Enhancement of 5-aminolaevulinic acid-induced photodynamic therapy in normal rat colon using hydroxypyridinone iron-chelating agents

5-Aminolevulinic Acid as a Theranostic Agent for Tumor Fluorescence Imaging and Photodynamic Therapy

5-Aminolevulinic acid (ALA) is a naturally occurring amino acid synthesized in all nucleated mammalian cells. As a porphyrin precursor, ALA is metabolized in the heme biosynthetic pathway to produce protoporphyrin IX (PpIX), a fluorophore and photosensitizing agent. ALA administered exogenously bypasses the rate-limit step in the pathway, resulting in PpIX accumulation in tumor tissues. Such tumor-selective PpIX disposition following ALA administration has been exploited for tumor fluorescence diagnosis and photodynamic therapy (PDT) with much success. Five ALA-based drugs have now received worldwide approval and are being used for managing very common human (pre)cancerous diseases such as actinic keratosis and basal cell carcinoma or guiding the surgery of bladder cancer and high-grade gliomas, making it the most successful drug discovery and development endeavor in PDT and photodiagnosis. The potential of ALA-induced PpIX as a fluorescent theranostic agent is, however, yet to be fully fulfilled. In this review, we would like to describe the heme biosynthesis pathway in which PpIX is produced from ALA and its derivatives, summarize current clinical applications of ALA-based drugs, and discuss strategies for enhancing ALA-induced PpIX fluorescence and PDT response. Our goal is two-fold: to highlight the successes of ALA-based drugs in clinical practice, and to stimulate the multidisciplinary collaboration that has brought the current success and will continue to usher in more landmark advances.

Experimental investigation of a combinational iron chelating protoporphyrin IX prodrug for fluorescence detection and photodynamic therapy

Photodynamic therapy (PDT) is an oxygen-dependent, light-activated, and locally destructive drug treatment of cancer. Protoporphyrin IX (PpIX)-induced PDT exploits cancer cells' own innate heme biosynthesis to hyper-accumulate the naturally fluorescent and photoactive precursor to heme, PpIX. This occurs as a result of administering heme precursors (e.g., aminolevulinic acid; ALA) because the final step of the pathway (the insertion of ferrous iron into PpIX by ferrochelatase to form heme) is relatively slow. Separate administration of an iron chelating agent has previously been demonstrated to significantly improve dermatological PpIX-PDT by further limiting heme production. A newly synthesized combinational iron chelating PpIX prodrug (AP2-18) has been assessed experimentally in cultured primary human cells of bladder and dermatological origin, as an alternative photosensitizing agent to ALA or its methyl or hexyl esters (MAL and HAL respectively) for photodetection/PDT. Findings indicated that the technique of iron chelation (either through the separate administration of the established hydroxypyridinone iron chelator CP94 or the just as effective combined AP2-18) did not enhance either PpIX fluorescence or PDT-induced (neutral red assessed) cell death in human primary normal and malignant bladder cells. However, 500 µM AP2-18 significantly increased PpIX accumulation and produced a trend of increased cell death within epithelial squamous carcinoma cells. PpIX accumulation destabilized the actin cytoskeleton in bladder cancer cells prior to PDT and resulted in caspase-3 cleavage/early apoptosis afterwards. AP2-18 iron chelation should continue to be investigated for the enhancement of dermatological PpIX-PDT applications but not bladder photodetection/PDT.

Impact of thiosemicarbazones on the accumulation of PpIX and the expression of the associated genes

Thiosemicarbazone derivatives are known for their broad biological activity including their antitumor potency. The aim of the current study was to examine the effect of a novel series of non-toxic iron chelators on the accumulation of protoporphyrin IX after external 5-aminolevulonic acid administration. From this series we selected one the most promising derivative which causes a pronounced increase in the concentration of protoporphyrin IX. The increase of the photosensitizer concentration is necessary for the trigger the efficient therapeutic effect of the photodynamic reaction. For selected compound 2 we performed an examination of a panel of the genes that are involved in the heme biosynthesis and degradation. Results indicated the crucial roles of ferrochelatase and heme oxygenase in the described processes. Surprisingly, there was a strict dependence on the type of the tested cell line. A decrease in the expression of the two aforementioned enzymes after incubation with compound 2 and 5-aminolevulonic acid is a commonly known fact and we detected this trend for the MCF-7 and HCT 116 cell lines. However, we noticed the upregulation of the tested targets for the Hs683 cells. These unconventional results prompted us to do a more in-depth analysis of the described processes. In conclusion, we found that compound 2 is a novel, highly effective booster of photodynamic therapy that has prospective applications.

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.

Chemical approaches for the enhancement of 5-aminolevulinic acid-based photodynamic therapy and photodiagnosis

The development of photodynamic therapy and photodiagnosis with 5-aminolevulinic acid presents a number of challenges that can be addressed by applying chemical insight and a range of novel prodrug strategies.

