On the role of protoporphyrin IX photoproducts in photodynamic therapy

Photodynamic and Sonodynamic Therapy with Protoporphyrin IX: In Vitro and In Vivo Studies

Sono-photodynamic therapy is a promising anticancer technique based on the combination of sonodynamic and photodynamic therapy to improve the cancer treatment effectiveness. This study was aimed at analyzing the effects of the sono-photodynamic (SPD) activity on protoporphyrin IX (PpIX) solution and PpIX-loaded rat liver. In vitro, PpIX 5 μM solutions were irradiated with light (635 nm, 30-50 mW/cm²), ultrasound (1 MHz, 1-2 W/cm²) and both. The PpIX absorption spectra recorded over exposure time revealed that the PpIX decay rate induced by SPD activity (combined irradiation) was approximately the sum of those induced by photodynamic and sonodynamic activity. In vivo, rats were intraperitoneally injected with 5-aminolevulinic acid at the dose of 500 mg/kg weight. After 3 h of injection, the PpIX-loaded livers were irradiated with light (635 nm, 180 ± 9 J/cm²), ultrasound (1.0 MHz, 770 ± 40 J/cm²) and both using a single probe capable of illuminating and sonicating the liver simultaneously. After 30 h, the liver damage induced by each protocol was analyzed histologically. It was found that a greater necrosis depth was induced by the SPD activity. These results suggest that the SPD activity could improve the PpIX decay rate and have greater scope than photodynamic or sonodynamic activity. Further studies should be performed to gain a better understanding of this protocol.

Photoproducts of the Photodynamic Therapy Agent Verteporfin Identified via Laser Interfaced Mass Spectrometry

Verteporfin, a free base benzoporphyrin derivative monoacid ring A, is a photosensitizing drug for photodynamic therapy (PDT) used in the treatment of the wet form of macular degeneration and activated by red light of 689 nm. Here, we present the first direct study of its photofragmentation channels in the gas phase, conducted using a laser interfaced mass spectrometer across a broad photoexcitation range from 250 to 790 nm. The photofragmentation channels are compared with the collision-induced dissociation (CID) products revealing similar dissociation pathways characterized by the loss of the carboxyl and ester groups. Complementary solution-phase photolysis experiments indicate that photobleaching occurs in verteporfin in acetonitrile; a notable conclusion, as photoinduced activity in Verteporfin was not thought to occur in homogenous solvent conditions. These results provide unique new information on the thermal break-down products and photoproducts of this light-triggered drug.

Porphyrins and flavins as endogenous acceptors of optical radiation of blue spectral region determining photoinactivation of microbial cells

It is shown that exposure of suspensions of gram-positive Staphylococcus aureus, gram-negative Escherichia coli and yeast-like fungi Candida albicans to laser radiation of blue spectral region with 405 and 445 nm causes their growth inhibition without prior addition of exogenous photosensitizers. It is experimentally confirmed that compounds of flavin type capable of sensitizing the formation of reactive oxygen species can act as acceptors of optical radiation of blue spectral region determining its antimicrobial effect along with endogenous metal-free porphyrins (the role of endogenous porphyrins has been confirmed earlier by a number of researchers). The participation of these compounds in the antimicrobial effect of laser radiation is supported by the registration of porphyrin and flavin fluorescence in extracts of microbial cells upon excitation by radiation used to inactivate the pathogens. In addition, the intensity of the porphyrin fluorescence in extracts of microbial cells in the transition from radiation with λ = 405 nm to radiation with λ = 445 nm decreases by 15-30 times, whereas the photosensitivity of the cells under study in this transition decreases only 3.7-6.2 times. The contribution of porphyrin photosensitizers is most pronounced upon exposure to radiation with λ = 405 nm (absorption maximum of the Soret band of porphyrins), and flavins - upon exposure to radiation with λ = 445 nm (maximum in the flavin absorption spectrum and minimum in the absorption spectrum of porphyrins). The ratio between the intensity of the porphyrin and flavin components in the fluorescence spectrum of extracts depends on the type of microbial cells.

Comparative study of phototoxicity of protoporphyrin IX synthetic and extracted from ssp Rattus novergicus albinus rats toward murine melanoma cells

Protoporphyrin IX (PpIX) is a precursor of heme synthesis and is known to be an active photosensitizer and precursor of photosensitizers applied in photodynamic therapy (PDT) and photodynamic diagnostics (PDD). On irradiation with visible light, PpIX undergoes phototransformation, producing photoproducts which may also be phototoxic and increase its efficacy. The mechanism of PpIX phototransformation depends on environmental characteristics and can be different in vitro and in vivo. In this paper, we present a comparative study of the photoactivity of synthetic PpIX and PpIX extracted from the Harderian gland of ssp Rattus novergicus albinus rats, along with their photoproducts toward murine B16F-10 melanoma cells. It was observed that when irradiated with visible light the endogenous PpIX demonstrates photocytotoxicity ten times higher than the synthetic PpIX. The photoproduct of endogenous PpIX also possesses phototoxicity, though slightly lower than that of PpIX itself. The rate of cell internalization for both endogenous PpIX and its photoproduct was eightfold greater than that obtained for the synthetic porphyrin. This difference might result from a complexation of the native PpIX with some amphiphilic compounds during its synthesis within the Harderian glands, which facilitates the cell uptake of PpIX. Fluorescence microscopy images show that both endogenous and synthetic porphyrins are localized after uptake predominantly in the mitochondrial region of cells.

Photodynamic therapy in dermatology: Dundee clinical and research experience

Topical photodynamic therapy (PDT) is increasingly accepted and used as a highly effective treatment for superficial non-melanoma skin cancer and dysplasia. We describe the developments in topical PDT for the treatment of skin diseases in our own PDT Centre in Dundee, both clinically and from a research base. Improvements in PDT could be achieved by optimisation of photosensitiser and light delivery, and these goals underpin the aims of our centre. We hope to facilitate the dissemination of use of PDT in dermatology throughout Scotland and outline some of the progress in these areas.

