OPEN ACCESS
`
`
`
`molecules
`
`Molecules 2011, 16, 4140-4164; doi:10.3390/molecules16054140
` 
`
`ISSN 1420-3049
`www.mdpi.com/journal/molecules
`
`Review
`Aminolevulinic Acid (ALA) as a Prodrug in Photodynamic
`Therapy of Cancer
`
`Małgorzata Wachowska 1, Angelika Muchowicz 1, Małgorzata Firczuk 1,
`Magdalena Gabrysiak 1, Magdalena Winiarska 1, Małgorzata Wańczyk 1, Kamil Bojarczuk 1 and
`Jakub Golab 1,2,*
`
`1 Department of Immunology, Centre of Biostructure Research, Medical University of Warsaw,
`Banacha 1A F Building, 02-097 Warsaw, Poland
`2 Department III, Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw,
`Poland
`
`* Author to whom correspondence should be addressed; E-Mail: jakub.golab@wum.edu.pl;
`Tel. +48-22-5992199; Fax: +48-22-5992194.
`
`Received: 3 February 2011 / Accepted: 3 May 2011 / Published: 19 May 2011
`
`
`Abstract: Aminolevulinic acid (ALA) is an endogenous metabolite normally formed in the
`mitochondria from succinyl-CoA and glycine. Conjugation of eight ALA molecules yields
`protoporphyrin IX (PpIX) and finally leads to formation of heme. Conversion of PpIX to
`its downstream substrates requires the activity of a rate-limiting enzyme ferrochelatase.
`When ALA is administered externally the abundantly produced PpIX cannot be quickly
`converted to its final product - heme by ferrochelatase and therefore accumulates within
`cells. Since PpIX is a potent photosensitizer this metabolic pathway can be exploited in
`photodynamic therapy (PDT). This is an already approved therapeutic strategy making
`ALA one of the most successful prodrugs used in cancer treatment.
`
`Key words: 5-aminolevulinic acid; photodynamic therapy; cancer; laser; singlet oxygen
`
`
`
`1. Introduction
`
`
`Photodynamic therapy (PDT) is a minimally invasive therapeutic modality used in the management
`of various cancerous and pre-malignant diseases. It involves the systemic administration of a non-toxic
`photosensitizing (PS) drug, which accumulates in host and tumor cells, and subsequent illumination of
`
`

`

`OPEN ACCESS
`
`
`
`molecules
`
`Molecules 2011, 16, 4140-4164; doi:10.3390/molecules16054140
` 
`
`ISSN 1420-3049
`www.mdpi.com/journal/molecules
`
`Review
`Aminolevulinic Acid (ALA) as a Prodrug in Photodynamic
`Therapy of Cancer
`
`Małgorzata Wachowska 1, Angelika Muchowicz 1, Małgorzata Firczuk 1,
`Magdalena Gabrysiak 1, Magdalena Winiarska 1, Małgorzata Wańczyk 1, Kamil Bojarczuk 1 and
`Jakub Golab 1,2,*
`
`1 Department of Immunology, Centre of Biostructure Research, Medical University of Warsaw,
`Banacha 1A F Building, 02-097 Warsaw, Poland
`2 Department III, Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw,
`Poland
`
`* Author to whom correspondence should be addressed; E-Mail: jakub.golab@wum.edu.pl;
`Tel. +48-22-5992199; Fax: +48-22-5992194.
`
`Received: 3 February 2011 / Accepted: 3 May 2011 / Published: 19 May 2011
`
`
`Abstract: Aminolevulinic acid (ALA) is an endogenous metabolite normally formed in the
`mitochondria from succinyl-CoA and glycine. Conjugation of eight ALA molecules yields
`protoporphyrin IX (PpIX) and finally leads to formation of heme. Conversion of PpIX to
`its downstream substrates requires the activity of a rate-limiting enzyme ferrochelatase.
`When ALA is administered externally the abundantly produced PpIX cannot be quickly
`converted to its final product - heme by ferrochelatase and therefore accumulates within
`cells. Since PpIX is a potent photosensitizer this metabolic pathway can be exploited in
`photodynamic therapy (PDT). This is an already approved therapeutic strategy making
`ALA one of the most successful prodrugs used in cancer treatment.
