Distribution of tetrapyrrole photosensitizers among human plasma proteins

Current trends in pyrrole and porphyrin-derived nanoscale materials for biomedical applications

This article is written to provide an up-to-date review of pyrrole-based biomedical materials. Porphyrins and other tetrapyrrolic molecules possess unique magnetic, optical and other photophysical properties that make them useful for bioimaging and therapy. This review touches briefly on some of the synthetic strategies to obtain porphyrin- and tetrapyrrole-based nanoparticles, as well as the variety of applications in which crosslinked, self-assembled, porphyrin-coated and other nanoparticles are utilized. We explore examples of these nanoparticles' applications in photothermal therapy, drug delivery, photodynamic therapy, stimuli response, fluorescence imaging, photoacoustic imaging, magnetic resonance imaging, computed tomography and positron emission tomography. We anticipate that this review will provide a comprehensive summary of pyrrole-derived nanoparticles and provide a guideline for their further development.

Effect of albumin on the fluorescence quantum yield of porphyrin -based agents for fluorescent diagnostics

BACKGROUND

Among modern methods of tumor diagnosis, fluorescent methods are considered one of the most prospective. Diagnostic agents (DAs) spread throughout the body by the bloodstream, so, the DA molecules are often transported by albumins and can be affected by these proteins. In our study we evaluate the effect of complex formation between bovine serum albumin (BSA) and three fluorescence DA's (Photolon, Photoditazin and Dimegin) on their fluorescent quantum yields.

METHODS

Electron absorption spectroscopy and fluorescence spectroscopy were carried out to calculate fluorescence quantum yields of the DAs using Rhodamine 6G as a standard fluorescent dye.

RESULTS

For all three DA's dissolved in phosphate buffer with pH 7.5 (close to that of blood) the addition of albumin resulted in bathochromic shift of the Soret band as well as change of amplitudes of absorption bands. Similar changes were observed for fluorescence spectra of all DAs that are connected with complex formation between DA and albumin. The presence of isobestic point suggests that DA can present in the solution only in two states, free and BSA-bound. Chlorine-based DA's demonstrate about 1.5-times higher fluorescence quantum yield in PBS than Dimegin. Nevertheless, the addition of BSA to the solutions of all DA's decreases sharply their fluorescence quantum yield to approximately equal values.

CONCLUSION

The complex formation between DA and albumin equalize fluorescence efficacies of all studied DAs, so the results of photodymanic diagnostics using the specific DA will depend on other factors.

pH-Dependent Changes in the Mechanisms of Transport of Chlorine e6 and Its Derivatives in the Blood

We studied the effects of medium pH on steady-state distribution of chlorine e6 and its derivatives between the main transport proteins of human blood plasma. The decrease in medium pH from weakly alkaline (pH 7.4) to acid (pH 5.0) was followed by an increase in relative affinity of chlorines to lipoproteins and reduced their affinity to serum albumin. pH-Dependent changes in the parameters of distribution of photosensitizers between the plasma and blood cells was revealed. We discussed the role of charge and polarity degree of photosensitizer molecule in the mechanism of binding to serum albumin. A possible role of changes in hydrogen ion activity in the processes of selective accumulation of chlorines by tumor cells is discussed.

Strategies for selective delivery of photodynamic sensitisers to biological targets

Strategies for increasing the affinity of photodynamic sensitisers for specific tissues, cells and organisms are reviewed. Biological outcomes are evaluated and therapeutic potential assessed.

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.

The binding of analogs of porphyrins and chlorins with elongated side chains to albumin

In previous studies, we demonstrated that elongation of side chains of several sensitizers endowed them with higher affinity for artificial and natural membranes and caused their deeper localization in membranes. In the present study, we employed eight hematoporphyrin and protoporphyrin analogs and four groups containing three chlorin analogs each, all synthesized with variable numbers of methylenes in their alkyl carboxylic chains. We show that these tetrapyrroles' affinity for bovine serum albumin (BSA) and their localization in the binding site are also modulated by chain lengths. The binding constants of the hematoporphyrins and protoporphyrins to BSA increased as the number of methylenes was increased. The binding of the chlorins depended on the substitution at the meso position opposite to the chains. The quenching of the sensitizers' florescence by external iodide ions decreased as the side chains became longer, indicating to deeper insertion of the molecules into the BSA binding pocket. To corroborate this conclusion, we studied the efficiency of photodamage caused to tryptophan in BSA upon illumination of the bound sensitizers. The efficiency was found to depend on the side-chain lengths of the photosensitizer. We conclude that the protein site that hosts these sensitizers accommodates different analogs at positions that differ slightly from each other. These differences are manifested in the ease of access of iodide from the external aqueous phase, and in the proximity of the photosensitizers to the tryptophan. In the course of this study, we developed the kinetic equations that have to be employed when the sensitizer itself is being destroyed.

Photodynamic therapy: update 2006. Part 1: Photochemistry and photobiology

Photodynamic therapy (PDT) is a two-step therapeutic technique in which the topical or systemic delivery of photosensitizing drugs is followed by irradiation with visible light. Activated photosensitizers transfer energy to molecular oxygen, generating reactive oxygen species (ROS). The subsequent oxidation of lipids, amino acids and proteins induces cell necrosis and apoptosis. In addition, ROS indirectly stimulate the transcription and release of inflammatory mediators. The photosensitizers are selective, in that they penetrate and accumulate in tumour cells or in the endothelium of newly formed vessels while generally avoiding the surrounding healthy tissue. The mechanisms of penetration through the cell membrane and the pattern of subcellular localization strongly influence the type of cellular effect. The photobiology and photoimmunology of the haematoporphyrin (Hp) derivative and its purified, lyophilized and concentrated form porfimer sodium have been investigated over the past 30 years. However, interest in PDT in dermatology was not raised until the 1990s with the availability of a simple and effective technique, the topical application of aminolaevulinic acid (ALA) and its methyl ester (methyl aminolaevulinate, MAL) followed by irradiation with broadband red light. At the same time, several new 'second-generation' synthetic sensitizers (e.g. benzoporphyrin derivatives, phthalocyanines, chlorins and porphycenes) became available. These compounds are chemically pure, highly efficient, selective and safe, while offering the advantage that the generalized skin photosensitivity they produce lasts for only a short time. They are currently under clinical evaluation but have not yet been approved for clinical use. This paper provides an overview of the chemistry of the photosensitizers, the photobiology and photoimmunology of the photodynamic reaction as well as the photophysical characteristics of the light sources available for PDT.

