Therapeutic efficacy of liposome-encapsulated gentamicin in rat Klebsiella pneumoniae pneumonia in relation to impaired host defense and low bacterial susceptibility to gentamicin

Animal models of Klebsiella pneumoniae mucosal infections

Klebsiella pneumoniae is among the most relevant pathogens worldwide, causing high morbidity and mortality, which is worsened by the increasing rates of antibiotic resistance. It is a constituent of the host microbiota of different mucosa, that can invade and cause infections in many different sites. The development of new treatments and prophylaxis against this pathogen rely on animal models to identify potential targets and evaluate the efficacy and possible side effects of therapeutic agents or vaccines. However, the validity of data generated is highly dependable on choosing models that can adequately reproduce the hallmarks of human diseases. The present review summarizes the current knowledge on animal models used to investigate K. pneumoniae infections, with a focus on mucosal sites. The advantages and limitations of each model are discussed and compared; the applications, extrapolations to human subjects and future modifications that can improve the current techniques are also presented. While mice are the most widely used species in K. pneumoniae animal studies, they present limitations such as the natural resistance to the pathogen and difficulties in reproducing the main steps of human mucosal infections. Other models, such as Drosophila melanogaster (fruit fly), Caenorhabditis elegans, Galleria mellonella and Danio rerio (zebrafish), contribute to understanding specific aspects of the infection process, such as bacterial lethality and colonization and innate immune system response, however, they but do not present the immunological complexity of mammals. In conclusion, the choice of the animal model of K. pneumoniae infection will depend mainly on the questions being addressed by the study, while a better understanding of the interplay between bacterial virulence factors and animal host responses will provide a deeper comprehension of the disease process and aid in the development of effective preventive/therapeutic strategies.

Current developments and prospects of the antibiotic delivery systems

Antibiotics have remained the cornerstone for the treatment of bacterial infections ever since their discovery in the twentieth century. The uproar over antibiotic resistance among bacteria arising from genome plasticity and biofilm development has rendered current antibiotic therapies ineffective, urging the development of innovative therapeutic approaches. The development of antibiotic resistance among bacteria has further heightened the clinical failure of antibiotic therapy, which is often linked to its low bioavailability, side effects, and poor penetration and accumulation at the site of infection. In this review, we highlight the potential use of siderophores, antibodies, cell-penetrating peptides, antimicrobial peptides, bacteriophages, and nanoparticles to smuggle antibiotics across impermeable biological membranes to achieve therapeutically relevant concentrations of antibiotics and combat antimicrobial resistance (AMR). We will discuss the general mechanisms via which each delivery system functions and how it can be tailored to deliver antibiotics against the paradigm of mechanisms underlying antibiotic resistance.

Recent Advances in the Development of Antimicrobial Nanoparticles for Combating Resistant Pathogens

The rapid growth of harmful pathogens and their multidrug-resistance poses a severe challenge for health professionals and for the development of new healthcare products. Various strategies are exploited for the development of effective antimicrobial agents, and nanoparticles are a particularly promising class of materials in this respect. This review summarizes recent advances in antimicrobial metallic, polymeric, and lipid-based nanoparticles such as liposomes, solid lipid nanoparticles, and nanostructured lipid carriers. The latter materials in particular are engineered for antimicrobial agent delivery and act by encapsulation, receptor-based binding, and disruption of microbial adherence to cellular substrates. Potential strategies for the design of multifunctional antimicrobial nanocarriers, combining material chemistry and biological interface science, are also discussed.

Amikacin: Uses, Resistance, and Prospects for Inhibition

Aminoglycosides are a group of antibiotics used since the 1940s to primarily treat a broad spectrum of bacterial infections. The primary resistance mechanism against these antibiotics is enzymatic modification by aminoglycoside-modifying enzymes that are divided into acetyl-transferases, phosphotransferases, and nucleotidyltransferases. To overcome this problem, new semisynthetic aminoglycosides were developed in the 70s. The most widely used semisynthetic aminoglycoside is amikacin, which is refractory to most aminoglycoside modifying enzymes. Amikacin was synthesized by acylation with the l-(-)-γ-amino-α-hydroxybutyryl side chain at the C-1 amino group of the deoxystreptamine moiety of kanamycin A. The main amikacin resistance mechanism found in the clinics is acetylation by the aminoglycoside 6'-N-acetyltransferase type Ib [AAC(6')-Ib], an enzyme coded for by a gene found in integrons, transposons, plasmids, and chromosomes of Gram-negative bacteria. Numerous efforts are focused on finding strategies to neutralize the action of AAC(6')-Ib and extend the useful life of amikacin. Small molecules as well as complexes ionophore-Zn⁺² or Cu⁺² were found to inhibit the acetylation reaction and induced phenotypic conversion to susceptibility in bacteria harboring the aac(6')-Ib gene. A new semisynthetic aminoglycoside, plazomicin, is in advance stage of development and will contribute to renewed interest in this kind of antibiotics.

