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

Allele-specific collateral and fitness effects determine the dynamics of fluoroquinolone resistance evolution

A promising strategy to overcome the evolution of antibiotic-resistant bacteria is to use collateral sensitivity-informed antibiotic treatments that rely on cycling or mixing of antibiotics, such that that resistance toward one antibiotic confers increased sensitivity to the other. Here, focusing on multistep fluoroquinolone resistance in Streptococcus pneumoniae, we show that antibiotic resistance induces diverse collateral responses whose magnitude and direction are determined by allelic identity. Using mathematical simulations, we show that these effects can be exploited via combination treatment regimens to suppress the de novo emergence of resistance during treatment.

Collateral sensitivity (CS), which arises when resistance to one antibiotic increases sensitivity toward other antibiotics, offers treatment opportunities to constrain or reverse the evolution of antibiotic resistance. The applicability of CS-informed treatments remains uncertain, in part because we lack an understanding of the generality of CS effects for different resistance mutations, singly or in combination. Here, we address this issue in the gram-positive pathogen Streptococcus pneumoniae by measuring collateral and fitness effects of clinically relevant gyrA and parC alleles and their combinations that confer resistance to fluoroquinolones. We integrated these results in a mathematical model that allowed us to evaluate how different in silico combination treatments impact the dynamics of resistance evolution. We identified common and conserved CS effects of different gyrA and parC alleles; however, the spectrum of collateral effects was unique for each allele or allelic pair. This indicated that allelic identity can impact the evolutionary dynamics of resistance evolution during monotreatment and combination treatment. Our model simulations, which included the experimentally derived antibiotic susceptibilities and fitness effects, and antibiotic-specific pharmacodynamics revealed that both collateral and fitness effects impact the population dynamics of resistance evolution. Overall, we provide evidence that allelic identity and interactions can have a pronounced impact on collateral effects to different antibiotics and suggest that these need to be considered in models examining CS-based therapies.

Pharmacokinetics of ceftriaxone in freshwater crocodiles (Crocodylus siamensis) after intramuscular administration at two dosages

One of the major obstacles to the successful treatment of infectious disease in freshwater crocodile species is incorrect dosing of antibiotics. There are few reports on pharmacokinetics and dosage regimens of antimicrobial drugs in crocodiles. The purpose of the present study was to clarify the pharmacokinetic characteristics of ceftriaxone (CEF) in Siamese freshwater crocodiles (Crocodylus siamensis). Freshwater crocodiles, Crocodylus siamensis, in breeding farms were treated with a single intramuscular administration of CEF at two dosages, 12.5 and 25 mg/kg body weight (b.w.). Blood samples were collected at preassigned times up to 168 hr. The plasma concentrations of CEF were measured by a validated method through liquid chromatography tandem-mass spectrometry. CEF plasma concentrations were quantified up to 72 and 96 hr after low- and high-dose administration, respectively. The C_max values of CEF were 24.61 ± 5.15 µg/ml and 26.39 ± 2.81 µg/ml at dosages of 12.5 and 25 mg/kg b.w., respectively. The AUC_last values increased in a dose-dependent fashion. The half-life values were not statistically different between the groups (around 20 hr). The average binding percentage of CEF to plasma protein was 53.78 ± 2.11%. Based on the pharmacokinetic data, susceptibility break-point and the surrogate PK-PD index (T > MIC, 0.2 μg/ml), i.m. administration of CEF at a dose of 12.5 mg/kg b.w. might be appropriate for initiating treatment of susceptible bacterial infections in freshwater crocodiles.

Comparative Activity of Ceftriaxone, Ciprofloxacin, and Gentamicin as a Function of Bacterial Growth Rate Probed by Escherichia coli Chromosome Replication in the Mouse Peritonitis Model

Commonly used antibiotics exert their effects predominantly on rapidly growing bacterial cells; yet, the growth dynamics taking place during infection in a complex host environment remain largely unknown. Hence, a means to measure in situ bacterial growth rate is essential to predict the outcome of antibacterial treatment. We have recently validated chromosome replication as a readout of in situ bacterial growth rate during Escherichia coli infection in the mouse peritonitis model. By the use of two complementary methods (quantitative PCR and fluorescence microscopy) for differential genome origin and terminus copy number quantification, we demonstrated the ability to track bacterial growth rate, both on a population average level and on a single-cell level, from one single biological specimen. Here, we asked whether the in situ growth rate predicts antibiotic treatment effect during infection in the same model. Parallel in vitro growth experiments were conducted as a proof of concept. Our data demonstrate that the activities of the commonly used antibiotics ceftriaxone and gentamicin correlated with pretreatment bacterial growth rate; both drugs performed better during rapid growth than during slow growth. Conversely, ciprofloxacin was less sensitive to bacterial growth rate, both in a homogenous in vitro bacterial population and in a more heterogeneous in vivo bacterial population. The method serves as a platform to test any antibiotic's dependency on active in situ bacterial growth. Improved insight into this relationship in vivo could ultimately prove helpful in evaluating future antibacterial strategies.

Pharmacokinetics of ceftriaxone in Green sea turtles (Chelonia mydas) following intravenous and intramuscular administration at two dosages

Green sea turtles are widely distributed in tropical and subtropical waters. Adult green sea turtles face many threats, primarily from humans, including injuries from boat propellers, being caught in fishing nets, pollution, poaching, and infectious diseases. To the best of our knowledge, limited pharmacokinetic information to establish suitable therapeutic plans is available for green sea turtles. Therefore, the present study aimed to describe the pharmacokinetic characteristics of ceftriaxone (CEF) in green sea turtles, Chelonia mydas, following single intravenous and intramuscular administrations at two dosages of 10 and 25 mg/kg body weight (b.w.). Blood samples were collected at assigned times up to 96 hr. The plasma concentrations of CEF were measured by liquid chromatography tandem mass spectrometry. The concentrations of CEF in the plasma were quantified up to 24 and 48 hr after i.v. and i.m. administrations at dosages of 10 and 25 mg/kg b.w., respectively. The C_max values of CEF were 15.43 ± 3.71 μg/ml and 43.48 ± 4.29 μg/ml at dosages of 10 and 25 mg/kg, respectively. The AUC_last values increased in a dose-dependent fashion. The half-life values were 2.89 ± 0.41 hr and 5.96 ± 0.26 hr at dosages of 10 and 25 mg/kg b.w, respectively. The absolute i.m. bioavailability was 67% and 108%, and the binding percentage of CEF to plasma protein was ranged from 20% to 29% with an average of 24.6%. Based on the pharmacokinetic data, susceptibility break-point and PK-PD index (T > MIC, 0.2 μg/ml), i.m. administration of CEF at a dosage of 10 mg/kg b.w. might be appropriate for initiating treatment of susceptible bacterial infections in green sea turtles.

