Elucidating the Pharmacokinetics/Pharmacodynamics of Aerosolized Colistin against Multidrug-Resistant Acinetobacter baumannii and Klebsiella pneumoniae in a Mouse Lung Infection Model

Variability of murine bacterial pneumonia models used to evaluate antimicrobial agents

Antimicrobial resistance has become one of the greatest threats to human health, and new antibacterial treatments are urgently needed. As a tool to develop novel therapies, animal models are essential to bridge the gap between preclinical and clinical research. However, despite common usage of in vivo models that mimic clinical infection, translational challenges remain high. Standardization of in vivo models is deemed necessary to improve the robustness and reproducibility of preclinical studies and thus translational research. The European Innovative Medicines Initiative (IMI)-funded “Collaboration for prevention and treatment of MDR bacterial infections” (COMBINE) consortium, aims to develop a standardized, quality-controlled murine pneumonia model for preclinical efficacy testing of novel anti-infective candidates and to improve tools for the translation of preclinical data to the clinic. In this review of murine pneumonia model data published in the last 10 years, we present our findings of considerable variability in the protocols employed for testing the efficacy of antimicrobial compounds using this in vivo model. Based on specific inclusion criteria, fifty-three studies focusing on antimicrobial assessment against Pseudomonas aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii were reviewed in detail. The data revealed marked differences in the experimental design of the murine pneumonia models employed in the literature. Notably, several differences were observed in variables that are expected to impact the obtained results, such as the immune status of the animals, the age, infection route and sample processing, highlighting the necessity of a standardized model.

PKPD Modeling of the Inoculum Effect of Acinetobacter baumannii on Polymyxin B in vivo

The reduction in antimicrobial activity at high bacterial counts is a microbiological phenomenon known as the inoculum effect (IE). In a previous in vitro study, a significant IE was observed for polymyxin B (PMB) against a clinical isolate of Acinetobacter baumannii, and well described by a new pharmacokinetic-pharmacodynamic model. Few in vivo studies have investigated the impact of inoculum size on survival or antibiotic efficacy. Therefore, our objective was to confirm the influence of inoculum size of this A. baumannii clinical isolate on PMB in vivo effect over time. Pharmacokinetics and pharmacodynamics of PMB after a single subcutaneous administration (1, 15 and 40 mg/kg) were studied in a neutropenic murine thigh infection model. The impact of A. baumannii inoculum size (10⁵, 10⁶ and 10⁷ CFU/thigh) on PMB efficacy was also evaluated. In vivo PMB PK was well described by a two-compartment model including saturable absorption from the subcutaneous injection site and linear elimination. The previous in vitro PD model was modified to adequately describe the decrease of PMB efficacy with increased inoculum size in infected mice. The IE was modeled as a decrease of 32% in the in vivo PMB bactericidal effect when the starting inoculum increases from 10⁵ to 10⁷ CFU/thigh. Although not as important as previously characterized in vitro an IE was confirmed in vivo.

Spray-freeze-dried inhalable composite microparticles containing nanoparticles of combinational drugs for potential treatment of lung infections caused by Pseudomonas aeruginosa

The multi-drug resistance of Pseudomonas aeruginosa is an overwhelming cause of terminal and persistent lung infections in cystic fibrosis (CF) patients. Antimicrobial synergy has been shown for colistin and ivacaftor, and our study designed a relatively high drug-loading dry powder inhaler formulation containing nanoparticles of ivacaftor and colistin. The ivacaftor-colistin nanosuspensions (Iva-Col-NPs) were prepared by the anti-solvent method with different stabilizers. Based on the aggregation data, the formulation 7 (F7) with DSPG-PEG-OMe as the stabilizer was selected for further studies. The F7 consisted of ivacaftor, colistin and DSPG-PEG-OMe with a mass ratio of 1:1:1. The F7 powder formulation was developed using the ultrasonic spray-freeze-drying method and exhibited a rough surface with relatively high fine particle fraction values of 61.4 ± 3.4% for ivacaftor and 63.3 ± 3.3% for colistin, as well as superior emitted dose of 97.8 ± 0.3% for ivacaftor and 97.6 ± 0.5% for colistin. The F7 showed very significant dissolution improvement for poorly water soluble ivacaftor than the physical mixture. Incorporating two drugs in a single microparticle with synchronized dissolution and superior aerosol performance will maximize the synergy and bioactivity of those two drugs. Minimal cytotoxicity in Calu-3 human lung epithelial cells and enhanced antimicrobial activity against colistin-resistant P. aeruginosa suggested that our formulation has potential to improve the treatment of CF patients with lung infections.

