Photodynamic therapy in dermatology

ARTICLE

Historical aspects

The notion of "photodynamic effect" was introduced in 1904 by H. von Tappeiner, who described the fluorescence in protozoa after the application of aniline dyes [1]. The effect turned out to be oxygen-consuming and toxic to cells which prompted its application in tissue destruction. In 1905 A. Jesionek effectively used 5% eosin in treating human skin cancer [2]. Soon after that the studies on hematoporphyrin sensitisation occurred [3], but it took nearly 50 years before Schwartz et al. found that the hematoporphyrin applied in past experiments was a mixture of different porphyrins with different physical and biological properties [4]. That led to attempts to extract a single derivative, which could provide reproducible study results. The substance obtained was given the name of hematoporphyrin derivative (HPD). Since 1960 when Lipsone et al. [5] first described the properties of HPD, two new commercial products appeared on the pharmaceutical market: Photofrin^® (porfimer sodium) and Photosan^® (polyhematoporphyrin). Both of them represent a mixture of hematoporphyrin esters and ethers of different length, but obtained by different extraction procedures. So far only Photofrin^® is registered for clinical use in the US and Europe. The HPD group represents the first generation of sensitisers. The second generation includes newer synthetic products of different groups (porphines, porphycenes, chlorins, phtalocyanines and others) created on the basis of porphyrin or porphyrin-like ring, and have a better pharmacological profile. These substances are presently at different stages of clinical and preclinical investigation. The possibility of creating antibody-conjugated photosensitisers gave rise to the third generation [6], which could combine the high efficacy of the second generation products and improved selectivity to certain tumours, thus decreasing the damage to surrounding tissues. In 1990 Kennedy et al. [8] introduced a new sensitiser prodrug, 5-aminolevulinic acid (ALA). This substance, being a natural metabolite in the porphyrin pathway, is widely used to-day in the topical PDT in dermatology. The only available commercial product of ALA is Levulan^® (20% solution), which is registered in the US for the treatment of solar keratosis. At the same time many scientists and clinicians use ex tempore prepared ALA cream.

Specific distribution of many sensitisers with affinity to neoplastic tissues was firstly noticed by Policard [7], who described red fluorescence in experimental tumours in 1924. This tumour-localising property was applied as a diagnostic procedure to define the borders of some brain and skin tumours, seeking optimal margins of resection as well as to detect early stage carcinomas in the bladder and lungs. The term photodynamic diagnosis (PDD) is disputed by some authors, meanwhile the method itself increases the practical value of photodynamic effect and is readily used by many physicians.

Light, light dosimetry and light sources

The light applied in PDT belongs to the visible part of the spectrum (400-700 nm). In practice, as it is deduced from the physical, biochemical and immunological properties, PDT has low potential to induce mutations or to be tumour promoting. This was confirmed by the fact that normal and DNA repair-deficient fibroblasts showed no difference in PDT-induced cytotoxicity [9]. On the other hand regular sun protecting creams will not help to prevent photo-damage from residual phototoxicity, which is a common problem with the first generation of PDT sensitisers.

The penetration of light into tissues is not uniform for all wavelength bands [10] (Fig. 1). The blue and the green part of the spectrum can reach a maximum of 2 mm depth, when penetrating deeper its energy dramatically decreases and cannot produce an appropriate photodynamic effect. The red part of the spectrum (> 600 nm) is characterised by a much better penetration profile which prompted many authors to use red filters with incoherent light sources. The presence of pigmentation, however, can efficiently block the penetration of even the red part of the visible spectrum (such as in melanotic malignant melanoma). In this case tissue transparency is observed only at wavelengths over 780 nm [11]. Different tissue transparency to the light of different wavelengths could explain why, when using PDT, the green light is less painful than the red light [12].

The time of exposition to a light source depends on the light energy rate (W/cm²). The last value is obtained by a direct measurement. Multiplying the energy rate by time in seconds yields the dose of light energy in Joules per square centimetre (J/cm²), which reflects the possible PDT damage to tissues. Different light sources give different rates of light energy, but in order to avoid a heating effect and decrease exposure time, most authors use energy rates of 50 to 300 mW/cm². To give an idea of a light energy rate, it can be compared to summer midday sunlight, which may reach up to 70 to 90 mW/cm² in Europe. A standardised light dosimetry for PDT does not exist yet. Doses of light energy applied in PDT are commonly within 60-200 J/cm², though doses may vary from 25 to 500 J/cm² depending on indications, tissues and light sources.

