Pharmacokinetics of tobramycin in cystic fibrosis

The pharmacokinetics of antibiotics in cystic fibrosis

INTRODUCTION

Dosing of antibiotics in people with cystic fibrosis (CF) is challenging, due to altered pharmacokinetics, difficulty of lung tissue penetration, and increasing presence of antimicrobial resistance.

AREAS COVERED

The purpose of this work is to critically review original data as well as previous reviews and guidelines on pharmacokinetics of systemic and inhaled antibiotics in CF, with the aim to propose strategies for optimization of antibacterial therapy in both children and adults with CF.

EXPERT OPINION

For systemic antibiotics, absorption is comparable in CF patients and non-CF controls. The volume of distribution (Vd) of most antibiotics is similar between people with CF with normal body composition and healthy individuals. However, there are a few exceptions, like cefotiam and tobramycin. Many antibiotic class-dependent changes in drug metabolism and excretion are reported, with an increased total body clearance for ß-lactam antibiotics, aminoglycosides, fluoroquinolones, and trimethoprim. We, therefore, recommend following class-specific guidelines for CF, mostly resulting in higher dosages per kg bodyweight in CF compared to non-CF controls. Higher local antibiotic concentrations in the airways can be obtained by inhalation therapy, with which eradication of bacteria may be achieved while minimizing systemic exposure and risk of toxicity.

Optimization of anti-pseudomonal antibiotics for cystic fibrosis pulmonary exacerbations: V. Aminoglycosides

Intravenous (IV) anti-pseudomonal aminoglycosides (i.e., amikacin and tobramycin) have been shown to be tolerable and effective in the treatment of acute pulmonary exacerbations (APEs) in both pediatric and adult patients with cystic fibrosis. The aim of this review is to provide an evidence-based summary of pharmacokinetic/pharmacodynamic, tolerability, and efficacy studies utilizing IV amikacin, gentamicin, and tobramycin in the treatment of APE and to highlight areas where further investigation is needed. The Cystic Fibrosis Foundation Pulmonary Guidelines recommend that once-daily administration of aminoglycosides is preferred over three times per day in the treatment of an APE. The literature supports dosing ranges for amikacin and tobramycin of 30-35 and 7-15 mg/kg/day, respectively, given once daily, with subsequent doses determined by therapeutic drug concentration monitoring. The literature does not support the routine use of gentamicin in the treatment of APE due to a lack of studies showing efficacy and evidence indicating an increased risk of nephrotoxicity. Further studies are needed to determine the optimal dosing strategy of amikacin in the treatment of an APE, and to further identify risk factors and determinants that influence the development of P. aeruginosa resistance with once-daily administration of tobramycin.

Pharmacokinetic and tolerability profiles of tobramycin nebuliser solution 300 mg/4 ml administered by PARI eFlow(®) rapid and PARI LC Plus(®) nebulisers in cystic fibrosis patients

BACKGROUND

Tobramycin nebuliser solution (TNS) is indicated for maintenance therapy in cystic fibrosis (CF) patients with chronic Pseudomonas aeruginosa (PA) infections. Adherence to recommended therapy in CF has always been a challenge and new generation nebulisers are increasingly used "off label" to reduce the time required for inhalation, potentially improving patient compliance.

METHODS

In this open-label, randomised, multi-centre, two-period crossover study, 27 CF patients with PA infection received TNS 300 mg/4 mL (TNS4) via the PARI eFlow(®) rapid or PARI LC Plus(®) nebuliser twice daily for 28 days in two study periods separated by a 4-week washout. The pharmacokinetic profile in plasma and sputum were determined after single and multiple dose administration on Day 1 and Day 28, respectively. Nebulisation times and general safety and tolerability profiles were evaluated throughout the study.

RESULTS

Plasma tobramycin pharmacokinetic profiles were similar for the eFlow and LC Plus nebulisers both on Day 1 and Day 28. After multiple dose administration for 28 days, the eFlow/LC Plus ratio of geometric means for plasma C(max) and AUC(0-t), were 85.32 (90% CI, 61.24-118.86) and 87.44 (90% CI, 64.87-117.87), respectively. Despite the high variability, sputum tobramycin C(max) and AUC(0-t) for the eFlow on Day 28 tended to be higher than for the LC Plus (90% CI for the ratio, 86.11-226.45 and 81.81-236.71), respectively. Nebulisation times were significantly shorter for the eFlow with a median time for nebulisation of 5 min in comparison to 13 min for the LC Plus. Safety data confirmed a favourable safety profile for TNS4, with the majority of the findings being related to the underlying CF disease.

