Photodynamic therapy of skin cancers: sensitizers, clinical studies and future directives

Beyond the Barriers of Light Penetration: Strategies, Perspectives and Possibilities for Photodynamic Therapy

Photodynamic therapy (PDT) is a photochemistry based treatment modality that involves the generation of cytotoxic species through the interactions of a photosensitizer molecule with light irradiation of an appropriate wavelength. PDT is an approved therapeutic modality for several cancers globally and in several cases has proved to be effective where traditional treatments have failed. The key parameters that determine PDT efficacy are 1. the photosensitizer (nature of the molecules, selectivity, and macroscopic and microscopic localization etc.), 2. light application (wavelength, fluence, fluence rate, irradiation regimes etc.) and 3. the microenvironment (vascularity, hypoxic regions, stromal tissue density, molecular heterogeneity etc.). Over the years, several groups aimed to monitor and manipulate the components of these critical parameters to improve the effectiveness of PDT treatments. However, PDT is still misconstrued to be a surface treatment primarily due to the limited depths of light penetration. In this review, we present the recent advances, strategies and perspectives in PDT approaches, particularly in cancer treatment, that focus on increasing the 'damage zone' beyond the reach of light in the body. This is enabled by a spectrum of approaches that range from innovative photosensitizer excitation strategies, increased specificity of phototoxicity, and biomodulatory approaches that amplify the biotherapeutic effects induced by photodynamic action. Along with the increasing depth of understanding of the underlying physical, chemical and physiological mechanisms, it is anticipated that with the convergence of these strategies, the clinical utility of PDT will be expanded to a powerful modality in the armamentarium for the management of cancer.

Evaluation of Hydrogel Suppositories for Delivery of 5-Aminolevulinic Acid and Hematoporphyrin Monomethyl Ether to Rectal Tumors

We evaluated the potential utility of hydrogels for delivery of the photosensitizing agents 5-aminolevulinic acid (ALA) and hematoporphyrin monomethyl ether (HMME) to rectal tumors. Hydrogel suppositories containing ALA or HMME were administered to the rectal cavity of BALB/c mice bearing subcutaneous tumors of SW837 rectal carcinoma cells. For comparison, ALA and HMME were also administered by three common photosensitizer delivery routes; local administration to the skin and intratumoral or intravenous injection. The concentration of ALA-induced protoporphyrin IX or HMME in the rectal wall, skin, and subcutaneous tumor was measured by fluorescence spectrophotometry, and their distribution in vertical sections of the tumor was measured using a fluorescence spectroscopy system. The concentration of ALA-induced protoporphyrin IX in the rectal wall after local administration of suppositories to the rectal cavity was 9.76-fold (1 h) and 5.8-fold (3 h) higher than in the skin after cutaneous administration. The maximal depth of ALA penetration in the tumor was ~3–6 mm at 2 h after cutaneous administration. Much lower levels of HMME were observed in the rectal wall after administration as a hydrogel suppository, and the maximal depth of tumor penetration was <2 mm after cutaneous administration. These data show that ALA more readily penetrates the mucosal barrier than the skin. Administration of ALA as an intrarectal hydrogel suppository is thus a potential delivery route for photodynamic therapy of rectal cancer.

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

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

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

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

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

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Photodynamic cell killing was dependent primarily on ¹O₂.

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

New photosensitizers for photodynamic therapy

Photodynamic therapy (PDT) was discovered more than 100 years ago, and has since become a well-studied therapy for cancer and various non-malignant diseases including infections. PDT uses photosensitizers (PSs, non-toxic dyes) that are activated by absorption of visible light to initially form the excited singlet state, followed by transition to the long-lived excited triplet state. This triplet state can undergo photochemical reactions in the presence of oxygen to form reactive oxygen species (including singlet oxygen) that can destroy cancer cells, pathogenic microbes and unwanted tissue. The dual-specificity of PDT relies on accumulation of the PS in diseased tissue and also on localized light delivery. Tetrapyrrole structures such as porphyrins, chlorins, bacteriochlorins and phthalocyanines with appropriate functionalization have been widely investigated in PDT, and several compounds have received clinical approval. Other molecular structures including the synthetic dyes classes as phenothiazinium, squaraine and BODIPY (boron-dipyrromethene), transition metal complexes, and natural products such as hypericin, riboflavin and curcumin have been investigated. Targeted PDT uses PSs conjugated to antibodies, peptides, proteins and other ligands with specific cellular receptors. Nanotechnology has made a significant contribution to PDT, giving rise to approaches such as nanoparticle delivery, fullerene-based PSs, titania photocatalysis, and the use of upconverting nanoparticles to increase light penetration into tissue. Future directions include photochemical internalization, genetically encoded protein PSs, theranostics, two-photon absorption PDT, and sonodynamic therapy using ultrasound.

Time course of apoptosis induced by photodynamic therapy with PsD007 in LT12 acute myeloid leukemia cells

Apoptosis is one of the major mechanisms of photodynamic therapy (PDT) that leads to tumor degradation. Apoptosis-related genes and proteins function in a certain order and timing in the complex network of apoptosis. To further understanding of the apoptotic mechanism of PDT, this research examined the time course of apoptosis from PsD007 (a second-generation photosensitizer developed in China) induced PDT on the rat acute myeloid leukemia cell line LT12. MTT was used to detect the temporal dynamic of PDT killing effects and identified the "apoptotic window" of 2-24 h. Apoptosis showed a basal peak at 2 h, and the duration of apoptosis depended on PDT dose, which disappeared quickly at low concentrations but lasted to higher levels to 6 or 12 h at high concentrations as detected by flow cytometry. High-content imaging confirmed these results. An 84-gene apoptosis PCR array identified 15 genes with an expression level change of over twofold at 6 h post-PDT. Nine apoptosis-related genes showed changes in expression at 2-12 h after PDT. TNF family genes TNF and FASLG showed a maximal change of 3.47- and 4.42-fold from baseline. Key apoptosis proteins such as activated caspases showed strong up-regulation after PDT, with the expression peaks of cleaved caspase-7, caspase-9 and PARP at 4-6 h, and cleaved caspase-3 delayed to 6-12 h. Our findings help clarify the time course of apoptosis events in response to PDT treatment in a leukemia cell line and may help contribute to the clinical application of PDT in leukemia treatment.

