.88 +/- 0.05).

Efficient synthesis and photodynamic activity of porphyrin-saccharide conjugates: targeting and incapacitating cancer cells

Since the role of saccharides in cell recognition, metabolism, and cell labeling is well-established, the conjugation of saccharides to drugs is an active area of research. Thus, one goal in the use of saccharide-drug conjugates is to impart a greater specificity toward a given cell type or other targets. Although widely used to treat some cancers and age related macular degeneration, the drugs used in photodynamic therapy (PDT) display poor chemical selectivity toward the intended targets, and uptake by cells most likely arises from passive, diffusional processes. Instead, the specific irradiation of the target tissues, and the formation of the toxic species in situ, are the primary factors that modulate the selectivity in the present mode of PDT. We report herein a two-step method to make nonhydrolyzable saccharide-porphyrin conjugates in high yields using a tetra(pentafluorophenyl)porphyrin and the thio derivative of the sugar. As a demonstration of their properties, the selective uptake (and/or binding) of these compounds to several cancer cell types was examined, followed by an investigation of their photodynamic properties. As expected, different malignant cell types take up one type of saccharide-porphyrin conjugate preferentially over others; for example, human breast cancer cells (MDA-MB-231) absorb a tetraglucose-porphyrin conjugate over the corresponding galactose derivative. Doseametric studies reveal that these saccharide-porphyrin conjugates exhibit varying PDT responses depending on drug concentration and irradiation energy. (1) Using 20 microM conjugate and greater irradiation energy induces cell death by necrosis. (2) When 10-20 microM conjugate and less irradiation energy are used, both necrosis and apoptosis are observed. (3) Using 10 microM and the least irradiation energy, a significant reduction in cell migration is observed, which indicates a reduction in aggressiveness of the cancer cells.

Dynamics of interactions of photosensitizers with lipoproteins and membrane-models: correlation with cellular incorporation and subcellular distribution

The incorporation and subcellular localization of photosensitizers are critical determinants of their efficiency. Here, we correlate these properties with the interactions of photosensitizers with membrane-models and low density lipoproteins (LDL) in acellular systems. Focus was given on dynamics aspects. Two amphiphilic photosensitizers, deuteroporphyrin (DP) and aluminum phthalocyanine sulfonated on two adjacent isoindole units (AlPcS2a) were selected. The phthalocyanine was bound to LDL with an overall association constant around 5 x 10(7)M(-1). Biphasic association kinetics was indicative of two types of sites. The release of the phthalocyanine into the bulk aqueous medium occurred within less than a second. A similar behavior was found previously for deuteroporphyrin although its affinity was somewhat higher (5.5 x 10(8)M(-1)). Both compounds were previously characterized by high affinity for membrane-models and quick exchange with the bulk solution. However, they strongly differed by their rate of transfer through the lipid bilayer, in the range of seconds for the porphyrin, several hours for the phthalocyanine. In the case of the porphyrin, fluorescence microscopy on human fibroblasts showed diffuse labeling with no significant modification of the distribution upon vectorization by LDL. In contrast, the phthalocyanine was localized in intracellular vesicles. Vectorization by LDL favored lysosomal localization although little effect was found on the overall uptake as shown by extraction experiments. The role of lipoproteins in the cellular localization of photosensitizers is significantly more important for photosensitizers not freely diffusing through bilayers. The dynamics of the interactions of photosensitizers with membranes appears as an important determinant of their subcellular localization.

Effect of aggregation on bacteriochlorin a triplet-state formation: a laser flash photolysis study

Bacteriochlorin a (BCA) is a potential photosensitizer for photodynamic therapy of cancer. It has been shown previously that the photoefficiency of the dye is mainly dependent on singlet oxygen (1O2) generation. Nanosecond laser flash photolysis was used to produce and to investigate the excited triplet state of the dye in methanol, phosphate buffer and dimiristoyl-L-alpha-phosphatidylcholine (DMPC) liposomes. The transients were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yield of singlet-triplet intercrossing. The lifetime of the BCA triplet state was measured at room temperature. The triplet-state quantum yield is quite high in methanol (0.7) and in DMPC (0.4) but only 0.095 in phosphate buffer. In the last case, BCA is in a monomer-dimer equilibrium, and the low value of the quantum yield observed was ascribed to the fact the triplet state is only formed by the monomers.

