Antibiotic concentrations in serum, serum bactericidal activity, and results of therapy of streptococcal endocarditis in rabbits

A systematic review of optimal pharmacokinetic/pharmacodynamic parameters for beta-lactam therapy in infective endocarditis

BACKGROUND

Beta-lactam antibiotics are the mainstay of therapy for most bacterial causes of infective endocarditis (IE). Traditionally considered as agents with a broad therapeutic index, there is increasing recognition that standard doses may be subtherapeutic or toxic in critically ill patients. Optimizing therapy for efficacy requires a defined pharmacokinetic (PK)/pharmacodynamic (PD) target associated with clinical and microbiological cure.

OBJECTIVES

To elucidate the factors that influence beta-lactam PK and PD variability in IE and to examine optimal PK/PD target parameters for therapy.

METHODS

The review was conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Clinical and laboratory in vivo animal or human studies examining PK and/or PD of beta-lactam antibiotics in IE were eligible. Ovid MEDLINE, Embase and Cochrane Central Registry were searched using defined terms. The Office of Health Assessment and Translation (OHAT) tool was used for assessing risk of bias.

RESULTS

From 2677 abstracts, 62 articles were selected for review and synthesis, comprising: 45 animal studies investigating the broad categories of beta-lactam diffusion into vegetations, PK/PD determinants of outcome, mode of antibiotic delivery and synergistic impact of agents; and 17 human studies totalling 347 participants. Findings supported the importance of time-dependent killing for beta-lactams but heterogeneous data limited the determination of an optimal PK/PD target for IE treatment.

CONCLUSION

Beta-lactam PK and PD in endocarditis are variable and specific to the particular antibiotic-organism combination. Time-dependent killing is important, consistent with non-endocarditis studies, but there is little agreement on optimal drug exposure. Clinical studies examining PK/PD targets in endocarditis are required to further inform drug selection and dosing.

Oral antibiotics for infective endocarditis: a clinical review

Current guidelines for management of infective endocarditis (IE) advise 4-6 weeks of IV antibiotics. This is based on historical data from animal models, which set a precedent for high peak serum antimicrobial levels, thought to be only achievable with IV therapy. However, there has been increasing recent interest in oral antibiotics as an alternative to prolonged parenteral therapy, not limited to treatment of IE. This review examines the theory behind parenteral antibiotic administration with reference to the MICs of relevant pathogens. By comparing published serum antimicrobial levels after oral and IV administration we suggest that safe levels of commonly used antibiotics can be achieved orally. We have then reviewed the literature to date on oral antibiotics for IE. The largest randomized controlled trial (RCT) in this area, the POET trial, concluded that oral therapy was non-inferior to prolonged IV therapy in stable patients with left-sided IE. Additionally, there have been two smaller RCTs published, as well as a number of observational studies over the last 50 years, utilizing a variety of different patient groups, methods and treatment strategies. This body of evidence gives weight to a potential shift in practice towards oral therapy, primarily as a step-down treatment. We conclude that pharmacological data offer theoretical reassurance for the safety of oral therapy. This is coupled with a growing evidence base for non-inferiority of oral antimicrobials compared with prolonged parenteral therapy in practice.

