Antimicrobial chemotherapy and lung microdialysis: a review

A Systematic Review of the Pharmacokinetics and Pharmacodynamics of Novel Beta-Lactams and Beta-Lactam with Beta-Lactamase Inhibitor Combinations for the Treatment of Pneumonia Caused by Carbapenem-Resistant Gram-Negative Bacteria

BACKGROUND

Novel beta-lactams show activity against many multidrug-resistant Gram-negative bacteria that cause severe lung infections. Understanding pharmacokinetic/pharmacodynamic characteristics of these agents may help optimise outcomes in the treatment of pneumonia.

OBJECTIVES

To describe and appraise studies that report pulmonary pharmacokinetic and pharmacodynamic data of cefiderocol, ceftazidime/avibactam, ceftolozane/tazobactam, imipenem/cilastatin/relebactam and meropenem/vaborbactam.

METHODS

MEDLINE (PubMed), Embase, Web of Science and Scopus libraries were used for the literature search. Pulmonary population pharmacokinetic and pharmacokinetic/pharmacodynamic studies on adult patients receiving cefiderocol, ceftazidime/avibactam, ceftolozane/tazobactam, imipenem/cilastatin/relebactam, and meropenem/vaborbactam published in peer-reviewed journals were included. Two independent authors screened, reviewed and extracted data from included articles. A reporting guideline for clinical pharmacokinetic studies (ClinPK statement) was used for bias assessment. Relevant outcomes were included, such as population pharmacokinetic parameters and probability of target attainment of dosing regimens.

RESULTS

Twenty-four articles were included. There was heterogeneity in study methods and reporting of results, with diversity across studies in adhering to the ClinPK statement checklist. Ceftolozane/tazobactam was the most studied agent. Only two studies collected epithelial lining fluid samples from patients with pneumonia. All the other phase I studies enrolled healthy subjects. Significant population heterogeneity was evident among available population pharmacokinetic models. Probabilities of target attainment rates above 90% using current licensed dosing regiments were reported in most studies.

CONCLUSIONS

Although lung pharmacokinetics was rarely described, this review observed high target attainment using plasma pharmacokinetic data for all novel beta-lactams. Future studies should describe lung pharmacokinetics in patient populations at risk of carbapenem-resistant pathogen infections.

Effects of lung inflammation and injury on pulmonary tissue penetration of meropenem and vancomycin in a model of unilateral lung injury

OBJECTIVE

The timing and dosing of antimicrobial therapy are key in the treatment of pneumonia in critically ill patients. It is uncertain whether the presence of lung inflammation and injury affects tissue penetration of intravenously administered antimicrobial drugs. The effects of lung inflammation and injury on tissue penetration of two antimicrobial drugs commonly used for pneumonia were determined in an established model of unilateral lung injury.

METHODS

Unilateral lung injury was induced in the left lung of 13 healthy pigs through cyclic rinsing; the right healthy lung served as control. Infusions of meropenem and vancomycin were administered and concentrations of these drugs in lung tissue, blood, and epithelial lining fluid (ELF) were compared over a period of 6 h.

RESULTS

Median vancomycin lung tissue concentrations and penetration ratio were higher in inflamed and injured lungs compared with uninflamed and uninjured lungs (AUC0-6h: P = 0.003 and AUCdialysate/AUCplasma ratio: P = 0.003), resulting in higher AUC0-24/MIC. Median meropenem lung tissue concentrations and penetration ratio in inflamed and injured lungs did not differ from that in uninflamed and uninjured lungs (AUC0-6: P = 0.094 and AUCdialysate/AUCplasma ratio: P = 0.173). The penetration ratio for both vancomycin and meropenem into ELF was similar in injured and uninjured lungs.

CONCLUSION

Vancomycin penetration into lung tissue is enhanced by acute inflammation and injury, a phenomenon barely evident with meropenem. Therefore, inflammation in lung tissue influences the penetration into interstitial lung tissue, depending on the chosen antimicrobial drug. Measurement of ELF levels alone might not identify the impact of inflammation and injury.

Lung microdialysis and in vivo PK/PD integration of cefquinome against Actinobacillus pleuropneumoniae in a porcine experimental lung infection model

This study aim to explore the application of microdialysis in pharmacokinetic (PK) and pharmacodynamic (PD) integration of cefquinome against Actinobacillus pleuropneumoniae in a porcine experimental lung infection model. The model was established via intratracheal inoculation where average bacterial counts (CFU) in the lungs of infected pigs reached 6.57 log10 CFU/g after 3 h. The PK profiles of unbound cefquinome in lung dialysates were determined following intramuscular injection of single doses of 0.125, 0.25, 0.5, 1, 2, and 4 mg/kg. Lung dialysate samples were collected using microdialysis at a flow rate of 1.5 μL/min until 24 h. The PD studies were conducted over 24 h based on 10 intermittent dosing regimens and total daily doses ranged from 0.25 to 4 mg/kg and dosage intervals included 12 and 24 h. The lung tissue was collected after 24 h of treatment and homogenized for bacterial counts. The relationships between PK/PD parameters derived from lung dialysates and drug efficacy were analyzed using an inhibitory sigmoid Emax model. The percentage of time the free drug concentration exceeded the minimum inhibitory concentration (%fT > MIC) was the PK/PD index best describing the antimicrobial activity (R2 = 0.96) in the porcine experimental infection model. The %fT > MIC values required to achieve net bacterial stasis, 1, 2 and 3 log10 CFU/g reductions in the lung were 22.45, 28.86, 37.62, and 56.46%, respectively. Cefquinome exhibited time-dependent characteristics against A. pleuropneumoniae in vivo. These results provide valuable insights into the application of microdialysis in PK/PD integration model studies and optima regimen of cefquinome for the treatment of porcine respiratory diseases caused by A. pleuropneumoniae.

Understanding the nebulisation of antibiotics: the key role of lung microdialysis studies

BACKGROUND

Nebulisation of antibiotics is a promising treatment for ventilator-associated pneumonia (VAP) caused by multidrug-resistant organisms. Ensuring effective antibiotic concentrations at the site of infection in the interstitial space fluid is crucial for clinical outcomes. Current assessment methods, such as epithelial lining fluid and tissue homogenates, have limitations in providing longitudinal pharmacokinetic data.

