Photodynamic therapy against intracellular pathogens: Problems and potentials

A Systematic Evaluation of Curcumin Concentrations and Blue Light Parameters towards Antimicrobial Photodynamic Therapy against Cariogenic Microorganisms

Dental caries is a highly preventable and costly disease. Unfortunately, the current management strategies are inadequate at reducing the incidence and new minimally invasive strategies are needed. In this study, a systematic evaluation of specific light parameters and aqueous curcumin concentrations for antimicrobial photodynamic therapy (aPDT) was conducted. Aqueous solutions of curcumin were first prepared and evaluated for their light absorbance after applying different ~56 mW/cm2 blue light treatments in a continuous application mode. Next, these same light treatments as well as different application modes were applied to the curcumin solutions and the molar absorptivity coefficient, reactive oxygen species (ROS) release, minimum inhibitory concentration (MIC), and minimum bactericidal concentration (MBC) for Streptococcus mutans and the MIC and minimum fungicidal concentration (MFC) for Candida albicans were measured. After up to 1 min of light treatment, the molar absorptivity of curcumin when added to culture media was lower than that for water only; however, at higher energy levels, this difference was not apparent. There was a noted dependence on both ROS type and cariogenic microorganism species on the sensitivity to both blue light treatment and application mode. In conclusion, this study provides new information towards improving the agonistic potential of aPDT associated with curcumin against cariogenic microorganisms.

Antimicrobial photodynamic therapy effectively reduces Porphyromonas gingivalis infection in gingival fibroblasts and keratinocytes: An in vitro study

BACKGROUND

Porphyromonas gingivalis possess the ability to invade host cells which prevents this pathogen from eradication by conventional periodontal therapy. Recently, antimicrobial photodynamic therapy (aPDT) was introduced to periodontal treatment as a complementary antibacterial method. The aim of this study was to evaluate the effect of toluidine blue-O (TBO) mediated aPDT on the viability of P. gingivalis invading gingival fibroblasts and keratinocytes in an in vitro model of infection.

METHODS

Primary human gingival fibroblasts (PHGF) and telomerase immortalized gingival keratinocytes (TIGK) were infected with Pg ATCC 33277. Two concentrations of TBO (0.01 mg/mL, TBO-c1 and 0.001 mg/mL, TBO-c2) and a non-laser red light source (λ = 630 nm) were applied to treat both cell-adherent/intracellular Pg (the adhesion/invasion model) or exclusively the intracellular bacteria (the intracellular infection model).

RESULTS

The median viability of cell-adherent/intracellular Pg in infected keratinocytes declined from 1.88 × 10⁵ cfu/mL in infected cells treated with TBO without irradiation to 40 cfu/mL upon irradiation for 10 s with TBO-c1. At higher light doses a complete photokilling of P. gingivalis was observed. Pg from exclusively intracellular infection model was also efficiently eradicated as the residual viability dropped from 1.44 × 10⁵ cfu/mL in control samples to 160, 20 and 10 cfu/mL upon irradiation for 10, 20 and 30 s, respectively. In the infected fibroblasts irradiation significantly reduced bacterial viability but did not completely eradicate the intracellular pathogen.

CONCLUSIONS

Antimicrobial PDT is effective in reducing the viability of intracellular periopathogens, however those residing within gingival fibroblasts seems to attenuate the photokilling effectiveness of this method.

Synergistic Photoantimicrobial Chemotherapy of Methylene Blue-Encapsulated Chitosan on Biofilm-Contaminated Titanium

Intensive efforts have been made to eliminate or substantial reduce bacterial adhesion and biofilm formation on titanium implants. However, in the management of peri-implantitis, the methylene blue (MB) photosensitizer commonly used in photoantimicrobial chemotherapy (PACT) is limited to a low retention on the implant surface. The purpose of this study was to assess enhancive effect of water-soluble quaternary ammonium chitosan (QTS) on MB retention on biofilm-infected SLA (sandblasted, large grid, and acid-etched) Ti alloy surfaces in vitro. The effectiveness of QTS + MB with different concentrations in eliminating Gram-negative A. actinomycetemcomitans or Gram-positive S. mutans bacteria was compared before and after PACT. Bacterial counting and lipopolysaccharide (LPS) detection were examined, and then the growth of human osteoblast-like MG63 cells was evaluated. The results indicated that the synergistic QTS + MB with retention ability significantly decreased the biofilm accumulation on the Ti alloy surface, which was better than the same concentration of 1 wt% methyl cellulose (MC). More importantly, the osteogenic activity of MG63 cells on the disinfected sample treated by QTS + MB-PACT modality was comparable to that of sterile Ti control, significantly higher than that by MC + MB-PACT modality. It is concluded that, in terms of improved retention efficacy, effective bacteria eradication, and enhanced cell growth, synergistically, PACT using the 100 μg/mL MB-encapsulated 1% QTS was a promising modality for the treatment of peri-implantitis.