A Nanosystem Capable of Releasing a Photosensitizer Bioprecursor under Two-Photon Irradiation for Photodynamic Therapy

The applications of photodynamic therapy (PDT) are usually limited by photosensitizers' side effects and singlet oxygen's short half-life. Herein, a mitochondria-targeted nanosystem is demonstrated to enhance the PDT efficacy by releasing a bio-precursor of photosensitizer under two-photon irradiation. A phototriggerable coumarin derivative is first synthesized by linking 5-aminolevulinic acid (5-ALA, the bio-precursor) to coumarin; and the nanosystem (CD-ALA-TPP) is then fabricated by covalently incorporating this coumarin derivative and a mitochondria-targeting compound triphenylphosphonium (TPP) onto carbon dots (CDs). Upon cellular internalization, the nanosystem preferentially accumulates in mitochondria; and under one- or two-photon irradiation, it releases 5-ALA molecules that are then metabolized into protoporphyrin IX in mitochondria through a series of biosynthesis processes. The subsequent red light irradiation induces this endogenously synthesized photosensitizer to generate singlet oxygen, thereby causing oxidant damage to mitochondria and then the apoptosis of the cells. Analysis via 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyltetrazolium bromide (MTT) assays indicate that the novel PDT system exhibits enhanced cytotoxicity toward cancer cells. This study may offer a new strategy for designing PDT systems with high efficacy and low side effects.

Optimization of protoporphyrin IX skin delivery for topical photodynamic therapy: Nanodispersions of liquid-crystalline phase as nanocarriers

Nanodispersions of liquid-crystalline phases (NLPs) composed of monoolein and oleic acid were chosen as nanocarriers to improve the topical retention of the photosensitizer protoporphyrin IX (PpIX) and thereby optimize photodynamic therapy (PDT) using this photosensitizer. The nanodispersions were characterized by polarized light microscopy, small-angle X-ray diffraction and dynamic light scattering. The stability and encapsulation efficiency (EE%) of the nanodispersions were also evaluated. In vitro and in vivo skin penetration studies were performed to determine the potential of the nanodispersions for cutaneous application. In addition, skin penetration and skin irritancy (in an animal model) after in vivo application were visualized by fluorescence light microscopy. The nanodispersion obtained was characterized as a monodisperse system (~150.0 nm) of hexagonal liquid-crystalline phase, which provided a high encapsulation efficiency of PpIX (~88%) that remained stable over 90 days of investigation. Skin penetration studies demonstrated that the nanodispersion enhanced PpIX skin uptake 11.8- and 3.3-fold (in vitro) and 23.6- and 20.8-fold (in vivo) compared to the PpIX skin uptake of control formulations, respectively. In addition, the hexagonal phase nanodispersion did not cause skin irritation after application for two consecutive days. Overall, the results show that the nanocarrier developed is suitable for use in topical PDT with PpIX.

Photodynamic therapy with a novel porphyrin-based photosensitizer against human gastric cancer

The objective of the present study was to evaluate the effects of novel porphyrin-based photosensitizer meso-5-[ρ-diethylene triamine pentaacetic acid- aminophenyl]-10,15,20-triphenyl-porphyrin (DTP)-mediated photodynamic therapy (PDT) on the HGC27 and SNU-1 human gastric cancer cell lines. The absorption spectrum of DTP was analyzed using a microplate spectrophotometer. The HGC27 or SNU-1 cells were incubated with DTP and exposed to illumination by a 650-nm laser. The experiments were divided into four groups: A blank control, cells treated with DTP without light, cells exposed to laser light without DTP and cells treated with a combination of DTP and light together. The phototoxicity of DTP was analyzed by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide assay. Cell apoptosis was detected by flow cytometry and Hoechst 33342 staining. In addition, the intracellular distribution of DTP was investigated by laser scanning confocal microscopy. DTP-PDT demonstrated marked phototoxicity towards HGC27- and SNU-1 cells. The rate of cell death increased significantly in a DTP concentration-dependent and light dose-dependent manner, with maximum mortality rates of 74.14 and 67.76%, respectively. There were significant differences between the therapeutic and control groups (P<0.01). In addition, the growth of cells treated with DTP or laser light alone was not inhibited. Further evaluation revealed that, following DTP-PDT, HGC27 and SNU-1 cells demonstrated notable apoptotic changes, including condensed chromatin, fragmented nuclei and apoptotic bodies, and the percentage of apoptotic cells was significantly higher than that of the control groups (P<0.01). Furthermore, confocal laser scanning microscopy revealed that DTP localized to the lysosomes but not mitochondria in the two types of tumor cell. In conclusion, significant phototoxicity and reduced cytotoxicity in dark conditions make the novel photosensitizer DTP a promising potential PDT drug for future use in the treatment of human gastric cancer.

Phototoxic Activity and DNA Interactions of Water-Soluble Porphyrins and Their Rhenium(I) Conjugates

In the search for alternative photosensitizers for use in photodynamic therapy (PDT), herein we describe two new water-soluble porphyrins, a neutral fourfold-symmetric compound and a +3-charged tris-methylpyridinium derivative, in which either four or one [1,4,7]-triazacyclononane (TACN) units are connected to the porphyrin macrocycle through a hydrophilic linker; we also report their corresponding tetracationic Re(I) conjugates. The in vitro (photo)toxic effects of the compounds toward the human cell lines HeLa (cervical cancer), H460M2 (non-small-cell lung carcinoma), and HBL-100 (non-tumorigenic epithelial cells) are reported. Three of the compounds are not cytotoxic in the dark up to 100 μm, and the fourfold-symmetric couple revealed very good phototoxic indexes (PIs). The intracellular localization of all derivatives was studied in HeLa cells by confocal fluorescence microscopy. Although low nuclear localization was observed for some of them, it still prompted us to investigate their capacity to bind both quadruplex and duplex DNA; we observed significant selectivity in the tris-methylpyridinium derivatives for G-quadruplex interactions.

Assessment of ALA-induced PpIX production in porcine skin pretreated with microneedles

Photodynamic therapy (PDT) is used for skin treatments of premalignant and cancer lesions and recognized as a non-invasive technique that combines tissue photosensitization and subsequent exposure to light to induce cell death. However, it is limited to the treatment of superficial lesions, mainly due to the low cream penetration. Therefore, the improvement of transdermal distribution of aminolevulinic acid (ALA) is needed. In this study, the kinetics and homogeneity of production of ALA-induced PpIX after the skin pre-treatment with microneedles rollers of 0.5, 1.0 and 1.5 mm length were investigated. An improvement in homogeneity and production of PpIX was shown in a porcine model. Widefield fluorescence imaging three hours after the topical application of ALA-cream in the combined treatment with microeedles rollers.

