Structure-function relationships of Nile blue (EtNBS) derivatives as antimicrobial photosensitizers

Photodynamic therapy for ESKAPE pathogens: An emerging approach to combat antimicrobial resistance (AMR)

The increase in antimicrobial resistance, particularly in ESKAPE pathogens, has resulted in the dire need for new therapeutic approaches. ESKAPE is an acronym for a group of bacteria that are responsible for a majority of nosocomial and community acquired infections. The acronym stands for Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species. These pathogens are known for their ability to develop resistance to multiple antibiotics, making them difficult to treat thus posing a significant threat to public health. In light of the alarming consequences of antimicrobial resistance, it has been estimated that, in the absence of a substantial increase in the rate of development of new effective drugs, the number of casualties related to these infections will increase from about 700,000 in 2016 up to nearly 10,000,000 in 2050 [1]. One potential strategy to treat these pathogens is photodynamic therapy (PDT). In the early 20th century, Oscar Raab observed the phototoxicity of acridine red against Paramecium caudatum, while Tappenier and Jesionek demonstrated the photodynamic effects of eosin for treating cutaneous diseases. These discoveries laid the foundation for Photodynamic Therapy (PDT), which utilizes a non-toxic photosensitizer (PS) followed by targeted light irradiation for treatment [2]. PDT involves the use of a photosensitizer, a light source, and oxygen to eliminate highly active infectious pathogens such as bacteria, viruses, and fungi. PDT is known to possess several advantages including localized treatment and fewer side effects. Various photosensitizers and light sources have been assessed in different strains, showing promising results suggesting PDT to be a promising potential treatment option. PDT utilizes PS compounds with suitable light absorption that showcase effective results against the pathogens in vitro and in vivo, including BODIPY derivatives, Methylene Blue, and other dyes like porphyrin derivatives, phthalocyanines, indole derivatives, Photophrin, etc., inhibiting the growth of infections, for both in planktonic cells and in biofilms. Combination of PDT with other therapies like efflux pump inhibitors or quorum sensing inhibitors has also proven to be efficacious. However, this domain further needs to be assessed before it reaches the society.

A S-substituted Nile Blue-derived bifunctional near-infrared fluorescent probe for in vivo carboxylesterase imaging-guided photodynamic therapy of hepatocellular carcinoma

The development of theranostic probes that integrate both diagnostic and therapeutic functions still remains an intractable challenge in precise cancer treatment. Herein, a novel bifunctional near-infrared (NIR) fluorescent probe (CEP1) for carboxylesterase (CE) imaging and photodynamic therapy (PDT) of hepatocellular carcinoma (HCC) has been firstly developed and successfully applied in vitro and in vivo. The probe was constructed by introducing carbamate as both the recognition unit and the fluorescence quenching unit into the fluorophore S-substituted Nile Blue (ENBS) via a self-eliminating spacer with substituted chloride. It can be activated by CE and hydrolyzed into fluorescent ENBS, which recover fluorescence at about 700 nm, and can generate superoxide radical anions under NIR irradiation. Additionally, the probe could effectively distinguish tumor cells from normal cells by CE imaging of live cells. Furthermore, it could achieve CE imaging in vivo and significantly inhibits tumor growth by imaging-guided PDT. Therefore, this study offers a promising and attractive platform for activatable imaging-guided PDT of HCC.

Molecular Charge and Antibacterial Performance Relationships of Aggregation-Induced Emission Photosensitizers

Bacterial infections remain a major cause of morbidity worldwide due to drug resistance of pathogenic bacteria. Photodynamic therapy (PDT) has emerged as a promising approach to overcome this drug resistance. However, existing photosensitizers (PSs) are broad-spectrum antibacterial agents that dysregulate the microflora balance resulting in undesirable side effects. Herein, we synthesized a series of aggregation-induced emission (AIE)-active PSs with a lipophilic cationic AIE core with varying charges, named TBTCP and its derivatives. The association of the difference in their molecular charge with the antibacterial effects was systemically investigated. Among the derivatives presented, TBTCP-SF with the electronegative sulfonate group nulled its ability to bind to and ablate Gram-positive (G+) or Gram-negative (G-) bacteria. TBTCP-QY modified by electropositive quaternary ammonium facilitated binding and augmented the photodynamic antibacterial activity for both G+ and G- bacteria. TBTCP-PEG with hydrophilic neutral ligands selectively bound and inactivated G+ bacteria. Under white-light illumination, TBTCP-PEG ablated 99.9% methicillin-resistant Staphylococcus aureus (MRSA) and promoted wound healing in MRSA-infected mice, eliminating MRSA infection both in vitro and in vivo. Our work provides unprecedented insight into the utility of AIE-active PSs for highly targeted and efficient photodynamic ablation of either G+ or G- bacteria that can be translated to next-generation antibacterial materials.