Clinical pharmacokinetics of the PDT photosensitizers porfimer sodium (Photofrin), 2-[1-hexyloxyethyl]-2-devinyl pyropheophorbide-a (Photochlor) and 5-ALA-induced protoporphyrin IX

BACKGROUND AND OBJECTIVES

Photodynamic therapy (PDT) uses a photosensitizer activated by light, in an oxygen-rich environment, to destroy malignant tumors. Clinical trials of PDT at Roswell Park Cancer Institute (RPCI) use the photosensitizers Photofrin, Photochlor, and 5-ALA-induced protoporphyrin IX (PpIX). In some studies the concentrations of photosensitizer in blood, and occasionally in tumor tissue, were obtained. Pharmacokinetic (PK) data from these individual studies were pooled and analyzed. This is the first published review to compare head-to-head the PK of Photofrin and Photochlor.

STUDY DESIGN/MATERIALS AND METHODS

Blood and tissue specimens were obtained from patients undergoing PDT at RPCI. Concentrations of Photofrin, Photochlor, and PpIX were measured using fluorescence analysis. A non-linear mixed effects modeling approach was used to analyze the PK data for Photochlor (up to 4 days post-infusion; two-compartment model) and a simpler multipatient-data-pooling approach was used to model PK data for both Photofrin and Photochlor (at least 150 days post-infusion; three-compartment models). Physiological parameters were standardized to correspond to a standard (70 kg; 1.818 m2 surface area) man to facilitate comparisons between Photofrin and Photochlor.

RESULTS

Serum concentration-time profiles obtained for Photofrin and Photochlor showed long circulating half-lives, where both sensitizers could be found more than 3 months after intravenous infusion; however, estimated plasma clearances (standard man) were markedly smaller for Photofrin (25.8 ml/hour) than for Photochlor (84.2 ml/hour). Volumes of distribution of the central compartment (standard man) for both Photofrin and Photochlor were about the size (3.14 L, 4.29 L, respectively) of plasma volume, implying that both photosensitizers are almost 100% bound to serum components. Circulating levels of PpIX were generally quite low, falling below the level of instrument sensitivity within a few days after topical application of 5-ALA.

CONCLUSION

We have modeled the PK of Photochlor and Photofrin. PK parameter estimates may, in part, explain the relatively long skin photosensitivity attributed to Photofrin but not Photochlor. Due to the potential impact and limited experimental PK data in the PDT field further clinical studies of photosensitizer kinetics in tumor and normal tissues are warranted.

Fractionated Illumination Significantly Improves the Response of Superficial Basal Cell Carcinoma to Aminolevulinic Acid Photodynamic Therapy

Photodynamic therapy (PDT) of superficial basal cell carcinoma (sBCC) using topical 5-aminolevulinic acid (ALA) and a light fluence of 75-100 J cm(-2) yields unsatisfactory long-term results. In several animal models, illumination with two light fractions 2 hours apart was considerably more effective than single illumination. Response is further enhanced if the fluence of the first light fraction is reduced, although the cumulative fluence is maintained. We compared the response of sBCC to a single illumination and 2-fold illumination scheme in which two light fractions of 20 and 80 J cm(-2) are performed 4 and 6 hours after the application of a single dose of 20% ALA. We randomly assigned 154 patients with a total of 505 primary sBCC into two treatment groups. Two hundred and forty-three lesions were treated using a single illumination of 75 J cm(-2) at a fluence rate of 50 mW cm(-2). Fractionated PDT, at the same fluence rate, was performed on 262 lesions. The complete response (CR) following a 2-fold illumination scheme is significantly greater than that following a single light fraction (P=0.002, log-rank test). Twelve months after therapy, CR rate to a 2-fold illumination is 97%, whereas the CR to a single illumination is 89%.

The pH-dependent conformational transition of β-lactoglobulin modulates the binding of protoporphyrin IX

We have investigated the interaction between PPIX and beta-lactoglobulin (beta-lg) as a function of the pH of the solution. beta-lg is a small globular protein (MW approximately 18 kDa) with a very well characterized structure that reveals several possible binding sites for ligands. The interaction with beta-lg affects the photophysical properties of PPIX. The shift of PPIX emission maximum, excitation maximum and the increase of the fluorescence intensity is an indicator that binding between the porphyrin and beta-lg occurs. The binding constant appears to be modulated by the pH of the solution. Spectroscopic measurements do not reveal any significant energy transfer between the Trp residues of beta-lg and PPIX, however, fluorescence anisotropy decay measurements confirm the binding and the modulation introduced by the pH of the solution. Since beta-lg has been shown to be stable within the range of pH adopted in our experiments (5.0-9.0), the results suggest that PPIX binds a site affected by the pH of the solution. Because of the crystallographic evidence an obvious site is near the aperture of the interior beta-barrel however an alternative (or concurrent) binding site may still be present.

Photobleaching kinetics, photoproduct formation, and dose estimation during ALA induced PpIX PDT of MLL cells under well oxygenated and hypoxic conditions

Fluorescence photobleaching and photoproduct formation were investigated during delta-aminolevulinic acid (ALA) induced protoporphyrin IX (PpIX) PDT of MLL cells in vitro. Cells were incubated in either 0.1 or 1.0 mM ALA for 4 h and were treated with 532 nm or 635 nm light under well oxygenated or hypoxic conditions. Fluorescence spectra were acquired during treatment. Photobleaching and photoproduct formation were quantified using singular value decomposition fitting of fluorescence spectra to experimentally determined basis spectra for PpIX, photoprotoporphyrin (Ppp), product II (peak at 655 nm), and product III (peak at 618 nm). PpIX photobleaching occurred under both normal and hypoxic conditions. The photobleaching kinetics could not be explained by purely first- or second-order photobleaching kinetics, and were attributed to differences in PpIX binding at the two ALA incubation concentrations. Ppp was the main photoproduct and accumulated in higher levels in the absence of oxygen, likely a result of reduced Ppp photobleaching under hypoxia. Increases in product II fluorescence occurred mainly in the presence of oxygen. To assess potential fluorescence based PDT dose metrics, cell viability was measured at select times during treatment using a colony formation assay. Cell survival correlated well to changes in product II fluorescence, independent of oxygenation, sensitizer concentration, and treatment wavelength, suggesting that this product is primarily a result of singlet oxygen mediated reactions and may potentially be useful to quantify singlet oxygen dose during PDT.