Rescuing the Last-Line Polymyxins: Achievements and Challenges

Antibiotic resistance is a major global health challenge and, worryingly, several key Gram negative pathogens can become resistant to most currently available antibiotics. Polymyxins have been revived as a last-line therapeutic option for the treatment of infections caused by multidrug-resistant Gram negative bacteria, in particular Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacterales. Polymyxins were first discovered in the late 1940s but were abandoned soon after their approval in the late 1950s as a result of toxicities (e.g., nephrotoxicity) and the availability of "safer" antibiotics approved at that time. Therefore, knowledge on polymyxins had been scarce until recently, when enormous efforts have been made by several research teams around the world to elucidate the chemical, microbiological, pharmacokinetic/pharmacodynamic, and toxicological properties of polymyxins. One of the major achievements is the development of the first scientifically based dosage regimens for colistin that are crucial to ensure its safe and effective use in patients. Although the guideline has not been developed for polymyxin B, a large clinical trial is currently being conducted to optimize its clinical use. Importantly, several novel, safer polymyxin-like lipopeptides are developed to overcome the nephrotoxicity, poor efficacy against pulmonary infections, and narrow therapeutic windows of the currently used polymyxin B and colistin. This review discusses the latest achievements on polymyxins and highlights the major challenges ahead in optimizing their clinical use and discovering new-generation polymyxins. To save lives from the deadly infections caused by Gram negative "superbugs," every effort must be made to improve the clinical utility of the last-line polymyxins. SIGNIFICANCE STATEMENT: Antimicrobial resistance poses a significant threat to global health. The increasing prevalence of multidrug-resistant (MDR) bacterial infections has been highlighted by leading global health organizations and authorities. Polymyxins are a last-line defense against difficult-to-treat MDR Gram negative pathogens. Unfortunately, the pharmacological information on polymyxins was very limited until recently. This review provides a comprehensive overview on the major achievements and challenges in polymyxin pharmacology and clinical use and how the recent findings have been employed to improve clinical practice worldwide.

In vitro evaluation of drug delivery behavior for inhalable amorphous nanoparticle formulations in a human lung epithelial cell model

Respiratory tract infections caused by multidrug-resistant (MDR) Gram-negative bacteria such as Pseudomonas aeruginosa are serious burdens to public health, especially in cystic fibrosis patients. The combination of colistin, a cationic polypeptide antibiotic, and ivacaftor, a cystic fibrosis transmembrane regulator (CFTR) protein modulator, displays a synergistic antibacterial effect against P. aeruginosa. The primary aim of the present study is to investigate the transport, accumulation and toxicity of a novel nanoparticle formulation containing colistin and ivacaftor in lung epithelial Calu-3 cells. The cell viability results demonstrated that ivacaftor alone or in combination with colistin in the physical mixture showed significant toxicity at an ivacaftor concentration of 10 μg/mL or higher. However, the cellular toxicity was significantly reduced in the nanoparticle formulation. Ivacaftor transport into the cells reached a plateau rapidly as compared to colistin. Colistin transport across the Calu-3 cell monolayer was less than ivacaftor. A substantial amount (46-83%) of ivacaftor, independent of dose, was accumulated in the cell monolayer following transport from the apical into the basal chamber, whereas the intracellular accumulation of colistin was relatively low (2-15%). The nanoparticle formulation significantly reduced the toxicity of colistin and ivacaftor to Calu-3 cells by reducing the accumulation of both drugs in the cell and potential protective effects by bovine serum albumin (BSA), which could be a promising safer option for the treatment of respiratory infections caused by MDR P. aeruginosa.

Surface Composition and Aerosolization Stability of an Inhalable Combinational Powder Formulation Spray Dried Using a Three-Fluid Nozzle

PURPOSE

This study aims to understand the impact of spray drying nozzles on particle surface composition and aerosol stability.

METHODS

The combination formulations of colistin and azithromycin were formulated by 2-fluid nozzle (2 N) or 3-fluid (3 N) spray drying in a molar ratio of 1:1. A 3-factor, 2-level (2³) factorial design was selected to investigate effects of flow rate, inlet temperature and feed concentration on yield of spray drying and the performance of the spray dried formulations for the 3 N.

RESULTS

FPF values for the 2 N formulation (72.9 ± 1.9% for azithromycin & 73.4 ± 0.8% for colistin) were higher than those for the 3 N formulation (56.5 ± 3.8% for azithromycin & 55.1 ± 1.6% for colistin) when stored at 20% RH for 1 day, which could be attributed to smaller physical size for the 2 N. There was no change in FPF for both drugs in the 2 N formulation after storage at 75% RH for 90 days; however, there was a slight increase in FPF for colistin in the 3 N formulation at the same storage conditions. Surface enrichment of hydrophobic azithromycin was measured by X-ray photoelectron spectroscopy for both 2 N and 3 N formulations and interactions were studied using FTIR.

CONCLUSIONS

The 3-fluid nozzle provides flexibility in choosing different solvents and has the capability to spray dry at higher feed solid concentrations. This study highlights the impact of hydrophobic azithromycin enrichment on particle surface irrespective of the nozzle type, on the prevention of moisture-induced deterioration of FPF for hygroscopic colistin.

Agents of Last Resort: An Update on Polymyxin Resistance

Polymyxin resistance is a major public health threat, because the polymyxins represent last-line therapeutics for gram-negative pathogens resistant to essentially all other antibiotics. Minimizing any potential emergence and dissemination of polymyxin resistance relies on an improved understanding of mechanisms of and risk factors for polymyxin resistance, infection prevention and stewardship strategies, together with optimization of dosing of polymyxins (eg, combination regimens).