π-Extended Donor-Acceptor Porphyrins and Metalloporphyrins for Antimicrobial Photodynamic Inactivation

Free base, zinc and palladium π‐extended porphyrins containing fused naphthalenediamide units were employed as photosensitizers in antimicrobial photodynamic therapy (aPDT). Their efficacy, assessed by photophysical and in vitro photobiological studies on Gram‐positive bacteria, was found to depend on metal coordination, showing a dramatic enhancement of photosensitizing activity for the palladium complex.

Does pre-irradiation time influence the efficacy of antimicrobial photodynamic therapy?

Antimicrobial photodynamic therapy (aPDT) has emerged as a promising antimicrobial treatment to control microorganisms including those involved in oral diseases, especially dental caries. Hence, the aim of this study was to evaluate the influence of aPDT - pre-irradiation time (PIT), at different periods, on antimicrobial rate of Streptococcus mutans (S. mutans). A standard suspension of S. mutans UA159 was prepared and submitted at sensitization of 0.005 % methylene blue (MB) for 0, 1, 3 and 5 min (G1 - G4 groups, respectively) and irradiated with a red laser (660 nm; 321 J/cm²; 9 J; 90 s) afterward. A control group using PBS instead of MB was performed as well (G5). The number of colony-forming units (CFU)/mL was recorded, transformed into log₁₀ and analyzed by ANOVA and Tukey's test at a cutoff value at 0.05. Overall, the aPDT groups tested achieved a bacterial reduction > 1-log₁₀ when compared to G5 (p < 0.05) with no statistical difference among the different PIT tested. The need of PIT before aPDT application deserves attention, since its time reduction implies on shorter clinical approaches without compromising the photodynamic antibacterial efficacy in the in vitro parameters employed.

Photodynamic and peptide-based strategy to inhibit Gram-positive bacterial biofilm formation

The self-produced extracellular polymeric matrix of biofilms renders them difficult to eliminate once they are established. This makes the inhibition of biofilm formation key to successful treatment of biofilm infection. Antimicrobial photodynamic therapy (aPDT) and antimicrobial peptides offer a new approach as antibiofilm strategies. In this study sub-lethal doses of aPDT (with chlorin-e6 (Ce6-PDT) or methylene blue (MB-PDT)) and the peptides AU (aurein 1.2 monomer) or (AU)₂K (aurein 1.2 C-terminal dimer) were combined to evaluate their ability to prevent biofilm development by Enterococcus faecalis. Biofilm formation was assessed by resazurin reduction, confocal microscopy, and infrared spectroscopy. All treatments successfully prevented biofilm development. The (AU)₂K dimer had a stronger effect, both alone and combined with aPDT, while the monomer AU had significant activity when combined with Ce6-PDT. Additionally, it is shown that the peptides bind to the lipoteichoic acid of the E. faecalis cell wall, pointing to a possible key mechanism of biofilm inhibition.

Study of the efficacy of N-methyl glucamine antimoniate (SbV) associated with photodynamic therapy using liposomal chloroaluminium phthalocyanine in the treatment of cutaneous leishmaniasis caused by Leishmania (L.) amazonensis in C57BL6 mice

BACKGROUND

Pentavalent antimonials remain first-line drugs in the treatment of cutaneous leishmaniasis (CL); however, adverse effects and drug resistance have led to the search for less toxic and more effective treatments. As an alternative, topical phthalocyanine has been studied and its efficacy and low toxicity demonstrated. We aimed to study the in vivo efficacy of N-methyl glucamine antimoniate (NMG) associated with photodynamic therapy (PDT) with topical liposomal chloroaluminium phthalocyanine (AlClPC) in the treatment of experimental CL by L. amazonensis.

METHODS

Experimental study with 54 C57BL6 isogenic mice divided into 9 groups including uninfected control, untreated control, PDT with AlClPC + NMG at doses of 10 and 20 mgSb^V/Kg/day. The criteria to evaluate the treatment efficacy were: paw diameter, amastigote count, culture, viability test and parasite counts using MTT (3-bromo-4,5-dimethylthiazol-2,5-diphenyl-tetrazolium bromide).