The pH-dependent distribution of the photosensitizer chlorin e6 among plasma proteins and membranes: a physico-chemical approach

Decrease in interstitial pH of the tumor stroma and over-expression of low density lipoprotein (LDL) receptors by several types of neoplastic cells have been suggested to be important determinants of selective retention of photosensitizers by proliferative tissues. The interactions of chlorin e6 (Ce6), a photosensitizer bearing three carboxylic groups, with plasma proteins and DOPC unilamellar vesicles are investigated by fluorescence spectroscopy. The binding constant to liposomes, with reference to the DOPC concentration, is 6 x 10(3) M(-1) at pH 7.4. Binding of Ce6 to LDL involves about ten high affinity sites close to the apoprotein and some solubilization in the lipid compartment. The overall association constant is 5.7 x 10(7) M(-1) at pH 7.4. Human serum albumin (HSA) is the major carrier (association constant 1.8 x 10(8) M(-1) at pH 7.4). Whereas the affinity of Ce6 for LDL and liposomes increases at lower pH, it decreases for albumin. Between pH 7.4 and 6.5, the relative affinities of Ce6 for LDL versus HSA, and for membranes versus HSA, are multiplied by 4.6 and 3.5, respectively. These effects are likely driven by the ionization equilibria of the photosensitizer carboxylic chains. Then, the cellular uptake of chlorin e6 may be facilitated by its pH-mediated redistribution within the tumor stroma.

Identification of amino acid residues involved in heme binding and hemoprotein utilization in the Porphyromonas gingivalis heme receptor HmuR

We have previously identified and characterized a heme/hemoglobin receptor, HmuR, in Porphyromonas gingivalis. To analyze the conserved amino acid residues of HmuR that may be involved in hemin/hemoprotein binding and utilization, we constructed a series of P. gingivalis A7436 hmuR mutants with amino acid replacements and characterized the ability of these mutants to utilize hemin and hemoproteins. Site-directed mutagenesis was employed to introduce mutations H95A, H434A, H95A-H434A, YRAP420-423YAAA, and NPDL442-445NAAA into HmuR in both P. gingivalis and Escherichia coli. Point mutations at H95 and H434 and in the NPDL motif of HmuR resulted in decreased binding to hemin, hemoglobin, and human serum albumin-hemin complex. Notably, mutations of these conserved sites and motifs led to reduced growth of P. gingivalis when human serum was used as the heme source. Analysis using a three-dimensional homology model of HmuR indicated that H95, H434, and the NPDL motif are present on apical or extracellular loops of HmuR, while the YRAP motif is present on the barrel wall. Taken together, these results support a role for H95, H434, and the NPDL motif of the P. gingivalis HmuR protein in heme binding and utilization of serum hemoproteins and the HmuR YRAP motif in serum hemoprotein utilization.

Redistribution of Foscan? from plasma proteins to model membranes

Photodynamic therapy is a comparatively novel modality of tumours treatment that includes simultaneous action of photosensitizers, light and oxygen. Photosensitizer redistribution between plasma proteins and biomembranes define photosensitizers interaction with cells, their intracellular localization and kinetics of sensitizers accumulation in the tumour. Present study investigates the kinetics of Foscan release from plasma proteins to model membranes using fluorescence resonance energy transfer (FRET) from label, covalently bound to protein, to sensitizer. We have demonstrated very slow kinetics of Foscan release from protein complexes with rate constants of (1.7 +/- 0.1) x 10(-3) s(-1) for albumin and (1.6 +/- 0.3) x 10(-4) s(-1) for high-density lipoproteins (HDL). Foscan redistributes by both collision and diffusion-mediated transfer from complexes with HDL, with bimolecular rate constant k(out) = (8.8 +/- 1.4) x 10(-2) M(-1) s(-1). Thermodynamic considerations proposed that sensitizer release from HDL into the aqueous medium is unfavourable and collision mechanism appeared to be a preferred mode of transfer in biological environment. Slow rates of Foscan redistribution from plasma proteins should be considered while planning dosimetry protocol of Foscan-PDT.

Phthalocyanine-mediated photodynamic therapy induces cell death and a G0/G1 cell cycle arrest in cervical cancer cells

We have developed a series of novel photosensitizers which have potential for anticancer photodynamic therapy (PDT). Photosensitizers include zinc phthalocyanine tetra-sulphonic acid and a family of derivatives with amino acid substituents of varying alkyl chain length and degree of branching. Subcellular localization of these photosensitizers at the phototoxic IC(50) concentration in human cervical carcinoma cells (SiHa Cells) was similar to that of the lysosomal dye Lucifer Yellow. Subsequent nuclear relocalization was observed following irradiation with 665nm laser light. The PDT response was characterized using the Sulforhodamine B cytotoxicity assay. Flow cytometry was used for both DNA cell cycle and dual Annexin V-FITC/propidium iodide analysis. Phototoxicity of the derivatives was of the same order of magnitude as for tetrasulphonated phthalocyanine but with an overall trend of increased phototoxicity with increasing amino acid chain length. Our results demonstrate cell death, inhibition of cell growth, and G(0)/G(1) cell cycle arrest during the phthalocyanine PDT-mediated response.

