Intracellular activity of clinical concentrations of phenothiazines including thioridiazine against phagocytosed Staphylococcus aureus

In vitro antileishmanial activity of thioridazine on amphotericin B unresponsive/ sensitive Leishmania donovani promastigotes and intracellular amastigotes

The recent increase in the drug (liposomal amphotericin-B) unresponsive cases becomes hostile for the visceral leishmaniasis (VL) elimination target. The quest for new antileishmanial drugs is on the way and may demand more time. Meanwhile, drug repurposing is a quite promising option to explore further. We made such an attempt with thioridazine (TRZ), a first-line antipsychotic drug, which was reported for antimicrobial activity. In this study, we evaluated the drug activity of TRZ against amphotericin-B (Amp-B) sensitive and unresponsive Leishmania donovani promastigotes, as well as intracellular amastigotes (drug sensitive). We observed a potent antileishmanial activity of TRZ with significantly low half maximal inhibitory concentrations (IC50) on both the variants of promastigotes (0.61 ± 0.15 μM). These concentrations are comparable to the previously reported IC50 concentration of the current antileishmanial drug (Amp-B) against L. donovani. Light microscopy reveals the perturbations in promastigote morphology upon TRZ treatment. The in vitro studies on human macrophage cell lines determine the 50% cytotoxicity concentration (CC50) of TRZ on host cells as 20.046 μM and a half maximal effective concentration (EC50) as 0.91 μM during L. donovani infection, in turn selectivity index (SI) was calculated as 22.03 μM. Altogether, the results demonstrate that TRZ has the potential for drug repurposing and further studies on animal models could provide better insights for VL treatment.

Clinical significance of microbiota changes under the influence of psychotropic drugs. An updated narrative review

Relationship between drugs and microbiota is bilateral. Proper composition thus function of microbiota is a key to some medications used in modern medicine. However, there is also the other side of the coin. Pharmacotherapeutic agents can modify the microbiota significantly, which consequently affects its function. A recently published study showed that nearly 25% of drugs administered to humans have antimicrobial effects. Multiple antidepressants are antimicrobials,. and antibiotics with proven antidepressant effects do exist. On the other hand, antibiotics (e.g., isoniaside, minocycline) confer mental phenotype changes, and adverse effects caused by some antibiotics include neurological and psychological symptoms which further supports the hypothesis that intestinal microbiota may affect the function of the central nervous system. Here we gathered comprehensively data on drugs used in psychiatry regarding their antimicrobial properties. We believe our data has strong implications for the treatment of psychiatric entities. Nevertheless the study of ours highlights the need for more well-designed trials aimed at analysis of gut microbiota function.

Drug repurposing to overcome microbial resistance

Infections are a growing global threat, and the number of resistant species of microbial pathogens is alarming. However, the rapid development of cross-resistant or multidrug-resistant strains and the development of so-called 'superbugs' are in stark contrast to the number of newly launched anti-infectives on the market. In this review, I summarize the causes of antimicrobial resistance, briefly discuss different approaches to the discovery and development of new anti-infective drugs, and focus on drug repurposing strategy, which is discussed from all possible perspectives. A comprehensive overview of drugs of other indications tested for their in vitro antimicrobial activity to support existing anti-infective therapeutics is provided, including several critical remarks on this strategy of repurposing non-antibiotics to antibacterial drugs.

Priming with biocides: a pathway to antibiotic resistance?

AIMS

To investigate the priming effects of sub-inhibitory concentrations of biocides on antibiotic resistance in bacteria.

METHODS AND RESULTS

Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus were exposed to sub-inhibitory concentrations of biocides via a gradient plate method. Minimum inhibitory concentration (MIC) and antibiotic susceptibility were determined, and efflux pump inhibitors (thioridazine and chlorpromazine) were used to investigate antibiotic resistance mechanism(s). Escherichia coli displayed a 2-fold increase in MIC (32 to 64 mg l-1) to H₂ O₂ which was stable after 15 passages, but lost after 6 weeks, and P. aeruginosa displayed a 2-fold increase in MIC (64 to 128 mg l-1) to BZK which was also stable for 15 passages. There were no other tolerances observed to biocides in E. coli, P. aeruginosa or S. aureus, however stable cross-resistance to antibiotics was observed in the absence of a stable increased tolerance to biocides. Six-fold increases in MIC to cephalothin and four-fold to ceftriaxone and ampicillin were observed in hydrogen peroxide primed E. coli. Chlorhexidine primed S. aureus showed a four-fold increase in MIC to oxacillin, and glutaraldehyde-primed P. aeruginosa showed four-fold (sulphatriad) and eight-fold (ciprofloxacin) increases in MIC. Thioridazine increased the susceptibility of E. coli to cephalothin and cefoxitin by four and two-fold respectively, and both thioridazine and chlorpromazine increased the susceptibility S. aureus to oxacillin by eight and four-fold respectively.

CONCLUSIONS

These findings demonstrate that sub-inhibitory concentrations of biocides can prime bacteria to become resistant to antibiotics even in the absence of stable biocide tolerance and suggests activation of efflux mechanisms may be a contributory factor.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrates the effects of low-level exposure of biocides (priming) on antibiotic resistance even in the absence of obvious increased biocidal tolerance.