The hydroxypyridinone iron chelator CP94 increases methyl-aminolevulinate-based photodynamic cell killing by increasing the generation of reactive oxygen species

Methyl-aminolevulinate-based photodynamic therapy (MAL-PDT) is utilised clinically for the treatment of non-melanoma skin cancers and pre-cancers and the hydroxypyridinone iron chelator, CP94, has successfully been demonstrated to increase MAL-PDT efficacy in an initial clinical pilot study. However, the biochemical and photochemical processes leading to CP94-enhanced photodynamic cell death, beyond the well-documented increases in accumulation of the photosensitiser protoporphyrin IX (PpIX), have not yet been fully elucidated. This investigation demonstrated that MAL-based photodynamic cell killing of cultured human squamous carcinoma cells (A431) occurred in a predominantly necrotic manner following the generation of singlet oxygen and ROS. Augmenting MAL-based photodynamic cell killing with CP94 co-treatment resulted in increased PpIX accumulation, MitoSOX-detectable ROS generation (probably of mitochondrial origin) and necrotic cell death, but did not affect singlet oxygen generation. We also report (to our knowledge, for the first time) the detection of intracellular PpIX-generated singlet oxygen in whole cells via electron paramagnetic resonance spectroscopy in conjunction with a spin trap.

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Augmentation of MAL-based photodynamic cell killing with CP94 increases necrosis.

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CP94 augmentation increases generation of ROS, likely to be mitochondria-localised.

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PpIX-generated ¹O₂ was detected in whole cells by EPR spectroscopy.

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Photodynamic cell killing was dependent primarily on ¹O₂.

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Superoxide/other ROS also contributed to the efficacy of photodynamic cell killing.

Revisiting photodynamic therapy dosimetry: reductionist & surrogate approaches to facilitate clinical success

Photodynamic therapy (PDT) can be a highly complex treatment, with many parameters influencing treatment efficacy. The extent to which dosimetry is used to monitor and standardize treatment delivery varies widely, ranging from measurement of a single surrogate marker to comprehensive approaches that aim to measure or estimate as many relevant parameters as possible. Today, most clinical PDT treatments are still administered with little more than application of a prescribed drug dose and timed light delivery, and thus the role of patient-specific dosimetry has not reached widespread clinical adoption. This disconnect is at least partly due to the inherent conflict between the need to measure and understand multiple parameters in vivo in order to optimize treatment, and the need for expedience in the clinic and in the regulatory and commercialization process. Thus, a methodical approach to selecting primary dosimetry metrics is required at each stage of translation of a treatment procedure, moving from complex measurements to understand PDT mechanisms in pre-clinical and early phase I trials, towards the identification and application of essential dose-limiting and/or surrogate measurements in phase II/III trials. If successful, identifying the essential and/or reliable surrogate dosimetry measurements should help facilitate increased adoption of clinical PDT. In this paper, examples of essential dosimetry points and surrogate dosimetry tools that may be implemented in phase II/III trials are discussed. For example, the treatment efficacy as limited by light penetration in interstitial PDT may be predicted by the amount of contrast uptake in CT, and so this could be utilized as a surrogate dosimetry measurement to prescribe light doses based upon pre-treatment contrast. Success of clinical ALA-based skin lesion treatment is predicted almost uniquely by the explicit or implicit measurements of photosensitizer and photobleaching, yet the individualization of treatment based upon each patients measured bleaching needs to be attempted. In the case of ALA, lack of PpIX is more likely an indicator that alternative PpIX production methods must be implemented. Parsimonious dosimetry, using surrogate measurements that are clinically acceptable, might strategically help to advance PDT in a medical world that is increasingly cost and time sensitive. Careful attention to methodologies that can identify and advance the most critical dosimetric measurements, either direct or surrogate, are needed to ensure successful incorporation of PDT into niche clinical procedures.

Design and synthesis of 5-aminolaevulinic acid/3-hydroxypyridinone conjugates for photodynamic therapy: enhancement of protoporphyrin IX production and photo-toxicity in tumor cells

A series of ALA-HPO conjugates was prepared. One such conjugate was found to possess high phototoxicity.

Aminolevulinic Acid-Based Tumor Detection and Therapy: Molecular Mechanisms and Strategies for Enhancement

Aminolevulinic acid (ALA) is the first metabolite in the heme biosynthesis pathway in humans. In addition to the end product heme, this pathway also produces other porphyrin metabolites. Protoporphyrin (PpIX) is one heme precursor porphyrin with good fluorescence and photosensitizing activity. Because tumors and other proliferating cells tend to exhibit a higher level of PpIX than normal cells after ALA incubation, ALA has been used as a prodrug to enable PpIX fluorescence detection and photodynamic therapy (PDT) of lesion tissues. Extensive studies have been carried out in the past twenty years to explore why some tumors exhibit elevated ALA-mediated PpIX and how to enhance PpIX levels to achieve better tumor detection and treatment. Here we would like to summarize previous research in order to stimulate future studies on these important topics. In this review, we focus on summarizing tumor-associated alterations in heme biosynthesis enzymes, mitochondrial functions and porphyrin transporters that contribute to ALA-PpIX increase in tumors. Mechanism-based therapeutic strategies for enhancing ALA-based modalities including iron chelators, differentiation agents and PpIX transporter inhibitors are also discussed.

