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

Photodynamic anticancer activity evaluation of novel 5-aminolevulinic acid and 3-hydroxypyridinone conjugates

5-Aminolevulinic acid (ALA) and its derivatives, serving as the endogenous precursor of the photosensitizer (PS) protoporphyrin IX (PpIX), successfully applied in tumor imaging and photodynamic therapy (PDT). ALA and its derivatives have been used to treat actinic keratosis (AK), basal cell carcinoma (BCC), and improve the detection of superficial bladder cancer. However, the high hydrophilicity of ALA and the conversion of PpIX to heme have limited the accumulation of PpIX, hindering the efficiency and potential application of ALA-PDT. This study aims to evaluate the PDT activity of three rationally designed series of ALA-HPO prodrugs, which were based on enhancing the lipophilicity of the prodrugs and reducing the labile iron pool (LIP) through HPO iron chelators to promote PpIX accumulation. Twenty-four ALA-HPO conjugates, incorporating amide, amino acid, and ester linkages, were synthesized. Most of the conjugates, exhibited no dark-toxicity to cells, according to bioactivity evaluation. Ester conjugates 19a-g showed promoted phototoxicity when tested on tumor cell lines, and this increased phototoxicity was strongly correlated with elevated PpIX levels. Among them, conjugate 19c emerged as the most promising (HeLa, IC50 = 24.25 ± 1.43 μM; MCF-7, IC50 = 43.30 ± 1.76 μM; A375, IC50 = 28.03 ± 1.00 μM), displaying superior photodynamic anticancer activity to ALA (IC50 > 100 μM). At a concentration of 80 μM, the fluorescence intensity of PpIX induced by compound 19c in HeLa, MCF-7, and A375 cells was 18.9, 5.3, and 2.8 times higher, respectively, than that induced by ALA. In conclusion, cellular phototoxicity showed a strong correlation with intracellular PpIX fluorescence levels, indicating the potential application of ALA-HPO conjugates in ALA-PDT.

Fractionated Photofrin-Mediated Photodynamic Therapy Significantly Improves Long-Term Survival

This study investigates the effect of fractionated (two-part) PDT on the long-term local control rate (LCR) using the concentration of reactive oxygen species ([ROS]rx) as a dosimetry quantity. Groups with different fractionation schemes are examined, including a 2 h interval between light delivery sessions to cumulative fluences of 135, 180, and 225 J/cm2. While the total treatment time remains constant within each group, the division of treatment time between the first and second fractionations are explored to assess the impact on long-term survival at 90 days. In all preclinical studies, Photofrin is intravenously administered to mice at a concentration of 5 mg/kg, with an incubation period between 18 and 24 h before the first light delivery session. Fluence rate is fixed at 75 mW/cm2. Treatment ensues via a collimated laser beam, 1 cm in diameter, emitting light at 630 nm. Dosimetric quantities are assessed for all groups along with long-term (90 days) treatment outcomes. This study demonstrated a significant improvement in long-term survival after fractionated treatment schemes compared to single-fraction treatment, with the optimal 90-day survival increasing to 63%, 86%, and 100% vs. 20%, 25%, and 50%, respectively, for the three cumulative fluences. The threshold [ROS]rx for the optimal scheme of fractionated Photofrin-mediated PDT, set at 0.78 mM, is significantly lower than that for the single-fraction PDT, at 1.08 mM.

Standardized intraoperative 5-ALA photodynamic therapy for newly diagnosed glioblastoma patients: a preliminary analysis of the INDYGO clinical trial

PURPOSE

Glioblastoma (GBM) is the most aggressive malignant primary brain tumor. The unfavorable prognosis despite maximal therapy relates to high propensity for recurrence. Thus, overall survival (OS) is quite limited and local failure remains the fundamental problem. Here, we present a safety and feasibility trial after treating GBM intraoperatively by photodynamic therapy (PDT) after 5-aminolevulinic acid (5-ALA) administration and maximal resection.