Preparation and characterisation of the colistin-entrapped liposome driven by electrostatic interaction for intravenous administration

Potential use of liposome for polycationic colistin is hindered by their phospholipid membrane permeability. In this study, liposomes were modified with sodium cholesteryl sulphate (Chol-SO4(-)) for improving the colistin loading by enhancing the colistin-bilayer electrostatic attraction. We have evaluated two liposomes: colistin-entrapped liposome of Chol-SO4(-) (CCL) and coated Chol-SO4(-)/colistin complex liposome (CCCL). In comparison with CCL which formed large aggregates at Chol-SO4(-)/colistin charge ratio below 2:1, CCCL showed a smaller size less dependent on the charge ratio, probably arising from more colistin entrapped on the inner leaflet of bilayer. Both liposomes exhibited significantly increased entrapment efficiency as compared with the liposome without Chol-SO4(-). But colistin released upon dilution, implying free transfer of colistin through bilayers. Pharmacokinetics results showed the approximately four-fold increase in the plasma AUC0-8 h for CCCL and CCL as compared with colistin solution, showing potential benefit for infectious target localisation by prolonging the systemic circulation of colistin.

Improvement of the pharmacokinetics and in vivo antibacterial efficacy of a novel type IIa topoisomerase inhibitor by formulation in liposomes

Several useful properties of liposome-based formulations of various existing antibacterial drugs have been reported. These properties include lower MICs, improved pharmacokinetics, lower toxicity, selective distribution to infected tissues, and enhanced in vivo efficacy. Here we report in vivo studies of a liposomal formulation of a member of a novel class of antibacterial type II topoisomerase inhibitors, others of which have progressed to early phases of clinical trials. The free (i.e., nonliposomal) compound has broad-spectrum MICs but suboptimal pharmacokinetics in rats and mice, characterized by a high volume of distribution and rapid clearance. The liposomal formulation of the compound had essentially unchanged MICs but greatly reduced volume of distribution and clearance in rats and mice. In an in vivo mouse model of Staphylococcus aureus infection of one thigh, the liposomal compound localized preferentially to the infected thigh, whereas the free compound showed no preference for the infected versus the uninfected thigh. Most importantly, the liposomal compound had enhanced efficacy at clearing the infection compared with the free compound. Delivery of this class of compounds as liposomal formulations may offer clinical advantages compared with free compounds.

The enhanced permeability retention effect: a new paradigm for drug targeting in infection

Multidrug-resistant, Gram-negative infection is a major global determinant of morbidity, mortality and cost of care. The advent of nanomedicine has enabled tailored engineering of macromolecular constructs, permitting increasingly selective targeting, alteration of volume of distribution and activity/toxicity. Macromolecules tend to passively and preferentially accumulate at sites of enhanced vascular permeability and are then retained. This enhanced permeability and retention (EPR) effect, whilst recognized as a major breakthrough in anti-tumoral targeting, has not yet been fully exploited in infection. Shared pathophysiological pathways in both cancer and infection are evident and a number of novel nanomedicines have shown promise in selective, passive, size-mediated targeting to infection. This review describes the similarities and parallels in pathophysiological pathways at molecular, cellular and circulatory levels between inflammation/infection and cancer therapy, where use of this principle has been established.

A comparison of the in vitro antimicrobial activity of liposomes containing meropenem and gentamicin

The antimicrobial activity of eight cationic, two neutral and three anionic liposome compositions containing meropenem and gentamicin was tested in vitro in broth and serum medium. The cationic formulations showed better antibacterial efficacy against both Gram-positive and Gram-negative bacteria than the anionic and neutral ones, regardless of the encapsulated drug. The most effective formulations were the cationic PC/DOPE/DOTAP 3:4:3 and PC/Chol/DOTAP 3:4:3, as the MICs with meropenem were 2 to 4 times lower than those of the free drug.

