Photodynamic Therapy for Cancer and for Infections: What Is the Difference?

Antimicrobial photodynamic effect of the photosensitizer riboflavin, alone and in combination with colistin, against pandrug-resistant Pseudomonas aeruginosa clinical isolates

INTRODUCTION

Development of multi-, extensively-, and pandrug-resistant (MDR, XDR, and PDR) strains of Pseudomonas aeruginosa remains a major problem in medical care. The present study evaluated the effect of antimicrobial photodynamic therapy (aPDT) as a monotherapy and in combination with colistin against P. aeruginosa isolates.

METHODS

Two P. aeruginosa isolates recovered from patients with respiratory tract infections were examined in this study. Minimum inhibitory concentration (MIC) of colistin was determined by the colistin broth disk elution (CBDE) and the reference broth microdilution (rBMD) methods. aPDT was performed using the photosensitizer (Ps) riboflavin at several concentrations and a light-emitting diode (LED) emitting blue light for different irradiation times with or without colistin at 1/2 × MIC concentration.

RESULTS

Both PA1 and PA2 isolates were identified as colistin-resistant P. aeruginosa with a MIC ≥4 μg/mL by the CBDE and MICs of 512 μg/mL and 256 μg/mL, respectively, by the rBMD. In aPDT, neither riboflavin nor LED light alone had antibacterial effects. The values of colony forming units per milliliter (CFU/mL) in both isolates were significantly reduced by LED + Ps treatments in a time-dependent manner (LED irradiation time) and dose-dependent manner (Ps concentration). In comparison with control, treatment with Ps (50 μM) + LED (120 s) and Ps (100 μM) + LED (120 s) resulted in 0.27 log10 CFU/mL and 0.43 log10 CFU/mL reductions in PA1, and 0.28 log10 CFU/mL and 0.34 log10 CFU/mL reductions in PA2, respectively, (P < 0.01). The best results were obtained after the combination of aPDT followed by colistin, which increased bacterial reduction, resulting in a 0.41-0.7 log10 CFU/mL reduction for PA1 and 0.35-0.83 log10 CFU/mL reduction for PA2 (P = 0.001).

CONCLUSIONS

This study suggests the potential implications of aPDT in combination with antibiotics, such as colistin for treatment of difficult-to-treat P. aeruginosa infections.

Evaluation of antimicrobial photodynamic action of a pluronic and pectin based film loaded with methylene blue against methicillin resistantStaphylococcus aureus

Antimicrobial wound dressings play a crucial role in treatment of wound infections. However, existing commercial options fall short due to antibiotic resistance and the limited spectrum of activity of newly emerging antimicrobials against bacteria that are frequently encountered in wound infections. Antimicrobial photodynamic therapy (aPDT) is very promising alternative therapeutic approach against antibiotic resistant microbes such as methicillin resistantStaphylococcus aureus (MRSA). However, delivery of the photosensitizer (PS) homogeneously to the wound site is a challenge. Though polymeric wound dressings based on synthetic and biopolymers are being explored for aPDT, there is paucity of data regarding theirin vivoefficacy. Moreover, there are no studies on use of PS loaded, pluoronic (PL) and pectin (PC) based films for aPDT. We report development of a polymeric film for potential use in aPDT. The film was prepared using PL and PC via solvent casting approach and impregnated with methylene blue (MB) for photodynamic inactivation of MRSAin vitroandin vivo. Atomic force microscopic imaging of the films yielded vivid pictures of surface topography, with rough surfaces, pores, and furrows. The PL:PC ratio (2:3) was optimized that would result in an intact film but exhibit rapid release of MB in time scale suitable for aPDT. The film showed good antibacterial activity against planktonic suspension, biofilm of MRSA upon exposure to red light. Investigations on MRSA infected excisional wounds of mice reveal that topical application of MB loaded film for 30 min followed by red light exposure for 5 min (fluence; ∼30 J cm-2) or 10 min (fluence; ∼60 J cm-2) reduces ∼80% or ∼92% of bioburden, respectively. Importantly, the film elicits no significant cytotoxicity against keratinocytes and human adipose derived mesenchymal stem cells. Taken together, our data demonstrate that PS-loaded PL-PC based films are a promising new tool for treatment of MRSA infected wounds.

Dendrimers: Advancements and Potential Applications in Cancer Diagnosis and Treatment-An Overview

Cancer is a leading cause of death worldwide, and the main treatment methods for this condition are surgery, chemotherapy, and radiotherapy. These treatment methods are invasive and can cause severe adverse reactions among organisms, so nanomaterials are increasingly used as structures for anticancer therapies. Dendrimers are a type of nanomaterial with unique properties, and their production can be controlled to obtain compounds with the desired characteristics. These polymeric molecules are used in cancer diagnosis and treatment through the targeted distribution of some pharmacological substances. Dendrimers have the ability to fulfill several objectives in anticancer therapy simultaneously, such as targeting tumor cells so that healthy tissue is not affected, controlling the release of anticancer agents in the tumor microenvironment, and combining anticancer strategies based on the administration of anticancer molecules to potentiate their effect through photothermal therapy or photodynamic therapy. The purpose of this review is to summarize and highlight the possible uses of dendrimers regarding the diagnosis and treatment of oncological conditions.

The safety and efficiency of photodynamic therapy for the treatment of osteosarcoma: A systematic review of in vitro experiment and animal model reports

BACKGROUND

Osteosarcoma (OS) is an aggressive malignant bone tumour with high mortality. A poor prognosis is noted in patients with distal metastases or multidrug resistance. As an emerging antitumor strategy, photodynamic therapy (PDT) mediated by visible and near infrared light has attracted intensive attention given its target selectivity, remote controllability, minimal or non-invasive features. However, PDT also has obvious limitations. Specifically, due to the limited penetration of light, it is mainly used in the clinical treatment of superficial malignant tumours, such as musculoskeletal sarcomas and melanoma, but it has not been applied to the clinical treatment of deep malignant bone tumours except for a very small number of experiments on deep canine OS models.

MATERIALS AND METHODS

We searched for studies that focused on the effectiveness and safety of PDT for OS based on in vitro experiments and animal models in the last decade. A systematic search was conducted using electronic databases, including PubMed, ClinicalTrials.gov, and the Cochrane Library.

INCLUSION CRITERIA

(1) original research articles about PDT for OS; (2) articles in English; (3) in vitro or animal model research; and (4) detailed information, including cell name, fluence, irradiation wavelength, time of incubation with PS, duration between PS treatment and irradiation, and duration between irradiation and viability assays.