CONCLUSIONS

Plasma and sputum pharmacokinetic data in CF patients with chronic PA infection support comparable pulmonary delivery and safety of TNS4 administered using different nebulisers, with a significantly shorter nebulisation time for the eFlow.

Pharmacokinetic profile of once daily intravenous tobramycin in children with cystic fibrosis

AIM

Once-daily tobramycin in patients with cystic fibrosis (CF) is a more convenient dosing regimen than thrice daily dosing. There are limited data on the pharmacokinetic (PK) profile for once-daily tobramycin in patients with CF. The aim of this study was to define the PK parameters for once-daily tobramycin in children with CF and develop an algorithm for therapeutic drug monitoring dosing.

METHODS

CF patients admitted to hospital were commenced on once-daily intravenous tobramycin (12 mg/kg/day) and ticarcillin/clavulinic acid. Serum tobramycin levels were taken at 30 min, 2-4 h and 12 h post dose. Data points for the PK model included: age, sex, weight, tobramycin dose, time of tobramycin doses and levels, tobramycin levels. WinNonMix was used to obtain the PK parameters.

RESULTS

Forty-four children with 86 admissions who were aged 9 months-20 years were included. A one-compartment intravenous infusion model with first order elimination kinetics produced the best model. Population parameters were: volume of distribution (V(d)) = 0.267 L/kg (95% confidence interval (CL) 0.260-0.272), clearance (CL) 0.103 L/kg/h (95% CI 0.098-0.107) and half-life (t(1/2)) 1.82 (95% CI 1.77-1.88) h. Once the population model was established post hoc analysis was used to calculate individual subject predictions. Plots of individual prediction curves agreed well with observed values.

CONCLUSION

This study has established an algorithm for routine monitoring of once-daily tobramycin in children with CF. Satisfactory serum levels of tobramycin were obtained with a dose of 12 mg/kg/day and a regimen algorithm that uses only one measurement to monitor the plasma concentration is suggested.

Antibiotic use in Cystic Fibrosis

Chronic pulmonary infections contribute significantly to the morbidity and mortality of patients with CF. The primary pathogens are Pseudomonas aeruginosa (PA) and Staphylococcus aureus. Hemophilus influenzae has been isolated from a significant number of patients also. A number of the β-lactam and aminoglycoside antibiotics reportedly have altered pharmacokinetic variables in CF. Therapy of acute pulmonary deterioration consists of intravenous antibiotics for two weeks. Antibiotic selection is based on culture and sensitivity results. Currently, the combination of a broad-spectrum penicillin and an aminoglycoside seems to provide the best results. Prophylactic antibiotics are effective if the primary isolates are sensitive to the agents used. Chronic PA infections are problematic because effective oral agents are not available. Aerosolized antibiotics do not improve results over adequate systemic therapy for acute exacerbations. Questions regarding optimal dosages, frequency, and duration of therapy remain.

Normal volume of distribution of tobramycin in a mother and daughter with a CFTR splice mutation (1717 - 1G --> A)

A mother and daughter pair with CF who shared a splice mutation (1717 - 1G --> A) had a normal volume of distribution of tobramycin. The literature on tobramycin pharmacokinetics, which was published before the genetic defect was identified, is discussed. The authors speculate on the role of CFTR in the distribution of aminoglycosides and recommend that CFTR mutations should be clarified in all future studies of tobramycin pharmacokinetics in patients with CF.

Population pharmacokinetics of aminoglycoside antibiotics in patients with cystic fibrosis

The population pharmacokinetics of gentamicin and tobramycin were investigated in a group of 51 young adults with cystic fibrosis. Their ages ranged from 14-35 years, weights from 38-82 kg, and 27 of the patients were female. None of the patients had renal impairment, but 3 patients were treated in the intensive therapy unit (ITU) during one of their courses of therapy. Data comprised 219 courses of therapy and 544 concentrations (mean: 11 per patient). Concentration-time data were analyzed using a nonlinear mixed-effects model package (NONMEM) and were best described by a one-compartment model. Factors identified as potentially influencing aminoglycoside pharmacokinetics were added in a stepwise fashion and the best model found that drug clearance and volume of distribution were related to body surface area and admission to ITU. The mean population estimates were a clearance of 2.89 L/hr/m2 and a volume of distribution of 9.21 L/m2 with a 60% increase in patients who were admitted to ITU. Interpatient variability in clearance and volume were 14% and 8%, respectively. The results suggested that a dose of 120 mg/m2 should achieve an average 1 hour postdose peak of 10 mg/L and trough of <1 mg/L and that higher doses might be required in ITU patients.

