Oxygen monitoring during 5-aminolaevulinic acid induced photodynamic therapy in normal rat colon. Comparison of continuous and fractionated light regimes

Fractionated illumination after topical application of 5-aminolevulinic acid on normal skin of hairless mice: The influence of the dark interval

We have previously shown that light fractionation during topical aminolevulinic acid based photodynamic therapy (ALA-PDT) with a dark interval of 2h leads to a significant increase in efficacy in both pre-clinical and clinical PDT. However this fractionated illumination scheme required an extended overall treatment time. Therefore we investigated the relationship between the dark interval and PDT response with the aim of reducing the overall treatment time without reducing the efficacy. Five groups of mice were treated with ALA-PDT using a single light fraction or the two-fold illumination scheme with a dark interval of 30 min, 1, 1.5 and 2h. Protoporphyrin IX fluorescence kinetics were monitored during illumination. Visual skin response was monitored in the first seven days after PDT and assessed as PDT response. The PDT response decreases with decreasing length of the dark interval. Only the dark interval of 2h showed significantly more damage compared to all the other dark intervals investigated (P<0.05 compared to 1.5h and P<0.01 compared to 1h, 30 min and a single illumination). No relationship could be shown between the utilized PpIX fluorescence during the two-fold illumination and the PDT response. The rate of photobleaching was comparable for the first and the second light fraction and not dependent of the length of dark interval used. We conclude that in the skin of the hairless mouse the dark interval cannot be reduced below 2h without a significant reduction in PDT efficacy.

Comparing and combining light dose fractionation and iron chelation to enhance experimental photodynamic therapy with aminolevulinic acid

BACKGROUND AND OBJECTIVES

Enhancement of photodynamic therapy (PDT) with 5-aminolevulinic acid (ALA) has been demonstrated experimentally using light dose fractionation or CP94 iron chelation. This study extends this research.

STUDY DESIGN/MATERIALS AND METHODS

In normal rat colon, CP94 administration and light dose fractionation were independently and concurrently employed to enhance ALA-PDT. In colonic rat tumors, the most successful enhancement regimes were employed separately.

RESULTS

Independent use of light dose fractionation and iron chelation produced similar results in normal colon (2.4- and 2.9-fold more necrosis than controls, respectively). Using both techniques simultaneously produced fivefold enhancement. In the colonic tumors, light dose fractionation and iron chelation (using different parameters) produced two and five times the volume of necrosis, respectively.

CONCLUSIONS

Both techniques significantly enhanced ALA-PDT in the normal and neoplastic tissues investigated and produced similar levels of enhancement when comparable parameters were employed. Concurrent use of light dose fractionation and iron chelation in normal colon produced considerably more enhancement than either technique could achieve independently.

In vivo measurement of parameters of dosimetric importance during interstitial photodynamic therapy of thick skin tumors

A system for interstitial photodynamic therapy is used in the treatment of thick skin tumors. The system allows simultaneous measurements of light fluence rate, sensitizer fluorescence, and tissue oxygen saturation by using the same fibers as for therapeutic light delivery. Results from ten tumor treatments using delta-aminolevulinic acid (ALA)-induced protoporphyrin IX show a significant, treatment-induced increase in tissue absorption at the therapeutic wavelength, and rapid sensitizer photobleaching. The changes in oxy- and deoxyhemoglobin content are monitored by means of near-infrared spectroscopy, revealing a varying tissue oxygenation and significant changes in blood volume during treatment. These changes are consistent with the temporal profiles of the light fluence rate at the therapeutic wavelength actually measured. We therefore propose the observed absorption increase to be due to treatment-induced deoxygenation in combination with changes in blood concentration within the treated volume. A higher rate of initial photobleaching is found to correlate with a less pronounced increase in tissue absorption. Based on the measured signals, we propose how real-time treatment supervision and feedback can be implemented. Simultaneous study of the fluence rate, sensitizer fluorescence, and local tissue oxygen saturation level may contribute to the understanding of the threshold dose for photodynamic therapy.