EXCLUSION CRITERIA

(1) study was a review/systemic review article, patent, letter, or conference abstract/paper; (2) articles were not published in English; (3) studies containing overlapping or insufficient data.

RESULTS

We identified 201 publications, and 44 articles met the inclusion criteria and were included in the synthesis. Unfortunately, there are no relevant clinical reports of the use of PDT in the treatment of human OS. In these studies, 8 studies only employed in vivo experiments to evaluate the efficiency of PDT in an OS animal model, 19 studies exclusively performed in vitro viability assays of cells treated with PDT under different conditions, and 17 studies included in vitro cell experiments and in vivo animal OS models to evaluate the effect of PDT on OS in vivo and in vitro. All studies have shown that PDT is cytotoxic to OS cells or can inhibit the growth of OS in heterologous or homologous animal OS models but exhibits minimal cytotoxicity at a certain range of dosages.

CONCLUSION

Based on this systematic review, PDT can eradicate OS cells in cell culture and there is some evidence for efficacy in animal models. However, the ability for PDT to control human OS is unclear, the animal and human reports do not show evidence of human OS control, they just do show feasibility. The major issues concerning the potential for treatment of osteosarcoma with PDT are that adequate light should be transmitted to tumor loci and if the disease is caught before metastasis and irradiation of tumor sites is feasible, curative potential is there. Otherwise, PDT may be mainly palliative. To determine whether PDT can safely and efficiently be used in the clinical treatment of OS, many preclinical orthotopic animal OS models and OS models of multiple systemic metastases must be performed and interstitial PDT or intraoperative PDT may be a good and potential candidate for human OS treatment. If these problems can be well solved, PDT may be a potentially effective strategy for the treatment of OS patients.

The effectiveness of antimicrobial photodynamic therapy with prodigiosin against reference strains of Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa

Prodigiosin (PG) is a secondary metabolite of bacterial origin that is able to absorb the visible light and plays a role as a photosensitizer in photodynamic therapy (PDT). This in vitro study aimed to investigate the cytotoxicity of PG-mediated PDT against the reference strains of Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), and Pseudomonas aeruginosa (P. aeruginosa). The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of PG were determined. Each strain was then allocated into four groups as follows: G1: control (no treatment), G2: PG-treated groups that received different PG concentrations (1000-1.95 μM), G3: laser-treated group (wavelength: 520 nm, radiation dose: 187 J/cm²), and G4: PG-mediated PDT groups that were initially treated with different concentrations of PG and were then exposed to laser irradiation in the same way as the previous group. Finally, the number of colony-forming units per milliliter (CFU/mL) was calculated and analyzed using the SPSS software. PG had both bacteriostatic and bactericidal activities on the tested bacteria, with the maximum antibacterial effect being observed against S. aureus. In all bacterial strains, the maximum number of CFUs was observed in the control group followed by the laser-irradiated and PG-treated groups, but the differences were not statistically significant (p > 0.05). However, the utilization of PG-mediated PDT resulted in a significant decrease in the mean number of CFUs in all the tested bacteria (p < 0.0001). PG-mediated PDT had the potential to kill some bacterial strains in the laboratory. Yet, further studies are warranted to confirm its efficacy and safety to be applied in clinical settings.

Selenophene-Modified Boron Dipyrromethene-Based Photosensitizers Exhibit Photodynamic Inhibition on a Broad Range of Bacteria

Microorganisms are crucial for human survival in view of both mutualistic and pathogen interactions. The control of the balance could be achieved by use of the antibiotics. There is a continuous arms race that exists between the pathogen and the antibiotics. The emergence of multidrug-resistant (MDR) bacteria threatens health even for insignificant injuries. However, the discovery of new antibiotics is not a fast process, and the healthcare system will suffer if the evolution of MDR lingers in its current frequency. The cationic photosensitizers (PSs) provide a unique approach to develop novel, light-inducible antimicrobial drugs. Here, we examine the antimicrobial activity of innovative selenophene-modified boron dipyrromethene (BODIPY)-based PSs on a variety of Gram (+) and Gram (-) bacteria. The candidates demonstrate a level of confidence in both light-dependent and independent inhibition of bacterial growth. Among them, selenophene conjugated PS candidates (BOD-Se and BOD-Se-I) are promising agents to induce photodynamic inhibition (PDI) on all experimented bacteria: E. coli, S. aureus, B. cereus, and P. aeruginosa. Further characterizations revealed that photocleavage ability on DNA molecules could be potentially advantageous over extracellular DNA possessing biofilm-forming bacteria such as B. cereus and P. aeruginosa. Microscopy analysis with fluorescent BOD-H confirmed the colocalization on GFP expressing E. coli.

The effect of charge on Zn tetra morpholine porphyrin conjugated to folic acid-nitrogen doped graphene quantum dots for photodynamic therapy studies

Zinc tetra morpholine porphyrin (complex 2), and its quaternized derivative (complex 3) were synthesized and conjugated to folic acid nitrogen doped graphene quantum dots (FA-NGQDs) through π-π stacking to study their photodynamic therapy (PDT) efficacy. Photophysiochemical properties of complexes 2, 3, and their conjugates (2-FA-NGQDs, 3-FA-NGQDs) were studied. It was found that complex 3 had higher ϕ_Δ of 0.56 compared to complex 2 with ϕ_Δ of 0.24, and respective composites: 3-FA-NGQDs had higher ϕ_Δ compared to 2-FA-NGQDs. The PDT studies were conducted for nanoparticles (FA-NGQDs), complexes (2, 3), and respective composites (2-FA-NGQDs, and 3-FA-NGQDs) using MCF-7 breast cancer cell. Dark toxicity of all compounds was above 90% which is negligible. At a highest concentration of 40 µg/mL, 3-FA-NGQDs gave the lowest cell viability of 28% compared to all other conjugates and porphyrins alone.

