Pharmacodynamics of once-daily amikacin in various combinations with cefepime, aztreonam, and ceftazidime against Pseudomonas aeruginosa in an in vitro infection model

An In Vitro Coumarin-Antibiotic Combination Treatment of Pseudomonas aeruginosa Biofilms

The drug resistance of Pseudomonas aeruginosa is a worldwide problem due to its great threat to human health. A crude extract of Angelica dahurica has been proved to have antibacterial properties, which suggested that it may be able to inhibit the biofilm formation of P. aeruginosa; initial exploration had shown that the crude extract could inhibit the growth of P. aeruginosa effectively. After the adaptive dose of coumarin was confirmed to be a potential treatment for the bacteria’s drug resistance, “coumarin-antibiotic combination treatments” (3 coumarins—simple coumarin, imperatorin, and isoimperatorin—combined with 2 antibiotics—ampicillin and ceftazidime) were examined to determine their capability to inhibit P. aeruginosa. The final results showed that (1) coumarin with either ampicillin or ceftazidime significantly inhibited the biofilm formation of P. aeruginosa; (2) coumarin could directly destroy mature biofilms; and (3) the combination treatment can synergistically enhance the inhibition of biofilm formation, which could significantly reduce the usage of antibiotics and bacterial resistance. To sum up, a coumarin-antibiotic combination treatment may be a potential way to inhibit the biofilm growth of P. aeruginosa and provides a reference for antibiotic resistance treatment.

Amikacin Pharmacokinetic-Pharmacodynamic Analysis in Pediatric Cancer Patients

We performed pharmacokinetic-pharmacodynamic (PK-PD) and simulation analyses to evaluate a standard amikacin dose of 15 mg/kg once daily in children with cancer and to determine an optimal dosing strategy. A population pharmacokinetic model was developed from clinical data collected in 34 pediatric patients and used in a simulation study to predict the population probability of various dosing regimens to achieve accepted safety (steady-state unbound trough plasma concentration [fCmin] of <10 mg/liter)- and efficacy (free, unbound plasma concentration-to-MIC ratio [fCmax/MIC] of ≥8)-linked targets. In addition, an adaptive resistance PD (ARPD) model of Pseudomonas aeruginosa was built based on literature time-kill curve data and linked to the PK model to perform PK-ARPD simulations and compare results with those of the probability approach. Using the probability approach, an amikacin dose of 60 mg/kg administered once daily is expected to achieve the target fCmax/MIC in 80% of pediatric patients weighing 8 to 70 kg with a 97.5% probability, and almost all patients were predicted to have fCmin of <10 mg/liter. However, PK-ARPD simulation predicted that 60 mg/kg/day is unlikely to suppress bacterial resistance with repeated dosing. Furthermore, PK-ARPD simulation suggested that amikacin at 90 mg/kg, given in two divided doses (45 mg/kg twice a day), is expected to hit safety and efficacy targets and is associated with a lower rate of bacterial resistance. The disagreement between the two methods is due to the inability of the probability approach to predict development of drug resistance with repeated dosing. This originates from the use of PK-PD indices based on the MIC that neglects measurement errors, ignores the time course dynamic nature of bacterial growth and killing, and incorrectly assumes the MIC to be constant during treatment.

A review of the pharmacokinetics and pharmacodynamics of aztreonam

The monobactam aztreonam is currently being re-examined as a therapeutic agent in light of the global spread of carbapenem resistance in aerobic Gram-negative bacilli and aztreonam's stability to Ambler class B metallo-β-lactamases. Of particular interest are the pharmacokinetic and pharmacodynamic properties of aztreonam alone and in combination with β-lactamase inhibitors. The choice of inhibitor may vary depending on the spectrum of β-lactamases produced by Enterobacteriaceae. The monobactam ring is also being used to produce new developmental monobactams. Thus, a greater understanding of aztreonam pharmacokinetics and dynamics is of great relevance in drug development. This review summarizes the pharmacokinetic profile of aztreonam in man and its pharmacodynamics in human and pre-clinical studies when studied alone and with β-lactamase inhibitors.

Dual beta-lactam therapy for serious Gram-negative infections: is it time to revisit?

We are rapidly approaching a crisis in antibiotic resistance, particularly among Gram-negative pathogens. This, coupled with the slow development of novel antimicrobial agents, underscores the exigency of redeploying existing antimicrobial agents in innovative ways. One therapeutic approach that was heavily studied in the 1980s but abandoned over time is dual beta-lactam therapy. This article reviews the evidence for combination beta-lactam therapy. Overall, in vitro, animal and clinical data are positive and suggest that beta-lactam combinations produce a synergistic effect against Gram-negative pathogens that rivals that of beta-lactam-aminoglycoside or beta-lactam-fluoroquinolone combination therapy. Although the precise mechanism of improved activity is not completely understood, it is likely attributable to an enhanced affinity to the diverse penicillin-binding proteins found among Gram negatives. The collective data indicate that dual beta-lactam therapy should be revisited for serious Gram-negative infections, especially in light of the near availability of potent beta-lactamase inhibitors, which neutralize the effect of problematic beta-lactamases.

