Comparative in vitro percutaneous penetration of 5-aminolevulinic acid and two of its esters through excised hairless mouse skin

Daylight-PDT: everything under the sun

5-Aminolevulinic acid-based photodynamic therapy (ALA-PDT) was first implemented over three decades ago and has since been mainly part of clinical practice for the management of pre-cancerous and cancerous skin lesions. Photodynamic therapy relies on the combination of a photosensitizer, light and oxygen to cause photo-oxidative damage of cellular components. 5-Aminolevulinic acid (ALA) is a natural precursor of the heme biosynthetic pathway, which when exogenously administered leads to the accumulation of the photoactivatable protoporphyrin IX. Although, effective and providing excellent cosmetic outcomes, its use has been restricted by the burning, stinging, and prickling sensation associated with treatment, as well as cutaneous adverse reactions that may be induced. Despite intense research in the realm of drug delivery, pain moderation, and light delivery, a novel protocol design using sunlight has led to some of the best results in terms of treatment response and patient satisfaction. Daylight PDT is the protocol of choice for the management of treatment of multiple or confluent actinic keratoses (AK) skin lesions. This review aims to revisit the photophysical, physicochemical and biological characteristics of ALA-PDT, and the underlying mechanisms resulting in daylight PDT efficiency and limitations.

Topical hexylaminolevulinate and aminolevulinic acid photodynamic therapy: complete arteriole vasoconstriction occurs frequently and depends on protoporphyrin IX concentration in vessel wall

Vascular responses to photodynamic therapy (PDT) may influence the availability of oxygen during PDT and the extent of tumor destruction after PDT. However, for topical PDT vascular effects are largely unknown. Arteriole and venule diameters were measured before and after hexylaminolevulinate (HAL) and aminolevulinic acid (ALA) PDT and related to the protoporphyrin IX (PpIX) concentration in the vessel wall. A mouse skin fold chamber model and an intravital confocal microscope allowed direct imaging of the subcutaneous vessels underlying the treated area. In both HAL and ALA groups over 60% of arterioles constricted completely, while venules generally did not respond, except for two larger veins that constricted partially. Arteriole vasoconstriction strongly correlated with PpIX fluorescence intensity in the arteriole wall. Total PpIX fluorescence intensity was significantly higher for HAL than ALA for the whole area that was imaged but not for the arteriole walls. In conclusion, complete arteriole vasoconstriction occurs frequently in both HAL and ALA based topical PDT, especially when relatively high PpIX concentrations in arteriole walls are reached. Vasoconstriction will likely influence PDT effect and should be considered in studies on topical HAL and ALA-PDT. Also, our results may redefine the vasculature as a potential secondary target for topical PDT.

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.

Percutaneous absorption in diseased skin: an overview

The stratum corneum's (SC) functions include protection from external hazardous environments, prevention of water loss and regulation of body temperature. While intact skin absorption studies are abundant, studies on compromised skin permeability are less common, although products are often used to treat affected skin. We reviewed literature on percutaneous absorption through abnormal skin models. Tape stripping is used to disrupt water barrier function. Studies demonstrated that physicochemical properties influence the stripping effect: water-soluble drugs are more affected. Abrasion did not affect absorption as much. Freezing is commonly used to preserve skin. It does not seem to modify water absorption, but still increases the penetration of compounds. Comparatively, heating the skin consistently increased percutaneous absorption. Removing SC lipids may increase percutaneous absorption of drugs. Many organic solvents are employed to delipidize. Delipidization with chloroform-methanol increased hydrophilic compound permeability, but not lipophilic. Acetone pre-treatment enhanced hydrophilic compound penetration. More data is needed to determine influence on highly lipophilic compound penetration. Sodium lauryl sulfate (SLS) induces irritant dermatitis and is frequently used as a model. Studies revealed that SLS increases hydrophilic compound absorption, but not lipophilic. However, skin irritation with other chemicals increases lipophilic penetration as much as hydrophilic. Animal studies show that UV exposure increases percutaneous absorption whereas human studies do not. Human studies show increased penetration in psoriatic and atopic dermatitis skin. The data summarized here begin to characterize flux alteration associated with damaged skin. Understanding the degree of alteration requires interpretation of involved conditions and the enlarging of our database to a more complete physicochemical spectrum.

Photosensitiser delivery for photodynamic therapy. Part 1: Topical carrier platforms

BACKGROUND

Photodynamic therapy (PDT) is a medical treatment in which a combination of a photosensitising drug and visible light causes destruction of selected cells. Due to the lack of true selectivity of preformed photosensitisers for neoplastic tissue and their high molecular weights, PDT of superficial skin lesions has traditionally been mediated by topical application of the porphyrin precursor 5-aminolevulinic acid (ALA).

