Determination of rate constants of 1O2 consumption by 1O2 acceptors in weakly deactivating solvents

Development of a Fluorescent Probe for Measurement of Singlet Oxygen Scavenging Activity of Flavonoids

A turn-on fluorescent probe, HOCD-RB, for monitoring singlet oxygen (¹O₂) was developed by linking rhodamine B as fluorophore with dimethylhomoocoerdianthrone (HOCD) as ¹O₂ reaction site and fluorescence quencher due to the intramolecular energy transfer (ET) between rhodamine B and HOCD moieties. Upon exposure to ¹O₂ it rapidly forms endoperoxide with HOCD and turns on the fluorescence of rhodamine B by 18-fold. Taking advantage of the HOCD-RB probe that shows fast response, high sensitivity, and selectivity for ¹O₂, it is applied for imaging of endogenous ¹O₂ in living cells and the fluorometric assay for evaluating ¹O₂ quenching activity of selected common flavonoids found in our daily diets. The results show that the ¹O₂ scavenging activity of flavonoids depends on not only the structure of individual flavonoid but also the competitive interactions between mixed flavonoids. The best antioxidant capacity for individual and mixed flavonoids is epigallocatechin gallate and the mixture of catechin gallate with kaempferol, respectively. Overall, this work provided a new tool for detection and imaging of singlet oxygen activity in a biological system as well as an efficient fluorometric assay of ¹O₂ scavenging activity.

Laser photo-oxidative degradation of 4,6-dimethyldibenzothiophene

Degradation of 4,6-dimethyldibenzothiophene (DMDBT), persistent sulfur contaminant in fuel oils has been achieved in non-polar phase by laser-irradiating DMDBT alone and in the presence of hydrogen peroxide and molecular oxygen. The most efficient degradation in the presence of molecular oxygen is judged to be the first example of self-sensitized photo-oxygenation of thiophenes, in which DMDBT sequentially acts as 1O2 sensitizer and reactant.

Photodynamic therapy: basic principles and potential uses for the veterinary ophthalmologist

Photodynamic therapy (PDT) involves the use of photochemical reactions mediated through the interaction of photosensitizing agents, light and oxygen. PDT, while now commonly used in physician ophthalmology and oncology, is uncommonly used for the veterinary ophthalmic patient. It is an emerging new therapy in veterinary ophthalmology for the treatment of periocular tumors. This article reviews the basic principles of PDT to provide the veterinary ophthalmologic community with a succinct reference for this emerging treatment modality in our field.

Kinetics of the superficial perfusion and temperature in connection with photodynamic therapy of basal cell carcinomas using esterified and non-esterified 5-aminolaevulinic acid

BACKGROUND

Photodynamic therapy (PDT) is a local treatment modality with increasing indications for various malignant and non malignant diseases. The treatment parameters have not yet been optimized as there is a need for a better understanding of the process. The skin is an important target and serves as a good model for monitoring and evaluating the interaction of light with biological tissue.

OBJECTIVES

The tissue perfusion and the temperature of basal cell carcinomas were measured in connection with PDT in order to investigate the biological mechanisms involved.

METHODS

An infrared camera was used during the treatment to measure skin temperature and a laser Doppler perfusion imaging device was used to image the superficial perfusion before and after treatment. Six hours after topical application of 5-aminolaevulinic acid (ALA) or methyl esterified ALA (ALA-ME), 38 basal cell carcinomas were treated using light from a diode laser at 633 nm.

RESULTS

In the lesions, the perfusion immediately after PDT was similar to that before PDT. One hour after the treatment the perfusion in the lesion was increased 50% compared with before PDT. However, in the skin surrounding the lesions the perfusion was doubled immediately after PDT and was still increasing 1 h after treatment. A temperature increase in the lesions of about 1-3 degrees C was observed for light fluence rates of 100-150 mW cm-2. In all patients treated, a diffuse temperature increase was visible outside the lesions. In some of the patients, the outlines of the blood vessels surrounding the treated lesions became visible in the thermal images. Measurements of temperature on healthy volunteers not administered photosensitizer, but illuminated with light of the same fluence rate, showed a similar increase in temperature in the illuminated spots. However, no temperature increase was observed outside the illuminated area. No statistically significant differences were found between the measurements on patients treated with ALA and ALA-ME.