Development and qualification of a pharmacodynamic model for the pronounced inoculum effect of ceftazidime against Pseudomonas aeruginosa

Evidence is mounting in support of the inoculum effect (i.e., slow killing at large initial inocula [CFUo]) for numerous antimicrobials against a variety of pathogens. Our objectives were to (i) determine the impact of the CFUo of Pseudomonas aeruginosa on ceftazidime activity and (ii) to develop and validate a pharmacokinetic/pharmacodynamic (PKPD) mathematical model accommodating a range of CFUo. Time-kill experiments using ceftazidime at seven concentrations up to 128 mg/liter (MIC, 2 mg/liter) were performed in duplicate against P. aeruginosa PAO1 at five CFUo from 10(5) to 10(9) CFU/ml. Samples were collected over 24 h and fit by candidate models in NONMEM VI and S-ADAPT 1.55 (all data were comodeled). External model qualification integrated data from eight previously published studies. Ceftazidime displayed approximately 3 to 4 log(10) CFU/ml net killing at 10(6.2) CFUo and concentrations of 4 mg/liter (or higher), less than 1.6 log(10) CFU/ml killing at 10(7.3) CFUo, and no killing at 10(8.0) CFUo for concentrations up to 128 mg/liter. The proposed mechanism-based model successfully described the inoculum effect and the concentration-independent lag time of killing. The mean generation time was 28.3 min. The effect of an autolysin was assumed to inhibit successful replication. Ceftazidime concentrations of 0.294 mg/liter stimulated the autolysin effect by 50%. The model was predictive in the internal cross-validation and had excellent in silico predictive performance for published studies of P. aeruginosa ATCC 27853 for various CFUo. The proposed PKPD model successfully described and predicted the pronounced inoculum effect of ceftazidime in vitro and integrated data from eight literature studies to support translation from time-kill experiments to in vitro infection models.

The role of pharmacodynamic research in the assessment and development of new antibacterial drugs

Antibacterial resistance continues to increase world wide, with some bacterial pathogens exhibiting resistance to virtually all available drugs. As the plague of antibacterial resistance continues to grow and create serious therapeutic problems, it is essential that the development of new antibacterial agents continue. Pharmacodynamic research plays an important role in the development of new antibacterial agents, as pharmacodynamic data can help define the clinical potential of a new drug and identify the strengths and weaknesses in comparison to other drugs already on the market. Furthermore, pharmacodynamic experiments can help focus the clinical phases of drug development by providing key information on the pharmacodynamic parameters that influence efficacy and the pharmacodynamic targets that should be achieved to optimize clinical success. Characterization of these pharmacodynamic properties for a new drug in development can help direct the design of the best dose and dosing strategy for clinical trials. This review will focus on the tools, methods, and strategies used to characterize the pharmacodynamics of antibacterial agents and aide in their development for clinical use.

Pharmacodynamics of cefepime alone and in combination with various antimicrobials against methicillin-resistant Staphylococcus aureus in an in vitro pharmacodynamic infection model

Treatment options for gram-positive resistant bacteria are limited; therefore, efforts to evaluate therapy options in the critical care population are warranted. Cefepime has broad-spectrum activity against gram-negative and gram-positive organisms. We have previously demonstrated that the combination of cefepime with vancomycin, linezolid, or quinupristin-dalfopristin had an improved or enhanced effect against methicillin-resistant Staphylococcus aureus (MRSA). We investigated various regimens of cefepime alone and in combination against two clinical MRSA isolates (R2481 and R2484) in an established in vitro pharmacodynamic model. Human pharmacokinetic regimen simulations were as follows: cefepime, 2 g every 8 h (q8h) (C8) and 12 h (C12), continuous-infusion 2-g loading dose followed by 4 g alone or in combination with gentamicin and tobramycin (1.0 or 2.0 [G1 and G2 or TB1 and TB2] mg/kg of body weight q12h and 5.0 [G5 or TB5] mg/kg q24h), arbekacin (ARB) (100 mg q12h), linezolid (LIN) (600 mg q12h), tigecycline (TIG) (100 mg q24h), or daptomycin (DAP) (6 mg/kg q24h) for 48 h. The MICs for cefepime, gentamicin, tobramycin, ARB, LIN, TIG, and DAP for the two clinical MRSA isolates (R2481 and R2484) were 4 and 4, 0.25 and 0.5, 128 and 0.5, 0.5 and 0.125, 2 and 4, 0.25 and 0.25, and 0.0625 and 0.125 microg/ml, respectively. At 48 h, combinations of C12 and C8 plus ARB, G1, or G5 (range, -2.05- to -4.32-log(10) decrease) demonstrated enhanced lethality against R2481 (resistant to tobramycin) (P < 0.05). A similar relationship was demonstrated against R2484 with cefepime plus ARB, gentamicin, or tobramycin (range, -2.05- to -3.63-log(10) decrease) (P < 0.05). A 99.9% kill was achieved with cefepime plus aminoglycoside combinations as early as 2 h and maintained throughout the 48-h period. TIG was antagonistic when combined with C12 against both isolates. DAP alone achieved 99.9% kill for up to 48 h for both isolates and was the most active agent against R2481 and R2484 (-2.89- and -3.61-log(10) decrease at 48 h); therefore, combination therapy did not enhance lethality. Overall, the most potent combinations noted were cefepime in combination with low- and high-dose aminoglycosides. Further investigations with combination therapies are warranted.