Hypothetical framework for enhanced renal tubular secretion of drugs in cystic fibrosis

Several clinical studies demonstrate reduced serum concentrations of renally excreted drugs in patients with cystic fibrosis (CF). To explain this phenomenon, we propose a model supporting increased proximal tubular secretion of certain drugs in individuals with CF. We hypothesize that the chloride channel located on the apical surface of renal proximal tubular cells and controlled by the cystic fibrosis transmembrane conductance regulator (CFTR) operates suboptimally in CF patients, and that the abnormal CFTR decreases Cl- reabsorption, resulting in an increased concentration of Cl- in the tubular lumen. We postulate that, in an effort to maintain homeostasis, luminal Cl- moves intracellularly in exchange for organic anions. The result of stimulating this anion exchanger is an increased rate of organic anion secretion by the renal tubule. Hence, due to enhanced tubular secretion, individuals with CF demonstrate increased tubular clearance of organic anion drugs, resulting in lower steady state serum concentrations.

Routine monitoring of gentamicin serum concentrations in pediatric patients with normal renal function is unnecessary

OBJECTIVE

Due to increasing demands for cost containment within the healthcare system, we evaluated the need for routine gentamicin concentrations (i.e., peak and trough with third dose).

DESIGN

Single-institution study performed concurrently with hospitalization.

SETTING

A 225-bed pediatric teaching hospital.

PARTICIPANTS

The study population consisted of 150 hospitalized pediatric patients (53% medicine, 47% surgical patients) from 3 months to 15 years old with normal serum creatinine.

OUTCOME MEASURES

If the administered dose produced diagnoses-appropriate peak concentrations of at least 4 micrograms/mL or 5 micrograms/mL in bacteremia/septicemia and at least 6 micrograms/mL or 8 micrograms/mL in patients with pneumonia if trough serum gentamicin concentrations were less than 2 micrograms/mL, if the patient was noted by the attending physician to be clinically responding as well as objectively having a decreased white blood cell count and was afebrile, and if there was not an increase of 0.5 mg/dL or more in serum creatinine during the course of therapy.

RESULTS

Patients received a mean dose of gentamicin 2.51 +/- 0.14 mg/kg i.v. q8h, which resulted in a mean peak concentration of 6.1 +/- 1.7 micrograms/mL (range 2.4-11.7) and a mean trough concentration of 0.5 +/- 0.3 microgram/mL (range 0.1-1.8). Peak and trough concentrations were at least 4 micrograms/mL and less than 2 micrograms/mL in 96% and 100% of patients, respectively. No patient required a dosage change due to lack of clinical response.

CONCLUSIONS

Our data do not support the routine monitoring of gentamicin concentrations in pediatric patients older than 3 months of age who are receiving appropriate standard doses of gentamicin and have normal renal function.

Minimisation of aminoglycoside toxicity in patients with cystic fibrosis

BACKGROUND

There is evidence that administration of higher doses of aminoglycosides given less frequently improves the bactericidal effect and reduces the potential to cause side effects. To investigate this, a prospectively randomised open label therapeutic trial was undertaken in stratified groups of patients with cystic fibrosis to examine the efficacy and toxic potential of an aminoglycoside dosing regimen designed to generate high peak drug concentrations at 12 hourly intervals compared with conventional dosing at eight hourly intervals.

METHODS

Patients in group A received tobramycin eight hourly using a dose aimed at generating a peak concentration of 10 mg/l with trough concentrations below 2 mg/l, and those in group B received the total daily dose required to achieve eight hourly target concentrations administered as two equal 12 hourly doses. Clinical outcomes measured and assessed included vestibular symptoms, hearing and renal function, length of hospital stay, readmission rate, and mortality.

RESULTS

Twenty nine patients were recruited during a six month period, 20 to group A and nine to group B. The average peak tobramycin level was higher in group B (12.5 (2.2) mg/l) than in group A (7.9 (1.9) mg/l), whilst the average trough level was higher in group A (0.8 (0.3) mg/l) than in group B (0.5 (0.2) mg/l). There was a difference in the number of ototoxic events between patients in group A (seven of 18, 38.9%) and group B (none of eight), but no difference was found in other outcome measures assessed.

