Spectroscopic and photoacoustic studies of hypericin embedded in liposomes as a photoreceptor model

Box-Behnken design of thermo-responsive nano-liposomes loaded with a platinum(iv) anticancer complex: evaluation of cytotoxicity and apoptotic pathways in triple negative breast cancer cells

Herein, thermo-responsive liposomes (TLs) loaded with Asp (Asp/TLs) were produced by self-assembling DPPC, DSPE-PEG2000, and cholesterol. The preparation variables were optimized using the Box-Behnken design (BBD). The optimized Asp/TLs exhibited an average particle size of 114.05 ± 1.56 nm, PDI of 0.15 ± 0.015, zeta potential of -15.24 ± 0.65 mV, and entrapment efficiency (EE%) of 84.08 ± 2.75%. In addition, under physiological conditions, Asp/TLs showed spherical shape, outstanding stability and thermo-triggered the release of Asp at 38 °C, reaching the maximum Asp release at 40 °C. The MTT assay showed that the optimal Asp/TLs exhibited the highest cytotoxic activity upon exposure to mild hyperthermia (40 °C) against the invasive triple-negative breast cancer cell line (MDA-MB-231) when compared to other preparations. The IC50 of Asp/TLs (40 °C) was estimated at 0.9 μg mL-1, while that of free Asp (40 °C) was 3.83 μg mL-1. As such, the optimal Asp/TLs were shown to increase the cytotoxic activity of Asp by 4-fold upon exposure to mild hyperthermia. The IC50 values of Asp and Asp/TLs without exposure to 40 °C were 6.6 μg mL-1 and 186 μg mL-1, respectively. This indicated that Asp was released only when placed at 40 °C. The apoptosis assay revealed that Asp/TLs (40 °C) caused a remarkable increase in the percentage of cell population among both the late apoptosis and necrosis quartiles, as well as a significant decline in the viable cell quartile (P ≤ 0.001) when compared to Asp (40 °C). Asp/TLs (40 °C) and Asp (40 °C) could stimulate the intrinsic apoptosis pathway by upregulating the apoptotic genes Bak and Bax, while downregulating the anti-apoptotic genes, BCL-xL and BCL-2. The free Asp (40 °C) increased the gene expression of Bak and Bax by 4.4- and 5.2-folds, while reducing the expression of BCL-xL and BCL-2 by 50% and 73%, respectively. The optimal Asp TLs (40 °C) manifested more potent effects as demonstrated by the upregulation of Bak, Bax, and P53 by 5.6-, 7.2-, and 1.3-folds, as well as the downregulation of BCL-xL and BCL-2 by 70% and 85%, respectively. As such, the optimal Asp TLs (40 °C) treatment displayed the most potent cytotoxic profile and induced both apoptosis and necrosis in MDA-MB-231.

Thermosensitive liposomes encapsulating hypericin: Characterization and photodynamic efficiency

The potent photodynamic properties of Hypericin (Hyp) elicit a range of light-dose-dependent anti-tumor activities. However, its low water solubility hampers its broad application. Therefore, the administration of Hyp into biological systems requires drug carriers that would enable sufficient bioavailability. Stimuli-triggered nanocarriers, which are sensitive to endogenous or exogenous stimuli, have become an attractive replacement for conventional therapeutic regimens. Herein, we produced optimized Hyp thermosensitive liposomes (Hyp-TSL), self-assembled from DPPC, DSPC, DSPE-PEG2000. Hyp-TSL displayed a hydrodynamic diameter below 100 nm with an adequate encapsulation efficiency of 94.5 % and good colloidal stability. Hyp-TSL exhibited thermal sensitivity over a narrow range with a phase transition temperature of 41.1 °C, in which liposomal destruction was evident in AFM images after elevated temperature above the phase transition temperature. The uptake of TSL-Hyp into MDA-MB-231 cells was significantly increased with hyperthermic treatment of 42 °C when compared to the uptake at a average physiological temperature of 37 °C. Consequent enhancement of cellular reactive oxygen species was observed after hyperthermic treatment at 42 °C. The half-maximal inhibitory concentration of Hyp TSL was reduced by 3.8 fold after hyperthermic treatment at 42 °C in comparison to treatment at 37 °C. Hyp-TSL were considered safe for intravenous applications as compared by hemocompatibility studies, where coagulation time was <50 s and hemolytic potential was <10%. Conclusively, the enhancement in tumor drug availability correlated with improved therapeutic outcomes.