In vivo curative and protective potential of orally administered 5-aminolevulinic acid plus ferrous ion against malaria

5-Aminolevulinic acid (ALA) is a naturally occurring amino acid present in diverse organisms and a precursor of heme biosynthesis. ALA is commercially available as a component of cosmetics, dietary supplements, and pharmaceuticals for cancer diagnosis and therapy. Recent reports demonstrated that the combination of ALA and ferrous ion (Fe²⁺) inhibits the in vitro growth of the human malaria parasite Plasmodium falciparum. To further explore the potential application of ALA and ferrous ion as a combined antimalarial drug for treatment of human malaria, we conducted an in vivo efficacy evaluation. Female C57BL/6J mice were infected with the lethal strain of rodent malaria parasite Plasmodium yoelii 17XL and orally administered ALA plus sodium ferrous citrate (ALA/SFC) as a once-daily treatment. Parasitemia was monitored in the infected mice, and elimination of the parasites was confirmed using diagnostic PCR. Treatment of P. yoelii 17XL-infected mice with ALA/SFC provided curative efficacy in 60% of the mice treated with ALA/SFC at 600/300 mg/kg of body weight; no mice survived when treated with vehicle alone. Interestingly, the cured mice were protected from homologous rechallenge, even when reinfection was attempted more than 230 days after the initial recovery, indicating long-lasting resistance to reinfection with the same parasite. Moreover, parasite-specific antibodies against reported vaccine candidate antigens were found and persisted in the sera of the cured mice. These findings provide clear evidence that ALA/SFC is effective in an experimental animal model of malaria and may facilitate the development of a new class of antimalarial drug.

PNA-Rose Bengal Conjugates as Efficient DNA Photomodulators

Selective photoinduced modulation of DNA may provide a powerful therapeutic tool allowing spatial and temporal control of the photochemical reaction. We have explored the photoreactivity of peptide nucleic acid (PNA) conjugates that were conjugated to a highly potent photosensitizer, Rose Bengal (RB). In addition, a short PEGylated peptide (K-PEG8-K) was conjugated to the C-terminus of the PNA to improve its water solubility. A short irradiation (visible light) of PNA conjugates with a synthetic DNA resulted in highly efficient photomodulation of the DNA as evidenced by polyacrylamide gel electrophoresis (PAGE). In addition, a PNA-RB conjugate replacing K-PEG8-K with four l-glutamic acids (E4) was found to be photoinactive. Irradiation of active PNA-RB conjugates with synthetic DNA in D20 augments the photoactivity; supporting the involvement of singlet oxygen. PAGE, HPLC, and MALDI-TOF analyses indicate that PNA-DNA photo-cross-linking is a significant pathway in the observed photoreactivity. Selective photo-cross-linking of such PNA-RB conjugates may be a novel approach to selective photodynamic therapy (sPDT) as such molecules would be sequence-specific, cell-permeable, and photoactivated in the visible region.

The photodynamic antibacterial effects of silicon phthalocyanine (Pc) 4

The emergence of antibiotic-resistant strains in facultative anaerobic Gram-positive coccal bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA), is a global health issue. Typically, MRSA strains are found associated with institutions like hospitals but recent data suggest that they are becoming more prevalent in community-acquired infections. It is thought that the incidence and prevalence of bacterial infections will continue to increase as (a) more frequent use of broad-spectrum antibiotics and immunosuppressive medications; (b) increased number of invasive medical procedures; and (c) higher incidence of neutropenia and HIV infections. Therefore, more optimal treatments, such as photodynamic therapy (PDT), are warranted. PDT requires the interaction of light, a photosensitizing agent, and molecular oxygen to induce cytotoxic effects. In this study, we investigated the efficacy and characterized the mechanism of cytotoxicity induced by photodynamic therapy sensitized by silicon phthalocyanine (Pc) 4 on (a) methicillin-sensitive Staphylococcus aureus (MSSA) (ATCC 25923); (b) community acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) (ATCC 43300); and (c) hospital acquired methicillin-resistant Staphylococcus aureus (HA-MRSA) (PFGE type 300). Our data include confocal image analysis, which confirmed that Pc 4 is taken up by all S. aureus strains, and viable cell recovery assay, which showed that concentrations as low as 1.0 μM Pc 4 incubated for 3 h at 37 °C followed by light at 2.0 J/cm2 can reduce cell survival by 2-5 logs. These results are encouraging, but before PDT can be utilized as an alternative treatment for eradicating resistant strains, we must first characterize the mechanism of cell death that Pc 4-based PDT employs in eliminating these pathogens.

Porphyrin-based photosensitizers and the corresponding multifunctional nanoplatforms for cancer-imaging and phototherapy

This review article briefly describes: (a) the advantages in developing multifunctional nanoparticles for cancer-imaging and therapy, (b) the advantages and limitations of most of the porphyrin-based compounds in fluorescence imaging and photodynamic therapy (PDT), (c) problems associated with current Food and Drug Administration (FDA) approved photosensitizers, (d) challenges in developing in vivo target-specific PDT agents, (e) development of porphyrin-based nuclear-imaging agents (PET, SPECT) with an option of PDT, (f) the importance of light dosimetry in PDT, (g) the role of whole body or local hyperthermia in enhancing tumor-uptake, tumor-imaging and phototherapy and finally, (h) the advantages of photosensitizer-gold nanocages (Ps- Au NC) in photoacoustic and PDT.

Stem cells loaded with nanoparticles as a drug carrier for in vivo breast cancer therapy

A novel anti-cancer drug carrier, mesenchymal stem cells (MSCs) encapsulating drug-loaded hollow silica nanoparticles, is used to carry a photosensitizer drug and deliver it to breast tumors, due to the natural high tumor affinity of the MSCs, and inhibit tumor growth by photo dynamic therapy. This new strategy for delivering a photo sensitizer to tumors by using tumor-affinitive MSCs addresses the challenge of the accumulation of photosensitizer drugs in tumors in photodynamic therapy.

Progress of photodynamic therapy applications in the treatment of musculoskeletal sarcoma (Review)

Photodynamic therapy (PDT) has clinical approval for use as a minimally invasive therapeutic procedure that is able to exert selective cytotoxic activity toward pathological cells, particularly malignant cells. Following a number of recent technological improvements, PDT has been widely applied to the diagnosis and treatment of malignancies, including lung, esophageal, gastrointestinal, bladder, prostate, head and neck, oral and skin cancer. Studies have shown that osteosarcoma is a malignant tumor afflicting young adults worldwide, and recently, the incidence of bone and soft-tissue malignant tumors has been shown to be increasing, so the use of PDT has become an area of focus for the diagnosis and treatment of musculoskeletal sarcoma.

