An in vivo model for evaluation of the postantibiotic effect

Pharmacodynamic studies of vancomycin, metronidazole and fusidic acid against Clostridium difficile

BACKGROUND

Pharmacodynamic studies of antibiotics have attracted great interest in recent years. However, studies on the pharmacodynamics of different antibiotics against Clostridium difficile are scarce.

METHODS

The postantibiotic effects (PAE) and the postantibiotic sub-minimum inhibitory concentration (MIC) effects (PA SME) of vancomycin, metronidazole and fusidic acid were investigated by viable counts against three different strains of C. difficile. The killing rate and extent of the three antibiotics against the same strains were also studied by adding 2, 4, 8, 16 and 32x MIC of the three antibiotics, respectively.

RESULTS

Metronidazole exerted a very rapid bactericidal effect at concentrations of 8x MIC and above against all three strains investigated. Vancomycin gave overall less kill in comparison to metronidazole and was bacteriostatic against two of the three strains. Fusidic acid exerted a concentration-dependent killing against two of the strains. Vancomycin exerted short PAEs and PA SMEs against all three strains. Significantly longer PAEs and PA SMEs were noted for fusidic acid. Metronidazole gave similar short PAEs like vancomycin but longer PA SMEs were noted against two of the investigated strains.

CONCLUSION

Metronidazole exerted the most prominent bactericidal effect greater than fusidic acid and greater than vancomycin. Fusidic acid gave overall the longest PAEs and PA SMEs greater than metronidazole and greater than vancomycin.

Selection of resistant Streptococcus pneumoniae during penicillin treatment in vitro and in three animal models

Pharmacokinetic (PK) and pharmacodynamic (PD) properties for the selection of resistant pneumococci were studied by using three strains of the same serotype (6B) for mixed-culture infection in time-kill experiments in vitro and in three different animal models, the mouse peritonitis, the mouse thigh, and the rabbit tissue cage models. Treatment regimens with penicillin were designed to give a wide range of T(>MIC)s, the amounts of time for which the drug concentrations in serum were above the MIC. The mixed culture of the three pneumococcal strains, 10(7) CFU of strain A (MIC of penicillin, 0.016 micro g/ml; erythromycin resistant)/ml, 10(6) CFU of strain B (MIC of penicillin, 0.25 micro g/ml)/ml, and 10(5) CFU of strain C (MIC of penicillin, 4 micro g/ml)/ml, was used in the two mouse models, and a mixture of 10(5) CFU of strain A/ml, 10(4) CFU of strain B/ml, and 10(3) CFU of strain C/ml was used in the rabbit tissue cage model. During the different treatment regimens, the differences in numbers of CFU between treated and control animals were calculated to measure the efficacies of the regimens. Selective media with erythromycin or different penicillin concentrations were used to quantify the strains separately. The efficacies of penicillin in vitro were similar when individual strains or mixed cultures were studied. The eradication of the bacteria, independent of the susceptibility of the strain or strains or the presence of the strains in a mixture or on their own, followed the well-known PK and PD rules for treatment with beta-lactams: a maximum efficacy was seen when the T(>MIC) was >40 to 50% of the observation time and the ratio of the maximum concentration of the drug in serum to the MIC was >10. It was possible in all three models to select for the less-susceptible strains by using insufficient treatments. In the rabbit tissue cage model, a regrowth of pneumococci was observed; in the mouse thigh model, the ratio between the different strains changed in favor of the less-susceptible strains; and in the mouse peritonitis model, the susceptible strain disappeared and was overgrown by the less-susceptible strains. These findings with the experimental infection models confirm the importance of eradicating all the bacteria taking part in the infectious process in order to avoid selection of resistant clones.