Interaction of dicarboxylic metalloporphyrins with liposomes. The effect of pH on membrane binding revisited

The acid-base properties of Zn-hematoporphyrin IX (ZnHP) and Zn-mesoporphyrin IX (ZnMP) and the effect of pH on their binding to liposomes have been studied. The ionization constants for the two carboxylate groups of ZnHP were calculated by principal component analysis and are 5.7 +/- 0.1 and 6.9 +/- 0.05. The neutral species and the mono- and dianionic forms all bind to liposomes, but a strong pH effect on the binding constant was observed for both the investigated compounds. We also observed a decrease in the binding of the two anionic species when the membranes carried a negative charge. These results indicate that the porphyrins partition into the membrane with their carboxylic moieties near the lipid-water interface so that their deprotonation, leading to a charged molecule, does not prevent the insertion of the tetrapyrrole ring into the lipid environment of neutral liposomes.

The influence of pH on charged porphyrins studied by fluorescence and photoacoustic spectroscopy

We have studied the influence of solvent acidity on the aggregation/protonation behaviour of two charged tetraphenylporphyrins: positively charged tetrakis(N,N,N-trimethylanilinium-4-yl)porphyrin (TAP) and negatively charged tetrakis(4-sulfonatophenyl)porphyrin (TPPS4). Spectroscopic measurements (absorption, fluorescence, excitation and photoacoustic) have been made to follow the radiative and nonradiative relaxation processes of excited dye molecules at various pH values. The ability of these porphyrins to exist in aggregated and protonated forms was also investigated. It has been shown that TPPS4 exists in at least three spectroscopic forms: monomer (M1), dication (D1) and aggregated dication (AD), whereas TAP exists in two forms: monomer (M2) and dication (D2). These forms are characterised by different absorption and fluorescence properties. The short wavelength forms of the dyes (M1 and M2) were assigned to the monomeric forms and D1 and D2 were assigned to the protonated forms of TPPS4 and TAP, respectively; AD was identified as the aggregate of the protonated form of TPPS4. Each form shows deactivation of its excited state by nonradiative pathways-particularly high thermal deactivation was observed for the aggregates of the protonated form. Energy transfer between the monomeric form and the dication form has been demonstrated and Förster radii have been estimated (R0 = 31 and 44 A for TAP and TPPS4, respectively). Since protonation and aggregation of photoreceptors can strongly affect their photosensitizing effects (e.g. cellular uptake, singlet oxygen production) the results presented here may be important in the study of the function of photosensitizers in tumour tissues because, as is already known, the microenvironment in cancerous tissue is more acidic than in healthy cells.

Increase of the photosensitizing efficiency of the Bacteriochlorin a by liposome-incorporation

To describe the action mechanisms of Bacteriochlorin a (BCA), a second generation photosensitizer, in phosphate buffer (PB) and in dimyristoyl phosphatidylcholine (DMPC) liposomes we carried out oxygen consumption and ESR measurements. In PB, where BCA was in a monomer-dimer equilibrium, our results suggested that the oxygen consumption was related to the BCA monomers concentration in solution. Incorporation of BCA in DMPC liposomes, by promoting the monomerization of BCA, increased 9-fold the oxygen consumption in comparison to the value in PB. The use of specific singlet oxygen quenchers (Azide and 9,10-Anthracenedipropionic acid) in ESR and oxygen consumption experiments allowed us to assert that BCA was mainly a type II sensitizer when it was incorporated in DMPC. Finally, the cell survival of WiDr cells after a PDT treatment was measured for cells incubated with BCA in cell culture medium and cells incubated with BCA in DMPC. Irrespective of the dye concentration, the cell survival was lower when liposomes were used. This effect could be the result of a better BCA monomerization and/or a different BCA uptake in cells.

Spectroscopic Probing of the Acid–Base Properties and Photosensitization of a Fluorinated Phthalocyanine in Organic Solutions and Liposomes¶

A perfluorinated derivative of phthalocyanine was synthesized as the free base, hexadeca-(2,2,2-trifluoroethoxy) phthalocyanine (H2F48Pc), and as a zinc complex, hexadeca-(2,2,2-trifluoroethoxy)-phthalocyaninatozinc (ZnF48Pc), and their spectroscopic and photochemical properties were studied. The absorption bands are shifted bathochromically relative to simple phthalocyanines, exhibiting the longest wavelength band near 735 nm (H2F48Pc) and 705 (ZnF48Pc). The solvatochromism of both compounds was modeled by Reichardt's ET(30) parameter and Kamlet, Abboud and Taft multiparameter approach. The former, simpler, model was found to be adequate. We found that H2F48Pc undergoes unique basic and acidic titrations in organic solvents. These titration processes are accompanied by spectral changes that are explained on the basis of the chromophore's symmetry. Singular value decomposition was employed to resolve the spectra into the contributions of the species at various stages of protonation and to obtain the equilibrium constants. Nuclear magnetic resonance spectra (1H, 19F and 13C) for the free base were obtained in a tetrahydrofurand8 solution. The carbon spectrum, taken as a function of temperature, provided evidence for the presence of a tautomerization process, which switches the two internal hydrogens between the four central nitrogen atoms. As far as we know, this is the first report of the measurement of the free energy of activation for such process (delta G = 10.6-11.4 kcal mol-1 between 217 and 330 K) for a phthalocyanine, in solution. Like most other phthalocyanines these two compounds also act as photosensitizers and as generators of singlet molecular oxygen. The absolute quantum yields (phi delta) for ZnF48Pc was 0.58 +/- 0.01 in benzene and 0.35 +/- 0.01 in lipid vesicles. H2F48Pc had lower yields, 0.16 and 0.005, respectively. Either protonation or deprotonation of the pyrrole nitrogens in H2F48Pc lowered the phi delta.