Coping with the ESKAPE pathogens: Evolving strategies, challenges and future prospects

Globally, the ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) are the major cause of nosocomial infections. These pathogens are multidrug resistant, and their negative impacts have brought serious health challenges and economic burden on many countries worldwide. Thus, this narrative review exploits different emerging alternative therapeutic strategies including combination antibiotics, antimicrobial peptides ((AMPs), bacteriophage and photodynamic therapies used in the treatment of the ESKAPE pathogens, their merits, limitations, and future prospects. Our findings indicate that ESKAPE pathogens exhibit resistance to drug using different mechanisms including drug inactivation by irreversible enzyme cleavage, drug-binding site alteration, diminution in permeability of drug or drug efflux increment to reduce accumulation of drug as well as biofilms production. However, the scientific community has shown significant interest in using these novel strategies with numerous benefits although they have some limitations including but not limited to instability and toxicity of the therapeutic agents, or the host developing immune response against the therapeutic agents. Thus, comprehension of resistance mechanisms of these pathogens is necessary to further develop or modify these approaches in order to overcome these health challenges including the barriers of bacterial resistance.

Application of photodynamic therapy in immune-related diseases

Photodynamic therapy (PDT) is a therapeutic modality that utilizes photodamage caused by photosensitizers and oxygen after exposure to a specific wavelength of light. Owing to its low toxicity, high selectivity, and minimally invasive properties, PDT has been widely applied to treat various malignant tumors, premalignant lesions, and infectious diseases. Moreover, there is growing evidence of its immunomodulatory effects and potential for the treatment of immune-related diseases. This review mainly focuses on the effect of PDT on immunity and its application in immune-related diseases.

In Vitro and In Vivo Demonstration of Ultraefficient and Broad-Spectrum Antibacterial Agents for Photodynamic Antibacterial Chemotherapy

Increasing threats from both pathogenic infections and antibiotic resistance highlight the pressing demand for nonantibiotic agents and alternative therapies. Herein, we report several new phenothiazinium-based derivatives, which could be readily synthesized via fragment-based assembly, which exhibited remarkable bactericidal activities both in vitro and in vivo. Importantly, in contrast to numerous clinically and preclinically used antibacterial photosensitizers, these compounds were able to eliminate various types of microorganisms, including Gram-(+) Staphylococcus aureus (S. aureus), Gram-(-) Escherichia coli, multidrug-resistant S. aureus, and their associated biofilms, at low drug and light dosages (e.g., 0.21 ng/mL in vitro and 1.63 ng/cm² in vivo to eradicate S. aureus at 30 J/cm²). This study thus unveils the potential of these novel phenothiaziniums as potent antimicrobial agents for highly efficient photodynamic antibacterial chemotherapy.

Cytosolic NQO1 Enzyme-Activated Near-Infrared Fluorescence Imaging and Photodynamic Therapy with Polymeric Vesicles

The utilization of enzymes as a triggering module could endow responsive polymeric nanostructures with selectivity in a site-specific manner. On the basis of the fact that endogenous NAD(P)H:quinone oxidoreductase isozyme 1 (NQO1) is overexpressed in many types of tumors, we report on the fabrication of photosensitizer-conjugated polymeric vesicles, exhibiting synergistic NQO1-triggered turn-on of both near-infrared (NIR) fluorescence emission and a photodynamic therapy (PDT) module. For vesicles self-assembled from amphiphilic block copolymers containing quinone trimethyl lock-capped self-immolative side linkages and quinone-bridged photosensitizers (coumarin and Nile blue) in the hydrophobic block, both fluorescence emission and PDT potency are initially in the "off" state due to "double quenching" effects, that is, dye-aggregation-caused quenching and quinone-rendered PET (photoinduced electron transfer) quenching. After internalization into NQO1-positive vesicles, the cytosolic NQO1 enzyme triggers self-immolative cleavage of quinone linkages and fluorogenic release of conjugated photosensitizers, leading to NIR fluorescence emission turn-on and activated PDT. This process is accompanied by the transformation of vesicles into cross-linked micelles with hydrophilic cores and smaller sizes and triggered dual drug release, which could be directly monitored by enhanced magnetic resonance (MR) imaging for vesicles conjugated with a DOTA(Gd) complex in the hydrophobic bilayer. We further demonstrate that the above strategy could be successfully applied for activated NIR fluorescence imaging and tissue-specific PDT under both cellular and in vivo conditions.