Gallium Mesoporphyrin IX-Mediated Photodestruction: A Pharmacological Trojan Horse Strategy To Eliminate Multidrug-Resistant Staphylococcus aureus

One of the factors determining efficient antimicrobial photodynamic inactivation (aPDI) is the accumulation of a light-activated compound, namely, a photosensitizer (PS). Targeted PS recognition is the approach based on the interaction between the membrane receptor on the bacterial surface and the PS, whereas the compound is efficiently accumulated by the same mechanism as the natural ligand. In this study, we showed that gallium mesoporphyrin IX (Ga³⁺MPIX) provided dual functionality—iron metabolism disruption and PS properties in aPDI. Ga³⁺MPIX induced efficient (>5log₁₀ reduction in CFU/mL) bacterial photodestruction with excitation in the area of Q band absorption with relatively low eukaryotic cytotoxicity and phototoxicity. The Ga³⁺MPIX is recognized by the same systems as haem by the iron-regulated surface determinant (Isd). However, the impairment in the ATPase of the haem detoxification efflux pump was the most sensitive to the Ga³⁺MPIX-mediated aPDI phenotype. This indicates that changes within the metalloporphyrin structure (vinyl vs ethyl groups) did not significantly alter the properties of recognition of the compound but influenced its biophysical properties.

In Vivo Potentiation of Antimicrobial Photodynamic Therapy in a Mouse Model of Fungal Infection by Addition of Potassium Iodide

Antimicrobial photodynamic inactivation (aPDI) involves the use of a nontoxic dye or photosensitizer excited with visible light to produce reactive oxygen species that can kill all classes of microorganisms. Antimicrobial photodynamic therapy (aPDT) can be used in vivo as an alternative therapeutic strategy to treat localized infections due to its ability to selectively kill microbes while preserving host mammalian cells. aPDI can be potentiated by the addition of the nontoxic inorganic salt potassium iodide (KI). KI is an approved drug for antifungal therapy. The mechanism of potentiation with iodide is likely to be singlet oxygen addition to iodide to form iodine radicals, hydrogen peroxide, and molecular iodine. A previous chapter in this volume described potentiation of aPDI in vitro by addition of KI, while in this chapter we address the ability of KI to potentiate aPDT in vivo using an animal model of localized fungal infection. We employed oral candidiasis in immunosuppressed mice caused by a bioluminescent strain of Candida albicans and monitored by bioluminescence imaging.

In Vitro Potentiation of Antimicrobial Photodynamic Inactivation by Addition of Potassium Iodide

The current increase in antibiotic resistance worldwide and the emergence of microbial strains that are resistant to all known antibiotics have stimulated research into novel strategies such as aPDI that are thought to be unlikely to lead to the development of resistance. Although many studies have reported in vitro aPDI killing of microorganisms by a range of different photosensitizers, there are still limitations to the effectiveness of aPDI, and recurrence of bacterial growth may occur in animal studies after completion of the illumination. In this chapter we cover a novel and relatively simple method to improve the efficacy of aPDI against Gram-positive Staphylococcus aureus, Gram-negative Escherichia coli, and fungal yeast Candida albicans by the addition of potassium iodide, a nontoxic inorganic salt. Under some circumstances up to six-logs additional killing can be obtained.

Nanoparticle Systems for Cancer Phototherapy: An Overview

Photodynamic therapy (PDT) and photothermal therapy (PTT) are photo-mediated treatments with different mechanisms of action that can be addressed for cancer treatment. Both phototherapies are highly successful and barely or non-invasive types of treatment that have gained attention in the past few years. The death of cancer cells because of the application of these therapies is caused by the formation of reactive oxygen species, that leads to oxidative stress for the case of photodynamic therapy and the generation of heat for the case of photothermal therapies. The advancement of nanotechnology allowed significant benefit to these therapies using nanoparticles, allowing both tuning of the process and an increase of effectiveness. The encapsulation of drugs, development of the most different organic and inorganic nanoparticles as well as the possibility of surfaces' functionalization are some strategies used to combine phototherapy and nanotechnology, with the aim of an effective treatment with minimal side effects. This article presents an overview on the use of nanostructures in association with phototherapy, in the view of cancer treatment.

Insights on the polypyrrole based nanoformulations for photodynamic therapy

This review is written to endow updated information on polypyrrole based photosensitizers for the treatment of deadly diseases such as cancer and microbial infection. Tetrapyrrolic macromolecules such as porphyrins and phthalocyanines hold unique photophysical properties which make them very useful compounds for various biomedical applications. Besides their properties, they also have some limitations such as low water solubility, bioavailability, biocompatibility and lack of specificity, etc. Researchers are trying to overcome these limitations by incorporating photosensitizers into the different types of nanoparticles and improve the quality of photodynamic therapy. We have contributed to this field by synthesizing and developing polypyrrolic photosensitizer based nanoparticles for potential applications in antimicrobial and anticancer photodynamic activity. Throughout this review, newly synthesized and existing PSs conjugated/encapsulated/doped/incorporated with nanoparticles are emphasized, which are essential for current and future research themes. Also in this review, we briefly summarized the research work carried over the past few years by considering the porphyrin based photosensitizers as alternative therapeutic entities for the treatment of microbial infections, cancers, and many other diseases.

Diamond Nanoparticles-Porphyrin mTHPP Conjugate as Photosensitizing Platform: Cytotoxicity and Antibacterial Activity

Conjugation of photosensitizers (PS) with nanoparticles has been largely used as a strategy to stabilize PS in the biological medium resulting in photosensitizing nanoparticles of enhanced photoactivity. Herein, (Meso-5, 10, 15, 20-tetrakis (3-hydroxyphenyl) phorphyryn (mTHPP) was conjugated with diamond nanoparticles (ND) by covalent bond. Nanoconjugate ND-mTHPP showed suitable stability in aqueous suspension with 58 nm of hydrodynamic diameter and Zeta potential of -23 mV. The antibacterial activity of ND-mTHPP was evaluated against Escherichia coli for different incubation times (0-24 h). The optimal activity was observed after 2 h of incubation and irradiation (660 nm; 51 J/cm2) performed right after the addition of ND-mTHPP (100 μg/mL) to the bacterial suspension. The inhibitory activity was 56% whereas ampicillin at the same conditions provided only 14% of bacterial growth inhibition. SEM images showed agglomerate of ND-mTHPP adsorbed on the bacterial cell wall, suggesting that the antimicrobial activity of ND-mTHPP was afforded by inducing membrane damage. Cytotoxicity against murine embryonic fibroblast cells (MEF) was also evaluated and ND-mTHPP was shown to be noncytotoxic since viability of cells cultured for 24 h in the presence of the nanoconjugate (100 μg/mL) was 78%. Considering the enhanced antibacterial activity and the absence of cytotoxic effect, it is possible to consider the ND-mTHPP nanoconjugate as promising platform for application in antimicrobial photodynamic therapy (aPDT).

