Comparative antibiotic dose-effect relations at several dosing intervals in murine pneumonitis and thigh-infection models

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.

Preclinical and clinical evaluation of once-daily aminoglycoside chemotherapy

The rationale for and effectiveness of extended dosage intervals for aminoglycosides are reviewed. Aminoglycoside antibiotics have a prolonged postantibiotic effect against a variety of common Gram-negative and Gram-positive organisms: higher serum aminoglycoside levels are associated with a longer postantibiotic effect and increased bactericidal activity. Moreover once daily aminoglycoside administration may reduce the potential for adaptive postexposure resistance by allowing less contact time between bacteria and antibiotic. A longer dosage interval may decrease the risk of nephro- and ototoxicity. At least 33 published clinical trials suggest that once-daily administration of aminoglycosides and conventional regimens involving shorter dosage intervals are equally effective in patients with normal renal function and Gram-negative infections: besides, once-daily administration may reduce the frequency of aminoglycoside toxicity or delay it.

Determination of robust ocular pharmacokinetic parameters in serum and vitreous humor of albino rabbits following systemic administration of ciprofloxacin from sparse data sets by using IT2S, a population pharmacokinetic modeling program

Robust determination of the concentration-time profile of anti-infective agents in certain specialized compartments is often limited by the inability to obtain more than a single sample from such a site in any one subject. Vitreous humor and cerebrospinal fluid are obvious examples for which the determination of concentrations of anti-infective agents is limited. Advances in pharmacodynamics have pointed out the importance of understanding the profiles of drugs in the plasma and in specialized compartments in order to dose the drugs to obtain the best patient outcomes. Advances in population pharmacokinetic modeling hold the promise of allowing proper estimation of drug penetration into the vitreous (or other specialized compartment) with only a single vitreous sample, in conjunction with plasma sampling. We have developed a rabbit model which allows multiple samples of vitreous to be obtained without breaking down the blood-vitreous barrier. We have employed this model to test the hypothesis that robust estimates of vitreous penetration by the fluoroquinolone ciprofloxacin can be obtained from a traditional intensive plasma sampling set plus a single vitreous sample. We studied 33 rabbits which were receiving 40 mg of ciprofloxacin per kg of body weight intravenously as short infusions and from which multiple plasma and vitreous samples were obtained and assayed for ciprofloxacin content by high-performance liquid chromatography. Data were analyzed by the iterative two-stage population modeling technique (IT2S), employing the iterative two-stage program of Forrest et al. (Antimicrob. Agents Chemother. 37:1065-1072, 1993). Two data sets were analyzed: all plasma and vitreous samples versus all plasma samples and the initially obtained single vitreous sample. The pharmacokinetic parameter values identified were used to calculate the percent vitreous penetration as the ratio of the area under the concentration-time curve for the vitreous to that for the plasma. The values identified, 4% penetration for the full data set versus 3% penetration for the single vitreous sample data set, and their corresponding estimates were not statistically significantly different. We conclude that population modeling holds promise for the analysis of penetration of antimicrobiol agents into specialized spaces from which only single samples can be obtained, particularly for patients with whom robust plasma sampling can be performed.

Cephalothin and cefazolin in vitro antibacterial activity and pharmacokinetics in dogs

The periods of time that cephalothin and cefazolin serum concentration remained above minimum inhibitory concentration (MIC) for beta hemolytic, coagulase positive staphylococcal, and Escherichia coli clinical isolates were compared. Cephalothin and cefazolin were similarly very effective in vitro against staphylococcal isolates, with an MIC90 of 0.12 microgram/mL and 0.25 microgram/mL, respectively. In contrast, cefazolin was more effective than cephalothin against E coli isolates; the cefazolin MIC90 for E coli was 16 micrograms/mL and for cephalothin 64 micrograms/mL. Cefazolin (20 mg/kg intravenously [i.v.]) serum concentration remained more than MIC90 for E coli isolates significantly longer than serum concentration of cephalothin (40 mg/kg i.v.) (P < .001).

Antibiotic selection factors and description of a hospital-based outpatient antibiotic therapy program in the USA

A variety of pharmacodynamic, pharmacokinetic and drug stability factors can influence the choice of drug, the dosing regimen and the method of drug administration for out-patient parenteral antibiotic therapy (OPAT). Beta-lactam antibiotics exhibit little if any concentration-dependent killing and produce short-term or no persistent effects with most bacterial pathogens. Optimal dosing regimens for these agents should provide serum levels that continually exceed the minimal inhibitory concentration (MIC) of the pathogen. Beta-lactam agents with long half-lives (greater than 2 hours) can provide these levels with intermittent dosing once or twice daily. Beta-lactam agents with shorter half-lives can be administered by programmable pumps or by continuous infusion providing the drug is sufficiently stable to degradation in solution. Imipenem and ampicillin are examples of drugs with short half-lives that are unstable in solution and must be dosed intermittently. Intramuscular administration slows absorption and can also prolong the length of time during which serum levels exceed the MIC of infecting bacteria. Aminoglycosides and fluoroquinolones, on the other hand, exhibit concentration-dependent killing and produce prolonged persistent effects. Optimal dosage regimens of these drugs should maximize serum levels. Once-daily dosing regimens for the aminoglycosides meet this goal and also appear to reduce drug-induced nephrotoxicity. Application of these principles to drug selection and administration in a hospital-based OPAT program has provided efficacious therapy and a low incidence of adverse reactions in an elderly population distributed over a wide geographic area.

A multicentre, randomized comparative study of 500 mg versus 1,000 mg ceftazidime t.d.s. for treatment on gram-negative infections

A multicentre, randomized study was performed to compare the clinical and bacteriological efficacy of 500 mg ceftazidime i.v. t.d.s. with 1,000 mg ceftazidime i.v. t.d.s. for treatment of hospitalised, non-compromised patients with gram-negative infections. The study was conducted in ten hospitals in The Netherlands. Hospitalised patients with a suspected gram-negative lower respiratory tract infection, complicated urinary tract infection or septicaemia were included. Excluded were patients with neutropenia, limited life expectancy, or severe renal insufficiency as well as those on antibiotics in the 48 h prior to entry. Ceftazidime was administered via an intravenous infusion every 8 h. For patients with moderately impaired renal function the frequency was reduced to 12 h. Treatment was continued for as long as clinically indicated. Clinical response (cure, improvement or failure) and bacteriological response (elimination, persistence or non-evaluable) were assessed primarily by the investigator. Final assessments were made by a panel of experts without prior knowledge. In total 127 patients were randomized, 64 patients to the 500 mg group and 63 to the 1,000 mg group; 47 patients were excluded from evaluation, usually due to an incorrect diagnosis prior to randomization. Ultimately 37 patients of the 500 mg group and 43 patients of the 1,000 mg group were available for evaluation. Between these two groups of evaluable patients there were no significant differences in baseline characteristics, types of infection, isolated bacterial pathogens or treatment characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

Experience of once-daily aminoglycoside dosing using a target area under the concentration-time curve

BACKGROUND

Many centres are changing to once-daily aminoglycoside administration. However, proposed methods for this practice often have theoretical and practical difficulties. We have developed a method in which a target area under the concentration-time curve (AUC) is used instead of traditional peak and trough serum concentrations.

