Low density lipoprotein receptor pathway in the delivery of Photofrin: how much is it relevant for selective accumulation of the photosensitizer in tumors?

Critical PDT Theory VII: Preclinical Translation

The translation of photodynamic effects into clinical practice is a complex process that involves the pharmacokinetics of photosensitizing agents, light dosimetry and oxygenation levels. But even the 'translation' of basic photobiology into meaningful preclinical information can be challenging. Some thoughts on directions for progress in clinical trials are suggested.

Photodynamic therapy and associated targeting methods for treatment of brain cancer

Brain tumors, including glioblastoma multiforme, are currently a cause of suffering and death of tens of thousands of people worldwide. Despite advances in clinical treatment, the average patient survival time from the moment of diagnosis of glioblastoma multiforme and application of standard treatment methods such as surgical resection, radio- and chemotherapy, is less than 4 years. The continuing development of new therapeutic methods for targeting and treating brain tumors may extend life and provide greater comfort to patients. One such developing therapeutic method is photodynamic therapy. Photodynamic therapy is a progressive method of therapy used in dermatology, dentistry, ophthalmology, and has found use as an antimicrobial agent. It has also found wide application in photodiagnosis. Photodynamic therapy requires the presence of three necessary components: a clinically approved photosensitizer, oxygen and light. This paper is a review of selected literature from Pubmed and Scopus scientific databases in the field of photodynamic therapy in brain tumors with an emphasis on glioblastoma treatment.

Sonodynamic therapy induces oxidative stress, DNA damage and apoptosis in glioma cells

Malignant glioma remains one of the most challenging diseases to treat because of the invasive growth of glioma cells and the existence of the blood-brain barrier (BBB), which blocks drug delivery to the brain. New strategies are urgently needed to overcome these shortcomings and improve the outcomes. Ultrasound represents a promising noninvasive and reversible BBB opening approach and the related sonodynamic therapy (SDT) is rapidly emerging. This study aims to explore the ultrasound parameters for BBB opening and the cell killing effect of SDT in human glioma U373 cells by using a recently reported sonosensitizer, sinoporphyrin sodium (DVDMS). The in vitro BBB model indicated that SDT caused a time-dependent permeability increase, which peaked at 2 h post treatment and then recovered gradually. The results of toxicology tests showed significant U373 cell viability loss and apoptosis increase after DVDMS-SDT, accompanied by enhanced cleaved-caspase-3 level and DNA fragmentation, in which reactive oxygen species (ROS) were a major triggering intermediate during DVDMS-SDT. Furthermore, DVDMS-SDT produced DNA damage and the underlying mechanisms were evaluated, in order to provide a fundamental basis for DVDMS-SDT application in glioma therapy. The findings indicated that the DNA molecules could be temporarily regulated by SDT and DNA double-strand breaks (DSBs), which increased the difficulty of cellular self-repair, thus aggravating cell apoptosis and inhibiting glioma cell invasive growth. Therefore, this study supports the use of SDT as an alternative approach for glioma therapy.

Dye Sensitizers for Photodynamic Therapy

Photofrin^® was first approved in the 1990s as a sensitizer for use in treating cancer via photodynamic therapy (PDT). Since then a wide variety of dye sensitizers have been developed and a few have been approved for PDT treatment of skin and organ cancers and skin diseases such as acne vulgaris. Porphyrinoid derivatives and precursors have been the most successful in producing requisite singlet oxygen, with Photofrin^® still remaining the most efficient sensitizer (quantum yield = 0.89) and having broad food and drug administration (FDA) approval for treatment of multiple cancer types. Other porphyrinoid compounds that have received approval from US FDA and regulatory authorities in other countries include benzoporphyrin derivative monoacid ring A (BPD-MA), meta-tetra(hydroxyphenyl)chlorin (m-THPC), N-aspartyl chlorin e6 (NPe6), and precursors to endogenous protoporphyrin IX (PpIX): 1,5-aminolevulinic acid (ALA), methyl aminolevulinate (MAL), hexaminolevulinate (HAL). Although no non-porphyrin sensitizer has been approved for PDT applications, a small number of anthraquinone, phenothiazine, xanthene, cyanine, and curcuminoid sensitizers are under consideration and some are being evaluated in clinical trials. This review focuses on the nature of PDT, dye sensitizers that have been approved for use in PDT, and compounds that have entered or completed clinical trials as PDT sensitizers.