[Optimization of variables in photodynamic antifungal assays against Candida species]

Oral infections due to yeasts of the genus Candida are very common in patients with predisposing factors, such as physiological conditions or certain underlying diseases. Moreover, oral candidiasis can also evolve into disseminated forms. In this work, strains of the genus Candida were studied to establish the optimal radiation conditions for photosensitizing compounds, in a photodynamic antifungal therapy protocol. A total of 39 isolates were evaluated. The strains were exposed to twelve dyestuffs, eight radiation sources and three different exposure times. Orthotoluidine blue exhibited good photodynamic activity, in combination with exposure to a 20W reflector lamp LED (light-emitting diode) light for 60minutes. When considering the difficulties of using conventional antifungal drugs, the emergence of resistant strains, recurrences, and adverse reactions of certain commonly used drugs, the photodynamic antifungal therapy is a promising strategy for the treatment of localized infections.

Studies to explore the UVA photosensitizing action of 9-phenylacridine in cells by interaction with DNA

Acridine and its derivatives are well known for their DNA binding properties. In this report, we present our findings on evaluating different binding parameters of the interaction of 9-phenylacridine (ACPH) with DNA. Absorption spectroscopic studies including standard and reverse titration, the effects of ionic strength and temperature on titration, and Job plot analysis were done to calculate the binding constant and determine the different thermodynamic parameters and stoichiometry of the binding. Spectrofluorimetry and circular dichroism (CD) spectral titration were also utilized to confirm these findings. The results indicated that ACPH binds to DNA reversibly through non-electrostatic interactions by hydrogen bonding and van der Waals interactions. The binding constant and the number of binding sites were of the order 10³ M^-1 and ≈2, respectively with a binding stoichiometry of 1:4. The binding of ACPH with DNA was spontaneous, exothermic and enthalpy-driven. The extent of uptake of ACPH in B16 melanoma cells was estimated. As this compound absorbs in the UVA region, the effect of treatment with ACPH prior to UVA exposure was assessed to evaluate its phototoxicity in these cells. Our results indicated that the binding to DNA enhanced damage to sensitize cells to killing through apoptosis. Our findings indicated its potential to act as a photosensitizer.

Autophagy Regulation and Photodynamic Therapy: Insights to Improve Outcomes of Cancer Treatment

Cancer is considered an age-related disease that, over the next 10 years, will become the most prevalent health problem worldwide. Although cancer therapy has remarkably improved in the last few decades, novel treatment concepts are needed to defeat this disease. Photodynamic Therapy (PDT) signalize a pathway to treat and manage several types of cancer. Over the past three decades, new light sources and photosensitizers (PS) have been developed to be applied in PDT. Nevertheless, there is a lack of knowledge to explain the main biochemical routes needed to trigger regulated cell death mechanisms, affecting, considerably, the scope of the PDT. Although autophagy modulation is being raised as an interesting strategy to be used in cancer therapy, the main aspects referring to the autophagy role over cell succumbing PDT-photoinduced damage remain elusive. Several reports emphasize cytoprotective autophagy, as an ultimate attempt of cells to cope with the photo-induced stress and to survive. Moreover, other underlying molecular mechanisms that evoke PDT-resistance of tumor cells were considered. We reviewed the paradigm about the PDT-regulated cell death mechanisms that involve autophagic impairment or boosted activation. To comprise the autophagy-targeted PDT-protocols to treat cancer, it was underlined those that alleviate or intensify PDT-resistance of tumor cells. Thereby, this review provides insights into the mechanisms by which PDT can be used to modulate autophagy and emphasizes how this field represents a promising therapeutic strategy for cancer treatment.

Current Limitations and Recent Progress in Nanomedicine for Clinically Available Photodynamic Therapy

Photodynamic therapy (PDT) using oxygen, light, and photosensitizers has been receiving great attention, because it has potential for making up for the weakness of the existing therapies such as surgery, radiation therapy, and chemotherapy. It has been mainly used to treat cancer, and clinical tests for second-generation photosensitizers with improved physicochemical properties, pharmacokinetic profiles, or singlet oxygen quantum yield have been conducted. Progress is also being made in cancer theranostics by using fluorescent signals generated by photosensitizers. In order to obtain the effective cytotoxic effects on the target cells and prevent off-target side effects, photosensitizers need to be localized to the target tissue. The use of nanocarriers combined with photosensitizers can enhance accumulation of photosensitizers in the tumor site, owing to preferential extravasation of nanoparticles into the tumor vasculature by the enhanced permeability and retention effect. Self-assembly of amphiphilic polymers provide good loading efficiency and sustained release of hydrophobic photosensitizers. In addition, prodrug nanomedicines for PDT can be activated by stimuli in the tumor site. In this review, we introduce current limitations and recent progress in nanomedicine for PDT and discuss the expected future direction of research.

Photodynamic antimicrobial chemotherapy in mice with Pseudomonas aeruginosa-infected wounds

This study examined the antibacterial effect of protoporphyrin IX–ethylenediamine derivative (PPIX-ED)–mediated photodynamic antimicrobial chemotherapy (PPIX-ED-PACT) against Pseudomonas aeruginosa in vitro and in vivo. PPIX-ED potently inhibited the growth of Pseudomonas aeruginosa by inducing reactive oxygen species production via photoactivation. Atomic force microscopy revealed that PPIX-ED-PACT induced the leakage of bacterial content by degrading the bacterial membrane and wall. As revealed using acridine orange/ethidium bromide staining, PPIX-ED-PACT altered the permeability of the bacterial membrane. In addition, the antibacterial effect of PPIX-ED-PACT was demonstrated in an in vivo model of P. aeruginosa-infected wounds. PPIX-ED (100 μM) decreased the number of P. aeruginosa colony-forming units by 4.2 log₁₀. Moreover, histological analysis illustrated that the wound healing rate was 98% on day 14 after treatment, which was 10% higher than that in the control group. According to the present findings, PPIX-ED-PACT can effectively inhibit the growth of P. aeruginosa in vitro and in vivo.