Multidrug resistance crisis during COVID-19 pandemic: Role of anti-microbial peptides as next-generation therapeutics

The decreasing effectiveness of conventional drugs due to multidrug-resistance is a major challenge for the scientific community, necessitating development of novel antimicrobial agents. In the present era of coronavirus 2 (COVID-19) pandemic, patients are being widely exposed to antimicrobial drugs and hence the problem of multidrug-resistance shall be aggravated in the days to come. Consequently, revisiting the phenomena of multidrug resistance leading to formulation of effective antimicrobial agents is the need of the hour. As a result, this review sheds light on the looming crisis of multidrug resistance in wake of the COVID-19 pandemic. It highlights the problem, significance and approaches for tackling microbial resistance with special emphasis on anti-microbial peptides as next-generation therapeutics against multidrug resistance associated diseases. Antimicrobial peptides exhibit exceptional mechanism of action enabling rapid killing of microbes at low concentration, antibiofilm activity, immunomodulatory properties along with a low tendency for resistance development providing them an edge over conventional antibiotics. The review is unique as it discusses the mode of action, pharmacodynamic properties and application of antimicrobial peptides in areas ranging from therapeutics to agriculture.

Repositioning of non-antibiotic drugs as an alternative to microbial resistance: a systematic review

The global spread of microbial resistance coupled with high costs and slow pace in the discovery of a new antibiotic have made drug repositioning an attractive and promising alternative in the treatment of infections caused by multidrug resistant (MDR) microorganisms. The reuse involves the production of compounds with lower costs and development time, using diversified production technologies. The present systematic review aimed to present a selection of studies published in the last 20 years, which report the antimicrobial activity of non-antibiotic drugs that are candidates for repositioning, which could be used against the current microbial multidrug resistance. A search was performed in the PubMed, SciELO and Google Scholar databases using the following search strategies: [(drug repurposing) OR (drug repositioning) OR (repositioning) AND (non-antibiotic) AND (antibacterial activity) AND (antimicrobial activity)]. Overall, 112 articles were included, which explored the antimicrobial activity in antidepressants, antihypertensives, anti-inflammatories, antineoplastics, hypoglycemic agents, among other drugs. It was concluded that they have significant antimicrobial activity in vitro and in vivo, against standard strain and clinical isolates (Gram-negative and Gram-positive) and fungi. When associated with antibacterials, most of these drugs had their antibacterial activity enhanced. It was also a consensus of the studies included in this review that the presence of aromatic rings in the molecular structure contributes to antimicrobial activity. This review highlights the potential repositioning of several classes of non-antibiotic drugs as promising candidates for repositioning in the treatment of severe bacterial infections of MDR bacteria, extensively resistant (XDR) and pan-resistant (PDR) to drugs.

Antidepressants and Antipsychotic Agents as Repurposable Oncological Drug Candidates

Drug repurposing, also known as drug repositioning/reprofiling, is a relatively new strategy for the identification of alternative uses of well-known therapeutics that are outside the scope of their original medical indications. Such an approach might entail a number of advantages compared to standard de novo drug development, including less time needed to introduce the drug to the market, and lower costs. The group of compounds that could be considered as promising candidates for repurposing in oncology include the central nervous system drugs, especially selected antidepressant and antipsychotic agents. In this article, we provide an overview of some antidepressants (citalopram, fluoxetine, paroxetine, sertraline) and antipsychotics (chlorpromazine, pimozide, thioridazine, trifluoperazine) that have the potential to be repurposed as novel chemotherapeutics in cancer treatment, as they have been found to exhibit preventive and/or therapeutic action in cancer patients. Nevertheless, although drug repurposing seems to be an attractive strategy to search for oncological drugs, we would like to clearly indicate that it should not replace the search for new lead structures, but only complement de novo drug development.

Do phenothiazines possess antimicrobial and efflux inhibitory properties?

Antibiotic resistance is a global health concern; the rise of drug-resistant bacterial infections is compromising the medical advances that resulted from the introduction of antibiotics at the beginning of the 20th century. Considering that the presence of mutations within individuals in a bacterial population may allow a subsection to survive and propagate in response to selective pressure, as long as antibiotics are used in the treatment of bacterial infections, development of resistance is an inevitable evolutionary outcome. This, combined with the lack of novel antibiotics being released to the clinical market, means the need to develop alternative strategies to treat these resistant infections is critical. We discuss how the use of antibiotic adjuvants can minimise the appearance and impact of resistance. To this effect, several phenothiazine-derived drugs have been shown to potentiate the activities of antibiotics used to treat infections caused by Gram-positive and Gram-negative bacteria. Outside of their role as antipsychotic medications, we review the evidence to suggest that phenothiazines possess inherent antibacterial and efflux inhibitory properties enabling them to potentially combat drug resistance. We also discuss that understanding their mode of action is essential to facilitate the design of new phenothiazine derivatives or novel agents for use as antibiotic adjuvants.

