Adriana Casas

Mechanisms of resistance to photodynamic therapy.

Photodynamic therapy (PDT) involves the administration of a photosensitizer (PS) followed by illumination with visible light, leading to generation of reactive oxygen species. The mechanisms of resistance to PDT ascribed to the PS may be shared with the general mechanisms of drug resistance, and are related to altered drug uptake and efflux rates or altered intracellular trafficking. As a second step, an increased inactivation of oxygen reactive species is also associated to PDT resistance via antioxidant detoxifying enzymes and activation of heat shock proteins. Induction of stress response genes also occurs after PDT, resulting in modulation of proliferation, cell detachment and inducing survival pathways among other multiple extracellular signalling events. In addition, an increased repair of induced damage to proteins, membranes and occasionally to DNA may happen. PDT-induced tissue hypoxia as a result of vascular damage and photochemical oxygen consumption may also contribute to the appearance of resistant cells. The structure of the PS is believed to be a key point in the development of resistance, being probably related to its particular subcellular localization. Although most of the features have already been described for chemoresistance, in many cases, no cross-resistance between PDT and chemotherapy has been reported. These findings are in line with the enhancement of PDT efficacy by combination with chemotherapy. The study of cross resistance in cells with developed resistance against a particular PS challenged against other PS is also highly complex and comprises different mechanisms. In this review we will classify the different features observed in PDT resistance, leading to a comparison with the mechanisms most commonly found in chemo resistant cells.

Macromolecular delivery of 5-aminolaevulinic acid for photodynamic therapy using dendrimer conjugates

Intracellular porphyrin generation following administration of 5-aminolaevulinic acid (5-ALA) has been widely used in photodynamic therapy. However, cellular uptake of 5-ALA is limited by its hydrophilicity, and improved means of delivery are therefore being sought. Highly branched polymeric drug carriers known as dendrimers present a promising new approach to drug delivery because they have a well-defined structure capable of incorporating a high drug payload. In this work, a dendrimer conjugate was investigated, which incorporated 18 aminolaevulinic acid residues attached via ester linkages to a multipodent aromatic core. The ability of the dendrimer to deliver and release 5-ALA intracellularly for metabolism to the photosensitizer, protoporphyrin IX, was studied in the transformed PAM 212 murine keratinocyte and A431 human epidermoid carcinoma cell lines. Up to an optimum concentration of 0.1 mmol/L, the dendrimer was significantly more efficient compared with 5-ALA for porphyrin synthesis. The intracellular porphyrin fluorescence levels showed good correlation with cellular phototoxicity following light exposure, together with minimal dark toxicity. Cellular uptake of the dendrimer occurs through endocytic routes predominantly via a macropinocytosis pathway. In conclusion, macromolecular dendritic derivatives are capable of delivering 5-ALA efficiently to cells for sustained porphyrin synthesis. [Mol Cancer Ther 2007;6(3):876–85]

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

SummarySamples 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.

Photosensitization and mechanism of cytotoxicity induced by the use of ALA derivatives in photodynamic therapy

The use of more lipophilic derivatives of 5-aminolevulinic acid (ALA) is expected to have better diffusing properties, and after conversion into the parent ALA, to reach a higher protoporphyrin IX (PPIX) formation rate, thus improving the efficacy of topical photodynamic therapy (PDT). Here we have analysed the behaviour of 3 ALA derivatives (ALA methyl-ester, hexyl ester and a 2-sided derivative) regarding PPIX formation, efficiency in photosensitizing cells and mechanism of cellular death. The maximum amount of porphyrins synthesized from 0.6 mM ALA was 47 ± 8 ng/105cells. The same amount was formed by a concentration 60-fold lower of hexyl-ALA and 2-fold higher of methyl-ALA. The 2-sided derivative failed to produce PPIX accumulation. Applying a 0.6 J cm–2light dose, cell viability decreased to 50%. With the 1.5 J cm–2light dose, less than 20% of the cells survive, and higher light doses produced nearly total cell killing. Comparing the PPIX production and the induced phototoxicity, the more the amount of porphyrins, the greater the cellular killing, and PPIX formed from either ALA or ALA-esters equally sensitize the cells to photoinactivation. ALA-PDT treated cells exhibited features of apoptosis, independently on the pro-photosensitizer employed. ALA-PDT can be improved with the use of ALA derivatives, reducing the amount of ALA necessary to induce efficient photosensitization.