RESULTS

Treatment of CL with the association of NMG20 + PDT with AlClPC showed significant reduction of paw diameter, amastigote count, cultures, viability test and parasite counts. Parasite reduction occurred at the 10th and 20th days of treatment and 60 days after treatment ended, indicating that parasites did not multiply again. The NMG10 + PDT group with AlClPC presented results equivalent to gold-standard treatment (20 mgSb^V/kg/day). Biochemical and histopathological evaluation showed minor changes.

CONCLUSION

Treatment of CL caused by L. amazonensis with NMG20 mgSb^V/kg/day + PDT with AlClPC was more effective than the traditional NMG20 mgSb^V/kg/day.

Effects of antibacterial photodynamic therapy on salivary mutans streptococci in 5- to 6-year-olds with severe early childhood caries

Antibacterial photodynamic therapy (A-PDT) has been shown to kill oral bacteria in the planktonic culture, dental plaque, and biofilm. This study sought to assess the antimicrobial effect of A-PDT with toluidine blue O (TBO) and diode laser on salivary mutans streptococci in 5-6-year-olds with severe early childhood caries (SECC). This case-control study was conducted on 56 children with SECC divided into four groups, namely 0.1 mg/mL TBO, diode laser (633 nm, 20 mW, 6 J/cm²), combination of the two, and no intervention control group. A-PDT was performed on days 1 and 3. Salivary samples were collected before and after A-PDT on days 1 and 3, and 1 and 2 weeks after the second intervention (day 3). Samples were cultured on mitis salivarius agar, and after incubation, the colonies were counted. Data were subjected to repeated measures ANOVA, ANCOVA, and paired comparisons with least square difference and Tukey's test. Bacterial count significantly decreased on days 1 and 3, and 1 and 2 weeks after the second intervention. Bacterial count also decreased following the use of TBO and laser separately, but these reductions were not significant (P > 0.05). Within the limitations of this study, antimicrobial efficacy of TBO + laser was higher than that of diode laser or TBO alone. Durability of treatment increased with double-dose therapy. This modality may be used to decrease the colony count of salivary mutans streptococci in children with SECC.

Design and validation of an open-source modular Microplate Photoirradiation System for high-throughput photobiology experiments

Research in photobiology is currently limited by a lack of devices capable of delivering precise and tunable irradiation to cells in a high-throughput format. This limits researchers to using expensive commercially available or custom-built light sources which make it difficult to replicate, standardize, optimize, and scale experiments. Here we present an open-source Microplate Photoirradiation System (MPS) developed to enable high-throughput light experiments in standard 96 and 24-well microplates for a variety of applications in photobiology research. This open-source system features 96 independently controlled LEDs (4 LEDs per well in 24-well), Wi-Fi connected control and programmable graphical user interface (GUI) for control and programming, automated calibration GUI, and modular control and LED boards for maximum flexibility. A web-based GUI generates light program files containing irradiation parameters for groups of LEDs. These parameters are then uploaded wirelessly, stored and used on the MPS to run photoirradiation experiments inside any incubator. A rapid and semi-quantitative porphyrin metabolism assay was also developed to validate the system in wild-type fibroblasts. Protoporphyrin IX (PpIX) fluorescence accumulation was induced by incubation with 5-aminolevulinic acid (ALA), a photosensitization method leveraged clinically to destroy malignant cell types in a process termed photodynamic therapy (PDT), and cells were irradiated with 405nm light with varying irradiance, duration and pulsation parameters. Immediately after light treatment with the MPS, subsequent photobleaching was measured in live, adherent cells in both 96-well and a 24-well microplates using a microplate reader. Results demonstrate the utility and reliability of the Microplate Photoirradiation System to irradiate cells with precise irradiance and timing parameters in order to measure PpIx photobleaching kinetics in live adherent cells and perform comparable experiments with both 24 and 96 well microplate formats. The high-throughput capability of the MPS enabled measurement of enough irradiance conditions in a single microplate to fit PpIX fluorescence to a bioexponential decay model of photobleaching, as well as reveal a dependency of photobleaching on duty-cycle-but not frequency-in a pulsed irradiance regimen.