A simply and sensitive fluorometric method for determination of gentamicin in liposomal suspensions

A new method for measuring gentamicin in liposomes fluorometrically is described. The assay is based on the reaction between the amino groups in the gentamicin molecule and o-phthaldialdehyde (OPA), under basic pH conditions; the product's fluorescence can be read directly on a simple fluorimeter. The effects of several factors (time of reaction, volume of the OPA reagent, and product stability) were investigated. The standard curve was linear in the concentration range of 0.5-4.0microg, showing an excellent determination coefficient of r(2)=0.99. Additionally, the influence of different liposomal lipids on gentamicin determination was tested. Liposomal lipids containing no free amino groups (PC, Chol, DOTAP) have no influence on the reaction when present in the reaction mixture. In contrast, amino groups containing lipid (SA) showed intense method interference. Therefore, a method of lipid extraction was adapted to remove undesired lipids. The described method was successfully utilised during 2 years of liposomal gentamicin experiments.

Preparation and characterization of dehydration–rehydration vesicles loaded with aminoglycoside and macrolide antibiotics

Enhanced activity of liposomes-encapsulated antibiotics against clinical isolates of Pseudomonas aeruginosa has been documented with liposomes of low encapsulation efficiency. We sought to construct liposomes with high yield entrapment of aminoglycoside and macrolide antibiotics as well as favorable stability in storage and physiological conditions. Liposome-entrapped aminoglycosides (amikacin, gentamicin, tobramycin) and a macrolide (erythromycin) were prepared by a modified dehydration-rehydration vesicles (DRVs) method, and their particle size and entrapment efficiency were determined. We studied in vitro stability of these vesicles over a 48 h period at 4 and 37 degrees C in phosphate-buffered saline (PBS) and in plasma at 37 degrees C. The mean particle size of DRVs loaded with antibiotics varied from 163.37+/-38.44 to 259.83+/-11.80 nm with no significant difference in regard with the type of the antibiotics encapsulated. Encapsulation efficiency of DRVs loaded with amikacin, gentamicin, tobramycin, and erythromycin were 29.27+/-1.17, 33+/-0.76, 22.33+/-1.48 and 32.06+/-0.82% of initial amount of the drug, respectively. These vesicles were stable regardless of the experimental temperature. Indeed, the liposomes retained more than 75% of the initially encapsulated drugs for the study period of 48 h. DRVs incubated in plasma however, released more antibiotics than those incubated in PBS. In conclusion, using this modified DRV method, we obtained small sized vesicles with high yield entrapment for aminoglycoside and macrolide antibiotics. The technique may be utilized to overcome the low encapsulation efficiency associated with aminoglycoside and macrolide antibiotics.

Long-Circulating Sterically Stabilized Liposomes in the Treatment of Infections

The administration of antimicrobial agents encapsulated in long-circulating sterically stabilized liposomes results in a considerable enhancement of therapeutic efficacy compared with the agents in the free form. After liposomal encapsulation, the pharmacokinetics of the antimicrobial agents is significantly changed. An increase in circulation time and reduction in toxic side effects of the agents are observed. In contrast to other types of long-circulating liposomes, an important characteristic of these sterically stabilized liposomes is that their prolonged blood circulation time is, to a high degree, independent of liposome characteristics such as liposome particle size, charge and lipid composition (rigidity) of the bilayer, and lipid dose. This provides the opportunity to manipulate antibiotic release from these liposomes at the site of infection, which is important in view of the differences in pharmacodynamics of different antibiotics and can be done without compromising blood circulation time and degree of target localization of these liposomes. Depending on the liposome characteristics and the agent encapsulated, antibiotic delivery to the infected site is achieved, or the liposomes act as a micro-reservoir function for the antibiotic. In experimental models of localized or disseminated bacterial and fungal infections, the sterically stabilized liposomes have successfully been used to improve antibiotic treatment using representative agents of various classes of antibacterial agents such as the beta-lactams, the aminoglycosides, and the quinolones or the antifungal agent amphotericin B. Extensive biodistribution studies have been performed. Critical factors that contribute to liposome target localization in infected tissue have been elucidated. Liposome-related factors that were investigated were poly(ethylene glycol) density, particle size, bilayer fluidity, negative surface charge, and circulation kinetics. Host-related factors focused on the components of the inflammatory response.