Actinic keratoses, basal cell carcinoma, and squamous cell carcinoma: uncommon treatments

This contribution will discuss the treatment of actinic keratoses, basal cell carcinomas, and squamous cell carcinoma using methods that are not routinely established but have been used for a long period. The purpose of this article is simply to expand the reader's knowledge of what has been or could be used to treat such lesions. This article should not be interpreted as advocating the use of the methods discussed because there are obviously efficient well-established treatments for these lesions.

Construction and evaluation of a non-laser optical system for photodynamic process excitation

Purpose: To show that a non-laser light source can be constructed, using a 500 W Tungsten lamp and optical filters, and that this light source produces photodynamic effect via protoporphyrin IX (PpIX) similar to the effect produced by HeNe laser light. Methods: The broad band spectrum from a Tungsten lamp was filtered. Infrared and blue part of the spectrum was discarded by absorption process and the fraction of the spectrum, centered at the red portion, was filtered by an interference filter. Photodynamic effect was studied by the activity on endogenous PpIX of Harderian glands of Wistar rats. Twenty rats were used for the experiment. Each animal had its two Harderian gland surgically exposed, so one of them was treated with the system while the other was kept as control. After a 30 minutes period of treatment, the animals were sacrificed and their glands were removed for histological analysis. This analysis was compared to earlier published results obtained with HeNe laser light. Results: The resultant light source emission was centered around (636 ± 6.5) nm and gives up to 11.3 mW/cm² power density. It produces photodynamic effect in Harderian gland, observed either by fluorescence spectroscopy or by histological microscopy. Conclusion: There is no noticeable difference in Photodynamic effect results if activated by HeNe laser or by the proposed non-laser light source emitting at the red portion of the spectrum.

Photodynamic therapy for Bowen's disease and squamous cell carcinoma of the skin

BACKGROUND

Photodynamic therapy involves the activation by visible light of a previously administered photosensitizing agent in order to cause tumor necrosis. Skin tumors can be treated with topical photosensitizers and thus avoiding systemic side effects. In this study we evaluate the immediate and long-term effects of photodynamic therapy (PDT), using aminolevulinic acid (ALA) as a photosensitizer and a non-laser light source, on Bowen's disease (intra-epithelial squamous cell carcinoma) and on frank squamous cell carcinoma (SCC) of the skin.

METHODS

ALA in cream form (20%) was topically applied on biopsy-proven Bowen's disease or SCC of the skin. The lesions were covered with occlusive and light-shielding dressing. Sixteen hours later, they were submitted to a 10-min light session using Versa-Light™, a non-laser light source (spectral output of 580-720nm and 1250-1600nm, 100J/cm(2)). The initial evaluation was done 21 days post-treatment and every 3m thereafter. Patients that did not respond to treatment after two to three sessions were referred to surgery.

RESULTS

Forty Bowen's disease lesions (24 patients) and 43 SCC lesions (18 patients) underwent treatment. Median follow-up was 21±8m. No patient had any remarkable side effects. Thirty-four Bowen's disease (85%) lesions completely responded as did 32 SCC lesions (74%).

CONCLUSIONS

Our findings showed that PDT is highly effective in treating Bowen's disease and SCC lesions and can be used as a first treatment modality in so far as its use does not preclude the subsequent surgery recommended for the small percentage of failures.

Topical and light-based treatments for actinic keratoses

Actinic keratosis is currently believed to be an early stage in the evolution of squamous cell carcinoma. Active and intensive treatment of actinic keratosis may prevent the formation of invasive squamous cell carcinoma and potential metastases. While destructive methods of treatment of actinic keratosis remain the gold standard for the eradication of visible and palpable actinic keratoses, new medical therapies may accomplish this goal more comfortably and reliably for the patient. Newer topical medications, light therapy and photodynamic therapy are generating promising results that presage more widespread use in the future. These novel therapies for the early treatment of actinic keratosis may be administered in combination or serially, with the locus of treatment at any given time possibly restricted to a region of affected skin. Treatment of incipient or subclinical lesions may mitigate the risk of future squamous cell carcinomas lesions. Widespread actinic keratosis constitutes a persistent medical problem that requires long-term management. The role of traditional and novel treatments in the routine treatment of actinic keratosis will be determined by the efficacy, limitations and the practicality of each of these methods in individual patients. As the first stage of squamous cell carcinoma, actinic keratosis is worthy of prompt evaluation and active treatment.

Comparison of the photodynamic effect of exogenous photoprotoporphyrin and protoporphyrin IX on PAM 212 murine keratinocytes

A comparative study of the cellular photosensitizing properties of protoporphyrin IX (PpIX) and photoprotoporphyrin (Ppp) was carried out in the transformed murine keratinocyte cell line, PAM 212. Time-course fluorescence studies were performed to determine the rate of uptake by cells together with fluorescence microscopy. The sensitized cells were laser irradiated with a range of light doses at 635 or 670 nm to determine the phototoxicity of the two compounds and to investigate their relative fluorescence photobleaching properties. Ppp showed enhanced phototoxicity at both its optimal activation wavelength of 670 nm (eight times more phototoxic than PpIX activated at its optimal wavelength of 635 nm for the same fluence) and at 635 nm (three times more phototoxic than PpIX at the same wavelength), using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay. The photobleaching rate of Ppp in cells was found to be higher using 670 nm irradiation compared with that of PpIX at 635 nm irradiation. At 635 nm, however, the photobleaching rate of Ppp was comparable to that of PpIX. The photobleaching quantum yields of the two compounds in cells were found to be similar at approximately 5 x 10(-4), with the same value confirmed at both 670 and 635 nm irradiation for Ppp. The fluorescence lifetime of Ppp in cells was measured as 5.4 ns using time-correlated single photon counting.