Meso-substituted cationic porphyrins of biological interest. Photophysical and physicochemical properties in solution and bound to liposomes

A series of cationic porphyrins with 1-4 positive charges are studied: mono(N-methyl-4-pyridyl)triphenylporphine chloride [Mono], cis(N-methyl-4-pyridyl)diphenylporphine chloride [Cis], tri(N-methyl-4-pyridyl)monophenylporphine chloride [Tri] and tetra(N-methyl-4-pyridyl)porphine chloride [Tetra]. Their photophysical properties are measured in small unilamellar vesicles and compared with those in homogeneous solution. Liposomes of L-alpha-dimyristoyl-phosphatidylcholine (100 nm diameter) and L-alpha-dipalmitoyl-phosphatidylcholine (50 nm diameter) in phosphate-buffered saline (pH = 7.4) or D2O 0.15 M NaCl were used. The effect of the medium microheterogeinity is discussed. The triplet quantum yields in liposomes for all the porphyrins are about 0.7, similar to the value obtained for Tetra in aqueous media. The singlet molecular oxygen quantum yields for the hydrophilic compounds Tri and Tetra are greater than those of the hydrophobic ones, Mono and Cis. Also, association constants (KL) of the dyes to liposomes and their localization within the membranes are determined from fluorescence and fluorescence polarization measurements, respectively. KL values are in the range of 10(4)-10(5) M-1 for all the compounds, indicating that hydrophobic and coulombic interactions between porphyrins and liposomes are responsible for the dye association. Fluorescence polarization experiments indicate that Mono and Cis can penetrate into the lipidic phase, and that Tri and Tetra are located near the polar heads of the lipidic molecules.

Fluorescence, absorption and electron spin resonance study of bacteriochlorin a incorporation into membrane models

Analysis of the bacteriochlorin a absorption spectra suggests the existence of a monomer-dimer equilibrium, particularly intense in phosphate buffer and favored by a decrease of the pH. The dye in methanolic solution is predominantly in monomeric form. Fluorescence and electron spin resonance nitroxide spin labeling measurements indicate that incorporation into the lipid phase of dimyristoyl-L-alpha-phosphatidylcholine liposomes induces dye monomerization. Moreover, the molecules are bound in the external surface of the vesicles and a complete incorporation is ensured by a lipid-to-dye ratio greater than 125.

Kinetics of the interactions of a dicarboxylic porphyrin with unilamellar lipidic vesicles: interplay between bilayer thickness and pH in rate control

The transfer of a dicarboxylic porphyrin from phosphatidylcholine fluid-phase unilamellar vesicles towards albumin is studied focusing on bilayer thickness and pH effects. The kinetics of this process yield the rate constants for the porphyrin flip-flop from the inner to the outer hemileaflet and its exit towards aqueous medium. Phospholipids with monounsaturated 14-22 carbon chains are used. Interplay between bilayer thickness and pH for the control of the rate constants is observed. This results in the amplification, at physiological pH, of the effect of membrane thickness on the flip-flop and exit rates as compared to pH 8.5 and 6.5. These data are explained by the degree of porphyrin burying within the bilayer resulting from a compromise between favorable hydrophobic interactions with the hydrocarbon phase and unfavorable penetration of the polar carboxylic chains. The balance between the two effects depends particularly on the neutralization of one carboxylic chain. Considering the bilayer hydrophobicity profile and the porphyrin size, the optimization of hydrophobic interactions appears dependent on the bilayer thickness. The flip-flop and the exit are governed by neutralization and deprotonation of the carboxylic chains, respectively, the rate of these proton exchanges being dependent on the porphyrin location within the bilayer.