CONCLUSIONS

These results suggest that 12 hourly high peak aminoglycoside dosing may be less toxic than equivalent eight hourly dosing, without any apparent difference in efficacy.

Pregnancy in patients with cystic fibrosis

With increasing life span of patients with CF, more women with CF are becoming pregnant and others are seeking information about the risks involved during pregnancy and delivery. A striking limitation of the available information is the lack of large prospective studies of pregnant patients with CF matched for age and disease severity compared with their non-pregnant cohorts. A study investigating the effect of pregnancy on morbidity and mortality is being completed by the Cystic Fibrosis Foundation. We recommend that all women with CF be offered contraceptive measures and counseling on the maternal and fetal risks of pregnancy, including the genetic risks for the child. The issue of who will raise the child in the event of subsequent morbidity or maternal mortality should ideally be prospectively discussed.

Plasma concentrations and effects of salbutamol administered orally to patients with cystic fibrosis

1. To test whether cystic fibrosis (CF) altered the kinetics and dynamics of oral salbutamol, 11 patients with CF (19-33 years old; five females; FEV1: 37 +/- 12% of predicted value) and 10 healthy volunteers (20-41 years old; five females; FEV1: 99 +/- 14% of predicted value) received orally 4 mg salbutamol. 2. The estimated pharmacokinetic parameters of salbutamol in patients with CF were identical to those in healthy subjects. For instance, peak plasma concentrations of salbutamol were 10.5 +/- 2.6 (mean +/- s.d.) and 10.2 +/- 2.9 ng ml-1 (NS), and the area under salbutamol plasma concentrations as a function of time (AUC (0, 7 h)) was 43.0 +/- 9.3 ng ml-1 h and 43.3 +/- 12.7 ng ml-1 h (NS) in CF patients and in healthy subjects, respectively. Since on a mg kg-1 dose basis, CF patients received a dose 28% greater than healthy subjects, this lack of differences implies a decrease in the amount of salbutamol absorbed, or alternatively, an increase in both clearance and volume of distribution of salbutamol. 3. Salbutamol did not elicit bronchodilation in CF patients, but increased heart rate from 77 +/- 2 to 103 +/- 3 beats min-1 (P < 0.05). 4. Salbutamol decreased plasma potassium concentrations from 4.5 +/- 0.1 to 3.8 +/- 0.1 mmol l-1 in the CF group (P < 0.05) and from 4.1 +/- 0.2 to 3.4 +/- 0.1 mmol l-1 in the controls (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of anti-infective agents in paediatric patients

Various differences in drug disposition exist between children and adults. For example, the volume of distribution (Vd) for many drugs is larger in children than in adults. Other parameters, including excretion and elimination may be altered in children compared with adults. The penicillins and cephalosporins are used commonly for the treatment of infection in paediatric patients. The increased Vd in children contributes to the increased elimination half-life of these agents. Clearance of the acylureido-penicillins is increased in children with cystic fibrosis, a disease that decreases the elimination half-life for these drugs. Aminoglycosides distribute into extracellular fluid and their pharmacokinetic profile is affected by changes in Vd. The Vd for aminoglycosides is slightly higher in children than in adults. Children with cystic fibrosis, burns, or cancer have higher clearance rates and larger Vd values for aminoglycosides. Few data in the literature address the pharmacokinetics of other anti-infective agents, including vancomycin, teicoplanin, erythromycin, metronidazole, chloramphenicol, and cotrimoxazole (trimethoprim-sulfamethoxazole), in children. Similarly, there is little information regarding the pharmacokinetic profile of antivirals and antifungals in children. Dosage guidelines are available to enable the clinician to initiate anti-infective therapy in children. Subsequent dosage requirements may change based on the patient's current clinical condition. Although several studies have investigated the pharmacokinetics of anti-infectives in neonates and adults, data for children are limited. Therefore, further studies are required so that the ever growing arsenal of anti-infectives can be administered appropriately to children.