MicroRNAs associated with the efficacy of photodynamic therapy in biliary tract cancer cell lines

Photodynamic therapy (PDT) is a palliative treatment option for unresectable hilar biliary tract cancer (BTC) showing a considerable benefit for survival and quality of life with few side effects. Currently, factors determining the cellular response of BTC cells towards PDT are unknown. Due to their multifaceted nature, microRNAs (miRs) are a promising analyte to investigate the cellular mechanisms following PDT. For two photosensitizers, Photofrin® and Foscan®, the phototoxicity was investigated in eight BTC cell lines. Each cell line (untreated) was profiled for expression of n=754 miRs using TaqMan® Array Human MicroRNA Cards. Statistical analysis and bioinformatic tools were used to identify miRs associated with PDT efficiency and their putative targets, respectively. Twenty miRs correlated significantly with either high or low PDT efficiency. PDT was particularly effective in cells with high levels of clustered miRs 25-93*-106b and (in case of miR-106b) a phenotype characterized by high expression of the mesenchymal marker vimentin and high proliferation (cyclinD1 and Ki67 expression). Insensitivity towards PDT was associated with high miR-200 family expression and (for miR-cluster 200a/b-429) expression of differentiation markers Ck19 and Ck8/18. Predicted and validated downstream targets indicate plausible involvement of miRs 20a*, 25, 93*, 130a, 141, 200a, 200c and 203 in response mechanisms to PDT, suggesting that targeting these miRs could improve susceptibility to PDT in insensitive cell lines. Taken together, the miRNome pattern may provide a novel tool for predicting the efficiency of PDT and-following appropriate functional verification-may subsequently allow for optimization of the PDT protocol.

Mechanisms in photodynamic therapy: part one-photosensitizers, photochemistry and cellular localization

The use of non-toxic dyes or photosensitizers (PS) in combination with harmless visible light that is known as photodynamic therapy (PDT) has been known for over a hundred years, but is only now becoming widely used. Originally developed as a tumor therapy, some of its most successful applications are for non-malignant disease. In a series of three reviews we will discuss the mechanisms that operate in the field of PDT. Part one discusses the recent explosion in discovery and chemical synthesis of new PS. Some guidelines on how to choose an ideal PS for a particular application are presented. The photochemistry and photophysics of PS and the two pathways known as Type I (radicals and reactive oxygen species) and Type II (singlet oxygen) photochemical processes are discussed. To carry out PDT effectively in vivo, it is necessary to ensure sufficient light reaches all the diseased tissue. This involves understanding how light travels within various tissues and the relative effects of absorption and scattering. The fact that most of the PS are also fluorescent allows various optical imaging and monitoring strategies to be combined with PDT. The most important factor governing the outcome of PDT is how the PS interacts with cells in the target tissue or tumor, and the key aspect of this interaction is the subcellular localization of the PS. Examples of PS that localize in mitochondria, lysosomes, endoplasmic reticulum, Golgi apparatus and plasma membranes are given. Finally the use of 5-aminolevulinic acid as a natural precursor of the heme biosynthetic pathway, stimulates accumulation of the PS protoporphyrin IX is described.

The effects of protoporphyrin IX-induced photodynamic therapy with and without iron chelation on human squamous carcinoma cells cultured under normoxic, hypoxic and hyperoxic conditions

BACKGROUND

Photodynamic therapy requires the combined interaction of a photosensitiser, light and oxygen to ablate target tissue. In this study we examined the effect of iron chelation and oxygen environment manipulation on the accumulation of the clinically useful photosensitiser protoporphyrin IX (PpIX) within human squamous epithelial carcinoma cells and the subsequent ablation of these cells on irradiation.

METHODS

Cells were incubated at concentrations of 5%, 20% or 40% oxygen for 24h prior to and for 3h following the administration of the PpIX precursors aminolevulinic acid (ALA), methyl aminolevulinate (MAL) or hexylaminolevulinate (HAL) with or without the iron chelator 1,2-diethyl-3-hydroxypyridin-4-one hydrochloride (CP94). PpIX accumulation was monitored using a fluorescence plate reader, cells were irradiated with 37 J/cm(2) red light and cell viability measured using the neutral red uptake assay.

RESULTS

Manipulation of the oxygen environment and/or co-administration of CP94 with PpIX precursors resulted in significant changes in both PpIX accumulation and photobleaching. Incubation with 5% or 40% oxygen produced the greatest levels of PpIX and photobleaching in cells incubated with ALA/MAL. Incorporation of CP94 also resulted in significant decreases in cell viability following administration of ALA/MAL/HAL, with oxygen concentration predominantly having a significant effect in cells incubated with HAL.

CONCLUSIONS

Experimentation with human squamous epithelial carcinoma cells has indicated that the iron chelator CP94 significantly increased PpIX accumulation induced by each PpIX congener investigated (ALA/MAL/HAL) at all oxygen concentrations employed (5%/20%/40%) resulting in increased levels of photobleaching and reduced cell viability on irradiation. Further detailed investigation of the complex relationship of PDT cytotoxicity at various oxygen concentrations is required. It is therefore concluded that iron chelation with CP94 is a simple protocol modification with which it may be much easier to enhance clinical PDT efficacy than the complex and less well understood process of oxygen manipulation.