Antibiotic efficacy is linked to bacterial cellular respiration

Bacteriostatic and bactericidal antibiotic treatments result in two fundamentally different phenotypic outcomes--the inhibition of bacterial growth or, alternatively, cell death. Most antibiotics inhibit processes that are major consumers of cellular energy output, suggesting that antibiotic treatment may have important downstream consequences on bacterial metabolism. We hypothesized that the specific metabolic effects of bacteriostatic and bactericidal antibiotics contribute to their overall efficacy. We leveraged the opposing phenotypes of bacteriostatic and bactericidal drugs in combination to investigate their activity. Growth inhibition from bacteriostatic antibiotics was associated with suppressed cellular respiration whereas cell death from most bactericidal antibiotics was associated with accelerated respiration. In combination, suppression of cellular respiration by the bacteriostatic antibiotic was the dominant effect, blocking bactericidal killing. Global metabolic profiling of bacteriostatic antibiotic treatment revealed that accumulation of metabolites involved in specific drug target activity was linked to the buildup of energy metabolites that feed the electron transport chain. Inhibition of cellular respiration by knockout of the cytochrome oxidases was sufficient to attenuate bactericidal lethality whereas acceleration of basal respiration by genetically uncoupling ATP synthesis from electron transport resulted in potentiation of the killing effect of bactericidal antibiotics. This work identifies a link between antibiotic-induced cellular respiration and bactericidal lethality and demonstrates that bactericidal activity can be arrested by attenuated respiration and potentiated by accelerated respiration. Our data collectively show that antibiotics perturb the metabolic state of bacteria and that the metabolic state of bacteria impacts antibiotic efficacy.

Principles of use of antibacterial agents

The selection of an antimicrobial regimen is based on a number of factors, including the nature of the infection, the identity and susceptibility of the pathogens, host characteristics, and the pharmacokinetics and pharmacodynamics of antimicrobial agents. This article provides a comprehensive overview of these factors, with particular attention to pharmacokinetics and monitoring for efficacy and toxicity. A brief summary is also provided of some other topics discussed in detail elsewhere in this issue, such as susceptibility testing, pharmacodynamics, and pharmacokinetics-pharmacodynamics parameters.

The Importance of Bactericidal Drugs: Future Directions in Infectious Disease

BACKGROUND

Although a considerable amount of research has gone into the study of the role of bactericidal versus bacteriostatic antimicrobial agents in the treatment of different infectious diseases, there is no accepted standard of practice.

METHODS

A panel of infectious diseases specialists reviewed the available literature to try to define specific recommendations for clinical practice.

RESULTS

In infections of the central nervous system, the rapidity with which the organism is killed may be an important determinant, because of the serious damage that may occur during these clinical situations. The failure of bacteriostatic antibiotics to adequately treat endocarditis is well documented, both in human studies and in animal models.

CONCLUSION

The bulk of the evidence supports the concept that, in treating endocarditis and meningitis, it is important to use antibacterial agents with in vitro bactericidal activity. This conclusion is based on both human and animal data. The data to support bactericidal drugs' superiority to bacteriostatic drugs do not exist for most other clinical situations, and animal models do not support this concept in some situations. Clinicians should be aware that drugs that are bacteriostatic for one organism may in fact be bactericidal for another organism or another strain of the same organism.

Principles of selection and use of antibacterial agents. In vitro activity and pharmacology

The selection of an antimicrobial treatment regimen is based on many factors, including the nature of the infection, the identity and susceptibility pattern of the infecting organisms, and the pharmacokinetics and pharmacodynamics of the antibacterial drugs. This article discusses principles of susceptibility testing, pharmacology, and monitoring of therapy.

The bactericidal activity of gatifloxacin in plasma and urine

OBJECTIVE: To investigate the bactericidal activity of gatifloxacin in serum and urine against relevant pathogens. METHODS: Serum and urine samples obtained in a single rising dose pharmacokinetic study were investigated for bactericidal activity. The doses employed were placebo, and 200, 400, 600 and 800 mg of gatifloxacin. RESULTS: The titers obtained reflected the dose and susceptibility of the pathogen. In serum a titer of greater-than-or-equal1:8 was observed for Escherichia coli for >24 h after 400 mg; for Staphylococcus aureus and Acinetobacter baumanii the titer was greater-than-or-equal1:8 for between 12--24 h after 600 mg; for Serratia marcescens it was greater-than-or-equal1:8 for >12 h following 800 mg; and for Streptococcus pneumoniae it was greater-than-or-equal1:8 for 8 h following 800 mg. In urine, the titers were greater, and for Escherichia coli, Staphylococcus saprophyticus, Proteus mirabilis and Enterococcus faecalis the titer was >1:8 for 12--24 h after 200 mg and for 24--36 h after a 400-mg dose. Pseudomonas aeruginosa demonstrated a titer of >1:8 for 18--24 h following 600 mg. CONCLUSIONS: These data suggest that gatifloxacin should be efficacious for a wide range of systemic and urinary tract infections.