In vivo pharmacokinetics of 8-aminolevulinic acid-induced protoporphyrin IX during pre- and post-photodynamic therapy in 7,12-dimethylbenz(a)nthracene-treated skin carcinogenesis in Swiss mice: a comparison by three-compartment model

Delta-aminolevulinic acid-photodynamic therapy (ALA-PDT) has emerged as a useful technique in the treatment of superficial basal cell carcinoma, actinic keratosis, squamous cell carcinoma and tumors of other organs. Earlier reports mention that there is reappearance of protoporphyrin IX (PpIX) after photoirradiation of tumors. This property of reappearance of PpIX is being utilized to treat nodular tumors by fractionated light dose delivery. However, there is still no unanimously accepted reason for this reappearance phenomenon and the rate of resynthesis after PDT. On account of this, studies are carried out on the estimation of the pharmacokinetics of the ALA-induced PpIX in mice tumor models and the surrounding normal tissues before and after PDT. Further, a mathematical model based on a multiple compartment system is proposed to estimate the rate parameter for the diffusion of PpIX from the surrounding normal tissues into the tumor tissue (km) caused by photobleaching during PDT with irradiating fluences of 36.0 and 57.6 J/cm2. The km value at two different fluences, 36.0 and 57.6 J/cm2, are estimated as 3.0636+/-0.7083 h(-1) and 6.9231+/-2.17651 h(-1), respectively. Further, the rate parameter for the cleavage and efflux of ALA (k1) and the rate parameter for the evasion of PpIX from the tumor tissues after PDT (kt) were also estimated by fitting the experimental data to the developed mathematical model. The statistical significance of the estimated parameters was determined using Student's t-test. The experimental results and the rate parameters obtained using the proposed compartment model suggest that in addition to the earlier reported reasons, the invasion or diffusion of PpIX from the surrounding tissues to the tumor tissues after photoirradiation might also contribute to the reappearance of PpIX after PDT.

Protoporphyrin IX fluorescence kinetics in UV-induced tumours and normal skin of hairless mice after topical application of 5-aminolevulinic acid methyl ester

Accumulation of protoporphyrin IX (PpIX) was investigated in normal skin and UV-induced tumours in hairless mice after topical application of a cream containing 2, 8 or 16% of 5-aminolevulinic acid methyl ester (ALA-Me). Higher levels of PpIX were measured in tumours compared to normal skin. The maximal amount of PpIX was reached at 1.5, 3 and 4 h after 2, 8 and 16% ALA-Me application, respectively. Higher tumour to normal skin PpIX fluorescence ratios were measured after application of 8 and 16% ALA-Me than after application of 2%. After irradiation with a broad spectrum of visible light from a slide projector, more than 90% of PpIX was bleached by fluences of 36 and 48 J/cm2, at fluence rates of 10 and 40 mW/cm2 respectively. At these fluences, the PpIX photobleaching rate was significantly higher (P<0.05) in normal mouse skin than in tumours. In addition, for a given fluence, more PpIX was photobleached at the lower fluence rate (10 mW/cm2) than at the higher fluence rate (40 mW/cm2) in normal skin (P<0.001) as well as in tumours (P<0.05) after exposure to 24 J/cm2 of light. In conclusion, the highest tumour to normal skin PpIX ratio was observed 3 h after application of 8% ALA-Me, suggesting that light exposure should be performed at this time in order to achieve an optimal PDT effect in this tumour model.

Effects of photoproducts on the binding properties of protoporphyrin IX to proteins

Photosensitisers are the photoactive molecules used in photodynamic therapy (PDT) of cancer. Despite the importance of their interaction with polypeptides, only the binding to plasma proteins has been investigated in some detail. In our study we compared the binding of Protoporphyrin IX (a clinically useful photosensitiser) to an immunoglobulin G, with the binding to albumins. Binding to IgG is relevant because a possible method of increasing tumour specificity of photosensitisers is to bind them to tumour-specific antibodies. Binding constants to albumins and the immunoglobulin were comparable ( congruent with6 x 10(-6) M(-1)). The apparent number of PPIX molecules bound to each protein was also within a similar range (from 4 to 7). The absence of a shift in the emission spectrum of PPIX bound to IgG, however, indicates that either larger aggregates of PPIX bind to the immunoglobulin or that the binding site leaves PPIX exposed to the buffer. We observed that PPIX photoproducts compete with PPIX for the same binding sites. The number of PPIX molecules bound to each protein in the presence of photoproducts decreased by 50-80%. Due to the spectral overlap between PPIX and its photoproducts, the binding in the presence of photoproducts was investigated using Derivative Synchronous Fluorescence Spectroscopy (DSFS) to improve the spectral separation between chromophores in solution. We also concluded that fluorescence measurements underestimate the number of PPIX molecules binding each protein. In fact, non-linear Scatchard plots (in the case of albumin binding) by definition yield a minimum number of molecules attached to a protein. Moreover, the binding of large aggregates, formed by an unknown number of PPIX molecules, to IgG results in the underestimate of the number of molecules bound. The number of PPIX molecules bound to these proteins is also much larger than the number of sites estimated by protein fluorescence quenching.

Guidelines for topical photodynamic therapy: report of a workshop of the British Photodermatology Group

Topical photodynamic therapy (PDT) is effective in the treatment of certain non-melanoma skin cancers and is under evaluation in other dermatoses. Its development has been enhanced by a low rate of adverse events and good cosmesis. 5-Aminolaevulinic acid (ALA) is the main agent used, converted within cells into the photosensitizer protoporphyrin IX, with surface illumination then triggering the photodynamic reaction. Despite the relative simplicity of the technique, accurate dosimetry in PDT is complicated by multiple variables in drug formulation, delivery and duration of application, in addition to light-specific parameters. Several non-coherent and coherent light sources are effective in PDT. Optimal disease-specific irradiance, wavelength and total dose characteristics have yet to be established, and are compounded by difficulties comparing light sources. The carcinogenic risk of ALA-PDT appears to be low. Current evidence indicates topical PDT to be effective in actinic keratoses on the face and scalp, Bowen's disease and superficial basal cell carcinomas (BCCs). PDT may prove advantageous where size, site or number of lesions limits the efficacy and/or acceptability of conventional therapies. Topical ALA-PDT alone is a relatively poor option for both nodular BCCs and squamous cell carcinomas. Experience of the modality in other skin diseases remains limited; areas where there is potential benefit include viral warts, acne, psoriasis and cutaneous T-cell lymphoma. A recent British Photodermatology Group workshop considered published evidence on topical PDT in order to establish guidelines to promote the efficacy and safety of this increasingly practised treatment modality.