Denis TG, Hamblin MR. Photodynamic Therapy for Cancer and for Infections: What Is the Difference?

Photodynamic therapy (PDT) was discovered over one hundred years ago when it was observed that certain dyes could kill microorganisms when exposed to light in the presence of oxygen. Since those early days, PDT has mainly been developed as a cancer therapy and as a way to destroy proliferating blood vessels. However, recently it has become apparent that PDT may also be used as an effective antimicrobial modality and a potential treatment for localized infections. This review discusses the similarities and differences between the application of PDT for the treatment of microbial infections and for cancer lesions. Type I and type II photodynamic processes are described, and the structure-function relationships of optimal anticancer and antimicrobial photosensitizers are outlined. The different targeting strategies, intracellular photosensitizer localization, and pharmacokinetic properties of photosensitizers required for these two different PDT applications are compared and contrasted. Finally, the ability of PDT to stimulate an adaptive or innate immune response against pathogens and tumors is also covered.

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.

High level of low-density lipoprotein receptors enhance hypericin uptake by U-87 MG cells in the presence of LDL

The dependence of the uptake of hypericin (Hyp) by human glioma U-87 MG cells on the level of expression of low-density lipoprotein (LDL) receptors has been studied in this work. A special role of the LDL receptor-pathway for Hyp delivery to U-87 MG cells in the presence of LDL was revealed by the substantial increase of Hyp uptake in the situation, when the number of LDL receptors on the cell surface was elevated. Moreover, the colocalization experiments showed the lysosomal localization of Hyp following the uptake and that the concentration of Hyp in these organelles was enhanced in the cells with elevated number of LDL receptors when the incubation medium contained LDL. Both these findings suggest that LDL and LDL receptor-pathway play an important role in the delivery and accumulation of Hyp into the cells.

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.

Mechanisms in photodynamic therapy: Part three-Photosensitizer pharmacokinetics, biodistribution, tumor localization and modes of tumor destruction

Photodynamic therapy (PDT) has been known for over a hundred years, but is only now becoming widely used. Originally developed as cancer therapy, some of its most successful applications are for non-malignant disease. The majority of mechanistic research into PDT, however, is still directed towards anti-cancer applications. In the final part of series of three reviews, we will cover the possible reasons for the well-known tumor localizing properties of photosensitizers (PS). When PS are injected into the bloodstream they bind to various serum proteins and this can affect their phamacokinetics and biodistribution. Different PS can have very different pharmacokinetics and this can directly affect the illumination parameters. Intravenously injected PS undergo a transition from being bound to serum proteins, then bound to endothelial cells, then bound to the adventitia of the vessels, then bound either to the extracellular matrix or to the cells within the tumor, and finally to being cleared from the tumor by lymphatics or blood vessels, and excreted either by the kidneys or the liver. The effect of PDT on the tumor largely depends at which stage of this continuous process light is delivered. The anti-tumor effects of PDT are divided into three main mechanisms. Powerful anti-vascular effects can lead to thrombosis and hemorrhage in tumor blood vessels that subsequently lead to tumor death via deprivation of oxygen and nutrients. Direct tumor cell death by apoptosis or necrosis can occur if the PS has been allowed to be taken up by tumor cells. Finally the acute inflammation and release of cytokines and stress response proteins induced in the tumor by PDT can lead to an influx of leukocytes that can both contribute to tumor destruction as well as to stimulate the immune system to recognize and destroy tumor cells even at distant locations.