METHODS

Ten patients with newly diagnosed GBM were enrolled and treated between May 2017 and June 2018. The standardized therapeutic approach included maximal resection (near total or gross total tumor resection (GTR)) guided by 5-ALA fluorescence-guided surgery (FGS), followed by intraoperative PDT. Postoperatively, patients underwent adjuvant therapy (Stupp protocol). Follow-up included clinical examinations and brain MR imaging was performed every 3 months until tumor progression and/or death.

RESULTS

There were no unacceptable or unexpected toxicities or serious adverse effects. At the time of the interim analysis, the actuarial 12-months progression-free survival (PFS) rate was 60% (median 17.1 months), and the actuarial 12-months OS rate was 80% (median 23.1 months).

CONCLUSIONS

This trial assessed the feasibility and the safety of intraoperative 5-ALA PDT as a novel approach for treating GBM after maximal tumor resection. The current standard of care remains microsurgical resection whenever feasible, followed by adjuvant therapy (Stupp protocol). We postulate that PDT delivered immediately after resection as an add-on therapy of this primary brain cancer is safe and may help to decrease the recurrence risk by targeting residual tumor cells in the resection cavity. Trial registration NCT number: NCT03048240. EudraCT number: 2016-002706-39.

Metronomic photodynamic therapy using an implantable LED device and orally administered 5-aminolevulinic acid

Metronomic photodynamic therapy (mPDT) is a form of PDT that induces cancer cell death by intermittent continuous irradiation with a relatively weak power of light for a long duration (several days). We previously developed a wirelessly powered, fully implantable LED device and reported a significant anti-tumor effect of mPDT. Considering application in clinical practice, the method used for repeated administrations of photosensitizers required for mPDT should not have a high patient burden such as the burden of transvenous administration. Therefore, in this study, we selected 5-aminolevulinic acid (ALA), which can be administered orally, as a photosensitizer, and we studied the antitumor effects of mPDT. In mice with intradermal tumors that were orally administered ALA (200 mg/kg daily for 5 days), the tumor in each mouse was simultaneously irradiated (8 h/day for 5 days) using a wirelessly powered implantable green LED device (532 nm, 0.05 mW). Tumor growth in the mPDT-treated mice was suppressed by about half compared to that in untreated mice. The results showed that mPDT using the wirelessly powered implantable LED device exerted an antitumor effect even with the use of orally administered ALA, and this treatment scheme can reduce the burden of photosensitizer administration for a patient.

Photodynamic Therapy and the Biophysics of the Tumor Microenvironment

Targeting the tumor microenvironment (TME) provides opportunities to modulate tumor physiology, enhance the delivery of therapeutic agents, impact immune response and overcome resistance. Photodynamic therapy (PDT) is a photochemistry-based, nonthermal modality that produces reactive molecular species at the site of light activation and is in the clinic for nononcologic and oncologic applications. The unique mechanisms and exquisite spatiotemporal control inherent to PDT enable selective modulation or destruction of the TME and cancer cells. Mechanical stress plays an important role in tumor growth and survival, with increasing implications for therapy design and drug delivery, but remains understudied in the context of PDT and PDT-based combinations. This review describes pharmacoengineering and bioengineering approaches in PDT to target cellular and noncellular components of the TME, as well as molecular targets on tumor and tumor-associated cells. Particular emphasis is placed on the role of mechanical stress in the context of targeted PDT regimens, and combinations, for primary and metastatic tumors.

Improving in vitro photodynamic therapy through the development of a novel iron chelating aminolaevulinic acid prodrug

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A new combined iron chelating prodrug (AP2-18) has been synthesised and evaluated.

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AP2-18 significantly increased protoporphyrin IX accumulation in human skin cells.

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This enhancement translated into greater cytotoxicity on irradiation.

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Clinical AP2-18 application may improve future dermatological photodynamic therapy.

Background

Photodynamic therapy (PDT) is a light activated drug therapy that can be used to treat a number of cancers and precancers. It is particularly useful in its topical form in dermatology but improvement of efficacy is required to widen its application.

