A pharmacokinetic evaluation of concomitant administration of linezolid and aztreonam

Linezolid, a new oxazolidinone antimicrobial agent, has a spectrum of activity encompassing a wide variety of Grampositive bacteria. The purpose of this study was to evaluate the pharmacokinetics of linezolid and aztreonam, an antimicrobial agent with selective activity against Gram-negative bacteria, when given alone and in combination. Healthy subjects were randomized to receive single, 30-minute intravenous infusions of (1) linezolid 375 mg, (2) aztreonam 1000 mg, and (3) linezolid 375 mg plus aztreonam 1000 mg in an open-label, crossover manner. The only statistically significant differences observed with combination treatment relative to each drug alone were an increase in the maximum plasma concentration of linezolid (approximately 18%) and an approximate 7% decrease in the apparent elimination rate of aztreonam, neither of which are expected to be clinically significant. In healthy subjects, the combination of linezolid and aztreonam was safe and well tolerated compared with each agent used alone. Pharmacokinetic data demonstrate that coadministration of linezolid and aztreonam does not alter the disposition of either agent under single-dose conditions. Therefore, it is not expected that a dose alteration of either agent will be necessary in a clinical setting.

Pharmacokinetics and pharmacodynamics of aztreonam administered by continuous intravenous infusion

The pharmacodynamic parameter that appears to correlate best with a successful therapeutic outcome with beta-lactam antibiotics is the length of time the serum antibiotic concentration remains above the minimum inhibitory concentration (MIC) for the infecting pathogen. By maximizing this parameter, continuous administration of beta-lactam and related antibiotics by intravenous infusion could represent the optimal mode of drug administration. The pharmacokinetic and pharmacodynamic properties of ceftazidime administered by continuous intravenous infusion have been evaluated previously. Aztreonam is a monobactam antibiotic with similar pharmacokinetic and microbiologic activity to that of ceftazidime. This study evaluated the pharmacokinetic and pharmacodynamic characteristics of aztreonam administered as a continuous intravenous infusion in healthy volunteers against multiple clinical isolates. Five men and 3 women received 6 g of aztreonam administered by continuous intravenous infusion over 24 hours. Blood samples were collected before the infusion and at 0.5, 1 through 8, 12, 18, and 24 hours after the start of the infusion. Pharmacokinetic parameters were determined by standard equations. In vitro susceptibility testing was performed using National Committee for Clinical Laboratory Standards guidelines for 4 clinical isolates of gram-negative bacteria (2 each of Escherichia coli and Pseudomonas aeruginosa). Serum inhibitory titers (SITs) were determined in duplicate for each clinical isolate at 0 and 24 hours. The subjects' mean (+/- SD) age was 29.3+/-4.4 years; mean weight, 74.6+/-14.0 kg; and calculated mean creatinine clearance, 107+/-13 mL/min. For the pharmacokinetic parameters, mean (+/- SD) values were as follows: steady-state serum concentration, 40.9+/-8.8 microg/L; half-life, 1.5+/-0.4 hours; elimination rate constant, 0.50+/-0.13 hours(-1); steady-state volume of distribution, 0.18+/-0.04 L/kg; and total body clearance, 6.1+/-1.2 L/h. The MICs were 0.0625 and 0.125 microg/mL against the 2 E coli isolates and 4 microg/mL against both P aeruginosa isolates. The median SITs against the E. coli isolates were 1:256 and 1:512, and against the P. aeruginosa isolates were 1:8 and 1:16. At steady state, II subjects had serum concentrations of aztreonam > or =4 times the MIC for each organism. These findings suggest that further clinical study of the administration of aztreonam by continuous intravenous infusion is warranted.

Cefepime-aztreonam: a unique double beta-lactam combination for Pseudomonas aeruginosa

An in vitro pharmacokinetic model was used to determine if aztreonam could enhance the pharmacodynamics of cefepime or ceftazidime against an isogenic panel of Pseudomonas aeruginosa 164, including wild-type (WT), partially derepressed (PD), and fully derepressed (FD) phenotypes. Logarithmic-phase cultures were exposed to peak concentrations achieved in serum with 1- or 2-g intravenous doses, elimination pharmacokinetics were simulated, and viable bacterial counts were measured over three 8-h dosing intervals. In studies with cefepime and cefepime-aztreonam against the PD strain, samples were also filter sterilized, assayed for active cefepime, and assayed for nitrocefin hydrolysis activity before and after overnight dialysis. Against WT strains, the cefepime-aztreonam combination was the most active regimen, but viable counts at 24 h were only 1 log below those in cefepime-treated cultures. Against PD and FD strains, the antibacterial activity of cefepime-aztreonam was significantly enhanced over that of each drug alone, with 3.5 logs of killing by 24 h. Hydrolysis and bioassay studies demonstrated that aztreonam was inhibiting the extracellular cephalosporinase that had accumulated and was thus protecting cefepime in the extracellular environment. In contrast to cefepime-aztreonam, the pharmacodynamics of ceftazidime-aztreonam were not enhanced over those of aztreonam alone. Further pharmacodynamic studies with five other P. aeruginosa strains producing increased levels of cephalosporinase demonstrated that the enhanced pharmacodynamics of cefepime-aztreonam were not unique to the isogenic panel. The results of these studies demonstrate that aztreonam can enhance the antibacterial activity of cefepime against derepressed mutants of P. aeruginosa producing increased levels of cephalosporinase. This positive interaction appears to be due in part to the ability of aztreonam to protect cefepime from extracellular cephalosporinase inactivation. Clinical evaluation of this combination is warranted.