Utility of Photodynamic Therapy in Dentistry: Current Concepts

The significant growth in scientific and technological advancements within the field of dentistry has resulted in a wide range of novel treatment modalities for dentists to use. Photodynamic therapy (PDT) is an emerging, non-invasive treatment method, involving photosensitizers, light of a specific wavelength and the generation of singlet oxygen and reactive oxygen species (ROS) to eliminate unwanted eukaryotic cells (e.g., malignancies in the oral cavity) or pathogenic microorganisms. The aim of this review article is to summarize the history, general concepts, advantages and disadvantages of PDT and to provide examples for current indications of PDT in various subspecialties of dentistry (oral and maxillofacial surgery, oral medicine, endodontics, preventive dentistry, periodontology and implantology), in addition to presenting some images from our own experiences about the clinical success with PDT.

Iron and zinc ions, potent weapons against multidrug-resistant bacteria

Drug-resistant bacteria are becoming an increasingly widespread problem in the clinical setting. The current pipeline of antibiotics cannot provide satisfactory options for clinicians, which brought increasing attention to the development and application of non-traditional antimicrobial substances as alternatives. Metal ions, such as iron and zinc ions, have been widely applied to inhibit pathogens through different mechanisms, including synergistic action with different metabolic enzymes, regulation of efflux pumps, and inhibition of biofilm formation. Compared with traditional metal oxide nanoparticles, iron oxide nanoparticles (IONPs) and zinc oxide nanoparticles (ZnO-NPs) display stronger bactericidal effect because of their smaller ion particle sizes and higher surface energies. The combined utilization of metal NPs (nanoparticles) and antibiotics paves a new way to enhance antimicrobial efficacy and reduce the incidence of drug resistance. In this review, we summarize the physiological roles and bactericidal mechanisms of iron and zinc ions, present the recent progress in the research on the joint use of metal NPs with different antibiotics, and highlight the promising prospects of metal NPs as antimicrobial agents for tackling multidrug-resistant bacteria.

Application of photodynamic therapy drugs for management of glioma

Human gliomas are one of the most prevalent and challenging-to-treat adult brain tumors, and thus result in high morbidity and mortality rates worldwide. Current research and treatments of gliomas include surgery associated with conventional chemotherapy, use of biologicals, radiotherapy, and medical device applications. The selected treatment options are often guided by the category and aggressiveness of this deadly disease and the patient’s conditions. However, the effectiveness of these approaches is still limited due to poor drug efficacy (including delivery to desired sites), undesirable side effects, and high costs associated with therapies. In addition, the degree of leakiness of the blood–brain barrier (BBB) that regulates trafficking of molecules in and out of the brain also modulates accumulation of adequate drug levels to tumor sites. Active research is being pursued to overcome these limitations to obtain a superior therapeutic index and enhanced patient survival. One area of development in this direction focuses on the localized application of photodynamic therapy (PDT) drugs to cure brain cancers. PDT molecules potentially utilize multiple pathways based on their ability to generate reactive oxygen species (ROS) upon photoactivation by a suitable light source. In this communication, we have attempted to provide a brief overview of PDT and cancer, photoactivation pathways, mechanism of tumor destruction, effect of PDT on tumor cell viability, immune activation, various research attempted by applying PDT in combination with novel strategies to treat glioma, role of BBB and clinical status of PDT therapy for glioma treatment.

Role of Toluidine Blue-O Binding Mechanism for Photooxidation in Bioinspired Bacterial Membranes

The alarming increase in bacterial resistance to antibiotics has demanded new strategies for microbial inactivation, which include photodynamic therapy whose activity relies on the photoreaction damage to the microorganism membrane. Herein, the binding mechanisms of the photosensitizer toluidine blue-O (TBO) on simplified models of bacterial membrane with Langmuir monolayers of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) and 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG) were correlated to the effects of the photoinduced lipid oxidation. Langmuir monolayers of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) were also used as a reference of mammalian membranes. The surface pressure isotherms combined with polarization-modulated infrared reflection absorption spectroscopy revealed that TBO expands DOPC, DOPE, and DOPG monolayers owing to electrostatic interactions with the negatively charged groups in the phospholipids, with a stronger adsorption on DOPG, which has a net surface charge. Light irradiation made the TBO-containing DOPC and DOPE monolayers less unstable as a result of the singlet oxygen (¹O₂) reaction with the chain unsaturation and hydroperoxide formation. In contrast, the decreased stability of the irradiated TBO-containing DOPG monolayer suggests the cleavage of carbon chains. The anionic nature of DOPG allowed a deeper penetration of TBO into the chain region, favoring contact-dependent reactions between the excited triplet state of TBO and lipid unsaturations or/and hydroperoxide groups, which is the key for the cleavage reactions and further membrane permeabilization.

Photodynamically Active Electrospun Fibers for Antibiotic-Free Infection Control

Antimicrobial biomaterials are critical to aid in the regeneration of oral soft tissue and prevent or treat localized bacterial infections. With the rising trend in antibiotic resistance, there is a pressing clinical need for new antimicrobial chemistries and biomaterial design approaches enabling on-demand activation of antibiotic-free antimicrobial functionality following an infection that are environment-friendly, flexible and commercially viable. This study explores the feasibility of integrating a bioresorbable electrospun polymer scaffold with localized antimicrobial photodynamic therapy (aPDT) capability. To enable aPDT, we encapsulated a photosensitizer (PS) in polyester fibers in the PS inert state, so that the antibacterial function would be activated on-demand via a visible light source. Fibrous scaffolds were successfully electrospun from FDA-approved polyesters, either poly(ε-caprolactone (PCL) or poly[(rac-lactide)-co-glycolide] (PLGA), with encapsulated PS (either methylene blue (MB) or erythrosin B (ER)). These were prepared and characterized with regards to their loading efficiency (UV-vis spectroscopy), microarchitecture (SEM, porometry, and BET (Brunauer-Emmett-Teller) analysis), tensile properties, hydrolytic behavior (contact angle, dye release capability, degradability), and aPDT effect. The electrospun fibers achieved an ∼100 wt % loading efficiency of PS, which significantly increased their tensile modulus and reduced their average fiber diameter and pore size with respect to PS-free controls. In vitro, PS release varied between a burst release profile to limited release within 100 h, depending on the selected scaffold formulation, while PLGA scaffolds displayed significant macroscopic shrinkage and fiber merging, following incubation in phosphate buffered saline solution. Exposure of PS-encapsulated PCL fibers to visible light successfully led to at least a 1 log reduction in Escherichia coli viability after 60 min of light exposure, whereas PS-free electrospun controls did not inactive microbes. This study successfully demonstrates the significant potential of PS-encapsulated electrospun fibers as photodynamically active biomaterial for antibiotic-free infection control.