Advantages of liposomal delivery systems for anthracyclines

Liposomes, closed vesicular structures consisting of one or more lipid bilayers, have generated a great deal of interest as drug delivery vehicles. In particular, they have been investigated for their ability to improve the delivery of chemotherapeutic agents to tumors, in efforts to increase therapeutic efficacy and decrease toxicity to normal cells. Development of liposomal chemotherapeutic agents has, in the past, been hindered primarily by the rapid uptake of liposomes by the reticuloendothelial system. Numerous strategies that seek to either exploit or avoid this phenomenon have been used. As a result, several liposomal chemotherapeutic agents are now available in the clinic. STEALTH, a novel liposomal system coated with polyethylene glycol, avoids uptake by the reticuloendothelial system, thus improving drug delivery to the tumor while decreasing toxicity. In pegylated liposomal doxorubicin (Doxil/Caelyx [PLD]), this delivery system encapsulates doxorubicin within polyethylene glycol-coated liposomes, leading to promising new applications for a well-established drug. Liposome-encapsulated doxorubicin citrate complex (Myocet [NPLD]), another liposomal delivery system for doxorubicin, lacks the polyethylene glycol coating, resulting in much shorter circulation times than those of PLD. Daunorubicin citrate liposome (DaunoXome [DNX]) contains daunorubicin encapsulated in a smaller liposome of a different lipid composition. It has circulation times between those of PLD and NPLD. This article reviews the advantages of liposomal delivery systems in general and the divergent approaches that have been taken in developing these agents.

Evaluation of newly developed oxygen meters with multi-channels and disposable oxygen electrode sensors for antimicrobial susceptibility testing

The objective of this study was to investigate the applicability of new oxygen meters with multi-channels and disposable oxygen electrode sensors (DOX-96) on the antimicrobial susceptibility testing of clinical bacterial isolates. The oxygen amount in the wells of 96-well plates was converted into current through electrodes. Bacterial inoculation decreased the oxygen amount in the wells because viable bacteria consume the oxygen. On the other hand, a failure of bacteria to consume oxygen was observed in the presence of potent antimicrobial agents, representing a serious arrest of bacterial metabolism usually leading to stasis or death. Based on these results, the minimum inhibitory concentration was determined by DOX-96 (MICdox). The MICdox showed good agreement with MIC measured by the standard broth microdilution method (98.2%). DOX-96 was also useful for turbid samples such as Mueller-Hinton broth containing 0.1% lipid emulsion. The MICdox in turbid samples showed good agreement with those in clear samples (100.0%). These results indicate that the newly developed DOX-96 is very useful in antimicrobial susceptibility testing even in turbid clinical samples such as colloidal products and turbid biological components.

Ciprofloxacin in polyethylene glycol-coated liposomes: efficacy in rat models of acute or chronic Pseudomonas aeruginosa infection

In a previous study in experimental Klebsiella pneumoniae pneumonia, the therapeutic potential of ciprofloxacin was significantly improved by encapsulation in polyethylene glycol-coated ("pegylated") long-circulating (STEALTH) liposomes. Pegylated liposomal ciprofloxacin in high doses was nontoxic and resulted in relatively high and sustained ciprofloxacin concentrations in blood and tissues, and hence an increase in the area under the plasma concentration-time curve (AUC). These data correspond to data from animal and clinical studies showing that for fluoroquinolones the AUC/MIC ratio is associated with favorable outcome in serious infections. Clinical failures and the development of resistance are observed for marginally susceptible organisms like Pseudomonas aeruginosa and for which sufficient AUC/MIC ratios cannot be achieved. In the present study the therapeutic efficacy of pegylated liposomal ciprofloxacin was investigated in two rat models of Pseudomonas aeruginosa pneumonia. In the acute model pneumonia developed progressively, resulting in a rapid onset of septicemia and a high mortality rate. Ciprofloxacin twice daily for 7 days was not effective at doses at or below the maximum tolerated dose (MTD). However, pegylated liposomal ciprofloxacin either at high dosage or given at low dosage in combination with free ciprofloxacin on the first day of treatment was fully effective (100% survival). Obviously, prolonged concentrations of ciprofloxacin in blood prevented death of the animals due to early-stage septicemia in this acute infection. However, bacterial eradication from the left lung was not effected. In the chronic model, pneumonia was characterized by bacterial persistence in the lung without bacteremia, and no signs of morbidity or mortality were observed. Ciprofloxacin administered for 7 days at the MTD twice daily resulted in killing of more than 99% of bacteria in the lung; this result can also be achieved with pegylated liposomal ciprofloxacin given once daily. Complete bacterial eradication is never observed.

Long-circulating sterically stabilized liposomes as carriers of agents for treatment of infection or for imaging infectious foci

Liposomes are considered as potential carriers for biologically active compounds. One evident drawback of 'classical' liposomes is their fast elimination by cells of the mononuclear phagocyte system (MPS), primarily by liver and spleen. An important breakthrough in this respect is the development of long-circulating liposomes among which liposomes coated with polyethyleneglycol (PEG), the so-called 'sterically stabilized' liposomes (SSL). An important characteristic of SSL is that their prolonged blood residence time and infectious target localization is relatively independent of the lipid dose, particle size or lipid composition of the bilayer. SSL are applied as carriers of antimicrobial agents to achieve infectious target localization, to reduce side effects, or to serve as a micro-reservoir in the circulation. In addition, radiolabelled SSL are used to image infectious and inflammatory foci.