Penicillin pharmacodynamics in four experimental pneumococcal infection models

Clinical and animal studies indicate that with optimal dosing, penicillin may still be effective against penicillin-nonsusceptible pneumococci (PNSP). The present study examined whether the same strains of penicillin-susceptible pneumococci (PSP) and PNSP differed in their pharmacodynamic responses to penicillin by using comparable penicillin dosing regimens in four animal models: peritonitis, pneumonia, and thigh infection in mice and tissue cage infection in rabbits. Two multidrug-resistant isolates of Streptococcus pneumoniae type 6B were used, one for which the penicillin MIC was 0.016 microg/ml and the other for which the penicillin MIC was 1.0 microg/ml. Two additional strains of PNSP were studied in the rabbit. The animals were treated with five different penicillin regimens resulting in different maximum concentrations of drugs in serum (C(max)s) and times that the concentrations were greater than the MIC (T(>MIC)s). The endpoints were bacterial viability counts after 6 h of treatment in the mice and 24 h of treatment in the rabbits. Similar pharmacodynamic effects were observed in all models. In the mouse models bactericidal activity depended on the T(>MIC) and to a lesser extent on the Cmax/MIC and the generation time but not on the area under the concentration-time curve (AUC)/MIC. Maximal bactericidal activities were similar for both PSP and PNSP, being the highest in the peritoneum and blood (approximately 6 log10 CFU/ml), followed by the thigh (approximately 3 log10 CFU/thigh), and being the lowest in the lung (approximately 1 log10 CFU/lung). In the rabbit model the maximal effect was approximately 6 log10 CFU/ml after 24 h. In the mouse models bactericidal activity became marked when T(>MIC) was > or =65% of the experimental time and C(max) was > or =15 times the MIC, and in the rabbit model bactericidal activity became marked when T(>MIC) was > or =35%, Cmax was > or =5 times the MIC, and the AUC at 24 h/MIC exceeded 25. By optimization of the Cmax/MIC ratio and T(>MIC), the MIC of penicillin for pneumococci can be used to guide therapy and maximize therapeutic efficacy in nonmeningeal infections caused by PNSP.

Pharmacodynamic effects of subinhibitory antibiotic concentrations

The pharmacodynamics of antibiotics have become increasingly important for the determination of optimal dosing regimens. Studies over the past decade have demonstrated marked differences in the time course of antimicrobial activity for different classes of antibiotics both in vitro, in animals and in human trials. One of the explanations for the success of intermittent dosing regimens has been the delay in regrowth after the concentration has fallen under the MIC, the so called postantibiotic effect (PAE). In addition to the PAE, the success of discontinuous dosing regimens may be attributed to both the function of a normal host defence and to the effects of subinhibitory antibiotic concentrations (sub-MICs). It has been shown that there is a difference between the effects of sub-MICs following a suprainhibitory dose (postantibiotic sub-MIC effect; PA SME) and the effects of sub-MICs (SME) alone. It seems that the PA SME is more clinically relevant compared with the PAE, since exposure to suprainhibitory concentrations will always be followed by sub-MICs in vivo. A long PA SME could indicate that longer dosing intervals may be used for that antibiotic /bacterial combination and together with the known effects of sub-MICs on bacterial virulence and the influence of the immune system, it may explain the efficacy of antibiotics with short half-lives even of they are given infrequently.

Postantibiotic effect and postantibiotic sub-mic effect of dirithromycin and erythromycin against respiratory tract pathogenic bacteria