Investigation of Foscan® interactions with plasma proteins

The present study investigates the interaction of the second generation photosensitizer Foscan with plasma albumin and lipoproteins. Spectroscopic studies indicated the presence of monomeric and aggregated Foscan species upon addition to plasma protein solutions. Kinetics of Foscan disaggregation in albumin-enriched solutions were very sensitive to the protein concentration and incubation temperature. Kinetic analysis demonstrated that two types of Foscan aggregated species could be involved in disaggregation: dimers with a rate constant of k1 = (2.30+/-0.15) x 10(-3) s(-1) and higher aggregates with rate constants varying from (0.55+/-0.04) x 10(-3) s(-1) for the lowest to the (0.17+/-0.02) x 10(-3) s(-1) for the highest albumin concentration. Disaggregation considerably increased with the temperature rise from 15 degrees C to 37 degrees C. Compared to albumin, Foscan disaggregation kinetics in the presence of lipoproteins displayed poorer dependency on lipoprotein concentrations and smaller variations in disaggregation rate constants. Gel-filtration chromatography analysis of Foscan in albumin solutions demonstrated the presence of aggregated fraction of free, non-bound to protein Foscan and monomeric Foscan, bound to protein.

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.

Encapsulation of p-THPP into nanoparticles: cellular uptake, subcellular localization and effect of serum on photodynamic activity

The cellular uptake, localization and efflux of meso-tetra-(4-hydroxyphenyl)porphyrin (p-THPP)-loaded nanoparticles have been studied in EMT-6 tumor cells. The effect of blood serum on photocytotoxicity has also been evaluated. Sub-130 nm nanoparticles based on poly(D,L-lactide-co-glycolide) (PLGA) (50:50 PLGA and 75:25 PLGA) and poly(D,L-lactide) (PLA) have been examined in comparison with free p-THPP. For all formulations tested, uptake of photosensitizer into cells was dependent on concentration, time and temperature. All nanoparticulate formulations accumulated within the cells to a greater extent relative to free drug. Indeed, the fluorescence intensities measured on EMT-6 cells treated with p-THPP-loaded nanoparticulate formulations were at least two-fold higher than those obtained with free dye. Furthermore, the highest accumulation level was found with PLGA nanoparticles. Fluorescence microscopy revealed that endocytosis is a major intracellular sequestration mechanism of these p-THPP formulations and that these were localized into early and late endosomes. The efflux study performed on both nonirradiated and irradiated cells indicated that free and p-THPP-loaded nanoparticles gradually escaped from EMT-6 cells as a function of time. This was more pronounced when cells were treated with nanoparticles and irradiated, reflecting important photodamage. It was also found that regardless of the nanoparticulate formulations tested, p-THPP photocytotoxicity was influenced by the concentration of the serum.

Physiological parameters and biodistribution of 5,10,15,20-tetra (4-methoxyphenyl) porphyrin in rats

Physiological parameters on hepatic and renal functionality and biodistribution, accumulation and elimination, in different organs of the 5,10,15,20-tetra (4-methoxyphenyl) porphyrin (TMP) were determined in Wistar rats. The transport of TMP by low-density (LDL) and high-density lipoproteins (HDL) was also investigated. The photosensitizer is accumulated in the spleen, where its concentration is significantly increased 21 d post-injection; it also accumulates in the liver and in a lower proportion, in the duodenum, and poorly in brain and muscle. The urine and serum biochemical parameters reached normal values both in control and treated groups. The glomerular filtrate rate was not affected by the TMP treatment in any of the studied times. These results would indicate that the sensitizer does not modify the renal glomerular function. TMP is mainly eliminated from the organism via the bile-gut pathway. Considering the total amount of porphyrin bound to both lipoproteins (LDL and HDL) in comparison with the total value of the TMP in serum, it can be inferred that a large amount of the agent is transported by lipoproteins in the plasma. This study proves information about the behavior of TMP in vivo under dark conditions. The results can be used to design photodynamic treatments using this porphyrin model as the sensitizer.

The correlation between hydrophilicity of hypericins and helianthrone: internalization mechanisms, subcellular distribution and photodynamic action in colon carcinoma cells

The internalization mechanism and subcellular distribution of hypericin (Hyp), hypericin tetrasulfonic acid (HypS4) and 1,3,4,6-tetrahydroxyhelianthrone (Hel) were studied in murine colon carcinoma CT26 cells, in protein-free medium or in the presence of serum proteins. The correlation between the extent of uptake of the sensitizers by cells that were incubated in the presence of different serum components, and the internalization mechanisms, was studied. The results indicate that sensitizer internalization may be a result of three mechanisms: partitioning, pinocytosis and endocytosis, and as a direct consequence is targeted to specific subcellular sites. While Hyp and Hel, the two lipophilic sensitizers, were localized in the endoplasmic reticulum after protein-free internalization, the hydrophilic HypS4 was localized in the cytoplasmic membrane and in lysosomes. An endolysosomal internalization route was revealed for Hyp and Hel under serum-enriched conditions showing lysosomal localization, as for HypS4. The lysosomal accumulation of Hyp-serum and specifically Hyp-LDL points to an endocytotic mechanism which is supported by its higher uptake parameter in an LDL-enriched medium, compared to the medium with 10% serum. The different uptake parameters of Hyp to cells, with or without serum, reflect the different mechanisms. Smaller differences in the uptake parameter for HypS4 reflect the distinction between partitioning and endocytosis, which, in this case, are both targeted to the lysosomes. The same uptake parameter of Hel to cells incubated in media with or without serum indicates the absence of the endocytotic mechanism. The interrelationship between subcellular targeting and photodynamic treatment was shown for the three sensitizers Hyp was found to be the most efficient sensitizer for PDT under our illumination protocol and it was dependent on internalization and localization sites.

Photosensitizer delivery for photodynamic therapy of choroidal neovascularization

The present review examines the importance of improving photosensitizer delivery for choroidal neovascularization (CNV) in light of the clinical impact of photodynamic therapy (PDT) for CNV. An overview of the classes of available photosensitizers is provided and the properties governing photosensitizer uptake and circulation in serum are discussed. Current delivery systems, for example liposomal formulations as well as the use of the promising strategy of antibody targeted delivery as a strategy to improve PDT selectivity and efficiency for CNV treatment are described. A summary of the work using Verteporfin, tin ethyl purpurin and Lu-Tex--photosensitizers currently in clinical trials for CNV--is given.