`
`Key words: 5-aminolevulinic acid; photodynamic therapy; cancer; laser; singlet oxygen
`
`
`
`1. Introduction
`
`
`Photodynamic therapy (PDT) is a minimally invasive therapeutic modality used in the management
`of various cancerous and pre-malignant diseases. It involves the systemic administration of a non-toxic
`photosensitizing (PS) drug, which accumulates in host and tumor cells, and subsequent illumination of
`
`

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`Molecules 2011, 16
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`4141
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`the tumor site with visible light, corresponding to the appropriate photosensitizer absorption
`wavelength (Figure 1).
`
`Figure 1. Overview of PDT. Following photosensitizer administration it undergoes
`systemic distribution and selectively accumulates in the tumor. Illumination activates the
`photosensitizer and in the presence of molecular oxygen triggers a photochemical reaction
`that culminates in the production of 1O2.
`
`
`
`
`The excited photosensitizer contributes to the generation of singlet oxygen and other reactive
`oxygen species (Figure 2), which results in the oxidative damage to intracellular macromolecules and
`consequently leads to cell death. The mode of PDT-induced cell death is usually a mixture of apoptosis,
`necrosis and autophagy, with the dominance of a particular process depending on the PS (mainly its
`subcellular localization) as well as light fluence. It is generally agreed that apart from the direct
`cellular cytotoxicity, two other important factors contribute to the overall PDT effect: the vascular
`shutdown and local inflammatory reaction [1-4]. One of the major advantages of PDT over other
`anticancer treatment modalities is its high degree of selectivity. This is accomplished via the
`combination of two inactive components, visible light and a photosensitizing drug, which applied
`together in the presence of oxygen lead to generation of cytotoxic intermediates that effectively kill
`tumor cells [5].
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`Molecules 2011, 16
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`Figure 2. Types of oxidative reactions during PDT. Light with sufficient energy and
`wavelength matching absorption spectrum of the photosensitizer (PS) can activate a
`photochemical reaction that leads to formation of activated PS molecule (denoted by
`asterisk). Activated PS can lose its energy by emitting of visible light (fosforescence or
`fluorescence). Alternatively it may generate singlet oxygen in a type II reaction or free
`radicals in a type I reaction.
`
`As the PS alone is non-toxic and ineffective, to some extent it can be considered as a prodrug.
`However, additional selectivity of PDT may be achieved by the administration of a PS precursor. The
`only clinically approved example of such a compound is δ-aminolevulinic acid (ALA), a precursor of
`the natural photosensitizer phrotoporphyrin IX (PpIX). In contrast to exogenously administered PSs
`such as Photofrin, the photodynamically inactive, non-selective and non-toxic compound, ALA, is
`intracellularly metabolized to the photodynamically active PpIX. Subsequent illumination of the tumor
`site with red light activates PpIX, triggers the oxidative damage and induces cytotoxicity [6].
`
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`ALA is a naturally occurring compound, the early intermediate in the heme biosynthesis pathway.
`For therapeutic purposes ALA is administered topically or systemically and penetrates non-selectively
`into all cells, where it is metabolized to an active sensitizer PpIX. The bioactivation of ALA utilizes
`the enzyme machinery of the heme biosynthesis pathway. Although nearly all human cell types
`express the enzymes involved in the heme synthesis, a distinct activity of the enzymes in tumor as
`compared with normal cells leads to a higher PpIX accumulation within transformed cells [7].
`For the last two decades a substantial amount of research has been focused on the elucidation of the
`mechanism of ALA-PDT and the improvement of its therapeutic activity. ALA is a polar molecule and
`in physiological pH occurs mainly as a charged zwitterion, which accounts for its low lipid solubility
`and reduced bioavailability. Further modifications of ALA aimed at improving its cellular permeability,
`increased stability in physiologic pH, increased selectivity and limitation of side effects, are important
`challenges in order to extend the clinical use of ALA-PDT. Since the very first topical application of
`ALA in the treatment of basal cell carcinoma in 1990 [8], the clinical use of ALA-PDT is still growing.
`Nowadays, PDT with ALA and its esters is an approved treatment of several malignant and
`premalignant conditions such as actinic keratosis, basal cell carcinoma, Bowen’s disease, bladder
`cancer and others. This review presents the use of ALA and its derivatives as prodrugs in PDT and
`summarizes the preclinical and clinical results of the treatment.
