Kill kinetics of bacteria under fluctuating concentrations of various antibiotics. II. Description of experiments

A Diffusion-Based and Dynamic 3D-Printed Device That Enables Parallel in Vitro Pharmacokinetic Profiling of Molecules

The process of bringing a drug to market involves many steps, including the preclinical stage, where various properties of the drug candidate molecule are determined. These properties, which include drug absorption, distribution, metabolism, and excretion, are often displayed in a pharmacokinetic (PK) profile. While PK profiles are determined in animal models, in vitro systems that model in vivo processes are available, although each possesses shortcomings. Here, we present a 3D-printed, diffusion-based, and dynamic in vitro PK device. The device contains six flow channels, each with integrated porous membrane-based insert wells. The pores of these membranes enable drugs to freely diffuse back and forth between the flow channels and the inserts, thus enabling both loading and clearance portions of a standard PK curve to be generated. The device is designed to work with 96-well plate technology and consumes single-digit milliliter volumes to generate multiple PK profiles, simultaneously. Generation of PK profiles by use of the device was initially performed with fluorescein as a test molecule. Effects of such parameters as flow rate, loading time, volume in the insert well, and initial concentration of the test molecule were investigated. A prediction model was generated from this data, enabling the user to predict the concentration of the test molecule at any point along the PK profile within a coefficient of variation of ∼ 5%. Depletion of the analyte from the well was characterized and was determined to follow first-order rate kinetics, indicated by statistically equivalent (p > 0.05) depletion half-lives that were independent of the starting concentration. A PK curve for an approved antibiotic, levofloxacin, was generated to show utility beyond the fluorescein test molecule.

In vitro pharmacodynamic models to determine the effect of antibacterial drugs

In vitro pharmacodynamic (PD) models are used to obtain useful quantitative information on the effect of either single drugs or drug combinations against bacteria. This review provides an overview of in vitro PD models and their experimental implementation. Models are categorized on the basis of whether the drug concentration remains constant or changes and whether there is a loss of bacteria from the system. Further subdifferentiation is based on whether bacterial loss involves dilution of the medium or is associated with dialysis or diffusion. For comprehension of the underlying principles, experimental settings are simplified and schematically illustrated, including the simulations of various in vivo routes of administration. The different model types are categorized and their (dis)advantages discussed. The application of in vitro models to special organs, infections and pathogens is comprehensively presented. Finally, the relevance and perspectives of in vitro investigations in drug discovery and clinical research are elucidated and discussed.

Effect of antibiotic sequence on combination regimens against Pseudomonas aeruginosa in a multiple-dose, in vitro infection model

The goal of this study was to investigate the effect of antibiotic sequence on combination regimens against Pseudomonas aeruginosa in an in vitro infection model. Ceftazidime plus ciprofloxacin and ceftazidime plus tobramycin were dosed every 12 h for 48 h using simultaneous or staggered administration. Simultaneous dosing and ceftazidime followed by ciprofloxacin or tobramycin were significantly more active at both 24 h (p = 0.03) and 48 h (p < 0.0001) than ciprofloxacin or tobramycin followed by ceftazidime. Final bacterial kill was sixfold greater with the former regimens. This study showed that antibiotic sequence had a significant and class dependent effect on antibacterial response. The clinical relevance of these observations warrants further investigations in animal models.

Use of pharmacodynamic parameters to predict efficacy of combination therapy by using fractional inhibitory concentration kinetics

Combination therapy with antimicrobial agents can be used against bacteria that have reduced susceptibilities to single agents. We studied various tobramycin and ceftazidime dosing regimens against four resistant Pseudomonas aeruginosa strains in an in vitro pharmacokinetic model to determine the usability of combination therapy for the treatment of infections due to resistant bacterial strains. For the selection of an optimal dosing regimen it is necessary to determine which pharmacodynamic parameter best predicts efficacy during combination therapy and to find a simple method for susceptibility testing. An easy-to-use, previously described E-test method was evaluated as a test for susceptibility to combination therapy. That test resulted in a MICcombi, which is the MIC of, for example, tobramycin in the presence of ceftazidime. By dividing the tobramycin and ceftazidime concentration by the MICcombi at each time point during the dosing interval, fractional inhibitory concentration (FIC) curves were constructed, and from these curves new pharmacodynamic parameters for combination therapy were calculated (i.e., AUCcombi, Cmax-combi, T>MIC-combi, and T>FICi, where AUCcombi, Cmax-combi, T>MIC-combi, and T>FICi are the area under the FICcombi curve, the peak concentration of FICcombi, the time that the concentration of the combination is above the MICcombi, and the time above the FIC index, respectively). By stepwise multilinear regression analysis, the pharmacodynamic parameter T>FICi proved to be the best predictor of therapeutic efficacy during combination therapy with tobramycin and ceftazidime (R2 = 0.6821; P < 0.01). We conclude that for combination therapy with tobramycin and ceftazidime the T>FICi is the parameter best predictive of efficacy and that the E-test for susceptibility testing of combination therapy gives promising results. These new pharmacodynamic parameters for combination therapy promise to provide better insight into the rationale behind combination therapy.