The postantibiotic effect (PAE) of dirithromycin and erythromycin against strains Streptococcus pyogenes group A M12, NCTC P1800, Streptococcus pneumoniae 23, Staphylococcus aureus Oxford strain 209, Moraxella catarrhalis 15616 and Haemophilus influenzae 5590 was investigated in vitro and in vivo by use of the tissue cage model in rabbits. By exposing strains to 2.5-5 x MIC levels for 6 h or 12 h, both compounds induced in vitro PAEs of 1-9 h, and in two cases >20 h. Cultures in the PAE-phase were then re-exposed to subinhibitory concentrations (0.25 x MIC and 0.5 x MIC) of antibiotic and prolonged suppression of regrowth was obtained for 2->20 h. Following i.v. antibiotic treatment of rabbits (10 mg/kg or 20 mg/kg dirithromycin and 20 mg/kg or 40 mg/kg erythromycin) and bacterial infection of the implanted tissue cages in the same rabbit, the tissue cage fluid (TCF) was sampled 6 h after infection and regrowth was monitored by sampling from new tissue cages in untreated rabbits. These i.v. single doses of both antibiotics induced in vivo PAEs of >6 h, but <20 h against S. pyogenes. Suppression of regrowth in TCF was also obtained for > or = 20 h on infection with exposed S. pyogenes in the PAE-phase in newly implanted tissue cages in rabbits that had been treated with low doses of antibiotic to produce subinhibitory concentrations in the TCE Dirithromycin was in general as active as erythromycin in inducing PAE and in prolonging suppression of bacterial regrowth in the PAE phase.

Efficacy of beta-lactam antibiotics: integration of pharmacokinetics and pharmacodynamics

The study of pharmacokinetics teaches us how drugs are distributed and eliminated, whereas, pharmacodynamics looks at the relationship between drug concentration and drug activity. The free, nonprotein-bound fraction of the serum concentration can be used as a surrogate marker for the levels at the site of infection. It is tempting to use tissue levels for this purpose, but their use is fraught with difficulties. The single pharmacodynamic parameter that correlates best with therapeutic efficacy for beta-lactam antibiotics is time that free serum levels stay above the minimum inhibitory concentration (MIC). Recent clinical studies seem to confirm the value of time above MIC in predicting clinical and bacteriological outcome.

Pharmacodynamic effects of sub-MICs of benzylpenicillin against Streptococcus pyogenes in a newly developed in vitro kinetic model

The pharmacodynamic effects of benzylpenicillin against Streptococcus pyogenes were studied in a new in vitro kinetic model in which bacterial outflow was prevented by a filter membrane. Following the administration of an initial dose of antibiotic, decreasing concentrations were produced by dilution of the medium. A magnetic stirrer was placed above the filter to avoid blockage of the membrane and to ensure homogeneous mixing of the culture. Repeated samplings were easily provided through a silicon diaphragm. Streptococci were exposed to a single dose corresponding to 1.5, 10, 100, or 500 x the MIC of benzylpenicillin and also to an initial concentration of 10 x the MIC of benzylpenicillin, followed by exposure to a repeated dose after 8 h yielding 10 or 1.5 x the MIC. Experiments were also performed with 10 x the MIC of benzylpenicillin with a half-life of 3 h or an initial half-life of 1.1 h that was altered to 3 h at the time point at which the antibiotic concentrations and MIC intersected. Bacterial killing and regrowth were followed by determining viable counts. The post-MIC effect (PME) was defined as the difference in time for the numbers of CFU in the culture vessel to increase 1 log10 CFU/ml, calculated from the numbers obtained at the time when the antibiotic concentration had declined to the MIC, and the corresponding time for a control culture, grown in a glass tube without antibiotic, to increase 1 log10 CFU/ml. To determine how much of the PME was attributable to subinhibitory concentrations, penicillinase was added to a part of the culture drawn from the flask at the time when the antibiotic concentration had fallen to the MIC. The longest PME was found in the experiments in which the half-life was extended from 1.1 to 3 h at the MIC. This illustrated that sub-MICs are sufficient to prevent regrowth. However, when the half-life was 3 h during the whole experiment, the PME was shorter, indicating that when concentrations decline slowly penicillin-binding proteins will already be present in amounts sufficient for regrowth at the time when the MIC is reached. The PME may prove to be a more reliable factor than the in vitro postantibiotic effect or postantibiotic sub-MIC effect for the design of optimal dosing schedules, since the PME, like the in vivo postantibiotic effect, includes the effects of subinhibitory concentrations and therefore better reflects the clinical situation with fluctuating antibiotic concentrations.

