High dose treatment with antibiotics in cystic fibrosis--a reappraisal with special reference to the pharmacokinetics of beta-lactams and new fluoroquinolones in adult CF-patients

Novel Population Pharmacokinetic Approach to Explain the Differences between Cystic Fibrosis Patients and Healthy Volunteers via Protein Binding

The pharmacokinetics in patients with cystic fibrosis (CF) has long been thought to differ considerably from that in healthy volunteers. For highly protein bound β-lactams, profound pharmacokinetic differences were observed between comparatively morbid patients with CF and healthy volunteers. These differences could be explained by body weight and body composition for β-lactams with low protein binding. This study aimed to develop a novel population modeling approach to describe the pharmacokinetic differences between both subject groups by estimating protein binding. Eight patients with CF (lean body mass [LBM]: 39.8 ± 5.4kg) and six healthy volunteers (LBM: 53.1 ± 9.5kg) received 1027.5 mg cefotiam intravenously. Plasma concentrations and amounts in urine were simultaneously modelled. Unscaled total clearance and volume of distribution were 3% smaller in patients with CF compared to those in healthy volunteers. After allometric scaling by LBM to account for body size and composition, the remaining pharmacokinetic differences were explained by estimating the unbound fraction of cefotiam in plasma. The latter was fixed to 50% in male and estimated as 54.5% in female healthy volunteers as well as 56.3% in male and 74.4% in female patients with CF. This novel approach holds promise for characterizing the pharmacokinetics in special patient populations with altered protein binding.

[Pharmacokinetics and pharmacodynamics of antibiotics in intensive care]

Optimized dosage regimens of antibiotics have remained obscure since their introduction. During the last two decades pharmacokinetic(PK)-pharmacodynamic(PD) relationships, originally established in animal experiments, have been increasingly used in patients. The action of betalactams is believed to be governed by the time the plasma concentration is above the minimum inhibitory concentration (MIC). Aminoglycosides act as planned when the peak concentration is a multiple of the MIC and vancomycin seems to work best when the area under the plasma vs. time curve (AUC) to MIC has a certain ratio. Clinicians should be aware that these relationships can only be an indication in which direction dosing should go. Larger studies with sufficiently high numbers of patients and particularly severely sick patients are needed to prove the concepts. In times where all antibiotics can be measured with new technologies, the introduction of therapeutic drug monitoring (TDM) is suggested for ICUs (Intensive Care Unit). The idea of a central lab for TDM of antibiotics such as PEAK (Paul Ehrlich Antibiotika Konzentrationsmessung) is supported.

Optimization of anti-pseudomonal antibiotics for cystic fibrosis pulmonary exacerbations: VI. Executive summary

Acute pulmonary exacerbations (APE) are well-described complications of cystic fibrosis (CF) and are associated with progressive morbidity and mortality. Despite aggressive management with two or more intravenous anti-pseudomonal agents, approximately 25% of exacerbations will result in a loss of lung function. The aim of this review is to provide an overview of the classes of intravenous anti-pseudomonal antibiotics, the findings of anti-pseudomonal antibiotic utilization surveys, the current antibiotic dosing recommendations from the U.S. and Europe, and the pharmacokinetic (PK) and pharmacodynamic (PD) differences between CF and non-CF individuals. Anti-pseudomonal antibiotic classes include beta-lactams, aminoglycosides, fluoroquinolones, and colistimethate sodium. Recent surveys of antibiotic utilization in CF Foundation-accredited care centers have shown that a large number of centers are not following recommended dosing strategies despite published recommendations in the U.S. and Europe. The recommended doses for anti-pseudomonal antibiotics may be higher than FDA-approved doses due to PK and PD differences. As a large portion of CF patients will not regain their lung function following an APE, it seems possible that currently available anti-pseudomonal agents are being used sub-optimally. As new anti-pseudomonal agents are not currently available, we suggest the need to optimize antibiotic dosing and dosing regimens used to treat pulmonary exacerbations in an effort to improve outcomes for CF patients infected with Pseudomonas aeruginosa.

