Significantly increased lesion size by using the near-infrared photosensitizer 5,10,15,20-tetrakis (m-hydroxyphenyl)bacteriochlorin in interstitial photodynamic therapy of normal rat liver tissue.

BACKGROUND AND OBJECTIVE: Penetration of tissues by activating light ultimately limits the size of the lesions achievable in interstitial photodynamic therapy. Measurements of the wavelength-dependence of tissue optical properties suggest that substantial improvements may be possible, particularly in pigmented organs such as the liver, by using drugs absorbing at near infrared wavelengths. STUDY DESIGN/MATERIALS AND METHODS: In this study, the extent of light induced necrosis with the photosensitive agents Photofrin (activated at 632 nm), meta-tetra(hydroxyphenyl)chlorin (mTHPC) (activated at 652 nm) and 5,10,15,20-tetrakis(m-hydroxyphenyl)bacteriochlorin (mTHPBC) (activated at 740 nm) are compared in normal rat liver. Interstitial irradiation of mTHPBC-sensitized liver tissue resulted in significantly larger necrotic areas than irradiation of Photofrin and mTHPC-sensitised livers. CONCLUSION: The results illustrate the advantage of near-infrared photosensitizer activation and point to a specific role for mTHPBC in the interstitial treatment of liver tumours.

In vivo photodynamic characteristics of the near-infrared photosensitizer 5,10,15,20-tetrakis(M-hydroxyphenyl) bacteriochlorin.

This paper describes the photodynamic characteristics of the new near-infrared photosensitizer 5,10,15,20-tetrakis(m-hydroxyphenyl)bacteriochlorin (mTHPBC or SQN400) in normal rat and mouse tissues. A rat liver model of photodynamic tissue necrosis was used to determine the in vivo action spectrum and the dose-response relationships of tissue destruction with drug and light doses. The effect of varying the light irradiance and the time interval between drug administration and light irradiation on the biological response was also measured in the rat liver model. Photobleaching of mTHPBC was measured and compared with that of its chlorine analog (mTHPC) in normal mouse skin and an implanted mouse colorectal tumor. The optimum wavelength for biological activation of mTHPBC in rat liver was 739 nm. mTHPBC was found to have a marked drug-dose threshold of around 0.6 mg kg-1 when liver tissue was irradiated 48 h after drug administration. Below this administered drug dose, irradiation, even at very high light doses, did not cause liver necrosis. At administered doses above the photodynamic threshold the effect of mTHPBC-PDT was directly proportional to the product of the drug and light doses. No difference in the extent of liver necrosis produced by mTHPBC was found on varying the light irradiance from 10 to 100 mW cm-2. The extent of liver necrosis was greatest when tissue was irradiated shortly after mTHPBC administration and necrosis was absent when irradiation was performed 72 h or later after drug administration, suggesting that the drug was rapidly cleared from the liver. In vivo photobleaching experiments in mice showed that the rate of bleaching of mTHPBC was approximately 20 times greater than that of mTHPC. It is argued that this greater rate of bleaching accounts for the higher photodynamic threshold and this could be exploited to enhance selective destruction of tissues which accumulate the photosensitizer.

Monte carlo simulations of light distributions in an embedded tumour model: studies of selectivity in photodynamic therapy.

It is well known that photosensitisers in photodynamic therapy (PDT) tend to localise in greater concentrations in tumours. This attractive feature may help confer on PDT the potential to selectivity destroy tumours while sparing the surrounding normal tissue. In this paper Monte Carlo simulations were used to study light distributions in a simple model consisting of tumour embedded in surrounding normal tissue subjected to superficial irradiance. The Monte Carlo model was coded to allow modelling of arbitrary geometries and multiple tissue types. This permitted the use of different optical properties for tumour and normal tissue. Two simulations were run using optical coefficients appropriate to breast carcinoma in adipose tissue and liver tumour in liver. Contours of equal fluence were plotted against depth for both simulations. Contours of equal photodynamic dose (fluence×drug concentration) were plotted for various tumour/normal drug ratios. By assuming a threshold for necrosis it was possible to estimate the depth of damage in the normal tissue and tumour simultaneously. A greater depth of selective tumour damage was observed in the breast tissue simulation for a given drug ratio due to the higher penetration of light compared to the liver. For a tumour to normal ratio of 4:1 selective damage to a depth greater than 4 mm was observed in the breast simulation compared to almost 3 mm in the case of the liver model.

mTHPC Polymer Conjugates: The In Vivo Photodynamic Activity of Four Candidate Compounds.