With regard to the therapeutic results reported so far there is no difference between coherent and incoherent light sources [46]. Many authors use incoherent ones, such as halogen, xenon, tungsten and fluorescent lamps because of their low price and easy exploitation. Even the light of a common slide projector can give good clinical results [13], however lamps specifically designed for PDT are becoming more popular. Among those are PDT1200 (Waldmann, Germany) and Curelight (Photocure, Norway) with red filtered light, Saalmann PDT green light lamp (Germany) and BLU-U (DUSA, USA) ­ emitting blue light. The main argument against incoherent light sources is that along with absorbed wavebands they emit wavelengths which are beyond the absorption spectrum of sensitisers.

Each photosensitiser has its proper wavelengths of absorption, usually several (Fig. 2), and a wavelength of emission, which results in typical red photodynamic fluorescence. The highest peak of light absorption (and sensitiser activation), the so-called Soret band, is usually located in the blue and ultraviolet part, which does not penetrate deep into tissues. This made clinicians aim at the red absorption peak. Most effectively this can be solved with laser light sources. Copper and gold vapour lasers (CVL/578 nm, GVL/628 nm) represent the older laser generation which are becoming less common due to their high price, expensive exploitation and fixed wavelength which may fit only one sensitiser. Dye pumped tuneable lasers (argon-dye laser, neodymium:YAG-dye laser and others) can be used with several photosensitisers which makes their application more practical and flexible. Unfortunately, these systems are still rather immobile and expensive. The most recent decision in PDT light sources is portable and tuneable diode lasers (i.e. gallium-aliuminium-arsenid laser) which are created with respect to the absorption peaks of newer sensitisers.

Photosensitisers (Table I)

An ideal PDT photosensitiser should meet several criteria [25]: (1) chemical purity; (2) tumour selectivity; (3) fast accumulation in target tissues and rapid clearance; (4) activation at wavelengths which penetrate deep; (5) high potential of light activated tissue damage and (6) no dark toxicity. Unfortunately the majority of PDT sensitisers only partially meet these criteria.

HPD group

Porfimer sodium (Photofrin^®) is the only systemic photosensitiser with approved indications. Among these are esophageal and endobroncheal cancer [14]. The medication is applied via intravenous injection of 2 mg/kg with a subsequent irradiation after 2 days with monochromatic laser light (630 ± 3 nm) at the dose of 200-300 J/cm². The light treatment can be repeated 4-5 days after the initial porfimer injection, if necessary. The need to wait for 2 days after injection creates a certain problem for patients who must avoid light while being treated by porfimer-PDT. The residual photosensitivity usually lasts for 30 days after injection, though certain photoreactions were described after as long as 9 weeks [15]. This is the major problem with HPD group sensitisers. Other side effects include nausea, vomiting, shivering and hypotension. Another HPD medication is polyhematoporphyrin (Photosan^®). Both products share the same etherised hematoporphyrin monomer (Fig. 3) but differ in the concentration of dimeric and monomeric molecules which are almost absent in Photosan^® [16]. These molecules have the strongest affinity to skin [17], and thus could be responsible for long lasting skin photosensitivity.