Effect of ABCC2 and ABCG2 Gene Polymorphisms and CSF-to-Serum Albumin Ratio on Ceftriaxone Plasma and Cerebrospinal Fluid Concentrations

We measured ceftriaxone pharmacokinetics in patients' plasma and cerebrospinal fluid (CSF) and assessed the influence of biometric, demographic, genetic (ABCB1, ABCC2, ABCB11, ABCG2, and SLCO1A2 polymorphisms) and pathological features. Adult patients with signs and symptoms of central nervous system infections, receiving intravenous ceftriaxone, were enrolled. Ceftriaxone plasma and CSF concentrations were measured by high-precision liquid chromatographic methods; allelic discrimination was performed by real-time polymerase chain reaction. Forty-three patients were included: median ceftriaxone maximal concentration was 15,713 ng/mL in plasma and 3512 ng/mL in CSF with a CSF-to-plasma ratio of 0.3. ABCC2 1249 rs2273697 (P = .027) and ABCG2 1194+928 rs13120400 (P = .015) variants were significantly associated with CSF concentrations and CSF-to-plasma ratios. At linear regression analysis, CSF-to-serum albumin ratio was an independent predictor of ceftriaxone CSF concentrations (P = .001; also in those with intact blood-brain barrier: P = .031) and CSF-to-plasma ratio (P = .001; also in those with blood-brain barrier impairment: P = .040). We here report the role of transporters' genetic variants as well as of blood-brain barrier permeability in predicting ceftriaxone exposure in the central nervous system.

Animal models in the pharmacokinetic/pharmacodynamic evaluation of antimicrobial agents

Animal infection models in the pharmacokinetic/pharmacodynamic (PK/PD) evaluation of antimicrobial therapy serve an important role in preclinical assessments of new antibiotics, dosing optimization for those that are clinically approved, and setting or confirming susceptibility breakpoints. The goal of animal model studies is to mimic the infectious diseases seen in humans to allow for robust PK/PD studies to find the optimal drug exposures that lead to therapeutic success. The PK/PD index and target drug exposures obtained in validated animal infection models are critical components in optimizing dosing regimen design in order to maximize efficacy while minimize the cost and duration of clinical trials. This review outlines the key components in animal infection models which have been used extensively in antibiotic discovery and development including PK/PD analyses.

Pharmacokinetic-pharmacodynamic modeling of antibacterial drugs

Pharmacokinetic-pharmacodynamic (PKPD) modeling and simulation has evolved as an important tool for rational drug development and drug use, where developed models characterize both the typical trends in the data and quantify the variability in relationships between dose, concentration, and desired effects and side effects. In parallel, rapid emergence of antibiotic-resistant bacteria imposes new challenges on modern health care. Models that can characterize bacterial growth, bacterial killing by antibiotics and immune system, and selection of resistance can provide valuable information on the interactions between antibiotics, bacteria, and host. Simulations from developed models allow for outcome predictions of untested scenarios, improved study designs, and optimized dosing regimens. Today, much quantitative information on antibiotic PKPD is thrown away by summarizing data into variables with limited possibilities for extrapolation to different dosing regimens and study populations. In vitro studies allow for flexible study designs and valuable information on time courses of antibiotic drug action. Such experiments have formed the basis for development of a variety of PKPD models that primarily differ in how antibiotic drug exposure induces amplification of resistant bacteria. The models have shown promise for efficacy predictions in patients, but few PKPD models describe time courses of antibiotic drug effects in animals and patients. We promote more extensive use of modeling and simulation to speed up development of new antibiotics and promising antibiotic drug combinations. This review summarizes the value of PKPD modeling and provides an overview of the characteristics of available PKPD models of antibiotics based on in vitro, animal, and patient data.

Immunogenicity in mice and non-human primates of the Group A Streptococcal J8 peptide vaccine candidate conjugated to CRM197

Vaccine development for Group A streptococcal (GAS) infection has been extensively focused on the N-terminal hypervariable or the C-terminal conserved regions of the M protein, a major virulence factor of GAS. We evaluated the immunogenicity and functional activity of the conserved C-terminal peptide vaccine candidate, J8, conjugated to CRM197, in two mouse strains: C3H (H2(k)) and Balb/c (H2(d)), and in rhesus macaques. Mice were immunized with J8-CRM197 formulated with Amorphous Aluminum Hydroxyphosphate Sulfate Adjuvant (AAHSA), and non-human primates were immunized with J8-CRM197 formulated with AAHSA, ISCOMATRIX (TM) adjuvant, or AAHSA/ISCOMATRIX adjuvant. J8-CRM197 was immunogenic in mice from both H2(k) and H2(d) backgrounds, and the antibodies generated bound to the surface of four different GAS serotypes and had functional bacterial opsonic activity. Mice immunized with J8-CRM197/AAHSA demonstrated varying degrees of protection from lethal challenge. We also demonstrated that J8-CRM197 is immunogenic in non-human primates. Our data confirm the utility of J8 as a potential GAS vaccine candidate and demonstrate that CRM197 is an acceptable protein carrier for this peptide.

The capsule of Streptococcus pneumoniae contributes to virulence in the insect model Manduca sexta

The polysaccharide capsule of Streptococcus pneumoniae is one of the most important virulence factors responsible for human infections and in mouse infection models as well. Larvae of Manduca sexta were used as an alternative animal model in order to test the impact of the pneumococcal capsule on virulence in the insect host. The unencapsulated S. pneumoniae strain R6 was able to cause disease and induce killing in the larvae, and similar results were obtained with related commensal species. However, using the same dose of S. pneumoniae, encapsulated strains including the type 2 D39 strain, the progenitor of R6, and genetically unrelated S. pneumoniae strains of serotype 2, 4, 6B, 23F and 19A, all had increased virulence potential compared to the R6 strain. Between 20 and 70% of the larvae were affected after 96 h compared to 12% observed with R6. Two type 6B S. pneumoniae strains were more virulent compared to the other strains. S. pneumoniae R6 transformants producing the type 6B capsule showed a similar elevated disease potential, confirming the contribution of the pneumococcal polysaccharide capsule to virulence in M. sexta.

Local epicardial inotropic drug delivery allows targeted pharmacologic intervention with preservation of myocardial loading conditions

Local myocardial application of inotropes may allow the study of pharmacologically augmented central myocardial contraction in the absence of confounding peripheral vasodilating effects and alterations in heart loading conditions. Novel alginate epicardial (EC) drug releasing platforms were used to deliver dobutamine to the left ventricle of rats. Pressure-volume analyses indicated that although both local and systemic intravenous (i.v.) use of inotropic drugs increase stroke volume and contractility, systemic infusion does so through heart unloading. Conversely, EC application preserves heart load and systemic blood pressure. EC dobutamine increased indices of contractility with minimal rise in heart rate and lower reduction in systemic vascular resistance than i.v. infusion. Drug sampling showed that dobutamine concentration was 650-fold higher in the anterior wall than in the inferior wall. The plasma dobutamine concentration with local delivery was about half as much as with systemic infusion. These data suggest that inotropic EC delivery has a localized effect and augments myocardial contraction by different mechanisms than systemic infusion, with far fewer side effects. These studies demonstrate a pharmacologic paradigm that may improve heart function without interference from effects on the vasculature, alterations in heart loading, and may ultimately improve the health of heart failure patients.

Influence of antidrug antibodies on plectasin efficacy and pharmacokinetics

Plectasin is a 4.4-kDa antimicrobial peptide with the potential to be a treatment of infections caused by gram-positive bacteria. Since plectasin is a large molecule compared to conventional antibiotics, the development of antidrug antibodies (ADAs) could be anticipated. The immunogenic properties of plectasin were assessed through immunization studies. In mice treated for 5 days with one to two daily subcutaneous doses of plectasin, no antibody response was observed. If the animals were immunized again, after a rest period, low levels of antibodies developed in approximately half the animals. Additionally, mice were immunized with plectasin in Freund's incomplete adjuvant (FIA). Ninety-two percent of these mice developed ADAs after repeated immunizations, with two-thirds having high levels of antibodies. An agar diffusion bioassay showed that sera from animals immunized with plectasin did not inhibit the efficacy of the drug, while hyperimmune sera from animals in which an immune response was provoked by immunization with plectasin in FIA reduced the efficacy of plectasin at the lowest concentration tested. Studies in the murine peritonitis model showed an excellent efficacy of plectasin for the treatment of Streptococcus pneumoniae infections both in naïve animals and in animals with ADAs. No difference in bacterial counts was seen when the animals were treated with plectasin at 2.5 mg/kg of body weight, a dose below the expected therapeutic level. When animals were treated with plectasin at 0.625 mg/kg, the effect was reduced but not neutralized in animals with high levels of ADAs. No animals showed signs of hypersensitivity or injection site reactions toward plectasin, and the half-life of the compound did not vary between animals with and without antibodies.

