Beta-lactam activity against penicillin-resistant Streptococcus pneumoniae strains exhibiting higher amoxicillin versus penicillin minimum inhibitory concentration values: an in vitro pharmacodynamic simulation

BACKGROUND

Activity of simulated serum concentrations after oral therapy with 400 mg cefditoren pivoxil b.i.d., 500 mg cefuroxime axetil b.i.d. and 875/125 mg amoxicillin/clavulanic acid b.i.d. and t.i.d. regimens was explored over 24 h against Streptococcus pneumoniae.

METHODS

Computerized pharmacodynamic simulations were performed against strains with penicillin/amoxicillin/cefuroxime/cefditoren minimum inhibitory concentrations (MICs, microg/ml) and serotypes: strain 1 (0.25/0.12/1/0.12; serotype 6A), strain 2 (2/4/ 2/0.25; serotype 6B), strain 3 (4/16/4/0.5; serotype 14), and strain 4 (4/16/8/1; serotype 14).

RESULTS

Bactericidal activity (> or =3 log(10) reduction) at 12 and 24 h was obtained against all strains with cefditoren, against strains 1 and 2 with cefuroxime and amoxicillin/clavulanic acid t.i.d., but only against strain 1 with amoxicillin/clavulanic acid b.i.d.. Bactericidal activity at 24 h was related to T > MIC of >30% dosing interval, 1.7-2.0 log(10) reductions with T > MIC of 20-30%, and <1 log(10) reduction or regrowth with T > MIC of 0%.

CONCLUSIONS

It is difficult to achieve pharmacodynamic coverage and bactericidal activity by physiological concentrations of oral beta-lactams against penicillin-resistant pneumococcal strains exhibiting higher amoxicillin versus penicillin MICs. Cefditoren may offer alternatives.

Levofloxacin vs. azithromycin pharmacodynamic activity against S. pneumoniae and H. influenzae with decreased susceptibility to amoxicillin/clavulanic acid

Resistant clones/phenotypes are putting into question the activity of commonly used beta-lactams, thus prompting the need for alternative options. A 500 mg levofloxacin vs. azithromycin once daily pharmacodynamic simulation was performed against 10(8) cfu/ml of four Streptococcus pneumoniae strains (exhibiting higher amoxicillin than penicillin MIC) and four Haemophilus influenzae strains: beta-lactamase producing, BLNAR (beta-lactamase-negative ampicillin-resistant) and BLPACR (beta-lactamase-positive amoxicillin/clavulanate-resistant). High levofloxacin AUC/MIC values for H. influenzae, and values of 50-100 for S. pneumoniae produced a >5 log(10) reduction at 24h for all strains. Azithromycin AUC/MIC values of approximately 10 were needed to obtain a 2-3 log(10) reduction of S. pneumoniae initial inocula, but lower AUC/MIC values (of approximately 6) obtained > or =3 log(10) reduction against all strains of H. influenzae. While in vitro simulated serum concentrations of levofloxacin were bactericidal at the end of the dosing interval against all S. pneumoniae strains and azithromycin against the susceptible ones, both antimicrobials achieved this endpoint against the BLNAR and BLPACR strains.

influence of TEM-1 beta-lactamase on the pharmacodynamic activity of simulated total versus free-drug serum concentrations of cefditoren (400 milligrams) versus amoxicillin-clavulanic acid (2,000/125 milligrams) against Haemophilus influenzae strains exhibiting an N526K mutation in the ftsI gene