Synergism between tobramycin and ceftazidime against a resistant Pseudomonas aeruginosa strain, tested in an in vitro pharmacokinetic model

Synergism between two antibiotics is usually tested by a checkerboard titration technique, or by time-kill methods. Both methods have the disadvantage that synergism is determined at constant concentrations of the antibiotics, which do not reflect reality in vivo. In the present study we determined whether synergism between tobramycin and ceftazidime can be found at declining concentrations below the MIC, and whether change in dosing sequence of the antibiotics would result in differences in killing. Three monotherapy and six combination therapy schedules were tested in an in vitro pharmacokinetic model, using a Pseudomonas aeruginosa resistant to both antibiotics. During all q8h dosing schedules the peak concentration (Cmax) was adjusted to the MIC for the strain of both antibiotics. During all monotherapy regimens bacterial growth was present, while all six combination therapy schedules showed significant killing. At t = 24 h there were no differences between all combination therapy schedules, but at t = 8 h the two combination therapy schedules with administration of tobramycin once daily showed a significantly faster killing. By using the area under the killing curve (AUKC) as a parameter for synergistic killing, simultaneous combination therapy starting with tobramycin once daily was significantly better than all other regimens. We conclude that there is synergism between tobramycin and ceftazidime at declining antibiotic concentrations below the MIC, resulting in a pronounced killing of a resistant Pseudomonas strain. Infections due to resistant Pseudomonas strains could possibly be treated by a synergistic combination of these drugs.

Improved efficacy with nonsimultaneous administration of first doses of gentamicin and ceftazidime in vitro

First doses of aminoglycoside and beta-lactam antibiotics, when used in combination, are usually given simultaneously; however, nonsimultaneous administration may be more efficacious. We used a dynamic in vitro model, which simulates in vivo serum kinetics, to assess the effect of spacing the first doses of gentamicin and ceftazidime used against Pseudomonas aeruginosa ATCC 27853 and two clinical isolates of P. aeruginosa, PA1 and PA2. The following dose regimens against P. aeruginosa ATCC 27853 were compared: (i) gentamicin given alone, (ii) ceftazidime given alone, (iii) gentamicin and ceftazidime given simultaneously, (iv) gentamicin followed by ceftazidime at 15 or 50 min or at 2, 4, or 8 h, and (v) ceftazidime which was followed by gentamicin at 4 h. The effects of regimen iii and the 4-h interval in regimen iv against PA1 and PA2 were also compared. Initial peak concentrations used were 8 mg/liter for gentamicin and 80 mg/liter for ceftazidime, with drug half-lives of 2.5 and 1.8 h, respectively. Compared with simultaneous administration, nonsimultaneous administration (regimens iv and v) produced greater overall bacterial killing and was associated with a delay in bacterial regrowth (p < 0.005) of up to 6.6 to 8.3 h, regardless of the order in which the drugs were given. The optimal interval between gentamicin and ceftazidime doses, which maximized initial bactericidal effect and the time before regrowth, appeared to be 2 to 4 h.

Time-survival studies for quantifying effects of azlocillin and tobramycin on Pseudomonas aeruginosa

Time-survival studies were conducted to estimate the effects of azlocillin and tobramycin on Pseudomonas aeruginosa NCIMB 8295 (in the exponential phase of growth) at concentrations ranging from one-quarter to twice the MIC. The effects of the individual agents and their combinations were determined by measuring the viable counts (CFU per milliliter) over a 24-h period. The typical pattern observed from the plot of the logarithm of the CFU per milliliter against time was an initial rapid killing; this was followed by a period of stasis and regrowth. Initial rates of killing by tobramycin were concentration dependent, whereas this was not the case with azlocillin. From the time-survivor plots, the area under the curve for viable bacteria was also calculated. It offered a useful method of interpreting the results of time-kill studies, taking the overall pattern of killing and regrowth into consideration. The area under the curve for viable bacteria was concentration dependent for both antibiotics. A 2(2) factorial experimental design was used to analyze the joint effects of azlocillin and tobramycin. In such a factorial experiment, an interaction between two factors, in this case, azlocillin and tobramycin concentrations, is shown by a change in the slope of the plot when the concentration of the interactant is changed. Analysis of variance showed that the combination was synergistic at low concentrations, but this was not significant when the concentration of either interactant was increased.