MAIN BODY

Lung microdialysis, an invasive research technique predominantly used in animals, involves inserting probes into lung parenchyma to measure antibiotic concentrations in interstitial space fluid. Lung microdialysis offers unique advantages, such as continuous sampling, regional assessment of antibiotic lung concentrations and avoidance of bronchial contamination. However, it also has inherent limitations including the cost of probes and assay development, the need for probe calibration and limited applicability to certain antibiotics. As a research tool in VAP, lung microdialysis necessitates specialist techniques and resource-intensive experimental designs involving large animals undergoing prolonged mechanical ventilation. However, its potential impact on advancing our understanding of nebulised antibiotics for VAP is substantial. The technique may enable the investigation of various factors influencing antibiotic lung pharmacokinetics, including drug types, delivery devices, ventilator settings, interfaces and disease conditions. Combining in vivo pharmacokinetics with in vitro pharmacodynamic simulations can become feasible, providing insights to inform nebulised antibiotic dose optimisation regimens. Specifically, it may aid in understanding and optimising the nebulisation of polymyxins, effective against multidrug-resistant Gram-negative bacteria. Furthermore, lung microdialysis holds promise in exploring novel nebulisation therapies, including repurposed antibiotic formulations, bacteriophages and immunomodulators. The technique's potential to monitor dynamic biochemical changes in pneumonia, such as cytokines, metabolites and inflammation/infection markers, opens avenues for developing theranostic tools tailored to critically ill patients with VAP.

CONCLUSION

In summary, lung microdialysis can be a potential transformative tool, offering real-time insights into nebulised antibiotic pharmacokinetics. Its potential to inform optimal dosing regimen development based on precise target site concentrations and contribute to development of theranostic tools positions it as key player in advancing treatment strategies for VAP caused by multidrug-resistant organisms. The establishment of international research networks, exemplified by LUMINA (lung microdialysis applied to nebulised antibiotics), signifies a proactive step towards addressing complexities and promoting multicentre experimental studies in the future.

Population pharmacokinetic modeling of the influence of chronic and acute biofilm-forming Pseudomonas aeruginosa lung infection on ciprofloxacin free pulmonary and epithelial lining fluid concentrations

We previously reported that ciprofloxacin (CIP) free lung interstitial concentrations are decreased by biofilm-forming Pseudomonas aeruginosa pulmonary chronic (14 d) infection. To get a better understanding on the influence of infection on CIP lung distribution, in the present study free lung interstitial fluid and epithelial lining fluid (ELF) concentrations were determined by microdialysis in biofilm-forming P. aeruginosa acutely (2 d) and chronically infected (14 d) Wistar rats following CIP 20 mg/kg i.v. bolus dosing. A popPK model was developed, using NONMEM® (version 7.4.3) with FOCE+I, with plasma data described as a three-compartment model with first-order elimination. For lung data inclusion, the model was expanded to four compartments and ELF concentrations were described as a fraction of lung levels estimated as a distribution factor (ƒD). Acute infection had a minor impact on plasma and lung CIP distribution and both infection stages did not alter ELF drug penetration. Probability of target attainment of ƒAUC0-24/MIC ≥ 90 using 20 mg q8h, equivalent to 400 mg q8h in humans, showed that CIP free concentrations in plasma are adequate to successfully treat lung infections. However, lung and ELF free interstitial concentrations might be insufficient to result in efficacious treatment of biofilm-forming P. aeruginosa chronic infection. However, lung and ELF free interstitial concentrations might be insufficient to result in efficacious treatment of biofilm-forming P. aeruginosa chronic infection.

Should Airway Interstitial Fluid Be Used to Evaluate the Pharmacokinetics of Macrolide Antibiotics for Dose Regimen Determination in Respiratory Infection?

Macrolide antibiotics are important drugs to combat infections. The pharmacokinetics (PK) of these drugs are essential for the determination of their optimal dose regimens, which affect antimicrobial pharmacodynamics and treatment success. For most drugs, the measurement of their concentrations in plasma/serum is the surrogate for drug concentrations in target tissues for therapy. However, for macrolides, simple reliance on total or free drug concentrations in serum/plasma might be misleading. The macrolide antibiotic concentrations of serum/plasma, interstitial fluid (ISF), and target tissue itself usually yield very different PK results. In fact, the PK of a macrolide antibiotic based on serum/plasma concentrations alone is not an ideal predictor for the in vivo efficacy against respiratory pathogens. Instead, the PK based on drug concentrations at the site of infection or ISF provide much more clinically relevant information than serum/plasma concentrations. This review aims to summarize and compare/discuss the use of drug concentrations of serum/plasma, airway ISF, and tissues for computing the PK of macrolides. A better understanding of the PK of macrolide antibiotics based on airway ISF concentrations will help optimize the antibacterial dose regimen as well as minimizing toxicity and the emergence of drug resistance in clinical practice.

Optimizing antibiotic dosing regimens for nosocomial pneumonia: a window of opportunity for pharmacokinetic and pharmacodynamic modeling

INTRODUCTION

Determining antibiotic exposure in the lung and the threshold(s) needed for effective antibacterial killing is paramount during development of new antibiotics for the treatment of nosocomial pneumonia, as these exposures directly affect clinical outcomes and resistance development. The use of pharmacokinetic and pharmacodynamic modeling is recommended by regulatory agencies to evaluate antibiotic pulmonary exposure and optimize dosage regimen selection. This process has been implemented in newer antibiotic development.

AREAS COVERED

This review will discuss the basis for conducting pharmacokinetic and pharmacodynamic studies to support dosage regimen selection and optimization for the treatment of nosocomial pneumonia. Pharmacokinetic/pharmacodynamic data that supported recent hospital-acquired bacterial pneumonia/ventilator-associated bacterial pneumonia indications for ceftolozane/tazobactam, ceftazidime/avibactam, imipenem/cilastatin/relebactam, and cefiderocol will be reviewed.

EXPERT OPINION

Optimal drug development requires the integration of preclinical pharmacodynamic studies, healthy volunteers and ideally patient bronchoalveolar lavage pharmacokinetic studies, Monte-Carlo simulation, and clinical trials. Currently, plasma exposure has been successfully used as a surrogate for lung exposure threshold. Future studies are needed to identify the value of lung pharmacodynamic thresholds in nosocomial pneumonia antibiotic dosage optimization.