Strategies to Combat Multidrug-Resistant and Persistent Infectious Diseases

Antibiotic failure is one of the most worrying health problems worldwide. We are currently facing an international crisis with several problematic facets: new antibiotics are no longer being discovered, resistance mechanisms are occurring in almost all clinical isolates of bacteria, and recurrent infections caused by persistent bacteria are hampering the successful treatment of infections. In this context, new anti-infectious strategies against multidrug-resistant (MDR) and persistent bacteria, as well as the rescue of Food and Drug Administration (FDA)-approved compounds (drug repurposing), are being explored. Among the highlighted new anti-infectious strategies, in this review, we focus on antimicrobial peptides, anti-virulence compounds, phage therapy, and new molecules. As drugs that are being repurposed, we highlight anti-inflammatory compounds, anti-psychotics, anti-helminthics, anti-cancerous drugs, and statins.

In vitro and in vivo toxicity evaluation of non-neuroleptic phenothiazines, antitubercular drug candidates

The phenothiazine-derived antipsychotic drugs, such as chlorpromazine and thioridazine, are bactericidal against drug-sensitive and drug-resistant strains of Mycobacterium tuberculosis, but produce undesirable side effects at clinically relevant doses. We have previously modified four novel phenothiazines and maintained their antimycobacterial activity. This study evaluated the pharmacological and toxicity profiles of these novel non-neuroleptic phenothiazines, PTZ3, PTZ4, PTZ31 and PTZ32, for their metabolic stability, kinetic solubility and potential cytotoxic effects in vitro. To further support the safet use of these drug candidates, the in vivo pharmacological and toxicity profiles were assessed in C57BL/6 mice via single or repeated oral gavage. In acute toxicity studies, all four modified phenothiazines showed favourable safety in mice. When treated daily with 100 mg/kg of PTZ3 and PTZ4 for 2 weeks, mice displayed no signs of toxicity. Alternatively, treatment with PTZ31 resulted in 20% mortality with no toxicity evident in biochemical or histological analysis, while exposure to PTZ32 resulted in a 45% survival with increased serum concentrations of uric acid and alkaline phosphatase. The combined non-neuroleptic and antimycobacterial effects of the novel phenothiazines PTZ3, PTZ4, PTZ31 and PTZ32 demonstrated favourable pharmacological and toxicity profiles in this study, highlight the potential of these compounds as suitable anti-tuberculosis drug candidates.

Psychotropics and the Microbiome: a Chamber of Secrets…

The human gut contains trillions of symbiotic bacteria that play a key role in programming different aspects of host physiology in health and disease. Psychotropic medications act on the central nervous system (CNS) and are used in the treatment of various psychiatric disorders. There is increasing emphasis on the bidirectional interaction between drugs and the gut microbiome. An expanding body of evidence supports the notion that microbes can metabolise drugs and vice versa drugs can modify the gut microbiota composition. In this review, we will first give a comprehensive introduction about this bidirectional interaction, then we will take into consideration different classes of psychotropics including antipsychotics, antidepressants, antianxiety drugs, anticonvulsants/mood stabilisers, opioid analgesics, drugs of abuse, alcohol, nicotine and xanthines. The varying effects of these widely used medications on microorganisms are becoming apparent from in vivo and in vitro studies. This has important implications for the future of psychopharmacology pipelines that will routinely need to consider the host microbiome during drug discovery and development.

Repurposing psychiatric drugs as anti-cancer agents

Cancer is a major public health problem and one of the leading contributors to the global disease burden. The high cost of development of new drugs and the increasingly severe burden of cancer globally have led to increased interest in the search and development of novel, affordable anti-neoplastic medications. Antipsychotic drugs have a long history of clinical use and tolerable safety; they have been used as good targets for drug repurposing. Being used for various psychiatric diseases for decades, antipsychotic drugs are now reported to have potent anti-cancer properties against a wide variety of malignancies in addition to their antipsychotic effects. In this review, an overview of repurposing various psychiatric drugs for cancer treatment is presented, and the putative mechanisms for the anti-neoplastic actions of these antipsychotic drugs are reviewed.

Boosting Effect of 2-Phenylquinoline Efflux Inhibitors in Combination with Macrolides against Mycobacterium smegmatis and Mycobacterium avium

The identification of efflux inhibitors to be used as adjuvants alongside existing drug regimens could have a tremendous value in the treatment of any mycobacterial infection. Here, we investigated the ability of four 2-(4'-propoxyphenyl)quinoline Staphylococcus aureus NorA efflux inhibitors (1-4) to reduce the efflux activity in Mycobacterium smegmatis and Mycobacterium avium strains. All four compounds were able to inhibit efflux pumps in both mycobacterial species; in particular, O-ethylpiperazinyl derivative 2 showed an efflux inhibitory activity comparable to that of verapamil, the most potent mycobacterial efflux inhibitor reported to date, and was able to significantly reduce the MIC values of macrolides against different M. avium strains. The contribution of the M. avium efflux pumps MAV_1406 and MAV_1695 to clarithromycin resistance was proved because they were found to be overexpressed in two M. avium 104 isogenic strains showing high-level clarithromycin resistance. These results indicated a correlation between increased expression of efflux pumps, increased efflux, macrolide resistance, and reduction of resistance by efflux pump inhibitors such as compound 2. Additionally, compound 2 showed synergistic activity with clarithromycin, at a concentration below the cytotoxicity threshold, in an ex vivo experiment against M. avium 104-infected macrophages. In summary, the 2-(4'-propoxyphenyl)quinoline scaffold is suitable to obtain compounds endowed with good efflux pump inhibitory activity against both S. aureus and nontuberculous mycobacteria.