CONCLUSIONS

The increased perfusion in the area surrounding the lesions after PDT, as seen by perfusion and temperature measurements, is the result of an inflammatory reaction to the PDT process. However, directly after PDT the perfusion in the lesions was the same as before irradiation. The combination of these observations suggests the presence of local blood stasis during and immediately after the treatment. The temperature measurements showed that the increased temperature was well below the temperature limit of hyperthermal damage. Furthermore, the measurements indicate that the increase in temperature was primarily a consequence of the heat absorbed in the tissue.

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.

Light-dependent oxygen consumption in bacteriochlorophyll-serine-treated melanoma tumors: on-line determination using a tissue-inserted oxygen microsensor

Successful application of anticancer therapy, and especially photodynamic therapy (PDT) mediated by type II (PDTII) processes, depends on the oxygen content within the tumor before, during and after treatment. The high consumption of oxygen during type II PDT imposes constraints on therapy strategies. Although rates of oxygen consumption and repletion during PDTII were suggested by theoretical studies, direct measurements have not been reported. Application of a novel oxygen sensor allowed continuous and direct in situ measurements (up to a depth of 8-9 mm from the tumor surface and for several hours) of temporal variations in the oxygen partial pressure (pO2) during PDT. Highly pigmented M2R mouse melanoma tumors implanted in CD1 nude mice were treated with bacteriochlorophyll-serine (Bchl-Ser; a new photodynamic reagent) and were subjected to fractionated illumination (700 < lambda < 900 nm) at a fluence rate of 12 mW cm-2. This illumination led to total oxygen depletion with an average consumption rate of 7.2 microM(O2) s-1. Spontaneous reoxygenation (at an average rate of 2.5 microM(O2)/s) was observed during the following dark period. These rates are in good agreement with theoretical considerations (Foster et al., Radiat. Res. 126, 296, 1991 and Henning et al., Radiat. Res. 142, 221, 1995). The observed patterns of oxygen consumption and recovery during prolonged periods of light/dark cycles were interpreted in terms of vasculature damage and sensitizer clearance. The presented data support the previously suggested advantages of fractionated illumination for type II photodynamic processes.

New applications in photodynamic therapy. Introduction

Photodynamic therapy is now recognized as a legitimate therapy for both palliative and potentially curative treatment of solid tumors. This represents a major achievement for investigators who have committed their careers to PDT. Scientific research characteristically runs well ahead of accepted views and technologies. This Symposium-in-Print perhaps provides a snapshot of the potential for this technology during the next two decades.

Photodynamic therapy of oral cancer. A review of basic mechanisms and clinical applications

Photodynamic therapy (PDT) is an experimental cancer treatment modality. PDT is based on the accumulation of a photosensitive dye in premalignant and malignant lesions. A certain period of time after the dye has been administered, tumor tissue may contain more of the sensitizer then the surrounding normal tissues. When tissue containing the sensitizer is exposed to light of a proper wavelength and dose, a photochemical reaction between sensitizer and light will occur. The activated photosensitizer reacts with available oxygen which subsequently damages cells and eventually may cause necrosis of the tumor. Photosensitizers can also be used for fluorescence detection. If a tumor contains more of the photosensitizer than the surrounding normal tissue, its fluorescence can potentially be utilized to detect tumors. Analogous to PDT, this can therefore be referred to as photodynamic detection (PDD). This paper reviews the basic mechanisms and clinical applications of PDT and PDD. Emphasis is placed on PDD and PDT with the photosensitizer Photofrin for detection and treatment of premalignant epithelial lesions and squamous cell carcinomas of the oral mucosa.