AIMS

To analyse our experience with the target AUC method in the first 100 courses of once-daily aminoglycoside administration in the Christchurch, New Zealand hospitals.

METHODS

Following a starting dose of 5-7 mg/kg, administered by 30-minute infusion, the AUC was calculated using two serum aminoglycoside concentrations taken at one and six-14 hours after the start of the infusion. Dose adjustment was made to correct for any difference between the calculated AUC and a target AUC (72-101 mg.1(-1).h). The method was assessed for practicality and precision in 100 courses of treatment. The incidence of aminoglycoside toxicity was documented.

RESULTS

The mean final dose of 6.68 mg/kg, and AUC of 92.8 mg.1(-1).h, were significantly different from the mean starting dose and AUC of 5.67 mg/kg and 74.0 mg.1(-1).h, respectively. The method appeared to be more precise than empirical dosing at achieving the target AUC even though the final recommended dose had more variability than the starting dose. Although the study was uncontrolled, observed nephrotoxicity (2%) and ototoxicity (up to 6.9%) were no greater than expected from the results of other studies. There were no deaths related to antibiotic failure.

CONCLUSIONS

The AUC method was practical, and more appropriate for once-daily dosing than the conventional method of aiming for target peak and trough concentrations. Dose adjustment can be made before the next dose.

Experimental Streptococcus pneumoniae infection in mice for studying correlation of in vitro and in vivo activities of penicillin against pneumococci with various susceptibilities to penicillin

The purpose of the study was to investigate the correlation of in vitro activity with the in vivo effect and the pharmacokinetics of penicillin in the treatment of infections with pneumococci with various susceptibilities to penicillin. We used 10 pneumococcal strains for which penicillin MICs ranged from 0.016 to 8 micrograms/ml. Time-kill curve experiments were performed with all strains. We found that the effect of penicillin in vitro is concentration independent, with a maximum effect at two to four times the MIC for penicillin-susceptible as well as penicillin-resistant pneumococci. The mouse peritonitis model with an inoculum of approximately 10(6) CFU, to which mucin was added, resulted in a reproducible lethal infection with the pneumococci. The 50% effective dose was determined for each strain, and we found a highly significant correlation between the log MIC and the log 50% effective dose of penicillin against these strains (P < 0.01). Furthermore, it was shown that the most important pharmacokinetic parameter determining the effect of penicillin in vivo was the time that the concentration of penicillin in serum was greater than the MIC.

Interrelationship between pharmacokinetics and pharmacodynamics in determining dosage regimens for broad-spectrum cephalosporins

The broad-spectrum cephalosporins exhibit time-dependent bactericidal activity and produce prolonged postantibiotic effects only with staphylococci. The duration of time that serum levels exceed the minimum inhibitory concentration (MIC) is the important pharmacodynamic parameter correlating with efficacy for these drugs. Maximal efficacy for cephalosporins in several animal infection models is approached when serum levels are above the MIC for 60%-70% of the dosing interval for Enterobacteriaceae and streptococci and for 40%-50% of the dosing interval for Staphylococcus aureus. Based on MIC90 values of 0.5 microgram/ml for enteric bacilli and 4 micrograms/ml for S. aureus, these time above MIC goals can be easily met in infected and/or elderly patients following 1-2 g of cefotaxime at 12-h intervals. Full knowledge of the interrelationships between pharmacokinetics and pharmacodynamics is important for determining effective dosage regimens for the broad-spectrum cephalosporins.

Pharmacodynamic (kinetic) considerations in the treatment of moderately severe infections with cefotaxime

Information about the pharmacodynamics of beta-lactams has accumulated rapidly over the last 20 years, and their application to cefotaxime are discussed in this review. Application of pharmacodynamics requires an integration of the pharmacokinetic and in vitro properties of the agent. Cefotaxime is similar to other beta-lactams in that it has little concentration-dependent killing and produces no postantibiotic effect against Gram-negative bacteria. However, it has a microbiologically active metabolite, deascetylcefotaxime, which can show synergy, partial synergy, or an additive effect in combination with the parent drug. More than any other technique, animal models have been able to elucidate the pharmacokinetic parameters that predict efficacy in vivo. They have shown that for beta-lactams it is the time that levels exceed the minimum inhibitory concentration (MIC) that is the most important determinant of efficacy. For bacteria to have no postantibiotic effect, plasma levels need to exceed the MIC for the whole of the dosing interval to achieve maximum killing at the site of infection. When applying these concepts as the most stringent criteria for efficacy using pharmacokinetic values from young, healthy volunteers, it can be shown that organisms with MICs of < or = 0.03 microgram/ml for a 1-g dose and 0.06 microgram/ml for a 2-g dose to achieve optimum efficacy with 12-h dosing of cefotaxime. However, two clinical studies have demonstrated trough levels much greater than would be predicted from these pharmacokinetic values, as a result of the effects of decreased renal function accompanying sepsis and older age. These studies showed that organisms with MICs < or = 1 microgram/ml for a 1-g dose or 2 micrograms/ml for a 2-g 12-h dose were covered for the whole of the dosing interval. Thus, all strains of Enterobacteriaceae and pathogenic Neisseria spp. that lack resistance mechanisms to third-generation cephalosporins would be covered using 12-h dosing schedules.

Beta-lactam antibiotics: is continuous infusion the preferred method of administration?

OBJECTIVE

To examine the pharmacodynamic properties of the beta-lactam class of antibiotics and the rationale for their continuous infusion (CI), and to explore reasons that this mode of administration has not replaced intermittent infusion as the standard of practice.

DATA SOURCES

A Medline search of the English-language literature evaluating CI administration of beta-lactam antibiotics was conducted. Bibliographic searches of these articles also were performed.

STUDY SELECTION

Because there were few human trials, all available trials were considered for review. A cross section of clinical trials, animal studies, and in vitro studies examining the impact of the route of antibiotic administration was selected for each pharmacodynamic variable addressed.

DATA SYNTHESIS

The support for CI as the preferred method of beta-lactam administration comes primarily from in vitro and animal data. Most beta-lactam antibiotics do not demonstrate concentration-dependent killing and have an appreciable postantibiotic effect only against gram-positive cocci. Their efficacy appears to be optimized by maintaining suprainhibitory concentrations throughout the dosing interval. Therefore, CI of beta-lactams could potentially enhance the efficacy of treatment or allow less drug to be used on a daily basis. This has yet to be demonstrated convincingly in human clinical trials. Comparative trials need to continue to explore the impact of the method of administration on patient outcomes such as duration and cost of therapy, as well as morbidity and mortality.