Mechanisms of 5‐aminolevulic acid ester uptake in mammalian cells

The porphyrin precursor 5‐aminolevulinic acid (ALA) is being widely used in photodynamic therapy of cancer. Improvement in ALA delivery has been sought through the use of ALA derivatives, in particular the esterification of ALA with aliphatic alcohols, which in certain cases can improve cellular penetration and selectivity. ALA uptake systems appear to be distinctive for each cell type. The LM3 mammary adenocarcinoma cell line takes ALA up by BETA transporters. In this work, we investigated ALA derivative transport systems through the inhibition of radiolabelled ALA uptake in the LM3 cells. We also performed inhibition studies of γ‐aminobutyric acid (GABA) uptake. The more lipohilic ALA derivatives hexyl‐ALA and undecanoyl‐ALA inhibit ALA uptake, whereas methyl‐ALA, R, S‐ALA‐2‐(hydroxymethyl)tetrahydropyranyl ester and the dendron aminomethane tris methyl 5‐ALA does not inhibit ALA uptake. A similar pattern was found for GABA, except that the dendron inhibited GABA uptake. However, hexyl‐ALA and undecanoyl‐ALA are not taken up by BETA transporters, but by simple diffusion, although they still inhibit ALA uptake by binding to the cell membrane. These results show that different modifications to the ALA molecule lead to different uptake mechanisms. Whereas ALA is taken up by BETA transporters, none of the ALA derivatives shares the same mechanism. Knowledge of the mechanisms of ALA derivatives entry into the cells is essential to understand and improve ALA‐mediated PDT and to the design of new ALA derivatives that may be taken up at a higher rate than ALA.

The influence of the vehicle on the synthesis of porphyrins after topical application of 5‐aminolaevulinic acid. Implications in cutaneous photodynamic sensitization

Background  The optimal vehicle to ensure adequate penetration of 5‐aminolaevulinic acid (ALA) for its use in photodynamic therapy (PDT) of skin lesions has not been determined. Objectives We aimed to study the effects of ALA in various vehicle formulations [saline lotion with and without dimethylsulphoxide (DMSO), cream, liposomes and vaseline] after topical application in a murine subcutaneous adenocarcinoma model. Methods The effect of DMSO on porphyrin synthesis and ALA penetration through the skin was studied by measuring the uptake of 14C label from ALA, ALA and porphobilinogen accumulation, and some haem enzyme activities. The tissue distribution and kinetics of porphyrin synthesis after topical application of ALA entrapped in large multilamellar liposomes was also determined. Results ALA in saline lotion, alone or with 10% DMSO, proved to be the most efficient vehicle for tumour porphyrin accumulation (mean ± SD 1·75 ± 0·25 and 2·09 ± 0·39 µg g−1, respectively), whereas cream and liposomes induced lower levels and identical porphyrin accumulation (0·60 µg g−1). Using ALA + DMSO saline lotion, a higher porphyrin accumulation was found in skin overlying the tumour tissue and in the first 2 mm of tumour, probably due to increased ALA penetration, or greater interconversion to porphyrins, or greater retention of ALA and/or porphyrins. Conclusions These findings reinforce the importance of the vehicle in topical ALA‐based PDT, and explain the mechanism of action of DMSO in enhancing protoporphyrin IX biosynthesis in superficial lesions.

Rational Design of 5-Aminolevulinic Acid Derivatives Aimed at Improving Photodynamic Therapy

5-aminolevulinic acid (ALA) is the first intermediate in heme biosynthesis and is therefore a precursor of protoporphyrin IX (PpIX). PpIX is used as an endogenous photosensitizer in photodynamic therapy (PDT). Several chemical modifications have been made, both on the amino and carboxyl groups of ALA to induce higher PpIX production and photosensitisation. Esterification of ALA with aliphatic lineal and cyclic alcohols was found to reduce the amount of ALA required for photosensitization. Esterification by aliphatic alcohols with carbohydrate chains equal or lower than C4 leads to porphyrin accumulation lower than ALA, whereas equal or longer than C6 chains leads to greater synthesis of porphyrin. A branch point in the alcohol located next to the site of ester cleavage limits access of the esters to the esterase active site, resulting in lower PpIX production. ALA esters of the polyethylenglycol family can induce high levels of PpIX, with some selectivity for endothelial cells toward tumor cells. On the basis of the differential expression of some aminopeptidases in tumor vasculature when compared to normal vasculature, some ALA-pseudopeptides were synthesized. In a rational design of ALA derivatives, the transport mechanism of these aminoacids into the cell is central. Due to the similar characteristics between ALA and GABA transport, a novel approach for designing new ALA derivatives which could penetrate more easily into tumoral cells, would be to take into account the structures of the inhibitors of GABA transport.