Genetically Encoded Photosensitizers as Light-Triggered Antimicrobial Agents

Diseases caused by multi-drug resistant pathogens have become a global concern. Therefore, new approaches suitable for treating these bacteria are urgently needed. In this study, we analyzed genetically encoded photosensitizers (PS) related to the green fluorescent protein (GFP) or light-oxygen-voltage (LOV) photoreceptors for their exogenous applicability as light-triggered antimicrobial agents. Depending on their specific photophysical properties and photochemistry, these PSs can produce different toxic ROS (reactive oxygen species) such as O₂^•− and H₂O₂ via type-I, as well as ¹O₂ via type-II reaction in response to light. By using cell viability assays and microfluidics, we could demonstrate differences in the intracellular and extracellular phototoxicity of the applied PS. While intracellular expression and exogenous supply of GFP-related PSs resulted in a slow inactivation of E. coli and pathogenic Gram-negative and Gram-positive bacteria, illumination of LOV-based PSs such as the singlet oxygen photosensitizing protein SOPP3 resulted in a fast and homogeneous killing of these microbes. Furthermore, our data indicate that the ROS type and yield as well as the localization of the applied PS protein can strongly influence the antibacterial spectrum and efficacy. These findings open up new opportunities for photodynamic inactivation of pathogenic bacteria.

Tetracyclines: light-activated antibiotics?

Tetracyclines are well established antibiotics but show phototoxicity as a side effect. Antimicrobial photodynamic inactivation uses nontoxic dyes combined with harmless light to destroy microbial cells by reactive oxygen species. Tetracyclines (demeclocycline and doxycycline) can act as light-activated antibiotics by binding to bacterial cells and killing them only upon illumination. The remaining tetracyclines can prevent bacterial regrowth after illumination has ceased. Antimicrobial photodynamic inactivation can be potentiated by potassium iodide. Azide quenched the formation of iodine, but not hydrogen peroxide. Demeclotetracycline (but not doxycycline) iodinated tyrosine after light activation in the presence of potassium iodide. Bacteria are killed by photoactivation of tetracyclines in the absence of oxygen. Since topical tetracyclines are already used clinically, blue light activation may increase the bactericidal effect.

Encapsulation of Gold Nanorods with Porphyrins for the Potential Treatment of Cancer and Bacterial Diseases: A Critical Review

Cancer and bacterial diseases have been the most incidental diseases to date. According to the World Health Report 2018, at least every family is affected by cancer around the world. In 2012, 14.1 million people were affected by cancer, and that figure is bound to increase to 21.6 million in 2030. Medicine therefore sorts out ways of treatment using conventional methods which have been proven to have many side effects. Researchers developed photothermal and photodynamic methods to treat both cancer and bacterial diseases. These methods pose fewer effects on the biological systems but still no perfect method has been synthesized. The review serves to explore porphyrin and gold nanorods to be used in the treatment of cancer and bacterial diseases: porphyrins as photosensitizers and gold nanorods as delivery agents. In addition, the review delves into ways of incorporating photothermal and photodynamic therapy aimed at producing a less toxic, more efficacious, and specific compound for the treatment.

Sodium nitrite potentiates antimicrobial photodynamic inactivation: possible involvement of peroxynitrate

We have recently shown that a wide range of different inorganic salts can potentiate antimicrobial photodynamic inactivation (aPDI) and TiO2-mediated antimicrobial photocatalysis. Potentiation has been shown with azide, bromide, thiocyanate, selenocyanate, and most strongly, with iodide. Here we show that sodium nitrite can also potentiate broad-spectrum aPDI killing of Gram-positive MRSA and Gram-negative Escherichia coli bacteria. Literature reports have previously shown that two photosensitizers (PS), methylene blue (MB) and riboflavin, when excited by broad-band light in the presence of nitrite could lead to tyrosine nitration. Addition of up to 100 mM nitrite gave 6 logs of extra killing in the case of Rose Bengal excited by green light against E. coli, and 2 logs of extra killing against MRSA (eradication in both cases). Comparable results were obtained for other PS (TPPS4 + blue light and MB + red light). Some bacterial killing was obtained when bacteria were added after light using a functionalized fullerene (LC15) + nitrite + blue light, and tyrosine ester amide was nitrated using both "in" and "after" modes with all four PS. The mechanism could involve formation of peroxynitrate by a reaction between superoxide radicals and nitrogen dioxide radicals; formation of the latter species was demonstrated by spin trapping with nitromethane.

Comparison of thiocyanate and selenocyanate for potentiation of antimicrobial photodynamic therapy

We have previously shown that antimicrobial photodynamic therapy (aPDT) mediated by different photosensitizers (PS) can be potentiated by a variety of inorganic salts. Potassium thiocyanate (KSCN) potentiated aPDT mediated by methylene blue (MB), while potassium selenocyanate (KSeCN) potentiated aPDT mediated by MB, Rose Bengal and the anionic porphyrin 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin dihydrochloride. However, the mechanisms of action that were proposed were fundamentally different. In the present study, we compare these two salts (KSCN and KSeCN) with different light-activated PS and different oxidative reactions for killing gram-positive and gram-negative bacteria. Overall KSeCN was more powerful than KSCN, and worked with a wider range of PS, while KSCN only worked with phenothiazinium salts. KSeCN produced killing when cells were added after light suggesting production of a semistable species called selenocyanogen (SeCN)2 . We tested three different oxidative reactions that can all potentially kill bacteria: lead tetraacetate (Pb[OAc]4 ); Fenton reagent (hydrogen peroxide [H2 O2 ] and ferrous sulfate) H2 O2 and horseradish peroxidase (HRP). In every case, KSeCN was substantially more effective (several logs) than KSCN in potentiating the bacterial killing. We conclude that (SeCN)2 is the mediator for aPDT using KSeCN, while sulfur trioxide radical anion is the mediator for KSCN using phenothiaziums. For H2 O2 /HRP with KSCN, hypothiocyanite is proposed to be the antibacterial agent in the literature, while hyposelenocyanite is said not to exist. Pb[OAc]4 is known to produce (SeCN)2 from KSeCN as well as the analogous (SCN)2 from KSCN. The mediators from Fenton reaction are unclear (pseudohalogen radical ions?) Both KSCN (which occurs naturally in the human body) and KSeCN may be clinically applicable.

Inorganic Salts and Antimicrobial Photodynamic Therapy: Mechanistic Conundrums?