CONCLUSIONS

Results of many animal and in vitro studies suggest that CI may be the optimal method of beta-lactam administration. Clinical trials need to further document the impact of the method of beta-lactam administration on the incidence of adverse effects, emergence of bacterial resistance, and patient outcome. Pharmacodynamic studies defining target beta-lactam concentrations, the practicality of CI in patients requiring multiple intravenous fluids and medications, and the pertinence of this issue when beta-lactam antibiotics are used as sole agents or in combination with other antimicrobials require further exploration.

The disposition of gentamicin in equine plasma, synovial fluid and lymph

Plasma (P), synovial fluid (SF) and lymph (L) concentrations of gentamicin were studied in two trials. A lymph vessel in the hindlimb was cannulated. The day after surgery (trial A), P and L samples were collected for 12 h after intravenous injection of gentamicin sulphate at 2.2 mg/kg dose rate. Approximately 48 h after surgery (trial B), the fetlock joint of the cannulated hindlimb was catheterized and P, SF and L samples collected for 12 h after a similar intravenous injection. The kinetic parameters were similar to those in other reports and did not differ between trials (P < 0.05). The P, L and SF disposition profiles were similar. The 95% confidence interval for P & L concentrations overlapped 2-3 h after injection. Thereafter, parallelism between L and P concentrations was observed, but L concentrations were on average 60% higher than P concentrations, and elimination from L was slower than from P. The mean L/SF and P/SF ratios were 1.54 +/- 0.2 and 1.25 +/- 0.2, 2-4 h after injection. Gentamicin elimination from SF appeared to be slower than from L and P. Lymph cannulation is a viable technique for antibiotic disposition studies. A sample of any of the fluids 3 h after injection was representative of the others. While SF concentrations were of limited value for predicting tissue fluid (L) concentrations 3-8 after injection, P concentrations were a useful index.

The importance of pharmacokinetic/pharmacodynamic surrogate markers to outcome. Focus on antibacterial agents

Pharmacokinetic/pharmacodynamic surrogate relationships have been used to describe the antibacterial activity of various classes of antimicrobial agents. Studies that have evaluated these relationships were reviewed to determine which of these surrogate markers were further dependent on antimicrobial class. The fluoroquinolone and aminoglycoside agents exhibit concentration-dependent killing. Studies have demonstrated that peak serum concentration: minimum inhibitory concentration (MIC) and area under the serum concentration-time curve (AUC): MIC ratios are important predictors of outcome for these antimicrobial agents. Area under the inhibitory concentration-time curve (AUIC24) [i.e. AUC24/MIC] is a useful parameter for describing efficacy for these agents, while an adequate peak concentration: MIC ratio seems necessary to prevent selection of resistant organisms. For beta-lactam antibiotics, the duration of time that the serum concentration exceeds the MIC (T > MIC) was the significant pharmacokinetic/pharmacodynamic surrogate in cases where the bacterial inoculum was low, or where very sensitive organisms were tested. However, in studies using more resistant organisms or larger inoculum sizes there is some concentration-dependence to the observed effect. Studies using reasonable dosage intervals have demonstrated covariance between T > MIC and AUC/MIC ratio for beta-lactam antibiotics. Since glycopeptide antibiotics display relatively slow but concentration-independent killing, and are cell wall active agents similar to beta-lactams, it has been presumed that T > MIC is the important pharmacokinetic surrogate related to efficacy for these agents. Some studies have shown that a concentration multiple of the MIC may be necessary for successful outcome with vancomycin. AUIC24 may prove to be an important pharmacokinetic surrogate if both time and concentration are indeed important parameters. To select an appropriate antimicrobial agent, the clinician must consider many patient-specific as well as organism-specific factors. Utilisation of known pharmacokinetic/pharmacodynamic surrogate relationships should help to optimise treatment outcome.

Postantibiotic effects and postantibiotic sub-MIC effects of roxithromycin, clarithromycin, and azithromycin on respiratory tract pathogens

Pharmacodynamic parameters have become increasingly important for the determination of the optimal dosing schedules of antibiotics. In this study, the postantibiotic effects (PAEs), the postantibiotic sub-MIC effects (PA SMEs), and the sub-MIC effects (SMEs) of roxithromycin, clarithromycin, and azithromycin on reference strains of Streptococcus pyogenes group A, Streptococcus pneumoniae, and Haemophilus influenzae were investigated. The PAE was induced by 2x MICs (S. pneumoniae) or 10x MICs of the different drugs for 2 h, and the antibiotics were eliminated by washing and dilution. The PA SMEs were studied by addition of 0.1, 0.2, and 0.3x MICs during the postantibiotic phase of the bacteria, and the SMEs were studied by exposition of the bacteria to the drugs at the sub-MICs only. Growth curves were followed by viable counts for 24 h. The SMEs were generally very short. A PAE of 2.9 to 8 h was noted for all antibiotics against all strains. Clarithromycin induced a statistically significantly shorter PAE on S. pneumoniae than did roxithromycin and azithromycin and did so also against H. influenzae in comparison with azithromycin. The PA SMEs were long and varied at 0.3x MIC between 6.4 19.6 h. This pronounced suppression of regrowth of bacteria which are first treated with a suprainhibitory concentration of antibiotics and then reexposed to sub-MIC levels indicates that long dosing intervals for macrolides and azalides can be allowed.

Pharmacokinetic contributions to postantibiotic effects. Focus on aminoglycosides

The postantibiotic effect (PAE) refers to a period of time after complete removal of an antimicrobial during which there is no growth of the target organism. The PAE appears to be a feature of most antimicrobial agents and has been documented with a variety of common bacterial pathogens. Various factors influence the presence or duration of the PAE including the type of organism, type of antimicrobial, concentration of antimicrobial, duration of antimicrobial exposure, antimicrobial combinations, and inoculum and medium used. beta-Lactams demonstrate a PAE against Gram-positive cocci, but produce only a short PAE with Gram-negative bacilli. Antimicrobial agents that inhibit RNA or protein synthesis have a PAE against Gram-positive cocci and Gram-negative bacilli. In vivo studies of aminoglycosides suggest that area under the plasma concentration-time curve is the pharmacokinetic parameter that best correlates with clinical efficacy. This is thought to be due to the concentration-dependent killing and PAE possessed by these antimicrobials. Animal and human studies have reported that once-daily administration of aminoglycoside is as effective as, or more effective than, and possibly less toxic than traditional multiple daily administration.