Population pharmacokinetic comparison and pharmacodynamic breakpoints of ceftazidime in cystic fibrosis patients and healthy volunteers

Despite the promising activity of ceftazidime against Pseudomonas aeruginosa and Burkholderia cepacia, there has not yet been a study that directly compared the pharmacokinetics (PK) of ceftazidime in cystic fibrosis (CF) patients and healthy volunteers by population PK methodology. We assessed the population PK and PK/pharmacodynamic (PD) breakpoints of ceftazidime in CF patients and healthy volunteers. Eight CF patients (total body weight [WT] [average +/- standard deviation] = 42.9 +/- 18.4 kg) and seven healthy volunteers (WT = 66.2 +/- 4.9 kg) received 2 g ceftazidime as a 5-min intravenous infusion. High-performance liquid chromatography (HPLC) was used for drug analysis, and NONMEM (results reported), S-ADAPT, and NPAG were used for parametric and nonparametric population PK modeling. We considered linear and allometric body size models to scale clearance and volume of distribution. Monte Carlo simulations were based on a target time of non-protein-bound plasma concentration of ceftazidime above MIC of > or =65%, which represents near-maximal killing. Unscaled total clearance was 19% lower in CF patients, and volume of distribution was 36% lower. Total clearance was 7.82 liters/h for CF patients and 6.68 liters/h for healthy volunteers with 53 kg fat-free mass. Allometric scaling by fat-free mass reduced the between-subject variability by 32% for clearance and by 18 to 26% for volume of both peripheral compartments compared to linear scaling by WT. A 30-min ceftazidime infusion of 2 g/70 kg WT every 8 h (q8h) achieved robust (> or =90%) probabilities of target attainment (PTAs) for MICs of < or =1 mg/liter in CF patients and < or =3 mg/liter in healthy volunteers. Alternative modes of administration achieved robust PTAs up to markedly higher MICs of < or =8 to 12 mg/liter in CF patients for 5-h infusions of 2 g/70 kg WT q8h and < or =12 mg/liter for continuous infusion of 6 g/70 kg WT daily.

Comparison of the pharmacokinetics and pharmacodynamic profile of carumonam in cystic fibrosis patients and healthy volunteers

Our objectives were to compare the pharmacokinetics (PK) of carumonam, a monobactam, between cystic fibrosis (CF) patients and healthy volunteers and assess its pharmacodynamic profile. We studied 10 adult CF patients and 18 healthy volunteers of similar body size (dose: 2.166 g of carumonam as 15-min intravenous infusion). High performance liquid chromatography with ultraviolet detection (HPLC-UV) was used for drug analysis and NONMEM (ICON, Ellicot City, MD) for population PK and Monte Carlo simulation with targets between > or =20% and 100% free time above MIC (fT > MIC). Unscaled renal clearance was 24% higher in CF patients. Lean body mass and creatinine clearance explained the difference in average clearance and volume of distribution between both subject groups. For a daily dose of 6 g per 70 kg of total body weight, 15-min infusions q8h achieved robust (>90%) probabilities of target attainment (PTAs) (target, 60% fT > MIC) for MICs < or =3 mg/L in CF patients and < or =6 mg/L in healthy volunteers. At the same dose, 4-h infusions q8h achieved robust PTAs up to markedly higher MICs < or =8 to 12 mg/L in CF patients and < or =16 mg/L in healthy volunteers.

Systematic comparison of the population pharmacokinetics and pharmacodynamics of piperacillin in cystic fibrosis patients and healthy volunteers