A Cleavage Reaction of Benzothiazole with Cyclic 1,3-Dicarbonyls in the Presence of Potassium Dihydrogen Phosphate

A facile and convenient synthesis of phenothiazine derivatives has been achieved by condensation reaction of benzothiazole derivatives with 1,3-dicarbonyls in the presence of potassium dihydrogen phosphate (KH2PO4) in moderate to good yields under mild conditions.

Energetics of pathogenic bacteria and opportunities for drug development

The emergence and spread of drug-resistant pathogens and our inability to develop new antimicrobials to overcome resistance has inspired scientists to consider new targets for drug development. Cellular bioenergetics is an area showing promise for the development of new antimicrobials, particularly in the discovery of new anti-tuberculosis drugs where several new compounds have entered clinical trials. In this review, we have examined the bioenergetics of various bacterial pathogens, highlighting the versatility of electron donor and acceptor utilisation and the modularity of electron transport chain components in bacteria. In addition to re-examining classical concepts, we explore new literature that reveals the intricacies of pathogen energetics, for example, how Salmonella enterica and Campylobacter jejuni exploit host and microbiota to derive powerful electron donors and sinks; the strategies Mycobacterium tuberculosis and Pseudomonas aeruginosa use to persist in lung tissues; and the importance of sodium energetics and electron bifurcation in the chemiosmotic anaerobe Fusobacterium nucleatum. A combination of physiological, biochemical, and pharmacological data suggests that, in addition to the clinically-approved target F1Fo-ATP synthase, NADH dehydrogenase type II, succinate dehydrogenase, hydrogenase, cytochrome bd oxidase, and menaquinone biosynthesis pathways are particularly promising next-generation drug targets. The realisation of cellular energetics as a rich target space for the development of new antimicrobials will be dependent upon gaining increased understanding of the energetic processes utilised by pathogens in host environments and the ability to design bacterial-specific inhibitors of these processes.

Characterization of the type 2 NADH:menaquinone oxidoreductases from Staphylococcus aureus and the bactericidal action of phenothiazines

Methicillin-resistant Staphylococcus aureus (MRSA) is currently one of the principal multiple drug resistant bacterial pathogens causing serious infections, many of which are life-threatening. Consequently, new therapeutic targets are required to combat such infections. In the current work, we explore the type 2 Nicotinamide adenine dinucleotide reduced form (NADH) dehydrogenases (NDH-2s) as possible drug targets and look at the effects of phenothiazines, known to inhibit NDH-2 from Mycobacterium tuberculosis. NDH-2s are monotopic membrane proteins that catalyze the transfer of electrons from NADH via flavin adenine dinucleotide (FAD) to the quinone pool. They are required for maintaining the NADH/Nicotinamide adenine dinucleotide (NAD(+)) redox balance and contribute indirectly to the generation of proton motive force. NDH-2s are not present in mammals, but are the only form of respiratory NADH dehydrogenase in several pathogens, including S. aureus. In this work, the two putative ndh genes present in the S. aureus genome were identified, cloned and expressed, and the proteins were purified and characterized. Phenothiazines were shown to inhibit both of the S. aureus NDH-2s with half maximal inhibitory concentration (IC50) values as low as 8μM. However, evaluating the effects of phenothiazines on whole cells of S. aureus was complicated by the fact that they are also acting as uncouplers of oxidative phosphorylation. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

UV-light effects on cytochrome c modulated by the aggregation state of phenothiazines

The present study shows the factors that modulate the photodamage promoted by phenothiazines. Cytochrome c was irradiated with UV light for 120 min, over a pH range from 4.0 to 8.0, in the absence and in the presence of different concentrations of thioridazine (TR) and fluphenazine (FP). In the absence of phenothiazines, the maximal rate of a Soret band blue shift (nm/min) from 409 to 406 nm was obtained at pH 4.0 (0.028 nm/min). The presence of phenothiazines at the concentration range 10-25 µmol/L amplified and accelerated a cytochrome c blue shift (409 to 405 nm, at a rate = 0.041 nm/min). Above 25 µmol/L, crescent concentrations of phenothiazines contributed to cytochrome c protection with (maximal at 2500 µmol/L). Scanning electronic microscopy revealed the formation of nanostructures. The pH also influenced the effect of low phenothiazine concentrations on cytochrome c. Thus, the predominance of phenothiazine-promoted cytochrome c damage or protection depends on a balance of the following factors: the yield of photo-generated drug cation radicals, which is favored by acidic pH; the stability of the cation radicals, which is favored by the drug aggregation; and the cytochrome c structure, modulated by the pH.