Vancomycin or vancomycin plus netilmicin for methicillin- and gentamicin-resistant Staphylococcus aureus aortic valve experimental endocarditis

Using a rabbit model of aortic valve endocarditis, we studied the efficacy of vancomycin alone or in combination with netilmicin and/or rifampin against a methicillin- and gentamicin-resistant strain of Staphylococcus aureus (MGRSA). Antibiotics were given for 6 to 12 days, as follows: vancomycin (15 mg/kg of body weight every 12 h [BID] intravenously), vancomycin plus netilmicin (2.5 mg/kg BID intramuscularly), vancomycin plus rifampin (10 mg/kg BID intramuscularly), and vancomycin plus netilmicin plus rifampin at the same routes, dosages, and schedules mentioned above. Netilmicin was given to two additional groups at a higher dosage (6 mg/kg every 24 h intramuscularly) alone or in combination with vancomycin (15 mg/kg BID intravenously) for 12 days. All regimens resulted in undetectable bacterial counts in a significant proportion of vegetations (except netilmicin alone) or reduced the bacterial counts in the vegetations compared with the counts in the untreated controls (P<0.01 to P<0.001). No resistance to rifampin or netilmicin developed during therapy. It is concluded that in the treatment of experimental aortic valve endocarditis caused by MGRSA (i) vancomycin as monotherapy is as efficacious as the triple combination, (ii) the addition of netilmicin (once daily or BID) to vancomycin does not improve the efficacy of the latter antibiotic, even in the presence of rifampin, and (iii) a 12-day course in more effective than a 6-day one, but not at a statistically significant level.

Platelet aggregation by strains of enterococci

The platelet aggregation capability of whole cells of Enterococcus faecalis, E. faecium and E. avium was tested. The optimum ratios of bacteria to platelets in E. faecalis (strain SMU-37), E. faecium (strain SMU-138) and E. avium (strain SMU-197) were 1.0, 1.2 and 2.0, respectively. During the platelet aggregation induced by the three strains of enterococci, 65-69% of total serotonin was released. The aggregation was totally inhibited by ethylenediaminetetraacetate (10 mM) and apyrase (1 mg/ml), while no effect was shown by aspirin (10 mM), indomethacin (10 mM) and quinacrine (1 mM). By pretreatment of platelet-poor plasma with heat (56 C, 30 min) or zymosan, the reactivities with platelets of each strain of species were markedly diminished. These results suggest that enterococci-induced platelet aggregation was an ion-dependent, cyclooxygenase-insensitive event, and plasma component(s) was (were) required for the reaction.

Value of the serum bactericidal test in management of patients with bacterial endocarditis

A review is given of the available clinical data on the prognostic value of the serum bactericidal test in the treatment of patients with bacterial endocarditis. It is concluded that the test, even when performed in a standardized manner, does not provide useful information for the majority of patients with bacterial endocarditis. Until further clinical data are available, routine performance of the test in patients with bacterial endocarditis is not recommended.

Pathogenesis and treatment of endocarditis

The rabbit model has been invaluable for in vivo studies in the pathogenesis and treatment of bacterial endocarditis. Both of the features of the mature bacterial vegetations in this rabbit model, i.e., absence of phagocytosis and decreased metabolic activity, provide evidence to support the concept that a rapidly bactericidal antimicrobial agent provides the optimal approach to the successful treatment of endocarditis. Imipenem, a carbapenem with a very broad spectrum of in vitro activity, has been shown to be rapidly bactericidal in animals and highly effective in the treatment of experimental bacterial endocarditis. In addition, twenty-six patients with endocarditis, caused largely by Staphylococcus aureus, have been successfully treated with imipenem/cilastatin.