Methods

An ester between aminolaevulinic acid (ALA) and CP94 was synthesised (AP2-18) and experimentally evaluated to determine whether protoporphyrin IX (PpIX)-induced PDT effectiveness could be improved. A biological evaluation of AP2-18 was conducted in cultured human primary cells with both PpIX fluorescence and cell viability (as determined via the neutral red assay) being assessed in comparison to the PpIX prodrugs normally utilised in clinical practice (aminolaevulinic acid (ALA) or its methyl ester (MAL)) either administered alone or with the comparator iron chelator, CP94.

Results

No significant dark toxicity was observed in human lung fibroblasts but AP2-18 significantly increased PpIX accumulation above and beyond that achieved with ALA or MAL administration +/- CP94 in both human dermal fibroblasts and epithelial squamous carcinoma cells. On light exposure, the combined hydroxypyridinone iron chelating ALA prodrug AP2-18 generated significantly greater cytotoxicity than any of the other treatment parameters investigated when the lowest concentration (250 μM) was employed.

Conclusions

Newly synthesised AP2-18 is therefore concluded to be an efficacious prodrug for PpIX-induced PDT in these dermatologically relevant human cells, achieving enhanced effects at lower concentrations than currently possible with existing pharmaceuticals.

Design of Bifunctional Dendritic 5-Aminolevulinic Acid and Hydroxypyridinone Conjugates for Photodynamic Therapy

Iron chelators have recently attracted interest in the field of photodynamic therapy (PDT) owing to their role in enhancement of intracellular protoporphyrin IX (PpIX) generation induced by 5-aminolevulinic acid (ALA) via the biosynthetic heme cycle. Although ALA is widely used in PDT, cellular uptake of ALA is limited by its hydrophilicity. In order to improve ALA delivery and enhance the PpIX production, several dendrimers incorporating both ALA and 3-hydroxy-4-pyridinone (HPO) were synthesized. The ability of the dendrimers to enter cells and be metabolized to the PpIX photosensitizer was studied in several human cancer cell lines. The dendrimers were found to be significantly more efficient than ALA alone in PpIX production. The higher intracellular PpIX levels showed a clear correlation with enhanced cellular phototoxicity following light exposure. Dendritic derivatives are therefore capable of efficiently delivering both ALA and HPO, which act synergistically to amplify in vitro PpIX levels and enhance PDT efficacy.

An experimental investigation of a novel iron chelating protoporphyrin IX prodrug for the enhancement of photodynamic therapy

Objectives

Non‐melanoma skin cancers are the most frequently occurring type of cancer worldwide. They can be effectively treated using topical dermatological photodynamic therapy (PDT) employing protoporphyrin IX (PpIX) as the active photosensitising agent as long as the disease remains superficial. Novel iron chelating agents are being investigated to enhance the effectiveness and extend the applications of this treatment modality, as limiting free iron increases the accumulation of PpIX available for light activation and thus cell kill.

Methods

Human lung fibroblasts (MRC‐5) and epithelial squamous carcinoma (A431) cells were treated with PpIX precursors (aminolaevulinic acid [ALA] or methyl‐aminolevulinate [MAL]) with or without the separate hydroxypyridinone iron chelating agent (CP94) or alternatively, the new combined iron chelator and PpIX producing agent, AP2‐18. PpIX fluorescence was monitored hourly for 6 hours prior to irradiation. PDT effectiveness was then assessed the following day using the lactate dehydrogenase and neutral red assays.

Results

Generally, iron chelation achieved via CP94 or AP2‐18 administration significantly increased PpIX fluorescence. ALA was more effective as a PpIX‐prodrug than MAL in A431 cells, corresponding with the lower PpIX accumulation observed with the latter congener in this cell type. Addition of either iron chelating agent consistently increased PpIX accumulation but did not always convey an extra beneficial effect on PpIX‐PDT cell kill when using the already highly effective higher dose of ALA. However, these adjuvants were highly beneficial in the skin cancer cells when compared with MAL administration alone. AP2‐18 was also at least as effective as CP94 + ALA/MAL co‐administration throughout and significantly better than CP94 supplementation at increasing PpIX fluorescence in MRC5 cells as well as at lower doses where PpIX accumulation was observed to be more limited.