In vitro activities of antibiotic combinations against clincal isolates of Pseudomonas aeruginosa

Combination therapy has been recommended to treat Pseudomonas aeruginosa infections worldwide. The purpose of the present study was to determine the in vitro activities of piperacillin, cefepime, aztreonam, amikacin, and ciprofloxacin alone and in combination against 100 clinical isolates of P. aeruginosa from one medical center in southern Taiwan. The combination susceptibility assay was performed using the checkerboard technique. The percentage of resistance of P. aeruginosa to single agents in our study was relatively high for the Asia-Pacific area, except to aztreonam. Piperacillin plus amikacin exhibited the highest potential for synergy (59/100) in this study. Moreover, a high percentage of synergism was also noted with amikacin combined with cefepime (7/100) or aztreonam (16/100). The combination of two beta-lactams, such as cefepime with piperacillin, and aztreonam with cefepime or piperacillin, showed synergistic effects against some P. aeruginosa isolates. Although ciprofloxacin is a good anti-pseudomonal agent, a very low potential for synergy with other antibiotics was demonstrated in this study. No antagonism was exhibited by any combination in our study. Among piperacillin-resistant strains, there was synergy with a beta-lactam plus amikacin, including the combination of piperacillin and amikacin. However, the combination of two beta-lactams, such as piperacillin and cefepime or aztreonam, did not have any synergistic activity against these strains. In summary, the combinations of amikacin with the tested beta-lactams (piperacillin, aztreonam, cefepime) had a greater synergistic effect against P. aeruginosa, even piperacillin-resistant strains, than other combinations. Understanding the synergistic effect on clinical strains may help clinicians choose better empirical therapy in an area with high prevalence of multidrug-resistant P. aeruginosa.

In vivo efficacy of continuous infusion versus intermittent dosing of ceftazidime alone or in combination with amikacin relative to human kinetic profiles in a Pseudomonas aeruginosa rabbit endocarditis model

Ceftazidime and amikacin were administered in a Pseudomonas aeruginosa rabbit endocarditis model using computer-controlled intravenous (iv) infusion pumps to simulate human serum concentrations for the following regimens: continuous (constant rate) infusion of 4, 6 or 8 g of ceftazidime over 24 h or intermittent dosing of 2 g every 8 h either alone or in combination with amikacin (15 mg/kg once daily). The in vivo activities of these regimens were tested on four Pseudomonas aeruginosa strains. Animals were killed 24 h after the beginning of treatment. Efficacy was assessed by comparing the effects of the different groups on bacterial counts in vegetations for each strain tested. For a susceptible reference strain (ATCC 27853; MICs of ceftazidime and amikacin 1 and 2 mg/L, respectively), continuous infusion of 4 g alone or with amikacin was as effective as intermittent dosing with amikacin. For a clinical isolate producing an oxacillinase (MICs of ceftazidime and amikacin 8 and 32 mg/L, respectively), continuous infusion of 6 g was equivalent to intermittent dosing. For a clinical isolate producing a TEM-2 penicillinase (MIC of ceftazidime and amikacin 4 mg/L), continuous infusion of 6 g, but not intermittent dosing, had a significant in vivo effect. For a clinical isolate producing an inducible, chromosomally encoded cephalosporinase (MIC of ceftazidime and amikacin 8 and 4 mg/L, respectively), neither continuous infusion nor intermittent dosing proved effective. Determination of ceftazidime concentrations in vegetations showed that continuous infusion produced tissue concentrations at the infection site far greater than the MIC throughout the treatment. It is concluded that continuous infusion of the same total daily dose provides significant activity as compared with fractionated infusion. This study confirms that a concentration of 4-5 x MIC is a reasonable therapeutic target in most clinical settings of severe P. aeruginosa infection.

Pharmacokinetics and pharmacodynamics of cefepime administered by intermittent and continuous infusion

OBJECTIVE

This study assessed the pharmacokinetics and pharmacodynamics of cefepime administered by intermittent and continuous infusion against clinical isolates of Pseudomonas aeruginosa, Enterobacter cloacae, and Staphylococcus aureus.

BACKGROUND

Because beta-lactam antibiotics exhibit time-dependent bactericidal activity and lack prolonged postantibiotic effects against many bacteria, the goal of therapy is to maintain serum drug concentrations above the minimum inhibitory concentration (MIC) for the relevant pathogen over most of the dosing interval. Continuous infusion is a mode of drug administration that can provide serum drug concentrations continuously above the MIC for most bacterial pathogens.

METHODS

Twelve healthy volunteers were enrolled. Each received cefepime 2 g by intermittent bolus q12h and, on another day, was randomly assigned to receive 4 or 3 g administered by continuous infusion over 24 hours.