The aim of this study was to explore bactericidal activity of total and free serum simulated concentrations after the oral administration of cefditoren (400 mg, twice daily [bid]) versus the oral administration of amoxicillin-clavulanic acid extended release formulation (2,000/125 mg bid) against Haemophilus influenzae. A computerized pharmacodynamic simulation was performed, and colony counts and beta-lactamase activity were determined over 48 h. Three strains were used: ampicillin-susceptible, beta-lactamase-negative ampicillin-resistant (BLNAR) (also resistant to amoxicillin-clavulanic acid) and beta-lactamase-positive amoxicillin-clavulanic acid-resistant (BLPACR) strains, with cefditoren MICs of < or =0.12 microg/ml and amoxicillin-clavulanic acid MICs of 2, 8, and 8 microg/ml, respectively. Against the ampicillin-susceptible and BLNAR strains, bactericidal activity (> or =3 log(10) reduction) was obtained from 6 h on with either total and free cefditoren or amoxicillin-clavulanic acid. Against the BLPACR strain, free cefditoren showed bactericidal activity from 8 h on. In amoxicillin-clavulanic acid simulations the increase in colony counts from 4 h on occurred in parallel with the increase in beta-lactamase activity for the BLPACR strain. Since both BLNAR and BLPACR strains exhibited the same MIC, this was due to the significantly lower (P < or = 0.012) amoxicillin concentrations from 4 h on in simulations with beta-lactamase positive versus negative strains, thus decreasing the time above MIC (T>MIC). From a pharmacodynamic point of view, the theoretical amoxicillin T>MIC against strains with elevated ampicillin/amoxicillin-clavulanic acid MICs should be considered with caution since the presence of beta-lactamase inactivates the antibiotic, thus rendering inaccurate theoretical calculations. The experimental bactericidal activity of cefditoren is maintained over the dosing interval regardless of the presence of a mutation in the ftsI gene or beta-lactamase production.

Effects of human albumin and serum on the in vitro bactericidal activity of cefditoren against penicillin-resistant Streptococcus pneumoniae

OBJECTIVES

Attempts to interpret antibiotic pharmacodynamics using reported protein binding may underestimate true activity. To elucidate this issue we examined bacterial killing kinetics at cefditoren concentrations equal to C(max) in the presence of 90% human serum or albumin at physiological concentrations.

METHODS

Killing curves (final inocula of approximately 10(7) cfu/mL, cefditoren concentration of 4.2 mg/L) were performed against Streptococcus pneumoniae strains exhibiting cefditoren MICs (mg/L) of 0.12 (strain 1), 0.25 (strain 2) and 0.5 (strain 3) in different media: (i) C(max)-MH, Mueller-Hinton broth plus 5% lysed horse blood (MH), (ii) C(max)-HS, MH broth with a final human serum concentration of 90%; and (iii) C(max)-HAlb, MH broth with 4 g/dL human albumin. Killing curves were also performed with a final cefditoren concentration of 0.5 mg/L (similar to free-drug C(max) considering 88% protein binding) in MH broth (12% C(max)).

RESULTS

No significant differences were found between the different media or concentrations with strain 1 (log(10) reductions >or=4 at 24 h). Against strains 2 and 3, we observed significantly higher initial inocula decreases at 24 h for C(max)-HS as compared with C(max)-HAlb or 12% C(max). Bactericidal activity (>or=3 log(10) reduction) was obtained at 24 h against the three strains only with C(max)-HS and C(max)-MH.

CONCLUSIONS

The presence of physiological concentrations of human albumin, but not 90% human serum, limited bactericidal activity as did the use of concentrations similar to free-drug C(max), suggesting that extrapolation of active drug from total drug by using the protein binding rate is not an accurate method to study antibacterial activity.

Are β-lactam breakpoints adequate to define non-susceptibility for all Haemophilus influenzae resistance phenotypes from a pharmacodynamic point of view?

OBJECTIVES

To investigate the bactericidal activity, against Haemophilus influenzae strains exhibiting different resistance phenotypes, of simulated serum concentrations obtained in humans after administration of 400 mg of cefditoren twice daily, 500 mg of cefuroxime twice daily, 875/125 mg of co-amoxiclav twice daily or 875/125 mg of co-amoxiclav three times daily.

METHODS

An in vitro computerized pharmacodynamic simulation was carried out and colony counts determined over 24 h. Four H. influenzae strains were used, one ampicillin-susceptible strain (Strain 1) and three ampicillin-resistant strains following CLSI and BSAC breakpoints: one beta-lactamase-positive strain with an MIC of co-amoxiclav of 0.5 mg/L (Strain 2), one beta-lactamase-negative ampicillin-resistant strain (BLNAR; ampicillin MIC = 16 mg/L) (Strain 3) and one beta-lactamase-positive strain with an MIC of co-amoxiclav of 4 mg/L (Strain 4). All strains were susceptible to cefuroxime and co-amoxiclav according to current CLSI breakpoints, but Strains 3 and 4 were resistant according to BSAC breakpoints. All strains exhibited cefditoren MIC <or= 0.12 mg/L.