Photophysical characterization of Hypericin-loaded in micellar, liposomal and copolymer-lipid nanostructures based F127 and DPPC liposomes

Hypericin (Hy) compound presents a high photoactivity in photodynamic therapy (PDT), photodiagnosis and theranostics applications. The maintenance of this compound in monomeric form could undermine the potential benefits of its photophysical and photodynamic activity. In this study, we demonstrated that the Hy formulated in a system based on the use of the F127 copolymer and the 1,2-dipalmitoyl-sn-3-glycerol-phosphatidylcholine (DPPC) as micelles, liposomal vesicles and Copolymer-Lipid coated systems, have improved its photophysical properties for many clinical modalities. Based on the results of the triplet state lifetime values (τ_t), the singlet oxygen quantum yield (Φ_Δ¹O₂), the fluorescence lifetime (τ_F) and the fluorescence quantum yield (Φ_F), all Hy formulations had its photophysical properties described in different models of drug delivery systems (DDS). In addition, the transient spectra profile of those formulations was unaffected by the Hy incorporation process, except for the liposomal system, which demonstrated to be the less stable one by flash photolysis technique. The cytotoxic effects of those formulations were also investigated for CaCo-2 and HaCat cells line. The cytotoxic concentrations for 50% (CC₅₀) were 0.56, 1.05, 1.33 and 4.80 µmol L^-1 for Copolymer-Lipid/Hy, DPPC/Hy, F127/Hy and ethanol/Hy for CaCo-2 cells, respectively, and 0.69, 2.02, 1.45 and 1.16 µmol L^-1 for Copolymer-Lipid/Hy, DPPC/Hy, F127/Hy and ethanol/Hy for HaCat cells, respectively. The F127 copolymer had a significant role in many photophysical parameters determined for Copolymer-Lipid/Hy coated system. Although all those formulations had shown satisfactory results, Copolymer-Lipid/Hy proved to be superior in many aspects, being the most promising formulation for PDT, photodiagnosis and theranostics applications.

Hypericin photodynamic activity in DPPC liposomes - part II: stability and application in melanoma B16-F10 cancer cells

Hypericin (Hyp) is considered a promising photosensitizer for Photodynamic Therapy (PDT), due to its high hydrophobicity, affinity for cell membranes, low toxicity and high photooxidation activity. In this study, Hyp photophysical properties and photodynamic activity against melanoma B16-F10 cells were optimized using DPPC liposomes (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) as a drug delivery system. This nanoparticle is used as a cell membrane biomimetic model and solubilizes hydrophobic drugs. Hyp oxygen singlet lifetime (τ) in DPPC was approximately two-fold larger than that in P-123 micelles (Pluronic™ surfactants), reflecting a more hydrophobic environment provided by the DPPC liposome. On the other hand, singlet oxygen quantum yield values (Φ_Δ¹O₂) in DPPC and P-123 were similar; Hyp molecules were preserved as monomers. The Hyp/DPPC liposome aqueous dispersion was stable during fluorescence emission and the liposome diameter remained stable for at least five days at 30 °C. However, the liposomes collapsed after the lyophilization/rehydration process, which was resolved by adding the lyoprotectant Trehalose to the liposome dispersion before lyophilization. Cell viability of the Hyp/DPPC formulation was assessed against healthy HaCat cells and high-metastatic melanoma B16-F10 cells. Hyp incorporated into the DPPC carrier presented a higher selectivity index than the Hyp sample previously solubilized in ethanol under the illumination effect. Moreover, the IC₅₀ was lower for Hyp in DPPC than for Hyp pre-solubilized in ethanol. These results indicate the potential of the formulation of Hyp/DPPC for future biomedical applications in PDT treatment.