Thioridazine induces major changes in global gene expression and cell wall composition in methicillin-resistant Staphylococcus aureus USA300

Subinhibitory concentrations of the neuroleptic drug thioridazine (TDZ) are well-known to enhance the killing of methicillin-resistant Staphylococcus aureus (MRSA) by β-lactam antibiotics, however, the mechanism underlying the synergy between TDZ and β-lactams is not fully understood. In the present study, we have examined the effect of a subinhibitory concentration of TDZ on antimicrobial resistance, the global transcriptome, and the cell wall composition of MRSA USA300. We show that TDZ is able to sensitize the bacteria to several classes of antimicrobials targeting the late stages of peptidoglycan (PGN) synthesis. Furthermore, our microarray analysis demonstrates that TDZ modulates the expression of genes encoding membrane and surface proteins, transporters, and enzymes involved in amino acid biosynthesis. Interestingly, resemblance between the transcriptional profile of TDZ treatment and the transcriptomic response of S. aureus to known inhibitors of cell wall synthesis suggests that TDZ disturbs PGN biosynthesis at a stage that precedes transpeptidation by penicillin-binding proteins (PBPs). In support of this notion, dramatic changes in the muropeptide profile of USA300 were observed following growth in the presence of TDZ, indicating that TDZ can interfere with the formation of the pentaglycine branches. Strikingly, the addition of glycine to the growth medium relieved the effect of TDZ on the muropeptide profile. Furthermore, exogenous glycine offered a modest protective effect against TDZ-induced β-lactam sensitivity. We propose that TDZ exposure leads to a shortage of intracellular amino acids, including glycine, which is required for the production of normal PGN precursors with pentaglycine branches, the correct substrate of S. aureus PBPs. Collectively, this work demonstrates that TDZ has a major impact on the cell wall biosynthesis pathway in S. aureus and provides new insights into how MRSA may be sensitized towards β-lactam antibiotics.

A comparative analysis of in vitro and in vivo efficacies of the enantiomers of thioridazine and its racemate

A long list of chemotherapeutical drugs used in the treatment of the peripheral and the central nervous systems possess anti-microbial activity. Some of these neurotropic compounds are chiral, with the one stereo isomeric form exaggerating reduced neurotropism. This is the case for the levorotatory form of thioridazine. The phenothiazine thioridazine is an interesting compound, characterized by exhibiting a significant growth inhibiting activity on a wide array of micro-organisms. Thioridazine is characterized by another challenging feature, because the compound is concentrated in certain human tissue cells. The present study describes a comparative study of the two enantiomers as well as the racemic form of thioridazine. The study exploits the stereochemical aspect and the in vitro and in vivo potential of these compounds, with a focus on the effects on Gram negative organism Salmonella enterica serover Typhimurium. In summary, the results of this study yielded a significant antibacterial activity of all forms of thioridazine, indicating the levorotatory (–)- form to be superior in terms of both its in vitro and in vivo efficacies.

Role of Phenothiazines and Structurally Similar Compounds of Plant Origin in the Fight against Infections by Drug Resistant Bacteria

Phenothiazines have their primary effects on the plasma membranes of prokaryotes and eukaryotes. Among the components of the prokaryotic plasma membrane affected are efflux pumps, their energy sources and energy providing enzymes, such as ATPase, and genes that regulate and code for the permeability aspect of a bacterium. The response of multidrug and extensively drug resistant tuberculosis to phenothiazines shows an alternative therapy for the treatment of these dreaded diseases, which are claiming more and more lives every year throughout the world. Many phenothiazines have shown synergistic activity with several antibiotics thereby lowering the doses of antibiotics administered to patients suffering from specific bacterial infections. Trimeprazine is synergistic with trimethoprim. Flupenthixol (Fp) has been found to be synergistic with penicillin and chlorpromazine (CPZ); in addition, some antibiotics are also synergistic. Along with the antibacterial action described in this review, many phenothiazines possess plasmid curing activities, which render the bacterial carrier of the plasmid sensitive to antibiotics. Thus, simultaneous applications of a phenothiazine like TZ would not only act as an additional antibacterial agent but also would help to eliminate drug resistant plasmid from the infectious bacterial cells.

Phenothiazines as anti-tubercular agents: mechanistic insights and clinical implications

INTRODUCTION

Tuberculosis (TB) chemotherapy has been rendered ineffective by the emergence of multi-drug resistant (MDR), extensively drug resistant (XDR) and totally drug resistant strains reinforcing the need for the development of new drugs as a global health priority. Reconsidering phenothiazines for the improvement of TB chemotherapy seems to be a rational option especially in view of their role as inhibitors of type II NADH dehydrogenase, a key component of respiratory chain of Mycobacterium tuberculosis, thus raising the speculation that they can be effective against latent TB as well.

AREAS COVERED

This article offers a detailed description of the chemotherapeutic efficacy of phenothiazine compounds against susceptible, drug resistant and latent TB. Furthermore, their clinical implications and molecular mechanisms of action have been reviewed extensively.

EXPERT OPINION

Phenothiazines are currently being evaluated for the treatment of TB and have been shown to be effective against M. tuberculosis through a number of in vitro, ex vivo and in vivo studies. In addition, recent clinical studies have implicated their role in the treatment of MDR/XDR TB also. Therefore, phenothiazines, particularly thioridazine, hold great potential to be considered as safe and effective antimycobacterial agents in near future.