Nonlinear photodynamic therapy: method of pulsed oxygen depletion

The photodynamic therapy technique involving pulsed oxygen depletion (POD) in tissue by long high-energy pulses of light was studied theoretically. The possibility of creating a uniform distribution of a therapeutic dose throughout a tumor using both surface and interstitial irradiation is shown. Possible thickness of the treated tissue layer is estimated. The comparison with other methods of nonlinear photodynamic therapy is made.

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.

Clinical system for interstitial photodynamic therapy with combined on-line dosimetry measurements

A system for interstitial photodynamic therapy with delta-aminolaevulinic acid and multiple optical fibers has been developed. The system enables photodynamic treatment of large embedded tumor volumes and utilizes real-time measurements to allow on-line dosimetry. Important parameters such as light fluence rate, sensitizer fluorescence intensity, and changes in local blood oxygen saturation are measured with the same fibers that deliver the therapeutic light. Data from the first clinical treatments on nodular basal cell carcinomas indicate a major treatment-induced light absorption increase, rapid sensitizer photobleaching, and a relatively constant global tissue oxygen saturation level during the treatment.

The vascular response to photodynamic therapy with ATX-S10Na(II) in the normal rat colon

The mechanism of tissue damage from photodynamic therapy (PDT) may be cellular, vascular or both, depending on the photosensitising agent and the treatment conditions. Well established photosensitisers like porfimer sodium have an optimum drug light interval of two days and may cause skin photosensitivity lasting several weeks. ATX-S10Na(II) is a new photosensitiser that remains largely in the vasculature after systemic administration and clears from the body within a few hours. The present study looks at the factors controlling the extent of PDT necrosis using ATX-S10Na(II) and correlates these with changes in the circulation after PDT. Normal Wistar rats were sensitised with ATX-S10Na(II), 2 mg/kg. At laparotomy, a laser fibre was positioned just touching the colonic mucosa and 50 J light at 670 nm delivered varying the drug light interval (0.5-24 h) and light delivery regime (100 mW continuous, 20 mW continuous or 100 mW in five fractions). Some animals were killed at three days to document the area of necrosis, others received fluorescein shortly prior to death (from a few minutes to three days after PDT) to outline the zone of PDT induced vascular shutdown. Maximum necrosis was seen with the shortest drug light interval (0.5 h), with no effect by 6 h. Fractionating the light or lowering the power did not increase the necrosis. The area of fluorescein exclusion increased over the first 2 h after PDT (in contrast to the re-perfusion seen with other photosensitisers) and correlated with the area of necrosis. PDT with ATX-S10Na(II) is most effective with a drug light interval of less than one hour. It induces irreversible vascular shutdown that extends after completion of light delivery and which is largely independent of the light delivery regime.

Application of lower fluence rate for less microvasculature damage and greater cell-killing during photodynamic therapy

During the process of photodynamic therapy (PDT), problems arise such as stasis or occlusion of microvasculature, tumor oxygen depletion, and photosensitizer bleaching. This study shows that the first problem could be reduced by using a lower fluence rate light source in PDT. Microvasculature damage was studied experimentally in hematoporphyrin derivative-mediated PDT against light fluence rate, and, to some extent, less microvasculature damage was induced under 75 mW/cm2 illumination than under 150 mW/cm2. Histology of vessels at the end of PDT showed that vessel damage could be observed in both groups, indicating that the microvasculature would eventually be damaged as long as the administration of light fluence was sufficient and regardless of the illuminating fluence rates. Thus microvasculature damage induced by low fluence rate illumination could also be effective in tumor control after PDT. The cell-killing experiment was performed in vitro and designed so that cell-killing rate was influenced only by light characteristics. The higher cell-killing rate caused by 75 mW/cm2 illumination indicated that lower fluence rate light could enhance the light absorbency or decrease the bleaching of photosensitizer.