Variation in postantibiotic effect of clindamycin against clinical isolates of Staphylococcus aureus and implications for dosing of patients with osteomyelitis

Initial measurements of postantibiotic effect (PAE) were made by a standard laboratory method (exposure to 1 mg of clindamycin per liter for 1 h). The range of PAE for 21 strains of Staphylococcus aureus isolated from osteomyelitis patients was 0.4 to 3.9 h, which markedly exceeded the coefficient of variation for the method (6 to 19%). Exposure of S. aureus to three doses of clindamycin at 8-h intervals had no consistent effect on either PAE or MIC. The PAE was dependent on both concentration and duration of exposure to clindamycin: for example, the PAEs for one strain were 1.7 h after exposure to 1 mg/liter for 1 h, 2.4 h after exposure to 4 mg/liter for 1 h, and 5.9 h after exposure to 4 mg/liter for 3 h. Pharmacokinetic simulations showed that the dose required to maintain free serum clindamycin concentrations above the MIC was 300 mg 6 hourly after oral administration (95% confidence interval, 243 to 301 mg) and 1.2 g 6 hourly (95% confidence interval, 305 to 1,145 mg) after intravenous (i.v.) administration. The duration of PAE would have to be at least 2.4 h to allow an increase in the oral dose interval to 8 h or to allow i.v. administration of 300 mg 6 hourly. Additional PAE experiments were performed with the three strains for which PAEs are the shortest after exposure to 1 mg/liter for 1 h (0.4 to 1.2 h). The PAE for these three strains increased markedly to 4.4 to 6.7 h following exposure to 2 mg/liter for 6 h (to mimic the area under the concentration-time curve from 0 to 6 h after a 300-mg dose). These data suggest that oral clindamycin could be administered at 300 mg 8 hourly in the treatment of S. aureus infection, whereas the i.v. dose interval should be 6 h. These suggestions should be confirmed by performing clinical trials.

The postantibiotic effect of selected antibiotics on Staphylococcus aureus Newbould 305 from bovine intramammary infection

The postantibiotic effect (PAE) was determined for selected antibiotics against Staphylococcus aureus Newbould 305 originating in vivo from mastitic milk and compared with the PAE for the same S. aureus strain cultured in vitro. The PAE was measured at 2 and 4 times the MIC for 1 and 2 h of exposure. The PAE of penicillin, pirlimycin, and tilmicosin were reduced against S. aureus 305 originating in vivo compared with S. aureus 305 grown in vitro. The PAE of cephapirin was increased against S. aureus originating in vivo. Minimal effect on PAE was noted for novobiocin. The PAE for rifampicin extended beyond the limits of the test parameters (> 180 min) for all antibiotic concentrations, media, and exposure times tested, except at 2 times the MIC and at 1 h exposure when the PAE was reduced to 60 min for S. aureus 305 originating in vivo. The PAE of an antibiotic may be an important consideration in determining therapy intervals and antibiotic concentrations for treatment of bovine mastitis.

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.

The in vivo and in vitro postantibiotic effect of aminoglycosides using a clinically isolated micro-organism

We reported a case of abscess in which Serratia marcescens was isolated as the causative organism. We measured the postantibiotic effect (PAE) of dibekacin (DKB) and gentamicin (GM) against S. marcescens and studied the relationship between the clinical effect and the PAE. The minimal inhibitory concentration (MIC) of DKB against S. marcescens was 6.25 micrograms/mL and the serum concentration 30 min after infusion of 100 mg DKB was 5.99 micrograms/mL. The abscess was cured by the administration of DKB every 12 h. The PAE in vivo was 2.5, 2.9 and 3.3 h when DKB was administered at 50 mg/kg, 100 mg/kg and 200 mg/kg, respectively. This PAE is one of the reasons that infection can be effectively treated with intermittent administration, even if the serum concentration is below the MIC.