Clearance mechanism of protoporphyrin IX from mouse skin after application of 5-aminolevulinic acid

BACKGROUND

5-Aminolevulinic acid (ALA) or its esters mediated photodynamic therapy (PDT) is the most widely practiced form of PDT in dermatology. One of its advantages is that undesirable photosensitization lasts only for 24-48 h. In order to optimize ALA-PDT it is necessary to understand the mechanisms of intracellular production and clearance of PpIX (efflux from cells into blood stream and/or its conversion into haem). The aim of this study is to investigate the factors controlling the clearance of intracellular PpIX from healthy skin of mice.

METHODS

PpIX was induced in mouse skin by topical or systemic application of ALA or by topical application of the iron chelator ethylenediaminetetraacetic acid (EDTA). Fluorescence spectroscopy was used to study PpIX kinetics in alive and dead skin.

RESULTS

Topical application of ALA or EDTA leads to porphyrin production in living skin, but not in excised skin. The clearance rates of PpIX from alive and dead skin were the same in the absence of an intracellular ALA pool. The clearance half-life of EDTA-induced PpIX was 4-7 times longer than that of PpIX after application of ALA.

CONCLUSIONS

Skin temperature and intracellular iron availability strongly affect PpIX clearance, while ALA application mode (topical versus systemic) and skin viability (dead versus alive) have no influence on PpIX decay. These results demonstrate that the clearance kinetics of PpIX from skin are determined mostly by the conversion of PpIX into haem, while the cellular efflux of PpIX into blood plays a minor role.

Biomodulatory approaches to photodynamic therapy for solid tumors

Photodynamic Therapy (PDT) uses a photosensitizing drug in combination with visible light to kill cancer cells. PDT has an advantage over surgery or ionizing radiation because PDT can eliminate tumors without causing fibrosis or scarring. Disadvantages include the dual need for drug and light, and a generally lower efficacy for PDT vs. surgery. This minireview describes basic principles of PDT, photosensitizers available, and aspects of tumor biology that may provide further opportunities for treatment optimization. An emerging biomodulatory approach, using methotrexate or Vitamin D in combination with aminolevulinate-based PDT, is described. Finally, current clinical uses of PDT for solid malignancies are reviewed.

An in vitro comparison of the effects of the iron-chelating agents, CP94 and dexrazoxane, on protoporphyrin IX accumulation for photodynamic therapy and/or fluorescence guided resection

Photodynamic therapy (PDT) utilizes the combined interaction of a photosensitizer, light and molecular oxygen to ablate tumor tissue. Maximizing the accumulation of the photosensitizer protoporphyrin IX (PpIX) within different cell types would be clinically useful. Dermatological PpIX-induced PDT regimes produce good clinical outcomes but this currently only applies when the lesion remains superficial. Also, as an adjuvant therapy for the treatment of primary brain tumors, fluorescence guided resection (FGR) and PDT can be used to highlight and destroy tumor cells unreachable by surgical resection. By employing iron chelators PpIX accumulation can be enhanced. Two iron-chelating agents, 1,2-diethyl-3-hydroxypyridin-4-one hydrochloride (CP94) and dexrazoxane, were individually combined with the porphyrin precursors aminolevulinic acid (ALA), methyl aminolevulinate (MAL) and hexyl aminolevulinate (HAL). Efficacies of the iron-chelating agents were compared by recording the PpIX fluorescence in human squamous epithelial carcinoma cells (A431) and human glioma cells (U-87 MG) every hour for up to 6 h. Coincubation of ALA/MAL/HAL with CP94 resulted in a greater accumulation of PpIX compared to that produced by coincubation of these congeners with dexrazoxane. Therefore the clinical employment of iron chelation, particularly with CP94 could potentially increase and/or accelerate the accumulation of ALA/MAL/HAL-induced PpIX for PDT or FGR.

Photodynamic therapy of cancer: an update

Photodynamic therapy (PDT) is a clinically approved, minimally invasive therapeutic procedure that can exert a selective cytotoxic activity toward malignant cells. The procedure involves administration of a photosensitizing agent followed by irradiation at a wavelength corresponding to an absorbance band of the sensitizer. In the presence of oxygen, a series of events lead to direct tumor cell death, damage to the microvasculature, and induction of a local inflammatory reaction. Clinical studies revealed that PDT can be curative, particularly in early stage tumors. It can prolong survival in patients with inoperable cancers and significantly improve quality of life. Minimal normal tissue toxicity, negligible systemic effects, greatly reduced long-term morbidity, lack of intrinsic or acquired resistance mechanisms, and excellent cosmetic as well as organ function-sparing effects of this treatment make it a valuable therapeutic option for combination treatments. With a number of recent technological improvements, PDT has the potential to become integrated into the mainstream of cancer treatment.

