In vivo activities and penetration of the two components of the streptogramin RP 59500 in cardiac vegetations of experimental endocarditis

We evaluated the in vivo activity and the diffusion of radiolabelled RP 57669 (RPI) and RP 54476 (RPII), the two components of the injectable streptogramin RP 59500, alone or in combination, in aortic vegetations from experimental endocarditis in rabbits. RPI and RPII demonstrated in vitro bacteriostatic and bactericidal synergy against a clinical strain of Staphylococcus aureus resistant to methicillin and susceptible to erythromycin. In experimental staphylococcal endocarditis, RP 59500 was as effective as vancomycin and significantly more effective than RPI (P < 0.01) and RPII (P < 0.05). Autoradiography studies showed different patterns of distribution into cardiac vegetations infected with Streptococcus sanguis for [14C]RPI and [14C]RPII. [14C]RPI was homogeneously distributed throughout the vegetations whereas [14C]RPII showed a decreasing gradient of concentration between the periphery and the core of the vegetation, with an approximately 2:1 ratio. [14C]RPI diffused approximately 2 to 4 times more than [14C]RPII into the core of the vegetations. Since the injected ratio of RPI and RPII is 30:70 in RP 59500, the actual RPI:RPII ratio in the core of the vegetation may range from 0.8 to 1.7, a ratio which remains compatible with the in vivo synergism demonstrated between the two components.

Fusidic acid alone or in combination with vancomycin for therapy of experimental endocarditis due to methicillin-resistant Staphylococcus aureus

The usefulness of fusidic acid, alone or combined with vancomycin, was investigated for the therapy of experimental endocarditis caused in rabbits by a methicillin-resistant strain of Staphylococcus aureus. In vitro killing curves showed an indifferent interaction between the two antibiotics. In vivo, vancomycin alone was as effective as a vancomycin-fusidic acid combination (P < 0.05 versus control animals). No resistance to fusidic acid emerged during combination therapy. Fusidic acid alone was not effective. Resistance emerged in 5 of 12 animals treated with fusidic acid alone and was responsible for antibacterial failure. Fusidic acid alone was effective (P < 0.001) and did not select resistant strains if therapy was started when animals retained a smaller inoculum. We concluded that the vancomycin-fusidic acid combination exhibited no advantage over vancomycin alone in this model.

Pharmacokinetic and pharmacodynamic aspects of therapy of experimental endocarditis

Bacterial endocarditis is difficult to cure because of poor penetration of antibiotics into infected vegetations, altered metabolic state of bacteria within the lesion, and absence of adequate host-defense cellular response, that could cooperate with antibiotic action. Two main aspects are discussed for their effect on human therapy: (1) the kinetics of antibiotic diffusion into vegetations, with a special reference to the data obtained with autoradiography, and (2) the specificity of some pharmacodynamic aspects of antibiotics in endocarditis, including the clinical consequences of these two parameters on antibiotic dosing regimens and length of therapy.

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

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

Influence of low-level resistance to vancomycin on efficacy of teicoplanin and vancomycin for treatment of experimental endocarditis due to Enterococcus faecium

Emergence of vancomycin-resistant strains among enterococci raises a new clinical challenge. Rabbits with aortic endocarditis were infected with Enterococcus faecium BM4172, a clinical strain resistant to low levels of vancomycin (MIC, 16 micrograms/ml) and susceptible to teicoplanin (MIC, 1 micrograms/ml), and against its susceptible variant E. faecium BM4172S obtained in vitro by insertional mutagenesis (MICs, 2 and 0.5 micrograms/ml, respectively). Control animals retained 8 to 10.5 log10 CFU/g of vegetation. We evaluated in this model the efficacy of vancomycin (30 mg/kg of body weight; mean peak and trough serum levels, 27 and 5 micrograms/ml, respectively), teicoplanin (standard dose, 10 mg/kg; mean peak and trough levels, 23 and 9 micrograms/ml, respectively; and high dose, 20 mg/kg; mean peak and trough levels, 63 and 25 micrograms/ml, respectively), gentamicin (6 mg/kg; mean peak and trough levels, 8.6 and less than 0.1 micrograms/ml, respectively), alone or in combination, given every 12 h intramuscularly for 5 days. Teicoplanin standard dose was as active as vancomycin against both strains. Vancomycin was not effective against E. faecium BM4172 but was highly effective against E. faecium BM4172S (7.5 +/- 1.1 log10 CFU/g of vegetation versus 4.9 +/- 1.0 log10 CFU/g of vegetation for vancomycin against E. faecium BM4172 and E. faecium BM4172S, respectively; P = 0.0012). A high dose of teicoplanin was more effective than vancomycin against E. faecium BM4172 (4.4 +/- 1.8 log10 CFU/g of vegetation versus 7.5 +/- 1.1 log10 CFU/g of vegetation for teicoplanin high dose and vancomycin, respectively; P less than 0.05). Against E. faecium BM4172 glycopeptide-gentamicin combinations were the most effective regimens in vitro and in vivo (2.8 +/- 0.7 and 3.5 +/- 1.3 log10 CFU/g of vegetation for vancomycin plus gentamicin and teicoplanin standard dose plus gentamicin, respectively; P < 0.05 versus single-drug regimens). We concluded that high-dose teicoplanin or the combination of a glycopeptide antibiotic plus gentamicin was effective against experimental infection due to E. faecium with low-level resistance to vancomycin.