Photocytotoxicity of hypocrellin B (HB) was enhanced by liposomalization in vitro

Hypocrellin B (HB) was encapsulated into a phosphatidylcholine liposome. Encapsulation of HB into liposomes not only improved the delivery of this photosensitizer but also increased its photodynamic efficacy compared to free HB molecules. Liposomal HB showed a higher cellular uptake than free HB as measured by confocal microscopy and was internalized into cultured HeLa cells by caveolar endocytosis, which was lipid-raft-dependent. Cell viability measurements demonstrated that liposomal HB was more phototoxic to HeLa cells than free HB as a result of the higher concentration of intracellular HB delivered by the liposomal formulation. The encapsulation of HB influenced the cell death pathway by an increased rate of necrotic cells after irradiation versus free HB, and a Type II (singlet oxygen) mechanism was responsible for the photocytotoxicity.

Kinetics of hypericin association with low-density lipoproteins

Steady-state and time-resolved fluorescence spectroscopy have been used for the study of the incorporation kinetics of hypericin (Hyp) into low-density lipoproteins (LDL). Biphasic kinetics of Hyp association with LDL was observed when solutions of Hyp and LDL were mixed at various concentration ratios. The rapid phase of Hyp incorporation is completed within seconds, while the slow phase lasts several minutes. The relative contributions of the individual phases show that a higher amount of Hyp molecules (65%) are incorporated into LDL in the second phase. The kinetics of the incorporation of Hyp into LDL particles preloaded with Hyp (Hyp/LDL=25:1) was also investigated. The decreased intensity of Hyp fluorescence is a sign of the formation of Hyp aggregates after penetration of additional Hyp molecules into Hyp/LDL=25:1 complex. The time dependence of Hyp fluorescence was measured after mixing the complex Hyp/LDL =200:1 with appropriate amounts of free LDL molecules. For each final Hyp/LDL ratio, an increase in the intensity and lifetime of Hyp fluorescence was observed, suggesting a monomerization of Hyp aggregates. The half-time of Hyp transfer from Hyp/LDL complex to LDL particles is similar to the half-time of the slow phase of Hyp incorporation into free LDL particles.

Accumulation and interaction of hypericin in low-density lipoprotein--a photophysical study

The accumulation and interaction of hypericin with the biologically important macromolecule, low-density lipoprotein (LDL), is investigated using various steady-state and time-resolved fluorescence measurements. It is concluded that multiple hypericins can penetrate considerably deeply into the LDL molecule. Up to approximately 20 nonaggregated hypericin molecules can enter LDL; but upon increasing the hypericin concentration, the fluorescence lifetime of hypericin decreases drastically, suggesting most likely the self-quenching of aggregated hypericin. There is also evidence of energy transfer from tryptophans of the constituent protein, apoB-100, to hypericin in LDL. The results demonstrate the ability of LDL to solubilize hypericin (a known photosensitizer) in nonaggregated form, which has implications for the construction of drug delivery systems.

Glutathione S-transferase P1-1 expression modulates sensitivity of human kidney 293 cells to photodynamic therapy with hypericin

Photodynamic therapy (PDT) relies on light-dependent, tissue-targeted, oxidative stress in tumors that have accumulated a photosensitizing drug. Glutathione S-transferases (GSTs) are often up-regulated in tumors and they modulate oxidative stress by several isoform-dependent mechanisms. GSTs, therefore, are potential confounding factors in PDT. Therefore, we examined this possibility in human kidney 293 cells transfected with a plasmid encoding either green fluorescent protein alone (pIRES-GFP) or both GFP and GSTP1-1 (pIRES-GFP-GSTP). Cells were cultured and treated with light alone, the sensitizer hypericin (HYP) alone, or light and HYP. Cells harboring pIRES-GFP-GSTP exhibited a modest 2-fold increase in GSTP1-1 expression over control cells. On the basis of flow cytometry and microscopy, the light-dependent toxicity of HYP was reduced in cells over-expressing GSTP1-1. Paradoxically, the decreased toxicity in the cells with GSTP1-1 over-expression occurred concomitantly with a modest approximately 2-fold increase in cellular uptake of the drug. Immunoprecipitation of HYP and Western analysis indicated that GSTP1-1 is a major intracellular-binding site for HYP. These results are the first to demonstrate GST expression as a confounding variable of photodynamic therapy. Further, a high-affinity GST inhibitor reversed the GSTP1-1-dependent resistance, suggesting the possible utility of pharmacological strategies to optimize PDT.