Respiratory tract infections cause 90% of premature mortality in patients with cystic fibrosis (CF). Treatment of Pseudomonas aeruginosa infection is often very problematic. Piperacillin-tazobactam has good activity against P. aeruginosa, but its pharmacokinetics (PK) in CF patients has not been compared to the PK in healthy volunteers in a controlled clinical study. Therefore, we compared the population PK and pharmacodynamics (PD) of piperacillin between CF patients and healthy volunteers. We studied 8 adult (median age, 20 years) CF patients (average total body weight [WT], 43.1 +/- 7.8 kg) and 26 healthy volunteers (WT, 71.1 +/- 11.8 kg) who each received 4 g piperacillin as a 5-min intravenous infusion. We determined piperacillin levels by high-performance liquid chromatography, and we used NONMEM for population PK and Monte Carlo simulation. We used a target time of nonprotein-bound concentration above the MIC of 50%, which represents near-maximal bacterial killing. Unscaled total clearance was 25% lower, and the volume of distribution was 31% lower in CF patients. Allometric scaling by lean body mass reduced the unexplained (random) between-subject variability in clearance by 26% compared to the variability of linear scaling by WT. A standard dosage regimen of 3 g/70 kg body WT every 4 h as a 30-min infusion (daily dose, 18 g) achieved a robust (> or =90%) probability-of-target attainment (PTA) for MICs of < or =12 mg/liter in CF patients and < or =16 mg/liter in healthy volunteers. Alternative modes of administration allowed a marked dose reduction to 9 g daily. Prolonged (4-h) infusions of 3 g/70 kg WT every 8 h and continuous infusion (daily dose, 9 g), achieved a robust PTA for MICs of < or =16 mg/liter in both groups. Piperacillin achieved PTA expectation values of 64% and 89% against P. aeruginosa infection in CF patients, based on susceptibility data from two German CF clinics.

Pharmacokinetics of aztreonam in healthy subjects and patients with cystic fibrosis and evaluation of dose-exposure relationships using monte carlo simulation

Aztreonam (AZM) is a monobactam antibiotic with a high level of activity against gram-negative micro-organisms, including Pseudomonas aeruginosa. We evaluated AZM pharmacokinetics and pharmacokinetic-pharmacodynamic relationships in patients with cystic fibrosis (CF) and healthy subjects. Pharmacokinetic data in eight CF patients and healthy subjects that were matched for age, gender, weight, and height were obtained and analyzed by using the nonparametric adaptive grid algorithm. Probabilities of target attainment using percentages of time of unbound concentration above the MIC (fT>MIC) were obtained by using a Monte Carlo simulation. AZM total body clearance was significantly higher in CF patients (100.1 +/- 17.1 versus 76.2 +/- 7.4 ml/min in healthy subjects; P < 0.01). The pharmacokinetic parameter estimates for terminal half-life (1.54 +/- 0.17 h [mean +/- the standard deviation]) and volume of distribution (0.20 +/- 0.02 liters/kg in patients with CF patients were not different from those in healthy subjects. Monte Carlo simulations with a target of a fT>MIC of 50 to 60% at a dose of 1,000 mg every 8 h indicated a clinical breakpoint of 4 mg/liter and 1 to 2 mg/liter for healthy subjects and CF patients, respectively. This study using matched controls showed that AZM total body clearance and not the volume of distribution is higher in CF patients as a result of increased renal clearance. Pharmacokinetic parameter estimates in healthy subjects resulted in a clinical susceptibility breakpoint of < or =4 mg/liter for a dose of 1,000 mg every 8 h. Patients suspected of having high clearance rates, such as CF patients, should be monitored closely, with dosing regimens adjusted accordingly.

Can the enhanced renal clearance of antibiotics in cystic fibrosis patients be explained by P-glycoprotein transport?

PURPOSE

To investigate in vitro if P-glycoprotein (P-gp) transport can differentiate between antibiotic drugs exhibiting increased active renal clearance (CL(r)) in cystic fibrosis (CF) patients (i.e., dicloxacillin, trimethoprim) and drugs that do not exhibit this phenomenon (i.e.. cefsulodin, sulfamethoxazole).

METHODS

Transport studies were carried out in MDCK (wild type) and MDR1-MDCK (P-gp overexpressing) cells that were grown to confluence on Transwell inserts. [14C]-mannitol transport and transepithelial electrical resistance (TEER) were measured to validate the integrity of the cells. Drug concentrations were analyzed using HPLC.

RESULTS

Dicloxacillin and trimethoprim are substrates of P-gp (B-->A/A-->B ratios in MDR1-MDCK cells are 32 and 50, respectively). P-gp inhibitors (i.e., GG918, cyclosporine, ketoconazole, vinblastine) decreased the B-->A transport of dicloxacillin and trimethoprim and increased the A-->B transport of trimethoprim while non-P-gp inhibitors (e.g., PAH) had no effect. In contrast, cefsulodin and sulfamethoxazole are not substrates of P-gp (B-->sA/A-->B values in MDCK and MDR1-MDCK cells are -1).