Pharmacokinetics of aztreonam in critically ill surgical patients

The pharmacokinetics of aztreonam in critically ill surgical patients with serious gram-negative infections were studied. Blood samples were taken before and at 30 minutes, 2.5 hours, and 5 hours after a dose of aztreonam 2 g i.v. every six hours. All patients had received at least two aztreonam doses before the dosage interval being studied. Aztreonam concentrations were measured by high-performance liquid chromatography. Aztreonam's pharmacokinetics, the severity of illness, and patient outcomes were examined. A total of 28 patients with 111 serum aztreonam concentrations were included in the analysis. The patients were young (mean age, 35 years) and predominantly male. The mean APACHE II score was 19.3, and 22 patients had sepsis. Four patients died. The mean volume of distribution (V) of 0.35 L/ kg was nearly twice the previously reported steady-state value for healthy volunteers (0.18 L/kg) and was highly variable. A slightly higher than normal mean V, 0.22 L/ kg, was seen in a subset of six patients whose infection occurred earlier in their intensive care and who had lower APACHE II scores. While with some antibiotics the elevated V would imply difficulty in achieving therapeutic drug levels, 99 (89%) of the 111 concentrations were at or above the in vitro susceptibility breakpoint of 8 micrograms/mL. Despite observations of markedly increased and highly variable V in critically ill surgical patients, a standard dosage of aztreonam was usually sufficient to maintain adequate serum drug levels.

Differential distributions in tissues and efficacies of aztreonam and ceftazidime and in vivo bacterial morphological changes following treatment

The differential tissue distributions of aztreonam and ceftazidime within fibrin clots infected with Pseudomonas aeruginosa, Enterobacter cloacae, and Serratia marcescens, their efficacies, and the in vivo bacterial morphological changes induced by these drugs were evaluated. Rabbits were given intravenously a single dose of 100 mg of either agents/kg of body weight. In the cores of the clots, the peak levels of both drugs were much lower than those observed in the peripheries and in serum. Aztreonam's half-lives within the peripheries and in the cores of the fibrin clots were up to six times higher than observed in serum, while ceftazidime's half-lives in clots were twice that observed in serum. This resulted in a much greater penetration ratio for aztreonam than for ceftazidime. Both drugs controlled the growth of P. aeruginosa in vivo, but E. cloacae and S. marcescens responded better to ceftazidime. Morphological changes were more abundant in the peripheries than in the cores of the clots. In the control group, P. aeruginosa's morphology in the cores was different than that in the peripheries of the clots. Against P. aeruginosa, aztreonam did induce morphological changes in the cores while ceftazidime did not. Electron microscopic studies revealed that morphological changes associated with aztreonam seemed different than those of ceftazidime. Along with elongation of bacteria, more bow tie and herniated bacteria were observed with aztreonam. Though both agents selectively affect PBP 3, as manifested by elongated bacteria, they induce in the peripheries of the clots thickening, breaks, and detachment in bacterial cell walls, alterations which are generally associated with antibiotics affecting PBP 1a and 1b.

The bioavailability of injectable, amorphous form of aztreonam

Amorphos injectable form of aztreonam (BIOKTAM) was prepared. It was shown that after intramuscular or intravenous administration there are not any considerable differences in the bioavailability of aztreonam from BIOKTAM and of the drug from AZACTAM.

Pharmacokinetics of aztreonam and imipenem in critically ill patients with pneumonia

STUDY OBJECTIVE

To evaluate the pharmacokinetic profiles of aztreonam and imipenem in critically ill trauma patients with pneumonia.

METHODS

Trauma patients in intensive care units who were intubated within 3 days of hospital admission were eligible for the study. Patients with the clinical diagnosis of pneumonia were consecutively randomized to receive either aztreonam plus vancomycin or imipenem-cilastatin. Serial blood samples were taken and sputum was collected to determine aztreonam and imipenem concentrations after 2-3 days and 7-8 days of therapy. Pharmacokinetics of both agents were estimated and compared with estimates from healthy volunteers.

RESULTS

Twenty patients were enrolled in the study, 10 patients received imipenem-cilastatin, and 10 received aztreonam plus vancomycin. Steady-state volume of distribution (Vss) for aztreonam at 2-3 days and 7-8 days was significantly greater in patients than in historical controls, whereas the Vss for imipenem was greater at 2-3 days. The beta-half-life for aztreonam at both sampling periods was significantly greater in patients than in controls. No significant changes in pharmacokinetics occurred over time for either antibiotic. Sputum concentrations of aztreonam and imipenem were highly variable when sampled 2 hours after the infusion.