Low-cost light-induced therapy to treat rickettsial infection

Antimicrobial photodynamic therapy uses a nontoxic photosensitizer with the assistance of harmless visible light to activate the photosensitizer. Consequently, the excited state of the photosensitizer interacts with molecular oxygen to produce reactive oxygen species, which have the antimicrobial effect. In this study, we evaluated the effect of photodynamic therapy on Vero cells infected with rickettsia using methylene blue as a photosensitizer along with red light. A significant reduction (by 96%) in the number of viable Rickettsia slovaca was determined by quantitative RT-PCR 48 h after the treatment with methylene blue followed by 30 min of red light excitation. A statistically significant reduction of R. slovaca was also recorded with pretreatment (by 99%). To the best of our knowledge, this result is the first one in the literature to confirm the effectiveness of photodynamic therapy for the elimination of R. slovaca and to suggest this technique as a good supportive treatment for rickettsial infections.

Antimicrobial Photodynamic therapy enhanced by the peptide aurein 1.2

In the past few years, the World Health Organization has been warning that the post-antibiotic era is an increasingly real threat. The rising and disseminated resistance to antibiotics made mandatory the search for new drugs and/or alternative therapies that are able to eliminate resistant microorganisms and impair the development of new forms of resistance. In this context, antimicrobial photodynamic therapy (aPDT) and helical cationic antimicrobial peptides (AMP) are highlighted for the treatment of localized infections. This study aimed to combine the AMP aurein 1.2 to aPDT using Enterococcus faecalis as a model strain. Our results demonstrate that the combination of aPDT with aurein 1.2 proved to be a feasible alternative capable of completely eliminating E. faecalis employing low concentrations of both PS and AMP, in comparison with the individual therapies. Aurein 1.2 is capable of enhancing the aPDT activity whenever mediated by methylene blue or chlorin-e6, but not by curcumin, revealing a PS-dependent mechanism. The combined treatment was also effective against different strains; noteworthy, it completely eliminated a vancomycin-resistant strain of Enterococcus faecium. Our results suggest that this combined protocol must be exploited for clinical applications in localized infections as an alternative to antibiotics.

Antibacterial photodynamic therapy with 808-nm laser and indocyanine green on abrasion wound models

Infections with pathogens could cause serious health problems, such as septicemia and subsequent death. Some of these deaths are caused by nosocomial, chronic, or burn-related wound infections. Photodynamic therapy (PDT) can be useful for the treatment of these infections. Our aim was to investigate the antibacterial effect of indocyanine green (ICG) and 808-nm laser on a rat abrasion wound model infected with the multidrug resistant Staphylococcus aureus strain. Abrasion wounds were infected with a multidrug resistant clinical isolate of S. aureus. ICG concentrations of 500, 1000, and 2000 μg∕ml were applied with a 450 J∕cm2 energy dose. Temperature change was monitored by a thermocouple system. The remaining bacterial burden was determined by the serial dilution method after each application. Wounds were observed for 11 days posttreatment. The recovery process was assessed macroscopically. Tissue samples were also examined histologically by hematoxylin–eosin staining. Around a 90% reduction in bacterial burden was observed after applications. In positive control groups (ICG-only and laser-only groups), there was no significant reduction. The applied energy dose did not cause any thermal damage to the target tissue or host environment. Results showed that ICG together with a 808-nm laser might be a promising antibacterial method to eliminate infections in animals and accelerate the wound-healing process.

Photodynamic potential of curcumin and blue LED against Streptococcus mutans in a planktonic culture

BACKGROUND

The photodynamic therapy (PDT) involves the use of light of specific wavelength to activate a nontoxic photosensitizing agent or dye in the presence of oxygen for eradication of target cells. In dentistry, this therapy is used to suppress the growth of microorganisms involved directly with dental decay and periodontitis process. There are evidences that curcumin dye is able to control microbial activity when illuminated with specific wavelength. The purpose of this study was to evaluate the in vitro efficacy of PDT using curcumin dye (Cur-C) in combination with a blue LED (L) device on a planktonic model of Streptococcus mutans (S. mutans).

METHODS

Suspensions (0.5 mL) containing S. mutans at 1×10(7)CFU mL(-1) were prepared and divided into 4 groups: Group C-L- (control: no treatment and 1 experimental condition), Group C+L- (curcumin at 3 different concentrations: 2000; 4000 and 8000 μM and 3 experimental conditions), Group C-L+ (LED at 3 different dosages: 24, 48 and 72 Jcm(-2) and 3 experimental conditions), and Group C+L+ (PDT group: curcumin at respective concentrations combined to LED dosages and 9 experimental conditions). Samples of each experimental condition were cultured in Petri dishes of BHI agar. Incubation in micro-aerophilia at 37°C for 48 h was performed for subsequent visual counting of CFU/mL. Data were transformed into log10 and analyzed by two-way ANOVA and Tukey's test at p<0.05.