In vivo efficacy of a broad-spectrum cephalosporin, ceftriaxone, against penicillin-susceptible and -resistant strains of Streptococcus pneumoniae in a mouse pneumonia model

The increasing emergence of penicillin-resistant (Pr) strains of Streptococcus pneumoniae could pose a therapeutic problem in the next few years. Ceftriaxone (CRO), a broad-spectrum cephalosporin, exhibits a smaller increase in MICs against Pr S. pneumoniae strains than amoxicillin (AMO) (usually referred as to the "gold standard" therapy for pneumococcal infections). Therefore, we compared their respective efficacies in a leukopenic Swiss mouse model of pneumococcal pneumonia. Infection was induced with two serotype 19 strains: a penicillin-susceptible (Ps) strain (MICs of < 0.01 for penicillin, 0.03 for AMO, and 0.03 for CRO) and a Pr strain (MICs of 4 for penicillin, 2 for AMO, and 0.5 for CRO). Untreated mice died within 2 or 3 days. Against the Ps strain, the minimal protective dose (two subcutaneous injections at 12-h intervals for 3 days) for both CRO and AMO was 5 mg/kg of body weight (87% survivors). Ten-fold-increased doses of CRO (50 mg/kg) gave similar protection (75% survivors) against the Pr strain, whereas 20- and 40-fold-increased doses of AMO protected 0 and 34% of the animals, respectively, against the Ps strain. CRO had a marked and prolonged antibacterial effect in the lungs (2.7-log-unit reduction of CFU in 24 h after a single 50-mg/kg injection) against the Pr strain in comparison with AMO. A standard dosage of 50 mg of CRO per kg in mice resulted in peak levels in serum and protein binding comparable to those observed with 1 g given intravenously in humans. This dosage remained effective against a highly Pr S. pneumoniae strain in this model. The microbiological activity and pharmacodynamic and pharmacokinetic properties of CRO (time during which concentrations exceed the MIC for the test pathogen [delta t MIC], > or less than 8 h; and peak/MIC ratio, >90 for free active drug) accounted for its efficacy relative to AMO (50 mg/kg: delta t MIC, <2; peak/MIC ratio, <25) against the highly Pr S. pneumoniae strain used in this study.

Animal models of endocarditis

Bacterial endocarditis is a difficult infection to cure, due to poor penetration of antibiotics into infected vegetations, altered metabolic state of bacteria within the lesion, and absence of adequate host-defense cellular response which could cooperate with antibiotic action. The contribution of animal models to a better understanding of the pathophysiology of the infection and to definition and improvement of therapeutic regimens of endocarditis in humans remains of great importance due to the difficulties encountered in clinical trials. The advantage of the experimental model is that besides the fact that it closely simulates the characteristics of the infection in humans, it provides clear endpoints which allow statistical comparisons among different therapeutic regimens: number of bacteria per gram of tissue, frequency of emergence of resistance, positivity of blood cultures, death vs. survival rates, and percentage of relapses after treatment has been stopped. All these parameters are more sensitive and more easy to study than in humans. The animal model has definitively established that bactericidal therapy is warranted and that in vitro susceptibility tests, especially those evaluating the killing rate, have a good predictive value on the therapeutic outcome. Two main aspects are discussed for their relevance to human therapy: (i) the kinetics of antibiotic diffusion into vegetations, with special reference to data obtained with autoradiography; and (ii) the specificity of some pharmacodynamic aspects of antibiotics in endocarditis, including the clinical consequences of these two parameters with respect to antibiotic dosing regimens and length of therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Optimal times above MICs of ceftibuten and cefaclor in experimental intra-abdominal infections

The duration of time that serum drug levels remain above the MIC (time above the MIC) for the pathogen has been shown to be the most significant parameter determining the efficacies of beta-lactam antibiotics. In the described study, we investigated the optimal time above the MIC of ceftibuten and cefaclor using a nonneutropenic mouse model of intra-abdominal infections caused by Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Streptococcus pneumoniae. The abilities of the drugs to protect mice against the organisms were determined in mouse protection tests, and the doses were fractionated to produce various dosing regimens with different times above the MIC. All drug-organism combinations showed a significant correlation (r > 0.9) between drug efficacy and the time above the MIC. Also, with ceftibuten treatment, the different dosing regimens that produced equal times above the MIC resulted in the same efficacy, whereas with cefaclor, an apparent dose-dependent effect was observed. These results showed that for a 100% recovery from K. pneumoniae and E. coli infections, the optimal times above the MIC with ceftibuten treatment were 2.2 and 1.6 h, respectively. Relatively high doses of both antibiotics were required to ensure recovery from S. pneumoniae infections. In vitro time-kill studies demonstrated that cefaclor exhibits a marked inoculum effect against the pathogens, and there was a concentration-dependent killing at a large inoculum size. On the other hand, ceftibuten showed no inoculum effect. It is suggested that optimization of both dose and time above the MIC appears to be necessary for the treatment of S. aureus infections with cefaclor, and this may apply to other beta-lactams tht exhibit marked inoculum effects.

In vivo bactericidal activities of ciprofloxacin and pefloxacin in an experimental model of Serratia marcescens endocarditis

The critical concentrations of pefloxacin and ciprofloxacin in serum, corresponding to the lowest concentration in serum able to achieve a 2-log-unit reduction in the CFU in vegetations after a 24-h exposure at a steady-state concentration obtained by a continuous intravenous infusion, were determined in an experimental model of Serratia marcescens endocarditis in rabbits. In vitro data showed that the MICs of ciprofloxacin and pefloxacin were 0.06 and 0.25 mg/liter, respectively. The killing curves indicated a maximum killing rate at a concentration four times that of the MICs. In vivo, the critical concentrations of pefloxacin and ciprofloxacin in serum were 0.4 and 0.24 mg/liter, respectively, corresponding to a concentration of four times the MICs.

Clinical pharmacokinetics of cefotetan

Cefotetan is a 7-alpha-methoxy beta-lactam. A long serum half-life and resistance to beta-lactamase hydrolysis have made cefotetan an attractive chemotherapeutic agent, and the results of clinical trials worldwide have demonstrated its efficacy in a wide variety of clinical situations. Cefotetan can be administered intravenously (bolus or infusion) or intramuscularly with lidocaine (lignocaine) 0.5%. Mean peak plasma concentrations are almost linearly related to dose. The volume of distribution is between 8 and 13L and is not different from other cephalosporins. No accumulation is seen after repeated doses and no metabolite has been detected in either plasma or urine. Total body clearance is 1.8 to 2.9 L/h. Renal clearance accounts for about 64 to 84% of a dose, and 75% of a dose is excreted in the urine within 24 hours. The plasma elimination half-life is between 3 and 4 hours after intravenous and intramuscular doses. Half-life is considerably prolonged in patients with renal impairment (up to 10 hours). Cefotetan concentrations are likely to be active against susceptible bacteria in most tissues and body fluids. Breast milk and cerebrospinal fluid concentrations are low. The recommended dosage is 1g every 12 hours, increasing to 2g in severe infections and 3g in life-threatening infections. In surgical prophylaxis, a single dose of 2g is given with the induction of anaesthesia; an additional dose of 2g may be administered 12 hours later. In children over 6 months, the recommended dosage is 30 mg/kg given 12-hourly. In patients with a creatinine clearance of 10 to 40 ml/min (0.6 to 2.4 L/h), the dose is halved or the dosage interval is doubled. When creatinine clearance is less than 10 ml/min (0.6 L/h), the dose is quartered or the dosage interval quadrupled.