Monitoring PDT-induced damage using spectrally resolved reflectance imaging of tissue oxygenation

Photodynamic therapy (PDT) with chlorin e6 (Chl) was monitored in vivo using vital microscopy and Fourier transform spectral imaging (FT-SI). Mammary C26 colon carcinoma, implanted intradermally in a mouse, was irradiated at 650 nm with various radiant exposures, 3 h after administration of 5 mg/kg Chl. The photodynamic response (PDR) in the skin flap with tumor was expressed as microcirculation disturbances (thrombi formation, multiple embolizations, arteriolar occlusion and venous stasis) and, dependent on the radiant exposure, was transient or permanent. These biological manifestations were accompanied by a change in hemoglobin (Hb)/oxyhemoglobin (HbO2) absorption spectra obtained in vivo by FT-SI. False-color mapping of hemoglobin oxygen saturation (OS) visualized the alteration of tissue oxygenation. The results demonstrate, for the first time, that FT-SI can serve as a sensitive non-invasive tool for OS monitoring of PDT effects.

Hypericin-mediated Photocytotoxic Effect on HT-29 Adenocarcinoma Cells Is Reduced by Light Fractionation with Longer Dark Pause Between Two Unequal Light Doses

The present study demonstrates the in vitro effect of hypericin-mediated PDT with fractionated light delivery. Cells were photosensitized with unequal light fractions separated by dark intervals (1 or 6 h). We compared the changes in viability, cell number, survival, apoptosis and cell cycle on HT-29 cells irradiated with a single light dose (12 J/cm(2)) to the fractionated light delivery (1 + 11 J/cm(2)) 24 and 48 h after photodynamic treatment. We found that a fractionated light regime with a longer dark period resulted in a decrease of hypericin cytotoxicity. Both cell number and survival were higher after light sensitization with a 6-h dark interval. DNA fragmentation occurred after a single light-dose application, but in contrast no apoptotic DNA formation was detected with a 6-h dark pause. After fractionation the percentage of cells in the G1 phase of the cell cycle was increased, while the proportion of cells in the G2 phase decreased as compared to a single light-dose application, i.e. both percentage of cells in the G1 and G2 phase of the cell cycle were near control levels. We presume that the longer dark interval after the irradiation of cells by first light dose makes them resistant to the effect of the second illumination. These findings confirm that the light application scheme together with other photodynamic protocol components is crucial for the photocytotoxicity of hypericin.

Carbogen breathing significantly enhances the penetration of red light in murine tumours in vivo

We report results of experiments that evaluated the influence of oxygenation on the penetration of red light in tissue, with particular emphasis on 630 and 650 nm laser wavelengths commonly used in photodynamic therapy (PDT) of solid tumours. Direct measurements in tissue-simulating phantoms comprised of intact human erythrocytes suspended in a scattering emulsion demonstrated significant enhancements in fluence rate at depths of 0.5-2 cm from the irradiated surface when the cells were fully oxygenated versus fully deoxygenated. The 630 and 650 nm fluence rates at depth in the homogeneous phantoms continued to increase when examined over a continuous range of oxygen partial pressures from 0 to 160 Torr. When considered as a function of haemoglobin oxygen saturation, the largest increases in fluence rate were observed as the saturation increased beyond 70%. Dramatic increases in optical fluence rate were measured at the base of 1-cm-thick subcutaneous EMT6 mammary carcinomas in vivo when the tumour-bearing mouse was subjected to carbogen through a nose cone. These results indicate that improved tumour oxygenation is important in PDT not only for the maintenance of the oxygen-dependent photochemistry but, through the effects reported here, may also enable more efficient treatment of thicker lesions.