Intracellular activity of antibiotics against Staphylococcus aureus in a mouse peritonitis model

Antibiotic treatment of Staphylococcus aureus infections is often problematic due to the slow response to therapy and the high frequency of infection recurrence. The intracellular persistence of staphylococci has been recognized and could offer a good explanation for these treatment difficulties. Knowledge of the interplay between intracellular antibiotic activity and the overall outcome of infection is therefore important. Several intracellular in vitro models have been developed, but few experimental animal models have been published. The mouse peritonitis/sepsis model was used as the basic in vivo model exploring a quantitative ex vivo extra- and intracellular differentiation assay. The intracellular presence of S. aureus was documented by electron microscopy. Five antibiotics, dicloxacillin, cefuroxime, gentamicin, azithromycin, and rifampin (rifampicin), were tested in the new in vivo model; and the model was able to distinguish between their extra- and intracellular effects. The intracellular effects of the five antibiotics could be ranked as follows as the mean change in the log(10) number of CFU/ml (Delta log(10) CFU/ml) between treated and untreated mice after 4 h of treatment: dicloxacillin (3.70 Delta log(10) CFU/ml) > cefuroxime (3.56 Delta log(10) CFU/ml) > rifampin (1.86 Delta log(10) CFU/ml) > gentamicin (0.61 Delta log(10) CFU/ml) > azithromycin (0.21 Delta log(10) CFU/ml). We could also show that the important factors during testing of intracellular activity in vivo are the size, number, and frequency of doses; the time of exposure; and the timing between the start of infection and treatment. A poor correlation between the intracellular accumulation of the antibiotics and the actual intracellular effect was found. This stresses the importance of performing experimental studies, like those with the new in vivo model described here, to measure actual intracellular activity instead of making predictions based on cellular pharmacokinetic and MICs.

A Pilot Study of an Anti-MRSA Bio-Engineered Lacteal Complex (Anti-MRSA BLC) in a Murine Septicemia Model

Methicillin-resistant Staphylococcus aureus (MRSA) is an important pathogen of humans and other animals, causing septicemia, abcessation, toxemia, and other infectious diseases. Refined bioengineered lacteal complex (BLC), made specifically against MRSA, is a novel complex of low molecular weight immunogenic and antimicrobial molecules. It was evaluated in vivo using a mouse model of MRSA-induced peritonitis. Intraperitoneal dosing of anti-MRSA BLC demonstrated a therapeutic effect (83% survival) against an intraperitoneal MRSA challenge that caused 100% mortality in untreated animals. Anti-MRSA BLC is a promising therapeutic modality for MRSA infection.

Pharmacokinetic parameters of ceftriaxone after single intravenous and intramuscular administration in camels (Camelus Dromedarius)

The purpose of this study was to investigate the plasma disposition kinetics of ceftriaxone in female camels (n=5) following a single intravenous (i.v.) bolus or intramuscular (i.m.) injections at a dosage of 10mg kg(-1) body weight in all animals. A crossover design was carried out in two phases separated by 15 days. Jugular blood samples were collected serially for 48h and the plasma was analysed by high-performance liquid chromatography (HPLC). Following single i.v. injections the plasma concentration time curves of ceftriaxone were best fitted to a two-compartment model. The drug was rapidly distributed with half-life of distribution t(1/2alpha) of 0.24+/-0.01h and moderately eliminated with elimination rate constant and elimination half-life of 0.27+/-0.13h(-1) and 2.57+/-0.52h, respectively. The volume of distribution at steady state (V(dss)) was 0.32+/-0.01lkg(-1) and the total body clearance (Cl(tot)) was 0.11+/-0.01lkg(-1)h(-1), respectively. Following i.m. administration, the mean T(max), C(max), t(1/2el) and AUC values for plasma data were 1.03+/-0.23h, 21.54+/-2.61microg ml(-1), 1.76+/-0.03h and 85.82+/-11.21microg ml(-1)h(-1), respectively. The i.m. bioavailability was 93.42+/-21.4% and the binding percentage of ceftriaxone to plasma protein was moderate, ranging from 33% to 42% with an average of 34.5%.

Estimating amoxicillin influx/efflux in chinchilla middle ear fluid and simultaneous measurement of antibacterial effect

Understanding the transport process and the factors that control the influx/efflux of antibiotics between plasma and middle ear fluid is essential in optimizing the antimicrobial efficacy in the treatment of acute otitis media. In this study, an experimental chinchilla model with the application of a microdialysis technique was utilized to evaluate amoxicillin middle ear distribution kinetics. Amoxicillin solutions at various doses were instilled into the middle ear with a simultaneous intravenous bolus dose. Unbound amoxicillin levels were monitored by microdialysis in both ears. Serial phlebotomy provided samples for the measurement of unbound amoxicillin concentration in plasma ultrafiltrates. In infected chinchillas, discrete middle ear fluid samples were plated and cultured to characterize Streptococcus pneumoniae growth-kill kinetics. Noncompartmental analysis was used to estimate distributional and elimination clearances assuming linear pharmacokinetics. A nonlinear Michaelis-Menten equation was also used to determine the efflux clearance (from middle ear fluid to plasma) in a mammillary compartment model. No difference was observed in amoxicillin pharmacokinetics between control and infected chinchillas. Influx clearance was (4.6 +/- 2.4) x 10(-3) ml/min-kg and significantly lower than the efflux clearance estimated as (19.2 +/- 9.7) x 10(-3) ml/min-kg (P < 0.002). Nonlinear kinetics was observed in the locally dosed ear. The microdialysis procedure did not interfere with the bacterial growth-kill profile, thereby enabling pharmacokinetic and pharmacodynamic evaluation concurrently. In conclusion, the results suggested that the distribution equilibrium of amoxicillin in the middle ear favors efflux to plasma over influx. An active transport mechanism across middle ear mucosal epithelium may be involved in amoxicillin distribution.

Low-dose aerosol model of pneumococcal pneumonia in the mouse: utility for evaluation of antimicrobial efficacy

Current mouse models of pneumococcal infection have two disadvantages: (1) those that are not based on lung infections do not take into account the tissue pharmacokinetics of drugs in the lung parenchyma; and (2) those that are pneumonia models typically use large infectious doses to produce fulminant infections. The objective of this study was to determine the utility of a low-dose aerosol pneumonia model for evaluation of antimicrobial efficacy. Mice infected with penicillin-susceptible or non-susceptible pneumococci were left untreated or treated for 2.5 days with ertapenem in a range of doses. Efficacy was determined by the change in log10 colony-forming unit (CFU) counts and survival. Low-dose aerosol infection with the penicillin-susceptible strain 6303 produced an indolent pneumonia that was reliably lethal 1-2 weeks after infection. Ertapenem demonstrated bactericidal activity and prevented mortality over a range of doses after infection with strain 6303, but demonstrated only bacteriostatic activity at the highest doses used against the more resistant 1980 strain. A beneficial effect on survival was seen at doses approaching bioequivalence with the standard human dosage. The low-dose aerosol model of pneumococcal pneumonia in the mouse is a viable alternative model for the evaluation of antimicrobial efficacy. It may be particularly useful in the evaluation of drugs that concentrate in the alveolar epithelial lining fluid or lung parenchyma.