The hydroxypyridinone iron chelator CP94 can enhance PpIX-induced PDT of cultured human glioma cells

Photodynamic therapy (PDT) with the pro-drugs 5-aminolevulinic acid (ALA) or methyl aminolevulinate (MAL) utilizes the combined interaction of a photosensitizer, light and molecular oxygen to ablate tumor tissue. To potentially increase accumulation of the photosensitizer, protoporphyrin IX (PpIX), within tumor cells an iron chelator can be employed. This study analyzed the effects of ALA/MAL-induced PDT combined with the iron chelator 1, 2-diethyl-3-hydroxypyridin-4-one hydrochloride (CP94) on the accumulation of PpIX in human glioma cells in vitro. Cells were incubated for 0, 3 and 6h with various concentrations of ALA/MAL with or without CP94 and the resulting accumulations of PpIX, which naturally fluoresces, were quantified prior to and following light irradiation. In addition, counts of viable cells were recorded. The use of CP94 in combination with ALA/MAL produced significant enhancements of PpIX fluorescence in human glioma cells. At the highest concentrations of each prodrug, CP94 enhanced PpIX fluorescence significantly at 3h for ALA and by more than 50% at 6h for MAL. Cells subsequently treated with ALA/MAL-induced PDT in combination with CP94 produced the greatest cytotoxicity. It is therefore concluded that with further study CP94 may be a useful adjuvant to photodiagnosis and/or PpIX-induced PDT treatment of glioma.

Influence of ceramide 2 on in vitro skin permeation and retention of 5-ALA and its ester derivatives, for Photodynamic Therapy

Photodynamic therapy (PDT) based on topical 5-aminolevulinic acid (5-ALA), an endogenous precursor of protoporphyrin, is an interesting approach for the treatment of skin cancer. However, 5-ALA is a hydrophilic molecule and such a characteristic limits its appropriate cutaneous penetration and retention. In this way, more lipophilic molecules, such as esterified 5-ALA derivatives, have been under investigation in order to improve the skin penetration of this molecule. Drug formulation can also alter 5-ALA skin penetration. Therefore, the aim of this work was to study the influence of ceramide 2 - the main lipid of the SC- on the cutaneous delivery of 5-ALA and its ester derivatives in vitro, using Franz diffusion cell. The skin permeation of all studied drugs was decreased in the presence of ceramide, representing a desirable characteristic in order to avoid the risk of systemic side effects. Nevertheless, the SC and [epidermis + dermis] retention after 16 h has also been decreased in the presence of ceramide, as compared to control. In conclusion, ceramide was not a good adjuvant, meaning that research of other vehicles could be useful to improve cutaneous delivery of 5-ALA.

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.

Enhancement of methyl-aminolevulinate photodynamic therapy by iron chelation with CP94: an in vitro investigation and clinical dose-escalating safety study for the treatment of nodular basal cell carcinoma

PURPOSE

Methyl-aminolevulinate (MAL) photodynamic therapy (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. Combining iron chelators with MAL has the potential to increase the accumulation of the photosensitizer protoporphyrin IX (PpIX) by reducing its bioconversion to heme. This paper investigates this method of enhancement both in vitro and for the first time clinically for the treatment of nodular basal cell carcinoma (BCC).

METHODS

Enhancement of MAL-induced PpIX accumulation by the iron chelator CP94 was quantified fluorometrically in human cultured cells (including three dermatological cell types). An open, dose-escalating, pilot study was then conducted in patients with nodular BCC, to determine the safety of this pharmacological modification.

RESULTS

Large enhancements in PpIX accumulation were observed in the cultured cells when co-incubated with the iron chelator CP94. Clinically the addition of CP94 was found to be feasible and safe. In addition greater reductions in tumor depth were observed in the CP94 co-incubated tumors.

CONCLUSION

Iron chelation by CP94 is an effective enhancer of MAL-induced PpIX accumulation in vitro. This method of enhancement was safely applied to a clinical PDT protocol with no unexpected adverse effects reported. Although the clinical investigation was only intended to be a small pilot to assess safety, enhancements in tumor clearance were observed both clinically and histologically when CP94 was included in the photosensitizing cream.

Milestones in the development of photodynamic therapy and fluorescence diagnosis

Many reviews on PDT have been published. This field is now so large, and embraces so many sub-specialties, from laser technology and optical penetration through diffusing media to a number of medical fields including dermatology, gastroenterology, ophthalmology, blood sterilization and treatment of microbial-viral diseases, that it is impossible to cover all aspects in a single review. Here, we will concentrate on a few basic aspects, all important for the route of development leading PDT to its present state: early work on hematoporphyrin and hematoporphyrin derivative, second and third generation photosensitizers, 5-aminolevulinic acid and its derivatives, oxygen and singlet oxygen, PDT effects on cell organelles, mutagenic potential, the basis for tumour selectivity, cell cooperativity, photochemical internalization, light penetration into tissue and the significance of oxygen depletion, photobleaching of photosensitizers, optimal light sources, effects on the immune system, and, finally, future trends.

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.