Photoinactivation of ESKAPE pathogens: overview of novel therapeutic strategy

The emergence of antimicrobial drug resistance requires development of alternative therapeutic options. Multidrug-resistant strains of Enterococcus spp., Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumanii, Pseudomonas aeruginosa and Enterobacter spp. are still the most commonly identified antimicrobial-resistant pathogens. These microorganisms are part of the so-called 'ESKAPE' pathogens to emphasize that they currently cause the majority of hospital acquired infections and effectively 'escape' the effects of antibacterial drugs. Thus, alternative, safer and more efficient antimicrobial strategies are urgently needed, especially against 'ESKAPE' superbugs. Antimicrobial photodynamic inactivation is a therapeutic option used in the treatment of infectious diseases. It is based on a combination of a photosensitizer, light and oxygen to remove highly metabolically active cells.

Influence of phenothiazines, phenazines and phenoxazine on cation transport in Candida albicans

AIMS

(i) To analyse the increase in calcium ion uptake caused by several cationic dyes on Candida albicans, (ii) to postulate a mechanism, (iii) to define the effects of Zn ions on the phenomenon, and (iv) to propose the use of the dyes or their derivatives against C. albicans.

METHODS AND RESULTS

Cells were grown in yeast peptone dextrose medium and starved. We measured the hydrophobic solvent/water partition coefficients and the dyes uptake by the cells and found no correlation with their hydrophobicity. Most of the dyes caused an increase in K+ efflux (in correlation with a decrease in 86 Rb+ uptake), and a raise in Ca2+ uptake except for those used as Zn salts, but not of their HCl salts. Respiration and acidification of the medium were modified only with few dyes and interestingly, when exposing cultures to nile blue, neutral red and toluidine blue ZnCl2 a decrease in C. albicans growth was observed.

CONCLUSIONS

We propose a general mechanism for the stimulation of Ca2+ uptake by the dyes used. Some of the dyes tested might be used as agents against C. albicans, probably combined with other agents. Moreover, the effects of Zn ions on Ca2+ uptake and on cell growth open possibilities of further studies, not only of their effects, but also of the mechanism of Ca2+ transport in C. albicans and other yeasts.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study, in conjunction with previously published results, contribute to the basic research regarding ion transport in C. albicans and the role of zinc in this process. Besides, suggests the additional use of dyes, along with other antifungals agents, as combined therapy against candidiasis. Derived dyes from those used also might be possible therapeutic agents against this disease.

Dissociation of zinc phthalocyanine aggregation on bacterial surface is key for photodynamic antimicrobial effect

Antimicrobial photodynamic therapy (aPDT) is an effective mean for killing bacteria in this era of increasing multi-antibiotic resistance, and possesses a number of unique advantages. Much effort has been devoted to the development a key component of aPDT photosensitizers (PSs). We synthesized a series of PSs with different positive charges (ZnPc(Lys)[Formula: see text], where [Formula: see text] 3, 5, 7, and studied their antibacterial activities and mechanisms against Escherichia coli (E. coli). Interestingly, the ZnPc(Lys)[Formula: see text] derivative showed stronger antibacterial effect (MIC = 25.3 [Formula: see text]M) than the other two PSs (MICs = 50.6 [Formula: see text]M), even though this PS did not have the highest uptake on bacteria among these PSs. It was ZnPc(Lys)[Formula: see text] that possessed the highest bacterial uptake. ZnPc(Lys)[Formula: see text] was found to have the highest monomeric fractions (62.0%) on bacteria surface than the other two PSs (37.9% for [Formula: see text] 3 and 33.9% [Formula: see text] = 7). These results clearly demonstrate that PS conformation on bacterial surface as a key parameter determining antibacterial efficacy of PSs. Other mechanistic aspects of photodynamic effects, including PS binding kinetics, bacterial surface hydrophobicity, zeta potential of bacteria, membrane permeability and bacterial signaling pathways were also studied.

Synthetic, small-molecule photoantimicrobials - a realistic approach

The search for suitable, low-molecular weight photoantimicrobials for use in infection control has strong foundations in conventional antiseptic research from the early-mid 20^th Century. Many examples of dyes exist having conventional antimicrobial activity among the azine, acridine and triphenylmethane families which have since also been found to exhibit photosensitising capabilities. The prior employment of these examples in human antisepsis provides a practical basis in terms of low host toxicity, while extant structure-activity relationships for conventional antimicrobial activity can support the development of similar relationships for photoactivated cell killing. The range of chromophores covered allows progress to be made both in topical and deeper, fluid-involved infections.

Breaching the wall: morphological control of efficacy of phthalocyanine-based photoantimicrobials

In this paper, photophysical, theoretical and biological studies are combined, highlighting the importance of different characteristics for designing new and more effective PSs.