Targeted intracellular delivery of photosensitizers to enhance photodynamic efficiency

Photodynamic therapy (PDT) is a novel treatment, used mainly for anticancer therapy, that depends on the retention of photosensitizers (PS) in tumour cells and irradiation of the tumour with appropriate wavelength light. Photosensitizers are molecules such as porphyrins and chlorins that, on photoactivation, effect strongly localized oxidative damage within target cells. The PS used for PDT localize in various cytoplasmic membranous structures, but are not found in the most vulnerable intracellular sites for reactive oxygen species, such as the cell nucleus. The experimental approaches discussed in the present paper indicate that it is possible to design highly efficient molecular constructs, PS carriers, with specific modules conferring cell-specific targeting, internalization, escape from intracellular vesicles and targeting to the most vulnerable intracellular compartments, such as the nucleus. Nuclear targeting of these PS-carrying constructs results in enhanced photodynamic activity, maximally about 2500-fold that of free PS. Future work is intended to optimize this approach to the point at which tumour cells can be killed rapidly and efficiently, while minimizing normal cell and tissue damage.

Increased binding of chlorin e6 to lipoproteins at low pH values

It is well known that the extracellular pH in tumors is lower than that of normal tissue. This has been proposed to be one of the reasons for the tumor selective uptake of several photosensitizers. Photosensitizers like chlorin e(6) are bound to blood components and delivered to different sites in the organism. Thus, the effect of pH on their interaction with human plasma needs to be studied in order to understand a possible role of the acidic microenvironment in tumors for the drug distribution. Increasing amounts of human plasma in the sample resulted in a gradual red shift of the fluorescence emission maxima of chlorin e(6), indicating binding of the drug to some of the plasma components. Titration showed that the drug-plasma interaction was pH-dependent. The titration curve had an inflection point at 7.4+/-0.1. The relative distribution of the drug among plasma components, as found after ultracentrifugation of chlorin e(6)-doped plasma in a salt gradient, showed more binding of the drug to nonlipoproteins than to lipoprotein classes at both pH values studied (6.5 and 7.4). A decrease in the pH was connected with a significant increase in drug-lipoprotein binding. The pH of the environment affects chlorin e(6)-plasma interaction and the distribution of the drug among different plasma components. The results of this study indicate a possible role of the acidic microenvironment in tumors for the preferential uptake and retention of several photosensitiziers.

Photodynamic therapy

Photodynamic therapy involves administration of a tumor-localizing photosensitizing agent, which may require metabolic synthesis (i.e., a prodrug), followed by activation of the agent by light of a specific wavelength. This therapy results in a sequence of photochemical and photobiologic processes that cause irreversible photodamage to tumor tissues. Results from preclinical and clinical studies conducted worldwide over a 25-year period have established photodynamic therapy as a useful treatment approach for some cancers. Since 1993, regulatory approval for photodynamic therapy involving use of a partially purified, commercially available hematoporphyrin derivative compound (Photofrin) in patients with early and advanced stage cancer of the lung, digestive tract, and genitourinary tract has been obtained in Canada, The Netherlands, France, Germany, Japan, and the United States. We have attempted to conduct and present a comprehensive review of this rapidly expanding field. Mechanisms of subcellular and tumor localization of photosensitizing agents, as well as of molecular, cellular, and tumor responses associated with photodynamic therapy, are discussed. Technical issues regarding light dosimetry are also considered.

Meso-tetraphenylporphyrin dimer derivative as a potential photosensitizer in photodynamic therapy

Studies on the synthesis, preliminary in vivo biological activity, singlet oxygen and fluorescence yields of a dimeric porphyrin (D1) are described. The pharmacokinetic behavior and photodynamic properties of the dimer D1 were examined in Balb/c mice bearing an MS-2 fibrosarcoma. Compound D1 shows a high selectivity for tumor localization (tumor/peritumoral tissue ratios of dye concentration ranging between ca 100 and 140 at 24 h after drug administration of 5.0-1.0 mg kg-1 into DL-alpha-dipalmitoylphosphatidylcholine liposomes). The phototherapeutic efficiency of dimer D1 was evaluated by following the growth curves of fibrosarcoma irradiated with red light (600-700 nm) with a total dose of 400 J cm-2, at 24 h after intravenous injection. Photodynamic therapy-treated tumors showed a significant delay in growth as compared to untreated control mice. The results obtained suggest that the porphyrin dimer D1 may be a promising candidate for further use in PDT experiments.

Correlation between site II-specific human serum albumin (HSA) binding affinity and murine in vivo photosensitizing efficacy of some Photofrin components

Human serum albumin (HSA) is one of the key components in human blood that may influence drug distribution. As such, it is important to know the affinity of any drug for albumin. Previously, Photofrin, a mixture of monomeric, dimeric and oligomeric porphyrins, has been subjected to HSA binding studies. However, due to its complex nature, binding studies on Photofrin or other hematoporphyrin derivatives with HSA are inconclusive. In this report, the binding properties of some components (dimers and trimers) of Photofrin and the relationship between murine photosensitizing efficacy and those binding properties were investigated. The interaction of these porphyrins with HSA was investigated by direct ultrafiltration and fluorescent titration techniques with fluorescent probes such as dansyl-L-proline (DP), which is known to interact selectively with site II on HSA. Porphyrins also were tested for antitumor activity in a mouse model following intravenous administration and exposure to laser light. Together, the results suggest that the photosensitizers that were preferentially bound to site II of HSA were most effective at controlling murine tumor regrowth.