CONCLUSIONS

Our in vitro studies suggest that P-glycoprotein may play a role in increasing renal clearance of drug substrates in CF patients. Dicloxacillin and trimethoprim. which are both substrates of P-gp, show increased active renal clearance in CF patients while cefsulodin and sulfamethoxazole, which are not P-gp substrates, do not show increased active renal clearance in CF patients.

Drug disposition in cystic fibrosis

There are many pathological changes in patients with cystic fibrosis (CF) which can lead to alterations in drug disposition. Although, in patients with CF, the extent of drug absorption varies widely and the rate of absorption is slower, bioavailability is not altered. Plasma protein binding for the majority of drugs studied did not differ in patients with CF compared with control groups. The difference in volume of distribution of most drugs between patients with CF and healthy individuals vanished when corrected for lean body mass. Despite hepatic dysfunction, patients with CF have enhanced clearance of many, but not all, drugs. Phase I mixed-function oxidases are selectively affected: cytochrome P450 (CYP) 1A2 and CYP2C8 have enhanced activity, while other CYP isoforms such as CYP2C9 and CYP3A4 are unaffected. Increased phase II activities are also demonstrated: glucuronyl transferase, acetyl transferase (NAT1) and sulfotransferase. The increased hepatic clearance of drugs in the presence of CF may be the consequence of disease-specific changes in both enzyme activity and/or drug transport within the liver. The renal clearance (CLR) of many drugs in patients with CF is enhanced although there has been no pathological abnormality identified which could explain this finding: glomerular filtration rate and tubular secretion appear normal in patients with CF. The precise mechanisms for enhanced drug clearance in patients with CF remain to be elucidated. The optimisation of antibiotic therapy in patients with CF includes increasing the dose of beta-lactams by 20 to 30% and monitoring plasma concentrations of aminoglycosides. The appropriate dosage of quinolones has not been definitively established.

Utilization of salivary concentrations of ciprofloxacin in subjects with cystic fibrosis

Ciprofloxacin, an orally-absorbed fluoroquinolone is effective against multiply resistant Pseudomonas aeruginosa in cystic fibrosis patients. It is the only practicable agent against extraintestinal salmonellosis and shigellosis in developing countries. However, concern with the risk of arthropathy in young children has restricted its use in pediatrics. Pharmacokinetic studies with ciprofloxacin are limited in the pediatric population. As a result, the dose and frequency of administration are not established in children. In this study the possibility of using salivary concentrations as surrogate measure of serum concentrations was investigated. A pediatric formulation of the drug (125 mg per capsule) was prepared and compared to 250 mg tablets. Relative bioavailability was 105% (tablet/capsule). The time to peak salivary concentration and elimination rate from saliva were significantly different from serum (p < 0.01 and p < 0.05 respectively). The linear regression analysis of post-peak concentrations in serum and saliva yielded a slope of 1.25 and correlation coefficient of 0.83. It was also found that salivary concentrations may be contaminated from drug retained in the oral cavity. The conclusion was drawn that salivary concentrations could not be reliably used as a surrogate measure of serum levels for therapeutic drug monitoring.

Optimisation of antibiotic therapy in cystic fibrosis patients. Pharmacokinetic considerations

Antibiotic therapy plays a central role in the medical management of patients with cystic fibrosis. While totally convincing efficacy data are lacking, antibiotics probably have a pronounced beneficial effect on both morbidity and mortality. Much has been learned in the past 20 years about antibiotic use in this population. At the same time, new antimicrobial agents with the potential to treat this condition have become available for use. The pharmacokinetics of a number of antibiotic classes, including beta-lactams, aminoglycosides and quinolones, are altered in this patient population. Increased total body clearance is a common occurrence but is not always changed enough to warrant altered dosages. Nonetheless, in light of altered pharmacokinetics in the cystic fibrosis population, appropriate dosage and monitoring parameters for a number of antibiotics have been determined.