Correlation of tobramycin-induced inhibition of protein synthesis with postantibiotic effect in Escherichia coli

Scant data exist on intracellular events during aminoglycoside-induced postantibiotic effect (PAE). We examined DNA, RNA, and protein syntheses after tobramycin exposure using [3H]thymidine, [14C]uracil, and [14C]alanine incorporation in a clinical Escherichia coli strain. Late-log-phase bacteria in oxygenated minimal salts medium at 37 degrees C were exposed to tobramycin (7.5 micrograms/ml) (twice the MIC) for 30 min. Tobramycin caused a kill of 2 log10 CFU/ml prior to drug removal by filtration and a 5-h PAE, measured by viable counts. Excess amounts of labelled precursors were added to tobramycin-exposed organisms during, immediately after, and at various intervals following exposure. In the presence of tobramycin, DNA, RNA, and protein syntheses were sequentially inhibited within 1 generation time. Following drug removal, both DNA and RNA syntheses promptly resumed, suggesting readily dissociable nonspecific binding to DNA and RNA. However, total protein synthesis did not resume until 4 h later. beta-Galactosidase activity, a measure of functional enzymatic protein synthesis, was also inhibited for 4 h after drug removal. Bacterium length, measured by confocal microscopy, increased during PAE. Two distinct populations eventually emerged: one that returned to control dimensions and one that remained excessively elongated by the end of PAE (2.5 microns versus 4.0 microns; P < 0.05). We hypothesize that only viable cells return to the control morphology. Flow cytometry showed enhanced DNA complexity during PAE, consistent with either impaired cellular protein synthesis in viable cells or perturbations in dying cells. In summary, duration of PAE correlated with inhibition of total and functional protein synthesis but not DNA or RNA synthesis.

Comparative efficacies of ofloxacin, cefotaxime, and doxycycline for treatment of experimental epididymitis due to Escherichia coli in rats

The in vivo efficacy of ofloxacin was compared with those of cefotaxime and doxycycline in a rat model of epididymitis due to Escherichia coli. Treatment was started 24 h after infection and was continued for 7 days. Ofloxacin reduced the numbers of E. coli organisms in the epididymides significantly more than the other therapeutic regimens and cured the infection more frequently. Histopathological changes in the epididymides of ofloxacin-treated animals were significantly less severe than those observed in untreated animals. Doxycycline was less effective than ofloxacin but significantly reduced the titers of organisms in rat epididymides. In contrast, despite excellent in vitro activity, cefotaxime failed to reduce the magnitude of infection. The results of this study suggest that ofloxacin may be a very effective antimicrobial agent for the treatment of epididymitis due to E. coli.

Qualitative susceptibility tests versus quantitative MIC tests

Qualitative susceptibility categories show reasonable, but incomplete, correlation with therapeutic outcome. Studies using quantitative MIC tests have demonstrated that treatment failures within the susceptible category are associated with higher minimum inhibitory concentrations (MICs) than with therapeutic successes. Other trials have exhibited enhanced response for increasing ratios of a pharmacokinetic parameter to MIC (for example, peak level to MIC ratio for aminoglycosides). Dose-response studies in animal infection models also demonstrate an excellent correlation between the dose of drug required for a given response and the infecting organism MIC. These studies suggest that the use of quantitative MIC tests may enable more individualization of the therapeutic regimen, especially in regards to dose and dosing frequency, than provided by qualitative category susceptibility tests. However, there are only rare studies that have used MIC results or pharmacokinetic parameters to improve efficacy. Furthermore, these studies have not consistently documented enhanced clinical efficacy. MICs can also be used to reduce drug dosage and cost of antimicrobial therapy for very susceptible organisms. Additional studies are clearly needed to define the full potential of the quantitative MIC test result.

Continuous infusion of cefazolin is superior to intermittent dosing in decreasing infection after hemorrhagic shock

Standard doses of antibiotic administered by intermittent infusions after hemorrhagic shock have decreased efficacy in combating infection. This study compared identical quantities of cefazolin administered after shock as intermittent doses or as continuous infusions in a subcutaneous abscess model. One hour after resuscitation from shock, rats were inoculated with 2 x 10(8) Staphylococcus aureus subcutaneously on the dorsum and divided into three groups: (1) control rats, which received no drug treatment; (2) rats in the intermittent group, which received cefazolin at either 30 or 60 mg/kg intraperitoneally, 30 minutes prior to inoculation, then every 8 hours for three doses, and (3) rats in the continuous infusion group, which received cefazolin at either 30 or 60 mg/kg intraperitoneally, 30 minutes prior to inoculation, followed by cefazolin, 90 or 180 mg/kg, intraperitoneally by continuous infusion more than 24 hours after inoculation. Seven days after the inoculation, abscess number, diameter, and weight were measured. Rats that received either dosage of cefazolin intermittently had the same abscess rate after shock as control rats. Rats that received a continuous infusion of cefazolin at either dose had 56% fewer abscesses than control rats. Abscess diameter and weight decreased with increasing quantities of cefazolin, and abscesses were always smaller in rats receiving the continuous infusion. There were no differences in peak subcutaneous cefazolin levels between the intermittent and continuous groups. Continuous infusion provided significantly more cefazolin to the tissue than an equivalent quantity of cefazolin delivered as intermittent doses. These data demonstrate that continuous infusion of cefazolin provided more antibiotic to the tissue and was superior to intermittent injection in reducing infection after hemorrhagic shock.

Pharmacodynamics of antimicrobial agents as a basis for determining dosage regimens

Pharmacodynamic parameters, such as the rate of bactericidal activity with increasing drug concentrations, post-antibiotic effect, sub-MIC effects, post-antibiotic leukocyte enhancement and first-exposure effect, more accurately describe the time course of antimicrobial activity than the MIC and MBC. Aminoglycosides and quinolones exhibit concentration-dependent killing and induce prolonged post-antibiotic effects. The amount of drug rather than the dosing frequency determines the efficacy of these drugs. However, high peak levels can reduce the emergence of resistance, and once-daily dosing of aminoglycosides can also reduce nephrotoxicity and ototoxicity. On the other hand, beta-lactam antibiotics show time-dependent killing and produce prolonged post-antibiotic effects only with staphylococci. The frequency of drug administration is an important determinant of outcome for these drugs, as the duration of time serum levels exceed the MIC is the major determinant of efficacy.