RESULTS

Group C+L+, in specific experimental conditions, demonstrated a log bacterial reduction 70% higher than Group C-L-. Both groups C-L+ and C+L- presented a slight decrease in log bacterial counting.

CONCLUSION

This in vitro method was able to reduce the number of S. mutans in a planktonic suspension.

Antimicrobial photodynamic therapy of resistant bacterial strains by indocyanine green and 809-nm diode laser

OBJECTIVE

This research aimed to investigate the bactericidal effect of indocyanine green (ICG) with 809-diode laser on wild type and resistant strains of Staphylococcus aureus and Pseudomonas aeruginosa in vitro.

BACKGROUND DATA

ICG and 809 nm combination can be a powerful tool for the treatment of wound-infecting, antibiotic-resistant bacteria.

METHODS

The effect of ICG and 809 nm laser light on wild type and resistant strains of S. aureus and P. aeruginosa was examined in vitro. ICG concentrations and laser doses were initially optimized for wild type S. aureus and P. aeruginosa. After determining the most effective ICG concentrations with specified light dose, they were applied on resistant strains. Viable bacterial cells were counted by serial dilution method.

RESULTS

Photodynamic therapy (PDT) with ICG was totally efficient to kill all of these bacterial strains, and light/ICG alone did not cause any lethal effect on any of the strains. Optimum ICG laser doses varied with respect to the bacteria type: 84 J/cm(2) of light dose with 6 μg/mL of ICG concentration caused more than 95% killing of wild type S. aureus strains. The same bactericidal effect was achieved with a lower amount of ICG (4 μg/mL) on resistant strain S. aureus. Optimum parameters for 99% killing of wild type P. aeruginosa were 125 μg/mL ICG and 252 J/cm(2) of light dose. Similarly, their bactericidal effect was stronger on resistant strain; 100 μg/mL ICG with 252 J/cm(2) was enough to cause a 99% decrease in viable cells.

CONCLUSIONS

The combination of ICG and 809 nm laser light was found as an effective antibacterial method to destroy antibiotic-resistant strains of gram-positive and gram-negative bacteria.

Synthesis, properties and photodynamic activities of some zinc(II) phthalocyanines against Escherichia coli and Staphylococcus aureus

Two new zinc phthalocyanine derivatives bearing four 3,5-di-tert-butyl-4-hydroxyphenyl (ZnPc1) and 3,5-dimethylphenoxy (ZnPc2) have been synthesized and proved by elemental analyses and UV-vis, 1H NMR, FTIR and MALDI-TOF mass spectra as spectroscopic determination. Thermal stabilities of these neutral Zn -phthalocyanines were performed by thermal gravimetric analysis and, significantly, were found stable up to 373 °C for ZnPc1 and 550 °C for ZnPc2. In addition, the photostability of the sensitizers was quite successful within 240 min. Furthermore, photodynamic therapy has been investigated using these neutral phthalocyanines. Singlet oxygen generation capacities of ZnPc1 and ZnPc2 were studied using 1,3-diphenyl-iso-benzofuran (50.0 μM) as a selective singlet oxygen trap in DMSO and both of them demonstrated very high singlet oxygen generation capacity. Photodynamic therapy is of considerable interest for its potential as an antimicrobial therapy on the grounds that the photodynamic activity of these compounds was tested against a Gram-negative bacteria, Escherichia coli and a Gram-positive bacteria, Staphylococcus aureus. Suspensions of the microorganisms were irradiated for 240 min in the presence of ZnPc1 (with hydrophilic group) and ZnPc2 phthalocyanines. In order to simulate solar radiation, we used a 750 W xenon lamp. Minimum photosensitizer concentration was used as 0.1 mg.10 mL-1 in 1% dimethylsulfoxide/phosphate buffer saline solution (DMSO/PBS). Moreover, the photostability of these compounds has been investigated and the effect of the amount of DMSO tested against selected bacteria. In the dark, with and without Pcs, bacterial inactivation did not occur. Bacterial inactivation by light with ZnPc1 was observed in response to Gram-negative bacteria E. coli and Gram-positive bacteria S. aureus, whereas the photoinactivation studies with ZnPc2 have revealed that the lack of its activity is due to its poor affinity for either of the organisms. These results suggest that a neutral amphiphilic photosensitizer may be easily used in an application concerning photoinactivation of bacterial cells as well as ionic photosensitizers.