Silicon phthalocyanine 4 phototoxicity in Trichophyton rubrum

Trichophyton rubrum is the leading pathogen that causes long-lasting skin and nail dermatophyte infections. Currently, topical treatment consists of terbinafine for the skin and ciclopirox for the nails, whereas systemic agents, such as oral terbinafine and itraconazole, are also prescribed. These systemic drugs have severe side effects, including liver toxicity. Topical therapies, however, are sometimes ineffective. This led us to investigate alternative treatment options, such as photodynamic therapy (PDT). Although PDT is traditionally recognized as a therapeutic option for treating a wide range of medical conditions, including age-related macular degeneration and malignant cancers, its antimicrobial properties have also received considerable attention. However, the mechanism(s) underlying the susceptibility of dermatophytic fungi to PDT is relatively unknown. As a noninvasive treatment, PDT uses a photosensitizing drug and light, which, in the presence of oxygen, results in cellular destruction. In this study, we investigated the mechanism of cytotoxicity of PDT in vitro using the silicon phthalocyanine (Pc) 4 [SiPc(OSi(CH3)2(CH2)3N(CH3)2)(OH)] in T. rubrum. Confocal microscopy revealed that Pc 4 binds to cytoplasmic organelles, and upon irradiation, reactive oxygen species (ROS) are generated. The impairment of fungal metabolic activities as measured by an XTT (2,3-bis[2-methoxy-4-nitro-5-sulfophenyl]-2H-tetrazolium-5-carboxyanilide inner salt) assay indicated that 1.0 μM Pc 4 followed by 670 to 675 nm light at 2.0 J/cm(2) reduced the overall cell survival rate, which was substantiated by a dry weight assay. In addition, we found that this therapeutic approach is effective against terbinafine-sensitive (24602) and terbinafine-resistant (MRL666) strains. These data suggest that Pc 4-PDT may have utility as a treatment for dermatophytosis.

Target-selective phototherapy using a ligand-based photosensitizer for type 2 cannabinoid receptor

Phototherapy is a powerful, noninvasive approach for cancer treatment, with several agents currently in clinical use. Despite the progress and promise, most current phototherapy agents have serious side effects as they can lead to damage to healthy tissue, even when the photosensitizers are fused to targeting molecules due to nonspecific light activation of the unbound photosensitizer. To overcome these limitations, we developed a phototherapy agent that combines a functional ligand and a near infrared phthalocyanine dye. Our target is type 2 cannabinoid receptor (CB2R), considered an attractive therapeutic target for phototherapy given it is overexpressed by many types of cancers that are located at a surface or can be reached by an endoscope. We show that our CB2R-targeted phototherapy agent, IR700DX-mbc94, is specific for CB2R and effective only when bound to the target receptor. Overall, this opens up the opportunity for development of an alternative treatment option for CB2R-positive cancers.

Lasers as an approach for promoting drug delivery via skin

INTRODUCTION

Using lasers can be an effective drug permeation-enhancement approach for facilitating drug delivery into or across the skin. The controlled disruption and ablation of the stratum corneum (SC), the predominant barrier for drug delivery, is achieved by the use of lasers. The possible mechanisms of laser-assisted drug permeation are the direct ablation of the skin barrier, optical breakdown by a photomechanical wave and a photothermal effect. It has been demonstrated that ablative approaches for enhancing drug transport provide some advantages, including increased bioavailability, fast treatment time, quick recovery of SC integrity and the fact that skin surface contact is not needed. In recent years, the concept of using laser techniques to treat the skin has attracted increasing attention.

AREAS COVERED

This review describes recent developments in using nonablative and ablative lasers for drug absorption enhancement. This review systematically introduces the concepts and enhancement mechanisms of lasers, highlighting the potential of this technique for greatly increasing drug absorption via the skin. Lasers with different wavelengths and types are employed to increase drug permeation. These include the ruby laser, the erbium:yttrium-gallium-garnet laser, the neodymium-doped yttrium-aluminum-garnet laser and the CO2 laser. Fractional modality is a novel concept for promoting topical/transdermal drug delivery. The laser is useful in enhancing the permeation of a wide variety of permeants, such as small-molecule drugs, macromolecules and nanoparticles.

EXPERT OPINION

This potential use of the laser affords a new treatment for topical/transdermal application with significant efficacy. Further studies using a large group of humans or patients are needed to confirm and clarify the findings in animal studies. Although the laser fluence or output energy used for enhancing drug absorption is much lower than for treatment of skin disorders and rejuvenation, the safety of using lasers is still an issue. Caution should be used in optimizing the feasible conditions of the lasers in balancing the effectiveness of permeation enhancement and skin damage.

Photosensitizer fluorescence and singlet oxygen luminescence as dosimetric predictors of topical 5-aminolevulinic acid photodynamic therapy induced clinical erythema

The need for patient-specific photodynamic therapy (PDT) in dermatologic and oncologic applications has triggered several studies that explore the utility of surrogate parameters as predictive reporters of treatment outcome. Although photosensitizer (PS) fluorescence, a widely used parameter, can be viewed as emission from several fluorescent states of the PS (e.g., minimally aggregated and monomeric), we suggest that singlet oxygen luminescence (SOL) indicates only the active PS component responsible for the PDT. Here, the ability of discrete PS fluorescence-based metrics (absolute and percent PS photobleaching and PS re-accumulation post-PDT) to predict the clinical phototoxic response (erythema) resulting from 5-aminolevulinic acid PDT was compared with discrete SOL (DSOL)-based metrics (DSOL counts pre-PDT and change in DSOL counts pre/post-PDT) in healthy human skin. Receiver operating characteristic curve (ROC) analyses demonstrated that absolute fluorescence photobleaching metric (AFPM) exhibited the highest area under the curve (AUC) of all tested parameters, including DSOL based metrics. The combination of dose-metrics did not yield better AUC than AFPM alone. Although sophisticated real-time SOL measurements may improve the clinical utility of SOL-based dosimetry, discrete PS fluorescence-based metrics are easy to implement, and our results suggest that AFPM may sufficiently predict the PDT outcomes and identify treatment nonresponders with high specificity in clinical contexts.