Subcellular localization of sulfonated tetraphenyl porphines in colon carcinoma cells by spectrally resolved imaging

Subcellular localization of the dye, 5,10,15,20-tetra(4-sulfonatophenyl)porphine (TPPS4) and the more hydrophobic dye, 5,10,15,20-tetra(1-sulfonatophenyl)porphine (TPPS1), in murine colon carcinoma cells was studied by spectrally resolved imaging (SRI) combined with image processing techniques. Spectrally resolved imaging enabled the acquisition of multipixel fluorescence spectra (> 10(4)) from a single cell. Demarcation of specific localization sites and segregation of the irrelevant fluorescence were based on the pixel spectra and by operating the functions of spectral similarity mapping (SSM), principal component analysis (PCA) and spectral classification. The SRI revealed the fine details of the photochemical process that clarify some aspects of subcellular damage. The SRI depicted the differences between TPPS4 and TPPS1 with respect to their initial localization and their fate at the end of the photochemical effect. The dye TPPS4 was localized initially in lysosomal vesicles, and upon irradiation fluorescence was seen in the nucleus as well as in vesicles. Some of the vesicles were closely related to the nucleus, as resolved by SSM, PCA and spectral classification. Additional light exposure stimulated relocalization of TPPS4 into the nucleus as well as into the nucleolus, which was clearly depicted by SSM and PCA. Spectral classification showed a third, weak residual cytoplasmic array around the nucleus. The dye TPPS1 concentrated in a Golgi-like complex and was resolved in the nuclear envelope and in small vesicles: it was not redistributed into other compartments upon photosensitization. Serum supplementation to the incubation media of colon carcinoma cells treated with TPPS4 or TPPS1 did not change the localization patterns. Pixel spectra of the two dyes in the cells showed spectral shifts and expanded shoulders due to microenvironmental effects. Thus, the chemical nature of the sulfonated phenyl porphines, and not their interaction with serum proteins, was the main determinant of their binding to the lysosomes, nucleus, nucleolus, nuclear envelope or Golgi.

Role of delivery vehicles for photosensitizers in the photodynamic therapy of tumours

The use of photosensitizing drugs associated with different types of delivery vehicle has received strong interest within the field of the photodynamic therapy of tumours. Lipid-based delivery vehicles, such as liposomes and oil emulsions, allow the administration of water-insoluble photosensitizers, widening the choice of photosensitizers potentially useful for treating tumours. In some cases, these delivery vehicles increase the selectivity of tumour targeting by favouring photosensitizer uptake in tumour tissue. However, a higher selectivity of tumour targeting could be obtained through the association of photosensitizers with delivery vehicles which can interact preferentially or specifically with tumour cells. With this aim in mind, low-density lipoproteins (LDLs) and monoclonal antibodies, in particular, are regarded as the most promising delivery systems for anticancer drugs. Some pharmacokinetic studies with LDL-associated photosensitizers have demonstrated a higher tumour uptake compared with the same photosensitizers delivered with other formulations. Monoclonal antibody-coupled photosensitizers have been tested mainly in vitro, and have shown a high selectivity towards cells expressing specific antigens. Only a limited number of reports are available on the biodistribution of immunoconjugated photosensitizers and on their selectivity in vivo, so that their importance for the selectivity of tumour targeting has not yet been defined.

Tumour-localising and -photosensitising properties of a novel zinc(II) octadecylphthalocyanine

1,4,8,11,15,18,22,25-Octadecylphthalocyaninato zinc(II), ZnODPc, incorporated into a Cremophor emulsion, was assayed for its pharmacokinetic and phototherapeutic properties in Balb/c mice bearing an intramuscularly transplanted MS-2 fibrosarcoma. The phthalocyanine was injected intravenously (i.v.) in three doses, i.e. 1.46, 0.73 and 0.37 mumol kg-1 body weight. In all cases, the octadecyl-substituted phthalocyanine showed an unusually high affinity for serum low-density lipoproteins (LDLs) and a high efficiency and selectivity of tumour targeting: the maximum accumulation in the tumour occurred at 24 h after injection, whereas no detectable amount of phthalocyanine was recovered from the muscle, i.e. the peritumoral tissue, between 1 h and 1 week after injection. At the same time, low amounts of phthalocyanine were recovered from skin and then only at short times after injection, with skin photosensitivity rapidly disappearing and the phthalocyanine present in the serum only. Tumour photosensitisation studies were carried out at 24 h after administration of 1.46 mumol kg-1 ZnODPc and showed that this phthalocyanine has a very high phototherapeutic efficiency; this is probably a consequence of the multiple mechanisms by which the phthalocyanine induces tumour damage, involving both direct modification of malignant cells and impairment of blood flow, as well as the alteration of a variety of subcellular components, such as mitochondria, the rough endoplasmic reticulum, the perinuclear membrane and, occasionally, cell nuclei. Tumour necrosis appears to be the consequence of both random cell death and apoptosis.

Structure and biodistribution relationships of photodynamic sensitizers

Photodynamic therapy (PDT) has, during the last quarter century, developed into a fully fledged biomedical field with its own association, the International Photodynamic Association (IPA) and regular conferences devoted solely to this topic. Recent approval of the first PDT sensitizer, Photofrin (porfimer sodium), by health boards in Canada, Japan, the Netherlands and United States for use against certain types of solid tumors represents, perhaps, the single most significant-indicator of the progress of PDT from a laboratory research concept to clinical reality. The approval of Photofrin will undoubtedly encourage the accelerated development of second-generation photosensitizers, which have recently been the subject of intense study. Many of these second-generation drugs show significant differences, when compared to Photofrin, in terms of treatment times postinjection, light doses and drug doses required for optimal results. These differences can ultimately be attributed to variations in either the quantum efficiency of the photosensitizer in situ, which is in turn affected by aggregation state, localized concentration of endogenous quenchers and primary photophysics of the dye, or the intratumoral and intracellular localization of the photosensitizer at the time of activation with light. The purpose of this review is to bring together data relating to the biodistribution and pharmacokinetics of second-generation sensitizers and attempt to correlate this with structural and electronic features of these molecules. As this requires a clear knowledge of photosensitizer structure, only chemically well-characterized compounds are included, e.g. Photofrin and crude sulfonated phthalocyanines have been excluded as they are known to be complex mixtures. Nonporphyrin-based photosensitizers, e.g. rose bengal and the hypericins, have also been omitted to allow meaningful comparisons to be made between different compounds. As the intracellular distribution of photosensitizers to organelles and other subcellular structures can have a large effect on PDT efficacy, a section will be devoted to this topic.