Pharmacokinetics of two once-daily parenteral cephalexin formulations in dogs

The aims of this study were to describe and compare the pharmacokinetic profiles and T(>MIC90) of two commercially available once-daily recommended cephalexin formulations in healthy adult dogs administered by the intramuscular (i.m.) route. Six beagle dogs received a 10 mg/kg dose of an 18% parenteral suspension of cephalexin of laboratory A (formulation A) and laboratory B (formulation B) 3 weeks apart. Blood samples were collected in predetermined times after drug administration. The main pharmacokinetic parameters were (mean +/- SD): AUC((0-infinity)), 72.44 +/- 15.9 and 60.83 +/- 13.2 microg.h/mL; C(max), 10.11 +/- 1.5 and 8.50 +/- 1.9 microg/mL; terminal half-life, 3.56 +/- 1.5 and 2.57 +/- 0.72 h and MRT((0-infinity)), 5.86 +/- 1.5 and 5.36 +/- 1.2 h for formulations A and B, respectively. T(>MIC90) was 63.1 +/- 14.7 and 62.1 +/- 14.7% of the dosing interval for formulations A and B, respectively. Median (range) for t(max) was 2.0 (2.0-3.0) h and 3.0 (2.0-4.0) for formulations A and B, respectively. Geometric mean ratios of natural log-transformed AUC((0-infinity)) and C(max) and their 90% confidence intervals (CI) were 0.84 (0.72-0.98) and 0.83 (0.64-1.07), respectively. The plasma profiles of cephalexin following the administration of both formulations were similar. No statistical differences between pharmacokinetic parameters or T(>MIC90) were observed, however, bioequivalence between both formulations could not be demonstrated, as lower 90% CI failed to fell within the selected range of 80-125% for bioequivalence.

Experimental study on the efficacy of combinations of glycopeptides and β-lactams against Staphylococcus aureus with reduced susceptibility to glycopeptides

OBJECTIVES

The combination of glycopeptides and beta-lactams has been proposed as an alternative therapy against infections due to Staphylococcus aureus with reduced susceptibility to glycopeptides, though its role is still controversial. Our aim was to evaluate the efficacy (decrease in bacterial concentration after 24 h therapy) of these combinations both in vitro and in vivo.

METHODS

Four strains of S. aureus with different glycopeptide susceptibility (MICs of vancomycin from 1 to 8 mg/L) were used. In vitro experiments were performed by means of time-kill curves while we used the mouse peritonitis model for in vivo evaluation.

RESULTS

Combinations of glycopeptides and beta-lactams showed synergy in in vitro time-kill curves against the four staphylococcal strains, the highest efficacy being detected against the glycopeptide-intermediate S. aureus (GISA) strain (MIC = 8 mg/L) (Deltalog 24 h = -3.19 cfu/mL for vancomycin at 1/2 x MIC and oxacillinat 1/64 x MIC versus -0.56 cfu/mL for vancomycin alone at 1/2 x MIC). On the other hand, no significant increase in efficacy was observed in vivo in the experimental model. The efficacy of the combinations decreased in correlation to the decreasing susceptibility of the strains to glycopeptides, showing only residual activity against the GISA strain (Deltalog 24 h = -1.42 cfu/mL for vancomycin and cloxacillin versus -1.22 cfu/mL for vancomycin).

CONCLUSIONS

In the in vivo setting we were unable to demonstrate the synergism between glycopeptides and beta-lactams observed in vitro; nor did combinations show antagonism against any of the strains. Though the usefulness of these combinations cannot be totally ruled out in highly specific clinical conditions, it seems unlikely that they will provide a serious therapeutic alternative in most hGISA and GISA infections in the coming years.

Evaluation of a single dose versus a divided dose regimen of amoxycillin in treatment of Actinobacillus pleuropneumoniae infection in pigs

The theory of a time-dependent effect of amoxycillin was examined in a model of porcine Actinobacillus pleuropneumoniae (Ap)-infection using clinically relevant dosage regimens. Twenty hours after infection of fourteen pigs, when clinical signs of pneumonia were present, one group of pigs received a single dose of amoxycillin (20 mg/kg, i.m.), whereas another group received four doses of 5 mg/kg injected at 8-h intervals. A similar AUC of the plasma amoxycillin concentration versus time curve was obtained in the two groups, whereas the maximum concentration was threefold higher using the single high dose. Plasma amoxycillin was above the MIC for twice as long using the fractionated dosage scheme. The condition of the animals was evaluated by clinical and haematological observations combined with quantification of biochemical infection markers: C-reactive protein, zinc and ascorbic acid. Within 48 h of treatment, the pigs in both treatment groups recovered clinically. No significant differences in the time-course of clinical observations or plasma concentrations of the biomarkers of infection were observed between the two treatments. In conclusion, the efficacy of these two dosage regimens of amoxycillin was not significantly different in treatment of acute Ap-infection in pigs.

A Mouse Peritonitis Model for the Study of Glycopeptide Efficacy in GISA Infections

In recent years, the emergence of Staphylococcus aureus strains with reduced susceptibility to glycopeptides has raised considerable concern. We studied the efficacy of vancomycin and teicoplanin, as well as cloxacillin and cefotaxime, against the infection caused by four S. aureus strains with different glycopeptide and beta-lactam susceptibilities (strains A, B, C, and D; MICs for vancomycin of 1, 2, 4, and 8 microg/ml respectively), using a modified model of mouse peritonitis. This optimized model appeared to be straightforward and reproducible, and was able to detect low differences in bacterial killing between antibiotics and also between different S. aureus strains. Bactericidal activities in peritoneal fluid for vancomycin, teicoplanin, cloxacillin, and cefotaxime decreased from -2.98, -2.36, -3.22, and -3.57 log(10) cfu/ml, respectively, in infection by strain A (MICs for vancomycin and cloxacillin of 1 and 0.38 microg/ml, respectively) to -1.22, -0.65, -1.04, and +0.24 in peritonitis due to strain D (MICs for vancomycin and cloxacillin of 8 and 1,024 microg/ml). Our data confirm the superiority of beta-lactams against methicillin-susceptible S. aureus and show that bactericidal activity of glycopeptides decreases significantly with slight increases in MICs; this finding suggests a reduced efficacy of glycopeptides in the treatment of serious glycopeptide-intermediate S. aureus infections.

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.

Suboptimal antibiotic dosage as a risk factor for selection of penicillin-resistant Streptococcus pneumoniae: in vitro kinetic model

Optimizing pharmacokinetic/pharmacodynamic indices of antibiotics to obtain clinical and microbiological efficacy is essential, but dosing regimens must also be tailored to minimize the risk for emergence of resistance. The aim of the present study was to investigate whether certain concentrations of benzylpenicillin are critical for the selection of resistant subpopulations. A mixed culture of Streptococcus pneumoniae containing ca. 90% susceptible (MIC = 0.031 mg/liter), 9% intermediate (MIC = 0.25 mg/liter), and 1% resistant (MIC = 8 mg/liter) was studied in an in vitro kinetic model. The time that concentrations exceeded the MIC (T>MIC) for the three strains in the culture was varied by different initial concentrations of benzylpenicillin. Samples for viable counts were withdrawn at different times during 24 h and seeded on blood agar plates and on selective antibiotic-containing plates. The T>MIC varied from 46 to 100% for the susceptible strain, from 6 to 100% for the intermediate strain, and from 0 to 48% for the resistant strain. Our study, which may mimic the clinical situation with carriage of a mixed population of S. pneumoniae with different antibiotic susceptibilities, has shown that selection of resistant bacteria may easily occur if dosing regimens are only targeted toward fully susceptible strains.