Identification of factors affecting in vivo aminoglycoside activity in an experimental model of gram-negative endocarditis

Aminoglycoside bactericidal activity during the first 24 h of treatment probably is a determining parameter in the prognosis of severe gram-negative infections in immunocompromised patients. To identify the predictive factors involved in the definition of the best therapeutic regimen for Enterobacter cloacae and Serratia marcescens infections, we studied different gentamicin, tobramycin, and amikacin regimens by using an experimental model of rabbit endocarditis. Two factors appear to play an important role in predicting in vivo efficacy: (i) the level of in vivo bactericidal activity, which can differ widely from one aminoglycoside to another for the same bacterial strain and from one strain to another of the same species, and (ii) the critical serum drug concentration (CSC, in milligrams per liter), defined as the lowest serum antibiotic concentration capable of producing a significant CFU reduction (P less than 0.05) in endocarditis vegetations 24 h after the beginning of a continuous infusion. Stepwise regression analysis showed that for gentamicin and S. marcescens, the area under the concentration-time curve above the CSC and then the time above the CSC are the determining parameters for efficacy (R = 0.69; F = 13.5; P = 0.001), whereas for amikacin and S. marcescens, the time above the CSC and then the area under the concentration-time curve above the CSC predict efficacy (R = 0.74; F = 24.0; P = 0.0001). The lowest CSC is that of amikacin (about 8 mg/liter); those of gentamicin and tobramycin are about 15 mg/liter. In severe S. marcescens infections, intermittent amikacin dosing offers excellent bactericidal activity within the first 24 h.

Considerations in dosage selection for third generation cephalosporins

Pharmacokinetic parameters of third generation cephalosporins vary widely, requiring different dosage regimens and adjustment methods for each agent. Although their antibacterial spectrum favours their usage in infections caused by aerobic Gram-negative organisms, due to their limited post-antibiotic effect against these organisms, dosage regimens should ensure that free drug concentrations at the site of infection remain above the minimum inhibitory concentration for as much of the dosage interval as possible in patients with normal host defence mechanisms and for the entire dosage interval in immunocompromised patients. Altered protein binding encountered in various disease states can affect both microbiological and pharmacokinetic properties especially for drugs with high protein binding. Since the concentrations at the site of action are often different from those in serum, a higher or lower range of dosages needs to be selected depending on the target site. Decreased renal function affects the elimination of most third generation cephalosporins, whereas the presence of hepatic disease does not generally necessitate dosage adjustment. Because of the complex age-related physiological changes in paediatric and elderly patients, dosage should be adjusted on the basis of the reported pharmacokinetic data in these populations. The usual recommended dose may or may not be optimal in a given condition depending on the complex interactions between pharmacokinetic, microbiological and other host factors.

Pharmacokinetics and antibacterial activity of daily gentamicin

Twenty full term neonates with suspected bacterial infection were randomly assigned to a once daily or a twice daily dosage regimen with gentamicin (4 mg/kg/day). Concomitantly all patients were treated with ampicillin (200 mg/kg/day). The gentamicin concentration time curves were analysed by an open two compartment model under steady state conditions on day 4 of treatment. The mean theoretical maximum serum concentration in the group taking gentamicin once daily (10.9 micrograms/ml) was significantly higher than in the group taking it twice daily (7.4 micrograms/ml). Potentially toxic serum concentrations were never reached. Mean trough concentrations were comparable in both groups (once daily 0.8 micrograms/ml; twice daily 1.0 micrograms/ml). Urinary alanine aminopeptidase excretion increased during and even two days after end of treatment in both groups without any significant differences. The results of the dynamic in vitro model revealed that both dosage schedules showed comparable bactericidal effects on pathogens inhibited by low concentrations of gentamicin like Escherichia coli and Staphylococcus aureus. However the once daily regimen was significantly superior in isolates with high minimal inhibitory concentrations.

Impact of the duration of infection on the activity of ceftazidime, gentamicin and ciprofloxacin in Klebsiella pneumoniae pneumonia and septicemia in leukopenic rats

An experimental Klebsiella pneumoniae pneumonia and septicemia in leukopenic rats was used to study the impact of the duration of infection on the bactericidal activity of ceftazidime, gentamicin and ciprofloxacin. It appeared that the number of bacteria persisting after a single intravenous injection progressively increased with delay of antibiotic administration up to 3 h after bacterial inoculation with each of the drugs tested. This effect was most pronounced for ciprofloxacin. An inoculum effect could not explain this decrease in bacterial killing. It was also observed that a single injection with a particular dose of each of the respective drugs did not kill all the Klebsiella pneumoniae organisms in the lung. Persisting bacteria did not represent a preexisting less susceptible subpopulation selected after antibiotic administration. In further experiments the impact of delay of the start of treatment on the efficacy of ceftazidime or ciprofloxacin after administration for a period of four days with intramuscular injections at 6 h intervals was investigated. Treatment was started at 5, 12 or 24 h after bacterial inoculation. The therapeutic efficacy of both drugs decreased with the increase of duration of infection, which may be at least in part due to the progressive number of bacteria persisting after antibiotic administration. These data underline the need to start antimicrobial treatment as soon as possible.

Single daily dosing of antibiotics: importance of in vitro killing rate, serum half-life, and protein binding

The relative importance of pharmacokinetic and pharmacodynamic parameters for the feasibility of a single daily dose (SDD) of antibiotics remains to be established. Therefore, we studied the relationship between in vitro bacteriological parameters (MIC, MBC, and killing rate [KR], defined as the reduction in the inoculum within 3 h), pharmacokinetic parameters (t1/2 and protein binding [PB], and in vivo antibacterial effect of a single antibiotic dose in an experimental rabbit model of Escherichia coli endocarditis. Nine antibiotics were investigated: two aminoglycosides, two quinolones, and five beta-lactams. For each drug, the minimal effective dose (MED) (in milligrams per kilogram) was defined as the lowest dose able to achieve a significant difference (P less than 0.05) of CFU in the vegetations in comparison with controls 24 h after a single intravenous injection. Aminoglycosides and quinolones had the lowest MEDs, followed by beta-lactams. Univariate regression analysis showed that KR was the major determinant of MED. A stepwise regression analysis showed that t1/2 significantly improved the predictive value of KR, while PB, MIC, and MBC did not. The final equation was MED = 1,586-238 KR-297 t1/2 (r = 0.90, P = 0.01). We concluded that the pharmacodynamic parameters (especially the high KR) of aminoglycosides and quinolones explained their low MEDs and might allow SDD. In contrast, the low KR of beta-lactams emphasized the critical importance of a long t1/2, as for ceftriaxone, allowing the use of this beta-lactam alone in SDD.

Mathematical examination of dual individualization principles (I): Relationships between AUC above MIC and area under the inhibitory curve for cefmenoxime, ciprofloxacin, and tobramycin

Traditional antibiotic dosage adjustments target predetermined serum concentrations, whereas a host of in vitro studies and recent clinical trials establish that bacteria vary in their susceptibility. Dual individualization, which considers the variance in both antibiotic pharmacokinetics and bacterial susceptibility, has been employed to describe different rates of bacterial eradication in relation to varying serum concentrations. In patients with nosocomial pneumonia, one of the model compounds studied was cefmenoxime, where a target six-hour area under the serum concentration-time curve (AUC) of 140 micrograms.h/mL above minimum inhibitory concentration (MIC) was previously associated with bacterial eradication in an average of four days. The target AUC value of 140 micrograms.h/mL above MIC is unique to cefmenoxime. Ideally, there should be a dual individualized target useful to adjust the dose of any antibiotic. Computer simulations performed to evaluate this hypothesis suggested that each antibiotic had a unique value for target AUC above MIC. These simulations indicated that an optimal AUC above MIC was about 80 percent of the total AUC above the MIC. Predictable rates of bacterial eradication would presumably result from maintaining these relationships across the range of bacterial susceptibility and the range of serum concentration profiles. Each antibiotic has a unique and different 24-hour AUC over MIC value associated with bacterial eradication in 4 days. For cefmenoxime, the target was 540 area units over MIC per 24 hours, tobramycin with 34 area units, and ciprofloxacin with 23 area units per 24 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation between in vitro and in vivo activity of antimicrobial agents against gram-negative bacilli in a murine infection model