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.

Synergy and cumulated killing effect of the penems FCE 22101 and FCE 25199 in combination with gentamicin against bacteria isolated from septicaemia

Blood isolates of Enterococcus faecalis, Streptococcus sanguis, Staphylococcus aureus, E. coli and Klebsiella oxytoca were tested for their synergistic and cumulated killing effect (CKE) with the new penems FCE 22101 or FCE 25199 in combination with gentamicin. The tissue cage model in rabbits was used to study the CKE in vivo after antibiotic treatment of the bacteria in vitro. Synergy was observed within two to seven h with all isolates in early logarithmic phase, except with S. aureus, which was rapidly killed by the penems alone. After one h treatment with the antibiotic combinations in vitro, a CKE was demonstrated for up to six h both in vitro and in vivo. The magnitude of the CKE differed between strains and in vitro vs. in vivo.

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.

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.

Postantibiotic effect of the penem FCE 22101 against selected gram-positive and gram-negative bacteria in vitro and in vivo by the use of a tissue cage model in rabbits

Isolates of Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae and Klebsiella pneumoniae were tested for their bactericidal activity and postantibiotic effect (PAE) with the new penem FCE 22101. The tissue cage model in rabbits was used to study PAE in vivo. The bactericidal activity against all four species was shown to be in the range of 0.05-4.0 mg/l. A 99.9% killing effect at MBC concentrations was reached within 2 hours with S. pneumoniae and K. pneumoniae and within 6-8 hours with S. aureus and H. influenzae. After in vitro exposure by FCE 22101 a PAE in vitro and in vivo was obtained against S. aureus, S. pneumoniae and H. influenzae strains but no PAE could be demonstrated against K. pneumoniae. FCE 22101 showed a good bactericidal activity and PAE against the strains investigated, except for K. pneumoniae.

Postantibiotic effects of imipenem, norfloxacin, and amikacin in vitro and in vivo

The postantibiotic effects (PAEs) of imipenem and norfloxacin were tested against Staphylococcus aureus, Streptococcus (Enterococcus) faecalis, Escherichia coli, and Pseudomonas aeruginosa. Amikacin was tested against the same bacteria except Streptococcus faecalis. For in vitro tests, a viable count-washing method was used, and for in vivo tests, the thread technique in normal mice was used. All three drugs produced PAEs of 1.1 to 3.8 h in vitro and 1.4 to 4.3 h in vivo against the pathogens tested. In vitro and in vivo results correlated well. The PAE had a significantly (P less than 0.01 to 0.001) longer duration in vivo than in vitro, but the PAE of imipenem on Staphylococcus aureus was longer in vitro. The PAE was not due to residual antibiotics at the site of infection, and no PAE was obtained if at any time the antibiotic concentration at the infection site reached the MIC for the pathogen tested. The results indicate that the presence of a PAE may enable antibiotics to be given more intermittently without a loss of efficacy and that the PAE can only be induced if the level of the antibiotic exceeds the MIC for the pathogen in question for at least several minutes.

Postantibiotic and bactericidal effect of imipenem against Pseudomonas aeruginosa

The postantibiotic effect of imipenem on Pseudomonas aeruginosa was studied at different inocula using one ATCC strain and four clinical isolates. The postantibiotic effect was measured using two different methods: viable counts and bioluminescence assay of intracellular bacterial ATP. The postantibiotic effect could be demonstrated with both methods (viable counts 1-2 h, ATP assay 3-5 h) for all strains at an inoculum of 10(6) CFU/ml. When the inoculum was raised to 10(8) CFU/ml, no postantibiotic effect could be observed with either method using routine growth conditions. This disappearance of the postantibiotic effect coincided with a loss of bactericidal effect of imipenem when high inocula were used. Improved oxygenation of the cultures restored the bactericidal and postantibiotic effects of imipenem at high inocula.