Daily variations in ceftriaxone pharmacokinetics in rats

The aim of this study was to determine whether the time of day ceftriaxone was administered modified its pharmacokinetics. Ceftriaxone was given intraperitoneally at either 0400, 1000, 1600, and 2200 h to Sprague-Dawley rats synchronized under a light-dark cycle of 12 h of light and 12 h of dark. Pharmacokinetic parameters were analyzed for the presence of a 24-h rhythm. Results showed significant daily variations (P < 0.05) in ceftriaxone clearance, with the highest values during the dark phase. It is concluded that time-dependent variations in ceftriaxone pharmacokinetics may affect the therapeutic efficacy of current once-daily dosing schedules.

Microbiologic effectiveness of time- or concentration-based dosing strategies in Streptococcus pneumoniae

This in vitro study evaluated the pharmacodynamic performance of levofloxacin using different dosing strategies against both a levofloxacin-sensitive (MIC = 1 mg/liter) and -resistant (MIC = 16 mg/liter) strain of Streptococcus pneumoniae. The strain was genotypically characterized by a mutation in gyrA and two mutations in parE; resistance was shown not to be efflux-mediated. The purpose of this study was to determine if simulated levofloxacin dosing strategies focused either on time or concentration would affect microbiologic outcome. Differing peak concentration/MIC ratios (1,2, and 10), T>MIC (3.6,9.6,15.6, and 24 h corresponding to 15, 40, 65, and 100% of the 24-h dosing interval), and AUC/MIC ratios (13-180) were generated by varying dosing strategies. Initial bacterial inocula were decreased by 99.9% in each experiment conducted. Despite the wide variation in exposure levels, in terms of AUC/MIC, Cp-max/MIC, and T>MIC, the kill portions of the bacterial density curves were super-imposable between all permutations of antibiotic exposure. However, there appeared to be an AUC/MIC breakpoint (35-40) defining bacterial regrowth. Over a 10-fold concentration range, levofloxacin appeared to kill S. pneumoniae in a concentration-independent fashion. When given in concentrations suitable to achieve specified pharmacodynamic endpoints (AUC/MIC >/=35), levofloxacin demonstrated the ability to eradicate both a levofloxacin-resistant and levofloxacin-sensitive strain of S. pneumoniae in the in vitro model.

How predictive is PK/PD for antibacterial agents?

The pharmacodynamic (PD) parameters most often used in studies of antibiotic effect include the following relationships between the antibiotic concentration curve in serum as a surrogate marker for the antibiotic concentration at the infection site, the peak/minimal inhibitory concentration (MIC) ratio, the area under the curve (AUC)/MIC ratio and the duration of time the concentration exceeds the MIC (T(>MIC)). The MIC plays an important role also as a PD marker, and its precision in this respect is discussed. The predictive role of T(>MIC) is important for drugs showing minimal concentration dependent effect such as the beta-lactam antibiotics, the macrolides and others. The time can be calculated as the chronological time measured or as the (cumulative) per cent of the dosing interval covered by the dose. Several clinical studies have confirmed this relationship. It can be deduced from experimental as well as clinical studies that there is a minimal effective time (MET), which needs to be covered by the antibiotic concentration at the site of infection in order to achieve cure. Dosing according to this MET will result in the least antibiotic needed for the shortest duration. In several cases a single dose will suffice to cover the MET. If this is not possible the antibiotic should be dosed in a way, that each dose will surpass the MIC for at least 40-50% of the dosing interval. For antibiotics with a clear concentration-dependent bacterial killing effect the most important pharmacokinetic/pharmacodynamic (PK/PD) index is the peak/MIC ratio (or the AUC/MIC ratio). This is the case for aminoglycosides and fluoroquinolones, and for both classes a peak/MIC ratio of at least 10 within the first 24 h of treatment has been shown to result in around 90% bacteriological as well as clinical cure. One consequence of clinical dosing has been the once-a-day (OD) dosing for aminoglycosides, which is the standard mode of therapy in many countries. Clinical studies in the field of antibacterial PD are still relatively scarce, and much information is needed to enable relevant dosing strategies for all types of antibiotics against all common infections and micro-organisms.

Pharmacokinetics of ceftriaxone administered by the intravenous, intramuscular or subcutaneous routes to dogs

The purpose of this study was to investigate the pharmacokinetics of ceftriaxone after single intravenous (i.v.), intramuscular (i.m.) and subcutaneous (s.c.) doses in healthy dogs. Six mongrel dogs received ceftriaxone (50 mg/kg) by each route in a three-way crossover design. Blood samples were collected in predetermined times after drug administration. Results are reported as mean +/- standard deviation (SD). Total body clearance (Cl(t)) and apparent volume of distribution (V(z)) for the i.v. route were 3.61 +/- 0.78 and 0.217 +/- 0.03 mL/kg, respectively. Terminal half-life harmonic mean (t(1/2 lambda)) was 0.88; 1.17 and 01.73 h for the i.v., i.m and s.c. routes, respectively. Mean peak serum concentration (C(max)) was 115.10 +/- 16.96 and 69.28 +/- 14.55 microg/mL for the i.m and s.c. routes, respectively. Time to reach C(max) (t(max)) was 0.54 +/- 0.24 and 1.29 +/- 00.64 h for the i.m and s.c. routes, respectively. Mean absorption time (MAT) was 1.02 +/- 0.64 and 2.23 +/- 00.73 h for the i.m and s.c. routes, respectively. Bioavailability was 102 +/- 27 and 106 +/- 14% for the i.m and s.c. routes, respectively. Statistically significant differences were determined in C(max), t(max), MAT and t(1/2 lambda) of s.c. administered ceftriaxone when compared with the i.v and i.m. routes. These findings suggest that once or twice s.c. or i.m. daily administered ceftriaxone should be adequate to treat most susceptible infections in dogs.

Prediction of in-vivo efficacy by in-vitro early bactericidal activity with oral beta-lactams, in a dose-ranging immunocompetent mouse sepsis model, using strains of Streptococcus pneumoniae with decreasing susceptibilities to penicillin

Killing curves and a sepsis model were performed with Streptococcus pneumoniae strains (MICs of penicillin = 0.01, 1, 2 and 4 mg/L) to assess the in vivo effect of in vitro early bactericidal activity. Optimal bactericidal concentration (OBC) was defined as the minimal concentration needed to obtain the maximal bactericidal activity during the sampling time for colony counting in killing curves. Animals were treated with amoxycillin, cefuroxime or cefpodoxime every 8 h for 48 h, with doses ranging from 2.5 to 50 mg/kg. ED100 (minimal antibiotic dose obtaining a 100% survival) was used as efficacy endpoint. Cmax/MIC, AUC/MIC and deltaT >MIC did not accurately predict efficacy against the most resistant strains, deltaT >OBC being the most predictive efficacy parameter indicating the in vivo effect of early bactericidal activity. Lower deltaT >OBC values for amoxycilin vs oral cehalosporins were needed for efficacy. The higher early bactericidal activity of amoxycillin may explain its higher in vivo efficacy.