We have recently discovered that the photodynamic action of many different photosensitizers (PSs) can be dramatically potentiated by addition of a solution containing a range of different inorganic salts. Most of these studies have centered around antimicrobial photodynamic inactivation that kills Gram-negative and Gram-positive bacteria in suspension. Addition of non-toxic water-soluble salts during illumination can kill up to six additional logs of bacterial cells (one million-fold improvement). The PSs investigated range from those that undergo mainly Type I photochemical mechanisms (electron transfer to produce superoxide, hydrogen peroxide, and hydroxyl radicals), such as phenothiazinium dyes, fullerenes, and titanium dioxide, to those that are mainly Type II (energy transfer to produce singlet oxygen), such as porphyrins, and Rose Bengal. At one extreme of the salts is sodium azide, that quenches singlet oxygen but can produce azide radicals (presumed to be highly reactive) via electron transfer from photoexcited phenothiazinium dyes. Potassium iodide is oxidized to molecular iodine by both Type I and Type II PSs, but may also form reactive iodine species. Potassium bromide is oxidized to hypobromite, but only by titanium dioxide photocatalysis (Type I). Potassium thiocyanate appears to require a mixture of Type I and Type II photochemistry to first produce sulfite, that can then form the sulfur trioxide radical anion. Potassium selenocyanate can react with either Type I or Type II (or indeed with other oxidizing agents) to produce the semi-stable selenocyanogen (SCN)₂. Finally, sodium nitrite may react with either Type I or Type II PSs to produce peroxynitrate (again, semi-stable) that can kill bacteria and nitrate tyrosine. Many of these salts (except azide) are non-toxic, and may be clinically applicable.

Comparison of two functionalized fullerenes for antimicrobial photodynamic inactivation: Potentiation by potassium iodide and photochemical mechanisms

A new fullerene (BB4-PPBA) functionalized with a tertiary amine and carboxylic acid was prepared and compared with BB4 (cationic quaternary group) for antimicrobial photodynamic inactivation (aPDI). BB4 was highly active against Gram-positive methicillin resistant Staphylococcus aureus (MRSA) and BB4-PPBA was moderately active when activated by blue light. Neither compound showed much activity against Gram-negative Escherichia coli or fungus Candida albicans. Therefore, we examined potentiation by addition of potassium iodide. Both compounds were highly potentiated by KI (1-6 extra logs of killing). BB4-PPBA was potentiated more than BB4 against MRSA and E. coli, while for C. albicans the reverse was the case. Addition of azide potentiated aPDI mediated by BB4 against MRSA, but abolished the potentiation caused by KI with both compounds. The killing ability after light decayed after 24 h in the case of BB4, implying a contribution from hypoiodite as well as free iodine. Tyrosine was readily iodinated with BB4-PPBA plus KI, but less so with BB4. We conclude that the photochemical mechanisms of these two fullerenes are different. BB4-PPBA is more Type 2 (singlet oxygen) while BB4 is more Type 1 (electron transfer). There is also a possibility of direct bacterial killing by electron transfer, but this will require more study to prove.

Can light-based approaches overcome antimicrobial resistance?

The relentless rise of antibiotic resistance is considered one of the most serious problems facing mankind. This mini-review will cover three cutting-edge approaches that use light-based techniques to kill antibiotic-resistant microbial species, and treat localized infections. First, we will discuss antimicrobial photodynamic inactivation using rationally designed photosensitizes combined with visible light, with the added possibility of strong potentiation by inorganic salts such as potassium iodide. Second, the use of blue and violet light alone that activates endogenous photoactive porphyrins within the microbial cells. Third, it is used for "safe UVC" at wavelengths between 200 nm and 230 nm that can kill microbial cells without damaging host mammalian cells. We have gained evidence that all these approaches can kill multidrug resistant bacteria in vitro, and they do not induce themselves any resistance, and moreover can treat animal models of localized infections caused by resistant species that can be monitored by noninvasive bioluminescence imaging. Light-based antimicrobial approaches are becoming a growing translational part of anti-infective treatments in the current age of resistance.

Antimicrobial photodynamic inactivation is potentiated by the addition of selenocyanate: Possible involvement of selenocyanogen?

We previously showed that antimicrobial photodynamic inactivation (aPDI) of Gram-positive and Gram-negative bacteria mediated by the phenothiazinium dye, methylene blue (MB), was potentiated by the addition of potassium thiocyanate (10 mM). The mechanism was suggested to involve a singlet oxygen-mediated reaction with SCN to form sulfite and cyanide and then to produce sulfur trioxide radical anion. We now report that potassium selenocyanate (concentrations up to 100 mM) can also potentiate (up to 6 logs of killing) aPDI mediated by a number of different photosensitizers (PS): MB, rose bengal and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin dihydrochloride (as low as 200 nM). When a mixture of selenocyanate with these PS in solution was illuminated and then bacteria were added after the light, there was up to 6 logs of killing (Gram-negative > Gram-positive) but the antibacterial species decayed rapidly (by 20 minutes). Our hypothesis to explain this antibacterial activity is the formation of selenocyanogen (SeCN)2 by reaction with singlet oxygen (1 O2 ) as shown by quenching of 1 O2 by SeCN and increased photoconsumption of oxygen. The fact that lead tetraacetate reacted with SeCN (literature preparation of (SeCN)2 ) also produced a short-lived antibacterial species supports this hypothesis.

Antimicrobial photodynamic therapy mediated by methylene blue and potassium iodide to treat urinary tract infection in a female rat model

Drug-resistant urinary tract infections (UTIs) are difficult and sometimes impossible to treat. Many UTIs are caused by uropathogenic Escherichia coli (UPEC). We developed an intact rat model of UTI, by catheterizing female rats and introducing a bioluminescent UPEC strain into the female rat bladder which lasted for up to six days. We recently showed that antimicrobial photodynamic inactivation (aPDI) of a bacterial infection mediated by the well-known phenothiazinium salt, methylene blue (MB) could be strongly potentiated by addition of the non-toxic salt potassium iodide (KI). In the intact rat model we introduced MB into the bladder by catheter, followed by KI solution and delivered intravesicular illumination with a diffusing fiber connected to a 1 W 660 nm laser. Bioluminescent imaging of the bacterial burden was carried out during the procedure and for 6 days afterwards. Light-dose dependent loss of bioluminescence was observed with the combination of MB followed by KI, but recurrence of infection was seen the next day in some cases. aPDT with MB + KI gave a significantly shorter duration of infection compared to untreated controls. aPDT with MB alone was the least effective. No signs of aPDT damage to the bladder lining were detected. This procedure to treat urinary tract infections without antibiotics by using already approved pharmaceutical substances (MB and KI) may have clinical applicability, either initially as a stand-alone therapy, or as an adjunct to antibiotic therapy by a rapid and substantial reduction of the bacterial burden.

