Sequential Time-Kill, a Simple Experimental Trick To Discriminate between Pharmacokinetics/Pharmacodynamics Models with Distinct Heterogeneous Subpopulations versus Homogenous Population with Adaptive Resistance.

Experiments were conducted with polymyxin B and two Klebsiella pneumonia isogenic strains (the wild type, KP_WT, and its transconjugant carrying the mobile colistin resistance gene, KP_MCR-1) to demonstrate that conducting two consecutive time-kill experiments (sequential TK) represents a simple approach to discriminate between pharmacokinetics/pharmacodynamics models with two heterogeneous subpopulations or adaptive resistance.

Semi-mechanistic PK/PD modelling of combined polymyxin B and minocycline against a polymyxin-resistant strain of Acinetobacter baumannii.

OBJECTIVES: To expand on previous reports of synergy between polymyxin B (PMB) and minocycline (MIN) against Acinetobacter baumannii; and to gain insight into the qualitative and quantitative determinants of their synergy. METHODS: A semi-mechanistic pharmacokinetic/pharmacodynamic (PK/PD) model was developed on the basis of data from in vitro time-kill experiments with determination of resistant bacterial count to describe the effects of PMB and MIN alone and in combination. The model was enriched by complementary experiments providing information on the characteristics of the resistant subpopulation. RESULTS: The model successfully described the data and made possible quantification of the strength of interaction between the two drugs and formulation of hypotheses about the mechanisms of the observed interaction. The effect of the combination was driven by MIN, with PMB acting as an helper drug; simulations at clinically achievable concentrations showed that 1.5 mg/L MIN +0.2 mg/L PMB is expected to produce sustained killing over 30 hours, while 0.3 mg/L MIN +1 mg/L PMB is met by bacterial regrowth. Interaction equations showed that maximal synergy is reached for PMB concentrations ≥0.1 mg/L and MIN concentrations ≥1 mg/L. CONCLUSIONS: Semi-mechanistic PK/PD modelling was used to investigate the quantitative determinants of synergy between PMB and MIN on a PMB-resistant A. baumannii strain. The developed model, improving on usual study techniques, showed asymmetry in the drug interaction, as PMB acted mostly as a helper to MIN, and provided simulations as a tool for future studies.

The inoculum effect of Escherichia coli expressing mcr-1 or not on colistin activity in a murine model of peritonitis.

OBJECTIVES: Colistin often remains the last resort antibiotic active against carbapenemase-producing Enterobacteriaceae. However, while in vitro inoculum effect has been reported, therapeutic relevance of this phenomenon remains questioned. METHODS: Ten E. coli strains were used that included the wild-type CFT073 and its transconjugant CFT073-MCR-1 and eight susceptible clinical isolates. Mice with peritonitis were treated for 24 h with colistin sulfate. Bacterial loads were determined in peritoneal fluid (PF) and spleen and colistin-resistant mutants were detected. RESULTS: MICs of colistin against the eight susceptible clinical strains and CFT073 ranged from 0.125 to 0.5 mg/L with an inoculum of 105 CFU/mL and from 2 to 4 mg/L with a 107 CFU/mL inoculum; 5/9 strains with an MIC of 4 mg/L were considered resistant according to EUCAST breakpoint (resistance, > 2 mg/L). When the bacterial load of wild-type CFT073 inoculated in mice increased from 107 to 108 CFU: i) mean log10 CFU reduction generated by colistin in PF and spleen decreased from 5.8/mL and 3.1/g, respectively, (p < 0.01) to 0.9/mL and 0.8/g, respectively (NS); ii) mice survival rate decreased from 15/15 (100%) to 6/15 (40%) (p = 0.017); and iii) proportion of mice with selection of colistin-resistant mutants increased from 4/15 to 15/15 (p < 0.01). These results were comparable to those obtained when peritonitis was produced with a 107 CFU bacterial load of E. coli CFT073 expressing mcr-1, for which the mean log10 CFU reductions were 3.5/mL and 0.6/g in PF and spleen, respectively (NS), and survival rate was 8/15 (53%) (p < 0.01 versus survival of mice infected with wild-type CFT073). CONCLUSIONS: Phenotypic colistin resistance in wild-type E. coli due to an increase in inoculum size had a therapeutic impact in mice with peritonitis that was comparable to that observed when the mcr-1 gene was expressed.