RESULTS

Bacterial counts of Strains 1 and 2 were >or= 6 log(10) reduced with all antibiotics tested at 12 and 24 h. Against Strains 3 and 4, log(10) reductions at 12 and 24 h were significantly higher for cefditoren versus cefuroxime (P < 0.01) (although both exhibited bactericidal activity, i.e. >or= 3 log(10) reduction) and versus the two co-amoxiclav regimens (P < 0.001) (that exhibited negligible initial inocula reductions).

CONCLUSIONS

Cefditoren exhibited the highest bactericidal activity maintained over time against ampicillin-resistant H. influenzae, regardless of beta-lactamase production and/or BLNAR phenotype. From the pharmacodynamic perspective, BSAC breakpoints seem more adequate to define or detect BLNAR strains.

[Influence of the displacement of protein binding by ibuprofen in the activity of a third-generation cephalosporin against Streptococcus pneumoniae]

The clinical significance of protein binding remains to be fully elucidated. The aim of this study was to evaluate the effect in the in vitro bactericidal activity of cefditoren through killing curves at Cmax concentrations against three Streptococcus pneumoniae strains (cefditoren MICs of 0.12, 0.25 and 0.5 mg/l) with or without human albumin (4 g/dl) and ibuprofen at Cmax concentrations (32.3 mg/l) and 10 times the Cmax (323 mg/l). Cefditoren was rapidly bactericidal (3 log(10) CFU/ml reduction) against the three strains at 4.2 mg/l concentration in Mueller-Hinton broth plus 5% lysed horse blood. In presence of human albumin, this effect was maintained against the most susceptible strain (MIC = 0.12 mg/l). Regrowths were observed with higher MIC values. The presence of ibuprofen (32.3 mg/l) slightly delayed regrowth while the increase of ibuprofen concentration up to 10 x Cmax recovered the bactericidal activity against all strains. The activity of an antimicrobial with high protein binding should not be linked exclusively with the theoretical unbound fraction extrapolated from the plasma concentration. The role of protein binding antagonists merits analysis due to their frequent use associated with cephalosporins in respiratory tract infections.

A pharmacodynamic approach to antimicrobial activity in serum and epithelial lining fluid against in vivo-selected Streptococcus pneumoniae mutants and association with clinical failure in pneumonia

OBJECTIVES

Emergence of resistance may be prevented by killing both the parental infecting strain and subsequent less susceptible step-mutants. The present study analyses eradication and resistance selection in Streptococcus pneumoniae with moxifloxacin, levofloxacin and azithromycin, using a parental serotype 3 clinical strain (strain A) and its correspondent step-mutant derivatives resistant to these antibiotics (B, C, D), which were selected in vivo in a patient with pneumonia.

METHODS

Moxifloxacin, levofloxacin and azithromycin MICs were 1, 2 and 0.5 mg/L for the parental strain; 4, 16 and 4 mg/L for isolate B; and 4, 16 and >128 mg/L for isolates C and D, respectively. A pharmacokinetic computerized device was used to simulate serum and epithelial lining fluid (ELF) concentrations. Initial inoculum was approximately 10(8) cfu/mL. Population analysis profiles were performed using plates with increasing antimicrobial concentrations.

RESULTS

In ELF simulations, moxifloxacin showed a bactericidal pattern against all isolates with a minority (approximately 100 cfu/mL) of the surviving population (isolates B, C and D) growing on plates with moxifloxacin concentrations just above those in ELF. Levofloxacin and azithromycin showed a bactericidal pattern only against isolate A, with the whole population of isolates B, C and D growing on plates with levofloxacin concentrations higher (16-64 mg/L) than those in ELF and in plates with azithromycin concentrations as high as 2048 mg/L (for isolates C and D).