Laser immunotherapy for cutaneous squamous cell carcinoma with optimal thermal effects to enhance tumour immunogenicity

BACKGROUND

Laser immunotherapy is a new anti-cancer therapy combining photothermal therapy and immunostimulation. It can eliminate the tumours by damaging tumour cells directly and promoting the release of damage-associated molecular patterns (DAMPs) to enhance tumour immunogenicity. The aim of this study was to investigate the thermal effects of laser immunotherapy and to evaluate the effectiveness and safety of laser immunotherapy for cutaneous squamous cell carcinoma (cSCC).

METHODS

The cell viability and the DAMPs productions of heat-treated cSCC A431 cells in different temperatures were investigated. Laser immunotherapy with the optimal thermal effect for DAMPs production was performed on SKH-1 mice bearing ultraviolet-induced cSCC and a patient suffering from a large refractory cSCC.

RESULTS

The temperature in the range of 45-50 °C killing half of A431 cells had an optimal thermal effect for the productions of DAMPs. The thermal effect could be further enhanced by local application of imiquimod, an immunoadjuvant. Laser immunotherapy eliminated most tumours and improved the survival rate of the ultraviolet-induced cSCC-bearing SKH-1 mice (p < 0.05). The patient with cSCC treated by laser immunotherapy experienced a significant tumour reduction after laser immunotherapy increased the amounts of infiltrating lymphocytes in the tumour. No obviously adverse effect was observed in the mice experiment or in the clinical application.

CONCLUSIONS

Our results strongly indicate that laser immunotherapy with optimal thermal effects is an effective and safe treatment modality for cSCC.

Photodynamic antimicrobial chemotherapy (PACT) using toluidine blue inhibits both growth and biofilm formation by Candida krusei

Among non-albicans Candida species, the opportunistic pathogen Candida krusei emerges because of the high mortality related to infections produced by this yeast. The Candida krusei is an opportunistic pathogen presenting an intrinsic resistance to fluconazol. In spite of the reduced number of infections produced by C. krusei, its occurrence is increasing in some groups of patients submitted to the use of fluconazol for prophylaxis. Photodynamic antimicrobial chemotherapy (PACT) is a potential antimicrobial therapy that combines visible light and a nontoxic dye, known as a photosensitizer, producing reactive oxygen species (ROS) that can kill the treated cells. The objective of this study was to investigate the effects of PACT, using toluidine blue, as a photosensitizer on both growth and biofilm formation by Candida krusei. In this work, we studied the effect of the PACT, using TB on both cell growth and biofilm formation by C. krusei. PACT was performed using a light source with output power of 0.068 W and peak wavelength of 630 nm, resulting in a fluence of 20, 30, or 40 J/cm². In addition, ROS production was determined after PACT. The number of samples used in this study varied from 6 to 8. Statistical differences were evaluated by analysis of variance (ANOVA) and post hoc comparison with Tukey-Kramer test. PACT inhibited both growth and biofilm formation by C. krusei. It was also observed that PACT stimulated ROS production. Comparing to cells not irradiated, irradiation was able to increase ROS production in 11.43, 6.27, and 4.37 times, in the presence of TB 0.01, 0.02, and 0.05 mg/mL, respectively. These results suggest that the inhibition observed in the cell growth after PACT could be related to the ROS production, promoting cellular damage. Taken together, these results demonstrated the ability of PACT reducing both cell growth and biofilm formation by C. krusei.

Potentiation of antimicrobial photodynamic inactivation by inorganic salts

INTRODUCTION

Antimicrobial photodynamic inactivation (aPDI) involves the use of non-toxic dyes excited with visible light to produce reactive oxygen species (ROS) that can destroy all classes of microorganisms including bacteria, fungi, parasites, and viruses. Selectivity of killing microbes over host mammalian cells allows this approach (antimicrobial photodynamic therapy, aPDT) to be used in vivo as an alternative therapeutic approach for localized infections especially those that are drug-resistant. Areas covered: We have discovered that aPDI can be potentiated (up to 6 logs of extra killing) by the addition of simple inorganic salts. The most powerful and versatile salt is potassium iodide, but potassium bromide, sodium thiocyanate, sodium azide and sodium nitrite also show potentiation. The mechanism of potentiation with iodide is likely to be singlet oxygen addition to iodide to form iodine radicals, hydrogen peroxide and molecular iodine. Another mechanism involves two-electron oxidation of iodide/bromide to form hypohalites. A third mechanism involves a one-electron oxidation of azide anion to form azide radical. Expert commentary: The addition of iodide has been shown to improve the performance of aPDT in several animal models of localized infection. KI is non-toxic and is an approved drug for antifungal therapy, so its transition to clinical use in aPDT should be straightforward.

Advances in antimicrobial photodynamic inactivation at the nanoscale

The alarming worldwide increase in antibiotic resistance amongst microbial pathogens necessitates a search for new antimicrobial techniques, which will not be affected by, or indeed cause resistance themselves. Light-mediated photoinactivation is one such technique that takes advantage of the whole spectrum of light to destroy a broad spectrum of pathogens. Many of these photoinactivation techniques rely on the participation of a diverse range of nanoparticles and nanostructures that have dimensions very similar to the wavelength of light. Photodynamic inactivation relies on the photochemical production of singlet oxygen from photosensitizing dyes (type II pathway) that can benefit remarkably from formulation in nanoparticle-based drug delivery vehicles. Fullerenes are a closed-cage carbon allotrope nanoparticle with a high absorption coefficient and triplet yield. Their photochemistry is highly dependent on microenvironment, and can be type II in organic solvents and type I (hydroxyl radicals) in a biological milieu. Titanium dioxide nanoparticles act as a large band-gap semiconductor that can carry out photo-induced electron transfer under ultraviolet A light and can also produce reactive oxygen species that kill microbial cells. We discuss some recent studies in which quite remarkable potentiation of microbial killing (up to six logs) can be obtained by the addition of simple inorganic salts such as the non-toxic sodium/potassium iodide, bromide, nitrite, and even the toxic sodium azide. Interesting mechanistic insights were obtained to explain this increased killing.