`
`2. Metabolism of ALA
`
`2.1. Heme Biosynthesis
`
`The synthesis of ALA is the first and rate-limiting step in the biosynthesis of heme [9-11]. ALA is
`normally synthesized in mitochondria in the condensation reaction between glycine and succinyl-CoA
`(Figure 3). The reaction is catalyzed by ALA synthase (ALAS) and requires pyridoxal-5-phosphate
`(PLP) as a cofactor. In mammals, two isoforms of ALA synthase have been identified: ALAS1, which
`is a housekeeping enzyme and ALAS2, which is expressed only in erythroid precursors [12].
`After being synthesized ALA reaches cytosol, where it undergoes a condensation reaction. The
`
`reaction occurs between two ALA molecules with the aid of zinc-dependent enzyme – aminolevulinate
`
`dehydratase (ALAD) – and leads to the formation of porphobilinogen (PBG). ALAD, also known as
`porphobilinogen synthase (PBGS) comprises four homodimers, each of them having one active
`site [13]. Two molecules of ALA bind non-symmetrically to the active site, finally leading to the
`synthesis of PBG [14].
`The next step in heme biosynthesis involves combining four molecules of PBG to form an unstable
`tetrapyrolle - hydroxymethylbilane (HMB). The reaction is catalyzed by porphobilinogen deaminase
`(PBDG), an enzyme containing dipyrromethane in its active site. Dipyrromethane is a co-factor
`covalently bound to the enzyme and consists of two PBG molecules. Four additional molecules of
`PBG attach to dipyrromethane leading to the formation of hexapyrolle. Afterwards, in the hydrolytic
`reaction, cleavage of the distal tetrapyrolle occurs, resulting in the release of HMB [15].
`Hydroxymethylbilane can then enter two pathways. The first one uses uroporphyrinogen III synthase
`(URO3S) to close the HMB macrocycle leading to conversion of tetrapyrolle to uroporphyrinogen III.
`Alternatively, HMB can undergo spontaneous cyclization, which leads to the formation of
`uroporphyrinogen I [16].
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`Molecules 2011, 16
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`Figure 3. Heme biosynthesis. The graphic presents the most important steps in the heme
`biosynthetic pathway.
`
`
`
`
`Uroporphyrinogen decarboxylase (UROD) catalyzes decarboxylation of all four acetate side chains
`of uroporphyrinogen III to methyl groups [17]. The product of this reaction – coproporphyrinogen III
`is transported to the intermembrane space of mitochondria probably by peripheral-type benzodiazepine
`receptors (PBR) [18,19]. Coproporphyrinogen oxidase (CPO) then catalyzes the conversion of
`coporoporphyrinogen III to protoporporphyrinogen IX with the release of H2O2 and CO2. The reaction
`provides vinyl groups by oxidative decarboxylation of propionate groups on pyrolle rings A and
`B [20]. There are two forms of CPO: oxygen-dependent found in aerobic organisms, and
`oxygen-independent, in anaerobic and facultative bacteria [21]. In mammals, oxygen-dependent CPO
`is originally synthesized in cytosol with unusually long N-terminal sequence targeting it to the
`mitochondria [22]. Finally, CPO is situated in the mitochondrial intermembrane space with a fraction
`loosely bound to the inner membrane [23].
`The next intermediate in heme biosynthesis, protoporphyrin IX (PpIX), is synthesized in the
`mitochondria and requires FAD-containing protoporphyrinogen oxidase (PPO) [24,25]. PPO catalyzes
`conversion of protoporphyrinogen IX to PpIX in the six-electron oxidation. The reaction requires
`oxygen as a terminal electron acceptor and leads to removal of six hydrogens from protoporphyrinogen
`IX [7]. Ferrochelatase (FECH), another rate-limiting enzyme, is responsible for insertion of Fe2+ into
`PpIX. The reaction occurs on the inner surface of the inner mitochondrial membrane [26]. This stage
`leads to the formation of the final product – heme – and completes the heme biosynthetic pathway.