How to Use Nebulized Antibiotics in Severe Respiratory Infections

Difficult-to-treat pulmonary infections caused by multidrug-resistant (MDR) pathogens are of great concern because their incidence continues to increase worldwide and they are associated with high morbidity and mortality. Nebulized antibiotics are increasingly being used in this context. The advantages of the administration of a nebulized antibiotic in respiratory tract infections due to MDR include the potential to deliver higher drug concentrations to the site of infection, thus minimizing the systemic adverse effects observed with the use of parenteral or oral antibiotic agents. However, there is an inconsistency between the large amount of experimental evidence supporting the administration of nebulized antibiotics and the paucity of clinical studies confirming the efficacy and safety of these drugs. In this narrative review, we describe the current evidence on the use of nebulized antibiotics for the treatment of severe respiratory infections.

Pharmacokinetics and soft-tissue distribution of tebipenem pivoxil hydrobromide using microdialysis: a study in healthy subjects and patients with diabetic foot infections

OBJECTIVE

Tebipenem pivoxil hydrobromide is a novel oral carbapenem prodrug of tebipenem, the active moiety. We assessed tebipenem steady-state pharmacokinetics in the skin and soft tissue in healthy subjects and infected patients with diabetes using in vivo microdialysis.

METHODS

Six healthy subjects and six patients with an ongoing diabetic foot infection (DFI) received tebipenem pivoxil hydrobromide 600 mg orally every 8 h for three doses. A microdialysis probe was inserted in the thigh of healthy subjects or by the wound margin in patients. Plasma and dialysate samples were obtained immediately prior to the third dose and sampled over 8 h.

RESULTS

Tebipenem plasma protein binding (mean ± SD) was 50.2% ± 2.4% in healthy subjects and 53.5% ± 5.6% in infected patients. Mean ± SD tebipenem pharmacokinetic parameters in plasma for healthy subjects and infected patients were: maximum free concentration (fCmax), 3.74 ± 2.35 and 3.40 ± 2.86 mg/L, respectively; half-life, 0.88 ± 0.11 and 2.02 ± 1.32 h; fAUC0-8, 5.61 ± 1.64 and 10.01 ± 4.81 mg·h/L. Tebipenem tissue AUC0-8 was 5.99 ± 3.07 and 8.60 ± 2.88 mg·h/L for healthy subjects and patients, respectively. The interstitial concentration-time profile largely mirrored the free plasma profile within both populations, resulting in a penetration ratio of 107% in healthy subjects and 90% in infected patients.

CONCLUSIONS

Tebipenem demonstrated excellent distribution into skin and soft tissue of healthy subjects and patients with DFI following oral administration of 600 mg of tebipenem pivoxil hydrobromide.

Probability of Target Attainment of Tobramycin Treatment in Acute and Chronic Pseudomonas aeruginosa Lung Infection Based on Preclinical Population Pharmacokinetic Modeling

Biofilms and infectious process may alter free antimicrobial concentrations at the site of infection. Tobramycin (TOB), an aminoglycoside used to treat lung infections caused by Pseudomonas aeruginosa, binds to alginate present in biofilm extracellular matrix increasing its minimum inhibitory concentration (MIC). This work aimed to investigate the impact of biofilm-forming P. aeruginosa infection on TOB lung and epithelial lining fluid (ELF) penetration, using microdialysis, and to develop a population pharmacokinetic (popPK) model to evaluate the probability of therapeutic target attainment of current dosing regimens employed in fibrocystic and non-fibrocystic patients. The popPK model developed has three compartments including the lung. The ELF concentrations were described by a penetration factor derived from the lung compartment. Infection was a covariate in lung volume (V3) and only chronic infection was a covariate in central volume (V1) and total clearance (CL). Simulations of the recommended treatments for acute and chronic infection achieved >90% probability of target attainment (PTA) in the lung with 4.5 mg/kg q24h and 11 mg/kg q24h, respectively, for the most prevalent P. aeruginosa MIC (0.5 mg/mL). The popPK model was successfully applied to evaluate the PTA of current TOB dosing regimens used in the clinic, indicating the need to investigate alternative posology.

Pharmacokinetic/Pharmacodynamic Optimization of Hospital-Acquired and Ventilator-Associated Pneumonia: Challenges and Strategies

Hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP) are correlated with high mortality rates worldwide. Thus, the administration of antibiotic therapy with appropriate dosing regimen is critical. An efficient antibiotic is needed to maintain an adequate concentration at the infection site, for a sufficient period of time, to achieve the best therapeutic outcome. It can, however, be challenging for antibiotics to penetrate the pulmonary system due to the complexity of its structure. Crossing the blood alveolar barrier is a difficult process determined by multiple factors that are either drug related or infection related. Thus, the understanding of pharmacokinetics/pharmacodynamics (PK/PD) of antibiotics identifies the optimum dosing regimens to achieve drug penetration into the epithelial lining fluid at adequate therapeutic concentrations. Critically ill patients in the ICU can express augmented renal clearance (ARC), characterized by enhanced renal function, or may have renal dysfunction necessitating supportive care such as continuous renal replacement therapy (CRRT). Both ARC and CRRT can alter drug elimination, thus affecting drug concentrations. PK of critically ill patients is less clear due to the multiple variabilities associated with their condition. Therefore, conventional dosing regimens often lead to therapeutic failure. Another major hurdle faced in optimizing treatment for HAP/VAP is the reduction of the in vitro potency. Therapeutic drug monitoring (TDM), if available, may allow health care providers to personalize treatment to maximize efficacy of the drug exposures while minimizing toxicity. TDM can be of significant importance in populations whom PK are less defined and for resistant infections to achieve the best therapeutic outcome.