Treatment-induced changes in tumor oxygenation predict photodynamic therapy outcome

Photodynamic therapy (PDT) requires oxygen to cause tumor damage, yet therapy itself can deplete or enhance tumor oxygenation. In the present work we measured the PDT-induced change in tumor oxygenation and explored its utility for predicting long-term response to treatment. The tissue hemoglobin oxygen saturation (SO(2)) of murine tumors was noninvasively measured by broadband diffuse reflectance spectroscopy. In initial validation studies, the oxyhemoglobin dissociation curve for mouse blood was accurately recreated based on measurements during deoxygenation of a tissue phantom of mouse erythrocytes. In vivo studies exhibited excellent correlation between carbogen-induced changes in SO(2) and pO(2) of radiation-induced fibrosarcoma tumors measured by reflectance spectroscopy and the Eppendorf pO(2) histograph, respectively. In PDT studies radiation-induced fibrosarcoma tumor SO(2) was measured immediately before and after Photofrin-PDT (135 J/cm(2), 38 mW/cm(2)). Animals were subsequently followed for tumor growth to a volume of 400 mm(3) (time-to-400 mm(3)) or the presence of tumor cure (no tumor growth at 90 days after treatment). In animals that recurred, the PDT-induced change in tumor SO(2), i.e., relative-SO(2) (SO(2) after PDT/SO(2) before PDT) was positively correlated with treatment durability (time-to-400 mm(3)). The predictive value of relative-SO(2) was confirmed in a second group of animals with enhanced pre-PDT oxygenation due to carbogen breathing. Furthermore, when all of the animals were considered (those that recurred and those that were cured) a highly significant association was found between increasing relative-SO(2) and increasing probability of survival, i.e., absence of recurrence. As independent variables, the SO(2) after PDT, the pre-PDT tumor volume, and light penetration depth all failed to predict response. As an independent variable, the SO(2) before PDT demonstrated a weak negative association with treatment durability; this association was driven by a correlation between decreasing pre-PDT SO(2) and increasing relative-SO(2). These data suggest that monitoring of PDT-induced changes in tumor oxygenation may be a valuable prognostic indicator.

Application of lower fluence rate for less microvasculature damage and greater cell-killing during photodynamic therapy

During the process of photodynamic therapy (PDT), problems arise such as stasis or occlusion of microvasculature, tumor oxygen depletion, and photosensitizer bleaching. This study shows that the first problem could be reduced by using a lower fluence rate light source in PDT. Microvasculature damage was studied experimentally in hematoporphyrin derivative-mediated PDT against light fluence rate, and, to some extent, less microvasculature damage was induced under 75 mW/cm(2) illumination than under 150 mW/cm(2). Histology of vessels at the end of PDT showed that vessel damage could be observed in both groups, indicating that the microvasculature would eventually be damaged as long as the administration of light fluence was sufficient and regardless of the illuminating fluence rates. Thus microvasculature damage induced by low fluence rate illumination could also be effective in tumor control after PDT. The cell-killing experiment was performed in vitro and designed so that cell-killing rate was influenced only by light characteristics. The higher cell-killing rate caused by 75 mW/cm(2) illumination indicated that lower fluence rate light could enhance the light absorbency or decrease the bleaching of photosensitizer.

Correlation of real-time haemoglobin oxygen saturation monitoring during photodynamic therapy with microvascular effects and tissue necrosis in normal rat liver