Biodistribution of long-circulating PEG-liposomes in a murine model of established subcutaneous abscesses

The biodistribution of long-circulating PEG-liposomes in a subcutaneous mouse model of established mixed infection abscesses was investigated to assess their possible role as drug carriers in the treatment of small, undrainable intra-abdominal abscesses. There was a 10-30-fold greater localisation of (67)Ga-labelled PEG-liposomes in abscesses compared to uninfected normal skin samples. Over 3% of the injected dose (ID) of liposomes was present in the abscesses 24 h after liposome administration in contrast to 0.1% in normal skin sections. The percentage ID present in the liver, spleen and kidneys was 17%, 4% and 2% per organ respectively. Five days after liposome injection, 2% ID could still be recovered from the abscesses. Using colloidal gold-labelled PEG-liposomes, it was shown that there was a 4-fold greater density of liposome clusters in the subcutaneous tissue surrounding the capsule than in the core of the abscesses. The clusters within the abscesses were distributed evenly. We conclude that PEG-liposomes localise to a significant degree at the infection focus in our mouse model and may provide a new approach to the antimicrobial treatment of intra-abdominal abscesses.

Encapsulation of DNA in negatively charged liposomes and inhibition of bacterial gene expression with fluid liposome-encapsulated antisense oligonucleotides

Antisense therapy for the treatment of bacterial infections is a very attractive alternative to overcome drug resistance problems. However, the penetration of antisense oligonucleotides into bacterial cells is a major huddle that has delayed research and application in this field. In the first part of this study, we defined efficient conditions to encapsulate plasmid DNA and antisense oligonucleotides in a fluid negatively charged liposome. Subsequently, we evaluated the potential of liposome-encapsulated antisense oligonucleotides to penetrate the bacterial outer membrane and to inhibit gene expression in bacteria. It was found that 48.9+/-12% and 43.5+/-4% of the purified plasmid DNA and antisense oligonucleotides were respectively encapsulated in the liposomes. Using fluorescence-activated cell sorting analysis, it was shown, after subtraction of the fluorescence values due to the aggregation phenomenon measured at 4 degrees C, that about 57% of bacterial cells had integrated the encapsulated antisense oligonucleotides whereas values for free antisenses were negligible. The uptake of the encapsulated anti-lacZ antisense oligonucleotides resulted in a 42% reduction of beta-galactosidase compared to 9% and 6% for the encapsulated mismatch antisense oligonucleotides and the free antisense oligonucleotides respectively. This work shows that it is possible to encapsulate relatively large quantities of negatively charged molecules in negative fluid liposomes and suggests that fluid liposomes could be used to deliver nucleic acids in bacteria to inhibit essential bacterial genes.

Improved efficacy of ciprofloxacin administered in polyethylene glycol-coated liposomes for treatment of Klebsiella pneumoniae pneumonia in rats

Animal and clinical data show that high ratios of the area under the concentration-time curve and the peak concentration in blood to the MIC of fluoroquinolones for a given pathogen are associated with a favorable outcome. The present study investigated whether improvement of the therapeutic potential of ciprofloxacin could be achieved by encapsulation in polyethylene glycol (PEG)-coated long-circulating sustained-release liposomes. In a rat model of unilateral Klebsiella pneumoniae pneumonia (MIC = 0.1 microg/ml), antibiotic was administered at 12- or 24-h intervals at twofold-increasing doses. A treatment period of 3 days was started 24 h after inoculation of the left lung, when the bacterial count had increased 1,000-fold and some rats had positive blood cultures. The infection was fatal within 5 days in untreated rats. Administration of ciprofloxacin in the liposomal form resulted in delayed ciprofloxacin clearance and increased and prolonged ciprofloxacin concentrations in blood and tissues. The ED(50) (dosage that results in 50% survival) of liposomal ciprofloxacin was 3.3 mg/kg of body weight/day given once daily, and that of free ciprofloxacin was 18.9 mg/kg/day once daily or 5.1 mg/kg/day twice daily. The ED(90) of liposomal ciprofloxacin was 15.0 mg/kg/day once daily compared with 36.0 mg/kg/day twice daily for free ciprofloxacin; 90% survival could not be achieved with free ciprofloxacin given once daily. In summary, the therapeutic efficacy of liposomal ciprofloxacin was superior to that of ciprofloxacin in the free form. PEG-coated liposomal ciprofloxacin was well tolerated in relatively high doses, permitting once daily administration with relatively low ciprofloxacin clearance and without compromising therapeutic efficacy.