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`Molecules 2011, 16
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`2.2. Heme Degradation
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`The enzyme responsible for heme degradation is heme oxygenase (HO). HO exists in two izoforms,
`significantly different in their regulatory mechanism: HO-1 is an inducible form whereas HO-2 is
`expressed constitutively [27,28]. They both catalyze the same reaction, which leads to the production
`of biliverdin, carbon monoxide (CO) and iron [29,30]. The reaction involves formation of HO-heme
`complex and reduction of ferric heme-iron to Fe2+ by NADPH:cytochrome p-450 reductase [31].
`Afterwards, three oxygenation cycles occur leading to the production of ferribiliverdin IXa complex
`[32,33]. The iron of the complex undergoes reduction, which results in the release of free iron and
`biliverdin [31] Then, biliverdin can be further reduced to bilirubin by NADPH-dependent biliverdin
`reductase [34].
`
`2.3. Effect of Exogenous ALA Administration
`
`PpIX is a strong photosensitizer, which assembles in mitochondria of tumor cells leading to their
`damage after light exposure. Although all enzymes involved in the heme biosynthetic pathway are
`necessary, only two of them: ALAS1 and ferrochelatase are considered to be rate-limiting. Normally,
`the activity of ALAS1, is regulated by heme through the negative feedback mechanism. Heme binds to
`the heme-regulatory motif (HRM) in mitochondrial targeting sequence of ALAS and therefore
`prevents transport of ALAS1 precursor to mitochondria [35]. Moreover, there is evidence that heme
`not only regulates ALAS1 mitochondrial import but also attenuates its transcription [36,37]. Normally,
`the feedback mechanism leads to the production of PpIX in such amounts that can be efficiently
`converted to heme by ferrochelatase. Exogenous administration of ALA bypasses the natural
`regulation that heme exerts on ALA synthesis, which leads to increased production of PpIX [7]. The
`efficacy of ferrochelatase is then too low to convert excessively produced PpIX to heme, which results
`in the accumulation of PpIX within cells. About 4-6 hours after administration of ALA, when PpIX is
`already synthesized, target cells are exposed to light, which leads to excitation of the photosensitizer
`and formation of 1O2 that exerts cytotoxic effects.
`
`2.4. Selectivity of PpIX Accumulation in Tumor Tissue in Response to ALA Administration
`
`PpIX has been found to preferentially accumulate in tumor as compared with normal cells. This
`phenomenon can be explained by differences in heme biosynthetic pathway between non-malignant
`and malignant cells. It has been shown that decreased activity of ferrochelatase [38-41] and limited
`availability of iron [42] in tumor cells contribute to increased PpIX accumulation. Enhanced activity of
`enzymes leading to production of PpIX, such as ALAD [43], UROD [43], or PBGD [38,43-44] has
`also been observed in tumor cells.
`
`2.5. Modifications of Heme Biosynthetic Pathway and Its Influence on ALA-PDT
`
`Many studies have been performed in order to better understand the significance of individual parts
`of heme biosynthesis for ALA-PDT. The role of ALAD has been demonstrated by Feuerstein et al.
`who showed that silencing this enzyme with shRNA decreased synthesis of protoporphyrin IX in K562
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`erythroleukemic cells and resulted in impaired PDT outcome [45]. Similar effects have been presented
`for inhibition of PBGD with Pb2+ [46]. Furthermore, it has been revealed that pretreatment of LNCaP
`tumor cells with methotrexate significantly improved ALA-PDT efficacy. This effect was related to
`increased synthesis of coproporphyrinogen oxidase, which stimulated the accumulation of PpIX [47].
`Moreover, studies in DBA/2 mice pretreated with PPO inhibitor revealed increased accumulation of
`cytosolic PpIX which potentiated the effect of ALA-PDT [48].
`A significant amount of research has focused on the influence of ferrochelatase activity on the
`efficacy of ALA-PDT. Improved PDT outcome has been shown both after targeted knockdown of
`ferrochelatase by siRNA [49] and after pretreatment of tumor cells with ferrochelatase inhibitors [50].
`Enhancement of PDT effects has been also observed after removal of iron with different iron-chelating
`substances such as desferrioxamine [51] or CP94 [52].
`All these data suggest that efficacy of ALA-PDT is mainly regulated by amount of protoporphyrin
`IX. Therefore, attempts directed either towards its increased production or slower conversion to heme
`seem to be rational approaches to improve the outcome of the therapy.