Fluorination of phthalocyanine substituents: Improved photoproperties and enhanced photodynamic efficacy after optimal micellar formulations

A fluorinated phthalocyanine and its non-fluorinated analogue were selected to evaluate the potential enhancement of fluorination on photophysical, photochemical and redox properties as well as on biological activity in cellular and animal models. Due to the pharmacological relevance, the affinity of these phthalocyanines towards biological membranes (logP_ow) as well as their primary interaction with human serum albumin (HSA) or low-density lipoprotein (LDL) were determined. Water-dispersible drug formulation of phthalocyanines via Pluronic^®-based triblock copolymer micelles was prepared to avoid self-aggregation effects and to improve their delivery. The obtained results demonstrate that phthalocyanines incorporation into tunable-polymeric micelles significantly enhanced their cellular uptake and their photocytotoxicity. The improved biodistribution and photodynamic efficacy of the phthalocyanines-triblock copolymer conjugates was also confirmed in vivo in CT26 bearing BALB/c mice. PDT with both compounds led to tumor growth inhibition in all treated animals. Fluorinated phthalocyanine 2 turned out to be the most effective anticancer agent as the tumors of 20% of mice treated regressed completely and did not appear for over one year after treatment.

Non-conventional therapeutics for oral infections

As our knowledge of host-microbial interactions within the oral cavity increases, future treatments are likely to be more targeted. For example, efforts to target a single species or key virulence factors that they produce, while maintaining the natural balance of the resident oral microbiota that acts to modulate the host immune response would be an advantage. Targeted approaches may be directed at the black-pigmented anaerobes, Porphyromonas gingivalis and Prevotella intermedia, associated with periodontitis. Such pigments provide an opportunity for targeted phototherapy with high-intensity monochromatic light. Functional inhibition approaches, including the use of enzyme inhibitors, are also being explored to control periodontitis. More general disruption of dental plaque through the use of enzymes and detergents, alone and in combination, shows much promise. The use of probiotics and prebiotics to improve gastrointestinal health has now led to an interest in using these approaches to control oral disease. More recently the potential of antimicrobial peptides and nanotechnology, through the application of nanoparticles with biocidal, anti-adhesive and delivery capabilities, has been explored. The aim of this review is to consider the current status as regards non-conventional treatment approaches for oral infections with particular emphasis on the plaque-related diseases.

The chemistry and biology of guanidine natural products

The present review discusses the isolation, structure determination, synthesis, biosynthesis and biological activities of secondary metabolites bearing a guanidine group.

Antibacterial photodynamic therapy: overview of a promising approach to fight antibiotic-resistant bacterial infections

Antibacterial photodynamic therapy (APDT) has drawn increasing attention from the scientific society for its potential to effectively kill multidrug-resistant pathogenic bacteria and for its low tendency to induce drug resistance that bacteria can rapidly develop against traditional antibiotic therapy. The review summarizes the mechanism of action of APDT, the photosensitizers, the barriers to PS localization, the targets, the in vitro-, in vivo-, and clinical evidence, the current developments in terms of treating Gram-positive and Gram-negative bacteria, the limitations, as well as future perspectives. Relevance for patients: A structured overview of all important aspects of APDT is provided in the context of resistant bacterial species. The information presented is relevant and accessible for scientists as well as clinicians, whose joint effort is required to ensure that this technology benefits patients in the post-antibiotic era.

Structure-activity relationship study of anticancer thymidine-quinoxaline conjugates under the low radiance of long wavelength ultraviolet light for photodynamic therapy

Thymidine quinoxaline conjugate (dT-QX) is a thymidine analog with selective cytotoxicity against different cancer cells. In this study, the structure activity relationship study of dT-QX analogs was carried out under the low radiance of black fluorescent (UVA-1) light. Significantly enhanced cytotoxicity was observed under UVA-1 activation among analogs containing both thymidine and quinoxaline moieties with different length of the linker, stereochemical configuration and halogenated substituents. Among these analogs, the thymidine dichloroquinoxaline conjugate exhibited potent activity under UVA-1 activation as the best candidate with EC50 at 0.67 μM and 1.3 μM against liver and pancreatic cancer cells, respectively. In contrast, the replacement of thymidine moiety with a galactosyl residue or the replacement of quinoxaline moiety with a fluorescent pyrenyl residue or a simplified diketone structure resulted in the full loss of activity. Furthermore, it was revealed that the low radiance of UVA-1 at 3 mW/cm(2) for 20 min was sufficient enough to induce the full cytotoxicity of thymidine dichloroquinoxaline conjugate and that the cytotoxic mechanism was achieved through a rapid and steady production of reactive oxygen species.