Intracellular Accumulation of Novel and Clinically Used TB Drugs Potentiates Intracellular Synergy

The therapeutic repertoire for tuberculosis (TB) remains limited despite the existence of many TB drugs that are highly active in in vitro models and possess clinical utility. Underlying the lack of efficacy in vivo is the inability of TB drugs to penetrate microenvironments inhabited by the causative agent, Mycobacterium tuberculosis, including host alveolar macrophages. Here, we determined the ability of the phenoxazine PhX1 previously shown to be active against M. tuberculosis in vitro to differentially penetrate murine compartments, including plasma, epithelial lining fluid, and isolated epithelial lining fluid cells. We also investigated the extent of permeation into uninfected and M. tuberculosis-infected human macrophage-like Tamm-Horsfall protein 1 (THP-1) cells directly and by comparing to results obtained in vitro in synergy assays. Our data indicate that PhX1 (4,750 ± 127.2 ng/ml) penetrates more effectively into THP-1 cells than do the clinically used anti-TB agents, rifampin (3,050 ± 62.9 ng/ml), moxifloxacin (3,374 ± 48.7 ng/ml), bedaquiline (4,410 ± 190.9 ng/ml), and linezolid (770 ± 14.1 ng/ml). Compound efficacy in infected cells correlated with intracellular accumulation, reinforcing the perceived importance of intracellular penetration as a key drug property. Moreover, we detected synergies deriving from redox-stimulatory combinations of PhX1 or clofazimine with the novel prenylated amino-artemisinin WHN296. Finally, we used compound synergies to elucidate the relationship between compound intracellular accumulation and efficacy, with PhX1/WHN296 synergy levels shown to predict drug efficacy. Collectively, our data support the utility of the applied assays in identifying in vitro active compounds with the potential for clinical development. IMPORTANCE This study addresses the development of novel therapeutic compounds for the eventual treatment of drug-resistant tuberculosis. Tuberculosis continues to progress, with cases of Mycobacterium tuberculosis (M. tuberculosis) resistance to first-line medications increasing. We assess new combinations of drugs with both oxidant and redox properties coupled with a third partner drug, with the focus here being on the potentiation of M. tuberculosis-active combinations of compounds in the intracellular macrophage environment. Thus, we determined the ability of the phenoxazine PhX1, previously shown to be active against M. tuberculosis in vitro, to differentially penetrate murine compartments, including plasma, epithelial lining fluid, and isolated epithelial lining fluid cells. In addition, the extent of permeation into human macrophage-like THP-1 cells and H37Rv-infected THP-1 cells was measured via mass spectrometry and compared to in vitro two-dimensional synergy and subsequent intracellular efficacy. Collectively, our data indicate that development of new drugs will be facilitated using the methods described herein.

Inferring pulmonary exposure based on clinical PK data: accuracy and precision of model-based deconvolution methods

Determining and understanding the target-site exposure in clinical studies remains challenging. This is especially true for oral drug inhalation for local treatment, where the target-site is identical to the site of drug absorption, i.e., the lungs. Modeling and simulation based on clinical pharmacokinetic (PK) data may be a valid approach to infer the pulmonary fate of orally inhaled drugs, even without local measurements. In this work, a simulation-estimation study was systematically applied to investigate five published model structures for pulmonary drug absorption. First, these models were compared for structural identifiability and how choosing an inadequate model impacts the inference on pulmonary exposure. Second, in the context of the population approach both sequential and simultaneous parameter estimation methods after intravenous administration and oral inhalation were evaluated with typically applied models. With an adequate model structure and a well-characterized systemic PK after intravenous dosing, the error in inferring pulmonary exposure and retention times was less than twofold in the majority of evaluations. Whether a sequential or simultaneous parameter estimation was applied did not affect the inferred pulmonary PK to a relevant degree. One scenario in the population PK analysis demonstrated biased pulmonary exposure metrics caused by inadequate estimation of systemic PK parameters. Overall, it was demonstrated that empirical modeling of intravenous and inhalation PK datasets provided robust estimates regarding accuracy and bias for the pulmonary exposure and pulmonary retention, even in presence of the high variability after drug inhalation.

Which Analysis Approach Is Adequate to Leverage Clinical Microdialysis Data? A Quantitative Comparison to Investigate Exposure and Reponse Exemplified by Levofloxacin

Purpose

Systematic comparison of analysis methods of clinical microdialysis data for impact on target-site drug exposure and response.

Methods

39 individuals received a 500 mg levofloxacin short-term infusion followed by 24-h dense sampling in plasma and microdialysate collection in interstitial space fluid (ISF). ISF concentrations were leveraged using non-compartmental (NCA) and compartmental analysis (CA) via (ii) relative recovery correction at midpoint of the collection interval (midpoint-NCA, midpoint-CA) and (ii) dialysate-based integrals of time (integral-CA). Exposure and adequacy of community-acquired pneumonia (CAP) therapy via pharmacokinetic/pharmacodynamic target-attainment (PTA) analysis were compared between approaches.

Results

Individual AUC_ISF estimates strongly varied for midpoint-NCA and midpoint-CA (≥52.3%CV) versus integral-CA (≤32.9%CV) owing to separation of variability in PK parameters (midpoint-CA = 46.5%–143%CV_PK, integral-CA = 26.4%–72.6%CV_PK) from recovery-related variability only in integral-CA (41.0%–50.3%CV_recovery). This also led to increased variability of AUC_plasma for midpoint-CA (56.0%CV) versus midpoint-NCA and integral-CA (≤33.0%CV), and inaccuracy of predictive model performance of midpoint-CA in plasma (visual predictive check). PTA analysis translated into 33% of evaluated patient cases being at risk of incorrectly rejecting recommended dosing regimens at CAP-related epidemiological cut-off values.

Conclusions

Integral-CA proved most appropriate to characterise clinical pharmacokinetics- and microdialysis-related variability. Employing this knowledge will improve the understanding of drug target-site PK for therapeutic decision-making.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11095-021-02994-1.

Correlation and clinical relevance of animal models for inhaled pharmaceuticals and biopharmaceuticals

Nonclinical studies are fundamental for the development of inhaled drugs, as for any drug product, and for successful translation to clinical practice. They include in silico, in vitro, ex vivo and in vivo studies and are intended to provide a comprehensive understanding of the inhaled drug beneficial and detrimental effects. To date, animal models cannot be circumvented during drug development programs, acting as surrogates of humans to predict inhaled drug response, fate and toxicity. Herein, we review the animal models used during the different development stages of inhaled pharmaceuticals and biopharmaceuticals, highlighting their strengths and limitations.