Pharmacodynamic modelling of ß-lactam/ß-lactamase inhibitor checkerboard data: illustration with aztreonam-avibactam.

OBJECTIVES: Checkerboard experiments followed by fractional inhibitory concentration (FIC) index determinations are commonly used to assess in vitro pharmacodynamic interactions between combined antibiotics, but FIC index cannot be determined in case of antibiotic/non-active compound combinations. The aim of this study was to use a simple modelling approach to quantify the in vitro activity of aztreonam-avibactam, a new ß-lactam-ß-lactamase inhibitor combination. METHODS: MIC checkerboard experiments were performed with 12 Enterobacteriaceae with diverse ß-lactamases profiles. Aztreonam MICs in the absence and presence of avibactam at different concentrations (ranging from 0.0625 to 4 mg/L) were determined. Aztreonam MIC versus avibactam concentrations were fitted by an inhibitory Emax model with a baseline effect parameter. RESULTS: A concentration-dependent relationship was observed with a steep initial reduction of aztreonam MIC at low avibactam concentrations and reaching a maximum at higher avibactam concentrations that was adequately fitted by the model. Maximum avibactam effect was characterized by the ratio of aztreonam MICs in the absence of avibactam (MIC0) and when avibactam concentration tends toward infinity (MIC∞), and this ratio ranged between 90 and 10 068 depending on the strain. Avibactam potency was characterized by avibactam concentrations corresponding to 50% of the maximum effect (IC50 values between 0.00022 and 0.053 mg/L). CONCLUSIONS: An inhibitory Emax model with a baseline effect could quantify maximum avibactam effect and potency among various strains. This simple modelling approach can be used to compare the activity of other combinations of antibiotics with non-antibiotic drugs when FIC index is inappropriate.

In vitro biopharmaceutical evaluation of ciprofloxacin/metal cation complexes for pulmonary administration.

Pulmonary delivery of fluoroquinolones (FQs) is an interesting approach to treat lung infections as it may lead to high local concentrations while minimizing systemic exposure. However, FQs have a rapid diffusion through the lung epithelium giving the pulmonary route no advantage compared to the oral route. Interactions between FQs and metal cations form complexes which limit the diffusion through the epithelial barrier and would reduce the absorption of FQs and maintain high concentrations in the lung. The effects of this complexation depend on the FQ and the metal cations and optimum partners should be selected through in vitro experiments prior to aerosol drug formulation. In this study, CIP was chosen as a representative FQ and 5 cations (Ca2+, Mg2+, Zn2+, Al3+, Cu2+) were selected to study the complexation and its effects on permeability, antimicrobial efficacy and cell toxicity. The results showed that the apparent association constants between CIP and cations ranked with the descending order: Cu2+>Al3+>Zn2+>Mg2+>Ca2+. When a target of 80% complexation was reached with the adequate concentrations of cations, the CIP permeability through the Calu-3 lung epithelial cells was decreased of 50%. Toxicity of the CIP on the Calu-3 cells, with an EC50 evaluated at 7µM, was not significantly affected by the presence of the cations. The minimum inhibitory concentration of CIP for Pseudomonas aeruginosa was not affected or slightly increased in the range of cation concentrations tested, except for Mg2+. In conclusion, permeability was the main parameter that was affected by the metal cation complexation while cell toxicity and antimicrobial activity were not or slightly modified. Cu2+, with the highest apparent constant of association and with no effect on cell toxicity and antimicrobial activity of the CIP, appeared as a promising cation for the development of a controlled-permeability formulation of CIP for lung treatment.