Benzophenothiazine and benzoporphyrin derivative combination phototherapy effectively eradicates large murine sarcomas

The tumoricidal effects of photochemotherapy with two photosensitizers, 5-ethylamino-9-diethylaminobenzo[a] phenothiazinium chloride (EtNBS) and benzoporphyrin derivative monoacid ring A (BPD-MA), were evaluated separately and in combination against the EMT-6 fibrosarcoma implanted subcutaneously in BALB/c mice. Animals carrying tumors 8-10 mm in diameter were divided into eight different groups (approximately 20/group) and subjected to various photoirradiation and drug conditions. The tumor response to photodynamic therapy (PDT) was measured as the mean tumor wet weight 2 weeks post-PDT. The combination treatment with 5.25 mg/kg EtNBS and 2.5 mg/kg BPD-MA followed by photoirradiation with 100 J/cm2 at 652 nm and then by 100 J/cm2 at 690 nm resulted in a 95% reduction in the average tumor weights compared to controls (no light, no drugs) with 76% of the mice being tumor free 2 weeks post-PDT. Because treatment with EtNBS or BPD-MA at twice the light dose and drug concentration resulted in either no significant reduction in tumor weights or increased the lethality of treatment, respectively, the data suggest that the enhanced PDT effect observed with the combination of drugs is synergistic rather than additive. Histology of tumors 24 h post-PDT with the combination of drugs showed nearly complete destruction of the tumor mass with little or no damage to the vasculature and no extravasation of red blood cells. There was no damage to the normal skin adjacent to the tumor. Fluorescence microscopy of EMT-6 cells incubated in vitro with the two photosensitizers revealed that they were localized to different intracellular compartments. The fluorescence pattern from frozen tumor tissue slices following the in vivo administration of the photosensitizers indicated a greater intracellular localization for EtNBS vs BPD-MA.

The effects of thromboxane inhibitors on the microvascular and tumor response to photodynamic therapy

Vascular stasis and tissue ischemia are known to cause tumor cell death in several experimental models after photodynamic therapy (PDT); however, the mechanisms leading to this damage remain unclear. Because previous studies indicated that thromboxane release is implicated in vessel damage, we further examined the role of thromboxane in PDT. Rats bearing chondrosarcoma were injected with 25 mg/kg Photofrin (intravenously) 24 h before treatment. Light (135 J/cm2, 630 nm) was delivered to the tumor area after injection of one of the following inhibitors: (1) R68070: a thromboxane synthetase inhibitor; (2) SQ-29548: a thromboxane receptor antagonist; and (3) Flunarizine: an inhibitor of platelet shape change. Systemic thromboxane levels were determined. Vessel constriction and leakage were evaluated by intravital microscopy. Tumor response was assessed after treatment. Thromboxane levels were decreased more than 50% with SQ-29548 as compared to controls. Thromboxane levels in animals given R68070 and Flunarizine remained at baseline levels. SQ-29548 and R68070 reduced vessel constriction compared to controls, while Flunarizine totally prevented vessel constriction. R68070 and SQ-29548 inhibited vessel permeability compared to PDT controls; Flunarizine did not. Animals given these inhibitors showed markedly reduced tumor cure. These results indicate that the release of thromboxane is linked to the vascular response in PDT.

The effects of photodynamic therapy using differently substituted zinc phthalocyanines on vessel constriction, vessel leakage and tumor response

The effects of four different zinc phthalocyanines were studied during and after photodynamic therapy (PDT). Measurements of vessel constriction, vessel leakage, tumor interstitial pressure, eicosanoid release, and tumor response of chondrosarcoma were made in Sprague-Dawley rats. Animals were injected intravenously with 1 mumol/kg of mono-, di-, or tetrasulfonated zinc phthalocyanine, or 1 mumol/kg of a zinc phthalocyanine substituted with four tertiary butyl groups. Tissues were exposed to 400 J/cm2 670 nm light 24 h after photosensitizer injection. An additional group of animals was given indomethacin before treatment. The use of the monosulfonated and tertiary butyl substituted zinc phthalocyanines in PDT caused the release of specific eicosanoids, caused vessel constriction, and induced venule leakage and increases in tumor interstitial pressure. Tumor cures of 27% and 7% were observed. Photodynamic therapy using the disulfonated zinc phthalocyanine did not induce vessel constriction or the release of eicosanoids, however, tumor cure was 43%. The use of the tetrasulfonated zinc phthalocyanine caused intermediate effects between the mono- and disulfonated compounds. The administration of indomethacin to animals completely inhibited the effects of PDT using the monosulfonated compound but had minimal effects on treatment using the disulfonated compound. This suggests that the monosulfonated and disulfonated compounds act by different mechanisms of destruction.