Elucidation of the tumoritropic principle of hypericin

Hypericin is a potent agent in the photodynamic therapy of cancers. To better understand its tumoritropic behaviour, we evaluated the major determinants of the accumulation and dispersion of hypericin in subcutaneously growing mouse tumours. A rapid exponential decay in tumour accumulation of hypericin as a function of tumour weight was observed for each of the six tumour models investigated, and a similar relationship was found between tumour blood flow and tumour weight. Moreover, there was a close correlation between the higher hypericin uptake in RIF-1 tumours compared to R1 tumours and tumour vessel permeability. To define the role of lipoproteins in the transport of hypericin through the interstitial space, we performed a visual and quantitative analysis of the colocalization of hypericin and DiOC18-labelled lipoproteins in microscopic fluorescent overlay images. A coupled dynamic behaviour was found early after injection (normalised fluorescence intensity differences were on the whole less than 10%), while a shifted pattern in localisation of hypericin and DiOC18 was seen after 24 h, suggesting that during its migration through the tumour mass, hypericin is released from the lipoprotein complex. In conclusion, we were able to show that the tumour accumulation of hypericin is critically determined by a combination of biological (blood flow, vessel permeability) and physicochemical elements (affinity for interstitial constituents).

Fluorescence Spectroscopic Study of Hypericin-photosensitized Oxidation of Low-density Lipoproteins

By means of UV-VIS absorption and fluorescence spectroscopy, we demonstrate that the photosensitizer hypericin (Hyp) interacts nonspecifically with low-density lipoproteins (LDL), most probably with the lipid fraction of LDL. The molar ratio of monomeric Hyp binding to nonoxidized LDL and mildly oxidized LDL is 30:1. Increasing the Hyp concentration further leads to the formation of Hyp aggregates inside the LDL molecule. We also demonstrate that photoactivated Hyp oxidizes LDL in a light dose and excitation wavelength dependent manner. The level of oxidation of LDL depends on the amount of Hyp inside the LDL molecule. The maximum of the photosensitized oxidation of the LDL by Hyp is achieved for a 30:1 molar ratio, which corresponds to the maximum concentration of monomeric form of Hyp in LDL.

Focal length variability and protein leakage as tools for measuring photooxidative damage to the lens

Hypericin is the ingredient used to standardize the popular over-the-counter antidepressant medication St. John's Wort. Because hypericin readily produces singlet oxygen and other excited state intermediates, it is a very efficient phototoxic agent in the eye that can potentially induce the development of the cataract photooxidative mechanism. Hypericin absorbs in the UV and visible ranges, binds to the lens crystallins (alpha, beta and gamma) and damages these proteins through a photooxidative mechanism. Effects were measured previously using fluorescence, UV and mass spectrometry. We report here two additional methods to monitor lens damage: (1) measuring focal length variability using a ScanTox instrument and (2) measuring protein leakage from the damaged lens. Because nonenzymic glycation results in free radical production, we chose to use elevated glucose concentrations as a convenient model for studying oxidative stress. To compare and contrast photooxidative damage against oxidative damage to the lens, we also measured the focal length variability and protein leakage induced by the presence of elevated glucose concentrations. We found that the total accumulated protein leakage was positively correlated (r = 0.9) with variability in focal length. Lenses treated with hypericin and irradiated with UVB had an increase in focal length variability as compared with the lenses that were only UVB-irradiated. Lenses without UVB irradiation had much lower focal length variability than irradiated lenses. For non-hypericin-treated lenses, UVB-irradiated lenses had a larger variability (4.58 mm) than the unirradiated lenses (1.78 mm). The lenses incubated in elevated glucose concentrations had a focal length variability (3.23 mm) equivalent to that of the unirradiated hypericin-treated lenses (3.54 mm). We conclude that photooxidative damage by hypericin results in changes in the optical properties of the lens, protein leakage and finally cataract formation. In contrast to this, high concentrations of glucose induced protein leakage but not changes in optical properties or the opacity associated with a cataract. This work provides further evidence that people should protect their eyes from intense sunlight when taking St. John's Wort.