Hepatic drug metabolism in cystic fibrosis: recent developments and future directions

OBJECTIVE

To review the most current information pertaining to hepatic drug metabolism in patients with cystic fibrosis (CF) and to explore the possible association between CF and specific pathways for the hepatic biotransformation of xenobiotics.

DATA SOURCES

A MEDLINE search (key terms: cystic fibrosis, pharmacokinetics, metabolism, pharmacogenetics) was used to identify pertinent literature, including reviews. Research findings from the author's laboratory are also presented.

STUDY SELECTION

Only recently reported (from 1988 to present), controlled, clinical investigations of hepatic drug metabolism in patients with CF are included. These investigations examined a mechanistic basis for altered drug biotransformation. Although uncontrolled clinical trials, case reports, and review articles are not included in the discussion, appropriate reference citations are made to these works.

DATA EXTRACTION

Data from well-designed, controlled, clinical and basic investigations of altered hepatic drug biotransformation in patients with CF are summarized and discussed. New data from an ongoing study concerning the renal excretion of antipyrine metabolites in these patients are presented.

DATA SYNTHESIS

In vivo studies of the formation clearance for metabolites of fleroxacin, sulfamethoxazole, and theophylline clearly demonstrate increased activity for important P-450 isoenzymes. These data are supported by an in vitro study that confirmed increased microsomal metabolism of theophylline to 1-methylxanthine, 3-methylxanthine, and 1,3-dimethyluric acid in a liver specimen from a patient with CF. These findings not only substantiate disease-specific increases in hepatic phase I biotransformation in patients with CF, but also verify the premise of substrate specificity for this pharmacogenetic phenomenon. Likewise, pharmacokinetic studies of drugs that undergo significant hepatic phase II biotransformation (e.g., furosemide, lorazepam, ibuprofen) appear to support increased hepatic drug clearance in patients with CF. This assertion has also been confirmed by a study of acetaminophen disposition, which demonstrated significantly increased formation clearance of the sulfate and glucuronide conjugates of the drug. Finally, the marked increase in the plasma clearance of indocyanine green, a pharmacologic probe for the biliary uptake and excretion of drugs, lends credence to the assertion that increased hepatic clearance of drugs in the presence of CF may be the consequence of disease-specific changes in both enzyme activity and/or drug transport within the liver.

CONCLUSIONS

Investigations of drug biotransformation in CF have revealed disease-specific increases in the formation of drug metabolites. Future application of techniques in molecular biology and biochemical pharmacology will need to characterize the mechanistic basis for altered drug metabolism in CF and expand our knowledge of the relationship between drug metabolism phenotype and genotype; the impact of growth, development, and disease severity on drug metabolism; the potential role of CF gene products (i.e., CFTR) on intrahepatic drug transport and biotransformation; and the pharmacogenetic determinants of substrate specificity for hepatic drug metabolism in CF.

Increased oral bioavailability of ciprofloxacin in cystic fibrosis patients

The altered pharmacokinetic properties of, e.g., aminoglycosides in cystic fibrosis patients have to be considered when pulmonary exacerbations are treated. Since reported data on ciprofloxacin, a fluorinated quinolone, are conflicting, we compared intravenous and oral administration in cystic fibrosis patients when treating them for mild symptoms of pulmonary infection. All of the patients were colonized with Pseudomonas species. Ciprofloxacin was administered orally (15 mg/kg of body weight) or intravenously (6 mg/kg) twice a day for at least 10 days during separate treatment periods. Five healthy volunteers received single intravenous and oral doses. Pharmacokinetic evaluations were performed at first dose and at steady state. The results showed that cystic fibrosis patients have increased oral bioavailability of ciprofloxacin (80% in cystic fibrosis patients versus 57% in volunteers) and increased total clearance (688 ml/min in CF patients versus 528 ml/min in volunteers). Our data indicate that the pharmacokinetic properties of ciprofloxacin are altered in cystic fibrosis patients with mild symptoms of pulmonary exacerbations and that the changes most probably are due to cystic fibrosis per se or to the impact of chronic infection.