OBJECTIVE

This article aims to review the traditional formulation-based approaches taken to topical delivery of ALA and discusses the more innovative strategies investigated for enhancement of PDT mediated by topical application of ALA and preformed photosensitisers.

METHODS

All of the available published print and online literature in this area was reviewed. As drug delivery of agents used in PDT is still something of an emerging field, it was not necessary to go beyond literature from the last 30 years.

RESULTS/CONCLUSION

PDT of neoplastic skin lesions is currently based almost exclusively on topical application of simple semisolid dosage forms containing ALA or its methyl ester. Until expiry of patents on the current market-leading products, there is unlikely to be a great incentive to engage in design and evaluation of innovative formulations for topical PDT, especially those containing the more difficult-to-deliver preformed photosensitisers.

Size and Surface Charge Effect of 5-Aminolevulinic Acid-Containing Liposomes on Photodynamic Therapy for Cultivated Cancer Cells

5-Aminolevulinic acid (ALA)-containing liposomes having various average diameters and/or positive surface charges were prepared, and their photodynamic therapy (PDT) efficacy for murine thymic lymphoma cells, EL-4 cells, cultivated in vitro was investigated. The PDT efficacy for EL-4 cells and the accumulation of ALA-induced protoporphyrin IX (PpIX) in the cells increased with a decrease in the average diameter of liposomes. In particular, the ALA-containing liposomes smaller than 63.5 nm in diameter promoted the PDT efficacy in comparison with that of ALA alone. We also found no significant changes in PDT efficacy and PpIX accumulation with increasing positive surface charges of liposomes.

Bioavailability of aminolaevulinic acid and methylaminolaevulinate in basal cell carcinomas: a perfusion study using microdialysis in vivo

BACKGROUND

Photodynamic therapy is becoming a popular treatment for superficial nonmelanoma precancerous and cancerous lesions, showing excellent cosmetic results. Nevertheless, the reported cure rates vary and the transdermal penetration of drugs has been discussed as a limiting factor, particularly for treatment of nodular basal cell carcinoma (BCC).

OBJECTIVES

To investigate the transdermal penetration of aminolaevulinic acid (ALA) and methylaminolaevulinate (MAL) in BCC in vivo using a microdialysis technique. The different prodrugs were compared and the effect of curettage was studied.

METHODS

Twenty patients with 27 histologically verified BCCs (13 superficial, 14 nodular) were included. All lesions were located at the front of the body (head and face excluded). The first 10 patients included were treated with MAL (13 BCCs), and the following 10 patients with ALA (14 BCCs). A light curettage was performed on every second lesion (curettage, n = 13; noncurettage, n = 14). Microdialysis catheters were inserted into the tumours at tissue depths varying from 0.4 to 1.9 mm. Dialysates were collected at 15-30-min intervals for 4 h and the interstitial concentrations of MAL and ALA were determined using high-performance liquid chromatography.

RESULTS

No significant difference in interstitial drug concentration was observed between lesions treated with ALA or MAL during the 4-h measurement period. However, for the lesions with deeper catheter locations, i.e. at or below 1 mm (n = 11), drug concentrations above the detection limit were obtained in only six lesions. All but one BCC with superficial catheter location, i.e. < 1 mm (n = 16), exhibited detectable drug concentration (P = 0.026). The interstitial peak concentrations were reached within 90 min in 23 of the 27 BCCs, but were not found to be correlated with the depth of the catheters. No difference was found when comparing superficial and nodular BCCs, and the effect of curettage was found to be negligible.

CONCLUSIONS

The results imply that there is no significant difference in transdermal penetration of ALA and MAL in tumour tissue. Detectable levels of drug were not obtained in almost 50% of the lesions where catheters were situated 1-1.9 mm in the lesion. Curettage was not found to affect the interstitial concentration, indicating that penetration of drug indeed might be a problem when treating BCCs thicker than 1 mm.

Derivatives of 5-Aminolevulinic Acid for Photodynamic Therapy

Photodynamic therapy (PDT) is a clinical treatment that combines the effects of visible light irradiation with subsequent biochemical events that arise from the presence of a photosensitising drug (possessing no dark toxicity) to cause destruction of selected cells. Today, the most common agent used in dermatological PDT is 5-aminolevulinic acid (ALA). As a result of its hydrophilic character, ALA penetrates skin lesions poorly when applied topically. Its systemic bioavailability is limited and it is known to cause significant side effects when given orally or intravenously. Numerous chemical derivatives of ALA have been synthesised with the aims of either improving topical penetration or enhancing systemic bioavailability, while reducing side effects. In vitro cell culture experiments with ALA derivatives have yielded promising results. However, if ALA derivatives are to demonstrate meaningful clinical benefits, a rational approach to topical formulation design is required, along with a systematic study aimed at uncovering the true potential of ALA derivatives in photodynamic therapy. With respect to systemic ALA delivery, more study is required in the developing area of ALA-containing dendrons and dendrimers.