Lung Collapse during Mini-Thoracotomy Reduces Penetration of Cefuroxime to the Tissue: Interstitial Microdialysis Study in Animal Models

Background: Single-lung ventilation facilitates surgical exposure during minimally invasive cardiac surgery. However, a deeper knowledge of antibiotic distribution within a collapsed lung is necessary for effective antibiotic prophylaxis of pneumonia. Patients and Methods: The pharmacokinetics/pharmacodynamics (PK/PD) of cefuroxime were compared between the plasma and interstitial fluid (ISF) of collapsed and ventilated lungs in 10 anesthetized pigs, which were ventilated through a double-lumen endotracheal cannula. Cefuroxime (20 mg/kg) was administered in single 30-minute intravenous infusion. Samples of blood and lung microdialysate were collected until six hours post-dose. Ultrafiltration, in vivo retrodialysis, and high-performance liquid chromatography-tandem mass spectrometry were used to determine plasma and ISF concentrations of free drug. The concentrations were examined with non-compartmental analysis and compartmental modeling. Results: The concentration of free cefuroxime in ISF was lower in the non-ventilated lung than the ventilated one, evidenced by a lung penetration factor of 47% versus 63% (p < 0.05), the ratio between maximum concentrations (65%, p < 0.05), and the ratio between the areas under the concentration-time curve (78%, p = 0.12). The time needed to reach a minimum inhibitory concentration (MIC) was 30%-40% longer for a collapsed lung than for a ventilated one. In addition, a delay of 10-40 minutes was observed for lung ISF compared with plasma. The mean residence time values (ISF collapsed lung > ISF ventilated lung > plasma) could explain the absence of practically important differences in the time interval with the concentration of cefuroxime exceeding the MICs of sensitive strains (≤4 mg/L). Conclusion: The concentration of cefuroxime in the ISF of a collapsed porcine lung is lower than in a ventilated one; furthermore, its equilibration with plasma is delayed. Administration of the first cefuroxime dose earlier or at a higher rate may be warranted, as well as dose intensification of the perioperative prophylaxis of pneumonia caused by pathogens with higher MICs.

Lung Pharmacokinetics of Tobramycin by Intravenous and Nebulized Dosing in a Mechanically Ventilated Healthy Ovine Model

BACKGROUND

Nebulized antibiotics may be used to treat ventilator-associated pneumonia. In previous pharmacokinetic studies, lung interstitial space fluid concentrations have never been reported. The aim of the study was to compare intravenous and nebulized tobramycin concentrations in the lung interstitial space fluid, epithelial lining fluid, and plasma in mechanically ventilated sheep with healthy lungs.

METHODS

Ten anesthetized and mechanically ventilated healthy ewes underwent surgical insertion of microdialysis catheters in upper and lower lobes of both lungs and the jugular vein. Five ewes were given intravenous tobramycin 400 mg, and five were given nebulized tobramycin 400 mg. Microdialysis samples were collected every 20 min for 8 h. Bronchoalveolar lavage was performed at 1 and 6 h.

RESULTS

The peak lung interstitial space fluid concentrations were lower with intravenous tobramycin 20.2 mg/l (interquartile range, 12 mg/l, 26.2 mg/l) versus the nebulized route 48.3 mg/l (interquartile range, 8.7 mg/l, 513 mg/l), P = 0.002. For nebulized tobramycin, the median epithelial lining fluid concentrations were higher than the interstitial space fluid concentrations at 1 h (1,637; interquartile range, 650, 1,781, vs. 16 mg/l, interquartile range, 7, 86, P < 0.001) and 6 h (48, interquartile range, 17, 93, vs. 4 mg/l, interquartile range, 2, 9, P < 0.001). For intravenous tobramycin, the median epithelial lining fluid concentrations were lower than the interstitial space fluid concentrations at 1 h (0.19, interquartile range, 0.11, 0.31, vs. 18.5 mg/l, interquartile range, 9.8, 23.4, P < 0.001) and 6 h (0.34, interquartile range, 0.2, 0.48, vs. 3.2 mg/l, interquartile range, 0.9, 4.4, P < 0.001).

CONCLUSIONS

Compared with intravenous tobramycin, nebulized tobramycin achieved higher lung interstitial fluid and epithelial lining fluid concentrations without increasing systemic concentrations.

Low penetrance of antibiotics in the epithelial lining fluid. The role of inhaled antibiotics in patients with bronchiectasis

Plasma drug concentrations, spectrum of antibacterial activity and minimum inhibitory concentration (MIC) had been widely considered as markers of the efficacy of antibiotics. Nonetheless, in several cases, antibiotics characterized by all these features were ineffective for the treatment of respiratory tract infections. A typical paradigm represented the case of patients with bronchiectasis who do not always benefit from antibiotics and thus experiencing increased sputum production, worse quality of life, more rapid forced expiratory volume in the first second (FEV1) decline, more frequent exacerbations and increased mortality rates, especially those with Pseudomonas aeruginosa (P. aeruginosa) chronic infection. Subsequently, penetrance of antibiotics in the epithelial lining fluid has gradually emerged as another key factor for the outcome of antibiotic treatment. Given that a plethora of antibiotics presented with poor or intermediate penetrance in the epithelial lining fluid, inhaled antibiotics targeting directly the site of infection emerged as a new option for patients with respiratory disorders including patients with bronchiectasis. This review article intends to summarize the current state of knowledge for the penetrance of antibiotics in the epithelial lining fluid and present results from clinical trials of inhaled antibiotics in patients with bronchiectasis of etiology other than cystic fibrosis.

A research pathway for the study of the delivery and disposition of nebulised antibiotics: an incremental approach from in vitro to large animal models

Background

Nebulised antibiotics are frequently used for the prevention or treatment of ventilator-associated pneumonia. Many factors may influence pulmonary drug concentrations with inaccurate dosing schedules potentially leading to therapeutic failure and/or the emergence of antibiotic resistance. We describe a research pathway for studying the pharmacokinetics of a nebulised antibiotic during mechanical ventilation using in vitro methods and ovine models, using tobramycin as the study antibiotic.