Monitoring photoproduct formation and photobleaching by fluorescence spectroscopy has the potential to improve PDT dosimetry with a verteporfin-like photosensitizer

In current clinical practice, photodynamic therapy (PDT) is carried out with prescribed drug doses and light doses as well as fixed drug-light intervals and illumination fluence rates. This approach can result in undesirable treatment outcomes of either overtreatment or undertreatment because of biological variations between different lesions and patients. In this study, we explore the possibility of improving PDT dosimetry by monitoring drug photobleaching and photoproduct formation. The study involved 60 mice receiving the same drug dose of a novel verteporfin-like photosensitizer, QLT0074, at 0.3 mg/kg body weight, followed by different light doses of 5, 10, 20, 30, 40 or 50 J/cm2 at 686 nm and a fluence rate of 70 mW/cm2. Photobleaching and photoproduct formation were measured simultaneously, using fluorescence spectroscopy. A ratio technique for data processing was introduced to reliably detect the photoproduct formed by PDT on mouse skin in vivo. The study showed that the QLT0074 photoproduct is stable and can be reliably quantified. Three new parameters, photoproduct score (PPS), photobleaching score (PBS) and percentage photobleaching score (PBS%), were introduced and tested together with the conventional dosimetry parameter, light dose, for performance on predicting PDT-induced outcome, skin necrosis. The statistical analysis of experimental results was performed with an ordinal logistic regression model. We demonstrated that both PPS and PBS improved the prediction of skin necrosis dramatically compared to light dose. PPS was identified as the best single parameter for predicting the PDT outcome.

In vivo mTHPC photobleaching in normal rat skin exhibits unique irradiance-dependent features

We report measurements performed on the normal skin of rats in vivo, which provide information on the photobleaching kinetics and mechanisms of the photosensitizer meso-tetrahydroxyphenyl chlorin (mTHPC). Loss of mTHPC fluorescence was monitored using in vivo fluorescence spectroscopy during photodynamic therapy (PDT) performed using 650 nm laser irradiation. The bleaching was evaluated for irradiances of 5, 20 and 50 mW cm(-2). Two distinct phases of mTHPC photobleaching were observed. In the first phase there was no obvious irradiance dependence in the loss of fluorescence vs fluence. The second phase was initiated by an irradiance-dependent discontinuity in the slope of the bleaching curve, after which the photobleaching rates showed an irradiance dependence consistent with an oxygen-dependent reaction process. To investigate the unusual shape of the in vivo bleaching curves, we measured the PDT-induced changes in O2 concentrations in mTHPC-sensitized spheroids irradiated with 2, 5 and 20 mW cm(-2) of 650 nm light. The oxygen concentration data indicated no unusual features within the range of fluences where the discontinuities in fluorescence were observed during in vivo spectroscopy. The fluorescence from the in vivo bleaching experiments thus reports a phenomenon that is not reported by measurements of the photochemical oxygen consumption in the spheroids.

The photosensitizing effect of the photoproduct of protoporphyrin IX

The photodynamic effect of a photoproduct of protoporphyrin IX (PpIX) induced by 5-aminolevulinic acid (ALA) was investigated in WiDr cells, a human adenocarcinoma cell line. The fluorescence excitation and emission spectra of PpIX and the photoproduct were measured. After 1, 3 or 5 min exposure of the ALA-incubated cells to 140 mW/cm(2) light at 635 nm, the photoproduct--the chlorin photoprotoporphyrin (Ppp), had an emission band around 670 nm. The Ppp excitation peak at 670 nm is well separated from the PpIX peak at 635 nm. The outcome of photodynamic therapy (PDT) was determined by measuring intracellular fluorescence intensity of propidium iodide (PI) 2 h following PDT and methylene blue (MB) staining 24 h following PDT. A significant increase in the fluorescence intensity of PI was noted when the ALA-loaded cells were exposed to 670 nm light after exposure to 635 nm, indicating enhanced cell membrane inactivation induced by the photodynamic action of the photoproduct. However, the fraction of the cells that survived following the same treatment as measured by MB staining was not significantly affected based on an analysis of variance. The fluorescence of PpIX decayed significantly during 635 nm light exposure. Exposure to light at 670 nm does not lead to any photodegradation of PpIX. The fluorescence of Ppp was bleached during 670 nm light exposure. Exposure of Ppp at 670 nm gives no PpIX back. Thus, the phototransformation of PpIX to Ppp is probably not a reversible process.

Chlorin-type photosensitizers photochemically derived from vinyl porphyrins

The reaction of singlet oxygen with four vinyl-substituted dicarboxylic porphyrins, vinyldeuteroporphyrin (VD), ethylvinyldeuteroporphyrin (EVD), hydroxyethylvinyldeuteroporphyrin (HVD) and protoporphyrin (PP) in organic solutions is investigated. The main products, the "hydroxyaldehyde" chlorin-type derivatives, are formed with a concentration-dependent photochemical quantum yield that reaches a maximum of 7.4 +/- 1.6 x 10(-3). However, owing to the high turnover of singlet-oxygen production, these chlorin-type compounds are easily prepared photochemically with a chemical yield of 70% and little side product formation. In chemical ionization mass spectrometry, these compounds display an unusual fragmentation with a loss of 16 mass units. This is attributed to the loss of the oxygen bound to the saturated carbon of the modified pyrrole unit. All these compounds sensitize the formation of singlet oxygen with a yield around 0.8. They interact with singlet oxygen with rate constants of 5 x 10(6)-9 x 10(6) M-1 s-1, lower than those measured for vinyl porphyrins. These data are likely to help in the characterization of photoproducts of vinyl porphyrins relevant to photodynamic therapy (PP, HVD). As exemplified with VD and EVD, they also point out the reaction of singlet oxygen as an efficient route to chlorin-type photosensitizers.