`
`3. ALA Pharmacokinetics
`
`Although a detailed summary of ALA pharmacokinetics is beyond the scope of this review, several
`issues are briefly outlined here. ALA is usually administrated intravenously (i.v.) or topically. For the
`treatment of bladder cancer ALA is administered intravesically. Moreover, in contrast to other PS, oral
`ALA administration is also feasible. Although this route is preferred by patients, the bioavailability of
`oral ALA is generally lower then after intravenous administration owing to presystemic drug
`elimination [53]. Large biosynthetic PpIX capacity of gastrointestinal mucosal cells and hepatic first
`pass metabolism are two major factors responsible for the reduction of ALA bioavailability. Indeed,
`Dalton et al. observed that only 60% of orally administered ALA is absorbed [54]. Regardless of poor
`oral ALA availability, the tissue PpIX concentrations were comparable after i.v. or oral administration
`[54]. On the other hand, preferential mucosal accumulation of PpIX might be favorable in the
`treatment of tumors of the gastrointestinal tract. Also, limited PpIX accumulation in the underlying
`stroma may reduce damage of deeper layers and risk of perforation or stenosis [55]. In terms of topical
`ALA administration it should be emphasized that due to the fact that stratum corneum is the most
`important barrier for ALA skin penetration, the PpIX fluorescence is observed only at a depth of 0.3 to
`0.6 mm [56-58]. Therefore, a number of studies have focused on ALA derivatization to enhance its
`permeability through the lipid networks in the stratum corneum [59]. Moreover, since hyperkeratosis
`is an important negative factor in ALA uptake pretreatment with keratolytics, curettage/debulking, tape
`stripping, microdermabrasion or laser ablation can be used to improve ALA penetration [60].
`In patients with bladder cancer, the intravesical ALA application results in pharmacokinetic
`advantages in comparison with oral administration. The bladder concentration of ALA is
`approximately 20,000-fold higher in comparison to the systemic circulation. Moreover, only 1% of the
`intravesical dose is absorbed from the bladder by the systemic circulation. Therefore, after intravesical
`ALA administration no systemic phototoxicity should be observed [54].
`Identification of additional factors that affect ALA pharmacokinetics is necessary to design most
`effective ALA-PDT. Changes of numerous parameters of tumor cells as well as their surrounding
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`microenvironment may alter PpIX production in malignant lesion. For example, the influence of tissue
`temperature on PpIX accumulation was observed in several studies [61,62]. Other parameters that
`affect PpIX accumulation include pH [63-65], oxygen availability [63,66], illumination conditions, or
`thickness of striatum corneum [67].
`
`4. Modifications of ALA
`
`
`
`Apart from a great potential and numerous advantages of ALA as a PS precursor, a number of
`limitations have also been revealed. ALA has a hydrophilic character and does not penetrate efficiently
`through the skin nor cell membranes. In consequence the production of PpIX after topical ALA
`administration is restricted to a 2–3 mm surface of skin, which might be insufficient to elicit
`satisfactory photosensitization [6]. Moreover, hyperkeratotic lesions drastically diminish ALA
`penetration through the skin. Since the  hydrophilic nature of ALA limits its penetration ability
`restricting the use of ALA-PDT extensive studies were performed in order to facilitate its topical
`delivery. At least 77 various ALA modification and different carriers have been reported [68].
`The most successful ALA derivatives are its esters: methyl ester (methyl aminolevulinate, MAL)
`and hexyl ester (hexyl aminolevulinate, HAL). Elongation of a carbon chain attached to ALA results in
`increased lipophilicity and in consequence higher membrane and skin permeability. The advantage of
`ALA derivatives over ALA can be mainly ascribed to: (i) the rate at which these compounds reach the
`target site, (ii) the rate at which they reach the intracellular space and (iii) the rate of their enzymatic
`conversion into photoactive compounds. Basically, these modifications are favorable and confer
`improved skin penetration. Nevertheless, extensively lipophilic agents tend to accumulate in stratum
`corneum which results in a drop in their biological activity. Different PpIX production profiles as a
`function of drug concentration have been observed for ALA and its esters. ALA and MAL profiles are
`identical, but HAL profile is completely different reaching plateau value at lower concentration [69].