Photosensitized structural modifications of the lens protein alpha-crystallin: do all modifications impair chaperone-like activity?

Among chaperone-like functioning proteins, the lens alpha-crystallins are of particular interest because they are not renewed, and even minor alterations can hurt their function of maintaining the proper refractive index and avoiding cataract formation in the lens. Several reports have suggested the occurrence of remarkable structural modifications in lens proteins in the presence of endogenous and exogenous sensitizers upon exposure to light. In particular, it has been shown in vitro that hypericin, the active ingredient of Hypericum, can bind to and, in the presence of light, cause the photopolymerization of alpha-crystallin. On the basis of these results it has also been suggested that a subsequent significant impairment of the protein function can occur. Using absorption and emission spectroscopic techniques, as well as circular dichroism, we have studied the structural modifications of alpha-crystallin resulting from its interaction with hypericin after irradiation with visible light. To investigate the chaperone-like function of alpha-crystallin, the heat-induced aggregation kinetics of another lens protein, betaLow-crystallin, was monitored by measuring the apparent absorption due to scattering at 360 nm as a function of time, and no apparent damage to its functional role was observed. Spectroscopic results, on the contrary, show a prominent reduction in both tryptophan and hypericin fluorescence emission intensity after light irradiation, suggesting an alteration in the tryptophan microenvironment and a high degree of packing of the chromophore due to photoinduced modification of the molecular framework. Control experiments on alpha-crystallin structurally modified by light in the presence of hypericin indicated that the protein still retains its ability to chaperone both lens crystallins and insulin.

Photobleaching of hypericin bound to human serum albumin, cultured adenocarcinoma cells and nude mice skin

Hypericin is a promising photosensitizer for photodynamic therapy (PDT) characterized by a high yield of singlet oxygen. Photobleaching of hypericin has been studied by means of absorption and fluorescence spectroscopy in different biological systems: in human serum albumin solution, in cultured human adenocarcinoma WiDr cells and in the skin of nude mice. Prolonged exposure to light (up to 95 min, 100 mW/cm2) of wavelength around 596 nm induced fluence-dependent photobleaching of hypericin in all studied systems. The photobleaching was not oxygen dependent, and singlet oxygen probably played no significant role. Emission bands in the spectral regions 420-560 nm and above 600 nm characterize the photoproducts formed. An emission band at 615-635 nm was observed after irradiation of cells incubated with hypericin or of mouse skin in vivo but not in albumin solution. The excitation spectrum of these products resembled that of hypericin. Hypericin appears to be more photostable than most sensitizers used in PDT, including mTHPC and Photofrin.