Methods

In vitro studies using a laser diffractometer and a bacterial-viral filter were used to measure the effect of the type and size of tracheal tubes and antibiotic concentration on the particle size distribution of the tobramycin 400 mg (4 ml; 100 mg/ml) and 160 mg (4 ml, 40 mg/ml) aerosol and nebulised mass delivered. To compare the regional drug distribution in the lung of two routes (intravenous and nebulised) of drug administration of tobramycin 400 mg, technetium-99m-labelled tobramycin 400 mg with planar nuclear medicine imaging was used in a mechanically ventilated ovine model. To measure tobramycin concentrations by intravenous and nebulised tobramycin 400 mg (4 ml, 100 mg/ml) administration in the lung interstitial space (ISF) fluid and blood of mechanically ventilated sheep, the microdialysis technique was used over an 8-h duration.

Results

Tobramycin 100 mg/ml achieved a higher lung dose (121.3 mg) compared to 40 mg/ml (41.3 mg) solution. The imaging study with labelled tobramycin indicated that nebulised tobramycin distributed more extensively into each lung zone of the mechanically ventilated sheep than intravenous administration. A higher lung ISF peak concentration of tobramycin was observed with nebulised tobramycin (40.8 mg/l) compared to intravenous route (19.0 mg/l).

Conclusions

The research methods appear promising to describe lung pharmacokinetics for formulations intended for nebulisation during mechanical ventilation. These methods need further validation in an experimental pneumonia model to be able to contribute toward optimising dosing regimens to inform clinical trials and/or clinical use.

Quantitative Assessment of Drug Delivery to Tissues and Association with Phospholipidosis: A Case Study with Two Structurally Related Diamines in Development

Drug induced phospholipidosis (PLD) may be observed in the preclinical phase of drug development and pose strategic questions. As lysosomes have a central role in pathogenesis of PLD, assessment of lysosomal concentrations is important for understanding the pharmacokinetic basis of PLD manifestation and forecast of potential clinical appearance. Herein we present a systematic approach to provide insight into tissue-specific PLD by evaluation of unbound intracellular and lysosomal (reflecting acidic organelles) concentrations of two structurally related diprotic amines, GRT1 and GRT2. Their intratissue distribution was assessed using brain and lung slice assays. GRT1 induced PLD both in vitro and in vivo. GRT1 showed a high intracellular accumulation that was more pronounced in the lung, but did not cause cerebral PLD due to its effective efflux at the blood-brain barrier. Compared to GRT1, GRT2 revealed higher interstitial fluid concentrations in lung and brain, but more than 30-fold lower lysosomal trapping capacity. No signs of PLD were seen with GRT2. The different profile of GRT2 relative to GRT1 is due to a structural change resulting in a reduced basicity of one amino group. Hence, by distinct chemical modifications, undesired lysosomal trapping can be separated from desired drug delivery into different organs. In summary, assessment of intracellular unbound concentrations was instrumental in delineating the intercompound and intertissue differences in PLD induction in vivo and could be applied for identification of potential lysosomotropic compounds in drug development.

Insights on animal models to investigate inhalation therapy: Relevance for biotherapeutics

Acute and chronic respiratory diseases account for major causes of illness and deaths worldwide. Recent developments of biotherapeutics opened a new era in the treatment and management of patients with respiratory diseases. When considering the delivery of therapeutics, the inhaled route offers great promises with a direct, non-invasive access to the diseased organ and has already proven efficient for several molecules. To assist in the future development of inhaled biotherapeutics, experimental models are crucial to assess lung deposition, pharmacokinetics, pharmacodynamics and safety. This review describes the animal models used in pulmonary research for aerosol drug delivery, highlighting their advantages and limitations for inhaled biologics. Overall, non-clinical species must be selected with relevant scientific arguments while taking into account their complexities and interspecies differences, to help in the development of inhaled medicines and ensure their successful transposition in the clinics.

Considerations for effect site pharmacokinetics to estimate drug exposure: concentrations of antibiotics in the lung

Bronchoalveolar lavage (BAL) and microdialysis have become the most reliable and relevant methods for measuring lung concentrations of antibiotics, with the majority of BAL studies involving either healthy adult subjects or patients undergoing diagnostic bronchoscopy. Emphasis on the amount of drug that reaches the site of infection is increasingly recognized as necessary to determine whether a dose selection will translate to good clinical outcomes in the treatment of patients with pneumonia. Observed concentrations and/or parameters of exposure (e.g. area-under-the-curve) need to be incorporated with pharmacokinetic-pharmacodynamic indices so that rational dose selection can be identified for specific pathogens and types of pneumonic infection (community-acquired vs hospital-acquired bacterial pneumonia, including ventilator-associated bacterial pneumonia). Although having measured plasma or lung concentration-time data from critically ill patients to incorporate into pharmacokinetic-pharmacodynamic models is very unlikely during drug development, it is essential that altered distribution, augmented renal clearance, and renal or hepatic dysfunction should be considered. Notably, the number of published studies involving microdialysis and intrapulmonary penetration of antibiotics has been limited and mainly involve beta-lactam agents, levofloxacin, and fosfomycin. Opportunities to measure in high-resolution effect site spatial pharmacokinetics (e.g. with MALDI-MSI or PET imaging) and in vivo continuous drug concentrations (e.g. with aptamer-based probes) now exist. Going forward these studies could be incorporated into antibiotic development programs for pneumonia in order to further increase the probability of candidate success.

Population Pharmacokinetic Modeling as a Tool To Characterize the Decrease in Ciprofloxacin Free Interstitial Levels Caused by Pseudomonas aeruginosa Biofilm Lung Infection in Wistar Rats

Biofilm formation plays an important role in the persistence of pulmonary infections, for example, in cystic fibrosis patients. So far, little is known about the antimicrobial lung disposition in biofilm-associated pneumonia. This study aimed to evaluate, by microdialysis, ciprofloxacin (CIP) penetration into the lungs of healthy and Pseudomonas aeruginosa biofilm-infected rats and to develop a comprehensive model to describe the CIP disposition under both conditions. P. aeruginosa was immobilized into alginate beads and intratracheally inoculated 14 days before CIP administration (20 mg/kg of body weight). Plasma and microdialysate were sampled from different animal groups, and the observations were evaluated by noncompartmental analysis (NCA) and population pharmacokinetic (popPK) analysis. The final model that successfully described all data consisted of an arterial and a venous central compartment and two peripheral distribution compartments, and the disposition in the lung was modeled as a two-compartment model structure linked to the venous compartment. Plasma clearance was approximately 32% lower in infected animals, leading to a significantly higher level of plasma CIP exposure (area under the concentration-time curve from time zero to infinity, 27.3 ± 12.1 μg · h/ml and 13.3 ± 3.5 μg · h/ml in infected and healthy rats, respectively). Despite the plasma exposure, infected animals showed a four times lower tissue concentration/plasma concentration ratio (lung penetration factor = 0.44 and 1.69 in infected and healthy rats, respectively), and lung clearance (CL_lung) was added to the model for these animals (CL_lung = 0.643 liters/h/kg) to explain the lower tissue concentrations. Our results indicate that P. aeruginosa biofilm infection reduces the CIP free interstitial lung concentrations and increases plasma exposure, suggesting that plasma concentrations alone are not a good surrogate of lung concentrations.