Evaluation of ceftriaxone and other antibiotics against Escherichia coli, Pseudomonas aeruginosa, and Streptococcus pneumoniae under in vitro conditions simulating those of serious infections

In pursuit of an in vitro system capable of reliably predicting the activities of antibiotics in serious infections and in infections occurring in immunocompromised hosts, we evaluated the abilities of four drugs to achieve virtually complete killing of bacterial cells growing in human body fluids in amounts which are very high and close to those likely to be present in serious infections; drug concentrations varied with time as they vary in human bronchial secretions or blood or urine (dynamic concentrations). The rationale for such a test was (i) to set up in vitro conditions as close as possible to those the antibiotics encounter in serious infections and (ii) to hold the drugs capable of almost completely killing the bacteria used in the assay to be highly active in vitro and likely to be the most efficacious in the treatment of serious infections. Among the antibiotics used, ceftriaxone proved to be highly active under conditions simulating pulmonary infections and septicemias caused by Streptococcus pneumoniae (bacteria grown in bronchoalveolar fluid or blood; antibiotic concentrations varying with time as in human bronchial secretions or blood) and under conditions simulating blood and urinary infections caused by Escherichia coli (bacteria grown in human blood or urine; antibiotic concentrations varying as in the various fluids). Gentamicin (not tested against pneumococci) appeared to be highly active only under conditions simulating urinary infections caused by E. coli; aztreonam (not tested against pneumococci) and ampicillin (tested only against pneumococci) did not appear to be highly active under any of the test conditions. Only the combination of gentamicin plus either ceftriaxone or aztreonam appeared to be highly active under conditions simulating serious septicemias and urinary infections caused by Psudomonas aeruginosa.

Antimicrobial chemotherapy in patients with cystic fibrosis

The treatment of exacerbations of pulmonary infections due to Pseudomonas aeruginosa in patients with cystic fibrosis is unsatisfactory. Serum concentrations and urinary excretion of cephalexin, epicillin, azlocillin, ticarcillin, trimethoprim-sulfa and gentamicin useful in the treatment of these infections were investigated in cystic fibrosis patients suffering from pulmonary infections. The data were compared to those found in non-cystic fibrosis children treated with antibiotics for other reasons. Cephalexin and trimethoprim are absorbed at a slower rate; epicillin, azlocillin, ticarcillin sulfonamides were eliminated at a faster rate by the kidneys which was unique to patients with cystic fibrosis. Gentamicin is also eliminated faster. Further investigations disclosed that a considerable amount of drug is eliminated by tubular secretion in addition to the regular glomerular filtration in patients with cystic fibrosis. Creatinine clearance values were determined in these patients and found to be normal. By doubling the dose of gentamicin and administration as infusion over 90 min, higher serum and tissue concentrations were achieved without being in the toxic range. The clinical relevance of these investigations was determined in 36 patients and 48 episodes of infection with P. aeruginosa. Patients received gentamicin 4 mg/kg BW as i.v. infusion over 90 min q. 8 h and azlocillin or ticarcillin 120-160 mg/kg BW q. 8 h, applied 4 h later. In 14 patients respectively 27 episodes, pseudomonas was eradicated from the sputum for a minimum of three weeks, and in most of them for 12-24 weeks. No side effects were observed from the higher doses of aminoglycosides.

A pharmacodynamic model for the activity of antibiotics against microorganisms under nonsaturable conditions

An exact mathematical solution was derived to a pharmacodynamic model which illustrates bacterial survival in the presence of antibiotics. In this report the survival of Pseudomonas aeruginosa in the medium of an initial concentration of 0.64 mM (320 mg/L) of piperacillin [(2S,5R,6R)-6-[(R)-2-(4-ethyl-2,3-dioxo-1- piperazinecarboxyamido)-2-phenylacetamido]-3,3-dimethyl-7- oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylate] was well described by the derived model for up to 24 h. The bacterial killing by the antibiotic and apparent natural growth rate constants were 2955.3 h-1 X mol-1 and 0.5698 h-1, respectively. The functional equation was also fit to the data of ampicillin against Escherichia coli under simulated in vivo conditions. The optimal multiple dosing time and the minimum critical concentration to achieve antimicrobial action can be readily calculated from the developed model. Computer simulations were made to examine the effect on microbial survival of such factors as initial antibiotic concentration (Co), elimination half-life (t1/2), kill rate constant (K) of the antibiotic, and apparent growth rate constant (Kapp) of the test organism.