In vivo and in vitro evaluation of the cytotoxic effects of Photosan-loaded hollow silica nanoparticles on liver cancer

This study aimed to compare the inhibitory effects of photosensitizers loaded in hollow silica nanoparticles and conventional photosensitizers on HepG2 human hepatoma cell proliferation and determine the underlying mechanisms. Photosensitizers (conventional Photosan-II or nanoscale Photosan-II) were administered to in vitro cultured HepG2 hepatoma cells and treated by photodynamic therapy (PDT) with various levels of light exposure. To assess photosensitizers' effects, cell viability was determined by 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. In addition, apoptotic and necrotic cells were measured by flow cytometry and the expression of caspase-3 and caspase-9 evaluated by western blot. Finally, the in vivo effects of nanoscale and conventional photosensitizers on liver cancer were assessed in nude mice. Nanoscale Photosan-II significantly inhibited hepatoma cell viability in a concentration-dependent manner and this effect was more pronounced with high laser doses. Moreover, nanoscale photosensitizers performed better than the conventional ones under the same experimental conditions (p < 0.05). Flow cytometry data demonstrated that laser-induced cell death was markedly increased after treatment with nanoscale Photosan-II in comparison with free Photosan-II (p < 0.05). Activated caspase-3 and caspase-9 levels were significantly higher in cells treated with Photosan-II loaded in silica nanoparticles than free Photosan-II (p < 0.05). Accordingly, treatment with nanoscale photosensitizers resulted in improved outcomes (tumor volume) in a mouse model of liver cancer, in comparison with conventional photosensitizers. Hollow silica nanoparticles containing photosensitizer more efficiently inhibited hepatoma cells than photosensitizer alone, through induction of apoptosis, both in vivo and in vitro.

Phototriggerable liposomes: current research and future perspectives

The field of cancer nanomedicine is considered a promising area for improved delivery of bioactive molecules including drugs, pharmaceutical agents and nucleic acids. Among these, drug delivery technology has made discernible progress in recent years and the areas that warrant further focus and consideration towards technological developments have also been recognized. Development of viable methods for on-demand spatial and temporal release of entrapped drugs from the nanocarriers is an arena that is likely to enhance the clinical suitability of drug-loaded nanocarriers. One such approach, which utilizes light as the external stimulus to disrupt and/or destabilize drug-loaded nanoparticles, will be the discussion platform of this article. Although several phototriggerable nanocarriers are currently under development, I will limit this review to the phototriggerable liposomes that have demonstrated promise in the cell culture systems at least (but not the last). The topics covered in this review include (i) a brief summary of various phototriggerable nanocarriers; (ii) an overview of the application of liposomes to deliver payload of photosensitizers and associated technologies; (iii) the design considerations of photoactivable lipid molecules and the chemical considerations and mechanisms of phototriggering of liposomal lipids; (iv) limitations and future directions for in vivo, clinically viable triggered drug delivery approaches and potential novel photoactivation strategies will be discussed.

Highly deformable and highly fluid vesicles as potential drug delivery systems: theoretical and practical considerations

Vesicles that are specifically designed to overcome the stratum corneum barrier in intact skin provide an efficient transdermal (systemic or local) drug delivery system. They can be classified into two main groups according to the mechanisms underlying their skin interaction. The first group comprises those possessing highly deformable bilayers, achieved by incorporating edge activators to the bilayers or by mixing with certain hydrophilic solutes. The vesicles of this group act as drug carriers that penetrate across hydrophilic pathways of the intact skin. The second group comprises those possessing highly fluid bilayers, owing to the presence of permeation enhancers. The vesicles of this group can act as carriers of drugs that permeate the skin after the barrier of the stratum corneum is altered because of synergistic action with the permeation enhancers contained in the vesicle structure. We have included a detailed overview of the different mechanisms of skin interaction and discussed the most promising preclinical applications of the last five years of Transfersomes® (IDEA AG, Munich, Germany), ethosomes, and invasomes as carriers of antitumoral and anti-inflammatory drugs applied by the topical route.

Photodynamic agents with anti-metastatic activities

A new concept in multifunctional anticancer agents is demonstrated. Tetrakis-(diisopropyl-guanidino) zinc phthalocyanine (Zn-DIGP) exhibits excellent properties as a photodynamic therapy (PDT) agent, as well as potential anti-metastatic activities in vivo. Zn-DIGP exhibits good cellular uptake and low toxicity in the dark (EC50 > 80 μM) and is well tolerated upon its intravenous injection into mice at 8 mg/kg. Upon photoexcitation with red laser light (660 nm), Zn-DIGP exhibits a high quantum yield for singlet oxygen formation (Φ ≈ 0.51) that results in potent phototoxicity to cell cultures (EC50 ≈ 0.16 μM). Zn-DIGP is also capable of inhibiting the formation of tumor colonies in the lungs of C57BL/6 mice injected with B16F10 cells. Zn-DIGP therefore inhibits cancer growth by both light-dependent and light-independent pathways. The anti-metastatic activities of Zn-DIGP possibly result from its ability to interfere with the signaling between chemokine CXCL10 and the G protein-coupled receptor CXCR3. Zn-DIGP is a competitive inhibitor of CXCR3 activation (IC50 = 3.8 μM) and selectively inhibits downstream events such as CXCL10-activated cell migration. Consistent with the presence of feedback regulation between CXCR3 binding and CXCL10 expression, Zn-DIGP causes overexpression of CXCL10. Interestingly, Zn-DIGP binds to CXCR3 without activating the receptor yet is able to cause endocytosis and degradation of this GPCR. To the best of our knowledge, Zn-DIGP is the first PDT agent that can facilitate the photodynamic treatment of primary tumors while simultaneously inhibiting the formation of metastatic tumor colonies by a light-independent mode of action.

Triple-functional core-shell structured upconversion luminescent nanoparticles covalently grafted with photosensitizer for luminescent, magnetic resonance imaging and photodynamic therapy in vitro

Upconversion luminescent nanoparticles (UCNPs) have been widely used in many biochemical fields, due to their characteristic large anti-Stokes shifts, narrow emission bands, deep tissue penetration and minimal background interference. UCNPs-derived multifunctional materials that integrate the merits of UCNPs and other functional entities have also attracted extensive attention. Here in this paper we present a core-shell structured nanomaterial, namely, NaGdF(4):Yb,Er@CaF(2)@SiO(2)-PS, which is multifunctional in the fields of photodynamic therapy (PDT), magnetic resonance imaging (MRI) and fluorescence/luminescence imaging. The NaGdF(4):Yb,Er@CaF(2) nanophosphors (10 nm in diameter) were prepared via sequential thermolysis, and mesoporous silica was coated as shell layer, in which photosensitizer (PS, hematoporphyrin and silicon phthalocyanine dihydroxide) was covalently grafted. The silica shell improved the dispersibility of hydrophobic PS molecules in aqueous environments, and the covalent linkage stably anchored the PS molecules in the silica shell. Under excitation at 980 nm, the as-fabricated nanomaterial gave luminescence bands at 550 nm and 660 nm. One luminescent peak could be used for fluorescence imaging and the other was suitable for the absorption of PS to generate singlet oxygen for killing cancer cells. The PDT performance was investigated using a singlet oxygen indicator, and was investigated in vitro in HeLa cells using a fluorescent probe. Meanwhile, the nanomaterial displayed low dark cytotoxicity and near-infrared (NIR) image in HeLa cells. Further, benefiting from the paramagnetic Gd(3+) ions in the core, the nanomaterial could be used as a contrast agent for magnetic resonance imaging (MRI). Compared with the clinical commercial contrast agent Gd-DTPA, the as-fabricated nanomaterial showed a comparable longitudinal relaxivities value (r(1)) and similar imaging effect.