[Bacteriologic surveillance of patients with infectious endocarditis. Value and limitations of the determination of minimal inhibitory concentration and serum bactericidal activity]

This is a retrospective study of the records of 17 patients who suffered from infectious endocarditis with positive blood cultures, and concerns the possibilities of predicting immediate infectious signs in patients:--by testing each of the antibiotics used for the minimal inhibitory concentration;--by determining the peak of the serum bactericidal activity of patients. All the patients were under observation, as outpatients, for more than a year and proved completely cured of their infectious endocarditis. A satisfactory minimal inhibitory concentration does not necessarily prevent an infectious evolution in the immediate future. The determination of the peak serum bactericidal activity does not reflect faithfully and instantaneously the anti-infectious action of the treatment; this is shown by the excellent and rapid evolution 13 times out 20 although serum bactericidal activity was considered insufficient (less than 1/8). This level of 1/8 is not a guarantee of therapeutic efficiency. Finally the antiinfectious treatment has been, in the majority of cases, determined by clinical signs and in very few cases by serum bactericidal activity results.

Association between serum inhibitory and bactericidal concentrations and therapeutic outcome in bacterial endocarditis

Several recent reviews on the therapy of bacterial endocarditis have recommended that a serum inhibitory and/or bactericidal concentration (SIC/SBC) of 1:8 or more be achieved to ensure successful therapeutic outcome. We conducted a methodologic and statistical analysis of the available literature on endocarditis to determine the association between SIC/SBC titers of 1:8 or more and therapeutic outcome. We reviewed 17 studies published between 1948 and 1980 in which both SIC/SBC and therapeutic outcome were available. Factors that affect outcome, such as age, duration of symptoms, organism, and valve status, varied widely among the 226 patients. The methods used to measure SIC/SBC differed with respect to the time of obtaining the blood specimen relative to the antibiotic dose, size of the bacterial inoculum, type of broth, and definition of the bactericidal end-point. None of the 17 studies showed a significant association between SIC/SBC titers of 1:8 or more and survival or bacteriologic cure. Fifteen of the 17 also failed to demonstrate a significant association between SIC/SBC titers of 1:8 or more and medical cure. However, each of the studies that failed to demonstrate an association between SIC/SBC titers of 1:8 or more and improved therapeutic outcome had an insufficient sample size to confidently exclude a false-negative result. Analysis of the published data reveals insufficient evidence to demonstrate that SIC and SBC titers are of prognostic value in the therapy of patients with bacterial endocarditis.

Cefoperazone treatment of experimental endocarditis

Cefoperazone (10 mg/kg) and cephalothin (20 mg/kg) administered intramuscularly every 6 h were both effective in reducing the number of Staphylococcus aureus cells in vegetations in rabbits with endocarditis. Cefoperazone produced higher peak concentrations and greater bactericidal activity in serum than did cephalothin. Cefoperazone (40 mg/kg) administered every 6 h was significantly more effective than cefamandole (40 mg/kg) administered every 6 h in reducing the number of Enterobacter aerogenes cells in vegetations. Although cefamandole produced higher peak concentrations in serum, the serum bactericidal activity was greater with cefoperazone. The half-lives in serum were 0.64 h for cefoperazone and 0.46 h for cephalothin and cefamandole.

Prophylaxis against enterococcal endocarditis: comparison of the aminoglycoside component of parenteral antimicrobial regimens

Prophylactic antibiotics for the prevention of enterococcal endocarditis are recommended for patients with valvular heart disease undergoing surgery or instrumentation of the genitourinary and gastrointestinal tracts. To evaluate the most active aminoglycoside antibiotic to include in these regimens, we administered streptomycin, gentamicin, or amikacin, each in combination with ampicillin, to six healthy adult volunteers in a crossover manner. When the sera from the volunteers were tested for bactericidal activity against 16 strains of enterococci, the gentamicin-ampicillin combination produced higher serum bactericidal levels for a longer duration of time against more strains than the other two regimens. At 1 h after antibiotic administration (a time when surgical procedures are likely to be performed), mean geometric bactericidal titers against the enterococci were 1: 7.0 for the gentamicin-ampicillin regimen, as compared with 1:3.6 and 1:3.2 for the streptomycin-ampicillin and amikacin-ampicillin combinations, respectively. Despite the lower serum levels for gentamicin, we feel that this aminoglycoside should be used in combination with ampicillin for prophylactic regimens against enterococcal endocarditis.

