Haematoporphyrin and OO'-diacetylhaematoporphyrin binding by serum and cellular proteins. Implications for the clearance of these photochemotherapeutic agents by cells

Hemopexin: a review of biological aspects and the role in laboratory medicine

BACKGROUND

Hemopexin is a heme-binding plasma glycoprotein which, after haptoglobin, forms the second line of defense against hemoglobin-mediated oxidative damage during intravascular hemolysis. A decrease in plasma hemopexin concentration reflects a recent release of heme compounds in the extracellular compartment. Heme-hemopexin complexes are delivered to hepatocytes by receptor-mediated endocytosis after which hemopexin is recycled to the circulation.

METHODS OF ANALYSIS

Immunonephelometric and -turbidimetric hemopexin assays are available as more precise and rapid alternatives to the radial immunodiffusion technique.

INTERPRETATIONS

Hemopexin determinations are not subject to interference by in vitro hemolysis. Altered serum or plasma concentrations of hemopexin are found not only in hemolytic anemias but also in other conditions such as chronic neuromuscular diseases and acute intermittent porphyria. In laboratory medicine, while hemopexin determination in tandem with haptoglobin has potential applications in the assessment of intravascular hemolysis and allows for the monitoring of the severity of hemolysis after depletion of haptoglobin, its diagnostic utility is less clear in other pathological conditions. Further studies are necessary to fully establish the clinical significance of hemopexin determination.

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 activity of lutetium-texaphyrin in a mouse tumor system

BACKGROUND AND OBJECTIVE

New photosensitizers proposed for photodynamic therapy (PDT) treatment of tumors need to be evaluated in animal models to determine the parameters needed for treatment. They also need to be compared with existing photosensitizers for efficacy. We examined the PDT response to lutetium-texaphyrin (PCI-0123) in a mouse mammary adenocarcinoma model and compared it with the PDT response seen when using Photofrin.

STUDY DESIGN/MATERIALS AND METHODS

DBA/2 mice with SMT-F tumors were used to explore PCI-0123 toxicity, laser light dose, and drug dose effects on PDT response and to determine the most effective time for light application. The PDT response of PCI-0123-treated tumors was compared with that of Photofrin-treated tumors.

RESULTS

Treatment of tumors with 150 J/cm2 of 740 nm laser light 5-6 hr after PCI-0123 administration (40 mg/kg) resulted in a 100% response rate and a 55% cure rate. Tumors treated with 150 J/cm2 of 630 nm laser light 24 hr after Photofrin administration (10 mg/kg) resulted in a 67% response rate and a 16% cure rate.

CONCLUSION

PCI-0123 was found to be a more effective photosensitizer than Photofrin.

Acid-base properties of chlorin e6: relation to cellular uptake

Chlorins are attractive compounds for photodynamic therapy because of their high absorption in the red spectral region. In this study, the absorbance, fluorescence excitation and fluorescence emission spectra of chlorin e6 have been recorded as functions of pH in phosphate-buffered saline (PBS) solution with and without fetal calf serum (FCS). For pure PBS solutions, variation of the pH of the solution results in a shift of both the absorption and the fluorescence spectrum as well as in a decrease of the fluorescence intensity. Spectrophotometric and fluorimetric titration curves, based on observed changes, have been plotted. There is an indication of aggregate formation at low pH values (pH < 5). The presence of 5% FCS results in a shift of the titration curve, from an inflection point at about 6.5 to one at about 7.6. Pronounced spectral changes of the fluorescence emission spectra of protein-bound chlorin e6 (change of spectral shape, decrease of peak intensity) are also observed. The partition coefficients in the 1-octanol-water system increase with decreasing pH. Thus, relatively more of the drug is incorporated in the octanol phase at low pH. Cellular uptake of chlorin e6 in the presence of serum is significantly higher at pH 6.7 as compared with that at 7.3 and 7.6. We conclude that a change in the pH value of the surrounding medium leads to a change in the lipophilicity of chlorin e6. Such a change is likely to influence its binding to the serum proteins as well as its interaction with the plasma membrane of cells and may also be related to the selective tumor uptake of the drug.