Correlation of subcellular and intratumoral photosensitizer localization with ultrastructural features after photodynamic therapy

Photodynamic therapy (PDT) of cancer typically involves systemic administration of tumor-localizing photosensitizers followed 48-72 h later by exposure to light of appropriate wavelengths. Knowledge about the distribution of photosensitizers in tissues is still fragmentary. In particular, little is known as to the detailed localization patterns of photosensitizers in neoplastic and normal tissues as well as the relationship between such patterns and the actual targets for the photosensitizing effect. This review focuses on ultrastructural features seen in treated cells and tumors. An attempt is made to correlate these findings with the subcellular/intratumoral localization pattern of the photosensitizers in tumor cell lines in vitro and in tumor models in vivo. Several subcellular sites are main targets of PDT with different sulfonated aluminum phthalocyanines (AIPcSn) in the human tumor cell line LOX. Nuclei are not among the primary targets. Overall, the ultrastructural changes correlate well with the data about the subcellular localization patterns for each analogue of AIPcSn in the same cell line. Similar findings are also obtained for the family of sulfonated mesotetraphenylporphines (TPPSn) in the NHIK 3025 cell line. The mechanisms involved in the killing of tumors by PDT seem to be a complex interplay between direct and indirect (via vascular damage) effects on neoplastic cells according to the intratumoral localization pattern of the applied dye. Several factors can affect the localization pattern of a drug, such as its chemical character, the mode of drug delivery, the time interval between drug administration and light exposure, and tumor type. Furthermore, whether local immune reactions (such as macrophages) and apoptosis (programmed cell death) are involved in the destruction of neoplastic cells by PDT in vivo is still an enigma. A general model for PDT-induced tumor destruction is suggested.

Structure-photodynamic activity relationships of a series of 4-substituted zinc phthalocyanines

Radioiodinated zinc phthalocyanine including [125I]ZnPcI4 and differently sulfonated [65Zn]ZnPcS (ZnPcS4, ZnPcS3, ZnPcS2 and ZnPcS1.75, a mixture of adjacent di and 25% mono) were prepared in order to study cell uptake and release kinetics in EMT-6 cells. The same compounds were evaluated for their in vitro phototoxicity and the biological parameters were compared to partition coefficients to arrive at quantitative structure-activity relationships (QSAR). At 1 microM in 1% serum, at 37 degrees C, all dyes showed rapid cell uptake during the first hour followed by a slow accumulation phase. After 24 h, the highest cellular concentration was observed with the lipophilic ZnPcI4, followed by the amphiphilic ZnPcS2 and ZnPcS1.75. The hydrophilic ZnPcS4 and ZnPcS3 showed lower uptake. Dye release from dye-loaded cells during incubation in dye-free medium could reach up to 60% and was shown to depend mainly on the amount of drug incorporated rather than the type of compound. These results suggest that care should be taken in interpreting dye toxicity data, which involve in vitro cell manipulations in dye-free medium, particularly during in vitro-in vivo protocols. The EMT-6 cell survival after 1 h or 24 h incubation with 1 microM dye in 1% serum followed by exposure to red light was assessed by means of the colorimetric 3-(4,5-dimethylthiazol-2-yl)-diphenyl-tetrazolium bromide (MTT) assay. Photocytotoxicities correlated inversely with the tendencies of the dyes to aggregate. Increased dye uptake by the cells also correlated with their activities, except for the lipophilic ZnPcI4, which showed the highest cell uptake but little phototoxicity. The QSAR between phototoxicity and the log of the partition coefficients (phosphate-buffered saline and n-octanol) gave a parabola with optimal partition values corresponding to the adjacent sulfonated ZnPcS2.

Biodistribution and virus inactivation efficacy of a silicon phthalocyanine in red blood cell concentrates as a function of delivery vehicle

The silicon phthalocyanine, HOSiPcOSi(CH3)2(CH2)3N(CH3)2 (Pc 4), is a new photosensitizer that can inactivate lipid-enveloped viruses in red blood cell concentrates (RBCC) upon exposure to red light. Because Pc 4 is insoluble in water, it was delivered either as an emulsion in saline and cremophor EL (CRM) or as a solution in dimethyl sulfoxide (DMSO). In RBCC, Pc 4 added in either vehicle distributed between the plasma and red blood cells (RBC) in a ratio of 4:6, similar to the ratio of these components in RBCC 3:7 (i.e. a hematocrit of 70%). Light exposure did not affect this distribution and caused only marginal degradation of Pc 4 at a light dose that inactivates > 5 log10 vesicular stomatitis virus (VSV). Among human plasma proteins, Pc 4 bound mainly (about 70%) to lipoproteins and to a lesser extent to albumin and lower molecular weight proteins when delivered in DMSO. When delivered in CRM, distribution between lipoproteins and albumin became more even. Among the lipoproteins Pc 4 bound almost exclusively to very low-density lipoproteins (VLDL) when delivered in DMSO and to both VLDL and low-density lipoproteins when added in CRM. The rate of VSV inactivation was independent of the delivery vehicle but there was less RBC damage, as measured by hemolysis during storage, when Pc 4 was added in CRM. These results indicate that using CRM as emulsifier can enhance the specificity of Pc 4-induced photochemical decontamination of RBCC for transfusion.