We studied the relationship between in vitro susceptibility tests (MICs, MBCs) and in vivo activity of tobramycin, pefloxacin, ceftazidime, and imipenem against 15 gram-negative bacilli from five different species in a murine thigh infection model. Complete dose-response curves were determined for each antimicrobial agent against each strain, and three parameters of in vivo activity were defined: maximal attainable antimicrobial effect (i.e., reduction in log10 CFU per thigh compared with untreated controls) at 24 h (Emax), total dose required to reach 50% of maximal effect (P50), and total dose required to achieve a bacteriostatic effect (static dose). Pefloxacin demonstrated the greatest Emax (P less than 0.05). Tobramycin was the most potent antimicrobial agent, as indicated by its having the lowest static dose/MIC ratio (P less than 0.002). Log10 P50s and static doses correlated significantly with log10 MICs or MBCs for the 15 strains of each antibiotic (P less than 0.01) except imipenem (P greater than 0.50). The greater potency of imipenem against the three Pseudomonas aeruginosa strains than against strains of the family Enterobacteriaceae (P less than 0.01) explained this lack of correlation. A longer duration of postantibiotic effect for imipenem against P. aeruginosa (P = 0.02) contributed to its increased potency against these strains. We conclude that in vitro susceptibility tests correlated well with in vivo activity in this animal model and that variations in potency among the four antimicrobial agents could be explained by differences in pharmacokinetics or pharmacodynamic activity.

Application of mathematical model to multiple-dose experimental chemotherapy for fatal murine pneumonia

Two beta-lactam antibiotics, cefazolin and cefmenoxime, were administered for 7 days to mice with pneumonia caused by Klebsiella pneumoniae by using dosage regimens that would simulate multiple dosing in usual clinical treatments at dosing intervals of 8 or 12 h. Viable numbers of the bacteria in the lungs were measured at 12- or 24-h intervals. The mathematical model established in a previous single-dose study was applied in this study to explain the time courses of the changes in bacterial count over 7 days. However, because the error in viable count measurements was larger than that in the previous study, the time course of the changes in mean viable count was not regular and the viable count reduction rate changed during multiple dosing, and therefore it was difficult to explain the time course by repeated application of the mathematical model described previously. This study suggests that the changes in pharmacokinetic and pharmacodynamic parameters during multiple dosing need to be considered.

The postantibiotic effect: a review of in vitro and in vivo data

The term postantibiotic effect (PAE) refers to a period of time after complete removal of an antibiotic during which there is no growth of the target organism. The PAE appears to be a feature of most antimicrobial agents and has been documented with a variety of common bacterial pathogens. Several factors influence the presence or duration of the PAE including the type of organism, type of antimicrobial, concentration of antimicrobial, duration of antimicrobial exposure, and antimicrobial combinations. In vitro, beta-lactam antimicrobials demonstrate a PAE against gram-positive cocci but fail to produce a PAE with gram-negative bacilli. Antimicrobials that inhibit RNA or protein synthesis produce an in vitro PAE against gram positive cocci and also produce a PAE against gram-negative bacilli. In vitro methods used to determine the PAE include colony counts, optical density, and measurement of adenosine triphosphate in bacteria. The exact mechanisms by which antimicrobials induce the PAE have not been clearly delineated. Animal studies reveal in vivo PAEs in accordance with PAEs obtained in vitro for most organism/antimicrobial combinations. The clinical relevance of the PAE is probably most important when designing dosage regimens. The presence of a long PAE allows aminoglycosides to be dosed infrequently; the lack of an in vivo PAE suggests that beta-lactam antimicrobials require frequent or continuous dosing. Important questions remain to be answered concerning the PAE.

First-exposure adaptive resistance to aminoglycoside antibiotics in vivo with meaning for optimal clinical use

The first exposure of gram-negative bacilli to an aminoglycoside antibiotic in vitro induces a biphasic bactericidal response and adaptive drug resistance (G. L. Daikos, G. G. Jackson, V. T. Lolans, and D. M. Livermore, J. Infect. Dis. 162:414-420, 1990; G. G. Jackson, G. L. Daikos, and V. T. Lolans, J. Infect. Dis. 162:408-413, 1990). The therapeutic implications were examined in netilmicin treatment of a Pseudomonas aeruginosa infection of normal and neutropenic mice. For 2 h after the first dose, the bactericidal rates were rapid, 0.75, 1.0, and 1.5 log10 CFU/h with doses of 10, 30, and 60 mg/kg, respectively. Each twofold increase in dosage reduced the number of surviving bacteria fivefold. Between 2 and 6 h, the second-phase bactericidal rate was slow, less than or equal to 0.3 log10 CFU/h, regardless of the dose. In a multiple-dose regimen, the same amount of netilmicin given in one dose was 70 and 90% more effective than two or three doses, respectively. Doses calculated to keep the drug level in plasma above the MIC were less effective than regimens giving first exposure to a high drug concentration. Adaptive resistance occurred when doses were given more than 2 h after the start of treatment. Temporary survival of bacteremic neutropenic mice was 60 to 70% greater with a second dose at 2 h than after a longer interval. In a thigh infection of neutropenic mice treated every 2 h, doses 4, 6, and 8 h after the first one showed no bactericidal effect. A drug-free interval of 8 h (20 times the drug half-life) renewed bacterial susceptibility to drug action. The results in vivo confirm the biphasic bactericidal action and induction of adaptive resistance that characterized first exposure of gram-negative bacilli to aminoglycoside antibiotics. The phenomena have meaning for the optimum clinical use of aminoglycosides.

Impact of dosage schedule on the efficacy of gentamicin, tobramycin, or amikacin in an experimental model of Serratia marcescens endocarditis: in vitro-in vivo correlation

Aminoglycosides are usually considered to be concentration-dependent antibiotics and to have similar pharmacodynamic and pharmacokinetic properties. To verify the equivalent activity of the aminoglycosides on a susceptible strain, we tested the killing rate of three aminoglycosides (gentamicin, tobramycin, and amikacin) on one strain of Serratia marcescens both in vitro and in vivo by using a rabbit model of left-ventricle endocarditis. Despite, similar MICs, the time-kill curve of gentamicin was consistently better than those of amikacin and tobramycin, whatever the concentration of each antibiotic used (1, 2, 4, 8, 16, or 32 mg/liter), after a 5-h incubation. The in vivo bacterial reduction in the vegetations was measured 24 h after administration of an intravenous 48-mg/kg bolus of each antibiotic or at the end of a 24-h continuous intravenous infusion of the same dose. Gentamicin was significantly more effective when administered as a bolus than when administered as a continuous infusion (2.8 +/- 0.2 versus 6.4 +/- 1.5 log10 CFU/g of vegetation, respectively; P less than 0.01), whereas amikacin was more effective as a continuous infusion than as a bolus injection (3.6 +/- 2.0 versus 7.5 +/- 1.3 log10 CFU/g of vegetation, respectively; P less than 0.01). Tobramycin was not very effective, whatever the dosage tested (approximately 6.5 to 7 log10 CFU/g). These results suggest that concentration-dependent bactericidal activities, both in vitro and in vivo, may vary greatly among aminoglycosides despite similar MICs.