Liposomal delivery of photosensitising agents

Photodynamic therapy is a clinically approved treatment for cancer and noncancer diseases. This modality utilises light-activatable chemicals (photosensitising agents) to capture photons and use light energy for the production of cytotoxic reactive molecular species. Most photosensitisers that are in use clinically or in preclinical development are hydrophobic and tend to aggregate in the aqueous environment, which limits their delivery and photosensitising efficiency. Liposomal delivery of photosensitisers will often overcome or decrease these problems. In addition, as with chemotherapeutic agents, liposomal formulations of photo-sensitising agents may help to achieve better selectivity for tumour tissue compared with normal tissue. Over the past years, liposomal photosensitisers have emerged as therapeutic agents in many experimental studies, and have obtained approval for clinical applications. Recent progress in liposomal technology further opens up the possibility of generating more selectively targeted photosensitisers encapsulated in liposomes. This review will cover progress in the use of liposomal photosensitisers, summarise current liposomal formulations, and project future directions for the liposomal delivery of photosensitising agents.

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.

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.

Binding of temoporfin to the lipoprotein fractions of human serum

The binding of a new photosensitizer, temoporfin, to human serum lipoproteins was investigated. [14C]-Temoporfin (0.1-10 micrograms ml-1) was incubated with human serum for 30 min at room temperature or for 20 h at 4 degrees C, prior to stepwise density flotation to separate the lipoprotein fractions. The distribution of the drug was independent of the initial concentration or time and temperature of the incubation. The proportion of temoporfin in each fraction was: very low density lipoprotein 6%, low density lipoprotein 22%, lipoprotein(a) 17%, high density lipoprotein 39% and lipoprotein deficient serum 16%. Autoradiography of agarose gels showed that the drug was associated with the lipoprotein in the fractions. Fractionation of plasma samples collected from a patient after an intravenous infusion of temoporfin revealed a binding profile similar to that obtained in the in vitro study.

The importance of liposomes as models and tools in the understanding of photosensitization mechanisms

The various applications of liposomes in understanding photosensitization are described in this paper, with particular emphasis on the various kinds of information that these models allow to obtain in phototherapy. Liposomes are simple vesicles in which an aqueous phase is enclosed by a phospholipidic membrane. They are suitable models mimicking specific situations occurring in vivo and they allow study of the influence of physicochemical, photobiological and biochemical factors on the uptake of photosensitizers by tissues, their mechanisms of action and the subsequent photoinduced tumor necrosis. Moreover, solubilization of the sensitizer into the bilayer seems to improve its tumoral selectivity and its photodynamic efficiency.

Clinical pharmacokinetic studies of photofrin by fluorescence spectroscopy in the oral cavity, the esophagus, and the bronchi

BACKGROUND

To optimize photodynamic therapy (PDT) and photodetection of cancer, two important variables that must be considered are the uptake of the dye and the dye contrast between normal and neoplastic tissue after injection.

METHODS

To study these variables in a clinical context, an apparatus based on a noninvasive optical fiber that detects the dye by light-induced fluorescence (LIF) was constructed.

RESULTS

Studies on the pharmacokinetics of the fluorescent fraction of Photofrin in patients with early squamous cell carcinoma in the oral cavity, esophagus or bronchi show a signal contrast ranging from 1.5 to 3.5 a short time after intravenous injection that rapidly decreases and tends to unity (one) about 12 hours later. The magnitude of this contrast appears to correlate with the staging of the cancer, the more invasive tumors showing the highest contrast. The more invasive tumors also show the highest uptake. The oral cavity pharmacokinetics are similar to those found in the esophagus and the bronchi.

CONCLUSIONS

The oral cavity appears to be a good model, with easy access for optimizing photodetection and PDT in the esophagus and the bronchi. These pharmacokinetics can be used directly for optimizing photodetection. However, complementary information on the detailed localization of the drug by fluorescence microscopy and a correlation of these data with tumor necrosis efficacy are necessary to optimize PDT timing and therapeutic gain.