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

BACKGROUND AND OBJECTIVES

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

STUDY DESIGN/MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Targeted photodynamic therapy

BACKGROUND AND OBJECTIVES

Photodynamic therapy (PDT) is an emerging modality for the treatment of various neoplastic and non-neoplastic pathologies.

STUDY DESIGN/MATERIALS AND METHODS

PDT usually occurs when reactive oxygen species (ROS) generated from light-activated chemicals (photosensitizer, PS) destroy the target. For non-dermatologic applications the PS are delivered systemically and accumulate, at different concentrations, in most organs.

RESULTS AND CONCLUSION

Typically there is a modest enhanced accumulation of the PS in tumor tissues, providing a first level of selectivity. Additional selectivity is provided by the confined illumination of the target area with the appropriate wavelength of light. For the treatment of pathologies in complex anatomical sites, such as in the peritoneal cavity, where restricted illumination is difficult; improved targeting of the PS is necessary to prevent damage to the surrounding healthy tissue. This article will focus on targeted PDT.

Enhancement of photodynamic effect in normal rat keratinocytes by treatment with 1,25 dihydroxy vitamin D3

BACKGROUND

To better understand the pathogenesis of photodynamic therapy (PDT)-induced apoptosis cytosolic calcium [Ca2+]i was measured using cultured fetal rat keratinocytes (FRSKs). Moreover, the influence of 1,25 dihydroxy vitamin D3 (1,25(OH)2D3) with the action of increasing [Ca2+]i on the PDT effect was studied.

METHODS

FRSKs were treated with a medium containing the photosensitizer, aluminum phthalocyanine tetrasulfonate (AlPcTs), and were then exposed to selective visible light derived from a halogen lamp. Electrophoresis of DNA extracted from the PDT-treated cells revealed DNA fragmentation, a sign of apoptosis in cultured FRSKs under the condition with or without 1,25(OH)2D3.

RESULTS

PDT-treated FRSKs exhibited increased levels of [Ca2+]i; these levels were significantly elevated further by the treatment of cells with 1,25(OH)2D3. However, cells treated with ethylene glycol bis (b-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA), a chelator of extracellular calcium, prior to PDT did not show any DNA fragmentation either in the presence or absence of 1,25(OH)2D3.

CONCLUSION

PDT-induced apoptosis in FRSKs may be caused by the influx of extracellular calcium. Addition of 1,25(OH)2D3 clearly enhanced the DNA fragmentation in the cultured FRSKs, indicating the effect of increased [Ca2+]i. The combination therapy of AlPcTs-PDT with the administration of 1,25(OH)2D3 may contribute to the enhancement of the AlPcTs-PTD effect.

Enhanced effects of aminolaevulinic acid-based photodynamic therapy through local hyperthermia in rat tumours

The possibility of enhancing aminolaevulinic acid (ALA)-based photodynamic therapy (PDT) by simultaneous application of localised hyperthermia (HT) was evaluated. Treatments of rat DS-sarcomas included: (i) control, (ii) ALA administration (375 mg kg(-1), i.p.), no illumination, (iii) 'nonthermal' illumination, (iv) ALA-PDT: that is, ALA administration, 'nonthermal' illumination, (v) localised HT, 43 degrees C, 60 min (vi) ALA-PDT+HT: ALA administration with full spectrum irradiation resulting in ALA-PDT and HT. Tumour volume was monitored for 90 days or until a target volume (3.5 ml) was reached. No differences were seen between the first three groups, with all tumours reaching the target volume by 8-11 days. A total of 13 and 15% of tumours did not reach the target volume by day 90 following HT or ALA-PDT treatment, respectively. ALA-PDT+HT showed the greatest antitumour effect (P=0.0001), with 61% of the tumours not reaching the target volume. Viability and in vitro growth were also assessed in cells from tumours excised after treatment. ALA-PDT+HT reduced the fraction of viable tumour cells by 85%, and in vitro culture showed pronounced growth delay compared to control cells. These results demonstrate an enhanced antitumour effect upon ALA+HT, which appears to involve direct cell toxicity rather than solely vascular damage.

The effect of an iron chelating agent on protoporphyrin IX levels and phototoxicity in topical 5-aminolaevulinic acid photodynamic therapy

BACKGROUND

In 5-aminolaevulinic acid (ALA)-photodynamic therapy (PDT), the prodrug ALA is endogenously converted to the active sensitizer protoporphyrin IX (PpIX), while further conversion of PpIX to haem requires iron.

OBJECTIVES

To explore the potential of the iron chelator desferrioxamine (DFO) to enhance PpIX levels and phototoxicity in ALA-PDT.

METHODS

A series of six doses of 2% ALA solution was iontophoresed into the healthy skin of each ventral forearm of 10 volunteers. One arm was pretreated with 20% DFO in aqueous cream, while the control arm received aqueous cream alone, for 16 h. At 5 h following iontophoresis, skin-surface PpIX fluorescence was measured, following which the forearms were simultaneously irradiated with 100 J cm-2 broadband red light. The phototoxic reaction was assessed at 24 h postirradiation as the minimal phototoxic dose (MPD) and with quantification of erythema. Next, eight patients with two superficial basal cell carcinomas or two plaques of Bowen's disease of similar appearance received 20% ALA topically to one lesion and 20% ALA with 20% DFO to the other, for 3 h. Skin-surface PpIX fluorescence was measured at 5 h, following which lesions were irradiated with 100 J cm-2 broadband red light.