RESULTS

For the intermittent regimen, the mean (+/- SD) pharmacokinetic findings were: maximum serum concentration, 112.9 +/- 21.1 microg/mL; minimum serum concentration, 1.3 +/- 0.5 microg/mL; and half-life, 2.6 +/- 0.4 hours. For the 3- and 4-g continuous infusion regimens, steady-state serum concentrations (C(SS)) were 13.9 +/- 3.8 and 20.3 +/- 3.3 microg/mL, respectively. MICs ranged from 2 to 4, 0.125 to 8, and 2 to 8 microg/mL against P. aeruginosa, E. cloacae, and S. aureus, respectively. For the intermittent regimen, serum inhibitory titers (SITs) at 24 hours were > or = 1:2 in 46% of subjects against P. aeruginosa, 48% against E. cloacae, and 2% against S. aureus. For both continuous infusion regimens, SITs for each organism were > or = 1:2 in all subjects.

CONCLUSIONS

The intermittent regimen maintained serum concentrations above the MIC for P. aeruginosa and E. cloacae in > or = 92% (11/12) of subjects for > or = 70% of the dosing interval, provided the MIC was < or = 4 microg/mL. Both continuous infusion regimens provided a C(SS) above the MIC for all organisms. However, the C(SS) was > or = 4 times the MIC only if the MIC was < or = 2 microg/mL. Only the 4-g regimen provided such concentrations against isolates with an MIC of 4 microg/mL, and neither regimen provided such concentrations when the MIC was 8 microg/mL. These findings should be applied in comparative clinical studies.

Ventilator-associated pneumonia

Mechanically ventilated patients are at a substantially higher risk for developing nosocomial pneumonia. Overall, there is a relatively constant 1&!TN!150;3% risk per day of developing pneumonia while receiving mechanical ventilation. The sensitivity and specificity of clinical criteria alone for diagnosis of ventilator-associated pneumonias (VAP) is low. Several techniques have been developed to sample and quantitate the lower respiratory tract to improve the diagnostic yield. Gram-negative bacillary pneumonias account for the majority of the VAP. Strategies for prevention of VAP such as use of sucralfate for stress ulcer prophylaxis and selective decontamination of the digestive tract have been the focus of many clinical studies. Cost-effective preventive measures are needed to combat the increasing antimicrobial resistance, growing population of immunocompromised patients and increasing number of mechanically ventilated patients.

Cefepime-aztreonam: a unique double beta-lactam combination for Pseudomonas aeruginosa

An in vitro pharmacokinetic model was used to determine if aztreonam could enhance the pharmacodynamics of cefepime or ceftazidime against an isogenic panel of Pseudomonas aeruginosa 164, including wild-type (WT), partially derepressed (PD), and fully derepressed (FD) phenotypes. Logarithmic-phase cultures were exposed to peak concentrations achieved in serum with 1- or 2-g intravenous doses, elimination pharmacokinetics were simulated, and viable bacterial counts were measured over three 8-h dosing intervals. In studies with cefepime and cefepime-aztreonam against the PD strain, samples were also filter sterilized, assayed for active cefepime, and assayed for nitrocefin hydrolysis activity before and after overnight dialysis. Against WT strains, the cefepime-aztreonam combination was the most active regimen, but viable counts at 24 h were only 1 log below those in cefepime-treated cultures. Against PD and FD strains, the antibacterial activity of cefepime-aztreonam was significantly enhanced over that of each drug alone, with 3.5 logs of killing by 24 h. Hydrolysis and bioassay studies demonstrated that aztreonam was inhibiting the extracellular cephalosporinase that had accumulated and was thus protecting cefepime in the extracellular environment. In contrast to cefepime-aztreonam, the pharmacodynamics of ceftazidime-aztreonam were not enhanced over those of aztreonam alone. Further pharmacodynamic studies with five other P. aeruginosa strains producing increased levels of cephalosporinase demonstrated that the enhanced pharmacodynamics of cefepime-aztreonam were not unique to the isogenic panel. The results of these studies demonstrate that aztreonam can enhance the antibacterial activity of cefepime against derepressed mutants of P. aeruginosa producing increased levels of cephalosporinase. This positive interaction appears to be due in part to the ability of aztreonam to protect cefepime from extracellular cephalosporinase inactivation. Clinical evaluation of this combination is warranted.

Pharmacodynamics of ampicillin-sulbactam in an in vitro infection model against Escherichia coli strains with various levels of resistance

The activity of ampicillin-sulbactam against beta-lactamase-producing Escherichia coli has been questioned. Therefore, in this study, the killing activity of ampicillin-sulbactam was investigated in an in vitro infection model which simulates human pharmacokinetics. One ampicillin-sensitive strain (E. coli ATCC 25922, ampicillin-sulbactam MIC = 4/2 microg/ml) and three ampicillin-resistant TEM-1-producing strains with various levels of ampicillin-sulbactam resistance (EC11, MIC = 4/2 microg/ml; TIM2, MIC = 12/6 microg/ml; and GB85, MIC > 128/64 microg/ml) were studied. The E. coli strains were exposed to ampicillin-sulbactam at a starting inoculum of 6 to 7 log10 CFU/ml. Ampicillin-sulbactam was infused over 30 min to simulate doses of 3 and 1.5 g every 6 h for 24 h. The 3-g ampicillin-sulbactam dose was bactericidal against E. coli ATCC 25922, EC11, and TIM2. The 1.5-g dose displayed bactericidal activity against ATCC 25922 and EC11 similar to that of the higher dose but failed to kill TIM2 due to inadequate time above the MIC and increased MICs over 24 h. GB85 was highly resistant and grew similarly to controls. Despite an MIC at 10(7) CFU/ml indicating resistance (20/10 microg/ml), TIM2 was killed by the 3-g dose of ampicillin-sulbactam. Current MIC breakpoints may not adequately portray the activity of ampicillin-sulbactam, considering both the activity in in vitro infection models and clinical data.