Porphyrin bleaching and PDT-induced spectral changes are irradiance dependent in ALA-sensitized normal rat skin in vivo

Photobleaching kinetics of aminolevulinic acid-induced protoporphyrin IX (PpIX) were measured in the normal skin of rats in vivo using a technique in which fluorescence spectra were corrected for the effects of tissue optical properties in the emission spectral window through division by reflectance spectra acquired in the same geometry and wavelength interval and for changes in excitation wavelength optical properties using diffuse reflectance measured at the excitation wavelength. Loss of PpIX fluorescence was monitored during photodynamic therapy (PDT) performed using 514 nm irradiation. Bleaching in response to irradiances of 1, 5 and 100 mW cm-2 was evaluated. The results demonstrate an irradiance dependence to the rate of photobleaching vs irradiation fluence, with the lowest irradiance leading to the most efficient loss of fluorescence. The kinetics for the accumulation of the primary fluorescent photoproduct of PpIX also exhibit an irradiance dependence, with greater peak accumulation at higher irradiance. These findings are consistent with a predominantly oxygen-dependent photobleaching reaction mechanism in vivo, and they provide spectroscopic evidence that PDT delivered at low irradiance deposits greater photodynamic dose for a given irradiation fluence. We also observed an irradiance dependence to the appearance of a fluorescence emission peak near 620 nm, consistent with accumulation of uroporphyrin/coproporphyrin in response to mitochondrial damage.

Comparison of red and green light in the treatment of Bowen's disease by photodynamic therapy

BACKGROUND

A variety of protocols exist for the treatment of Bowen's disease by photodynamic therapy (PDT) using topical 5-aminolaevulinic acid (5-ALA).

OBJECTIVE

To determine the optimal wavelength (red or green light) for this treatment.

METHODS

A randomized comparison study of ALA-PDT using red (630 +/- 15 nm) or green (540 +/- 15 nm) light in the treatment of Bowen's disease.

RESULTS

The initial clearance rate for lesions treated by red light was 94% (30 of 32) in comparison with 72% (21 of 29) for those lesions receiving green light (P = 0.002). Over the following 12 months, there were two recurrences in the red light group and seven in the green light group reducing the clearance rates to 88% and 48%, respectively. The frequency and severity of pain experienced were similar between the two treatment groups. No hyperthermia, nor significant difference in lesional temperatures, was observed between the wavelengths studied.

CONCLUSION

Green light is less effective than red light, at a theoretically equivalent dose, in the treatment of Bowen's disease by topical ALA-PDT.

Topical delta-aminolevulinic acid-based photodynamic therapy for Japanese actinic keratoses

Based on the opinion that 5-aminolevulinic acid-based photodynamic therapy (ALA-PDT) may be disadvantageous to Oriental patients with dark skin, including Japanese, because the competing chromophore melanin inhibits the photochemical reaction, we assessed the therapeutic effects when Japanese AK were treated with ALA-PDT. Fifty-three lesions in ten patients were followed up for over one year. The cure rates of AK on the face including the neck and on extremities were 81.8% and 55.6%, respectively. The results were approximately the same cure rates as those reported for AK in Caucasian patients. However, a larger number of treatment sessions were required. In the red light range, incoherent light including 630 nm and 670 nm is superior to 630 nm laser light.

Phototransformations of 5-aminolevulinic acid-induced protoporphyrin IX in vitro: a spectroscopic study

Human adenocarcinoma cells of the line WiDr were incubated with 5-aminolevulinic acid to induce protoporphyrin IX (PpIX) and then exposed to laser light of wavelength 635 nm. The PpIX fluorescence decreased with increasing exposure. The decay rate was slightly dependent on the initial PpIX concentration. The PpIX fluorescence was halved by a fluence of about 40 J/cm2. Several fluorescing photoproducts were formed. The main one, supposedly the chlorine-type photoprotoporphyrin (Ppp), had a fluorescence excitation spectrum stretching out to about 680 nm with a maximum at around 668 nm. The formation kinetics of this product was dependent on the initial PpIX concentration. Moreover, it was selectively bleached by exposure to light at 670 nm. A photoproduct with an emission maximum at 652 nm, different from Ppp, remained after this exposure. Traces of a photoproduct(s) with fluorescence emission slightly blue-shifted compared with that of PpIX, supposedly water-soluble porphyrins, were also detected after light exposure.

A randomized paired comparison of photodynamic therapy and topical 5-fluorouracil in the treatment of actinic keratoses

BACKGROUND

Photodynamic therapy (PDT) has not been compared with topical 5-fluorouracil (5-FU) in the treatment of epidermal dysplasia.

OBJECTIVE

The purpose of this study was to assess the efficacy and tolerability of these two treatment modalities in 17 patients with actinic keratoses on the backs of the hands.

METHODS

Each patient's right and left hands were randomized to receive either a 3-week course of topical 5-FU applied twice per day or PDT using topical 5-aminolevulinic acid (5-ALA) and then, after 4 hours, irradiation with an incoherent light source consisting of a 1200 W metal halogen lamp emitting red light (580 to 740 nm). Each hand randomized for PDT received 150 J/cm(2). The observed median fluence rate was 86 mW/cm(2) (interquartile range, 53 to 100 mW/cm(2)). All patients were reviewed at 1, 4, and 24 weeks after starting treatment.

RESULTS

Fourteen of 17 patients (82%) completed the study. The mean lesional area treated with topical 5-FU decreased from 1390 mm(2) (standard deviation [SD], 1130) to 297 mm(2) (SD, 209). This represents a mean reduction in lesional area of 70% (confidence interval [CI], 61%-80%). The mean lesional area treated with topical PDT decreased from 1322 mm(2) (SD, 1280) to 291 mm(2) (SD, 274), representing a mean reduction in lesional area of 73% (CI, 61%-84%). The reduction in lesional area elicited by the two treatment methods was similar (CI, -25% to 17%). There was no statistically significant difference between the treatment methods in overall symptom scores for pain and redness.

CONCLUSION

One treatment with PDT using topical 5-ALA appears to be as effective and well tolerated as 3 weeks of twice-daily topical 5-FU, a cheap and widely available alternative.