Correlation of distribution of sulphonated aluminium phthalocyanines with their photodynamic effect in tumour and skin of mice bearing CaD2 mammary carcinoma

A chemical extraction assay and fluorescence microscopy incorporating a light-sensitive thermoelectrically cooled charge-coupled device (CCD) camera was used to study the kinetics of uptake, retention and localisation of disulphonated aluminium phthalocyanine (A1PcS2) and tetrasulphonated aluminium phthalocyanine (A1PcS4) at different time intervals after an i.p. injection at a dose of 10 mg kg-1 body weight (b.w.) in tumour and surrounding normal skin and muscle of female C3D2/F1 mice bearing CaD2 mammary carcinoma. Moreover, the photodynamic effect on the tumour and normal skin using sulphonated aluminium phthalocyanines (A1PcS1, A1PcS2, A1pcS4) and Photofrin was compared with respect to dye, dye dose and time interval between dye administration and light exposure. The maximal concentrations of A1PcS2 in the tumour tissue were reached 2-24 h after injection of the dye, while the amounts of A1PcS4 peaked 1-2 h after the dye administration. A1PcS2 was simultaneously localised in the interstitium and in the neoplastic cells of the tumour, whereas A1PcS4 appeared to localise only in the stroma of the tumour. The photodynamic efficiency (light was applied 24 h after dye injection at a dose of 10 mg kg-1 b.w.) of the tumours was found to decrease in the following order: A1PcS2 > A1PcS4 > Photofrin > A1PcS1. Furthermore, photodynamic efficacy was strongly dependent upon dye doses and time intervals between dye administration and light exposure: the higher the dose, the higher the photodynamic efficiency. The most efficient photodynamic therapy (PDT) of the tumour was reached (day 20 tumour-free) when light exposure took place 2 h after injection of A1PcS2 (10 mg kg-1). A dual intratumoral localisation pattern of the dye, as found for A1PcS2, seems desirable to obtain a high photodynamic efficiency. The kinetic patterns of uptake, retention and localisation of A1PcS2 and A1PcS4 are roughly correlated with their photodynamic effect on the tumour and normal skin.

Binding of etiopurpurin to human plasma proteins. Delivery in cremophor EL and dimethyl sulphoxide. III

Binding of the photosensitizer etiopurpurin (ET2) to human plasma was assessed, using conditions that would yield a high percentage of ET2 in the form of LDL-bound monomers which may favor photosensitizer tumor localization. Two delivery systems, Cremophor EL (CRM) and dimethyl sulphoxide (DMSO), were used. The binding of ET2 to CRM-modified lipoproteins was compared to the binding of the dye to the native proteins using delivery in DMSO. Plasma-bound monomers and unbound high density aggregates were shown to coexist. The density and rate of formation of the dye aggregates were correlated. The aggregates formed by delivery in DMSO could be partially converted into plasma-bound monomeric ET2. There was no mode-delivery-effect upon the distribution of monomeric ET2 among the plasma proteins. 70% of monomeric ET2 was bound to LDL and most of the remainder to HDL. In delivery in DMSO the yield of LDL-bound dye monomers (up to 30% of added ET2) increased with decreasing concentration of ET2 in the delivery solution and with increasing time of incubation (< or = 48 hr). Long incubation also induced changes in the densities of LDL and HDL. The yields of LDL-bound monomers (up to 40%) increased with increasing concentration of CRM-bound ET2. High yields of LDL-bound monomers were obtained using both modes of delivery. Although the aggregates associated with the two modes of delivery had different properties. The change in lipoprotein composition might be involved in the conversion of aggregates into plasma-bound monomers.

Effect of density-gradients on the binding of photosensitizing agents to plasma proteins

The binding of photosensitizing agents to low-density lipoprotein is considered an important factor in tumor localization. We examined the affinity of a group of photosensitizers, varying in charge and hydrophobicity, for LDL, under conditions designed to determine whether the high salt concentration involved in conventional KBr gradients affected the results. Density-gradients containing KBr vs D2O were evaluated; the latter can delineate VLDL and LDL from other plasma components, while the KBr gradient readily resolved VLDL, LDL, HDL and albumin. Distribution of the photosensitizers to plasma fractions was assessed, along with the effect of Cremophor EL, an emulsifier used for formulation of water-insoluble drugs. Both the D2O and KBr gradients provided similar results with regard to the affinity of anionic, neutral or cationic photosensitizers for LDL. The use of Cremophor EL for drug formulation was associated with an altered electrophoretic lipoprotein profile. In some cases, affinity of CRM-solubilized sensitizers for plasma components varied with the density-gradient employed. The high salt concentration used in density-gradient fractionation had little effect on the affinity of photosensitizing agents to low-density lipoprotein but may introduce artifacts when emulsifiers are used in drug formulation.

Separation of lipoproteins, albumin and gamma-globulin by single-step ultracentrifugation of human serum. Application. I: Binding of hematoporphyrin to human serum and to albumin

Previous studies of the serum binding of the photosensitizer hematoporphyrin (Hp) have given widely different results. The serum binding of Hp is therefore further illuminated by experiment and discussion. Ultracentrifugal separation of serum is improved and applied to study the binding of Hp to human serum and HSA. The observed distribution of Hp among the serum proteins is compared with the distribution expected from available association constants for Hp binding with individual proteins. The lipoprotein classes and the two major high density proteins (HDP), albumin and gamma-globulin, were separated in a NaCl-KBr gradient by single spin ultracentrifugation (SW 40; 30,000 rpm). HSA- and HDP-bound Hp were similarly distributed in the centrifuged gradient. Centrifugation of Hp-doped HSA separated the unbound Hp (75%) and the HSA-bound Hp (25%). The present association constant for the Hp-HSA complex (10(3)/M) was much lower than earlier published ones (10(6)/M) found by other techniques. The association of Hp with HDP in serum was much stronger than the association of Hp with the isolated HSA (electrophoretic grade). The estimated ratio of HSA-bound to LDL-bound HP in serum was at least 25 times larger than the experimental value. The percentage of LDL-bound Hp decreased with increasing Hp concentration. The serum binding of Hp is the same as that found previously using another rotor and another salt gradient (70.1 Ti, 70,000 rpm, NaCl-CsCl). LDL has high-affinity-low-capacity binding sites for Hp. HSA is the major HDP protein that binds Hp in human serum. The strength of the HSA-Hp complex may depend on the batch of HSA used and upon the absence/presence of other proteins. Proteins may interact in serum in manners that affect the binding of certain drugs. Neither the type of gradient salt nor the field of gravity affected the serum binding of Hp.