5-aminolevulinic acid

ALA is a natural precursor of a potent endogenous photosensitiser ­ protoporphyrin IX (PpIX). The biosynthesis of hem has two rate-limiting steps. The first one is the synthesis of ALA from glycine and succinyl-CoA. The second one is the incorporation of iron into PpIX by ferochelatase leading to the formation of hem, which is not photochemically active. Bypassing the first rate-limiting step results in accumulation of PpIX in tissues. This event can be noticed as red fluorescence in Wood's light. Tissues with damaged epidermal barrier as well as those with an increased metabolism rate, accumulate PpIX faster than healthy tissues. This is called tumour selectivity and is applied for diagnostic purposes, localising size and borders of certain tumours. ALA can be applied topically and systemically as well. Topical 20% ALA in o/w emulsion is usually kept for several hours under occlusion. The duration of occlusive dressing may vary depending on how deep the substance must penetrate. Due to its hydrophilic properties, ALA penetrates the intact epidermal barrier poorly, that is why after 4 hrs only skin appendages demonstrate the fluorescence typical for PpIX. To reach the deep dermis ALA must be kept under occlusion for 12 hrs [18]. In the case of damaged epidermis or in superficial lesions, 3 to 6 hrs should be sufficient. Since fluence energy diminishes as light passes to the deeper layers of the skin, it may be necessary to use higher doses of light (over 100 J/cm²) for dermal lesions. Meanwhile it has been shown that in epidermal lesions, such as actinic keratosis, a wide range of doses (10 to 150 J/cm²) resulted in similar clinical responses [19]. The absorption peaks of PpIX are 412, 506, 532, 580 and 635 nm, meaning that in superficial lesions full-spectrum light can be effectively used [13]. In case of monochromatic light sources it was shown that, on the contrary to Photofrin^®, which is best activated at 630 nm wavelength light, PpIX induced sensitisation gives a stronger reaction with 635 nm light [10, 20]. To improve ALA biological availability, ALA has been esterified with various alcohols, such as methanol and hexanol. Such ALA esters being more lipophilic than ALA itself, are obviously deesterified in tissues by esterases, resulting in a higher accumulation of PpIX in cells and a better PDT response [24]. Another approach to improve treatment outcomes in the case of thick cutaneous lesions is the combination of ALA-PDT with surgical debulking [22].

In the case of internal malignancies ALA can be used systemically. Its usual dose is 30-60 mg/kg with subsequent irradiation in 4 to 6 hrs [21]. Using higher doses (60 mg/kg) seems to increase tumour selectivity of PpIX concentration to 5:1 in certain tumours compared to surrounding normal tissues [23]. ALA plasma concentration reaches its peak at 60 min after oral administration, with a half-life of 50 min. Tissue photo-sensitisation returns to normal within 24 hrs. Although some patients may suffer from mild nausea or transient liver function abnormalities, it appears that systemic administration of ALA at doses not exceeding 60 mg/kg (oral) or 30 mg/kg (intravenous) does not result in any neurotoxic effect or other severe toxicity reactions [21].

TPPS₄ (Tetrasodium-TetraPhenylPorphineSulfonate). A synthetic pophyrine with high potency of photosensitisation. Unfortunately early reports of its neurotoxicity [26] prompted physicians to keep on the safe side, not administering it systemically. Even further investigations ascribing neurotoxicity to imperfect purification procedures [27], could not fully rehabilitate the substance. TPPS₄ is activated with the 630 nm light. Promising results were obtained after topical (2% solution) [28] or intralesional (0.15-0.3 mg in 0.2 ml of saline) [29] administration of the substance and same-day irradiation. This avoided possible side effects due to systemic toxicity and made the treatment course shorter.

Chlorins

m-THPC (meta-Tetrahydroxyphenylchlorin, Foscan™). A sensitiser with 652 nm absorption peak. The time between intravenous injection and irradiation is 3 to 4 days [35]. Indications under research are skin, head and neck cancers.

SnET₂ (Tin ethyl etiopurpurin, Purlytin™). The excitation wavelength is 660 nm. Light can be applied after 24 hrs of intravenous injection (0.8-1.6 mg/kg) [37]. Increased skin photosensitivity may last for 1 month and longer [34]. The substance is being investigated for application in many different fields including oncology, dermatology, ophthalmology and cardiology [38]. Topical formulation for psoriasis is still at the preclinical phase.

NPe₆ (N-Aspartyl-chlorin e6). This substance absorbs light of 664 nm, rapidly accumulates in tissues, so light can be applied after 4 hrs. The intravenous injection of 0.5-3.5 mg/kg results in a stronger response at higher doses and more selective tumour destruction at lower ones [36]. The medication has a rather safe profile and short-lasting skin photosensitivity. Fields of research include cutaneous and subcutaneous cancer as well as ophthalmological indications.