Photodynamic therapy (PDT) requires a photosensitising drug, light and oxygen. While it is known that the haemoglobin oxygen saturation (HbSat) can be altered by PDT, little has been done to correlate this with microvascular changes and the final biological effect. This report describes such studies on the normal liver of rats sensitised with aluminium disulphonated phthalocyanine. In total, 50 J of light at 670 nm, continuous or fractionated at 25 or 100 mW, was applied with a single laser fibre touching the liver surface. HbSat was monitored continuously 1.5-5.0 mm from the laser fibre using visible light reflectance spectroscopy (VLRS). Vascular shutdown was assessed by fluorescein angiography 2-40 min after light delivery. Necrosis was measured at post mortem 3 days after PDT. In all treatment groups at a 1.5 mm separation, HbSat fell to zero with little recovery after light delivery. At 2.5 mm, HbSat also decreased during light delivery, except with fractionated light, but then recovered. The greatest recovery of fluorescein perfusion after PDT was seen using 25 mW, suggesting an ischaemia/reperfusion injury. Necrosis was more extensive after low power and fractionated light than with 100 mW, continuous illumination. We conclude that VLRS is a useful technique for monitoring HbSat, although the correlation between HbSat, fluorescein exclusion and necrosis varied markedly with the light delivery regimen used.

Influence of uterine cervix shape on photodynamic therapy efficiency

The goal of practical photodynamic therapy (PDT) dosimetry is to optimize the distribution of a light dose delivered to tissue by selecting the irradiation time and geometry to match the geometry and optical properties of the tumor and surrounding tissue. Homogeneous irradiation is among one of the sources of correct PDT dosimetry. The goal of this study is to model and predict the influence of the shape of a treated organ in need of light dose correction. Thus efficiency of light delivery to the tissue volume is defined and calculated with shape factors of the uterine cervix as parameters. Two cases (parallel and divergent beam) of enlightening configuration are investigated. The calculations presented extend PDT dosimetry with the influence of the shape of the uterine cervix on PDT necrosis depth. This allows for photodynamic excitation light dose correction for more reliable treatments.

5-Aminolaevulinic acid-induced photodynamic therapy and photodetection in Barrett's esophagus

Barrett's esophagus is a precursor of adenocarcinoma of the esophagus. This cancer has the fastest growing incidence of any solid tumor in the Western world. Surveillance of Barrett's esophagus is routinely undertaken to detect early malignant transformation. However, ablative endoscopic treatments are available and these can obliterate the abnormal epithelium, allowing neosquamous regrowth. Photodynamic therapy using 5-aminolaevulinic acid (ALA) is such a technique. In this non-thermal method of ablation, ALA is metabolized to produce the photosensitizer protoprophyrin IX. This, together with light and oxygen, produces local tissue destruction. Fluorescence detection using ALA has also been used to identify areas of dysplasia and thus enhance positive biopsy yield. The use of ALA in photodynamic therapy and photodetection is reviewed.

Differences between cytotoxicity in photodynamic therapy using a pulsed laser and a continuous wave laser: study of oxygen consumption and photobleaching

Oxygen consumption at the targeted site has a significant effect on dosimetry in photodynamic therapy (PDT). However, oxygen consumption in PDT using a pulsed laser as a light source has not been clarified. We therefore investigated the dependence of cytotoxicity on the oxygen consumption and the photosensitizer photobleaching of PDT using a pulsed laser by comparing with that using a continuous wave (CW) laser. Mouse renal carcinoma cells (Renca) were incubated with a second-generation photosensitizer, PAD-S31. The cells were then irradiated with either a 670-nm nanosecond pulsed light from the 3rd harmonics of a Nd:YAG laser-pumped optical parametric oscillator with a peak fluence rate of approximately 1 MW/cm(2) at 30 Hz or a 670-nm CW diode laser with a total light dose of 40 J/cm(2). Regardless of laser source, cytotoxic effects exhibited cumulative dose responses to the photosensitizer ranging from 12 to 96 microg/ml. However, cytotoxic effect of PDT using the pulsed light was significantly less than that using the CW light with the photosensitizer concentrations of 24 and 48 microg/ml under identical fluence rates. During PDT, the cells exposed to the pulsed light consumed oxygen more slowly, resulting in a lower amount of oxygen consumption when compared with PDT using CW light. In accordance with oxygen consumption, the pulsed light induced significantly less photobleaching of the photosensitizer than the CW light did. These results indicate that the efficiency of PDT using pulsed light is less when compared with CW light, probably being related to suppressed oxygen consumption during the pulsed light irradiation.

