Pharmacodynamics, a tool for a better use of antibiotics?

Fosfomycin: the characteristics, activity, and use in critical care

Fosfomycin (C₃H₇O₄P) is a phosphonic acid derivative representing an epoxide class of antibiotics. The drug is a re-emerging bactericidal antibiotic with a wide range of actions against several Gram-positive and Gram-negative bacteria. Among the existing antibacterial agents, fosfomycin has the lowest molecular weight (138 Da), which is not structurally associated with other classes of antibiotics. In intensive care unit (ICU) patients, severe soft tissue infections (STIs) may lead to serious life-threatening problems, and therefore, appropriate antibiotic therapy and often intensive care management (ICM) coupled with surgical intervention are necessary. Fosfomycin is an antibiotic primarily utilized for the treatment of STIs in ICUs. Recently, fosfomycin has attracted renewed interest for the treatment of serious systemic infections caused by multidrug-resistant Enterobacteriaceae. In some countries, intravenous fosfomycin has been prescribed for various serious systemic infections, such as acute osteomyelitis, nosocomial lower respiratory tract infections, complicated urinary tract infections, bacterial meningitis, and bacteremia. Administration of intravenous fosfomycin can result in a sufficient concentration of the drug at different body regions. Dose modification is not required in hepatic deficiency because fosfomycin is not subjected to enterohepatic circulation.

Can Bioimpedance Determine the Volume of Distribution of Antibiotics in Sepsis?

The relationship between the volume of distribution, assessed according to the two-compartmental pharmacokinetic model, and extracellular water estimated by bioimpedance was studied in mechanically ventilated patients with sepsis and capillary leak. A prospective observational study was performed in a twenty-bed general intensive care unit in the university hospital.

Patients received either vancomycin (n=16) or netilmicin (n=12) for more than 48 hours. Those with ascites, pleural effusion, on renal replacement therapy or with haemodynamic instability were excluded. Serum concentrations of drugs were taken for pharmacokinetic analysis before, 1 hour and 4 hours after the 30 minute infusion. Bioimpedance measurement was performed at the time of the third sampling. The protocol was repeated after 24 hours. Fluid balance during the 24 hour interval was recorded.

Extracellular water was increased and represented 45.6 to 46.6% of total body water. Fluid balance correlated with the change of extracellular water (r=0.82, P<0.0001) and total body water (r=0.74, P<0.0001). Volumes of distribution of vancomycin (0.677±0.339 l/kg) and netilmicin (0.505±0.172 l/kg) were increased compared to normal values. A correlation was demonstrated between volume of distribution (Vdarea) of vancomycin and extra cellular water/total body ratio (r=0.70, P<0.0001). The central compartment distribution volume (V1) of netilmicin correlated with extracellular water/total body water ratio (r=0.60, P<0.003). Serum concentrations above the recommended therapeutic range were detected in 81.2% of patients on vancomycin and in 50% of patients on netilmicin.

Increased volumes of distribution can be estimated by the bioimpedance measurements but are not associated with requirements for higher dosage of the glycopeptide or aminoglycoside antibiotics.

Relevance of soft-tissue penetration by levofloxacin for target site bacterial killing in patients with sepsis

Antimicrobial therapy of soft tissue infections in patients with sepsis sometimes lacks efficiency, despite the documented susceptibility of the causative pathogen to the administered antibiotic. In this context, impaired equilibration between the antibiotic concentrations in plasma and those in tissues in critically ill patients has been discussed. To characterize the impact of tissue penetration of anti-infective agents on antimicrobial killing, we used microdialysis to measure the concentration-versus-time profiles of levofloxacin in the interstitial space fluid of skeletal muscle in patients with sepsis. Subsequently, we applied an established dynamic in vivo pharmacokinetic-in vitro pharmacodynamic approach to simulate bacterial killing at the site of infection. The population mean areas under the concentration-time curves (AUCs) for levofloxacin showed that levofloxacin excellently penetrates soft tissues, as indicated by the ratio of the AUC from time zero to 8 h (AUC(0-8)) for muscle tissue (AUC(0-8 muscle)) to the AUC(0-8) for free drug in plasma (AUC(0-8 plasma free)) (AUC(0-8 muscle)/AUC(0-8 plasma free) ratio) of 0.85. The individual values of tissue penetration and maximum concentration (C(max)) in muscle tissue were highly variable. No difference in bacterial killing of a select Staphylococcus aureus strain for which the MIC was 0.5 microg/ml was found between individuals after exposure to dynamically changing concentrations of levofloxacin in plasma and tissue in vitro. In contrast, the decrease in the bacterial counts of Pseudomonas aeruginosa (MIC = 2 microg/ml) varied extensively when the bacteria were exposed to levofloxacin at the concentrations determined from the individual concentration-versus-time profiles obtained in skeletal muscle. The extent of bacterial killing could be predicted by calculating individual C(max)/MIC and AUC(0-8 muscle)/AUC(0-8 plasma free) ratios (R = 0.96 and 0.93, respectively). We have therefore shown in the present study that individual differences in the tissue penetration of levofloxacin may markedly affect target site killing of bacteria for which MICs are close to 2 microg/ml.