CONCLUSIONS

Antimicrobial activity in pulmonary tissue against possible emerging resistant mutants during pneumonia treatment may prevent failures more than the solely activity against the S. pneumoniae parental infecting strain.

Effect of opsonophagocytosis mediated by specific antibodies on the co-amoxiclav serum bactericidal activity against Streptococcus pneumoniae after administration of a single oral dose of pharmacokinetically enhanced 2000/125 mg co-amoxiclav to healthy volunteers

OBJECTIVES

To measure the effect of opsonophagocytosis mediated by complement activated by specific antibodies on the co-amoxiclav serum bactericidal activity against Streptococcus pneumoniae strains with reduced susceptibility to beta-lactams (amoxicillin MICs of 2, 4, 8 and 16 mg/L).

METHODS

An open Phase I study measuring ex vivo bactericidal activity after anti-pneumococcal vaccination and an oral dose of 2000/125 mg sustained-release co-amoxiclav was carried out. The ex vivo bactericidal activity was investigated through killing curves over 3 h [assuring polymorphonuclear neutrophil (PMN) viability] with serum samples collected 1.5 h and 5 h after dosing. Global killing was measured as the area under the killing curve (AUKC; log cfu x h/mL). The AUKC of the control growth curve of S. pneumoniae in Hanks' balanced salt solution (AUKC(K)) was used as control. The effect of the presence of complement and/or PMN was evaluated by the difference in the AUKC(K) and the different AUKCs obtained in the presence of inactivated serum (AUKC(IS)), active serum (AUKC(S)), inactivated serum plus PMN (AUKC(IS+PMN)) and active serum plus PMN (AUKC(S+PMN)).

RESULTS

Significant differences were found in all cases between the bactericidal activity of active serum+PMN (AUKC(K) - AUKC(S+PMN)) and that of inactivated serum (AUKC(K) - AUKC(IS)) with therapeutic indexes ranging from 0.56 to 3.04. At 1.5 h after dosing, a significantly higher bactericidal activity of co-amoxiclav was obtained when opsonophagocytosis occurred (samples with active serum and PMN) than when not occurring (killing curves with inactivated serum and without PMN), for all amoxicillin non-susceptible strains.

CONCLUSIONS

The results of this ex vivo study suggest that the collaboration of co-amoxiclav and complement-mediated opsonophagocytosis activated by specific antibodies may lay new approaches to overcome in vivo amoxicillin non-susceptibility.

Is there a pharmacodynamic need for the use of continuous versus intermittent infusion with ceftazidime against Pseudomonas aeruginosa? An in vitro pharmacodynamic model

OBJECTIVES

In order to explore the pharmacodynamic need for continuous versus intermittent (three times a day) administration of ceftazidime in critically ill patients, a pharmacokinetic computerized device was used to simulate concentrations of ceftazidime in human serum after 6 g/day.

METHODS

Efficacy was measured as the capability of simulated concentrations over time to reduce initial inoculum against four strains of Pseudomonas aeruginosa. MICs of the strains matched NCCLS breakpoints: one susceptible strain (MIC = 8 mg/L), two intermediate strains (MIC = 16 mg/L) and one resistant strain (MIC = 32 mg/L). C(max) was 119.97+/-2.53 mg/L for intermittent bolus and C(ss) (steady-state concentration) was 40.38+/-0.16 mg/L for continuous infusion. AUC(0-24) was similar for both regimens (approximately 950 mg x h/L). Inhibitory quotients were three times higher for the intermittent administration whereas t > MIC was higher for continuous infusion (100%) versus intermittent administration (99.8%, 69% and 47.6% for the susceptible, intermediate and resistant strains, respectively).

RESULTS

Against the susceptible and intermediate strains, no differences were found between both regimens with > or = 3 log10 reduction from 8 to 24 h. Against the resistant strain, only the continuous infusion achieved this bactericidal activity in the same time period, minimizing the differences between resistant and susceptible strains. Significantly higher initial inoculum reduction at 32 h was obtained for the continuous versus the intermittent administration (83.35% versus 38.40%, respectively).