Sufficient PpIX production for PDT even with short contact time of topically applied 5-ALA in rabbit tongues

Although effectiveness of photodynamic therapy (PDT) after application of 5-aminolevulinic acid ointment to oral mucosal lesions has been reported, a consensus regarding recontact time of ALA applied to a lesion has been unreached. Hence, we determined the contact time of ALA required for protoporphyrin IX (PpIX) production efficient for full-blown PDT reaction. ALA ointment was topically applied to healthy rabbits' tongues for different periods and then washed out. On the surface of the tongue, 10-min contact of ALA maximized the PpIX-derived fluorescence. PpIX yield in a tissue specimen with 10-min contact of ALA reached 73% of that in a tissue specimen with 240-min contact. Histological observation showed that PpIX accumulation predominated in the basal layer, and the PDT effects were confined in the mucosal epithelium regardless of contact time. These results suggest that 5-aminolevulinic-acid-ointment-mediated PDT with short contact of ALA is potentially applicable for treating tongue epithelial lesions.

Aminolevulinic Acid-Photodynamic Therapy for Photorejuvenation

Light therapy is widely used in dermatology. An enhancement to his therapy is the use of a photosensitizing medication along with light therapy, known as photodynamic therapy (PDT). PDT is primarily used for the treatment of precancerous lesions, acne vulgaris, and nonmelonoma skin cancer, and has also been shown to improve the appearance of photodamaged skin. This article reviews the available literature for the treatment of photodamaged skin using PDT along with recent advancements in PDT for various dermatologic indications.

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%.

In vitro phototoxicity of 5-aminolevulinic acid and its methyl ester and the influence of barrier properties on their release from a bioadhesive patch

Topical administration of excess exogenous 5-aminolevulinic acid (ALA) leads to selective accumulation of the potent photosensitiser protoporphyrin IX (PpIX) in neoplastic cells, which can then be destroyed by irradiation with visible light. Due to its hydrophilicity, ALA penetrates deep lesions, such as nodular basal cell carcinomas (BCCs) poorly. As a result, more lipophilic esters of ALA have been employed to improve tissue penetration. In this study, the in vitro release of ALA and M-ALA from proprietary creams and novel patch-based systems across normal stratum corneum and a model membrane designed to mimic the abnormal stratum corneum overlying neoplastic skin lesions were investigated. Receiver compartment drug concentrations were compared with the concentrations of each drug producing high levels of PpIX production and subsequent light-induced kill in a model neoplastic cell line (LOX). LOX cells were found to be quite resistant to ALA- and M-ALA-induced phototoxicity. However, drug concentrations achieved in receiver compartments were comparable to those required to induce high levels of cell death upon irradiation in cell lines reported in the literature. Patches released significantly less drug across normal stratum corneum and significantly more across the model membrane. This is of major significance since the selectivity of PDT for neoplastic lesions will be further enhanced by the delivery system. ALA/M-ALA will only be delivered in significant amounts to the abnormal tissue. PpIX will only then accumulate in the neoplastic cells and the normal surrounding tissue will be unharmed upon irradiation.

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

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

Porphyrin Formation in Actinic Keratosis and Basal Cell Carcinoma after Topical Application of Methyl 5-Aminolevulinate

Photodynamic therapy using topical methyl 5-aminolevulinate (MAL) is a new treatment modality for basal cell carcinoma (BCC) and actinic keratosis (AK). MAL induces endogenous porphyrins, which act as photosensitizers. Pharmacokinetic studies of the porphyrin-inducing effect of MAL creams (Metvix) applied in different concentrations (16-160 mg/g) and application times are presented. Surface fluorescence measurements were used to monitor porphyrin accumulation in 18 superficial BCCs and 32 AKs. For both lesion types, the fluorescence increased during the first 13 of 28 hours of continuous MAL application. A 20-fold site-to-site variation was observed, and there were no significant MAL concentration dependencies. The selectivity between lesions and normal skin was 10-fold during the first hours and decreased throughout the application time. Fluorescence microscopy images of tissue sections from 32 nodular BCCs were analyzed to calculate the porphyrin content in tumor tissue as a function of depth. Significant correlation to MAL concentration was seen within the tumors treated for 3 hours. Increase to 18-hour MAL application enhanced the fluorescence levels in superficial tumor layers, but not in deep layers. In conclusion, application of the 160 mg/g cream for 3 hours gave advantageous porphyrin distributions for all types of lesions.