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.

Pharmacodynamics of glycopeptides in the mouse peritonitis model of Streptococcus pneumoniae or Staphylococcus aureus infection

The emergence of resistance to various antibiotics in pneumococci leaves the glycopeptides as the only antibiotics against which pneumococci have no resistance mechanism. This situation has led to a renewed interest in the use of glycopeptides. It has not yet been possible to conclude which one or more of the pharmacokinetic or pharmacodynamic (PK/PD) parameters are the most important and best predictors for the effects of treatment with glycopeptides in animal models or in humans. We used the mouse peritonitis model with immunocompetent mice and with Staphylococcus aureus and Streptococcus pneumoniae as infective organisms. A wide spectrum of different treatment regimens with vancomycin and teicoplanin was tested to study the pharmacodynamics of these drugs. In studies in which the single dose that protected 50% of lethally infected mice (ED(50)) was given as one dose or was divided into two doses, survival was significantly decreased when the dose was divided. The only statistically significant correlations between the percentage of survival of the mice after 6 days and each of the PK/PD parameters were for peak concentration (C(max))/MIC and S. aureus and for the free fraction of C(max) (C(max-free))/MIC and S. pneumoniae. For S. pneumoniae, the ED(50) for different dosing regimens increased with the number of doses given; e.g., the single-dose ED(50)s for vancomycin and teicoplanin were 0.65 and 0. 45 mg/kg, respectively, but the ED(50)s for dosing regimens with 2-h doses given for 48 h were 6.79 and 5.67 mg/kg, respectively. In experiments with 39 different vancomycin dosing regimens and 40 different teicoplanin dosing regimens against S. pneumoniae, the different PK/PD parameters were analyzed using logistic regression. The C(max-free)/MIC was one of two parameters that best explained the effect for both drugs; for vancomycin, the other important parameter was the AUC/MIC, and for teicoplanin, the other parameter was the time the free fraction of the drug is above the MIC. The effect analyzed as a function of C(max-free)/MIC disclosed thresholds with shifts from almost no effect to full effect at ratios of five to six for vancomycin and two to three for teicoplanin.

Imipenem, doxycycline and amikacin in monotherapy and in combination in Acinetobacter baumannii experimental pneumonia

Acinetobacter baumannii is a common cause of nosocomial pneumonia and other nosocomial infections. Multiresistant A. baumannii has also a high prevalence, which can make effective treatment difficult. We designed a new model of A. baumannii experimental pneumonia using C57BL/6 immunocompetent mice. This model was used to compare the efficacy of imipenem, doxycycline and amikacin in monotherapy, and the combination of imipenem plus amikacin and doxycycline plus amikacin. Doxycycline plus amikacin were synergic in vitro after 24 h incubation, whereas imipenem plus amikacin showed no in vitro synergy. The number of sterile lungs and the lung clearance of A. baumannii were greater in the group treated with imipenem than in those treated with amikacin or doxycycline in monotherapy (P < 0.05). The combination of imipenem plus amikacin and doxycycline plus amikacin was no more effective than imipenem alone in the clearance of organisms from lungs (2.42 +/- 1.46 cfu/g versus 2.7 +/- 1.5 cfu/g versus 1.23 +/- 1.02 cfu/g). These results suggest that the addition of amikacin does not improve the results obtained by imipenem monotherapy. Doxycycline plus amikacin is an alternative to imipenem in the therapy of A. baumannii pneumonia.

Treatment of pneumococcal pneumonia: the case for penicillin G

Although widely endorsed for specific treatment of pneumococcal pneumonia, penicillin G is seldom used for this purpose in clinical practice for at least three reasons: (1) concern about penicillin-resistant Streptococcus pneumoniae (PRSP) strains; (2) the difficulty of making an early etiologic diagnosis of pneumonia; and (3) lack of a clear consensus about the optimum dosage. Continuous infusion of 20-24 million units of penicillin per day provides serum levels of 16-20 microg/mL in persons with normal renal function. These levels easily exceed the minimum inhibitory concentrations (MICs) of penicillin G against most PRSP strains (4 microg/mL), which are actually strains with reduced susceptibility to penicillin. High-dose penicillin G therapy has not been shown to be therapeutically ineffective against pneumonia due to PRSP strains. However, the extent of penicillin resistance warrants continued monitoring, because strains exhibiting extremely high-level resistance (MIC > or = 8 microg/mL) would probably respond poorly if at all. Development and use of rapid, sensitive, specific ways to diagnose pneumococcal pneumonia could extend the usefulness of penicillin G, thus postponing the emergence of resistance to other antibiotics.

Comparison of pharmacodynamics of azithromycin and erythromycin in vitro and in vivo

In this study, we determined the efficacy of various dosing regimens for erythromycin and azithromycin against four pneumococci with different susceptibilities to penicillin in an in vitro pharmacokinetic model and in a mouse peritonitis model. The MIC was 0.03 microg/ml, and the 50% effective doses (determined after one dose) of both drugs were comparable for the four pneumococcal strains and were in the range of 1.83 to 6.22 mg/kg. Dosing experiments with mice, using regimens for azithromycin of one to eight doses/6 h, showed the one-dose regimen to give the best result; of the pharmacodynamic parameters tested (the maximum drug concentration in serum [Cmax], the times that the drug concentration in serum remained above the MIC and above the concentration required for maximum killing, and the area under the concentration time curve), Cmax was the best predictor of outcome. The bacterial counts in mouse blood or peritoneal fluid during the first 24 h after challenge were not correlated to survival of the mice. The serum concentration profiles obtained with mice for the different dosing regimens were simulated in the in vitro pharmacokinetic model. Here as well, the one-dose regimen of azithromycin showed the best result. However, the killing curves in vivo in mouse blood and peritoneal fluid and in the vitro pharmacokinetic model were not similar. The in vitro killing curves showed a decrease of 2 log10 within 2 and 3 h for azithromycin and erythromycin, respectively whereas the in vivo killing curves showed a bacteriostatic effect for both drugs. It is concluded that the results in terms of predictive pharmacodynamic parameters are comparable for the in vitro and in vivo models and that high initial concentrations of azithromycin favor a good outcome.

Amoxicillin middle ear fluid penetration and pharmacokinetics in children with acute otitis media

BACKGROUND

Acute otitis media (AOM) is a common childhood infectious disease. The efficacy of antibiotic dosing regimens is usually assessed by antibiotic plasma pharmacokinetics or middle ear fluid (MEF) concentration at one or two time points. Viral coinfection in AOM reduced antibacterial efficacy of antibiotics.

OBJECTIVE

To determine amoxicillin MEF penetration and pharmacokinetics in bacterial and combined bacterial and viral AOM.

METHODS

Thirty-four children with AOM were enrolled, and MEF was collected by tympanocentesis for bacterial culture and viral studies. Nasal wash and venous blood were also obtained for viral culture and serologic studies, respectively. Subjects were treated with amoxicillin 40 mg/kg/day orally, divided in equal doses every 8 h. During the second visit (48 to 72 h later) the subjects, with the regular morning amoxicillin dose withheld, were given an oral amoxicillin dose of 25 mg/kg. Thereafter two blood samples and one MEF sample by tympanocentesis were collected from each child at selected times between 0.5 and 4.0 h after dosing for bacterial and viral studies and amoxicillin concentration determination by high performance liquid chromatography.