Rationale behind high-dose amoxicillin therapy for acute otitis media due to penicillin-nonsusceptible pneumococci: support from in vitro pharmacodynamic studies

To evaluate whether increased doses of amoxicillin should be used to treat acute pneumococcal otitis media, an in vitro pharmacokinetic model was used to evaluate the killing of pneumococci by amoxicillin when middle ear pharmacokinetics were simulated. Logarithmic-phase cultures were exposed to peak concentrations of 3, 6, and 9 microg of amoxicillin per ml every 12 h, and an elimination half-life of 1.6 h was simulated. Changes in viable bacterial counts were measured over 36 h. All three doses rapidly decreased the viable bacterial counts of penicillin-susceptible strains below the 10-CFU/ml limit of detection by 6 to 10 h and maintained counts below this limit through 36 h. The 3-microg/ml peak dose was much less effective against two of three strains with intermediate penicillin resistance and all three penicillin-resistant strains, with bacterial counts approaching those in drug-free control cultures by 12 h. The 6-microg/ml peak dose completely eliminated two of three strains with intermediate penicillin resistance and maintained viable counts of the other nonsusceptible strains at 1.5 to 2 logs below the initial inoculum through 36 h. The 9-microg/ml peak dose was most effective, completely eliminating all three strains with intermediate penicillin resistance and maintaining the viable counts of the resistant strains at 3 to 4 logs below the original inoculum. The pharmacodynamics observed in this study suggest that peak concentrations of amoxicillin of 6 to 9 microg/ml may be sufficient for the elimination of penicillin-nonsusceptible pneumococcal strains causing otitis media, especially those with intermediate resistance to amoxicillin. In vivo pharmacokinetic studies are needed to determine if these levels can be achieved in middle ear fluid with amoxicillin at 70 to 90 mg/kg/day divided into two daily doses. If these levels are reliably achieved, then clinical studies are warranted.

Comparative efficacy of successive exposure of Pseudomonas aeruginosa to gentamicin and ceftazidime

Aminoglycoside and beta-lactam antibiotics, when used in combination, are usually given simultaneously, however, successive administration may be more efficient. The killing capacity was used to assess the effect of time intervals between low and high concentrations (2-8xMICs) of gentamicin and/or ceftazidime on Pseudomonas aeruginosa and to determine which drug is better to be administered first. The killing capacity after exposure to the antibiotic for 1 h were compared: (i) cells treated with gentamicin alone; (ii) cells treated with ceftazidime alone; and (iii) ceftazidime was added to (i) or (iv) gentamicin was added to (ii) at 0, 1 and 3 h of antibiotic removal. The bactericidal activity of gentamicin was potentiated and the viable cells decreased up to 6 h after antibiotic removal when the ceftazidime was added at O and at 1 h but the extent of bactericidal activity was reduced, when it was added at 3 h after gentamicin removal. Alternatively, treating the cells first with ceftazidime and then gentamicin was added after drug removal at O and at 1 h resulted in a marked decline in the viable cells, while addition of gentamicin after 3 h from ceftazidime removal, the extent of bactericidal activity was reduced. The non-treated cells with gentamicin started to grow heavily within 6 h of ceftazidime removal. No viable cells were detected after overnight incubation in cultures treated first with 6 or 8xMIC of gentamicin for 0.5 or 1 h. This in vitro study suggests that the optimum interval between gentamicin and ceftazidime doses, which gave the maximum bactericidal effect and the time before re-growth, appeared to be 1-2 h.

Influence of pH on the antimicrobial activity of clarithromycin and 14-hydroxyclarithromycin against Haemophilus influenzae using an in vitro pharmacodynamic model

Clarithromycin activity can be influenced by the pH of the surrounding environment. Evidence supports a reduced pH of middle ear fluid (MEF) and lung tissues in patients with otitis media and pneumonia, respectively. To evaluate the influence of pH on clarithromycin activity, an in vitro pharmacodynamic chamber model (PDCM) was used to generate bacterial time-kill curves for clarithromycin and a 2:1 ratio of clarithromycin and 14-hydroxyclarithromycin (HC) against Haemophilus influenzae at three different pH values: 7.2, 6.8, 6.4. Concentrations observed in MEF and lung tissues were simulated for clarithromycin alone and clarithromycin plus HC. Differences in activity at each pH were identified by comparing initial kill curve slopes and total log reduction. Experiments with amoxicillin-clavulanate were conducted as a reference. In simulated MEF regimens at pH 7.2, activity of clarithromycin alone improved by adding HC (additional 2 log10 reduction at 8 h); however, at pH values of 6.8 and 6.4, kill curves resembled growth controls. In simulated lung regimens, differences between clarithromycin alone and clarithromycin plus HC were insignificant; both produced a 2 log10 reduction at pH 7.2, and activity dramatically dropped to < 0.4 log10 as pH declined. In contrast, amoxicillin-clavulanate consistently produced a 3 log10 reduction over each pH value with more rapid initial kill relative to all clarithromycin regimens. These findings suggest the activity of clarithromycin against H. influenzae may be significantly compromised in respiratory tract infections involving a reduced pH. Trials with emphasis on clinical outcomes analysis will assist further in determining the significance of these experimental findings.