RESULTS

In healthy skin, PpIX fluorescence increased with increasing ALA dose at DFO-treated and untreated sites (P < 0.0005); PpIX fluorescence peak values were consistently higher in DFO-treated compared with control sites (P < 0.02). Erythema also correlated with ALA dose (P < 0.0005), but a significant difference between active and control sites occurred only at low ALA dose (P < 0.05). The median MPD appeared lower at the DFO-treated sites, at 6 mC vs. 12 mC (P = 0.06). In contrast, in lesional skin there was no consistent difference in PpIX fluorescence levels between those treated with and without DFO.

CONCLUSIONS

While iron chelation augmented ALA-PDT phototoxicity in normal skin, this occurred only at low ALA dose. Addition of DFO does not appear to confer additional benefit in ALA-PDT of nonmelanoma skin cancers.

Characterization of two isomeric beta-d-glucosiduronic acids derived from 1,2-diethyl-3-hydroxypyridin-4-one (CP94) in rat liver homogenate incubates

1,2-Diethyl-3-hydroxypyridin-4-one (CP94) is an orally active iron chelator with potential for use in photodynamictherapy. This investigation reports the formation and characterization of two isomeric glucuronides of CP94 in rat liver homogenate incubates. To assign the glucuronidation sites in the CP94 molecule, two O-methylated derivatives of CP94 have been synthesized. By comparing the spectral characteristics of the CP94 3-O- and 4-O-methyl derivatives with CP94 and the CP94 glucuronides formed during incubation, evidence was obtained which enabled the assignment of these two isomeric glucuronides to the 3-O-glucuronide and 4-O-glucuronide of CP94. It was found that the 3-O-glucuronide was the dominant CP94 metabolite under in-vitro conditions. In an attempt to understand the potential influence of structural variation on the glucuronidation of CP94 analogues, the 1-and 2-monoethyl derivatives of CP94 were investigated. The 2-monoethyl derivative of CP94 yielded only the 3-O-glucuronide in rat liver homogenate incubate, while no glucuronide was formed from the 1-monoethyl derivative. In addition, no glucuronide from the 3-O-methyl or 4-O-methyl derivatives of CP94 could be detected. The relevance of these findings to the development of new 3-hydroxypyridin-4-one iron chelators is discussed.

Photodynamic therapy of human Barrett's cancer using 5-aminolaevulinic acid-induced protoporphyrin IX: an in-vivo dosimetry study in athymic nude mice

OBJECTIVE

There has been a dramatic increase in recent years in the incidence of Barrett's oesophagus and the oesophageal adenocarcinoma associated with it. Alongside surgical treatment for early Barrett's carcinomas, endoscopic treatment procedures such as photodynamic therapy (PDT), which have much lower complication and mortality rates, will play an increasing role in the future. In this study, the effects of light energy dose, light fractionation and oxygenation on the efficacy of PDT were investigated for the first time in an in-vivo nude mice tumour model bearing a human Barrett's carcinoma.

DESIGN

A total of 387 NMRI strain (nu/nu) nude mice with thymic aplasia (total 53 controls) were transplanted with human Barrett's carcinoma and treated with laser light at 635 nm (light dose 0-200 J/cm2, fluence rate 400 mW/cm2). 5-Aminolaevulinic acid-induced protoporphyrin IX (5-ALA-PpIX) (100 mg 5-ALA/kg body weight administered orally) was used as the photosensitizer.

METHODS

Fractionation studies were performed at 0, 50, 100 and 150 J/cm2. The light dose was administered in four equal fractions divided by three irradiation-free intervals of 120 s. Oxygenation studies were carried out at 150 J/cm2 with simultaneous oxygen supply of 2, 6 and 8 l oxygen/min.

RESULTS

Dosimetry studies demonstrated a positive correlation between increase in light dose and tumour destruction up to 150 J/cm2 when using either continuous or fractionated light delivery. The optimal light energy dose was 150 J/cm2. Neither fractionation of light nor simultaneous oxygenation enhanced the efficacy of the PDT.

CONCLUSION

This is the first study in the literature that proves the efficiency of PDT with 5-ALA-PpIX in human Barrett's adenocarcinoma and that demonstrates an exact dosimetry of the optimal light energy dose (150 J/cm2). No general recommendation can be made for the use of fractionation or oxygenation in clinical PDT.

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

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

Design of clinically useful iron(III)-selective chelators

Iron overload is a serious clinical condition which can be largely prevented by the use of iron-specific chelating agents. Desferrioxamine-B, the most widely used iron chelator in haematology over the past 30 years, has a major disadvantage of being orally inactive. Consequently, the successful design of an orally active, nontoxic, selective iron chelator has become a much sought after goal. In order to identify an ideal iron chelator for clinical use, a range of specifications must be considered such as metal selectivity and affinity, kinetic stability of the complex, bioavailability and toxicity. A wide range of chelator types bind iron(III) and of these, hexa-, tri-, and bidentate are capable of providing iron(III) with the favoured octahedral environment. In this review, the comparative properties of such ligands are discussed, examples being selected from hydroxamates, aminocarboxylates, hydroxypyridinones, orthosubstituted phenols and triazoles.