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.

Photosensitizer targeting in photodynamic therapy. II. Conjugates of haematoporphyrin with serum lipoproteins

Conjugates between haematoporphyrin (HP) and human low-density lipoprotein (LDL), human high-density lipoprotein (HDL) and bovine HDL have been prepared, purified and characterized. HP-LDL is aggregated possibly via interparticle apoB protein cross-linking. HP-HDL human and bovine conjugates show different degrees of intraparticle apoA polypeptide cross-linking. Receptor-mediated endocytosis of HP-LDL by NIH 3T3 cells is inferred from the increased uptake observed when LDL receptors are upregulated. HP-LDL uptake into HT29 cells faces competition from unlabelled LDL, albeit at rather high doses. HP-HDL uptake is also inhibited by LDL, suggesting that both lipoprotein conjugates may have cell-surface binding sites in addition to the specific LDL (apoB) receptor. J774.2 macrophages avidly accumulate HP-LDL, retaining most of the fluorescence and some of the protein while degrading the remainder. Oxidized LDL species compete in these processes, with the major effect on protein degradation. Chloroquine has little effect on the fluorescence uptake but inhibits protein degradation (and hence enhances protein accumulation). HP-HDL is also avidly taken up by J774.2 cells, but in the case of the bovine material with a sigmoidal concentration dependence. This is consistent with prior aggregation before the particles can be endocytosed. P388.D1 cells, which appear to be less activated than the J774.2 line, take up less fluorescence and retain and degrade less protein, but still to higher extents than observed for non-phagocytic cells. We conclude that photosensitizer-lipoprotein conjugates can be taken up in large amounts by cells possessing scavenger receptors and/or phagocytic activity, and that this may be a means of targeting photodynamic therapy.

Photosensitizer targeting in photodynamic therapy. I. Conjugates of haematoporphyrin with albumin and transferrin

Conjugates of haematoporphyrin (HP) with serum albumin and transferrin were prepared, purified by gel filtration and characterized by high performance liquid chromatography (HPLC), polyacrylamide gel electrophoresis (PAGE) and spectroscopy. Although the fluorescence was somewhat quenched, the conjugates had similar singlet oxygen quantum yields to free porphyrin. The albumin conjugate (HP-BSA) could be divided into monomeric and cross-linked fractions. In NIH 3T3 and HT29 cells, native albumin could not compete with the uptake of HP-BSA and the uptake was greatly enhanced in the absence of serum and in the presence of poly-L-lysine. We infer that the conjugate was mostly associated with the plasma membrane in these cells. The uptake of HP-transferrin showed evidence of a receptor-mediated component in that it was partially inhibited by native protein and increased when transferrin receptors were upregulated by an iron chelator. J774 macrophage-like cells accumulated fluorescence from HP-BSA to a much higher degree than HT29 cells, even though the protein was extensively degraded (HT29 cells did not appear to degrade the protein). The time course of the photocytotoxicity of HP-BSA was prolonged in J774 cells, although their response to free porphyrins was similar to that seen in HT29 cells. Chloroquine inhibited protein degradation without having an effect on the fluorescence uptake. J774 cells acquired more fluorescence and degraded more protein when supplied with cross-linked HP-BSA compared with monomeric fraction. For a given fluorescence uptake, the cross-linked fraction was also more photocytotoxic. We conclude that macrophages can acquire photosensitizer-protein conjugates avidly and that these are delivered to the lysosomes. These types of conjugate may have applications in targeting fluorescent molecules for diagnostic imaging and for the photodynamic treatment of macrophage malignancies.