Kinetic fluorescence studies of 5-aminolaevulinic acid-induced protoporphyrin IX accumulation in basal cell carcinomas

Laser-induced fluorescence (LIF) investigations have been performed in connection with photodynamic therapy (PDT) of basal cell carcinomas and adjacent normal skin following topical application of 5-aminolaevulinic acid (ALA) in order to study the kinetics of the protoporphyrin IX (PpIX) build-up. Five superficial and 10 nodular lesions in 15 patients are included in the study. Fluorescence measurements are performed prior to the application of ALA, 2, 4 and 6 h post ALA application, immediately post PDT (60 J cm-2 at 635 nm), and 2 h after the treatment. Hence, the build-up, photobleaching and re-accumulation of PpIX can be followed. Superficial lesions show a maximum PpIX fluorescence 6 h post ALA application, whereas the intensity is already the highest 2-4 h after the application in nodular lesions. Immediately post PDT, the fluorescence contribution at 670 nm from the photoproducts is about 2% of the pre-PDT PpIX fluorescence at 635 nm. Two hours after the treatment, a uniform distribution of PpIX is found in the lesion and surrounding normal tissue. During the whole procedure, the autofluorescence of the lesions and the normal skin does not vary significantly from the values recorded before the application of ALA.

Protection against photodynamic therapy (PDT)-induced photosensitivity by fabric materials

"Special" highly protective fabrics are now available that offer broad-spectrum protection in preventing sunburn, and possibly other types of photodamage. It is important to know to what extent these fabrics are capable of protecting the wearer against skin cancer, photosensitivity disorders, and inadvertent phototoxic reactions from photodynamic therapy (PDT). We assess the ability of one such special (Solumbra) fabric and one "typical" summer fabric to provide protection against PDT phototoxicity produced in tape-stripped Sk-1 hairless mice by topical 5-aminolevulinic acid (ALA) and (primarily) visible light (360-800 nm). Since ALA-derived photosensitizers absorb most of the visible spectrum, results from these studies give a good indication of the photoprotective capability of these fabrics throughout this region. Mice were irradiated dorsally with a Kodak slide projector equipped with a 300 W tungsten-halogen lamp (I0 = 48.3 mW/cm2). After determining the minimal phototoxic dose (MPD) to be 1.40 +/- 0.4 J/cm2, we irradiated the tape-stripped ALA-sensitized mice through the stretched test fabrics with appropriate multiples of the MPD. The special fabric provided protection against 25-30 MPD visible light between 360-800 nm in 14/14 mice. The typical fabric failed to provide protection against 2.5 MPD of the same spectrum. No phototoxic or other adverse responses were seen with either the ALA or light control. In conclusion, the Solumbra fabric is much more protective against ALA photosensitization than the typical fabric. Both appear better at blocking UV than visible light.

Fluorescence photobleaching of ALA-induced protoporphyrin IX during photodynamic therapy of normal hairless mouse skin: the effect of light dose and irradiance and the resulting biological effect

The photobleaching of 5-aminolaevulinic acid (ALA)-induced protoporphyrin IX (PpIX) was investigated during superficial photodynamic therapy (PDT) in normal skin of the SKH HR1 hairless mouse. The effects of light dose and fluence rate on the dynamics and magnitude of photobleaching and on the corresponding PDT-induced damage were examined. The results show that the PDT damage cannot be predicted by the total light dose. Photobleaching was monitored over a wide range of initial PpIX fluorescence intensities. The rate of PpIX photobleaching is not a simple function of fluence rate but is dependent on the initial concentration of sensitizer. Also, at high fluence rates (50-150 mW/cm2, 514 nm) oxygen depletion is shown to have a significant effect. The rate of photobleaching with respect to light dose and the corresponding PDT damage both increase with decreasing fluence rate. We therefore suggest that the definition of a bleaching dose as the light dose that causes a 1/e reduction in fluorescence signal is insufficient to describe the dynamics of photobleaching and PDT-induced damage. We have detected the formation of PpIX photoproducts during the initial period of irradiation that were themselves subsequently photobleached. In the absence of oxygen, PpIX and its photoproducts are not photobleached. We present a method of calculating a therapeutic dose delivered during superficial PDT that demonstrates a strong correlation with PDT damage.

5-aminolaevulinic-acid-induced formation of different porphyrins and their photomodifications

The relative amounts of 5-aminolaevulinic acid (ALA)-induced hydrophobic and hydrophilic porphyrins produced in normal mouse tissue and solid Ehrlich ascites carcinoma in mice were investigated as a function of added glucose, light irradiation and restricted blood flow to the tumour. Protoporphyrin IX (PpIX) is the predominant porphyrin produced from exogenous ALA in tissues that have viable mitochondria. However, under special conditions which seem to be connected with a reduced viability of the mitochondria, the formation of water-soluble porphyrins can be observed in vivo. Fluorescence maxima of the water-soluble porphyrins are located in the region between 615 and 625 nm. Irradiation can further lead to the formation of PpIX photoproduct(s), showing a fluorescence emission band at 676 nm. The photobleaching of PpIX in normal tissue and tumour tissue is different under restricted blood flow. ALA-induced PpIX can be observed in the normal blood stream, and can be linked to a slower photobleaching than in tissue with restricted blood flow.

Implicit and explicit dosimetry in photodynamic therapy: a New paradigm

Dosimetry for photodynamic therapy (PDT) is becoming increasingly complex as more factors are identified which may influence the effectiveness of a given treatment. The simple prescription of a PDT treatment in terms of the administered photosensitizer dose, the incident light and the drug-light time interval does not account for patient-to-patient variability in either the photosensitizer uptake, tissue optical properties or tissue oxygenation, nor for the interdependence of the photosensitizer-light-tissue factors. This interdependence is examined and the implications for developing adequate dosimetry for PDT are considered. The traditional dosimetric approach, measuring each dose factor independently, and termed here 'explicit dosimetry', may be contrasted with the recent trend to use photosensitizer photobleaching as an index of the effective delivered dose, termed here 'implicit dosimetry'. The advantages and limitations of each approach are discussed, and the need to understand the degree to which the photobleaching mechanism is linked, or 'coupled', to the photosensitizing mechanism is analysed. Finally, the influence of the tissue-response endpoints on the optimal dosimetry methods is considered.