The in vivo photodynamic activities of four poly(ethylene glycol) (PEG) conjugates of the photosensitiser 5,10,15,20-tetrakis-(m-hydroxyphenyl)chlorin (mTHPC, temoporfin, Foscan(®)) were compared with that of mTHPC over a range of drug-light intervals using acute tumour necrosis and skeletal muscles swelling in a mouse model in order to ascertain the influence of linking group stability and PEG chain length on the photodynamic activity. The four compounds examined contained either PEG 2000 or PEG 5000 attached by carbonate or triazine linkages at the phenol hydroxyl groups of the mTHPC.All compounds tested caused tumour necrosis at drug-light intervals of between one and four days. mTHPC produced tumour necrosis of over 5 mm at drug-light intervals of 1 and 2 days with limited muscle damage at early drug-light intervals. The relatively labile carbonate-linked conjugates gave tumour necrosis similar to mTHPC but produced severe muscle and systemic phototoxicity on irradiation at 4-24 h after injection. The more stable triazine-linked conjugates produced no significant muscle damage at any of the drug-light intervals tested, but gave only limited tumour necrosis under the conditions tested. PEG chain length had relatively little effect on the patterns of bioactivity.It is concluded that both classes of mTHPC PEG conjugates may be suitable for photodynamic therapy if the problems of stability and early photosensitivity in the case of the carbonates and reduced potency in the case of the triazines can be overcome through improved formulations and PDT treatment regimens.

Characterization of a murine model for the rapid assessment of acute photodynamic response in tumour and muscle.

A murine implanted colorectal tumour model is described in which a measurement of tumour depth of necrosis is combined with a simultaneous measurement of muscle damage. The response of this model to a range of photodynamic therapy parameters was characterized using the chlorin photosensitizer mTHPC (temoporfin, Foscan(R)R). Both tumour depth of necrosis (measured directly and on histology) and muscle swelling correlate with the dose of photosensitizer and light, and are shown to be repeatable and consistent with published values obtained under similar conditions using established models.

Tissue photosensitizer detection by low-power remittance fluorimetry.

A simple adaptation of a commercial spectrofluorimeter which allows the semiquantitative determination of photodynamic therapy photosensitizer fluorescence in accessible tissues is described. Light from a xenon lamp is directed via a monochromator onto the tissue surface by a bifurcated random fibre bundle. Tissue fluorescence is directed to the emission monochromator and photomultiplier of the fluorimeter by the second limb of the fibre bundle. Although relatively simple, this device can be used to carry out a wide range of useful measurements in clinical and experimental photodynamic therapy. The sensitivity and reproducibility of the measurements were determined using mouse tumour and muscle tissue fluorescence measured in vivo compared with photosensitizer content measured by high performance liquid chromatography. As an illustration of the potential applications of such systems, the time courses of fluorescence in the skin of patients treated with the photosensitizers Photofrin(R) and metatetra(hydroxyphenyl)chlorin (mTHPC) (temoporfin) and the photobleaching of 5-aminolaevulinic acid-derived protoporphyrin IX during treatment, are described.

A comparison of novel light sources for photodynamic therapy.

A diode laser, light-emitting diode (LED) array bandwidth 25 nm, full width half maximum (FWHM) and filtered arc lamp (bandwidth 40 nm, FWHM), all with peak emission at about 650 nm, suitable for the photosensitizer tetra(meta-hydroxyphenyl)chlorin (mTHPC), were compared with a copper vapour laser pumped dye laser, using depth of necrosis in normal rat liver as a measure of photodynamic effect.A three-way comparison between a DL10K dye laser, the LED array and the filtered arc lamp resulted in mean depths of necrosis of 4.64, 4.29 and 4.04 mm, respectively, at 20 J cm(-2), the values for the laser and arc lamp being significantly different at the 5% level. A further comparison of a narrower linewidth DL20K dye laser with the LED array, using a light dose of 20 J cm(-2), showed a significant difference between the mean depths of necrosis of 4.97 and 4.05 mm, respectively (p=0.01).A final study, comparing the DL20K dye laser with the diode laser and a light dose of 10 J cm(-2), demonstrated no significant difference in depths of necrosis (3.23 and 3.25 mm, respectively). The results obtained in the three studies are attributed to the relative bandwidths of light emission for the various sources. A simple mathematical model is presented explaining the results in terms of the relative activation of the photosensitizer and the consequent threshold fluence required for the induction of necrosis.It is concluded that, in order to achieve the same depth of effect as a laser when using the broad band sources, the incident fluence would have to be approximately doubled. However, when the low cost and ease of use of the non-laser sources are taken into consideration, these devices are likely to find widespread applications in clinical photodynamic therapy.