RESULTS

Eleven (37%) children had only bacterial infection, 6 (20%) had viral infection only, 6 (20%) had both bacterial and viral infections and in 7 (23%) neither bacterial nor viral pathogens were recovered. MEF bacterial culture was positive in 23 of 40 ears (57.5%) before treatment with amoxicillin (40 mg/kg/day) and was still positive in 4 of 38 ears (10.5%) after 2 to 3 days of treatment. Amoxicillin plasma concentration reached its peak at 1.0 to 1.5 h after a 25-mg/kg oral dose. The estimated MEF concentration peak occurred 3.0 h after the dose with MEF concentrations ranging from undetectable to 20.6 microg/ml and a mean of approximately 9.5 microg/ml. Geometric mean amoxicillin concentrations were lowest in virus-infected children (2.7 microg/ml), nearly the same in culture-negative samples from children without viral infection (2.9 microg/ml), higher in children with combined bacterial and viral infection (4.1 microg/ml) and highest in children with bacterial-only infection (5.7 microg/ml).

CONCLUSIONS

MEF amoxicillin penetration tended to be lower in children with viral infection. The current amoxicillin dosing recommendation of 40 mg/kg/day in three divided dose is inadequate to effectively eradicate resistant Streptococcus pneumoniae, particularly during viral coinfection. A dosing regimen of 75 to 90 mg/kg/day is recommended for AOM.

Pharmacodynamics and bactericidal activity of ceftriaxone therapy in experimental cephalosporin-resistant pneumococcal meningitis

Adequate concentrations of beta-lactam antibiotics in cerebrospinal fluid (CSF) are difficult to achieve for meningitis caused by drug-resistant Streptococcus pneumoniae. Ceftriaxone in dosages of 150 or 400 mg/kg of body weight per day, given in one or two doses, was used for the treatment of experimental highly cephalosporin-resistant (MIC and MBC, 4 microg/ml) pneumococcal meningitis. The bacterial killing rate (delta log10 CFU per milliliter per hour) and pharmacokinetic indices, including percentage of time the antibiotic concentration exceeded the MBC during a 24-h period (T>MBC), CSF peak concentration above the MBC, and area under the concentration-time curve from 0 to 24 h above MBC, were measured and correlated. By multiple stepwise regression, only T>MBC independently predicted the bacterial killing rate. There was a direct linear correlation between T>MBC in CSF and the bacterial killing rate during the first 24 h of therapy (r = 0.87; P = 0.004). Sterilization of CSF was achieved only when the T>MBC was 95 to 100%. In the first 24 h, the 200-mg/kg/12-h regimen, compared with the 400-mg/kg/24-h regimen, was associated with a greater T>MBC (87% +/- 10% versus 60% +/- 22%; P = 0.03) and greater bacterial killing rate (0.2 +/- 0.04 versus 0.13 +/- 0.07; P = 0.003), confirming that ceftriaxone exhibits time-dependent bactericidal activity. After 24 h, the T>MBC and the CSF sterilization rates were similar whether ceftriaxone was given once or twice daily.

Activities of vancomycin and teicoplanin against penicillin-resistant pneumococci in vitro and in vivo and correlation to pharmacokinetic parameters in the mouse peritonitis model

The activities of glycopeptides against pneumococci were studied in vitro and in vivo. The MICs of two glycopeptides, vancomycin and teicoplanin, in different media against 10 strains of pneumococci with different susceptibilities to penicillin were determined. The MICs of teicoplanin were four times lower than those of vancomycin in Mueller-Hinton media supplemented with 5% blood, but the MICs were similar in mouse and human sera supplemented with 5% blood. The serum protein binding levels in mouse and human sera were 90% for teicoplanin in both and 25 and 35%, respectively, for vancomycin. The MICs for vancomycin and teicoplanin were only correlated in human serum (P < 0.001). The single doses giving protection to 50% of the animals in the mouse peritonitis model after a lethal challenge of pneumococci, the ED50s, were similar for vancomycin and teicoplanin, between 0.1 and 1 mg/kg of body weight for all 10 strains. The log ED50s were significantly correlated only to the log MICs of teicoplanin determined for mouse serum with 5% blood (P = 0.01) and to the log MICs of vancomycin determined by the E test (P = 0.03). Among the pharmacokinetic parameters analyzed at the ED50s, the most constant parameter was the time for which the drug concentration exceeded the MIC (T(>MIC)) when each drug was considered separately; however, when both drugs were considered together, the maximum concentration of drug in serum (Cmax) varied the least. This indicates that both these parameters are of importance for predicting the effect of the drugs. We conclude that the effect of glycopeptides was not influenced by the penicillin resistance of the pneumococci, either in vitro or in vivo, and that the superior activity of teicoplanin over that of vancomycin in vitro was abolished in vivo, an effect which probably was due to the high serum protein binding of teicoplanin. Both the pharmacokinetic parameters T(>MIC) and Cmax are important predicting the effect of glycopeptides, but the pharmacodynamics of glycopeptides are still not completely elucidated.

Treatment of experimental pneumonia due to penicillin-resistant Streptococcus pneumoniae in immunocompetent rats

A model of pneumonia due to Streptococcus pneumoniae resistant to penicillin was developed in immunocompetent Wistar rats and was used to evaluate the efficacies of different doses of penicillin, cefotaxime, cefpirome, and vancomycin. Adult Wistar rats were challenged by intratracheal inoculation with 3 x 10(9) CFU of one strain of S. pneumoniae resistant to penicillin (MICs of penicillin, cefotaxime, cefpirome, and vancomycin, 2, 1, 0.5, and 0.5 microg/ml, respectively) suspended in brain heart broth supplemented with 0.7% agar. The rats experienced a fatal pneumonia, dying within 5 days and with peak mortality (70 to 80%) occurring 48 to 72 h after infection, and the bacterial counts in the lungs persisted from 8.87 +/- 0.3 log10 CFU/g of lung at 24 h of the infection to 9.1 +/- 0.3 log10 CFU/g at 72 h. Four hours after infection the animals were randomized into the following treatment groups: (i) control without treatment, (ii) penicillin G at 100,000 IU/kg of body weight every 2 h, (iii) penicillin G at 250,000 IU/kg every 2 h, (iv) cefotaxime at 100 mg/kg every 2 h, (v) cefpirome at 200 mg/kg every 2 h, and (vi) vancomycin at 50 mg/kg every 8 h. Two different protocols were used for the therapeutic efficacy studies: four doses of beta-lactams and one dose of vancomycin or eight doses of beta-lactams and two doses of vancomycin. Results of the therapy for experimental pneumonia caused by penicillin-resistant S. pneumoniae showed that initially, all the antimicrobial agents tested had similar efficacies, but when we prolonged the treatment, higher doses of penicillin, cefotaxime, and cefpirome were more effective than penicillin at lower doses in decreasing the residual bacterial titers in the lungs. Also, when we extended the treatment, vancomycin was more efficacious than penicillin at lower doses but was less efficacious than higher doses of penicillin or cefpirome. The model that we have developed is simple and amenable for inducing pneumonia in immunocompetent rats and could be used to explore the pathophysiology and to evaluate optimal therapy of this infection in the immunocompetent host.