Conclusions

PpIX fluorescence levels, as well as PDT cell kill effects on irradiation can be significantly increased by pyridinone iron chelation, either via the addition of CP94 to the administration of a PpIX precursor or alternatively via the newly synthesized combined PpIX prodrug and siderophore, AP2‐18. The effect of the latter compound appears to be at least equivalent to, if not better than, the separate administration of its constituent parts, particularly when employing MAL to destroy skin cancer cells. AP2‐18 therefore warrants further detailed analysis, as it may have the potential to improve dermatological PDT outcomes in applications currently requiring enhancement. Lasers Surg. Med. 50:552–565, 2018. © 2018 The Authors. Lasers in Surgery and Medicine Published by Wiley Periodicals, Inc.

Comparing desferrioxamine and light fractionation enhancement of ALA-PpIX photodynamic therapy in skin cancer

Background:

Aminolevulinic acid (ALA)-based photodynamic therapy (PDT) provides selective uptake and conversion of ALA into protoporphyrin IX (PpIX) in actinic keratosis and squamous cell carcinoma, yet large response variations in effect are common between individuals. The aim of this study was to compare pre-treatment strategies that increase the therapeutic effect, including fractionated light delivery during PDT (fPDT) and use of iron chelator desferrioxamine (DFO), separately and combined.

Methods:

Optical measurements of fluorescence were used to quantify PpIX produced, and the total amount of PpIX photobleached as an implicit measure of the photodynamic dose. In addition, measurements of white light reflectance were used to quantify changes in vascular physiology throughout the PDT treatment.

Results:

fPDT produced both a replenishment of PpIX and vascular re-oxygenation during a 2 h dark interval between the first and second PDT light fractions. The absolute photodynamic dose was increased 57% by fPDT, DFO and their combination, as compared with PDT group (from 0.7 to 1.1). Despite that light fractionation increased oedema and scab formation during the week after treatment, no significant difference in long-term survival has been observed between treatment groups. However, outcomes stratified on the basis of measured photodynamic dose showed a significant difference in long-term survival.

Conclusions:

The assessment of implicit photodynamic dose was a more significant predictor of efficacy for ALA-PDT skin cancer treatments than prescription of an enhanced treatment strategy, likely because of high individual variation in response between subjects.

The importance of iron chelation and iron availability during PpIX-induced photodynamic therapy

Background:

Protoporphyrin IX (PpIX)-induced photodynamic therapy (PDT) is being utilised as a topical method of localised ablation of certain non-melanoma skin cancers and precancers. Standardised protocols have been implemented to good effect when the disease remains superficial but improvement is required to treat thicker or acrally located conditions. Concurrent administration of an iron chelator during PpIX-PDT has been demonstrated to increase cellular accumulation of PpIX by reducing its bioconversion to haem (an iron dependent process) thus increasing cell kill on subsequent irradiation. Iron however, can also play a role in reactive oxygen species (ROS) generation and limiting its availability via chemical chelation could theoretically reduce the efficacy of PpIX-PDT, so that a response less than that maximally feasible is produced.

Materials and methods:

The effects of iron availability and chelation on PpIX-PDT have therefore been investigated via fluorescence quantification of PpIX accumulation, single-cell gel electrophoresis (comet assay) measurement of ROS-induced DNA damage and trypan blue exclusion assessment of cell viability. Cultured human cells were utilised and incubated in standardised iron conditions with the PpIX precursor’s aminolaevulinic acid (ALA) or its methyl ester (MAL) in the presence or absence of either of the iron chelating agents desferrioxamine (DFO) or hydroxypyridinone (CP94), or alternatively iron sulphate as a source of iron.

Results:

ALA or MAL incubation was found to significantly increase cellular PpIX accumulation pre-irradiation as anticipated and this observation correlated with both significantly increased DNA damage and reduced cellular viability following irradiation. Co-incubation with either of the iron chelators investigated (DFO or CP94) significantly increased pre-irradiation PpIX accumulation as well as DNA damage and cell death on irradiation indicating the positive effect of iron chelation on the effectiveness of PpIX-induced PDT. The opposite effects were observed however, when the cells were co-incubated with iron sulphate, with significant reductions in pre-irradiation PpIX accumulation (ALA only) and DNA damage (ALA and MAL) being recorded indicating the negative effects excessive iron can have on PpIX-PDT effectiveness. Some dark toxicity produced by iron sulphate administration in non-irradiated control groups was also observed.