Mechanisms of cell killing in photodynamic therapy using a novel in vivo drug/in vitro light culture system

Photodynamic therapy of certain neoplasms has emerged as a promising form of cancer treatment. This type of therapy involves the exogenous administration of a photosensitizer with subsequent exposure to light. The ensuing photochemical reaction results in destruction of the tumor. Whether tumor cells are destroyed directly by the photodynamic treatment or indirectly as a result of destruction of the tumor microvascular bed is unknown. To address this question, methods were adapted to test whether combinations of a photosensitizer and light resulted in direct cell killing of precision cut tissue slices placed in culture. The major advantages of this culture system are that photosensitizers are administered in vivo, tissue slices produced in minutes, placed in culture medium, and irradiated in vitro. Any resulting cellular destruction occurs in the absence of a functioning vascular system and indicates that photodynamic therapy acts through a direct cell killing mechanism. Tissue slice viability was monitored by two standard methods: assay for intracellular potassium and morphological examination at the electron microscopic level. The effects of hematoporphyrin derivative and light were examined on tissue slices produced from a prostate adenocarcinoma transplanted into male Copenhagen rats. The data indicate that direct killing of tumor slices occurs and is dependent on the irradiation protocol used.

The mechanism of photosensitization in photodynamic therapy: chemiluminescence caused by photosensitization of porphyrins in saline containing human serum albumin

Chemiluminescence (CL) caused by photosensitization of porphyrins in phosphate buffered saline (PBS) solution containing 3% human serum albumin (HSA) was observed for the first time. Irrespective of porphyrins concerned, CL shows a spectrum ranging from 380 to 520 nm with a peak near 450 nm and decays almost single-exponentially with a lifetime of about 15 s. The intensity of CL depends on concentrations of porphyrins and HSA in PBS solution. We have examined a number of porphyrins and observed CL for the compounds with triplet lifetimes longer than 0.1 ms. The appearance and quenching of CL by photosensitization of porphyrin-HSA systems indicate that type II reaction by singlet oxygen occurs significantly in photodynamic therapy resulting in hypoxic regions in environments surrounding the sensitizer.

Preclinical examination of first and second generation photosensitizers used in photodynamic therapy

Numerous photosensitizers with absorption peaks spanning the 600-800 nm "therapeutic window" have been and continue to be synthesized. Structural modifications of the dyes can then be made in order to improve tumor deliverability and retention. Chemical alterations can also enhance the yields of light generated reactive oxygen species. Utilization of lipoproteins, emulsions and antibody conjugates can enhance the selectivity of drug localization. Most cell types and subcellular structures are highly photosensitive and biochemical analysis indicates that cellular target sites associated with PDT correlate with photosensitizer location. In vivo data suggest that vascular and direct tumor cell damage as well as systemic and local immunological reactions are involved in PDT responsiveness. Additional mechanistic, synthetic and developmental studies are required in order to fully appreciate the potentials of PDT. However, continued enthusiasm and support for basic PDT research (as observed during the past 8 years) will depend to a large extent on the outcome of the current clinical trials.

The mechanism of photosensitization in photodynamic therapy: phosphorescence behavior of porphyrin derivatives in saline solution containing human serum albumin

The phosphorescence properties, especially the dynamic behavior of metal free and metal complexed porphyrins, have been studied in phosphate buffered saline (PBS) containing 0-3% human serum albumin (HSA). 6,7-Bisaspartyl-2,4-bis (1-hexyloxyethyl)-deutero- porphyrin (DP) and its gallium(III), zinc(II), and indium(III) complexes are used as photosensitizers. Upon irradiation, a solution of porphyrins containing more than 0.1% HSA shows phosphorescence with a lifetime longer than 1 ms. With an increase in irradiation time, phosphorescence intensities and lifetimes of porphyrins increase, depending upon their concentrations and triplet lifetimes, and approach saturated values close to those under deaerated conditions. The experimental results may be interpreted in terms of hypoxia induced by photosensitization in a local environment surrounding the sensitizer. The hypoxia is caused by the reaction between proteins and singlet molecular oxygen generated by photosensitization of porphyrins. Phosphorescence behavior of sensitizers in HSA PBS solution gives significant information for classifying photosensitizers as to their efficacy for photodynamic therapy.