Oxidative stress effects of fullerene-porphyrin derivatives in photodynamic therapy

Due to their special and growing medical recent interest, the fullerenes started to be a very studied class of chemical compounds. In order to improve their water solubility and to reduce their cytotoxic characteristics, the fullerenes have been coupled in a system fullerene/PVP/porphyrin (C60/PVP/TPP) and its application in photodynamic therapy will be evaluated in this paper. The oxidative stress effects on photodynamic therapy with systems fullerene/poly-N-vinylpirrolidone/5,10,15,20-tetrakis(4-phenyl)porphyrin (C60/PVP/TPP) were tested on Wistar rats sub-cutaneously inoculated with Walker 256 carcinoma. The animals were irradiated with red light (λ = 685 nm; D = 50 J/cm2; 15 minutes) 24 h after intra-peritoneal administration of 10 mg/kg body weight of the system C60/PVP/TPP. After photodynamic therapy, the free radicals in tumors have been indirectly evaluated by lipid peroxides level (measured as thiobarbituric reactive substances) and protein carbonyls (indices of oxidative effects produced on susceptible biomolecules), both of them increasing in tumor tissues of animals 24 h after treatment. The levels of thiol groups and total antioxidant capacity have been determined in tumors, too, their decreasing values being the effect of the strong tumoral oxidative process.

Biomodulatory approaches to photodynamic therapy for solid tumors

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

Liposomes in topical photodynamic therapy

INTRODUCTION

Topical photodynamic therapy (PDT) refers to topical application of a photosensitizer onto the site of skin disease which is followed by illumination and results in death of selected cells. The main problem in topical PDT is insufficient penetration of the photosensitizer into the skin, which limits its use to superficial skin lesions. In order to overcome this problem, recent studies tested liposomes as delivery systems for photosensitizers.

AREAS COVERED

This paper reviews the use of different types of liposomes for encapsulating photosensitizers for topical PDT. Liposomes should enhance the photosensitizers' penetration into the skin, while decreasing its absorption into systemic circulation. Only few photosensitizers have currently been encapsulated in liposomes for topical PDT: 5-aminolevulinic acid (5-ALA), temoporfin (mTHPC) and methylene blue.

EXPERT OPINION

Investigated liposomes enhanced the skin penetration of 5-ALA and mTHPC, reduced their systemic absorption and reduced their cytotoxicity compared with free drugs. Their high tissue penetration should enable the treatment of deep and hyperkeratotic skin lesions, which is the main goal of using liposomes. However, liposomes still do not attract enough attention as drug carriers in topical PDT. In vivo studies of their therapeutic effectiveness are needed in order to obtain enough evidence for their potential clinical use as carriers for photosensitizers in topical PDT.

Intrinsic therapeutic applications of noble metal nanoparticles: past, present and future

Biomedical nanotechnology is an evolving field having enormous potential to positively impact the health care system. Important biomedical applications of nanotechnology that may have potential clinical applications include targeted drug delivery, detection/diagnosis and imaging. Basic understanding of how nanomaterials, the building blocks of nanotechnology, interact with the cells and their biological consequences are beginning to evolve. Noble metal nanoparticles such as gold, silver and platinum are particularly interesting due to their size and shape dependent unique optoelectronic properties. These noble metal nanoparticles, particularly of gold, have elicited a lot of interest for important biomedical applications because of their ease of synthesis, characterization and surface functionalization. Furthermore, recent investigations are demonstrating another promising application of these nanomaterials as self-therapeutics. To realize the potential promise of these unique inorganic nanomaterials for future clinical translation, it is of utmost importance to understand a few critical parameters; (i) how these nanomaterials interact with the cells at the molecular level; (ii) how their biodistribution and pharmacokinetics influenced by their surface and routes of administration; (iii) mechanism of their detoxification and clearance and (iv) their therapeutic efficacy in appropriate disease model. Thus in this critical review, we will discuss the various clinical applications of gold, silver and platinum nanoparticles with relevance to above parameters. We will also mention various routes of synthesis of these noble metal nanoparticles. However, before we discuss present research, we will also look into the past. We need to understand the discoveries made before us in order to further our knowledge and technological development (318 references).

Fluorescence localization and kinetics of mTHPC and liposomal formulations of mTHPC in the window-chamber tumor model

BACKGROUND AND OBJECTIVE

Foslip® and Fospeg® are liposomal formulations of the photosensitizer mTHPC, intended for use in Photodynamic Therapy (PDT) of malignancies. Foslip consists of mTHPC encapsulated in conventional liposomes, Fospeg consists of mTHPC encapsulated in pegylated liposomes. Possible differences in tumor fluorescence and vasculature kinetics between Foslip, Fospeg, and Foscan® were studied using the rat window-chamber model.

MATERIAL AND METHODS

In 18 rats a dorsal skin fold window chamber was installed and a mammary carcinoma was transplanted in the subcutaneous tissue. The dosage used for intravenous injection was 0.15 mg/kg mTHPC for each formulation. At seven time-points after injection (5 minutes to 96 hours) fluorescence images were made with a CCD. The achieved mTHPC fluorescence images were corrected for tissue optical properties and autofluorescence by the ratio fluorescence imaging technique of Kascakova et al. Fluorescence intensities of three different regions of interest (ROI) were assessed; tumor tissue, vasculature, and surrounding connective tissue.