The pharmacokinetic profile of a new generation of parenteral cephalosporin

Cefepime, a fourth-generation cephalosporin with activity against both gram-negative and gram-positive organisms, has been investigated in healthy subjects and in special patient populations, including the elderly and those whose ability to eliminate drugs may be compromised. This review of seven pharmacokinetic studies indicates that the pharmacokinetic disposition of cefepime, administered either intravenously or intramuscularly, is similar to that of other cephalosporins with regard to dose linearity, renal excretion, and low serum protein binding. The elimination half-life of intravenous cefepime has been found to be approximately 2 hours, and peak serum concentrations approach 82 microg/mL for a 1-g dose and 164 microg/mL for a 2-g dose. Total body clearance is approximately 120 mL/min, independent of dose, and >80% of the drug is excreted unchanged by the kidneys. Dosage adjustment is warranted for patients with renal insufficiency but not hepatic dysfunction. The pharmacokinetic disposition of cefepime is altered in the elderly but not enough to necessitate additional dosage adjustment. Tissue penetration studies indicate similarity to other cephalosporins with low protein binding.

Pharmacodynamics of ceftazidime administered as continuous infusion or intermittent bolus alone and in combination with single daily-dose amikacin against Pseudomonas aeruginosa in an in vitro infection model

We compared the pharmacodynamics and killing activity of ceftazidime, administered by continuous infusion and intermittent bolus, against Pseudomonas aeruginosa ATCC 27853 and ceftazidime-resistant P. aeruginosa 27853CR with and without a single daily dose of amikacin in an in vitro infection model over a 48-h period. Resistance to ceftazidime was selected for by serial passage of P. aeruginosa onto agar containing increasing concentrations of ceftazidime. Human pharmacokinetics and dosages were simulated as follows: half-life, 2 h; intermittent-bolus ceftazidime, 2 g every 8 h (q8h) and q12h; continuous infusion, 2-g loading dose and maintenance infusions of 5, 10, and 20 micrograms/ml; amikacin, 15 mg/kg q24h. There was no significant difference in time to 99.9% killing between any of the monotherapy regimens or between any combination regimen against ceftazidime-susceptible P. aeruginosa. Continuous infusions of 10 and 20 micrograms/ml killed as effectively as an intermittent bolus of 2 g q12h and q8h, respectively. Continuous infusion of 20 micrograms/ml and an intermittent bolus of 2 g q8h were the only regimens which prevented organism regrowth at 48 h, while a continuous infusion of 5 micrograms/ml resulted in the most regrowth. All of the combination regimens exhibited a synergistic response, with rapid killing of ceftazidime-susceptible P. aeruginosa and no regrowth. Against ceftazidime-resistant P. aeruginosa, none of the ceftazidime monotherapy regimens achieved 99.9% killing. The combination regimens exhibited the same rapid killing of the resistant strain as occurred with the susceptible strain; however, regrowth occurred with all regimens. The combination regimens of continuous infusion of 20 micrograms/ml plus amikacin and intermittent bolus q8h or q12h plus amikacin continued to be synergistic. Overall, continuous infusion monotherapy with ceftazidime at concentrations 4 to 5 and 10 to 15 times the MIC was as effective as an intermittent bolus of 2 g q12h (10 to 15 times the MIC) and q8h (25 to 35 times the MIC), respectively, against ceftazidime-susceptible P. aeruginosa. Combination therapy with amikacin plus ceftazidime, either intermittently q8h or by continuous infusion of 20 micrograms/ml, appeared to be effective and exhibited synergism against ceftazidime-resistant P. aeruginosa.

Bactericidal killing activities of cefepime, ceftazidime, cefotaxime, and ceftriaxone against Staphylococcus aureus and beta-lactamase-producing strains of Enterobacter aerogenes and Klebsiella pneumoniae in an in vitro infection model

Cefepime (CP) is a new injectable cephalosporin with a broad spectrum of activity and stability against common chromosomally and plasmid-mediated beta-lactamases. The bactericidal activities of CP, ceftazidime (CZ), cefotaxime (CTX), and ceftriaxone (CAX) against reference and clinical strains of Staphylococcus aureus, an isogenic pair of Enterobacter aerogenes strains (wild type and a CZ-resistant derepressed mutant), and a Klebsiella pneumoniae isolate possessing a TEM-10 beta-lactamase were investigated in a two-compartment pharmacodynamic in vitro infection model which simulates human pharmacokinetics. An inoculum of approximately 10(6) CFU/ml was used in all model experiments. Antibiotics were administered to simulate the following regimens: CP at 2 g every 12 h (q12h), CZ at 2 g q8h, CTX at 2 g q8h, and CAX at 2 g q24h. Human albumin was added during experiments with CAX and staphylococci to simulate protein binding. Samples were removed at multiple time points over a 48-h period to determine the inoculum size for time-kill curves. Development of resistance was detected by inoculating samples obtained at 0, 24, and 48 h onto antibiotic-containing agar plates. The time to 99.9% killing was used to compare drug regimens. Against staphylococci, the time to bacterial eradication was significantly delayed with CAX-albumin. All regimens had similar activities against the wild-type Enterobacter strain; however, regrowth was noted with CZ, CTX, and CAX against the CZ-resistant strain. There were no differences between the CP, CTX, and CAX regimens against K. pneumoniae. Of interest, no regrowth of any organism was noted with CP. These data indicate that CP has activity against S.aureus and CZ-resistant gram-negative bacilli.