Photobleaching during and re-appearance after photodynamic therapy of topical ALA-induced fluorescence in UVB-treated mouse skin

Photodynamic therapy (PDT) using protoporphyrin IX (PpIX) induced by topically applied 5-aminolevulinic acid (ALA) seems a promising alternative for the treatment of superficial non-melanoma skin cancer and actinic keratosis. In this study, the kinetics of new PpIX fluorescence arising after a PDT treatment that had photobleached the original fluorescence were determined. Our purpose was to examine the feasibility of multiple irradiations, following a single topical ALA application, to increase PDT efficacy. In addition, photobleaching during PDT and the fluorescence spectra during and after PDT were studied. As a model we used hairless mice with and without UVB-induced skin lesions. ALA was applied to the skin for 4 hr. An illumination was delivered either immediately after application or 6 hr after the end of the application (at interval of maximum fluorescence). During PDT, the fluorescence of normal skin decreased at a faster rate than the fluorescence of the skin lesions. In the fluorescence study after PDT, the areas treated immediately post-application showed a fluorescence increase over time similar to that in non-treated areas on the same mice. A remarkable result was that the fluorescence of areas treated at maximum fluorescence increased, whereas the fluorescence of non-treated areas did not increase over time. With both treatment intervals the new fluorescence showed a characteristic PpIX spectrum. Our results demonstrate that a second illumination, when new PpIX fluorescence has been formed, may increase PDT efficacy after topical ALA application. This finding has been demonstrated previously for systemic ALA administration.

Current status of photodynamic therapy in dermatology

The accessibility of skin to light treatment, as well as the expertise developed by dermatologists in laser surgery and phototherapy, creates an exciting opportunity for dermatologic PDT to become part of our standard therapeutic armamentarium. PDT appears to be viable alternative to conventional therapy for superficial BCC and Bowen's disease, although definitive controlled studies are lacking. The introduction of ongoing research developments, new photosensitizers, and better light sources into clinical PDT trials in the coming years will undoubtedly expand the range of indications for this novel form of therapy, particularly for nononcologic conditions.

5-Aminolevulinic acid-based photodynamic therapy. Clinical research and future challenges

BACKGROUND

Photodynamic therapy (PDT) for cancer patients has developed into an important new clinical treatment modality in the past 25-years. PDT involves administration of a tumor-localizing photosensitizer or photosensitizer prodrug (5-aminolevulinic acid [ALA], a precursor in the heme biosynthetic pathway) and the subsequent activation of the photosensitizer by light. Although several photosensitizers other than ALA-derived protoprophyrin IX (PpIX) have been used in clinical PDT, ALA-based PDT has been the most active area of clinical PDT research during the past 5 years. Studies have shown that a higher accumulation of ALA-derived PpIX in rapidly proliferating cells may provide a biologic rationale for clinical use of ALA-based PDT and diagnosis. However, no review updating the clinical data has appeared so far.

METHODS

A review of recently published data on clinical ALA-based PDT and diagnosis was conducted.

RESULTS

Several individual studies in which patients with primary nonmelanoma cutaneous tumors received topical ALA-based PDT have reported promising results, including outstanding cosmetic results. However, the modality with present protocols does not in general, appear to be superior to conventional therapies with respect to initial complete response rates and long term recurrence rates, particularly in the treatment of nodular skin tumors. Topical ALA-PDT does have the following advantages over conventional treatments: it is noninvasive; it produces excellent cosmetic results; it is well tolerated by patients; it can be used to treat multiple superficial lesions in short treatment sessions; it can be applied to patients who refuse surgery or have pacemakers and bleeding tendency; it can be used to treat lesions in specific locations, such as the oral mucosa or the genital area; it can be used as a palliative treatment; and it can be applied repeatedly without cumulative toxicity. Topical ALA-PDT also has potential as a treatment for nonneoplastic skin diseases. Systemic administration of ALA does not seem to be severely toxic, but the advantage of using this approach for PDT of superficial lesions of internal hollow organs is still uncertain. The ALA-derived porphyrin fluorescence technique would be useful in the diagnosis of superficial lesions of internal hollow organs.

CONCLUSIONS

Promising results of ALA-based clinical PDT and diagnosis have been obtained. The modality has advantages over conventional treatments. However, some improvements need to be made, such as optimization of parameters of ALA-based PDT and diagnosis; increased tumor selectivity of ALA-derived PpIX; better understanding of light distribution in tissue: improvement of light dosimetry procedure; and development of simpler, cheaper, and more efficient light delivery systems.

Light dosimetry in vivo

This paper starts with definitions of radiance, fluence (rate) and other quantities that are important with regard to in vivo light dosimetry. The light distribution in mammalian tissues can be estimated from model calculations using measured optical properties or from direct measurements of fluence rate using a suitable detector. A historical introduction is therefore followed by a brief discussion of tissue optical properties and of calculations using diffusion theory, the P3-approximation or Monte Carlo simulations. In particular the form of the scattering function is considered in relation to the fluence rate close to the tissue boundary, where light is incident. Non-invasive measurements of optical properties yield the absorption coefficient mu a and mu s(1 - g), where mu s is the scattering coefficient and g is the mean cosine of the scattering angle. An important question is whether this combination is sufficient, or whether g itself must be known. It appears that for strongly forward scattering, as in mammalian tissues, rather detailed knowledge of the scattering function is needed to reliably calculate the fluence rate close to the surface. Deeper in the tissue mu s (1 - g) is sufficient. The construction, calibration and use of fibre-optic probes for measurements of fluence rate in tissues or optical phantoms is discussed. At present, minimally invasive absolute fluence (rate) measurements seem to be possible with an accuracy of 10-20%. Examples are given of in vivo measurements in animal experiments and in humans during clinical treatments. Measurements in mammalian tissues, plant leaves and marine sediments are compared and similarities and differences pointed out. Most in vivo light fluence rate measurements have been concerned with photodynamic therapy (PDT): Optical properties of the same normal tissue may differ between patients. Tumours of the same histological type may even show different optical properties in a single patient. Treatment-induced changes of optical properties may also occur. Scattered light appears to contribute substantially to the light dose. All these phenomena emphasize the importance of in situ light measurements. Another important dosimetric parameter in PDT is the concentration and distribution of the photosensitizer. Apart from in vivo fluorescence monitoring, the photosensitizer part of in vivo PDT dosimetry is still in its infancy.