Importance of the aminoglycoside dosing regimen in the penicillin-netilmicin combination for treatment of Enterococcus faecalis-induced experimental endocarditis

The penicillin-aminoglycoside combination is recommended for the treatment of systemic enterococcal infections. However, the optimal dosing regimen of the aminoglycoside remains to be elucidated. We evaluated the efficacy of penicillin, alone or in combination with various dosing regimens of netilmicin, for the treatment of experimental left-sided Enterococcus faecalis endocarditis in rabbits. Animals were injected intramuscularly for 4 days with penicillin alone or in combination with netilmicin in one of the following regimens: netilmicin at a low dose (2 mg/kg of body weight every 8 h), netilmicin at a high dose (4 mg/kg every 8 h), or netilmicin at a single daily high dose (12 mg/kg every 24 h). MICs and MBCs were 3.1 and 6.2 micrograms/ml and 8 and 8 micrograms/ml for penicillin and netilmicin, respectively. A netilmicin concentration of 4 micrograms/ml was the lowest concentration that achieved synergism with penicillin, as shown by the kill-curve method. Mean peak levels of netilmicin in serum were 5.6 (netilmicin at 2 mg/kg), 9.8 (netilmicin at 4 mg/kg), and 20.6 (netilmicin at 12 mg/kg) micrograms/ml. Mean penicillin levels in serum were constantly above the MIC. Penicillin plus netilmicin at a high dose given three times daily was more effective (P less than 0.05) than any other regimen in reducing bacterial titers in vegetations and was the only treatment that induced a significant bactericidal activity in rabbit serum during the trough. We concluded that divided doses of aminoglycoside are more effective than the same total dose given once daily in combination with penicillin. Our data suggest that prolonged levels of aminoglycoside in serum might be important to exhibit the greatest in vivo efficacy of the combination against E. faecalis. They also indicate that use of a reduced total daily dose of aminoglycoside or an increase in the interval between each dose might reduce the efficacy of therapy in animals with this type of infection.

Results of a multicenter trial comparing imipenem/cilastatin to tobramycin/clindamycin for intra-abdominal infections

We designed a multicenter study to compare tobramycin/clindamycin to imipenem/cilastatin for intra-abdominal infections. We included the Acute Physiology and Chronic Health Evaluation (APACHE II) index of severity and excluded patients without established infection. Two hundred ninety patients were enrolled, of whom 162 were evaluable. Using logistic regression to analyze both outcome at the abdominal site of infection and outcome as mortality, we found a significant correlation for both with APACHE II score (p less than 0.0001 for both). Next we analyzed the residual effect of treatment assignment and found a significant improvement in outcome for imipenem/cilastatin-treated patients (p = 0.043). The differences in outcome were explained by a higher failure rate for patients with gram-negative organisms for tobramycin/clindamycin-treated patients (p = 0.018). This was reflected in a significantly higher incidence of fasciitis requiring reoperation and prosthetic fascial replacement. Maximum peak tobramycin levels were analyzed for 63 tobramycin/clindamycin patients harboring gram-negative organisms. For failures the maximum peak was 6.4 +/- 1.9 micrograms/mL, and time to maximum peak was 4.6 +/- 5.2 days. For successes the maximum peak was 6.1 +/- 1.7 micrograms/mL, occurring at 3.8 +/- 2.6 days. This study supports inclusion of severity scoring in statistical analyses of outcome results and supports the notion that imipenem/cilastatin therapy improves outcome at the intra-abdominal site of infection as compared to a conventionally prescribed amino-glycoside-based regimen.

Significance of tissue levels for prediction of antibiotic efficacy and determination of dosage

Determination of tissue levels is widely thought to provide useful information in the investigation of a new antibiotic, however this assumption is not fully justified. In this paper the following questions are covered. Firstly, the methodological problems encountered in the measurement of antibiotic levels in solid tissues and extravascular fluids are described. Secondly, the difficulties encountered in interpreting tissue concentrations are discussed, given the heterogeneity of distribution of drugs, potential intracellular penetration, binding to tissue proteins and local factors reducing antibacterial activity. Thirdly, the respective importance of animal and human data on tissue levels for prediction of efficacy is analysed. In some animal models it is possible to study the conditions affecting antibacterial activity and to obtain data relevant to the clinical situation. However, the inoculum effect, the metabolism of bacteria, the mechanism of the bactericidal action of drugs and the type of infected tissue are important factors to be taken into account. In humans, data obtained from infected tissue is sparse and contributes little to knowledge. Knowledge of tissue levels may be important for establishing whether data obtained in animals can be applied to humans or not. Fourthly, the importance of tissue levels in determining antibiotic dosages is evaluated. In humans, tissue levels are of limited value in the determination of unitary dose and dosage intervals in relation to the clinical response. Trough serum levels of free drug, half-life at the beta-phase and rate of in vitro killing are the most important parameters to be considered. Their relative importance depends mainly on the mode of action of the drug.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacodynamic properties of antibiotics: application to drug monitoring and dosage regimen design

The goal of antimicrobial chemotherapy is to effectively eradicate pathogenic organisms while minimizing the likelihood of drug-related adverse effects. In this era of cost containment, consideration should also be given to performing this task with the smallest total dose of drug and the shortest duration of therapy. Determination of the appropriate dose and dosing interval of an antimicrobial requires knowledge and integration of both its pharmacokinetic and pharmacodynamic properties.

Impact of dosage schedule of antibiotics on the treatment of serious infections

Experimental studies suggest that the importance of the antibiotic dosage schedule for therapeutic efficacy in severe infection and when host defences are impaired is related to the class of antibiotic. The efficacy of beta-lactams is mainly dependent on the maintenance of adequate antibiotic concentrations in plasma during the entire treatment interval, and not on high peak concentrations. The efficacy of aminoglycosides is related to the total dose administered, i.e., the area under the concentration-time curve, irrespective of the frequency of administration. This difference in efficacy between beta-lactams and aminoglycosides in relation to the dosage schedule correlate well with differences between both classes of antibiotics in kinetics of antibacterial activity in vitro and in vivo. Another factor relevant in this respect is the post-antibiotic effect (PAE) which means the suppression of bacterial regrowth at the end of the period of exposure to antibiotic.