Comparative serum bactericidal activities of ceftizoxime and cefotaxime against intermediately penicillin-resistant Streptococcus pneumoniae

In a randomized crossover study involving 12 healthy volunteers, 1 g of ceftizoxime or cefotaxime was administered intravenously every 12 h for a total of three doses on two separate weekends. The duration of serum bactericidal titers (SBTs) greater than 1:2 and the time serum drug concentrations remained above the MIC (T > MIC) were determined against three clinical isolates of Streptococcus pneumoniae with intermediate resistance to penicillin. The duration of SBTs and T > MIC for both antimicrobial agents exceeded 50% of the dosing interval for all isolates. Ceftizoxime's T > MIC was statistically greater than that of cefotaxime, indicating that its longer half-life in serum (1.7 h) compared with that of cefotaxime (approximately 1 h) compensates for its slightly lower microbiologic activity against the penicillin-resistant pneumococci tested in this study.

Impaired bacteriologic response to oral cephalosporins in acute otitis media caused by pneumococci with intermediate resistance to penicillin

BACKGROUND

Penicillin resistance of Streptococcus pneumoniae, one of the most common causes of acute otitis media, has recently increased and is now highly prevalent in many regions. However, its contribution to clinical failure still must be proved. Because the role of antibiotics in acute otitis media is to eradicate the pathogens present in the middle ear fluid, we conducted a randomized controlled study to determine bacterial eradication of pathogens in acute otitis media by two commonly used oral cephalosporins, cefuroxime axetil (30 mg/kg/day) and cefaclor (40 mg/kg/day).

METHODS

Patients 6 to 36 months old with pneumococcal otitis media seen in the Pediatrics Emergency Room were studied. An initial middle ear fluid culture was obtained at enrollment, and a second culture was obtained on Day 4 or 5 during treatment. Follow-up was done also on Days 10, 17 and 42 after initiation of treatment. In cases of clinical relapse a third culture was obtained.

RESULTS

In total 78 patients were enrolled, 41 in the cefuroxime axetil group and 37 in the cefaclor group. Of the 78 S. pneumoniae isolates 31 (40%) were intermediately penicillin-resistant (MIC 0.125 to 1.0 microgram/ml). Of the 47 patients with penicillin-susceptible organisms 3 (6%) had bacteriologic failure vs. 4 of 19 (21%) and 7 of 11 (64%) of those with MIC of 0.125 to 0.25 microgram/ml and 0.38 to 1.0 microgram/ml, respectively (P < 0.001). For intermediately resistant pneumococci, in 7 of 12 (58%) of those receiving cefaclor the isolate was not eradicated vs. only 4 of 19 (21%) of those receiving cefuroxime axetil (P = 0.084). MIC to the administered cephalosporin of > 0.5 microgram/ml was associated with bacteriologic failure. Clinical failure was observed in 9 of 14 (64%) patients with bacteriologic failure vs. 10 of 52 (19%) patients with bacteriologic eradication (P = 0.003).

CONCLUSION

Intermediately penicillin-resistant S. pneumoniae is associated with an impaired bacteriologic and clinical response of acute otitis media to cefaclor and cefuroxime axetil. This effect was more pronounced with cefaclor than with cefuroxime axetil.

In vivo activity and pharmacodynamics of amoxicillin in combination with fosfomycin in fibrin clots infected with highly penicillin-resistant Streptococcus pneumoniae

Using a clinical pneumococcal strain for which MICs were 4, 2, and 32 mg/liter for penicillin, amoxicillin, and fosfomycin, respectively, we studied the efficacies of these antibiotics alone and their combinations in the treatment of prolonged (48-h) experimental fibrin clot infection in rabbits. Treatments were as follows: amoxicillin IV at 20 mg/kg of body weight in one dose (Amo20), 50 mg/kg in one dose (Amo50), or two doses 6 h apart (Amo20 x 2 and Amo50 x 2); fosfomycin IV at a fixed dose of 50 mg/kg in one dose (Fos50) or two divided doses 6 h apart (Fos50 x 2); or the combinations of amoxicillin and fosfomycin with the same schedules. Maximum concentrations in clots were 2.03 +/- 1.02 and 2.13 +/- 0.33 mg/liter for Amo20 regimens, 3.7 +/- 1.9 and 4 +/- 1.3 mg/liter for Amo50 regimens, and 24 +/- 7 and 40 +/- 8 mg/liter for fosfomycin regimens, respectively. The mean half-lives of elimination from clots were between 2 and 3 h for amoxicillin regimens and between 5 and 7 h for fosfomycin. We observed the highest bacterial reductions (log10 CFU/gram) for Amo50 in two divided doses with or without fosfomycin. A significantly higher bacterial reduction than that with each monotherapy was observed when Amo20 was combined with fosfomycin in either one dose or two doses 6 h apart (0.16 +/- 0.8 and 1.64 +/- 1.6 log10 CFU/g for Amo20 in one and two doses, respectively, and 0.93 +/- 0.81 and 0.61 +/- 0.56 log10 CFU/g for fosfomycin in one and two doses, respectively, versus 3.46 +/- 1.26 and 3.16 +/- 1.31 log10 CFU/g for Amo20 plus fosfomycin in one and two doses, respectively [P < 0.001]). A time-dependent effect was observed with amoxicillin regimens. The time of regrowth was significantly delayed when amoxicillin was combined with fosfomycin. By using a multivariate analysis, we demonstrated that the most important parameter correlated to efficacy of the combination amoxicillin-fosfomycin was the length of the period during which the concentration of amoxicillin remained above the MIC. We demonstrated that the in vivo efficacy of the combination of amoxicillin and fosfomycin gave higher antibacterial effect than each monotherapy.

Noncompromised penicillin-resistant pneumococcal pneumonia CBA/J mouse model and comparative efficacies of antibiotics in this model

The present study confirms that CBA/J mice are susceptible to several clinical isolates of Streptococcus pneumoniae, including four of five penicillin-susceptible and all five penicillin-resistant strains tested, thus providing the first noncompromised animal model for penicillin-resistant S. pneumoniae pneumonia. In this model, doses of penicillin G of 0.6 mg/kg of body weight given six times at 1-h intervals produced effective pulmonary clearance of a penicillin-susceptible strain (penicillin G MIC, 0.015 microgram/ml), while doses of 40 mg/kg given six times at 1-h intervals were required to clear a penicillin-resistant strain (penicillin G MIC, 1 microgram/ml). Imipenem (MIC, 0.25 microgram/ml) was the most active antibiotic tested against the penicillin-resistant strain, with a calculated dose of 0.42 mg/kg given six times at 1-h intervals, resulting in a 2-log decrease in the number of pulmonary bacteria. Comparable effects were seen with vancomycin (MIC, 0.5 microgram/ml), cefotaxime (MIC, 0.5 microgram/ml), and penicillin G at doses of 3.3, 5.5, and 31.0 mg/kg given six times at 1-h intervals, respectively. The pharmacokinetic profile of vancomycin in infected lungs was superior to those of the other antibiotics, especially in regard to the elimination half-life (215.4 min for vancomycin versus 15.0, 14.5, and 14.5 min for penicillin G, cefotaxime, and imipenem, respectively). Both imipenem and vancomycin allowed 90% survival when 40-mg/kg doses were administered twice a day beginning 5 days after infection. Survival rates with penicillin G (160-mg/kg doses) and cefotaxime (40-mg/kg doses) were 40 and 30%, respectively, while no saline-treated mice survived. The present study shows that the CBA/J mouse pneumonia model may be useful for evaluating antibiotic efficacies against penicillin-resistant pneumococcal pneumonia in immunocompetent individuals. Our data suggest that imipenem and vancomycin may be the most active agents against penicillin-resistant S. pneumoniae pneumonia.