Porphyrin metabolism in rats intaking chemical carcinogens

This study was performed on rats receiving nitrosamine precursors as potent liver carcinogens in order to investigate and follow up the porphyrin metabolism during the intake of hepatocarcinogens. As clarified from our results, progressive increases in free erythrocyte porphyrins, erythrocyte protoporphyrins, haem content and hepatic total porphyrins were observed after 2 months of intaking these carcinogens and further increases were gradually observed in parallel with the continued intake of these chemicals, which was extended to 7 months. At the same time, such elevations were also observed in the activity of hepatic delta-aminolevulinic acid (ALA)-synthetase and uroporphyrinogen-1-synthetase either in hepatic tissues or erythrocytes of these carcinogen-subjected rats. However, significant inhibition was found in the activity of erythrocyte ALA-dehydratase, which reached 40.5% of the control values after 7 months. Therefore, these observations demonstrated that the intake of hepatocarcinogens may influence the rate of hepatic porphyrin and haem biosynthesis.

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.

Cellular delivery and retention of Photofrin II: the effects of interaction with human plasma proteins

The absorbance and fluorescence spectra of Photofrin II (PII) in the presence of albumin, globulins and lipoproteins from human plasma show that all of these proteins induce a degree of disaggregation of PII material. In addition, there are substantial rearrangements in the distribution of different fractions contained in PII and their binding to the protein. It is shown that these rearrangements have considerable impact on the uptake of PII by cultured cells and the ensuing retention of the drug in the cells. The information on the contribution of fluorescing and non-fluorescing components of PII in the cells was obtained by measuring first the PII fluorescence in suspensions of live cells, followed by chemical extraction of porphyrin material from the same cells. The interaction of PII with low density lipoproteins resulted in markedly lower levels of PII material retained in the cells, compared to protein-free drug exposure. Somewhat better but still inferior PII retention was observed with high density lipoproteins. The samples with very low density lipoproteins showed increased uptake of PII, but the subsequent retention of the drug was low, so that the remaining amount of the drug was not much different than in protein-free samples. The strongest inhibition of PII uptake was seen with albumin, with ensuing retention of PII not significantly different than in protein-free samples. The best retention of PII was observed with globulins, with approx. 25% higher total drug content retained in the cells after long-term clearance relative to protein-free samples.

Binding of porphyrin to human serum albumin. Structure-activity relationships

The equilibrium binding of hydroxyethyl vinyl deuteroporphyrin (HVD) and of irreversible porphyrin aggregates to human serum albumin was studied at the molecular level. This protein may function as an endogenous drug carrier for porphyrins in photodynamic therapy of tumours. HVD-protein binding studies revealed two types of binding sites, which are attributed to the two HVD isomers. The binding constant for the high-affinity isomer, 2.1 (+/- 0.3) x 10(8) M-1, is similar to that previously determined for protoporphyrin. At the same time the binding constant for the lower-affinity HVD isomer, 1.8(+/- 0.3) x 10(6) M-1, is similar to that previously determined for haematoporphyrin. Irreversible porphyrin aggregates were purified from the haematoporphyrin derivative and from Photofrin and are defined by spectral and chromatographic data. Gel-exclusion studies indicate that the dominant size of these aggregates is ten porphyrin monomeric units. The protein-binding constant of these aggregates is 1.7(+/- 0.2) x 10(5) M-1, with four binding sites per protein molecule. The distinction between the HVD isomers along the porphyrin-protein affinity sequence gives insight into the relationship of porphyrin structure to porphyrin-albumin binding. On the basis of this study an evaluation of human serum albumin as an endogenous carrier for porphyrins (at various aggregation states) in photodynamic therapy of tumours is presented.