BPD-MA (benzoporphyrin derivative-monoacid ring A, verteporpfin, Visudyne^TM) is a lipophylic compound with strong absorbance of 690 nm light [33]. Sufficient tumour concentration is achieved within 30 to 150 min after intravenous injection at doses 0.2-0.5 mg/kg. Due to rapid clearance from tissues, skin photosensitivity lasts only a few days after administration [34]. Clinical research with verteporfin-PDT is being performed in several directions, among which are age-related macular degeneration, skin cancer, psoriasis, arterial restenosis, rheumatoid arthritis and autoimmune disorders [14].

Phthalocyanines

Chloro-aluminum sulfonated phthalocyanine (CASPc) [30] and silicon-based phthalocyanines (Pc4, Pc10, Pc12 and Pc18) [31, 32] is a new group of synthetic photosensitisers which are still under research. These substances are activated by 670-700 nm light which penetrates deeper into tissues than 630 nm light; they show low dark toxicity, fast accumulation in tumours and high tumour selectivity. At the same time phthalocyanines have a low affinity to skin and are rapidly eliminated from tissues [73-75]. Light can be applied within several minutes (in ophthalmology) up to 3 hrs (in oncology).

Texaphyrins

A new group of cyclic photosensitisers based on the so-called expanded porphyrin. The structural modification to the molecule enabled the introduction of large metal ions in the centre of it and in this way the significant modification of the photochemical properties of the substance. This also led to increased stability of the sensitiser itself and shifted its absorption peak further to the red part of the spectrum (720-760 nm) providing even deeper penetration into tissues. Two substances: motexafin lutetium (Lutrin^®, Antrin^®) and motexafin gadolinium (Xcytrin^®) are still under clinical research. The first one intended for PDT, the second, for magnetic resonance imaging and radiosensitisation [39, 40]. Motexafin lutetium is a substance with a broad peak of absorption between 720 and 760 nm (centred at 732 nm). Its affinity to malignant lesions and atheromatous plaques is very strong [41], which minimises PDT damage to normal tissues. The medication is administered systemically (0.6-7.2 mg/kg) and allows early irradiation after 2 to 4 hrs [34]. One of its most important features is the absence of cutaneous phototoxicity. Promising results have been achieved treating atherosclerosis, subcutaneous metastases of malignant melanoma, Kaposi's sarcoma and epithelial skin cancer [41, 70-72].

Besides the above mentioned groups and products, a lot of new substances have been created, among them are porphycens, antracens, purpurins, hypericin, hypocrellin and chlorophyll derivatives [42]. The clinical importance of the majority of them will be defined in the future.

Mechanism of photodynamic effect

The development of PDT-induced tissue damage is not fully understood. The theory describing the interaction of photosensitiser, tissue and light includes 2 main phases [43]:

1) The formation of triplets of excited sensitiser molecule. Triplet stability and high quantum yield reflect the potential for tumour ablation.

2) Returning of a sensitiser to its ground state, transferring energy to the formation of singlet oxygen (type II reaction) or initiating free radical chain reactions with superoxide and hydrogen peroxide ions as well as hydroxyl radicals (type I reaction). This phase initiates the damaging effect of PDT which is realised via several pathways [44, 45]:

i. Cell necrosis and apoptosis. Lipid peroxydation and protein crosslinking affect cell membrane enzymes and transmembranous transport. This induces further accumulation of sensitiser and cell ballooning, while alteration of mitochondial membranes and related enzymes blocks cell respiration. At the same time this triggers the expression of apoptosis mediators and starts apoptotic cell changes.

ii. Microcirculation arrest. Damage of endothelial cells promotes thrombus formation and vascular stasis which also contributes to tumour ablation.

iii. Inflammation in the exposed tissue with accumulation of macrophages and myeloid cells follows the direct damage to cells and impaired circulation.

iv. Induction of host immune response. Release of tumour antigens resulting from inflammation stimulates host immune response even to poorly immunogenic processes.

These theories reflect only general events following PDT, while the detailed mechanism on different steps still needs further research.