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.

Pharmacodynamics of levofloxacin, ofloxacin, and ciprofloxacin, alone and in combination with rifampin, against methicillin-susceptible and -resistant Staphylococcus aureus in an in vitro infection model

The pharmacodynamic properties of levofloxacin (an optically active isomer of ofloxacin), ofloxacin, and ciprofloxacin, alone and in combination with rifampin, were evaluated over 24 to 48 h against clinical isolates of methicillin-susceptible and -resistant Staphylococcus aureus (MSSA 1199 and MRSA 494, respectively) in an in vitro infection model. The incidence of the emergence of resistance among the test strains was also determined. The fluoroquinolones were administered to simulate dosage regimens of 200 mg, 400 mg given intravenously (i.v.) every 12 h (q12h), and 400 and 800 mg given i.v. q24h. Rifampin was dosed at 600 mg i.v. q24h. Although the MICs and MBCs of the quinolones were similar (< or = 0.49 microgram/ml), levofloxacin was the most potent agent in time-kill studies on the basis of the time required to achieve a 99.9% reduction in the number of log10 CFU per milliliter (e.g., with the regimen of levofloxacin [400 mg q24h, 6.5 h] versus ofloxacin [12.5 h], P < 0.024, and levofloxacin versus ciprofloxacin [6.5 versus 9.0 h], P < 0.0017) against MSSA 1199. The killing activity of levofloxacin was similar to that of ofloxacin against MRSA 494 (time to achieve a 99.9% reduction in the number of log10 CFU per milliliter, 11.1 versus 13.8 h, respectively). Levofloxacin and ofloxacin dosed once daily demonstrated greater bactericidal activity than when they were dosed twice daily against MSSA 1199. Resistance to levofloxacin or ofloxacin was not observed with any dosage regimen. Furthermore, resistance to ofloxacin was not detected when the half-life was reduced from 6 to 3 h. Regrowth and stable resistance (65-fold increase in the MIC for MSSA 1199; 16-fold increase in the MIC for MRSA 494) were noted within 24 h of exposure to ciprofloxacin at 200 mg q12h. Combination therapy with rifampin prevented the emergence of resistance to ciprofloxacin. Neither DNA gyrase alteration nor an energy-dependent efflux process mediated by the norA gene appeared to be responsible for the resistance observed. Our data suggest that with levofloxacin there is a more rapid onset of bactericidal activity than with ofloxacin or ciprofloxacin against MSSA 1199 and that the activity of levofloxacin is similar to that of ofloxacin but better than that of ciprofloxacin against MRSA 494. Resistance was noted only after exposure to the low dose of ciprofloxacin. Resistance to ofloxacin did not develop even when the pharmacokinetics of the drug were set to equal those of ciprofloxacin, suggesting that ofloxacin differs from ciprofloxacin irrespective of time of exposure. The resistance to ciprofloxacin that developed in our vitro model may be mediated by the cfx-ofx locus, which has been shown to be associated with low-level fluoroquinolone resistance. Overall, levofloxacin demonstrated potent bactericidal activity against S. aureus, without the emergence of resistance in our infection model. Quinolones dosed once daily were more effective than equivalent dosages administered twice daily. The addition of rifampin was not synergistic but prevented the emergence of ciprofloxacin resistance.

Bactericidal activities of teicoplanin, vancomycin, and gentamicin alone and in combination against Staphylococcus aureus in an in vitro pharmacodynamic model of endocarditis

We adapted an in vitro pharmacodynamic model of infection to incorporate simulated endocardial vegetations. The bactericidal activities of teicoplanin, vancomycin, gentamicin, and various combinations of these drugs were studied against a strain of methicillin-susceptible Staphylococcus aureus obtained from a patient being treated for endocarditis at Detroit Receiving Hospital. Bacteria were grown overnight, concentrated, and added to a mixture of cryoprecipitate (80%) and thrombin (10%) to achieve approximately 5 x 10(9) CFU/g. Fibrin clots (8 to 10) were suspended into the model, removed at 24, 48, and 72 h in duplicate, weighed, and homogenized in 1.25% trypsin. Control experiments were conducted to characterize the growth kinetics. The following antibiotics were administered to simulate the pharmacokinetics of the drugs in humans: teicoplanin at 3 and 15 mg/kg of body weight, vancomycin at 15 mg/kg, and gentamicin at 1 mg/kg. Fibrin clot samples used to detect resistance were plated on antibiotic-containing tryptic soy agar plates. For the teicoplanin and vancomycin regimens, protein binding to cryoprecipitate, thrombin, and fibrin clot was determined to be 32, 43, and 50% and 26, 28, and 29%, respectively. In comparison with no treatment, vancomycin or teicoplanin at 15 mg/kg or either of these regimens combined with gentamicin significantly reduced bacterial counts (P < 0.0001). Monotherapy with teicoplanin at 3 mg/kg or gentamicin resulted in no killing activity. Combination treatment with teicoplanin at 3 mg/kg and gentamicin resulted in the killing of approximately 2 log10 CFU/g by 72 h and the development of resistance to gentamicin. The results obtained with the in vitro model of endocarditis are similar to the results reported by several investigators with the rabbit model of infective endocarditis. This unique infection model is useful for designing initial drug dosage regimens and may be predictive of drug efficacy against infective endocarditis.