`Research by Lee et al. clearly shows that ALA butenyl, pentenyl and hexenyl esters induce higher
`production of PpIX than ALA or MAL [70]. In order to obtain comparable amount of PpIX with
`ALA-induced porphyrin accumulation, approximately 100 times less concentration of ALA heptyl
`ester is required [71]. On the other hand, undecanoyl-ALA ester, which reveals favorable diffusing
`properties, results in lower PpIX production than ALA itself. Even with the liposomal formulation it
`reaches only standard ALA-mediated PpIX level [72]. The advantage of ALA esters in comparison to
`ALA-inducedPpIX formation is not a general rule, some researchers report that it depends on esterase
`activity which varies within the tissues or cell lines used in the studies [73,74].
`Administration of liposomes entrapping the ALA prodrug into tumor-bearing mice results in
`increment porphyrin biosynthesis as well as higher tumor to normal cells porphyrin ratio [75]. In vitro
`experiments demonstrated lipid sponge forms (thermodynamically stable, amphiphilic liquid) as a
`potential carrier for transdermal drug delivery [76]. It was also suggested that addition of dimethyl
`sulfoxide (DMSO) and ethylenediaminetetraacetic acid (EDTA) to ALA may enhance its delivery to
`hairless mice skin for topical PDT [77]. Conjugating 5-aminolevulinic acid to nanoparticles, including
`biocompatible gold (Au) and chitosan, was one of the solutions suggested to improve ALA delivery to
`the tumor [78,79]. Interestingly, Moan's group proposed topical bioadhesive patch systems that
`enhance selectivity of PpIX accumulation in tumor-bearing mice model [80].
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`5. Comparison of ALA with Porphyrin-based Photosensitizers
`
`An ideal photosensitizer should be a well-defined chemical compound characterized by a strong
`selective phototoxic effect and an ability to generate active forms of oxygen. Moreover, it should show
`high absorption coefficients in the red part of the electromagnetic spectrum (600–900 nm) allowing the
`light to penetrate the tissue deeply. Its rapid clearance and favorable ADME (absorption, distribution,
`metabolism, excretion) parameters are also of utmost importance. The demand for such a compound is
`a challenge that leads to a synthesis of numerous photosensitizers. They differ from one another in
`terms of chemical structure, which often determines their pharmacokinetics, intracellular localization
`[81] and cytotoxic effect [82]. Generally, the currently used and investigated photosensitizing agents
`can be classified in two groups: porphyrin derivatives and non-porphyrin-based photosensitizers. The
`porphyrin family can be divided into three generations: the first one refers to photosensitizers
`developed between 1970 and 1980 and includes hematoporphyrin derivative (HpD) and Photofrin®.
`The second generation includes porphyrin derivatives that were intended to overcome the limitations
`of the former and the third generation encompasses photosensitizing agents conjugated with antibodies
`and other biological targeting molecules. So far only five photosensitizers have been approved for
`clinical PDT – porfimer sodium (Photofrin®), 5-aminolevulinic acid (ALA, Levulan®), and its methyl
`ester MAL (Metvix®), temoporfin (Foscan®), verteporfin (Visudyne®) and talaporfin (Laserphyrin®;
`the latter only in Japan). ALA is the only photosensitizer registered for topical use, particularly useful
`for the local treatment of superficial skin lesions. ALA-PDT has demonstrated high efficacy, minimal
`side effects and excellent cosmetic effects in a wide range of benign and malignant conditions.
`ALA has several advantages over other photosensitizers, namely rapid metabolism and high
`selectivity for malignant lesions. Prompt systemic clearance of ALA-induced PpIX within 24 h
`eliminates prolonged photosensitivity and allows treatment to be repeated at regular intervals (as
`frequently as every 48 h) without cumulative effects and risk of damage to normal tissue. Nevertheless,
`significant disadvantages of ALA-PDT include pain associated with treatment [83], limited depth of
`tumor penetration [84,85], as well as individual variations among patients that affect ALA absorption
`and pharmacokinetics that influence effective ALA concentrations in the treated area [86]. Moreover,
`ALA-PDT appeared to be less efficient in destroying cutaneous lesions when compared with
`Photofrin-PDT [86]. A general comparison of ALA and different porphyrin-based photosensitizers,
`taking into account their physical and chemical properties as well as molecular mechanisms associated
`with the influence on tumor cells is provided in Table 1.