The antimicrobial activity of photodynamic therapy against Streptococcus mutans using different photosensitizers

Several photosensitizers have been used against oral bacteria without standardization. Singlet oxygen ((1)O(2)) is an aggressive chemical species that can kill cells through apoptosis or necrosis.

OBJECTIVE

to compare the antimicrobial activity of photodynamic therapy (PDT) with different photosensitizers at the same concentration against Streptococcus mutans. In addition, the (1)O(2) production of each photosensitizer was determined. The photosensitizers (163.5 μM) methylene blue (MB), toluidine blue ortho (TBO) and malachite green (MG) were activated with a light-emitting diode (LED; λ=636 nm), while eosin (EOS), erythrosine (ERI) and rose bengal (RB) were irradiated with a curing light (λ=570 nm). Light sources were operated at 24 J cm(-2). For each photosensitizer, 40 randomized assays (n=10 per condition) were performed under one of the following experimental conditions: no light irradiation or photosensitizer, irradiation only, photosensitizer only or irradiation in the presence of a photosensitizer. After treatment, serial dilutions of S. mutans were seeded onto brain heart infusion agar to determine viability in colony-forming units per milliliter (CFU mL(-1)). Generation of (1)O(2) was analyzed by tryptophan photooxidation, and the decay constant was estimated. Results were analyzed by one-way ANOVA and the Tukey-Kramer test (p<0.05). PDT with irradiation in the presence of the photosensitizers TBO and MG was effective in reducing S. mutans counts by 3 and 1.4 logs, respectively (p<0.01), compared to their respective untreated controls. MB generated 1.3 times more (1)O(2) than TBO, and both produced significantly higher concentrations of singlet oxygen than the other photosensitizers. Since in vitro bulk (1)O(2) production does not indicate that (1)O(2) was generated in the bacterial activity site, the bactericidal action against S. mutans cannot be related to in vitro singlet O(2) generation rate. In vitroS. mutans-experiments demonstrated TBO as the only photosensitizer that effectively reduced 99.9% of these microorganisms.

Temoporfin (Foscan®, 5,10,15,20-tetra(m-hydroxyphenyl)chlorin)--a second-generation photosensitizer

This review traces the development and study of the second-generation photosensitizer 5,10,15,20-tetra(m-hydroxyphenyl)chlorin through to its acceptance and clinical use in modern photodynamic (cancer) therapy. The literature has been covered up to early 2011.

Photodynamic therapy for cutaneous leishmaniasis: the effectiveness of topical phenothiaziniums in parasite eradication and Th1 immune response stimulation

Photodynamic therapy (PDT) is emerging as a therapeutic modality in the clinical management of cutaneous leishmaniasis (CL). The efficacy of PDT against CL has been demonstrated previously with aminolevulinic acid, although the prolonged terms of therapy were less than ideal, and the search for new photosensitizers (PS) is ongoing. However, phenothiaziniums have demonstrated high parasiticidal effects in vitro. The subject of our investigation is the in vivo activity of two PS, 5-ethylamino-9-diethylaminobenzo[a]phenoselenazinium chloride (EtNBSe) and (3,7-Bis(N,N-dibutylamino) phenothiazinium bromide (PPA904). The results of our comparative analysis of the efficacy of these two phenothiazinium analogues demonstrated a high antiparasitic activity of EtNBSe in vitro, and the higher efficacy of PPA904 in a mouse model of CL. The kinetics of photodestruction are different in parasite and mammalian cells, and with both dyes, the macrophages are more susceptible to photodynamic effects than L. major parasites. As the number of parasites in the lesions undergoes a biphasic change, temporarily increasing on days 2-4 and decreasing on days 5-7, more than one treatment is required within an interval of 5 to 7 days. We have also shown that PPA904-PDT can provide an immunomodulating, dose-dependent efflux on IL-12p70 production. This mechanism could be responsible for promoting a more rapid healing in PPA904-PDT treated mice. Our initial data indicate that phenothiaziniums exhibit a high parasiticidal effect in vivo against CL; this finding may be of use in establishing curative PDT regimens for future clinical trials.