Effect of human serum on killing activity of vancomycin and teicoplanin against Staphylococcus aureus

STUDY OBJECTIVE

To investigate the effects of pooled human serum (PHS) on the killing activity of vancomycin and teicoplanin against two isolates of Staphylococcus aureus from patients treated for endocarditis.

DESIGN

An in vitro assessment of antibiotic susceptibility and killing rates.

SETTING

An urban university teaching hospital.

PATIENTS

Pooled human serum from patients treated for endocarditis.

INTERVENTIONS

Two clinical isolates of Staphylococcus aureus were obtained from patients treated for endocarditis. Media consisted of cation-supplemented Mueller-Hinton broth alone and in 1:1 dilutions with PHS, 2-hour heat-inactivated PHS (HI-PHS), ultrafiltrate (UF), and 2-hour heat-inactivated ultrafiltrate (HI-UF). Heat inactivation of PHS and UF was accomplished by treatment at 56 degrees C for 2 hours.

MEASUREMENTS AND MAIN RESULTS

Killing curves with vancomycin and teicoplanin were performed using drug concentrations of 45 micrograms/ml and a starting inoculum of approximately 1 x 10(6) colony-forming units (cfu)/ml. Bactericidal rates (-log cfu/ml/hr) were calculated from the slope of the killing curves over 0-12 hours (mean 3-8 replicates).

CONCLUSIONS

The killing activity of vancomycin in PHS and HI-PHS against both isolates was significantly greater than all other media tested (p < 0.0001). Ultrafiltrate tended to reverse this enhancement effect. Addition of PHS or UF did not enhance teicoplanin's killing activity against either isolate. Further investigations in our laboratory will determine if the factor is antibiotic class or organism specific.

Evaluation of activity of temafloxacin against Bacteroides fragilis by an in vitro pharmacodynamic system

An in vitro pharmacodynamic system has been successfully adapted to simulate in vivo antimicrobial pharmacokinetics under anaerobic conditions. This system was used to perform time-kill kinetic studies which were designed to compare the activity of temafloxacin to ciprofloxacin and cefotetan against two strains of Bacteroides fragilis (ATCC 25285 and ATCC 23745). All experiments were performed as single-dose, 24-h, duplicate runs. Starting bacterial inocula of 10(7) CFU/ml were exposed to starting antimicrobial concentrations of 5 micrograms of temafloxacin per ml, 5 micrograms of ciprofloxacin per ml, and 100 micrograms of cefotetan per ml. Terminal half-lives of 8, 4, and 4 h were simulated for each antimicrobial agent. Temafloxacin was rapidly bactericidal against B. fragilis. Ciprofloxacin was not bactericidal (< 3 log10 unit decline in bacterial numbers) to either strain of B. fragilis. Cefotetan was bactericidal (> or = 3 log10 unit decline in bacterial numbers) to each strain but killed at a slower rate than temafloxacin. Times to 3 log10 unit declines of strain ATCC 25285 were 2, 4, and > 24 h, whereas those of strain ATCC 23745 were 4, 4, and > 24 h for temafloxacin, cefotetan, and ciprofloxacin, respectively. Total logarithmic declines of strain ATCC 25285 were > 4.5, > 4.5, and 2.9 log10 CFU/ml, whereas those of strain ATCC 23745 were 4.1, > 4.5, and 1.2 log10 CFU/ml for each drug, respectively. These and other studies demonstrated that temafloxacin showed potential as an agent that could have been further developed for use in the treatment of anaerobic infections. However, the drug was removed from the market by its manufacturer because of toxicity issues. Although the release of newer fluoroquinolones that possess significant activity against anaerobic bacteria does not appear imminent, the time-kill studies performed in this study demonstrate that further research is warranted in the development of fluoroquinolones which possess significant antianaerobic activity.

Elimination of quinolone antibiotic carryover through use of antibiotic-removal beads

To prove the utility of antibiotic-removal beads in separating antibiotics from bacterial samples, Escherichia coli ATCC 25922 was exposed to five separate quinolones before and after each was exposed to antibiotic-removal beads. Plates treated with antibiotic solutions that were exposed to beads demonstrated antibiotic removal, and plates treated with antibiotic solutions that were not exposed to beads demonstrated antibiotic carryover. After exposure to beads, fluoroquinolone concentrations decreased from 5 micrograms/ml to 0.14 micrograms/ml (ciprofloxacin), 0.04 micrograms/ml (temafloxacin), < 0.01 microgram/ml (ofloxacin), < 0.01 microgram/ml (sparfloxacin), and 0.02 micrograms/ml (clinafloxacin). These data indicate that antibiotic carryover can be successfully circumvented through the use of antibiotic-removal beads.