In vitro interaction of zinc(II)-phthalocyanine-containing liposomes and plasma lipoproteins

We have studied the interaction of small unilamellar liposomes containing zinc(II)-phthalocyanine (Zn-Pc) with human plasma lipoproteins. High-, low- and very low-density lipoproteins (HDL, LDL and VLDL), were purified from plasma and combined in amounts reflecting their natural abundance in plasma. After short periods of incubation at 37 degrees C, the bulk of Zn-Pc was incorporated into HDL and LDL; very little 14C-labelled palmitoyl oleoyl phosphocholine, the most abundant phospholipid in the formulation, was associated with lipoproteins. When liposomes were incubated in pooled plasma, 73%-85% of Zn-Pc and 27%-34% of radiolabelled phospholipid were recovered with HDL and LDL, indicating a possible role for plasma lipid transfer proteins in the incorporation of phospholipid into lipoproteins. Some Zn-Pc was also found in association with VLDL. The buoyant density of Zn-Pc liposomes increased in a dose-dependent fashion when the particles were incubated with plasma, and it is suggested that this was due, at least in part, to opsonization of liposomes by plasma proteins.

On the mechanism of the tumour-localising effect in photodynamic therapy

The proposed mechanisms by which tumours concentrate photosensitisers are reviewed. Tumour-associated macrophages have been shown by others to accumulate up to nine times the level of porphyrins as do tumour cells. Macrophages also take up and degrade oxidised or otherwise modified low-density lipoprotein (LDL). We propose that the interaction of photosensitisers with LDL is an important factor, leading to accumulation in macrophages. Uptake into these cells via liposomes and high-density lipoprotein is also possible. There may be three separate mechanisms for tumour destruction in photodynamic therapy: (i) direct damage to tumour cells; (ii) damage to the endothelial cells of the tumour microvasculature; and (iii) macrophage-mediated immune infiltration of the tumour. The association of photosensitisers with lipoproteins may accentuate the latter two (endothelial cells can also accumulate modified lipoproteins). Accumulation in macrophages may also largely explain the high porphyrin retention observed in atheromatous plaques.

Thermodynamics of the binding of hematoporphyrin ester, a hematoporphyrin derivative-like photosensitizer, and its components to human serum albumin, human high-density lipoprotein and human low-density lipoprotein

The phenomena of the high affinity of porphyrins to the human serum proteins, albumin, high-density lipoproteins (HDL) and low-density lipoproteins (LDL) is well established. Yet, evaluation of the activities of these proteins as endogenous porphyrin carriers, especially with respect to receptor-mediated porphyrin uptake into tumor cells, the merits of which are still in dispute, requires more quantitative protein-porphyrin binding data. As a continuation of previous studies on this issue, the binding of several porphyrin systems to each of the three proteins, employing previously developed spectral methodologies, was studied. The specific systems reported here are hematoporphyrin ester (HPE), which is a novel hematoporphyrin derivative (HPD)-like system, two porphyrin trimers (denoted O1 and O2) and a porphyrin dimer (denoted O3) isolated from HPE. Human serum albumin (HSA) was found to have a single high-affinity site for the monomeric components of HPE, with an equilibrium binding constant of 3.6 x 10(6). The equilibrium parameters determined for the binding of the three HPE-isolated oligomers to each of the serum proteins are: (1) Binding constants (Kb') of 2.3 x 10(6), 6.9 x 10(4) and 1.5 x 10(4) and number of sites per protein molecule (n) of 3, 1 and 5, for the binding of O1, O2 and O3, respectively, to HSA. (2) Kb' values of 15.5 x 10(3), 15.3 x 10(3) and 6.6 x 10(3) and n values of 1, 2 and 2, for the binding of O1, O2 and O3, respectively, to HDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular delivery and retention of photofrin: III. Role of plasma proteins in photosensitizer clearance from cells