Impact of two different colistin dosing strategies on healthy piglet fecal microbiota.

Colistin is often used in piglets but underdosing and overdosing are frequent. The impact of such administrations on fecal microbiota was studied. Piglets were given either underdoses of colistin by oral gavage for five days or overdoses by in-feed medication for 14days. The composition of fecal microbiota was studied by quantitative PCR, 16S rRNA sequencing, culture of Enterobacteriaceae, and quantification of short-chain fatty acids (SCFAs). The mean colistin concentrations during the treatment for underdosed and overdosed groups were 14.4µg/g and 64.9µg/g of feces respectively. Whatever the piglet and the sampling day, the two main phyla were Firmicutes and Bacteroidetes, The main families were Lactobacillaceae, Clostridiales, Lachnospiraceae and Ruminococcaceae. The main perturbation was the significant but transitory decrease in the Escherichia coli population during treatment, yet all the E. coli isolates were susceptible to colistin. Moreover, colistin did not affect the production of SCFAs. These results show that under- or overdoses of colistin do not result in any major disturbance of piglet fecal microbiota and rarely select for chromosomal resistance in the dominant E. coli population.

Population Pharmacokinetics of Colistin Methanesulfonate and Colistin in Critically Ill Patients with Acute Renal Failure Requiring Intermittent Hemodialysis.

Colistin is increasingly used as a last option for the treatment of severe infections due to Gram-negative bacteria in critically ill patients requiring intermittent hemodialysis (HD) for acute renal failure. Our objective was to characterize the pharmacokinetics (PK) of colistin and its prodrug colistin methanesulfonate (CMS) in this population and to suggest dosing regimen recommendations. Eight intensive care unit (ICU) patients who were under intermittent HD and who were treated by CMS (Colimycine) were included. Blood samples were collected between two consecutive HD sessions. CMS and colistin concentrations were measured by a specific chromatographic assay and were analyzed using a PK population approach (Monolix software). Monte Carlo simulations were conducted to predict the probability of target attainment (PTA). CMS nonrenal clearance was increased in ICU-HD patients. Compared with that of ICU patients included in the same clinical trial but with preserved renal function, colistin exposure was increased by 3-fold in ICU-HD patients. This is probably because a greater fraction of the CMS converted into colistin. To maintain colistin plasma concentrations high enough (>3 mg/liter) for high PTA values (area under the concentration-time curve for the free, unbound fraction of a drug [fAUC]/MIC of >10 and fAUC/MIC of >50 for systemic and lung infections, respectively), at least for MICs lower than 1.5 mg/liter (nonpulmonary infection) or 0.5 mg/liter (pulmonary infection), the dosing regimen of CMS should be 1.5 million international units (MIU) twice daily on non-HD days. HD should be conducted at the end of a dosing interval, and a supplemental dose of 1.5 MIU should be administered after the HD session (i.e., total of 4.5 MIU for HD days). This study has confirmed and complemented previously published data and suggests an a priori clear and easy to follow dosing strategy for CMS in ICU-HD patients.

New colistin population pharmacokinetic data in critically ill patients suggesting an alternative loading dose rationale.

Colistin is an old antibiotic that has recently gained a considerable renewal of interest as the last-line defense therapy against multidrug-resistant Gram-negative bacteria. It is administered as colistin methanesulfonate (CMS), an inactive prodrug, and it was shown that due to slow CMS conversion, colistin plasma concentrations increase very slowly after treatment initiation, which constitutes the rationale for a loading dose in critically ill patients. However, faster CMS conversion was observed in healthy volunteers but using a different CMS brand, which may also have a major impact on colistin pharmacokinetics. Seventy-three critically ill patients not undergoing dialysis received multiple doses of CMS. The CMS concentrations were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS), and a pharmacokinetic analysis was conducted using a population approach. We confirmed that CMS renal clearance and colistin concentrations at steady state are mostly governed by creatinine clearance, but we predict a typical maximum concentration of drug in serum (Cmax) of colistin close to 2 mg/liter, occurring 3 h after an initial dose of 2 million international units (MIU) of CMS. Accordingly, the estimated colistin half-life (t1/2) was relatively short (3.1 h), with rapid attainment of steady state. Our results are only partially consistent with other recently published results. We confirm that the CMS maintenance dose should be adjusted according to renal function in critically ill patients. However, much higher than expected colistin concentrations were observed after the initial CMS dose, with rapid steady-state achievement. These discrepancies challenge the pharmacokinetic rationale for a loading dose, which may still be appropriate for rapid bacterial eradication and an improved clinical cure rate.