Interactions with glutathione S-transferases of porphyrins used in photodynamic therapy and naturally occurring porphyrins

Several naturally occurring porphyrins and porphyrins used in photodynamic therapy inhibit glutathione S-transferase isoenzymes either purified from rat liver or lung or in cytosol from normal and from cancerous (Morris 7288C hepatoma) liver. Although differences occur in the type and amount of transferases in normal and cancerous liver and in the liver of rats bearing an extrahepatic tumour, these enzymes are potential binding sites for porphyrins. Porphyrin structure is an important factor in determining the affinity of binding, as shown by the relative inhibitory effectiveness. Of the dicarboxylic porphyrins in the mixture used clinically, OO'-diacetylhaematoporphyrin and monohydroxyethylmonovinyldeuteroporphyrin are more effective inhibitors than haematoporphyrin and protoporphyrin IX. Of the naturally occurring porphyrins the order of effectiveness is protoporphyrin IX (dicarboxylic) greater than coproporphyrin (tetracarboxylic) greater than uroporphyrin (octacarboxylic) and type I greater than type III isomers of both uroporphyrin and coproporphyrin, and the synthetic tetra-meso-phenylporphinetetrasulphonate is a better inhibitor (apparent Ki = 250 nM) than coproporphyrin, which contains a comparable number of negative charges. In addition, iron-porphyrin chelates are more effective inhibitors of the transferases, with 25-fold decrease in Ki value, than the free porphyrins. These results indicate that one means whereby porphyrins accumulate in tissues is the occupation of intracellular binding sites, such as the transferases. Since porphyrins inhibit the activity of these important detoxifying enzymes, there will be metabolic consequences to the cell.

Deuteroporphyrin-albumin binding equilibrium. The effects of porphyrin self-aggregation studied for the human and the bovine proteins

The binding equilibrium of deuteroporphyrin IX to human serum albumin and to bovine serum albumin was studied, by monitoring protein-induced changes in the porphyrin fluorescence and taking into consideration the self-aggregation of the porphyrin. To have control over the latter, the range of porphyrin concentrations was chosen to maker dimers (non-covalent) the dominant aggregate. Each protein was found to have one high-affinity site for deuteroporphyrin IX monomers, the magnitudes of the equilibrium binding constants (25 degrees C, neutral pH, phosphate-buffered saline) being 4.5 (+/- 1.5) X 10(7) M-1 and 1.7 (+/- 0.2) X 10(6) M-1 for human serum albumin and for bovine serum albumin respectively. Deuteroporphyrin IX dimers were found to bind directly to the protein, each protein binding one dimer, with high affinity. Two models are proposed for the protein-binding of porphyrin monomers and dimers in a porphyrin system having both species: a competitive model, where each protein molecule has only one binding site, which can be occupied by either a monomer or a dimer; a non-competitive model, where each protein molecule has two binding sites, one for monomers and one for dimers. On testing the fit of the data to the models, an argument can be made to favour the non-competitive model, the equilibrium binding constants of the dimers, for the non-competitive model (25 degrees C, neutral pH, phosphate-buffered saline), being: 8.0 (+/- 1.8) X 10(8) M-1 and 1.2 (+/- 0.6) X 10(7) M-1 for human serum albumin and bovine serum albumin respectively.

Differential interaction of porphyrins used in photoradiation therapy with ferrochelatase

The mechanism of porphyrin accumulation by tumours is not yet established. If metabolism aids porphyrin elimination, tumours, unlike normal tissues, may not metabolize porphyrins used clinically, such as proto-, haemato-, OO'-diacetyl-haemato- and monohydroxyethyl-monovinyl-deutero-porphyrin. Proto-, haemato- and monohydroxyethyl-monovinyl-deutero-porphyrin are substrates for the mitochondrial enzyme ferrochelatase (EC 4.99.1.1), which can form haem analogues from exogenous porphyrins. The Km values for proto-, haemato- and monohydroxyethyl-monovinyl-deutero-porphyrin are 11, 22 and 23 microM respectively. However, OO'-diacetyl-haematoporphyrin is an effective competitive inhibitor with Ki of 11 microM. Hepatic ferrochelatase specific activity is 5.9 and 5.5 nmol of haem/h per mg of protein respectively in normal Buffalo rat and in those bearing the extrahepatic Morris 7288C hepatoma, and is only 0.13 nmol/h per mg in the hepatomas. Therefore low ferrochelatase activity in cancerous cells may provide one means whereby some porphyrins accumulate in tumours, and the ability of certain porphyrins to act as ferrochelatase inhibitors may provide another.