Fluoroquinolones in the treatment of cystic fibrosis: a critical appraisal

While the appropriate usage of antibiotics in cystic fibrosis patients is still a matter of debate, the introduction of oral antipseudomonal antibiotics such as fluoroquinolones represents an eagerly awaited addition to the therapeutic armamentarium. Ciprofloxacin is the single agent most often studied and used in this population for treatment of pulmonary exacerbations. Altered pharmacokinetics of fluoroquinolones have been described in cystic fibrosis patients as for other drugs, and a higher dosage than usual is recommended. Open clinical trials have shown good efficacy of ciprofloxacin in acute infection. A few comparative trials have demonstrated that ciprofloxacin is as effective clinically as conventional intravenous agents. As with other agents, a lack of correlation between clinical improvement and bacteriologic evaluation has been observed. Ciprofloxacin (and possibly ofloxacin) are considered useful alternatives to parenteral agents in therapy of cystic fibrosis patients older than 18 years of age with exacerbations of pulmonary infection. Intermittent therapy with ciprofloxacin alternating with other conventional treatment appears to be a rational approach; clinical trials evaluating the alternation of fluoroquinolones with intravenous anti-pseudomonal therapy are necessary. Considering the potential for emergence of resistance and the not completely elucidated implication of increasing MICs during ciprofloxacin therapy, the duration of treatment should be limited to 2 to 4 weeks. In older children (12 to 18 years old), ciprofloxacin provides an alternative to intravenous agents when clinically justifiable. In view of the possibility of fluoroquinolone associated-arthropathy in younger children, ciprofloxacin should be used judiciously when no alternative agents are available or in life-threatening situations.

Selection of antibiotics for treatment and prophylaxis of staphylococcal infections in cystic fibrosis patients

Susceptibility in vitro of 179 staphylococcal strains from 107 cystic fibrosis patients against 31 antibiotics indicates that only teicoplanin, vancomycin and netilmicin were active against all strains. The use of betalactam antibiotics is impaired by 11.7% of methicillin-resistant strains. The bactericidal kinetics of cephalexin and flucloxacillin as determined in a pharmacodynamic model demonstrates the killing of strains resistant to cephalexin (MIC 8 mg/l to 32 mg/l) by flucloxacillin. For the rational selection of antistaphylococcal antibiotics for cystic fibrosis patients, both the MIC of the isolates and the concentration of the antibiotics in cystic fibrosis patients have to be considered.

The tubular excretion of benzylpenicillin in patients with cystic fibrosis

1. The tubular excretion of benzylpenicillin (BP) was investigated in six volunteers with cystic fibrosis. 2. The volunteers received a continuous infusion of BP at increasing infusion rates in order to maintain constant plasma concentrations at three different levels. Blood and urine samples were taken every 30 min. Sufficient urinary flow was ensured by a saline infusion (500 ml h-1). 3. The renal clearance of BP was calculated for the non-protein bound fraction of the drug. 4. Tubular clearance and tubular excretion rate were estimated from the renal clearance of the antibiotic minus the glomerular filtration rate; the latter was considered to be equal to creatinine clearance. 5. The data were analysed according to a Scatchard plot and values for ECu50 and maximal tubular excretion rate were calculated. The mean value of ECu50 was 89 +/- 24 (mg l-1 +/- s.d.) and that for the maximal tubular excretion rate was 2603 +/- 714 (mg h-1 +/- s.d.). The latter value was significantly less than that found in a previous study of healthy volunteers, but the ECu50 was similar. 6. It is concluded that the tubular excretory capacity for BP is decreased in patients with cystic fibrosis in direct relation to their low body weight.

Bacteriological effects of anti-Pseudomonas aeruginosa chemotherapy in cystic fibrosis

Strains of Pseudomonas aeruginosa resistant to clinically relevant antibiotics (beta-lactams, quinolones, aminoglycosides) were detectable in sputa of cystic fibrosis patients. Correlations between in vitro susceptibility and bacteriological results in vivo were demonstrated at a quantitative level. P. aeruginosa strains susceptible prior to antibiotic therapy were observed to become resistant towards each of the compounds used for treatment. We conclude that antibiotic therapy in cystic fibrosis has to be optimized by culture specific selection of the drugs and consecutive bacteriological follow-ups.