Pseudomonas aeruginosa infection in cystic fibrosis lung disease and new perspectives of treatment: a review.

Cystic fibrosis (CF) is a complex inherited disease which affects many organs, including the pancreas and liver, gastrointestinal tract and reproductive system, sweat glands and, particularly, the respiratory system. Pseudomonas aeruginosa is the main cause of chronic airway infection. In order to reduce morbidity and mortality due to lung infection by P. aeruginosa, aerosol antibiotics have been used to achieve high local concentrations in the airways and to reduce systemic toxicity. In the course of this review, the current treatments to control CF lung infections by P. aeruginosa are presented. Some innovative aerosol formulations such as liposomes and microspheres are herein reviewed, which may improve the efficiency of anti-pseudomonal agents, and ensure patients' compliance to treatments, by reducing dosing frequency and/or drug dose, while maintaining therapeutic efficacy, preventing the occurrence of bacterial resistance and/or reducing adverse effects due to their controlled-release properties.

A preclinical pharmacokinetic modeling approach to the biopharmaceutical characterization of immediate and microsphere-based sustained release pulmonary formulations of rifampicin.

A rifampicin-hydroxypropyl-beta-cyclodextrin (RIF-HPCD) complex solution and two RIF-loaded PLGA microspheres with slow or fast release rates were nebulized into the rat lungs for a comparative biopharmaceutical evaluation. A pharmacokinetic model was applied to model systemic RIF concentrations and to predict the RIF concentrations in the lung epithelial lining fluid (ELF). With intravenous RIF and nebulized RIF-HPCD, plasma profiles and predicted RIF ELF profiles were superimposed indicating that RIF diffused almost instantaneously through the broncho-alveolar barrier. 5h post administration RIF ELF predicted concentrations were in agreement with experimental concentrations determined using the broncho-alveolar lavage (BAL) sampling method. Microsphere formulations resulted in different plasma concentration profiles, demonstrating RIF sustained release. The PK model predicted the ELF concentrations to be much higher with microspheres than with nebulized and IV RIF, over a prolonged time period, which was confirmed by BAL sampling. In conclusion this work demonstrated the benefit of using sustained-release microspheres administered as aerosols to maintain, over a prolonged time period, high levels of pulmonary concentrations of drugs characterized by a rapid absorption through the broncho-alveolar barrier. Moreover, PK modeling was a useful tool to build concentration-versus-time profiles in non-readily accessible ELF compartment and to assess the biopharmaceutical properties of aerosol formulations for lung delivery.

Colistin in critically ill patients.

Colistin is a re-emerging old antibiotic that is used as a salvage treatment against multidrug-resistant Gram-negative infections. Because it is administrated as an inactive prodrug, colistin methanesulfonate (CMS) that undergoes rapid hydrolyze to colistin, pharmacokinetic studies using biological assays are unreliable. With the recent development of new assays using high performance liquid chromatography (HPLC) accurate pharmacokinetic of CMS and formed colistin is now available in various populations. This article aims to update previous reports on pharmacodynamics, pharmacokinetics, safety and clinical use of colistin, with a special focus on data useful to treat critically ill patients.

Colistin pharmacokinetics: the fog is lifting.