Hyperoxygenation enhances the tumor cell killing of photofrin-mediated photodynamic therapy

Tumor hypoxia, either preexisting or as a result of oxygen depletion during photodynamic therapy (PDT) light irradiation, can significantly reduce the effectiveness of PDT-induced cell killing. To overcome tumor hypoxia and improve tumor cell killing, we propose using supplemental hyperoxygenation during Photofrin-PDT. The mechanism for the tumor cure enhancement of the hyperoxygenation-PDT combination is investigated using an in vivo-in vitro technique. A hypoxic tumor model was established by implanting mammary adenocarcinoma in the hind legs of mice. Light irradiation (200 J/cm2 at either 75 or 150 mW/cm2), under various oxygen supplemental conditions (room air, carbogen, 100% normobaric or hyperbaric oxygen), was delivered to animals that received 12.5 mg/kg Photofrin 24 h before light irradiation. Tumors were harvested at various time points after PDT and grown in vitro for colony formation analysis. Treated tumors were also analyzed histologically. The results show that when PDT is combined with hyperoxygenation, the hypoxic condition could be improved and the cell killing rate at various time points after PDT could be significantly enhanced over that without hyperoxygenation, suggesting an enhanced direct and indirect cell killing associated with high-concentration oxygen breathing. This study further confirms our earlier observation that when a PDT treatment is combined with hyperoxygenation it can be more effective in controlling hypoxic tumors.

Monitoring in situ dosimetry and protoporphyrin IX fluorescence photobleaching in the normal rat esophagus during 5-aminolevulinic acid photodynamic therapy

Experimental therapies for Barrett's esophagus, such as 5-aminolevulinic acid (ALA)-based photodynamic therapy (PDT), aim to ablate the premalignant Barrett's epithelium. However, the reproducibility of the effects should be improved to optimize treatment. Accurate irradiation with light of a proper wavelength (633 nm), fluence and fluence rate has shown to be critical for successful ALA-PDT. Here, we have used in situ light dosimetry to adjust the fluence rate measured within the esophagus for individual animals and monitored protoporphyrin IX (PpIX) fluorescence photobleaching simultaneously. Rats were administered 200 mg kg-1 ALA (n = 14) or served as control (n = 7). Animals were irradiated with an in situ measured fluence rate of 75 mW cm-2 and a fluence of 54 J cm-2. However, this more accurate method of light dosimetry did not decrease the variation in tissue response. Large differences were also observed in the dynamics of PpIX fluorescence photobleaching in animals that received the same measured illumination parameters. We found that higher PpIX fluorescence photobleaching rates corresponded with more epithelial damage, whereas lower rates corresponded with no response. A two-phased decay in PpIX fluorescence could be identified in the response group, with a rapid initial phase followed by a slower rate of photobleaching. Non-responders did not show the rapid initial decay and had a significantly lower rate of photobleaching during the second phase of the decay (P = 0.012).

Fluorescence microspectroscopy technique for the study of intracellular protoporphyrin IX dynamics

We describe a technique designed to monitor the fluorescence dynamics of photosensitizers used in photodynamic therapy (PDT) at micrometer-scale locations within individual formalin-fixed cells. The accumulation of protoporphyrin IX (PpIX) within keratinocytes and fibroblasts. following incubation with 5-aminolaevulinic acid (ALA), is shown to be dependent upon both incubation time and cell proliferation status. Also, the process of photobleaching within these cells is demonstrated via the depletion in PpIX fluorescence emission during exposure to 532 nm light. All spectra show a progressive reduction of the 634 nm PpIX peak, following a bi-exponential decay that is consistent with a singlet oxygen mediated process. The rate of photobleaching, when plotted as a function of light dose, increases with reduced incident laser power. The generation of the hydroxyaldehyde-chlorin photoproduct (photoprotoporphyrin), as monitored by the increase in fluorescence emission centered on 672 nm, is also greatest when the lowest laser power is applied. When light is delivered in two fractions, PpIX fluorescence recovers during the dark period and there is an increase in bleaching rate at the onset of the second exposure. These results are qualitatively consistent with measurements performed in vivo, which demonstrate that the photodynamic dose is dependent upon fluence rate and oxygen status.