Targeting bacterial membrane function: an underexploited mechanism for treating persistent infections

Persistent infections involving slow-growing or non-growing bacteria are hard to treat with antibiotics that target biosynthetic processes in growing cells. Consequently, there is a need for antimicrobials that can treat infections containing dormant bacteria. In this Review, we discuss the emerging concept that disrupting the bacterial membrane bilayer or proteins that are integral to membrane function (including membrane potential and energy metabolism) in dormant bacteria is a strategy for treating persistent infections. The clinical applicability of these approaches is exemplified by the efficacy of lipoglycopeptides that damage bacterial membranes and of the diarylquinoline TMC207, which inhibits membrane-bound ATP synthase. Despite some drawbacks, membrane-active agents form an important new means of eradicating recalcitrant, non-growing bacteria.

Enhancement of vancomycin activity by phenothiazines against vancomycin-resistant Enterococcus faecium in vitro

The antimicrobial and resistance-reversal activities of seven phenothiazine derivatives were evaluated against vancomycin-sensitive Enterococcus faecalis ATCC 29212, vancomycin resistant E. faecalis ATCC 51299 and ten vancomycin-resistant E. faecium strains originating from human infections. Minimum inhibitory concentrations (MIC) of the compounds were determined by agar dilution method, and synergy between phenothiazines and vancomycin was investigated using Checkerboard (microbroth dilution) technique. We found that all enterococci strains, regardless of their susceptibility to vancomycin, were inhibited by phenothiazines at concentrations varying from 8 to 256 microg/ml, with thiethylperazine being the most potent inhibitory agent. Besides, all the phenothiazines showed partial synergy with vancomycin and could lessen MIC of vancomycin from 512 to 8 microg/ml at their sub-inhibitory concentrations. The highest reduction in MIC was observed with chlorpromazine (32 times); however, thiethylperazine and promethazine stood next (24 times). Although resistance modification was observed at concentrations higher than those that phenothiazines reach in vivo, the potential offered by non-antibiotics justify further animal experiments as well as clinical trials to establish their clinical relevance.

Inhibition of Listeria monocytogenes infection by neurological drugs

To gain insights into the cellular processes required for intracellular bacterial pathogenesis, we previously developed a generalisable screening approach to identify small molecule compounds that alter Listeria monocytogenes infection. In this report, a small molecule library enriched for compounds affecting neurological functions was screened and 68 compounds that disrupted L. monocytogenes infection of macrophages were identified. Many of these compounds were known antimicrobial agents, however 26 compounds were novel inhibitors of intracellular infection. Two of the compounds chosen for further study, the antipsychotic drug thioridazine and the calcium channel blocker bepridil, exhibited dose-dependent inhibition of vacuolar escape and intracellular replication of L. monocytogenes during infection of murine macrophages. These results suggest that clinically approved neurological drugs may provide a novel source of anti-infective agents that are suitable for development as therapeutics against intracellular bacterial infections.

Reversal of methicillin resistance in Staphylococcus aureus by thioridazine

OBJECTIVES

Thioridazine has been shown to reverse oxacillin resistance in methicillin-resistant Staphylococcus aureus (MRSA) in vitro. The aim of this study was to investigate whether thioridazine alone or in combination with oxacillin affects the transcription of the methicillin resistance gene mecA and the protein level of the encoded protein PBP2a.

METHODS

Viability of MRSA was determined in liquid media in the presence of oxacillin or thioridazine alone or in combination. Transcription of mecA was analysed by primer extension, and the protein level of PBP2a was analysed by western blotting in the presence of thioridazine and oxacillin.

RESULTS

We observed an increased susceptibility of MRSA towards oxacillin in the presence of thioridazine compared with bacteria grown with oxacillin or thioridazine alone. Transcription of mecA was reduced with increasing concentrations of thioridazine in the presence of a fixed amount of oxacillin. Furthermore, the protein level of PBP2a was reduced when bacteria were treated with the combination of oxacillin and thioridazine. The two drugs also affected the mRNA level of the beta-lactamase gene, blaZ.

CONCLUSIONS

The present study indicates that reversal of methicillin resistance by thioridazine in MRSA may be explained by a reduced transcription of mecA and blaZ, resulting in a reduced protein level of PBP2a.

The TB laboratory of the future: macrophage-based selection of XDR-TB therapeutics

Therapy of multidrug-resistant (MDR)-TB is highly problematic; that of extensively drug-resistant (XDR)-TB even more so. Both infections result in high mortality, especially if the patient is coinfected with HIV or presents with AIDS. Selection of therapy for these infections is limited and, for most situations, it is performed ‘blind’. However, there is a solution for the selection of effective therapy and this is presented herein. Ideal therapy of the patient infected with MDR-TB or XDR-TB can be determined a priori by the mycobacteriology laboratory. This would involve the isolation of the patient’s macrophages, the phagocytosis of the mycobacterial isolate and the presentation of the antitubercular agent to the macrophage–bacterium complex. This system is reviewed in its entirety and its potential and feasibility are supported by hard experimental demonstrations.