Diffusion-limited PBPK model for predicting pulmonary pharmacokinetics of florfenicol in pig

For most bacterial lung infections, the concentration of unbound antimicrobial agent in lung interstitial fluid has been thought to be responsible for antimicrobial efficacy. In this study, a diffusion-limited physiologically based pharmacokinetic (PBPK) model was developed to predict the pulmonary pharmacokinetics of florfenicol (FF) in pigs. The model included separate compartments corresponding to blood, diffusion-limited lung, flow-limited muscle, liver, and kidney and an extra compartment representing the remaining carcass. The absorption rate constant and renal and hepatic clearance of FF were determined in vivo. Other parameters were taken from the literature or optimized based on existing pharmacokinetic data. All mathematical operations during the development of the model were performed using acslXtreme version 3.0.2.1 (Aegis Technologies Group, Inc., Huntsville, AL, USA). The model accurately predicted the concentration-time courses of FF in lung interstitial fluid, serum, and plasma following different dosing schedules, except at the dose of 15 mg/kg. When compared with the tissue residue data, the model generally underestimated the FF concentration at the injection site, whereas it gave good predictions of FF concentrations in lung, liver, and kidney at early time points. The model predictions provide a scientific basis for the dosage regimen design of FF.

Utility of Microdialysis in Infectious Disease Drug Development and Dose Optimization

Adequate drug penetration to a site of infection is absolutely imperative to ensure sufficient antimicrobial treatment. Microdialysis is a minimally invasive, versatile technique, which can be used to study the penetration of an antiinfective agent in virtually any tissue of interest. It has been used to investigate drug distribution and pharmacokinetics in variable patient populations, as a tool in dose optimization, a potential utility in therapeutic drug management, and in the study of biomarkers of disease progression. While all of these applications have not been fully explored in the field of antiinfectives, this review provides an overview of how microdialysis has been applied in various phases of drug development, a focus on the specific applications in the subspecialties of infectious disease (treatment of bacterial, fungal, viral, parasitic, and mycobacterial infections), and developing applications (biomarkers and therapeutic drug management).

Cavitary penetration of levofloxacin among patients with multidrug-resistant tuberculosis

A better understanding of second-line drug (SLD) pharmacokinetics, including cavitary penetration, may help optimize SLD dosing. Patients with pulmonary multidrug-resistant tuberculosis (MDR-TB) undergoing adjunctive surgery were enrolled in Tbilisi, Georgia. Serum was obtained at 0, 1, 4, and 8 h and at the time of cavitary removal to measure levofloxacin concentrations. After surgery, microdialysis was performed using the ex vivo cavity, and levofloxacin concentrations in the collected dialysate fluid were measured. Noncompartmental analysis was performed, and a cavitary-to-serum levofloxacin concentration ratio was calculated. Twelve patients received levofloxacin for a median of 373 days before surgery (median dose, 11.8 mg/kg). The median levofloxacin concentration in serum (Cmax) was 6.5 μg/ml, and it was <2 μg/ml in 3 (25%) patients. Among 11 patients with complete data, the median cavitary concentration of levofloxacin was 4.36 μg/ml (range, 0.46 to 8.82). The median cavitary/serum levofloxacin ratio was 1.33 (range, 0.63 to 2.36), and 7 patients (64%) had a ratio of >1. There was a significant correlation between serum and cavitary concentrations (r = 0.71; P = 0.01). Levofloxacin had excellent penetration into chronic cavitary TB lesions, and there was a good correlation between serum and cavitary concentrations. Optimizing serum concentrations will help ensure optimal cavitary concentrations of levofloxacin, which may enhance treatment outcomes.

Pulmonary penetration of piperacillin and tazobactam in critically ill patients

Pulmonary infections in critically ill patients are common and are associated with high morbidity and mortality. Piperacillin–tazobactam is a frequently used therapy in critically ill patients with pulmonary infection. Antibiotic concentrations in the lung reflect target‐site antibiotic concentrations in patients with pneumonia. The aim of this study was to assess the plasma and intrapulmonary pharmacokinetics (PK) of piperacillin–tazobactam in critically ill patients administered standard piperacillin–tazobactam regimens. A population PK model was developed to describe plasma and intrapulmonary piperacillin and tazobactam concentrations. The probability of piperacillin exposures reaching pharmacodynamic end points and the impact of pulmonary permeability on piperacillin and tazobactam pulmonary penetration was explored. The median piperacillin and tazobactam pulmonary penetration ratios were 49.3 and 121.2%, respectively. Pulmonary piperacillin and tazobactam concentrations were unpredictable and negatively correlated with pulmonary permeability. Current piperacillin–tazobactam regimens may be insufficient to treat pneumonia caused by piperacillin–tazobactam–susceptible organisms in some critically ill patients.