Spectroscopic study of visible-light effects on hypericin-lens proteins systems

Molecular interactions between hypericin and alpha-, beta- and gamma-crystallin proteins have been studied by means of absorption and steady-state fluorescence spectroscopy, aiming to clarify if and how the pigment binds to the proteins and to investigate the effects of visible-light irradiation on these molecular systems. Such a study is a prerequisite for assessing the possibility of using hypericin as a mild antidepressant and/or as a photodynamic agent for the treatment of eye tumors and eye viral and bacterial diseases without side injuries to the lens. We have shown that in dark-kept samples, with increasing alpha-crystallin concentration, both the fluorescence emission intensity and the ratio of the absorption maxima around 590 and 550 nm of hypericin increase. These effects have been attributed to the monomerization of nonfluorescent hypericin aggregates caused by the binding of the pigment to alpha-crystallin. The binding constant of hypericin has been evaluated to be of the order of 3.0 (mg/mL)-1, corresponding to a dissociation constant of the order of 0.3 mg/mL. Following irradiation with light of wavelengths over 400 nm, at an irradiance of 20 mW/cm2, both tryptophan and hypericin fluorescence emission intensities decrease. These effects are suggested to be the consequence of a spatial rearrangement of the protein framework which takes place following the alpha-crystallin photopolymerization sensitized by hypericin itself described in the literature. For the sake of comparison hypericin has been studied also in the presence of beta H-, beta L- and gamma-crystallins at the same concentration.

Spectroscopic studies and photodynamic actions of hypocrellin B in liposomes

Hypocrellin B (HB), a lipid-soluble natural pigment of perylenequinone derivative, is considered as potential photosensitizer for photodynamic therapy. Liposomes loaded with HB can constitute a simple model system, appropriate for better understanding the photodynamic action of HB in vivo. The steady-state absorption and emission spectra, quantum yield and lifetime of fluorescence of HB incorporated into egg L-a-phosphatidyl-choline (EPC) liposome were examined. The photochemical properties (Type I and/or Type II) of HB have also been studied in aqueous dispersions of small unilamellar liposomes of EPC using electron paramagnetic resonance and spectrophotometric methods, respectively. The quantum yield of 1O2 generated by HB is ca 0.76 in chloroform solution and it did not change upon the incorporation of HB into liposomes of EPC. The superoxide anion radical was generated by the electron transfer from the anion radical of HB (HB.-) to oxygen. The disproportionation of O2.- can generate H2O2 and ultimately the highly reactive .OH via the Fenton reaction. It could be that the disproportionation proceeded too fast, so we could not detect O2.- directly in aqueous dispersions of liposome EPC. Moreover, the self-sensitized photooxygenation of HB embedded in liposomes was studied, and almost fully (87%) inhibiting this reaction of HB by p-benzoquinone (as the quencher of O2.-) in aqueous dispersion of liposome EPC indicated that the radical mechanism (Type I) might be mainly involved in this oxygenation. All these findings suggested that the photodynamic action of HB proceeded via both Type-I and -II mechanisms, but Type-I mechanism might play a more important role in the aqueous dispersion.

Photochromic polypeptides as synthetic models of biological photoreceptors: a spectroscopic study

L-Glutamic acid polypeptides containing photochromic nitrospiropyran bound to the side chains at various percentages ("local" concentration) have been synthesized and investigated as possible artificial models of biological photoreceptors. Absorption and fluorescence spectroscopy have been utilized to investigate the photophysical and photochemical properties of nitrospiropyrans, both inserted in the polypeptide chain and in solution as "free" dye. Conformational variations produced by dark storage and light exposure of the photochromic polypeptides have been studied by means of circular dichroism. Dark-kept "free" dyes in hexafluoro-2-propanol solution in the merocyanine form ("open" form) give rise to molecular aggregates, which have been characterized as merocyanine dimers. The equilibrium constant between the monomer and the dimer, K, and their molar extinction coefficients, epsilon, at several wavelengths have been determined. Fluorescence measurements on "free" and polypeptide-bound nitrospiropyrans suggest that the dimerization process between merocyanines is favored when the photochromic units are inserted in the polypeptide chain and that under these conditions an efficient energy transfer from the monomer (donor) to the dimer (acceptor) occurs. By varying "local" as well as total nitrospiropyran concentration, it has been shown that the dimeric species result from intermolecular interactions between photochromic groups inserted in the same polypeptide chain. The alpha-helix --> random coil transition of the polypeptide structure after dark storage has eventually been shown to be the result of the dimerization process and not of the dark isomerization per se from the "closed" spiropyran form to the "open" merocyanine form of the dye.