`
`Chemical
`classification
`Hematoporphyrin
`
`Table 1. Properties of selected photosensitizers.
`Photosensitizer/trade
`Clinical approval/
`Cellular
`name/company
`clinical trials
`localization
`Approved
`Plasma
`Porfimer sodium -
`membrane
`combination of monomers,
`Golgi
`dimers and oligomers of
`apparatus
`hematoporphyrin
`[139,140]
`derivative (around 85%
`oligomeric material and
`mixture of more than 60
`compounds)/ Photofrin
`/Axcan Pharma, QLT
`Pharmaceuticals
`
`Advantages
`Most commonly
`used photo-
`sensitizer, the
`longest clinical
`history and
`patient record,
`pain-free
`treatment
`
`Disadvantages
`Complex composition,
`slow clearance rate,
`prolonged
`photosensitivity up to 3
`months, low
`fluorescence quantum
`yield, low efficiency in
`ROS generation,
`limited penetration and
`efficacy in deep and
`bulky tumors
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`
`
`Protoporhyrin
`
`Pro drug (5-aminolevulinic
`acid - ALA) converted to
`photoactive protoporhyrin
`IX/ Levulan, Levulan
`Kerastick (for topical use)/
`Dusa Pharmaceuticals)
`
`Table 1. Cont. 
`Approved
`Mitochondria,
`cell
`membranes,
`cytosol,
`cytosolic
`membranes
`[141]
`
`
`
`Texaphyrins
`
`Porphycenes
`
`Aminolevulinic esters/
`Metvixia/Galderma
`Benzvix, Hervix used for
`photodiagnosis [142]
`(PhotoCure AS)
`Motexafin lutetium/
`Lutrin, Optrin, Antrin/
`Pharmacyclics
`
`
`Various porphycene
`derivatives, modifiable
`isomers of porphyrin/ NDA
`
`Approved
`
`
`
`Completed clinical
`trials phase I
`
`Phase II clinical trial
`of topical ATMPn
`(9-acetoxy-
`2,7,12,17-tetrakis(β-
`methoxyethyl)-
`porphycene)
`
`Primarily in
`lysosomal
`compartment
`[143]
`
`Mitochondria
`Lysosomes
`ER
`plasma
`membrane
`[144]
`
`Purpurins
`
`Tin etyl etiopurpuryn
`rostaporfin/ SnET2,
`Photrex, Purlytin/ Miravant
`Medical Technologies
`
`In clinical trials
`phase III
`
`
`Mitochondria
`lysosomes
`
`Pheophorbides
`
`WST-09 (padoporfin,
`palladium
`bacteriopherophorbide
`/Tookad and WST-
`11(padeliporphin) /Stakel/
`Steba Biotech
`
`WST11- phase I and
`II, WST-09 – phase
`II
`
`NDA
`
`4149
`
`Pain associated with
`treatment (need of
`local analgesia), need
`of prolonged contact
`period before
`illumination
`
`Painful treatment
`
`
`Severe pain during
`the phototherapy
`(need of local
`anesthesia)
`
`Photobiological
`properties still poorly
`explored
`
`Post-treatment pain,
`long-lasting photo-
`sensitization up to 14
`days [146], poor
`stability in water
`(need of formulation
`in lipid emulsions,
`which can lead to
`allergic reactions)
`Narrow time window
`available for light
`delivery (important
`in clinical settings)
`
`Easy synthesis
`and formulation,
`minimal
`photosensitivity
`for no more than
`24 hours (rapid
`clearance),
`excellent
`cosmetic results,
`especially eyelids,
`inexpensive,
`possibility of
`application
`without doctor’s
`supervision (oral
`and topical
`administration),
`can be
`administered at
`regular intervals
`(even every 48h),
`high selectivity
`due to
`metabolism of
`ALA in malignant
`cells and
`pilosebaceous
`units
`Improved skin
`penetration -
`greater selectivity
`
`Deep tissue
`penetration
`
`Efficient ROS
`generation,
`possibility of
`various structural
`and chemical
`modifications that
`improve half-life
`and
`enhance
`therapeutic
`efficiency
`Excellent
`cosmetics effect,
`effective in
`treatment of
`locally advanced
`metastatic
`malignancies
`[145]
`
`Little or no skin-
`associated
`sensitivity,
`greater tissue
`penetration [84],
`the possibility of
`repeated
`treatments
`
`

`

`Molecules 2011, 16
`
`
`
`Chlorins
`
`Talaporfin sodium, mono-
`L-aspartyl chlorine 6/
`NPe6, MACE, LS11,
`Laserphyrin, Photolon,
`Aptocine/
`Light Sciences Oncology
`
`Table 1. Cont. 