Clinical Pharmacology & Therapeutics (2014); 96 4, 438–448. doi:10.1038/clpt.2014.131

Biopharmaceutical characterization of nebulized antimicrobial agents in rats: 1. Ciprofloxacin, moxifloxacin, and grepafloxacin

The aim of this study was to evaluate the biopharmaceutical characteristics of three fluoroquinolones (FQs), ciprofloxacin (CIP), moxifloxacin (MXF), and grepafloxacin (GRX), after delivery via a nebulized aerosol to rats. Bronchoalveolar lavages (BAL) were conducted 0.5, 2, 4, and 6 h after FQ intravenous administration and nebulized aerosol delivery to estimate epithelial lining fluid (ELF) drug concentrations. Plasma drug concentrations were also measured, and profiles of drug concentrations versus time after intravenous administration and nebulized aerosol delivery were virtually superimposable, attesting for rapid and complete systemic absorption of FQs. ELF drug concentrations were systematically higher than corresponding plasma drug concentrations, whatever the route of administration, and average ELF-to-unbound plasma drug concentration ratios post-distribution equilibrium did not change significantly between the ways of administration and were equal: 4.0 ± 5.3 for CIP, 12.6 ± 7.3 for MXF, and 19.1 ± 10.5 for GRX (means ± standard deviations). The impact of macrophage lysis on estimated ELF drug concentrations was significant for GRX but reduced for MXF and CIP; therefore, simultaneous pharmacokinetic modeling of plasma and ELF drug concentrations was only performed for the latter two drugs. The model was characterized by a fixed volume of ELF (VELF), passive diffusion clearance (QELF), and active efflux clearance (CLout) between plasma and ELF, indicating active efflux transport systems. In conclusion, this study demonstrates that ELF drug concentrations of these three FQs are several times higher than plasma drug concentrations, probably due to the presence of efflux transporters at the pulmonary barrier level, but no biopharmaceutical advantage of FQ nebulization was observed compared with intravenous administration.

Intrapulmonary concentration of levofloxacin in patients with idiopathic pulmonary fibrosis

Patients with idiopathic pulmonary fibrosis (IPF) have significantly impaired pulmonary diffusion, which may affect the pulmonary concentration of many drugs, including antibiotics. In this study, we compared the difference in pulmonary levofloxacin (LVFX) concentration between patients with normal lung function and IPF. The IPF group included 10 patients with a proven diagnosis of IPF and a diffusing capacity for carbon monoxide ranging from 40% to 70% of predicted values. The control group included 10 patients with normal pulmonary function. Blood and bronchoalveolar lavage fluid (BALF) were taken at 3-3.5 h after fasting. LVFX (500 mg) was administered orally. LVFX concentrations in the serum and BALF were determined using HPLC-MS/MC. The level of LVFX in alveolar epithelial lining fluid (ELF) was calculated using the following formula: LVFX ELF = LVFX BALF × (Urea serum/Urea BALF). No significant differences in age, body weight, height, and calculated creatinine clearance and BALF retrieval rate were observed between groups. LVFX serum concentrations in the IPF and control groups were (5.97 ± 1.28) μg/ml and (6.84 ± 3.43) μg/ml, respectively (P = 0.4727). ELF concentration of LVFX in the control group was (27.81 ± 21.36) μg/ml, while the concentration in the IPF group was (10.17 ± 2.46) μg/ml, less than half of that in the controls (P = 0.0058). The intrapulmonary concentration of LVFX in IPF patients was lower than those with normal lung function. Notably, however, the ELF LVFX concentration following 500 mg once-daily exceeded the MIC90 of common respiratory pathogens. Excellent antibacterial efficacy of LVFX can be expected for IPF patients in the treatment of respiratory tract infections.

Effect of cardiopulmonary bypass on regional antibiotic penetration into lung tissue

The use of cardiopulmonary bypass (CPB) during cardiac surgery causes regional ventilation-perfusion mismatch, contributing to regional disturbances in antibiotic penetration into lung tissue. Ventilation-perfusion mismatch is associated with postoperative pneumonia, a frequent and devastating complication after cardiac surgery. In this prospective clinical animal study, we performed in vivo microdialysis to determine the effect of CPB on regional penetration of levofloxacin (LVX) into lung tissue. Six pigs underwent surgery with CPB (CPB group), and another six pigs underwent surgery without CPB (off-pump coronary artery bypass grafting; OPCAB group). LVX (750 mg) was administered intravenously to all pigs immediately after surgery. For regional measurements of LVX in pulmonary concentrations, microdialysis probes were inserted in both lungs of each pig. Pigs were placed in the right lateral position. Time versus concentration profiles of unbound LVX were measured in the upper and lower lung tissue and plasma in all pigs. In all pigs, maximum concentrations (Cmax) of LVX were significantly lower in the upper lung than in the lower lung (OPCAB, P = 0.035; CPB, P < 0.001). Median Cmax of LVX showed a significant difference in the upper versus lower lung in the CPB group (P < 0.05). No significant difference was found in the median Cmax of LVX in the upper and the lower lung in the OPCAB group (P = 0.32). Our data indicate that CPB affects perioperative regional antibiotic penetration into lung tissue. Common clinical antibiotic dosing schemes should be reevaluated in patients undergoing coronary artery bypass grafting with CPB.

Penetration of anti-infective agents into pulmonary epithelial lining fluid: focus on antibacterial agents

The exposure-response relationship of anti-infective agents at the site of infection is currently being re-examined. Epithelial lining fluid (ELF) has been suggested as the site (compartment) of antimicrobial activity against lung infections caused by extracellular pathogens. There have been an extensive number of studies conducted during the past 20 years to determine drug penetration into ELF and to compare plasma and ELF concentrations of anti-infective agents. The majority of these studies estimated ELF drug concentrations by the method of urea dilution and involved either healthy adult subjects or patients undergoing diagnostic bronchoscopy. Antibacterial agents such as macrolides, ketolides, newer fluoroquinolones and oxazolidinones have ELF to plasma concentration ratios of >1. In comparison, β-lactams, aminoglycosides and glycopeptides have ELF to plasma concentration ratios of ≤1. Potential explanations (e.g. drug transporters, overestimation of the ELF volume, lysis of cells) for why these differences in ELF penetration occur among antibacterial classes need further investigation. The relationship between ELF concentrations and clinical outcomes has been under-studied. In vitro pharmacodynamic models, using simulated ELF and plasma concentrations, have been used to examine the eradication rates of resistant and susceptible pathogens and to explain why selected anti-infective agents (e.g. those with ELF to plasma concentration ratios of >1) are less likely to be associated with clinical treatment failures. Population pharmacokinetic modelling and Monte Carlo simulations have recently been used and permit ELF and plasma concentrations to be evaluated with regard to achievement of target attainment rates. These mathematical modelling techniques have also allowed further examination of drug doses and differences in the time courses of ELF and plasma concentrations as potential explanations for clinical and microbiological effects seen in clinical trials. Further studies are warranted in patients with lower respiratory tract infections to confirm and explore the relationships between ELF concentrations, clinical and microbiological outcomes, and pharmacodynamic parameters.