Aminolevulinic acid derivatives and liposome delivery as strategies for improving 5-aminolevulinic acid-mediated photodynamic therapy.

Photodynamic Therapy employing 5-aminolevulinic acid (ALA) as a precursor of the photosensitizer Protoporphyrin IX has become a promising approach to treat superficial cancers. However, the hydrophilic nature of the ALA molecule somewhat limits the penetration through the skin as well as all cell membranes. Different attempts are currently under investigation to enhance ALA penetration, such as the development of new synthetic and more lipophilic molecules derived from ALA and the incorporation of ALA into lipophilic vehicles such as liposomes. Among the new synthesized molecules, we can find ALA esters, ALA aminoacid derivatives and ALA dendrimers. In general, there is consensus that the promising results obtained in vitro with ALA esters cannot be reproduced in vivo. However, ALA methyl ester (1) has been widely used for treatment of skin malignancies and ALA hexyl ester (15) proved to be more powerful than ALA in bladder imaging. ALA aminoacid derivatives have been designed to use specific cellular aminopeptidases to targeting tumors, and it was shown that they can be metabolized to ALA with some specificity.

Porphyrin synthesis from ALA derivatives for photodynamic therapy. In vitro and in vivo studies.

The aim of this work was to test in vitro and in vivo the efficacy of the derivatives of 5-aminolevulinic acid (ALA): hexyl-ALA (He-ALA), undecanoyl-ALA and R,S-2-(hydroximethyl)tetrahydropyranyl-ALA (THP-ALA) as pro-photosensitising agents. The compounds were assayed in a cell line derived from a murine mammary tumour, in tumour explants and after injection of the cells into mice. In vitro, undecanoyl-ALA and THP-ALA did not improve ALA efficacy in terms of porphyrin synthesis. On the other hand, half of the amount of ALA is required to obtain the same plateau amount of photosensitiser from He-ALA. However, this plateau value cannot be surpassed in spite of the four-times higher accumulation of ALA/He-ALA from the ALA derivative. This shows that He-ALA conversion to porphyrins but not He-ALA entry to the cells is limiting. Employing ionic exchange chromatography, we found that 80% of total uptake was He-ALA whereas only 20% was ALA. This suggests that the esterases, probably themselves regulated by the heme pathway, are limiting the conversion of ALA derivatives into porphyrins. A similar situation occurs with THP-ALA. Tumour explant porphyrin results correlate well with cell line data. However, i.p. injection of ALA derivatives to mice resulted in a lower porphyrin concentration in the tumour when compared to the administration of equimolar amounts of ALA, indicating that there should be retention of ALA derivatives either within the blood vessels in the initial phase of distribution and/or within the capillaries of the tumour.

Sustained and efficient porphyrin generation in vivo using dendrimer conjugates of 5-ALA for photodynamic therapy

The use of endogenous protoporphyrin IX (PpIX) after administration of 5-aminolaevulinic acid (ALA) has led to many applications in photodynamic therapy (PDT). However the efficacy of ALA-PDT is sub-optimal for thicker tumours and improved ALA delivery and therapeutic response are required. We have investigated the conjugation of ALA to a second-generation dxcendrimer for enhancing porphyrin synthesis in vitro and in vivo in a murine tumour model using systemic i.p. administration. In vitro, the dendrimer was more efficient than ALA for porphyrin synthesis at low concentrations in good correlation with higher cellular ALA dendrimer accumulation. In vivo, the porphyrin kinetics from ALA exhibited an early peak between 3 and 4 h in most tissues, whereas the dendrimer induced sustained porphyrin production for over 24 h and basal values were not reached until 48 h after administration. Integrated porphyrin accumulation from the dendrimer and ALA, at equivalent molar ratios, was comparable showing that the majority of ALA residues were liberated from the dendrimer. The porphyrin kinetics appear to be governed by the rate of enzymatic cleavage of ALA from the dendrimer, which is consistent with in vitro results. ALA dendrimers may be useful for metronomic PDT, and multiple low-dose ALA-PDT treatments.