RESULTS

The three mTHPC formulations showed marked differences in their fluorescence kinetic profile. After injection, vascular mTHPC fluorescence increased for Foslip and Fospeg but decreased for Foscan. Maximum tumor fluorescence is reached at 8 hours for Fospeg and at 24 hours for Foscan and Foslip with overall higher fluorescence for both liposomal formulations. Foscan showed no significant difference in fluorescence intensity between surrounding tissue and tumor tissue (selectivity). However, Fospeg showed a trend toward tumor selectivity at early time points, while Foslip reached a significant difference (P < 0.05) at these time points.

CONCLUSIONS

Our results showed marked differences in fluorescence intensities of Fospeg, Foslip, and Foscan, which suggest overall higher bioavailability for the liposomal formulations. Pegylated liposomes seemed most promising for future application; as Fospeg showed highest tumor fluorescence at the earlier time points.

Mechanisms of photodynamic inactivation of a gram-negative recombinant bioluminescent bacterium by cationic porphyrins

Photodynamic therapy is a very promising approach to inactivate pathogenic microorganisms. The photodamage of cells involves reactive oxygen species (ROS) which are generated in situ by two main mechanisms (type I and/or type II). The mechanism responsible for the photoinactivation (PI) of a bioluminescent recombinant Escherichia coli, induced by three different cationic porphyrins, was identified in this work using a rapid method based on the monitoring of the metabolic activity of this bacterium. The inhibitory effect of the photodynamic process in the presence of a singlet oxygen quencher (sodium azide) or free radical scavengers (d-mannitol and l-cysteine) was evaluated by exposing bacterial suspensions with 0.5 μM Tri-Py(+)-Me-PF, 5.0 μM Tetra-Py(+)-Me or 5.0 μM Tri-SPy(+)-Me-PF to white light. Strong bacterial protection was observed with sodium azide (100 mM) for the three cationic porphyrins. However, in the presence of Tri-Py(+)-Me-PF and Tetra-Py(+)-Me and the free radical scavengers (l-cysteine and d-mannitol) the reduction on the bacterial bioluminescence was significantly higher and similar to that obtained in their absence (5.4-6.0 log reduction). In the case of Tri-SPy(+)-Me-PF two distinct behaviours were observed when l-cysteine and d-mannitol were used as free radical scavengers: while the presence of l-cysteine (100 mM) lead to a bacterial protection similar to the one observed with sodium azide, in the presence of d-mannitol only a small protection was detected. The high inhibition of the PS activity by l-cysteine is not due to its radical scavenger ability but due to the singlet oxygen quenching by the sulfanyl group (-SH). In fact, the photodecomposition of 1,3-diphenylisobenzofuran in the presence of Tri-SPy(+)-Me-PF is completely suppressed when l-cysteine is present. The results obtained in this study suggest that singlet oxygen (type II mechanism) plays a very important role over free radicals (type I mechanism) on the PI process of the bioluminescent E. coli by Tri-Py(+)-Me-PF, Tetra-Py(+)-Me and Tri-SPy(+)-Me-PF. Although the use of scavengers is an adequate and simple approach to evaluate the relative importance of the two pathways, it is important to choose scavengers which do not interfere in both PI mechanisms. Sodium azide and d-mannitol seem to be good oxygen and free radical quenchers, respectively, to study the PI mechanisms by porphyrinic photosensitizers.

Retained topical delivery of 5-aminolevulinic acid using cationic ultradeformable liposomes for photodynamic therapy

5-Aminolevulinic acid (5-ALA), inducing photodynamic protoporphyrin (PpIX), is a hydrophilic molecule, resulting in leashing the capacity to cross tissue barriers like stratum corneum (SC) of skin. Here, we aimed to develop 5-ALA loaded ultradeformable liposomes (UDL) with different surface charges, and to investigate their physicochemical characteristics and capability for the skin penetration and retention of 5-ALA for topical photodynamic therapy (PDT). The effects of surface charges of UDL on in vitro permeation of 5-ALA and in vivo accumulation of 5-ALA-induced PpIX in viable skin were determined and then compared with conventional neutral liposomes (nLiposome). All UDL showed smaller particle size and better deformability than nLiposome. However, entrapment efficiency of 5-ALA was similar to each vesicle. Among vesicles, the cationic UDL (cUDL) demonstrated higher stability and permeability, and could deliver 5-ALA into deep skin tissue by topical application. Moreover, the 5-ALA loaded in cUDL was long retained, and induced more amount of PpIX in viable skin than those in other UDL and nLiposome. Considering that the conversion of 5-ALA into PpIX occurs preferentially in epidermis, these results suggested that topical delivery of 5-ALA loaded in cUDL could be an interesting proposal to optimize PDT related to 5-ALA.

Photodynamic therapy of cancer: an update

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

The in vitro PDT efficacy of a novel metallophthalocyanine (MPc) derivative and established 5-ALA photosensitizing dyes against human metastatic melanoma cells

BACKGROUND AND OBJECTIVE

Numerous worldwide clinical trials have shown that photodynamic therapy (PDT) represents an effective and safe modality for various skin disorders, but little research has been done in terms of its effect on malignant melanomas (MM). Thus, the aim of this study was to compare the effect of both established porphyrin photosensitizer 5-aminolevulinic acid (5-ALA) and novel metallophthalocyanine (MPc) photosensitizer on human metastatic skin cells which produce a MM.

MATERIALS AND METHODS

The cellular responses following PDT were assessed using changes in cell morphology, cell viability, cytotoxicity, apoptosis, and proliferation.

RESULTS

Findings reported that in vitro human MM cell line A375 (EACC no: 88113005) are highly sensitive to growth inhibition and apoptosis induction by the cytotoxic side-effects induced by MPc and 5-ALA photosensitizing treatments post-laser irradiation at 680 and 636 nm, respectively. The decrease of cell viability accompanied by an increased cytotoxicity and apoptotic and necrotic levels, with a time-dependant decrease in cellular proliferation was found to be far more significant for MPc-treated cells than 5-ALA-treated cells, since MPc was applied in far lower concentrations and exhibited far less photoxicity to control cells.

CONCLUSION

Hence, novel MPc proved to be the better photosensitizing dye for metastatic melanoma tumor destruction in combination with laser irradiation and is a particularly attractive photosensitizer since it exhibits so many ideal properties of a photosensitizing agent, thus further research of this possible anticancer agent could contribute to its potential application in PDT cancer treatment of MMs.