Enhancement of delta aminolevulinic acid-photodynamic therapy in vivo by decreasing tumor pH with glucose and amiloride

OBJECTIVES/HYPOTHESIS

Delta aminolevulinic acid (ALA)-induced protoporphyrin IX (PpIX) is a fluorescent sensitizer that permits detection and treatment of squamous cell carcinoma of the oral cavity. An exogenously induced decrease in tissue pH was evaluated for its effect in enhancing cellular uptake of ALA and facilitating its transformation into PpIX.

STUDY DESIGN

Mice grafted with HT29 colonic cancers had been given glucose and amiloride to modify the pH of tissues. Influence of pH changes has been evaluated on ALA-induced PPIX fluorescence by optic fiber spectrofluorimetry as well as on tumor growth.

METHODS

RESULTS

The pH in HT 29 tumor decreased from 7.1 to 6.67 (P < .05) after intraperitoneal injection of glucose and amiloride. The PpIX fluorescence ratios in tumor or muscle before, between, and 2 hours after glucose and amiloride injection were not higher than control ratios. Aminolevulinic acid-photodynamic therapy was more efficient on HT 29 tumor-bearing mice when the pH value was decreased with glucose and amiloride, showing a difference in the tumor growth index ratio from the 1st to 14th day of 22% between amiloride-glucose aminolevulinic acid-photodynamic therapy and aminolevulinic acid-photodynamic therapy alone (P < .05).

CONCLUSIONS

Glucose and amiloride did not change PpIX fluorescence in HT 29 tumor after intraperitoneal injection of aminolevulinic acid but enhanced aminolevulinic acid-photodynamic therapy efficacy. This was probably a result of mechanisms other than an increase in aminolevulinic acid cellular penetration and PpIX production, such as susceptibility to free radical toxicity or alteration of cellular repair enzymes under acidotic conditions. If a decrease of pH induces a more efficient photodynamic therapy as suggested by our results, an easier way to obtain this decrease than glucose and amiloride would be necessary for clinical applications.

Protoporphyrin IX fluorescence kinetics and localization after topical application of ALA pentyl ester and ALA on hairless mouse skin with UVB-induced early skin cancer

In order to improve the efficacy of 5-aminolevulinic acid-based (ALA) photodynamic therapy (PDT), different ALA derivatives are presently being investigated. ALA esters are more lipophilic and therefore may have better skin penetration properties than ALA, possibly resulting in enhanced protoporphyrin IX (PpIX) production. In previous studies it was shown that ALA pentyl ester (ALAPE) does considerably enhance the PpIX production in cells in vitro compared with ALA. We investigated the in vivo PpIX fluorescence kinetics after application of ALA and ALAPE to hairless mice with and without UVB-induced early skin cancer. ALA and ALAPE (20% wt/wt) were applied topically to the mouse skin and after 30 min, the solvent was wiped off and PpIX fluorescence was followed in time with in vivo fluorescence spectroscopy and imaging. At 6 and 12 h after the 30 min application, skin samples of visible lesions and adjacent altered skin (UVB-exposed mouse skin) and normal mouse skin were collected for fluorescence microscopy. From each sample, frozen sections were made and phase contrast images and fluorescence images were recorded. The in vivo fluorescence kinetics showed that ALAPE induced more PpIX in visible lesions and altered skin of the UVB-exposed mouse skin, but not in the normal mouse skin. In the microscopic fluorescence images, higher ALAPE-induced PpIX levels were measured in the stratum corneum, but not in the dysplastic layer of the epidermis. In deeper layers of the skin, PpIX levels were the same after ALA and ALAPE application. In conclusion, ALAPE does induce higher PpIX fluorescence levels in vivo in our early skin cancer model, but these higher PpIX levels are not located in the dysplastic layer of the epidermis.

Comparative effect of ALA derivatives on protoporphyrin IX production in human and rat skin organ cultures

Samples of human and rat skin in short-term organ culture exposed to ALA or a range of hydrophobic derivatives were examined for their effect on the accumulation of protoporphyrin IX (PpIX) measured using fluorescence spectroscopy. With the exception of carbobenzoyloxy-D-phenylalanyl-5-ALA-ethyl ester the data presented indicate that, in normal tissues, ALA derivatives generate protoporphyrin IX more slowly than ALA, suggesting that they are less rapidly taken up and/or converted to free ALA. However, the resultant depot effect may lead to the enhanced accumulation of porphyrin over long exposure periods, particularly in the case of ALA-methyl ester or ALA-hexyl ester, depending on the applied concentration and the exposed tissue. Addition of the iron chelator, CP94, greatly increased PpIX accumulation in human skin exposed to ALA, ALA-methyl ester and ALA-hexyl ester. The effect in rat skin was less marked.