CONCLUSION

Larger volumes of distribution were observed for both aztreonam and imipenem in trauma patients than in volunteers, suggesting that standard initial dosages of the antibiotics may result in lower concentrations in these critically ill patients. Both antibiotics penetrated into the sputum of most patients; however, the degree of penetration was highly variable in relation to serum concentrations.

Adverse effects of monobactams and carbapenems

Monobactams and carbapenems are 2 classes of beta-lactam antibiotics that were introduced in the 1980s. This review considers the monobactam aztreonam and the carbapenems imipenem and meropenem. Imipenem is administered together with cilastatin, which inhibits the enzymatic breakdown of imipenem in the kidney. The antibacterial activities of these drugs are quite different from older beta-lactams. Aztreonam is directed towards aerobic Gram-negative bacteria, especially Pseudomonas aeruginosa, while imipenem and meropenem are active against both aerobic and anaerobic Gram-positive and Gram-negative bacteria. Thus, these drugs should be reserved for patients who have a special need for them. They are also structurally different from older beta-lactams and possess different adverse drug reaction profiles. It was initially suggested that aztreonam would be less immunogenic than previous beta-lactams because reactive breakdown products acting as haptens are less likely to be formed. Clinical reports now support this assumption, and, in particular, cross hypersensitivity between aztreonam and other beta-lactams seems to be rare which makes the drug a useful therapeutic alternative. However, hypersensitivity to aztreonam does occur. The predominant concern in terms of adverse reactions to imipenem/cilastatin is the increased tendency to cause seizures compared with other beta-lactams. The risk of producing a seizure is highly associated with inadequate dose adjustment in relation to kidney function. If appropriate care is taken, seizures occur in less than 1% of patients treated. However, it is possible that concomitant administration of other drugs with neurotoxic profiles (e.g. theophylline and cyclosporin) given in overdose, may increase the risk of seizures.(ABSTRACT TRUNCATED AT 250 WORDS)

The monobactams

The monobactam antibiotics are synthetic compounds, although monocyclic beta-lactam compounds have been found in nature in various soil bacteria. Although additional orally and parenterally administered monobactams are under investigation, the first marketed monobactam was aztreonam. This agent has an antimicrobial spectrum similar to that of gentamicin and tobramycin, aminoglycoside antibiotics. Aztreonam, however, is not nephrotoxic, is weakly immunogenic, and has not been associated with disorders of coagulation. Aztreonam may be administered intramuscularly or intravenously; absorption after oral administration is poor. The primary route of elimination is the urine. The serum half-life of the drug in patients with normal renal function is 1.5 to 2.1 hours; the recommended dosing interval in patients with normal renal function is every 8 hours. Dosage adjustment is necessary in patients with renal impairment. The strictly gram-negative aerobic spectrum of aztreonam limits its use as a single empiric agent. Approved indications for its use include infections of the urinary tract or lower respiratory tract, intra-abdominal and gynecologic infections, septicemia, and cutaneous infections caused by susceptible organisms. Concurrent initial therapy with other antimicrobial agents is recommended before the causative organism (or organisms) has been determined in patients who are seriously ill and at risk for gram-positive or anaerobic infections.

Comparison of the effects of aztreonam and tigemonam against Escherichia coli and Klebsiella pneumoniae in vitro and in vivo

A study was performed to investigate the pharmacodynamics of aztreonam and tigemonam against Escherichia coli and Klebsiella pneumoniae in vitro and in vivo. The in vitro concentration-effect relationships were determined in short-term growth experiments. The in vivo dose-effect relationships were determined in an experimental thigh muscle infection in irradiated mice. In this model, E. coli was injected into one thigh muscle and K. pneumoniae was injected into the other. Throughout these experiments aztreonam was administered subcutaneously and tigemonam was administered orally. For analysis of the antibacterial pharmacodynamics, the following parameters were determined: the maximum effect as a parameter for efficacy, the 50% effective concentration (or dose) as a parameter for potency, and the slope of the concentration-effect relationship. To assess the relationship between the concentration of the antibiotic and the antibacterial effect in vivo, the pharmacokinetics of the two drugs in the plasma of mice were determined as well. The maximum in vitro and in vivo effects of aztreonam and tigemonam against both bacteria did not differ substantially. However, both drugs killed E. coli more effectively than K. pneumoniae, indicating that the maximum in vitro effect of these drugs against E. coli was higher than that against K. pneumoniae. The maximum in vivo effect of both drugs against E. coli was similar to that against K. pneumoniae. Furthermore, in vitro aztreonam was about twice as potent as tigemonam, but in vivo the reverse was the case. These findings were explained by pharmacokinetic differences between subcutaneously administered aztreonam and orally administered tigemonam, because concentrations of tigemonam in plasma remained at microbiologically active concentrations longer than those of aztreonam did.

New antibiotics: carbapenems, monobactams and quinolones

New beta lactams and the quinolone class of antibiotics represent major improvements in the therapy of moderate to severe infections. These newer antibiotics have an extended spectrum of antimicrobial activity, excellent pharmacokinetic properties and low toxicity. The beta lactams include carbapenems, represented by imipenem-cilastatin, and monobactams, represented by aztreonam. Norfloxacin and ciprofloxacin are potent quinolones.