CONCLUSIONS

These results stress the importance of optimizing t >MIC, even at peri-MIC concentrations, of ceftazidime against resistant strains. Local prevalence of resistance justifies, on a pharmacodynamic basis, electing for continuous infusion versus intermittent administration.

Evidence for estrogen receptor expression in germ cell and somatic cell subpopulations in the ovary of the newly hatched chicken

Estrogens are involved in the gonadal morphogenesis of vertebrates, and almost all hormonal effects of 17beta-estradiol are mediated through specific receptors. At the time of sexual differentiation in the chicken, or even before, there is evidence of the presence of estrogen receptors and the secretion of 17beta-estradiol. However, no information is available regarding the cellular types that express the estrogen receptor in the immature chick ovary. The present study analyzes estrogen receptor expression in germ and somatic cells of the ovary in the newly hatched chicken. Highly purified cell subpopulations of germ and somatic cells were evaluated for specific 17beta-estradiol nuclear binding. In addition, the estrogen receptor was localized at the ultrastructural level by the immunogold technique. Finally, reverse transcription and polymerase chain reaction procedures detected a steady-state level of mRNA for the estrogen receptor. Somatic cells including typical steroidogenic cells showed specific 17beta-estradiol nuclear binding, displayed the estrogen receptor, and possessed estrogen receptor transcripts. The same result was observed in primary oocytes, together with the ultrastructural localization of estrogen receptor in extended chromatin filaments. Our experimental data support the hypothesis that estrogens are involved in the function of somatic and germ cells subpopulations in the immature chicken ovary.

Correlation of the changes of the frequency of perichromatin granules with the RNA content of the interchromatin region of uterine cells in normal and ovariectomized rats. A high resolution in situ hybridization and stereological study

The changes in the number of perichromatin granules (PCG) and the alterations in the RNA content of the interchromatin and perichromatin regions caused by ovariectomy and estradiol injection were studied in rat endometrial fibroblast and myometrial muscle cells. Twelve rats were divided in four groups. A group of rats was fixed without any treatment, the other three groups were ovariectomized and processed 21 days after the operation. One of them was studied without further treatment, and two groups were injected intraperitoneally with 20 micrograms of 17 beta-estradiol hemisuccinate and fixed 0.5 and 2 h after the injection. The frequency of PCG was evaluated in preparations stained with EDTA procedure preferential for RNP. The alterations of RNA content were estimated by post-embedding high resolution in situ hybridization using a total DNA probe labeled with biotinilated nucleotides revealed by streptavidin coupled with 10 nm gold grains. Most of the non-nucleolar labeling is associated to RNP containing fibrils. Perichromatin and interchromatin granules are labeled to a lesser extent. Castration brings about a reduction of the number of PCG and of the numerical density of labeling in endometrial fibroblasts. The injection of estradiol causes a rapid increase in both parameters. On the contrary, the frequency of PCG and intensity of labeling of epithelial endometrial cells and in muscle cells increase after ovariectomy and are reduced by estradiol administration. These results suggest that estradiol may affect differentially various types of target cells in the same organ, and also that PCG are not the only nuclear compartment of pre-mRNA or mRNA altered by the changes in estradiol, the RNP containing fibrils located in the perichromatin and in the interchromatin regions are also involved.

Three-dimensional analysis of the arrangement of compact chromatin in the nucleus of G0 rat lymphocytes