Effects of light fractionation and different fluence rates on photodynamic therapy with 5-aminolaevulinic acid in vivo

To improve efficacy of photodynamic therapy (PDT) with intravenously administered 5-aminolaevulinic acid (ALA) fractionating the light dose or reducing the light intensity may be a possibility. Therefore, Syrian Golden hamsters were fitted with dorsal skinfold chambers containing an amelanotic melanoma (n=26). PDT was performed (100 mW cm(-2), 100 J cm(-2), continuously or fractionated, and 25 mW cm(-2), 100 J cm(-2); continuously or fractionated) using an incoherent light source following i.v. application of ALA. Following fractionated irradiation, the light was paused after 20 J cm(-2) for 15 min. Prior to and up to 24 h after PDT tissue, pO(2) was measured using luminescence lifetime imaging. The efficacy was evaluated by measuring the tumour volume of amelanotic melanoma cells grown subcutaneously in the back of Syrian Golden hamsters (n=36). Only high-dose PDT resulted in a significant decrease of pO(2). Irrespective of the mode of irradiation only high-dose PDT induced complete remission of all tumours (13 out of 13). It could be shown that low-dose PDT failed to induce a significant decrease of pO(2). No significant effect of fractionated irradiation was shown regarding the therapeutic efficacy 28 days after PDT. Thus performing a fractionated PDT with ALA or reducing the light intensity seems not to be successful in clinical PDT according to the present data.

Photodynamic therapy for Barrett's esophagus: a review

Barrett's esophagus is the major risk factor for the development of esophageal adenocarcinoma, which is increasing in incidence faster than any other cancer in the Western world. Barrett's esophagus has previously been considered an irreversible lesion that required life-long surveillance to detect malignant transformation. However, endoscopic ablative techniques to destroy the abnormal mucosa and allow squamous regeneration have now been developed. Photodynamic therapy (PDT) is a non-thermal technique where the interaction of a photosensitizer in the tissues and light of a known wavelength results in tissue destruction. It appears to be an effective tool for ablating dysplasia and superficial cancers in Barrett's esophagus. The status of PDT for this disease is reviewed.

Monitoring In Situ Dosimetry and Protoporphyrin IX Fluorescence Photobleaching in the Normal Rat Esophagus During 5-Aminolevulinic Acid Photodynamic Therapy¶

Experimental therapies for Barrett's esophagus, such as 5-aminolevulinic acid (ALA)-based photodynamic therapy (PDT), aim to ablate the premalignant Barrett's epithelium. However, the reproducibility of the effects should be improved to optimize treatment. Accurate irradiation with light of a proper wavelength (633 nm), fluence and fluence rate has shown to be critical for successful ALA-PDT. Here, we have used in situ light dosimetry to adjust the fluence rate measured within the esophagus for individual animals and monitored protoporphyrin IX (PpIX) fluorescence photobleaching simultaneously. Rats were administered 200 mg kg-1 ALA (n = 14) or served as control (n = 7). Animals were irradiated with an in situ measured fluence rate of 75 mW cm-2 and a fluence of 54 J cm-2. However, this more accurate method of light dosimetry did not decrease the variation in tissue response. Large differences were also observed in the dynamics of PpIX fluorescence photobleaching in animals that received the same measured illumination parameters. We found that higher PpIX fluorescence photobleaching rates corresponded with more epithelial damage, whereas lower rates corresponded with no response. A two-phased decay in PpIX fluorescence could be identified in the response group, with a rapid initial phase followed by a slower rate of photobleaching. Non-responders did not show the rapid initial decay and had a significantly lower rate of photobleaching during the second phase of the decay (P = 0.012).