Thioridazine: resurrection as an antimicrobial agent?

The emergence of multiresistant bacterial strains and the continuing burden of infectious disease globally point to the urgent need for novel affordable antimicrobial drugs. Thioridazine is a phenothiazine antipsychotic drug with well-recognized antimicrobial activity, but this property has not been harnessed for clinical use as a result of its central nervous system and cardiac side-effects. The cardiotoxicity of thioridazine has recently been shown to be structurally specific at a molecular level, whereas its antimicrobial properties are shared by a number of phenothiazine analogues. This raises the possibility that its enantiomers or its inactive metabolite, the ring sulphoxide, may act as a lead compound in the future development of antimicrobial drugs to face the new challenges in infectious disease.

Antibacterial activity of ergosterol peroxide against Mycobacterium tuberculosis: dependence upon system and medium employed

Ergosterol peroxide, cycloart-23-en-3beta,25-diol, vanillin and 4-hydroxybenzaldehyde have been isolated and characterized from a crude methanol extract of Euphorbia lagascae. Previous studies have shown contradictory results about the antibacterial activity of ergosterol peroxide against Mycobacterium tuberculosis. In order to clarify this question, the activity of this compound was tested against Mycobacterium tuberculosis H37Rv ATCC 27294 strain using two different systems: BACTEC 460TB (Bactec 460) and BACTEC MGIT 960 system (Bactec 960). The results obtained show that significant activity was demonstrable only with the Bactec 460 system. The lack of activity noted with the Bactec 960 system appears to be due to the much faster growth rate of the organism in the medium of this system as opposed to that of the Bactec 460 system. Ergosterol peroxide is also shown by the current study to be devoid of any activity against an antibiotic sensitive ATCC strain of Staphylococcus aureus.

Prolonged exposure of methicillin-resistant Staphylococcus aureus (MRSA) COL strain to increasing concentrations of oxacillin results in a multidrug-resistant phenotype

Our previous studies demonstrated that exposure of a bacterium to increasing concentrations of an antibiotic would increase resistance to that antibiotic as a consequence of activating efflux pumps. This study utilises the same approach; however, it employs the methicillin-resistant Staphylococcus aureus (MRSA) COL strain, which is highly resistant to oxacillin (OXA). MRSA COL was adapted to 3200 mg/L of OXA. Changes in resistance to other antibiotics were evaluated and efflux pump activity during the adaptation process was determined. MRSA COL was exposed to stepwise two-fold increases of OXA. At the end of each step, minimum inhibitory concentration determination for erythromycin (ERY) and other antibiotics was conducted. Reserpine (RES) was employed to evaluate whether resistance to ERY was dependent on efflux pump activity. Efflux pump activity was also evaluated using the ethidium bromide (EB) assay. DNA typing of the products of each culture step was conducted to assess purity. Serial exposure of MRSA COL to increasing concentrations of OXA resulted in increased resistance to ERY, which could be eliminated with RES. Evaluation of efflux pump activity by the EB method indicated increased efflux activity. Resistance to ERY was accompanied by resistance to kanamycin, amikacin, ofloxacin, norfloxacin, ciprofloxacin and rifampicin. This is the first time that a multidrug-resistant phenotype has been experimentally produced as a consequence of exposure of the organism to an antibiotic to which it is initially highly resistant.

Enhanced killing of intracellular multidrug-resistant Mycobacterium tuberculosis by compounds that affect the activity of efflux pumps

Whereas human neutrophils are effective and efficient killers of bacteria, macrophages such as those derived from monocytes are almost devoid of killing activity. Nevertheless, monocytes can be transformed into effective killers of mycobacteria or staphylococci when exposed to clinical concentrations of a phenothiazine or to inhibitors of efflux pumps (reserpine and verapamil), or to ouabain, an inhibitor of K(+) transport. Because the rates of multidrug-resistant Mycobacterium tuberculosis (MDR-TB) continue to escalate globally, and because no new effective drug has been made available for almost 40 years, compounds that enhance the killing activity of monocytes against MDR-TB are obviously needed. This review covers the specific characteristics of MDR-TB, identifies a variety of agents that address these characteristics and therefore have potential for managing MDR-TB. Because the mechanism by which these agents enhance the killing of intracellular bacteria is important for the intelligent design of new anti-tubercular agents, the review correlates the mechanisms by which these agents manifest their effects. Lastly, a model is presented which describes the mechanisms by which distinct efflux pumps of the phagosome-lysosome complex are inhibited by agents that are known to inhibit K(+) flux. The model also predicts the existence of a K(+) activated exchange (pump) that is probably located in the membrane that delineates the lysosome. This putative pump, which is immune to inhibitors of K+ flux, is identified as being the cause for the acidification of the lysosome thereby activating its hydrolytic enzymes. Because the non-killer macrophage can be transformed into an effective killer by a variety of compounds that inhibit K(+) transport, perhaps it would be wise to develop drugs that enhance the killing activity of these cells inasmuch as this approach would not be subject to any resistance, as is the eventual case for conventional antibiotics.