Enhanced porphyrin accumulation using dendritic derivatives of 5-aminolaevulinic acid for photodynamic therapy: An in vitro study

Intracellular porphyrin generation following administration of 5-aminolaevulinic acid has been widely used in photodynamic therapy for a range of malignant and certain non-malignant lesions. However, cellular uptake of 5-aminolaevulinic acid is limited by its hydrophilic nature and improved means of delivery are therefore being sought. Highly branched polymeric drug carriers known as dendrimers are a promising new approach to drug delivery. The aim of this study was to investigate the efficacy of dendrimers conjugated with 5-aminolaevulinic acid for porphyrin production in the transformed PAM 212 keratinocyte cell line and skin explants. Each dendritic derivative incorporated three 5-aminolaevulinic acid residues which were conjugated as esters via methyl or propyl linkers to a central tertiary carbon whose remaining terminal bore an amino, aminobenzyloxycarbonyl or nitro group. In the cell line, all compounds were more efficient at low concentrations compared to equimolar 5-aminolaevulinic acid for porphyrin production, with the most efficient incorporating the longer propyl linker. This compound was also the most lipophilic according to partition coefficient measurements. The intracellular porphyrin fluorescence levels showed good correlation with cellular phototoxicity following light exposure for all the compounds, together with minimal dark toxicity. Our findings indicate that the key factors influencing the efficacy of the dendritic derivatives are lipophilicity and steric hindrance within the dendritic structure which could restrict access to intracellular esterases for liberation of 5-aminolaevulinic acid. These findings should be taken into account in the design of larger dendrimers of 5-aminolaevulinic acid.

5-Aminolevulinic Acid Derivatives in Photomedicine: Characteristics, Application and Perspectives

The introduction of lipophilic derivatives of the naturally occurring heme precursor 5-aminolevulinic acid (5-ALA) into photomedicine has led to a true revival of this research area. 5-ALA-mediated photodynamic therapy (PDT) and fluorescence photodetection (FD) of neoplastic disease is probably one of the most selective cancer treatments currently known in oncology. To date, this method has been assessed experimentally for the treatment of various medical indications. However, the limited local bioavailability of 5-ALA has widely prevented its use in daily clinical practice. Although researchers were already aware of this drawback early during the development of 5-ALA-mediated PDT, only recently have well-established concepts in pharmaceutical science been adapted to investigate ways to overcome this drawback. Recently, two derivatives of 5-ALA, methylaminolevulinate (MAL) and hexylaminolevulinate (HAL), gained marketing authorization from the regulatory offices in Europe and Australia. MAL is marketed under the trade name Metvix for the treatment of actinic keratosis and difficult-to-treat basal cell carcinoma. HAL has recently been launched under the trade name Hexvix to improve the detection of superficial bladder cancer in Europe. This review will first present the fundamental concepts underlying the use of 5-ALA derivatives in PDT and FD from a chemical, biochemical and pharmaceutical point of view. Experimental evidences from preclinical data on the improvements and limits observed with 5-ALA derivatives will then be introduced. The state-of-the-art from clinical studies with 5-ALA esters will be discussed, with special emphasis placed on the process that led to the development of MAL in dermatology and to HAL in urology. Finally, we will discuss promising medical fields in which use of 5-ALA derivatives might potentially lead to further use of this methodology in photomedicine.

Chronic UVB exposure enhances in vitro percutaneous penetration of 5-aminulevulinic acid in hairless mouse skin

BACKGROUND AND OBJECTIVES

(Pre)cancerous skin lesions accumulate more protoporphyrin IX (PpIX) upon topical application of 5-aminolevulinic acid (ALA) than the surrounding normal skin. This might be the result of a higher percutaneous penetration of ALA into (pre)cancerous skin.

STUDY DESIGN/MATERIALS AND METHODS

ALA penetration through (1) healthy skin with intact stratum corneum, (2) healthy skin with reduced stratum corneum (i.e. tape stripped skin) and (3) diseased skin with dysplastic and thickened epidermis (chronically UVB-exposed skin) was determined in an in vitro model with hairless mouse skin.

RESULTS

More ALA had penetrated through chronically UVB-exposed skin than through normal non-exposed skin after 8 hours ALA application. The amount of ALA penetrated through chronically UVB-exposed skin was smaller than through tape stripped skin.

CONCLUSIONS

The stratum corneum barrier function is less effective in chronically UVB-exposed skin than in normal non-exposed skin, but more effective than in tape stripped skin. A higher penetration rate of ALA into (pre)cancerous lesions may be (partly) responsible for the greater accumulation of PpIX in such lesions.