`Approved
`Lysosomes
`
`
`
`
`
`
`Phthalocyanines
`
`
`Approved
`
`
`ER
`mitochondria
`
`Temoporfin, meta-
`tetrahydroxyphenylchlorine,
`mTHPC/
`Foscan/
` Biolitec Pharma
`
`
`HPPH 2-(1-hexyloxyethyl)-
`2-devinyl pyropheo-
`phorbide/ Photochlor/
`Roswell Park Cancer
`Institute
`Aluminium (III) phthalo-
`cyanine tetrasulphate,
`AlPcS4/Photosens (mixture
`of sulfonated aluminium
`phtalocyanines)/
`developed in Russia,
`General Physics Institute
`
`Mitochondria
`
`Mitochondria
`
`In naturally occurring
`veterinary tumors
`(cats and dogs),
`clinical trials phase I
`and II
`In naturally occurring
`veterinary tumors,
`several clinical trials
`in Russia
`
`
`
`Silicon phthalocyanine 4/
`Pc4/ Case Western Reserve
`University
`
`Ongoing clinical
`trials phase I
`
`Mitochondria
`ER
`
`4150
`
`In clinically needed
`high doses little
`selectivity to tumor
`tissue occurred
`[147]
`
`Accumulation in the
`skin, requires strict
`protection of the
`eyes and skin from
`sunlight for up to 6
`weeks, long drug-
`illumination
`interval, requires
`very precise
`illumination
`(reflected light can
`produce
`photodynamic
`reaction) and
`accurate dosimetry
`Phototoxicity not
`determined in higher
`doses
`
`Problems with
`purification,
`typically final
`product is a mixture
`of mono- di- tri- and
`tetrasulphonated
`derivatives, in water
`aggregate at
`relatively low
`concentrations
`which results in loss
`of photo-chemical
`activity
`Heterogeneous
`distribution within
`and between lesions
`detected by
`noninvasive
`spectroscopy [148]
`
`Excellent singlet
`oxygen yield, used
`in Litx therapy
`(Light Infusion
`Technology),
`where talaporfin is
`illuminated for
`prolonged time (1-
`3 h) locally with
`light-emitting
`diodes (LEDs)
`implanted in the
`tumor, minimal
`skin photo-
`sensitivity
`High singlet
`oxygen yield (low
`drug dose and low
`light dose -
`20J/cm2 are
`required to
`generate
`photodynamic
`reaction), low
`activation energy
`and short time
`treatment, long
`half-life in triplet
`state
`
`Minimal sunlight
`photosensitivity,
`relatively easy to
`synthesize
`
`High singlet
`oxygen yield with
`long-lived triplet
`states due to
`presence of
`aluminum atom,
`due to enhance
`fluorescence can
`be used for
`diagnostic
`purpose, minimal
`photosensitivity
`
`High singlet
`oxygen yield with
`long-lived triplet
`states due to
`presence of silicon
`atom, good
`efficacy in both
`preclinical and
`clinical studies,
`due to enhance
`fluorescence can
`be used for
`diagnostic
`purposes
`
`

`

`Molecules 2011, 16
`
`
`
`4151
`
`Benzoporphyrins Verteprofin/Visudyne
`/Novartis
`
`Table 1. Cont.
`Approved
`Mitochondria
`and ER
`
`Deep tissue
`penetration,
`minimal photo-
`sensitization up to
`48 h, effectiveness
`in neovascular
`lesions, successful
`in cutaneous
`lesions
`Abbreviations used: ER – endoplasmatic reticulum, NDA – no data available, ROS – reactiv