The arrangement of compact chromatin of G0 lymphocytes was studied in three-dimensional reconstructions of the ensemble of the chromatin and of individual compact chromatin bodies. Rat spleen was serially cut and sections were contrasted with procedures preferential for DNA. Electron microscopy images were digitized, processed, and displayed using a commercial software package, complemented by a system for three-dimensional reconstruction and analysis developed by us on an IBM-compatible microcomputer provided with an image acquisition board. The reconstructions showed a continuous layer of compact chromatin in contact with the nuclear envelope that prevents the automatic recognition of individual chromatin clumps. The ensemble of the arrangement of compact chromatin was found to be very similar in different lymphocytes. After morphological filtering procedures, the initial mass was divided into individual bodies of compact chromatin, which were tagged. Most of these bodies contact the nuclear envelope. The number of bodies as well as the number of contacts with the envelope are similar and correspond to a haploid number of chromosomes. The largest body is always the one containing nucleolus-associated chromatin. When the cell has two nucleoli, the nucleolus-associated chromatin bodies contact the envelope in diametrically opposed areas. This feature was also described in rat liver cells. It is concluded that: (a) the individualized compact chromatin bodies do not correspond to an entire chromosome or to a pair of chromosomes; (b) the arrangement of compact chromatin is not identical in each G0 lymphocyte, but there are patterns that are repeated with limited changes; and (c) there are common features that appear in different cell types of individuals of the same species.

Effects of testosterone on nuclear ribonucleoprotein components of prostate epithelial cells

The changes of the nuclear components caused by castration and testosterone injection were studied in epithelial cells of the ventral prostate of the rat. Castration drastically diminishes the nuclear and nucleolar volume, as well as the fraction of the nuclear volume occupied by non-nucleolar ribonucleoprotein (RNP) fibrils. However, in castrated animals the frequency of perichromatin granules (PCG) is 79% higher than in controls. Testosterone injection causes a reduction of the number of PCG to 33% of the castrated level in 15 min, and increases the non-nucleolar RNP fibrils. Other parameters such as nuclear and nucleolar volume and the relative volume of the compact chromatin present only small changes in a period of 2 h following the hormone administration. High resolution quantitative autoradiography demonstrates that the transportation of previously synthesized RNA increases steeper than the RNA synthesis. All these effects are similar to those caused by ovariectomy and estradiol injection on the nuclear structures of endometrial epithelial cells. These similarities and other observations suggest that PCGs contain mRNA, of a few genes, stored in the nucleus by a restriction of its transportation to the cytoplasm.

Evidence of repetitive patterns of chromatin distribution in cell nuclei of rat liver

Samples of rat livers were fixed in glutaraldehyde, contrasted en bloc with phosphotungstic acid, embedded in an epoxy resin and serially sectioned. The study of three-dimensional models of 20 complete nuclei shows that all of them share some general features: they have more than one nucleolus (2-4), an irregular layer of compact chromatin adjacent to the nuclear membrane and well-delimited clumps of chromatin both in the nuclear sap and surrounding the nucleoli. A space of 8 sections containing the central nucleolus and a lateral one was studied in detail. In this space, 8 clumps of compact chromatin were found in 17 nuclei and 9 clumps in the other 3 nuclei. No other number of clumps was found in those zones. In all the nuclei studied the compact chromatin surrounding the central nucleolus contacts the nuclear envelope. This contact takes place in a region almost diametrically opposed to the lateral nucleolus in 13 nuclei. In 7 nuclei, these structures were at angles between 50 and 125 degrees. These results support the existence of nonrandom repetitive patterns of chromatin distribution in liver cells.

A simple staining method for chromatin in electron microscopy compatible with serial sectioning

The study of the disposition of chromatin in the interphasic nucleus requires the combination of serial sectioning and a specific or preferential chromatin staining. The staining of chromatin with phosphotungstic acid (PTA) chromatin was originally employed on sections of glycolmethacrylate-embedded samples. As it is very difficult to obtain ribbons with this resin, we introduced a modification which consisted of staining the tissue after fixation and before dehydration, in order that epoxy resins can be applied. Several procedures were tried and the best results were attained in the following way. Standard fixation of samples no thicker than 1 mm was carried out with 2.5% glutaraldehyde at pH 7.2 for 1 or 2 h at room temperature; tissues were then rinsed three times with 0.2N HCl adjusted to pH 2.1-2.3 with 0.2N NaOH for 15 min. Staining was held with 3% W/V PTA in 1N HCl adjusted to the same pH. Samples were dehydrated in gradual ethyl alcohol concentrations and Epon-embedded. Post-staining on sections with uranyl-acetate and lead-citrate or other methods may be used to demonstrate other cell components and their relations to chromatin.