Human plasma proteins, albumin, globulins and low density (LDL), high density (HDL) and very low density (VLDL) lipoproteins were tested for their effects on retention of Photofrin and three other photosensitizers in cultured cells. This was assessed by incubating the cells, subsequent to the exposure to Photofrin, in the photosensitizer-free medium containing various concentrations of different plasma proteins. Photofrin clearance levels differed with individual plasma proteins and also were dependent on concentration of these proteins in the incubation medium. All of the proteins except VLDL promoted clearance of Photofrin taken up by the cells in the presence of 5% human serum. Subsequent to some Photofrin exposure conditions (in the presence of 5% fetal bovine serum, or in protein-free medium), albumin, in contrast to LDL, HDL and globulins, exhibited decreased capacity for promoting the photosensitizer clearance from the cells. The VLDL showed very little or no effect in promoting cellular clearance of Photofrin, tetraphenyl porphine tetrasulfonate (TPPS4), and di- and tetrasulfonated chloroaluminum phthalocyanine (AlPcS2 and AlPcS4, respectively). The LDL seem to be particularly effective in promoting clearance of Photofrin and AlPcS2 from the cells, whereas albumin and globulins were shown to be more effective than LDL and HDL in promoting the cellular clearance of TPPS4.

Binding of drugs to human plasma proteins, exemplified by Sn(IV)-etiopurpurin dichloride delivered in cremophor and DMSO

1. The mode-delivery-effect upon the binding of Sn(IV)-etiopurpurin dichloride (SnET2) in human plasma has been studied by ultracentrifugation, combined with absorption and fluorescence spectroscopy. SnET2 was delivered to plasma either in Cremophore EL (CRM) or in dimethyl sulfoxide (DMSO). To facilitate interpretation, optical, conductivity and aggregation properties of SnET2 were obtained for various solutions. 2. The second order rate constant for the aggregation of SnET2 monomers seemed to be remarkably small, of the order of 10(3) M-1 min-1. 3. SnET2 was bound as monomeric entities. Such entities had environmental-sensitive fluorescent properties dependent on the type of protein or solvent (DMSO, CRM, H2O) with which they interacted. 4. SnET2 showed saturable binding with high density subfraction(s) of high density lipoproteins and with one or more high density proteins. Complete or substantial saturation was achieved at the SnET2 level of 3.5 micrograms/ml. Such binding might be mediated by apolipoprotein D and alpha 1-acid glycoprotein. 5. There was little effect of SnET2 concentrations (3.5-35 micrograms SnET2/ml) upon the plasma binding of SnET2, irrespective of the mode of delivery. 6. The percentages of SnET2 bound to low density lipoproteins (LDL), high density lipoproteins (HDL), and high density proteins (HDP) were 10, 70 and 20%, respectively, for delivery in DMSO. The value for LDL also includes binding with very low density lipoproteins (VLDL). For delivery in CRM the corresponding values were 20, 50 and 30%. Apparently, CRM interacted with HDL entities and reduced their affinity for SnET2. 7. The distribution pattern of SnET2 among lipoproteins reflects interactions with apoproteins and/or with surface phospholipids rather than with core lipid constituents of lipoproteins. 8. Conductivity measurements showed that SnET2 was partly an ionic entity in water. 9. The plasma binding of SnET2 is compared with the corresponding binding of other drugs, both tetrapyrroles and nontetrapyrroles.

The role of lipoproteins in the distribution of tin etiopurpurin (SnET2) in the tumor-bearing rat

The role of plasma lipoproteins in the distribution of the photosensitizing agent tin etiopurpurin (SnET2) was examined in male rats bearing the N-[4-(5-nitro-2-furyl)-2- thiazolyl1bdformamide-induced tumor. Treatment with 17 alpha-ethinyl estradiol resulted in the depletion of total plasma cholesterol by > 70% and a corresponding decrease in plasma lipoproteins. To both control and estradiol-treated animals, a therapeutic dose (1.5 mg/kg) of SnET2 was administered and biodistribution measured 24 h later. Estradiol treatment was not associated with differences in the distribution of SnET2 to liver, skin or tumor, or in the pattern of affinity of SnET2 to plasma albumin and lipoprotein. These results indicate that a substantial decrease in circulating lipoprotein levels does not alter patterns of SnET2 biodistribution.