Effect of axial ligation and delivery system on the tumour-localising and -photosensitising properties of Ge(IV)-octabutoxy-phthalocyanines

Four Ge(IV)-octabutoxy-phthalocyanines (GePcs) bearing two alkyl-type axial ligands were assayed for their pharmacokinetic properties and phototherapeutic efficiency in Balb/c mice bearing an intramuscularly transplanted MS-2 fibrosarcoma. The GePcs were i.v. injected at a dose of 0.35 mumol kg-1 body weight after incorporation into either Cremophor emulsions or small unilamellar liposomes of dipalmitoyl-phosphatidylcholine (DPPC). Both the nature of the delivery system and the chemical structure of the phthalocyanine were found to affect the behaviour of the GePcs in vivo. Thus, Cremophor-administered GePcs invariably yielded a more prolonged serum retention and a larger association with low-density lipoproteins (LDLs) as compared with the corresponding liposome-delivered phthalocyanines. This led to a greater efficiency and selectivity of tumour targeting. These effects were more pronounced for those GePcs having relatively long alkyl chains (hexyl to decyl) in the axial ligands. Maximal tumour accumulation (0.67 nmol per g of tissue) was found for Ge-Pc(hexyl)2 at 24 h after injection. Consistently, the Ge-Pc(hexyl)2, administered via Cremophor, showed the highest phototherapeutic activity towards MS-2 fibrosarcoma.

Unusually high affinity of Zn(II)-tetradibenzobarrelenooctabutoxy-phthalocyanine for low-density lipoproteins in a tumor-bearing mouse

An important factor in determining the efficacy of photosensitizing compounds in photodynamic therapy of tumors is the level to which tumors take up the photosensitizers after systemic injection. This parameter seems to be related to the transport modalities of the photosensitizer in the bloodstream. In this work the photosensitizer Zn(II)-tetradibenzobarrelenooctabutoxyphthalocyanine was shown to have an unprecedentedly high association with low-density lipoproteins (71% of the phthalocyanine in the plasma) when delivered in Cremophor micelles to tumor-bearing mice. This was accompanied by a particularly high tumor uptake at 24 h post-injection.

Effect of the delivery system on the biodistribution of Ge(IV) octabutoxy-phthalocyanines in tumour-bearing mice

The pharmacokinetic properties of the Ge(IV)-octabutoxy-phthalocyanines (GePc) with two axially ligated triethylsiloxy (GePcEt) or trihexyl-siloxy (GePcHex) chains were studied in BALB/C mice bearing a transplanted MS-2 fibrosarcoma. The GePcs were delivered to mice after incorporation into unilamellar liposomes of dipalmitoyl phosphatidylcholine (DPPC) or in an emulsion of Cremophor-EL. The Cremophor delivered GePcs were cleared from the blood circulation at a much slower rate than the liposome-delivered GePcs. At the same time, Cremophor induced a slower and reduced uptake of the GePcs in the liver and spleen while it greatly enhanced the uptake in the tumour as compared to liposomes. Maximum tumour uptake was observed at 24 h post-injection and was equivalent to 0.67 and 0.50 nmol/g, respectively, for the Cremophor delivered GePcHex and GePcEt. The corresponding values for the liposome-delivered drugs were approximately one fourth of that observed with Cremophor.

Binding of etiopurpurin and tin-coordinated etiopurpurin to human plasma proteins. Delivery in cremophore EL and dimethyl sulfoxide (paper II)

Purpurins are potent hydrophobic photosensitizers in vivo. Cremopfore EL is an important vehicle for the administration of hydrophobic drugs. Mode-delivery-effects on the binding of etiopurpurin (ET2) to human plasma (LDL, HDL, and high density proteins, HDP) is studied for delivery in CRMaq and in DMSO by ultracentrifugation. A similar study of SnET2 is available (Kongshaug et al., 1993) and has been extended. In the absence of plasma, only nonfluorescent ET2 entities (aggregates) were present, a portion of which moved unaffected by gravity (small aggregates), the remainder according to their densities (high density aggregates). Aggregated ET2 showed, at high salt density, similar positions and halfwidths in the gradient, and similar adsorption properties as the aggregates in plasma-containing samples. In CRMaq (1 mg CRM/ml) the adsorptive loss of the dye affected only marginally the binding of fluorescent monomeric ET2. In this mode (i) 20% of ET2 was bound as monomers, about 70% of which to CRM-modified LDL, most of the remainder to CRM-modified HDL; (ii) such HDL also marginally bound small aggregates; (iii) only monomeric ET2 was bound to CRM-modified LDL. In delivery in DMSO, aggregated ET2 (98% of ET2 in the gradient) converted, post centrifugally, into minor amounts of HDL-bound monomeric ET2; LDL-bound ET2 included monomers (about 50%) and small aggregates, mainly dimers. The percentage binding of SnET2 to HDP was independent of the concentrations of CRMaq and HDL. Plasma-bound small aggregates (such as dimers) and plasma-unbound high density aggregates (mean densities of 1.13-1.19 g/ml) were substantially present in the plasma-containing samples. There were mode-delivery-effects upon the yields and properties of aggregated ET2, and upon the yields of plasma-bound monomeric ET2. Monomeric ET2 showed a remarkably high percentage binding to LDL and was similarly distributed among the lipoproteins as is total cholesterol. There was little or no real mode-delivery-effect upon the distribution of monomeric ET2 among the plasma proteins. The affinity of CRM-modified LDL for SnET2 was similar to that of HDL plus HDP in native plasma.