The phenothiazinium chromophore and the evolution of antimalarial drugs

The phenothiazinium salt methylene blue [3,7-bis(dimethylamino)phenothiazinium chloride] is the oldest known synthetic antimalarial drug, its clinical efficacy having been reported in 1891. The role of methylene blue in the evolution of the modern antimalarial armoury is often unappreciated, yet it can be linked directly to standard drugs such as chloroquine and its congeners. Also, in the face of increasing plasmodial resistance to modern antimalarials, phenothiazinium derivatives have again featured as lead compounds in drug research. The precise mode of action of methylene blue and its commercial analogues against Plasmodium spp. remains a cause for conjecture, having been variously described as nucleic acid intercalation, food vacuole basification, parasite redox cycle interference and haem polymerization inhibition. That the activity of the series may be due to more than one route - i.e. a multifactorial activity - underlines the utility of these compounds in antimalarial research either as single drugs or as adjuvants (partners in a drug combination), particularly in the face of resistant parasitic strains.

Inhibition of the Carpobrotus edulis methanol extract on the growth of phagocytosed multidrug-resistant Mycobacterium tuberculosis and methicillin-resistant Staphylococcus aureus

The Carpobrotus edulis methanol extract, inactive against the methicillin-resistant Staphylococcus aureus or the multidrug-resistant Mycobacterium tuberculosis, does inhibit the growth of these two bacteria once they are phagocytosed by monocyte derived human macrophages.

In vitro Activity of Phenothiazine Derivatives in Enterococcus faecalis and Enterococcus faecium

The antimicrobial activity of the phenothiazine derivatives thioridazine and prochlorperazine have been evaluated with 11 Enterococcus faecalis strains and 9 Enterococcus faecium strains, originating from human infections and animal faecal flora. We found that all E. faecalis and E. faecium strains, regardless of their susceptibility to commonly used antibiotics, were inhibited by thioridazine at a concentration of 16-32 microg/ml and by prochlorperazine at a concentration of 32-64 microg/ml. Combinations of the antibiotics vancomycin or ampicillin and thioridazine and prochlorperazine at subinhibitory concentrations, could render vancomycin- or ampicillin-resistant bacteria sensitive to each of the antibiotics. Verapamil and reserpine, inhibitors of P-glycoprotein-mediated multidrug resistance, did not reduce resistance. Our results outline modification of resistance in enterococci induced by phenothiazine derivatives unrelated to P-glycoprotein-mediated multidrug resistance.

Carpobrotus edulis methanol extract inhibits the MDR efflux pumps, enhances killing of phagocytosed S. aureus and promotes immune modulation

Although alkaloids from the family Aizoaceae have anticancer activity, species of this family have received little attention. Because these alkaloids also exhibit properties normally associated with compounds that have activity at the level of the plasma membrane, a methanol extract of Carpobrotus edulis, a common plant found along the Portuguese coast, was studied for properties normally associated with plasma membrane active compounds. The results of this study show that the extract is non-toxic at concentrations that inhibit a verapamil sensitive efflux pump of L5178 mouse T cell lymphoma cell line thereby rendering these multi-drug resistant cells susceptible to anticancer drugs. These non-toxic concentrations also prime THP-1 human monocyte-derived macrophages to kill ingested Staphylococcus aureus and to promote the release of lymphokines associated with cellular immune functions. The extract also induces the proliferation of THP-1 cells within 1 day of exposure to quantities normally associated with phytohaemagglutinin. The potential role of the compound(s) isolated from this plant in cancer biology is intriguing and is currently under investigation. It is supposed that the resistance modifier and immunomodulatory effect of this plant extract can be exploited in the experimental chemotherapy of cancer and bacterial or viral infections.

Clinical concentrations of thioridazine kill intracellular multidrug-resistant Mycobacterium tuberculosis

The phenothiazines chlorpromazine (CPZ) and thioridazine (TZ) have equal in vitro activities against antibiotic-sensitive and -resistant Mycobacterium tuberculosis. These compounds have not been used as anti-M. tuberculosis agents because their in vitro activities take place at concentrations which are beyond those that are clinically achievable. In addition, chronic administration of CPZ produces frequent severe side effects. Because CPZ has been shown to enhance the killing of intracellular M. tuberculosis at concentrations in the medium that are clinically relevant, we have investigated whether TZ, a phenothiazine whose negative side effects are less frequent and serious than those associated with CPZ, kills M. tuberculosis organisms that have been phagocytosed by human macrophages, which have nominal killing activities against these bacteria. Both CPZ and TZ killed intracellular antibiotic-sensitive and -resistant M. tuberculosis organisms when they were used at concentrations in the medium well below those present in the plasma of patients treated with these agents. These concentrations in vitro were not toxic to the macrophage, nor did they affect in vitro cellular immune processes. TZ thus appears to be a serious candidate for the management of a freshly diagnosed infection of pulmonary tuberculosis or as an adjunct to conventional antituberculosis therapy if the patient originates from an area known to have a high prevalence of multidrug-resistant M. tuberculosis isolates. Nevertheless, we must await the outcomes of clinical trials to determine whether TZ itself may be safely and effectively used as an antituberculosis agent.