Dermatological applications of PDT

Reports about the application of PDT in dermatology cover common skin cancer types and certain benign skin disorders (Table II), but most clinical experience concerns the treatment of solar keratosis (SK) and basal cell carcinoma. Treating SK with topical ALA-PDT [13] and full-spectrum light, complete response was obtained in 93% in head and neck lesions and 51% in forearms and hands at the dose of 50 J/cm². Similar results are presented by other authors [19, 21]. The effect of PDT treating BCC [21] depends on the BCC type. The best outcomes are observed in the superficial type of tumour, with the complete response rate of 87-92% after ALA-PDT and 18 months follow-up. Similar results (86%) are obtained after HPD-PDT. Nodular type of BCC responds less well, though results can be improved adding EDTA/DMSO to the vehicle (ethylendiamine tetraacetic acid and dimethylsulfoxide). These substances increase the penetration of ALA in tissues, inducing higher concentra-tion of PpIX and resulting in complete response in 55-91% vs 34-67% without EDTA/DMSO. Cutaneous SCC is not as sensitive as BCC to PDT with ALA or HPD. Average rate of complete response is around 70% [21]. This could be explained not just by tumour insensitivity to PDT but rather by insufficient concentration of sensitiser in the target tissue and by poor penetration of light. In this regard new compounds such as BPD-MA, SnET₂ and NPe₆ could give promising results since they show good tumour selectivity and are activated by deeper penetrating wavelengths (660-690 nm).

Comparing other treatment modalities with PDT for superficial lesions such as Bowen's disease, ALA-PDT was shown to be superior to cryotherapy in giving a higher rate of recovery after one treatment and resulting in no side effects such as ulceration and infection, observed after cryotherapy [50]. PDT can be effectively combined with radiotherapy [55] and surgery [56] in the management of extramammary Paget's disease. The beneficial effect could be achieved not only through tissue destruction, but also in the decreasing adhesiveness and metastatic potential of cancer cells [57]. Certain synergism was observed applying PDT with some antineoplastic drugs such as mitomycin C [59], adriamycin and methotrexate, while other medications (cyclophosphamide, thiotepa, vincristine, and 5-fluorouracil) did not result in significant increases of tumour response [60]. In the treatment of cutaneous malignant melanoma (MM), PDT with currently available photosensitisers cannot be accepted as an alternative treatment modality [34]. Despite good in vitro response of MM cells to PDT, in vivo results were short-lasting or unsuccessful. Palliative application of systemic PDT showed good results in Kaposi's sarcoma [54] and skin metastases of some tumours [29]. It could also provide a good alternative in the treatment of capillary hemangioma and produce effective vascular occlusion [64]. The absence of heat damage to the epidermis which is observed in conventional laser treatment could be beneficial with regard to the cosmetic outcomes.

Research of PDT applications in non-oncological skin disorders primarily concerns psoriasis [61]. Since no uniform protocol was applied in PDT of psoriasis, both systemic and topical sensitisation resulted in various responses [62, 63]. In contrast to oncological disorders, PDT of psoriasis requires multiple treatments and lower light doses, because the PDT effect is targeted rather at cell suppression, than at ablation. It was shown that in chronic plaque-stage psoriasis, PDT resulted in decreased production of interleukin 1beta and 6 as well as tumour necrosis factor alpha in peripheral mononuclear cells, which is similar to PUVA induced changes and suggests some common anti-inflammatory mechanisms of action [34]. Certain positive results were observed with PDT as an antiviral modality [65, 66]. Disorders due to herpes simplex and papilloma viruses could be a potential area of PDT application. Even in those cases where complete elimination of a virus is impossible, PDT can provide better disease control [67]. Among other possible dermatological indications of PDT one could name disorders of cutaneous adnexa, such as alopecia areata, hirsutism and acne vulgaris [34]. Unfortunately research in this field is far from sufficient as yet to make conclusions as to the usefulness of PDT.