Improvement of tumor response by manipulation of tumor oxygenation during photodynamic therapy

Photodynamic therapy (PDT) requires molecular oxygen during light irradiation to generate reactive oxygen species. Tumor hypoxia, either preexisting or induced by PDT, can severely hamper the effectiveness of PDT. Lowering the light irradiation dose rate or fractionating a light dose may improve cell kill of PDT-induced hypoxic cells but will have no effect on preexisting hypoxic cells. In this study hyperoxygenation technique was used during PDT to overcome hypoxia. C3H mice with transplanted mammary carcinoma tumors were injected with 12.5 mg/kg Photofrin and irradiated with 630 nm laser light 24 h later. Tumor oxygenation was manipulated by subjecting the animals to 3 atp (atmospheric pressure) hyperbaric oxygen or normobaric oxygen during PDT light irradiation. The results show a significant improvement in tumor response when PDT was delivered during hyperoxygenation. With hyperoxygenation up to 80% of treated tumors showed no regrowth after 60 days. In comparison, when animals breathed room air, only 20% of treated tumors did not regrow. To explore the effect of hyperoxygenation on tumor oxygenation, tumor partial oxygen pressure was measured with microelectrodes positioned in preexisting hypoxic regions before and during the PDT. The results show that hyperoxygenation may oxygenate preexisting hypoxic cells and compensate for oxygen depletion induced by PDT light irradiation. In conclusion, hyperoxygenation may provide effective ways to improve PDT efficiency by oxygenating both preexisting and treatment-induced cell hypoxia.

The role of reperfusion injury in photodynamic therapy with 5-aminolaevulinic acid--a study on normal rat colon

Reperfusion injury can occur when blood flow is restored after a transient period of ischaemia. The resulting cascade of reactive oxygen species damages tissue. This mechanism may contribute to the tissue damage produced by 5-aminolaevulinic acid-induced photodynamic therapy, if this treatment temporarily depletes oxygen in an area that is subsequently reoxygenated. This was investigated in the normal colon of female Wistar rats. All animals received 200 mg kg(-1) 5-aminolaevulinic acid intravenously 2 h prior to 25 J (100 mW) of 628 nm light, which was delivered continuously or fractionated (5 J/150 second dark interval/20 J). Animals were recovered following surgery, killed 3 days later and the photodynamic therapy lesion measured macroscopically. The effects of reperfusion injury were removed from the experiments either through the administration of free radical scavengers (superoxide dismutase (10 mg kg(-1)) and catalase (7.5 mg kg(-1)) in combination) or allopurinol (an inhibitor of xanthine oxidase (50 mg kg(-1))). Prior administration of the free radical scavengers and allopurinol abolished the macroscopic damage produced by 5-aminolaevulinic acid photodynamic therapy in this model, regardless of the light regime employed. As the specific inhibitor of xanthine oxidase (allopurinol) protected against photodynamic therapy damage, it is concluded that reperfusion injury is involved in the mechanism of photodynamic therapy in the rat colon.

The use of 5-aminolaevulinic acid as a photosensitiser in photodynamic therapy and photodiagnosis

Photodynamic therapy (PDT) is a treatment for cancer and pre-malignant conditions, which involves the administration of a photosensitising agent followed by exposure of the tissue to light. 5-Aminolaevulinic acid (ALA) is a naturally occurring compound in the haem biosynthetic pathway, which is metabolised to a photosensitive product, protoporphyrin IX (PpIX). The major advantage of ALA when compared to synthetic photosensitisers is the rapid metabolism, which significantly reduces the period of cutaneous photosensitivity. This review focuses on the development of ALA as a photosensitiser in photodynamic therapy and photodiagnosis, and the wide range of clinical applications in which ALA is now being used as a therapeutic modality.