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

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

Factors affecting duration of in-vivo postantibiotic effect for aminoglycosides against gram-negative bacilli

A murine thigh-infection model was used to determine the effect of certain host- and drug-related factors on the duration of the in-vivo postantibiotic effect (PAE) observed with aminoglycosides against Gram-negative bacilli. The role of neutrophils (PMNs), pharmacokinetics and variation among species and strains were studied. PAEs were quantitated after a single injection of gentamicin or amikacin. PAEs were several hours longer in normal mice than in neutropenic mice, in mice with renal impairment than in those with normal renal function, and with strains of Klebsiella pneumoniae than with strains of Escherichia coli, Serratia marcescens and Enterobacter cloacae. Among the 15 strains of Enterobacteriaceae studied, the duration of the in-vivo PAE did not correlate with MIC, duration of in-vitro PAE, and extent of in-vivo bactericidal activity. We conclude that prolonged PAEs are consistently observed in vivo with aminoglycosides against Enterobacteriaceae, and that this duration is enhanced in the presence of PMNs and by pharmacokinetic properties simulating those observed in humans.

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

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

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

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

Activity of sulbactam in combination with ceftriaxone in vitro and in experimental endocarditis caused by Escherichia coli producing SHV-2-like beta-lactamase

We studied the efficacy of sulbactam, a beta-lactamase inhibitor, in combination with ceftriaxone in vitro and in experimental endocarditis due to an Escherichia coli strain producing an extended-spectrum beta-lactamase most similar to SHV-2, a new mechanism of resistance to broad-spectrum cephalosporins among members of the family Enterobacteriaceae. In vitro, ceftriaxone demonstrated an important inoculum effect (MICs were 2 and 256 micrograms/ml with 5 X 10(5) and 5 X 10(7) CFU of inoculum per ml, respectively). Sulbactam inhibited the beta-lactamase degradation of ceftriaxone and enhanced the killing by ceftriaxone with both inocula tested. In vivo, sulbactam (100 mg/kg every 8 h) or ceftriaxone (15 or 30 mg/kg every 24 h) alone were ineffective after a 4-day therapy. The addition of sulbactam to ceftriaxone (15 mg/kg) or to the ceftriaxone (15 mg/kg)-netilmicin (6 mg/kg every 24 h) combination produced a reduction of 2 log10 CFU/g of vegetation greater than that produced by therapy without sulbactam. The sulbactam-ceftriaxone (30 mg/kg) combination produced a reduction of almost 5 log10 CFU/g of vegetation greater than that produced by single-drug therapy (P less than 0.01), sterilized five of eight vegetations (versus none of seven for ceftriaxone [30 mg/kg] alone; P less than 0.05), and was as effective as the ceftriaxone (15 mg/kg)-sulbactam-netilmicin combination. We concluded that (i) SHV-2 production was responsible for ceftriaxone failure in vivo, probably because of the high inoculum present in vegetations; (ii) sulbactam used in a regimen which provided levels in serum constantly above 4 micrograms/ml and a vegetation/serum peak ratio of approximately 1:3 enhanced the activity of a broad-spectrum cephalosporin in a severe experimental infection; and (iii) the highest dose of ceftriaxone in combination with sulbactam was as effective as the lowest dose of ceftriaxone plus sulbactam plus an aminoglycoside.

Ceftriaxone-netilmicin combination in single-daily-dose treatment of experimental Escherichia coli endocarditis

We evaluated the activities of ceftriaxone (15 mg/kg), netilmicin (6 mg/kg), and their combination given intramuscularly once daily for 4 days for the treatment of experimental Escherichia coli endocarditis in rabbits. In vitro, a greater rate of killing and an increased trough serum bactericidal titer (P less than 0.01) were achieved with the combination. In vivo, the combination had a greater bactericidal effect (P less than 0.01) and resulted in a greater number of sterile vegetations (P less than 0.05) than single-drug therapy. Thus, in vivo, an increased effect can be obtained despite a single daily dose of a long-acting cephalosporin and an aminoglycoside.

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

Animal studies that compare antibiotics have used only a limited number of doses administered at intervals chosen without regard for their pharmacodynamic effects of pharmacokinetic profiles. We compared the relative efficacy and potency of three beta-lactams and two aminoglycosides in lung and thigh-infection models in neutropenic mice by defining the maximum attainable antimicrobial effect at 24 h (Emax) and the total dose required to reach 50% of maximum effect (P50) at several dosing intervals. For beta-lactams, Emaxs were similar, whereas P50s increased 10- to 50-fold with longer intervals in both models. Aminoglycosides were significantly more bactericidal in the lung than in the thigh, and dosing interval had little impact on P50s in either model. Recognizing the variable impact of dosing interval on efficacy for different classes of antibiotics is mandatory for the proper design and interpretation of comparative trials.

Lithium causing a serious sinus-node dysfunction at therapeutic doses

The cardiovascular effects of lithium have been indexed in literature since 1900, most frequently in cases of intoxication. We recently studied the exceptional case of a patient showing a serious sinus node dysfunction during lithium treatment. The serum lithium levels were always within the therapeutic range. After two endocardial explorations and withdrawal of lithium, we found that the sinus node dysfunction was due to lithium and reversible after withdrawal of it. Some ignored dysfunctions could exist with a regularly controlled treatment.

[Concept of class in relation to first-generation cephalosporins]

The authors consider differences in answer of the first generation cephalosporins against several strains of Gram-negative and Gram-positive bacteria, and suggest a revision of the "class" concept as regards sensitivity response of the above mentioned cephalosporins.

[Activity of new cephalosporins against polyresistant bacterial strains]

Activities of four new Cephalosporins (Cephamandole, Cefoxitin, Cefuroxime, Cefotaxime) and of some of the most used Aminoglycosidic antibiotics were tested against Gram-negative bacteria (Pseudomonas and some Enterobacteriaceae) that are often formed to be multi-resistant. 266 strains were observed. An evident increase or resistance to Aminoglycosidic antibiotics was confirmed, while the new Cephalosporins demonstrated an remarkable activity against the majority of resistant bacteria, often superior to Aminoglycosidics.