Photosensitizers for photodynamic therapy: One-pot heterogeneous catalytic transfer reduction of porphyrins

A number of new porphyrin-based photosensitizers have been developed for Photodynamic Therapy (PDT) in recent years. Chlorins, which are a reduced form of porphyrins, show better potential of application since they have a stronger absorption band on the red region of the visible spectrum and, hence, a deeper penetration into tissues. We found that by using heterogeneous catalytic transfer reduction (CTR), meso-tetraphenylporphyrin (TPP) could be hydrogenated, although in modest yields, to meso-tetraphenylchlorin (TPC) in a single reaction step. Best reaction conditions were attained using formic acid or sodium phosphinate/water as hydrogen donors, tetrahydrofuran (THF) or toluene as solvent and 10% palladium on charcoal as catalyst.

Aminolevulinic acid-photodynamic therapy combined with topically applied vascular disrupting agent vadimezan leads to enhanced antitumor responses

The tumor vascular-disrupting agent (VDA) vadimezan (5,6-dimethylxanthenone-4-acetic acid, DMXAA) has been shown to potentiate the antitumor activity of photodynamic therapy (PDT) using systemically administered photosensitizers. Here, we characterized the response of subcutaneous syngeneic Colon26 murine colon adenocarcinoma tumors to PDT using the locally applied photosensitizer precursor aminolevulinic acid (ALA) in combination with a topical formulation of vadimezan. Diffuse correlation spectroscopy (DCS), a noninvasive method for monitoring blood flow, was utilized to determine tumor vascular response to treatment. In addition, correlative CD31-immunohistochemistry to visualize endothelial damage, ELISA to measure induction of tumor necrosis factor-alpha (TNF-α) and tumor weight measurements were also examined in separate animals. In our previous work, DCS revealed a selective decrease in tumor blood flow over time following topical vadimezan. ALA-PDT treatment also induced a decrease in tumor blood flow. The onset of blood flow reduction was rapid in tumors treated with both ALA-PDT and vadimezan. CD31-immunostaining of tumor sections confirmed vascular damage following topical application of vadimezan. Tumor weight measurements revealed enhanced tumor growth inhibition with combination treatment compared with ALA-PDT or vadimezan treatment alone. In conclusion, vadimezan as a topical agent enhances treatment efficacy when combined with ALA-PDT. This combination could be useful in clinical applications.

Oil components modulate the skin delivery of 5-aminolevulinic acid and its ester prodrug from oil-in-water and water-in-oil nanoemulsions

The study evaluated the potential of nanoemulsions for the topical delivery of 5-aminolevulinic acid (ALA) and methyl ALA (mALA). The drugs were incorporated in oil-in-water (O/W) and water-in-oil (W/O) formulations obtained by using soybean oil or squalene as the oil phase. The droplet size, zeta potential, and environmental polarity of the nanocarriers were assessed as physicochemical properties. The O/W and W/O emulsions showed diameters of 216-256 and 18-125 nm, which, respectively, were within the range of submicron- and nano-sized dispersions. In vitro diffusion experiments using Franz-type cells and porcine skin were performed. Nude mice were used, and skin fluorescence derived from protoporphyrin IX was documented by confocal laser scanning microscopy (CLSM). The loading of ALA or mALA into the emulsions resulted in slower release across cellulose membranes. The release rate and skin flux of topical drug application were adjusted by changing the type of nanocarrier, the soybean oil O/W systems showing the highest skin permeation. This formulation increased ALA flux via porcine skin to 180 nmol/cm(2)/h, which was 2.6-fold that of the aqueous control. The CLSM results showed that soybean oil systems promoted mALA permeation to deeper layers of the skin from ∼100 μm to ∼140 μm, which would be beneficial for treating subepidermal and subcutaneous lesions. Drug permeation from W/O systems did not surpass that from the aqueous solution. An in vivo dermal irritation test indicated that the emulsions were safe for topical administration of ALA and mALA.

Light-sensitive lipid-based nanoparticles for drug delivery: design principles and future considerations for biological applications

Radiation-based therapies aided by nanoparticles have been developed for decades, and can be primarily categorized into two main platforms. First, delivery of payload of photo-reactive drugs (photosensitizers) using the conventional nanoparticles, and second, design and development of photo-triggerable nanoparticles (primarily liposomes) to attain light-assisted on-demand drug delivery. The main focus of this review is to provide an update of the history, current status and future applications of photo-triggerable lipid-based nanoparticles (light-sensitive liposomes). We will begin with a brief overview on the applications of liposomes for delivery of photosensitizers, including the choice of photosensitizers for photodynamic therapy, as well as the currently available light sources (lasers) used for these applications. The main segment of this review will encompass the details of strategies used to develop photo-triggerable liposomes for their drug delivery function. The principles underlying the assembly of photoreactive lipids into nanoparticles (liposomes) and photo-triggering mechanisms will be presented. We will also discuss factors that limit the applications of these liposomes for in vivo triggered drug delivery and emerging concepts that may lead to the biologically viable photo-activation strategies. We will conclude with our view point on the future perspectives of light-sensitive liposomes in the clinic.

Novel patch-based systems for the localised delivery of ALA-esters

In photodynamic therapy (PDT) a combination of visible light and a sensitising drug causes the destruction of selected cells. Aminolaevulinic acid (ALA) has been widely used in topical PDT for over 15 years. However, ALA does not possess favourable physicochemical properties for skin penetration. Consequently, the clearance rates for difficult to treat lesions, such as nodular basal cell carcinomas are relatively low. For the first time, equimolar concentrations of ALA, methyl-ALA (m-ALA) and hexyl-ALA (h-ALA) have been incorporated into a bioadhesive patch-based system. In vitro penetration studies into excised porcine skin revealed that ALA patches containing relatively high loadings (226.7 micromol cm(-2)) were associated with significantly greater tissue concentrations (70.7 micromol cm(-3)) than patches containing m-ALA (16.3 micromol cm(-3)) or h-ALA (17.4 micromol cm(-3)). ALA was also found to be the most efficient inducer of protoporphyrin (PpIX) fluorescence in mice, in vivo (maximum mean fluorescence: ALA=236.2 a.u., m-ALA=175.1 a.u., h-ALA=193.5 a.u.). However, when the lipophilic hexylester was formulated in a pressure sensitive adhesive (PSA) patch, significantly higher PpIX levels were achieved compared to all bioadhesive systems tested. Of major importance, PSA patches containing relatively low h-ALA loadings induced high PpIX levels, which were localised to the application area. This study has highlighted the importance of rational selection of both the active agent and the delivery system. Bioadhesive preparations containing ALA are ideal for delivery to moist environments; whereas h-ALA-loaded PSA systems may facilitate enhanced delivery to dry areas of skin. In addition, owing to the relatively low loadings of h-ALA required in PSA patches, the costs of clinical PDT may potentially be reduced.