Use of the serum bactericidal titer to assess the adequacy of oral antibiotic therapy in the treatment of acute hematogenous osteomyelitis

Serum bactericidal titers against Staphylococcus aureus were measured in 63 children who were receiving mafcillin or methicillin intravenously, or dicloxacillin, penicillin, or cephalexin orally. The SBTs obtained following unit does of 25 mg/kg of dicloxacillin, 35 mg/kg of penicillin, or 25 mg/kg of cephalexin with probenecid were comparable to those seen following intravenous doses of 40 mg/kg nafcillin or methicillin. Twenty-two children with acute hematogenous osteomyelitis proven or presumed to be due to S. aureus were treated intravenously until point tenderness and fever had resolved, and then with oral therapy. The mean duration of intravenous therapy was 14 days. Oral doses were adjusted so that a peak SBT of greater than or equal to 1:16 and a trough SBT of greater than or equal to 1:2 were obtained in most children. No recurrences occurred. The SBT proved to be a practical means of assessing the adequacy of oral therapy in children with infections due to S. aureus.

Comparative study of anti-pseudomonas activity of azlocillin, mezlocillin, and ticarcillin

The anti-pseudomonas activities of azlocillin and mezlocillin were compared with that of ticarcillin. We measured the minimal inhibitory and minimal bactericidal concentrations of the three drugs against 20 different strains of Pseudomonas aeruginosa and found significantly lower values for azlocillin than for the other two drugs. We then infused 5 g of each drug into 10 volunteers on three consecutive days and determined the serum levels of the three antibiotics at 1-h intervals from 1 to 6 h after injection. The levels of azlocillin were significantly higher than those of mezlocillin and ticarcillin (at 1 h: 236.55 mug/ml +/- 12.9 for azlocillin, 192.45 mug/ml +/- 28.8 for mezlocillin, and 131.5 mug/ml +/- 10.9 for ticarcillin). The inhibitory and bactericidal activities of the sera obtained 1 and 6 h after the injection against the same 20 strains of P. aeruginosa demonstrated a significantly greater anti-pseudomonas activity of azlocillin when compared with mezlocillin and ticarcillin; mezlocillin and ticarcillin had approximately the same activity. The mean values for bactericidal activity against the strains tested were 1/32 for azlocillin, 1/8 for mezlocillin, and 1/8 for ticarcillin. Azlocillin thus appears to be a promising anti-pseudomonas drug and should be tested in clinical trials.

Rat model of experimental endocarditis

A simple model of infective endocarditis was produced in rats. With the aid of a guide wire, polyethylene catheters were passed into the left ventricle through the right carotid artery of Sprague-Dawley rats weighing 300 to 350 g. A volume of 1 ml of an overnight culture of Streptococcus mitis, Staphylococcus aureus, or Streptococcus faecalis was intravenously injected 1 to 2 days after catheterization. Bacterial titers of Streptococcus mitis in vegetations were about 10(4)-fold greater than in other tissues. Blood cultures were always positive after 6 h. Mortality was 19% at 1 week and 82% at 2 weeks. Catheters were pulled 24 h after infection, and vegetation titers of greater than 7.0 log10 colony-forming units per g were sustained at 5 days. In intravenously infected rats without catheters, blood and tissues were sterile after 3 to 5 days. With Staphylococcus aureus, vegetations had greater than 9.0 log10 colony-forming units and with Streptococcus faecalis 8.8 +/- 0.3 log10 colony-forming units per g at 2 days. The rat model of infective endocarditis should prove to be suitable for further pathological and therapeutic studies.