Conclusion:

Iron chelation and availability have therefore been observed to positively and adversely affect the PpIX-PDT process respectively and it is concluded that the effects of increased PpIX accumulation pre-irradiation produced via iron chelation outweigh any limitations reduced iron availability may have on the ability of iron to catalyse ROS generation/cascades following PpIX-induced PDT. Further investigation of iron chelation within dermatological applications where enhanced PpIX-PDT treatment effects would be beneficial is therefore warranted.

Focal therapy for prostate cancer. Alternative treatment

CONTEXT

The great controversy surrounding the treatment of localized prostate cancer is related with its possibilities of radical treatment or active surveillance. The objective of this paper is to analyze the rationale selection among current focal therapy modalities regarding tumor and patient selection.

EVIDENCE ACQUISITION

Current articles about advantages and disadvantages on the treatment of localized prostate cancer as well as information about focal therapy regarding tumour selection, characteristics and indications cited in MEDLINE search were reviewed.

SUMMARY OF EVIDENCE

Focal therapy standardized criteria must be: low risk tumors, PSA<10-15, Gleason score ≤ 6, and unilateral presentation all supported by image-guided biopsy and nuclear magnetic resonance (NMR). There are doubts about the suitability of focal therapy in cases of bilateralism or in those with Gleason score 3+4 or PSA>15.

CONCLUSIONS

Focal therapy is an alternative for localized prostate cancer treatment. However, some aspects of their diagnosis and selection criteria should be defined by prospective studies which should provide knowledge about the indication for focal therapy.

Experimental use of photodynamic therapy in high grade gliomas: a review focused on 5-aminolevulinic acid

Photodynamic therapy (PDT) consists of a laser light exposure of tumor cells photosensitized by general or local administration of a pharmacological agent. Nowadays, PDT is a clinically established modality for treatment of many cancers. 5-Aminolevulinic acid (ALA) induced protoporphyrin IX (PpIX) has proven its rational in fluoro-guided resection of malignant gliomas due to a selective tumor uptake and minimal skin sensitization. Moreover, the relatively specific accumulation of photosensitizing PPIX within the tumor cells has gained interest in the PDT of malignant gliomas. Several experimental and clinical studies have then established ALA-PDT as a valuable adjuvant therapy in the management of malignant gliomas. However, the procedure still requires optimizations in the fields of tissue oxygenation status, photosensitizer concentration or scheme of laser light illumination. In this extensive review, we focused on the methods and results of ALA-PDT for treating malignant gliomas in experimental conditions. The biological mechanisms, the effects on tumor and normal brain tissue, and finally the critical issues to optimize the efficacy of ALA-PDT were discussed.

Continuous or fractionated photodynamic therapy? Comparison of three PDT schemes for ovarian peritoneal micrometastasis treatment in a rat model

OBJECTIVE

This experimental study aimed to compare three illumination schemes to optimize hexaminolaevulinate (HAL)-PDT in a rat tumor model with advanced ovarian cancer.

MATERIALS AND METHODS

Peritoneal carcinomatosis was induced by intraperitoneal 5×10(6)NuTu-19 cells injection in 60 female rats Fisher 344. Carcinomatosis was obtained 50 days post-tumor induction. Four hours post-intraperitoneal HAL (Photocure ASA, Oslo, Norway) injection, three different schemes of PDT were performed during 25 min on a 1cm(2) area. (A) Fractionated illumination (n=20) with an on-off cycle ("on": 2 min and "off": 1 min) at 30mW cm(-2) until a fluence of 30J cm(-2), (B) continuous illumination (n=20) at 30mW cm(-2) with a fluence of (45J cm(-2)C) continuous illumination (n=20) at 20mW cm(-2) with a fluence of 30J cm(-2). Laser light was generated using a 532nm KTP laser (Laser Quantum, Stockport, UK). Biopsies were taken 24h after treatment. Quantitative histology was performed. Necrosis value was determined: 0-no necrosis to 4-full necrosis. Depth of necrosis was then measured for each sample and correlated to Necrosis value.