Inhalable Nanocomposite Microparticles with Enhanced Dissolution and Superior Aerosol Performance

Previous studies have shown that combining colistin (Col), a cationic polypeptide antibiotic, with ivacaftor (Iva), a cystic fibrosis (CF) drug, could achieve synergistic antibacterial effects against Pseudomonas aeruginosa. The purpose of this study was to develop dry powder inhaler formulations for co-delivery of Col and Iva, aiming to treat CF and lung infection simultaneously. In order to improve solubility and dissolution for the water-insoluble Iva, Iva was encapsulated into bovine serum albumin (BSA) nanoparticles (Iva-BSA-NPs). Inhalable composite microparticles of Iva-BSA-NPs were produced by spray-freeze-drying using water-soluble Col as the matrix material and l-leucine as an aerosol enhancer. The optimal formulation showed an irregularly shaped morphology with fine particle fraction (FPF) values of 73.8 ± 5.2% for Col and 80.9 ± 4.1% for Iva. Correlations between "D×ρtapped" and FPF were established for both Iva and Col. The amorphous solubility of Iva is 66 times higher than the crystalline solubility in the buffer. Iva-BSA-NPs were amorphous and remained in the amorphous state after spray-freeze-drying, as examined by powder X-ray diffraction. In vitro dissolution profiles of the selected DPI formulation indicated that Col and Iva were almost completely released within 3 h, which was substantially faster regarding Iva release than the jet-milled physical mixture of the two drugs. In summary, this study developed a novel inhalable nanocomposite microparticle using a synergistic water-soluble drug as the matrix material, which achieved reduced use of excipients for high-dose medications, improved dissolution rate for the water-insoluble drug, and superior aerosol performance.

Novel Polymyxin Combination with the Antiretroviral Zidovudine Exerts Synergistic Killing against NDM-Producing Multidrug-Resistant Klebsiella pneumoniae

Polymyxins are used as a last-line therapy against multidrug-resistant (MDR) New Delhi metallo-β-lactamase (NDM)-producing Klebsiella pneumoniae However, polymyxin resistance can emerge with monotherapy; therefore, novel strategies are urgently needed to minimize the resistance and maintain their clinical utility. This study aimed to investigate the pharmacodynamics of polymyxin B in combination with the antiretroviral drug zidovudine against K. pneumoniae Three isolates were evaluated in static time-kill studies (0 to 64 mg/liter) over 48 h. An in vitro one-compartment pharmacokinetic/pharmacodynamic (PK/PD) model (IVM) was used to simulate humanized dosage regimens of polymyxin B (4 mg/liter as continuous infusion) and zidovudine (as bolus dose thrice daily to achieve maximum concentration of drug in broth [C _max] of 6 mg/liter) against K. pneumoniae BM1 over 72 h. The antimicrobial synergy of the combination was further evaluated in a murine thigh infection model against K. pneumoniae 02. In the static time-kill studies, polymyxin B monotherapy produced rapid and extensive killing against all three isolates followed by extensive regrowth, whereas zidovudine produced modest killing followed by significant regrowth at 24 h. Polymyxin B in combination with zidovudine significantly enhanced the antimicrobial activity (≥4 log₁₀ CFU/ml) and minimized bacterial regrowth. In the IVM, the combination was synergistic and the total bacterial loads were below the limit of detection for up to 72 h. In the murine thigh infection model, the bacterial burden at 24 h in the combination group was ≥3 log₁₀ CFU/thigh lower than each monotherapy against K. pneumoniae 02. Overall, the polymyxin B-zidovudine combination demonstrates superior antimicrobial efficacy and minimized emergence of resistance to polymyxins.

Composite particle formulations of colistin and meropenem with improved in-vitro bacterial killing and aerosolization for inhalation

Antibiotic combination therapy is promising for the treatment of lower respiratory tract infections caused by multi-drug resistant Gram-negative pathogens. Inhaled antibiotic therapy offers the advantage of direct delivery of the drugs to the site of infection, as compared to the parenteral administrations. In this study, we developed composite particle formulations of colistin and meropenem. The formulations were characterized for particle size, morphology, specific surface area, surface chemical composition, in-vitro aerosolization performance and in-vitro antibacterial activity. The combinations demonstrated enhanced antibacterial activity against clinical isolates of Acinetobacter baumannii N16870 and Pseudomonas aeruginosa 19147, when compared with antibiotic monotherapy. Spray-dried meropenem alone showed a poor aerosolization performance as indicated by a low fine particle fraction (FPF) of 32.5 ± 3.3%. Co-spraying with colistin improved the aerosolization of meropenem with up to a two-fold increase in the FPF. Such improvements in aerosolization can be attributed to the enrichment of colistin on the surface of composite particles as indicated by X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), and the increases in particle porosity. Intermolecular interactions between colistin and meropenem were observed for the combination formulations as measured by FT-IR. In conclusion, our results show that co-spray drying with colistin improves the antibacterial activity and aerosol performance of meropenem and produces a formulation with synergistic bacterial killing.