Colistin is a re-emerging old antibiotic that is used to treat multidrug-resistant infections in critically ill patients. It corresponds to a mixture of at least 30 different compounds administered as inactive derivatives. Therefore, colistin pharmacokinetics are quite difficult to investigate and complex to predict. However specific chromatographic methods have been made available in recent years, leading to a series of modern pharmacokinetic studies after intravenous administration of the prodrug to critical-care patients; these have been conducted by a few groups and have only been recently published. The objective of this article was to conduct a critical review of these very informative modern pharmacokinetic studies and to provide prospective thoughts.

A survey on the use of gentamicin in infective endocarditis.

US and European guidelines recommend a daily divided gentamicin dose (3 mg/kg in two or three equally divided doses) for the treatment of infective endocarditis caused by staphylococci or enterococci, but once-daily dosing (3 mg/kg/day) is recommended for streptococcal endocarditis. However, studies have recommended the use of higher doses of gentamicin (4 or ≥5 mg/kg/day) administered once-daily. A survey was conducted in France by mailing a questionnaire to the 595 members of the French Infectious Disease Society regarding their gentamicin prescription patterns in infective endocarditis, focusing on the dosing regimen. The survey was answered by 137 physicians (23%). The proportions of physicians following guideline-based regimens were similar for each organism (30.9%, 38.8%, and 39.4% for staphylococci, enterococci, and streptococci, respectively [p=0.26]). In contrast, the proportions of physicians following literature-based regimens were significantly different for each organism (59.6%, 42.5%, and 27.7% for staphylococci, enterococci, and streptococci, respectively [p<0.001]). The number of years practicing and the type of practice (university vs. non-university hospital) did not influence the gentamicin dose or regimen. Although adherence to published guidelines for gentamicin administration in patients with infective endocarditis was poor, a large proportion of physicians who did not follow those guidelines used literature-based regimens.

Formulation and in vitro characterization of inhalable rifampicin-loaded PLGA microspheres for sustained lung delivery.

The solvent evaporation method with premix membrane homogenization was applied, with class-3 ethyl acetate as organic solvent, to produce narrowly size-distributed rifampicin (RIF)-loaded poly(lactide-co-glycolide) (PLGA) microspheres for sustained lung delivery as aerosol. Microsphere formulations (simple or multiple emulsions, different PLGA and RIF concentrations) and process parameters (transmembrane pressure, SPG membrane pore diameter) were investigated as their effects on RIF content, microsphere size, aerodynamic properties of the freeze-dried powder and in vitro release profiles. Narrowly size distributed microspheres with diameters from 2 to 8 µm, satisfactory RIF contents (from 4.9 to 16.5%), 80% RIF release from 12h to 4 days, and adequate aerodynamic properties were prepared from a multiple emulsion and using SPG membrane pore diameter of 19.9 µm. The premix membrane homogenization appeared to be a rapid and efficient method to prepare monodisperse drug-loaded microspheres suitable for lung delivery as sustained-release microsphere aerosol.

Pharmacokinetics of colistin and colistimethate sodium after a single 80-mg intravenous dose of CMS in young healthy volunteers.

Colistin pharmacokinetics (PK) was investigated in young healthy volunteers after a 1-h infusion of 80 mg (1 million international units (MIU)) of the prodrug colistin methanesulfonate (CMS). Concentration levels of CMS and colistin were determined in plasma and urine using a new chromatographic assay and analyzed simultaneously with a population approach after correcting the urine-related data for postexcretion hydrolysis of CMS into colistin. CMS and colistin have low volumes of distribution (14.0 and 12.4 liters, respectively), consistent with distribution being restricted to extracellular fluid. CMS is mainly excreted unchanged in urine (70% on average), with a typical renal clearance estimated at 103 ml/min-close to the glomerular filtration rate. Colistin elimination is essentially extrarenal, given that its renal clearance is 1.9 ml/min, consistent with extensive reabsorption. Colistin elimination is not limited by the formation rate because its half-life (3 h) is longer than that of CMS. The values of these pharmacokinetic parameters will serve as reference points for future comparisons with patients' data.