Photodynamic therapy of ovarian tumours and normal cells using 5,10,15,20-tetra-(3-carboxymethoxyphenyl)-chlorin

The photosensitizing ability of the second generation photosensitizer 5,10,15,20-tetra-(3-carboxymethoxyphenyl)-chlorin (m-TCMPC), a derivative of m-THPC, was tested on both three-dimensional multicellular spheroids of varying sizes and on monolayer cultures. These experiments were carried out on two spheroid-forming cell lines, A2780 (a human ovarian adenocarcinoma) and CHO (Chinese hamster ovarian cells). For both cell lines, photodynamic therapy (sensitizer plus visible light) treatments were carried out. The chlorin m-TCMPC was shown to have considerable promise as a photosensitizing agent. Cell kill was achieved for all situations tested, i.e. monolayer, 100, 500 and 750 microm spheroids. In addition no significant dark toxicity was observed.

H-dependent formation of 5-aminolaevulinic acid-induced protoporphyrin IX in fibrosarcoma cells

pH-Dependent variations in the fluorescence intensity of 5-aminolaevulinic acid-induced 5-aminolaevulinic acid protoporphyrin IX (PP IX) were compared with the cell viability following light irradiation. The fluorescence intensity was determined by flow cytometry and the cell activity was investigated by a colorimetric 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay 24 h after photodynamic treatment. The results obtained by fluorescence measurements clearly showed that pH values of the incubation medium containing (5-ALA) below and above pH 7.4 led to a significant decrease in the fluorescence intensity. The viability of cells incubated with 0.6 mM 5-ALA in a medium at pH 6.0 was unaffected on exposure to light at lambda = 635 nm up to 15 J cm-2. However, cells incubated at pH 7.4 (with the other treatment parameters the same) were nearly completely destroyed. In addition, depletion of intracellular PPIX was faster in physiological medium than in acid medium.

Cross-resistance to photofrin-mediated photodynamic therapy and UV light and recovery from photodynamic therapy damage in Rif-8A mouse fibrosarcoma cells measured using viral capacity

Photodynamic therapy (PDT) utilizes the localized delivery of light to activate a photosensitizing drug (such as Photofrin) which is selectively retained by the tumour tissues. The intrinsic in vitro sensitivity of tumour cells to PDT is thought to be an important determinant of clinical tumour response to PDT. In this work we show the feasibility of using a viral capacity assay for adenovirus (Ad) DNA synthesis as an indicator of cellular sensitivity to and recovery from Photofrin-mediated PDT. Rif-1 mouse fibrosarcoma cells and a PDT resistant derivative, Rif-8A, as well as Chinese hamster ovary (CHO) cells and CHO-MDR multi-drug resistant mutant cells were studied. Consistent with the clonogenic survival of these cells, the capacity of PDT-treated cells for Ad DNA synthesis was greater for Rif-8A compared to Rif-1 cells and for CHO-MDR compared to CHO-N cells. Delaying infection of the Rif cells from immediately after, to 6 hours after PDT, resulted in an increased capacity for Ad DNA synthesis, which was greater for Rif-8A compared to Rif-1 cells, suggesting that the increased resistance of Rif-8A cells to PDT results from an elevated recovery and/or repair of PDT damage. The capacity of UV-irradiated cells for Ad DNA synthesis was also greater for Rif-8A compared to Rif-1 cells indicating a cross-resistance of Rif-8A cells to UV. These results suggest some overlap in the types of cellular damage induced by UV and PDT and/or overlap in the pathways for the repair of UV and PDT damage in Rif cells.

A fluorescence quenching study on protoporphyrin IX in a model membrane system

The interaction of protoporphyrin IX (3,7,12,15-tetramethyl-8, 13-divinyl-2,18-porphyrine-dipropionic acid) (PPIX) with unilamellar dimyristoyl-L-alpha-phosphatidylcholine (DMPC) phospholipid vesicles has been studied by means of steady-state fluorescence quenching spectroscopy. The method of fluorescence-quenching-resolved spectroscopy has been applied in order to resolve the complex emission spectrum of a membrane-bound PPIX into two component spectra, attributed to distinct fluorophore species with different accessibilities to the iodide quencher. It is shown that PPIX associated with liposomes exists in two different microenvironments. One part of the fluorophore is embedded inside the lipid bilayer and is inaccessible to iodide. Its fluorescence spectrum exhibits the maximum characteristic of protoporphyrin found in the apolar medium. The other fraction of PPIX is located near the membrane surface, close to the polar phospholipid heads. Its emission is blue-shifted, resembling that of PPIX in a polar environment. It is quenched by iodide, although it reveals significant shielding from the quencher as compared to a buffer PPIX solution. Fluorescence quenching using 1-oxyl-4-oxo-2,2,6,6-tetramethyl-piperidine (TEMPONE) does not discriminate between the two protoporphyrin species. However, the accessibility of protoporphyrin IX to this quencher is much lower in a liposome system than in water.