Cefepime versus cefpirome: the importance of creatinine clearance

Standard dosage recommendations for beta-lactam antibiotics can result in very low drug levels in intensive care (IC) patients without renal dysfunction. We compared the pharmacokinetics of two fourth-generation cephalosporins, cefepime and cefpirome, and examined the relationship of drug clearance (CL) to creatinine clearance (CL(CR)). Two separate but similar pharmacokinetic studies (which used 2 g twice daily for each antibiotic) were conducted. Blood was sampled after an initial and a subsequent antibiotic dose. Drug plasma concentrations were measured, and pharmacokinetic analyses were conducted and compared. The pharmacokinetics of cefepime and cefpirome are similar in IC patients. Any differences in drug CL can largely be attributed to differences in CL(CR). Despite normal plasma creatinine concentrations, 54% of patients' antibiotic concentrations were less than the minimum inhibitory concentration (MIC) (4 mg/L) for >20% of the dosing interval. Thirty-four percent of patients had CL(CR) >144 mL/min (20% higher than the expected maximum of 120 mL/min). Only CL(CR) was an independent predictor of antibiotic CL. Time above MIC was predicted only by CL(CR). Some IC patients have a very large CL(CR), which results in very low levels of studied antibiotics. Either shortening the dosage interval or using continuous infusions would prevent low levels and keep troughs above the MIC for longer periods. In view of the lack of bedside measurement of cephalosporin levels, we suggest that more frequent use be made of CL(CR) to allow prediction of small concentrations clinically.

IMPLICATIONS

Some intensive care patients have very large creatinine clearances that result in very low levels of fourth-generation cephalosporins. Serum levels of these antibiotics need to be maintained (time > minimum inhibitory concentration is important). Because routine measurements of cephalosporin levels are generally unavailable, we suggest that more frequent use be made of creatinine clearances to allow prediction of low levels and, hence, alterations in dosing.

Plasma and tissue pharmacokinetics of cefpirome in patients with sepsis

OBJECTIVE

Broad initial antibiotic treatment is crucial for patients experiencing septic shock or severe sepsis. Fourth-generation beta-lactam antibiotics, such as cefpirome, are frequently favored in these conditions because of their low toxicity and wide antimicrobial coverage. From recent data, however, there is circumstantial evidence that one reason for the high mortality rate of patients with sepsis might be an impaired penetration of antimicrobial agents from the central compartment to the infectious focus. Thus, the present study aimed at describing penetration properties of cefpirome to the target site of many bacterial infections, which is the extracellular space fluid of soft tissues.

DESIGN

Prospective comparative study of two groups.

SETTING

An intensive care unit and research ward in a university hospital.

SUBJECTS

The study population included 12 patients with septic shock or severe sepsis and a control group of six overall age-matched healthy volunteers.

INTERVENTIONS

To measure cefpirome penetration into the interstitial space fluid of skeletal muscle, we employed microdialysis after single intravenous administration of 2.0 g of cefpirome to patients and healthy volunteers.

MEASUREMENTS AND MAIN RESULTS

Maximum concentration and area under the concentration vs. time values in interstitium were significantly lower in patients compared with the control group (p <.004). Cefpirome area under the concentration time values for plasma were 16.0 +/- 1.1 mg.min/mL (mean +/- sem) and 18.8 +/- 1.1 mg.min/mL in patients and healthy volunteers, respectively (p =.075, not significant). In both study groups, mean cefpirome concentrations in interstitium and plasma exceeded 28 microg/mL throughout the observation period of 240 mins and covered completely minimal inhibitory concentration values for a range of clinically relevant pathogens.

CONCLUSION

Cefpirome concentrations reached in tissue interstitium and plasma exceeded minimal inhibitory concentrations of most clinically relevant pathogens in patients with sepsis. Thus, cefpirome exhibits a tissue pharmacokinetic profile, which seems to be particularly valuable for the empirical therapy of patients with sepsis.