Applications of PDT in other specialities (Table II)

PDT has been applied in practically all common malignant and premalignant diseases of internal organs and CNS. Early cancer detection and intraoperative photodynamic diagnosis are helpful modalities and improve treatment outcomes. The new modification of PDT, the sensitisation to radiotherapy, emerged with the appearance of texaphyrins. Even if complete ablation of tumours sometimes receives sceptical comments, the palliative value of PDT is not argued. Among other progressive applications is PDT-mediated occlusion of ocular choroidal neovascularization in such disorders as macular degeneration and pathological myopia. The affinity of sensitisers to atherosclerotic plaques stimulated research in PDT-angioplasty and the prevention of arterial restenosis. Certain research has been performed in PDT-mediated synovial ablation suggesting a possible application in rheumatoid arthritis. The antiviral potential of PDT has been applied in virus inactivation in blood samples, which creates another safety barrier against HIV and hepatitis B viruses.

Adverse treatment effects

The most serious adverse effect of conventional PDT sensitisers is prolonged skin photoreactivity. Patients who received porfimer sodium must avoid exposure of skin and eyes to direct sunlight or bright indoor light (from examination lamps, including dental lamps, operating room lamps, unshaded light bulbs at close proximity, etc.) for 30 days [14]. However patients should not stay in a darkened room. Exposure of the skin to ambient indoor light could be beneficial, because the remaining drug will be inactivated gradually and safely through a photobleaching reaction. Patients are recommended to test their light reactivity by exposing a small area of skin to sunlight for 10 min, so this could help to decide if they are still photosensitive. If no photosensitivity reaction (erythema, edema, blistering) occurs within 24 hrs, the patient can gradually resume normal outdoor activities. If a reaction occurs with the limited skin test, the patient should continue precautions for another 2 weeks before retesting. The tissue around the eyes may be more sensitive, therefore, it is not recommended that the face be used for testing. Patients must be informed that ultraviolet sunscreens are of no value in protecting against photosensitivity reactions because photoactivation is caused by visible light.

Topically applied substances as well as newer products are free from this disadvantage. Another commonly observed undesirable reaction is pain during irradiation. Deeper penetration of light and sensitiser seems to produce a stronger pain sensation, though, being a subjective feeling, it remains individual in different patients. To manage pain during PDT, different modalities are applied, such as cooling with liquid nitrogen or using a fan, which also produce a both cooling and distracting effect. Meanwhile hyperthermia could be beneficial for PDT tumour ablation [69] since it increases tissue oxygenation and potentiates PDT effect. In severe cases of pain analgesia may be recommended. General undesirable effects after systemic administration of sensitisers include dizziness, diarrhea, headache and nausea, which are usually transient and do not need supplementary treatment.

Associated benefits of PDT

The good cosmetic results which are obtained due to selective tissue destruction and mobilisation of organism proper immune response is one of the main PDT advantages. From the pharmacological point of view, sensitisers show low toxicity and almost no interaction with other medications, making PDT a safe treatment modality. PDT is easy to apply in curved skin areas where surgery is difficult to carry out. It can be also administered for seriously ill patients in whom surgery with general anesthesia is not recommended. Treatment sessions can be easily performed and repeated if necessary on an out-patient basis which saves money and is convenient for patients. Using polychromatic light sources makes the PDT procedure fairly inexpensive.

Future perspectives

So far PDT is considered a new approach. The diversity of recently created photosensitisers shows its tendency to rapid development. At the same time the mechanism of PDT-mediated tissue response demands further research. The penetration of topically applied substances (such as ALA) can be improved with better vehicles, liposomal formulations, esterified derivatives and combination with DMSO/EDTA. Protocols for light delivery and doses of sensitisers still lack uniformity, which results in a broad range of cure rates. A comparative study could reveal the optimal dose-time combination and improve clinical outcomes. Better understanding of light distribution in tissues could help to create a standard for light presentation. PDT of bigger skin areas needs light sources with uniform intensity over the whole irradiated area. Unfortunately uneven intensity of the light beam emitted by the available PDT lamps makes it difficult to irradiate bigger skin areas and to dose light to convex and concave parts of the body. In superficial epidermal lesions full-spectrum light could be even more effective than filtered light and spare deeper parts of dermis from unnecessary damage. Bath-PDT may also be a possible substitute for bath-PUVA in certain conditions. The combination of PDT with surgery and chemotherapy could improve cure rates and cosmetic results. All the above-mentioned shows PDT as a promising treatment modality with a good potential for future development.

Article accepted on 4/9/00

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