RESULTS

HAL-PDT was efficient in producing necrosis irrespective of the scheme. Tumor destruction was superior with fractionated illumination compared to both continuous illumination schemes regarding to the depth of necrosis (213±113μm vs 154±133μm vs 171±155μm) (p<0.05) or to the full necrosis rate (50% vs 30% vs 10%) (p<0.0001).

CONCLUSION

Fractionated illumination during photodynamic therapy (PDT) was shown to improve tumor response. Fractionated illumination with short intervals should be considered for an effective PDT of advanced ovarian cancer.

Photodynamic therapy for focal ablation of the prostate

Although in early stages of clinical development, photodynamic therapy (PDT) shows promise in delivering focal treatment of both primary and post-radiotherapy prostate cancer. This article will review the mechanism of action of PDT, previous research using PDT for treating prostate cancer including the development of newer vascular-acting photosensitizers, and the potential advantages and disadvantages of PDT in delivering focal therapy.

Clinical investigation of the novel iron-chelating agent, CP94, to enhance topical photodynamic therapy of nodular basal cell carcinoma

BACKGROUND

Photodynamic therapy (PDT) involves the activation of a photosensitizer by visible light to produce activated oxygen species within target cells, resulting in their destruction. Evidence-based guidelines support the efficacy of PDT using topical 5-aminolaevulinic acid (ALA-PDT) in actinic keratoses, Bowen disease and basal cell carcinoma (BCC). Efficacy for nodular BCC appears inferior to that for superficial BCC unless prior debulking or repeat treatments are performed. Objectives The aim of this study was to assess the safety and efficacy of adding a novel iron-chelating agent, CP94 (1,2-diethyl-3-hydroxypyridin-4-one hydrochloride), to topical ALA, to temporarily increase the accumulation of the photosensitizer in the tumour.

METHODS

A mixed topical formulation of ALA + increasing concentrations of CP94 was used to carry out PDT on previously biopsied nodular BCC with no prior lesion preparation using standard light delivery. The area was assessed clinically and surgically excised 6 weeks later for histological examination.

RESULTS

Enhanced PDT using 40% CP94 resulted in significantly greater clearance rates in nodular BCC than with ALA-PDT alone, in our protocol of single-treatment PDT with no lesion preparation.

CONCLUSIONS

The results of this study demonstrate the safe and effective use of an enhanced ALA-PDT protocol for nodular BCC using CP94, with no adverse reactions to this modification. This is the first time this formulation has been used in patients. This formulation is now the focus of further study.

Direct Comparison of δ-Aminolevulinic Acid and Methyl-Aminolevulinate-Derived Protoporphyrin IX Accumulations Potentiated by Desferrioxamine or the Novel Hydroxypyridinone Iron Chelator CP94 in Cultured Human Cells

Aminolevulinic acid photodynamic therapy (ALA-PDT) is a cancer therapy that combines the selective accumulation of a photosensitizer in tumor tissue with visible light (and tissue oxygen) to produce reactive oxygen species. This results in cellular damage and ablation of tumor tissue. The use of iron chelators in combination with ALA has the potential to increase the accumulation of the photosensitizer protoporphyrin IX (PpIX) by reducing its bioconversion to heme. This study compares directly for the first time the effects of the novel hydroxypyridinone iron chelating agent CP94 and the more clinically established iron chelator desferrioxamine (DFO) on the enhancement of ALA and methyl-aminolevulinate (MAL)-induced PpIX accumulations in cultured human cells. Cultured human cells were incubated with a combination of ALA, MAL, CP94 and DFO concentrations; the resulting PpIX accumulations being quantified fluorometrically. The use of iron chelators in combination with ALA or MAL was shown to significantly increase the amount of PpIX accumulating in the fetal lung fibroblasts and epidermal carcinoma cells; while minimal enhancement was observed in the normal skin cells investigated (fibroblasts and keratinocytes). Where enhancement was observed CP94 was shown to be significantly superior to DFO in the enhancement of PpIX accumulation.

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.