Endotracheal and aerosol administrations of ceftazidime in patients with nosocomial pneumonia: pharmacokinetics and absolute bioavailability

Pharmacokinetic studies on ceftazidime, an aminothiazole cephalosporin with a wide spectrum of antibacterial activity, including activity against Pseudomonas aeruginosa, were performed in patients with nosocomial pneumonia. The concentration-time profiles of ceftazidime in plasma, urine, and bronchial secretions of 12 patients were investigated after intravenous (i.v.) (n = 12), endotracheal (n = 10), and aerosol (n = 5) administrations. In all cases a 1-g dose was administered. Concentrations of drug in all samples were assayed by high-performance liquid chromatography with UV detection. The elimination of the drug from the blood followed a biexponential (i.v. administration) or a monoexponential (endotracheal and aerosol administrations) decay, with an elimination half-life of 6 h and a total body clearance of 4.2 liters/h. The apparent volume of distribution was 0.36 liter/kg of body weight. Renal clearance of the drug accounted for 58% of the total clearance; 66% +/- 17.7%, 33.5% +/- 17.3%, and 6.59% +/- 3.45% of the administered dose were eliminated in urine as parent drug after i.v., endotracheal, and aerosol administrations, respectively. The absolute bioavailabilities were 0.47 and 0.08 for endotracheal and aerosol administrations, respectively. Very high concentrations were found in bronchial secretions after local administration. The MICs for 90% of the most important pathogens responsible for nosocomial infections were exceeded by concentrations in bronchial secretion for up to 12 h after i.v. infusion and for up to 24 h after endotracheal and aerosol administrations.

Aminoglycoside induced ototoxicity in patients with cystic fibrosis

The improved survival of cystic fibrosis (CF) patients is partly due to intensive treatment for their chronic infections. Treatment usually includes intravenous and nebulised aminoglycoside antibiotics and they receive a large cumulative dose of these antibiotics over their lifetime. There is little information in the literature on the prevalence of ototoxicity due to aminoglycoside in these patients. We performed pure tone audiometry on 43 CF patients aged 14-42 years. Seven (16%) had bilateral sensorineural hearing loss (SNHL) for high frequency sounds, consistent with aminoglycoside induced ototoxicity. However, only 2 of these patients had documented toxic serum levels in the past. The identification of bilateral SNHL in one in six adult CF patients is a cause for concern. It may be that the high cumulative dose of aminoglycosides received by these patients may be causing inner ear injury in the absence of specific episodes of toxic serum levels.

Aminoglycoside volume of distribution in pediatric patients

Pharmacokinetic parameters of three aminoglycoside antibiotics were studied retrospectively in 218 pediatric patients to determine an apparent volume of distribution (Vd) for this age group and to determine if Vd is significantly different in pediatric patients compared with adults. Data on patients considered for inclusion in the study were obtained from the files of the aminoglycoside monitoring services at Saskatoon University Hospital and Regina General Hospital. Both services use a computer program that calculates pharmacokinetic parameters using the Sawchuk-Zaske method. Children between the ages of 1 and 16 years with normal renal function from whom serum concentration had been obtained were included in the study. Exclusion criteria included abnormal or unstable renal function, cystic fibrosis, and pregnancy. The mean age of the pediatric group was 8.65 +/- 5.37 years. Average values for Vd and half-life were 0.34 L/kg and 2.3 h, respectively. No strong correlation was found between the Vd (L/kg) and age. The patients were subdivided into three age groups: 1-4.9 years, 5-9.9 years, and 10-16 years. Group 1 (1-4.9 years) had a larger Vd than the other groups and the Vd of all three groups were significantly different from the estimated Vd of 0.20 L/kg for adult patients. A "normal" pediatric value for the Vd of aminoglycosides could not be determined; however, the Vd in children is significantly larger than the Vd in adults and dosage regimens should be adjusted accordingly.

Rapid methylprednisolone clearance in a patient with cystic fibrosis

A pharmacokinetic investigation of administered corticosteroids was conducted in a 16-year-old girl with cystic fibrosis (CF). Equivalent doses of methylprednisolone were given orally and intravenously on consecutive days. Oral bioavailability was essentially 100 percent, ruling out impaired absorption. Drug half-life was shorter and both clearance and volume of distribution were greater in this patient than in normal young adults. From one patient it is impossible to conclude that this unusual disposition of corticosteroids is characteristic of CF. Given the interest in use of corticosteroids in this population, and the examples of other drugs with altered pharmacokinetics in patients with CF, further investigation is warranted.

Clinical pharmacology of antibiotics and other drugs in cystic fibrosis

The disposition of many drugs in cystic fibrosis is abnormal. In general, changes in pharmacokinetics include: increased volume of distribution, decreased plasma concentration, and enhanced renal and sometimes non-renal elimination of drugs. Pathophysiology of the disease important for drug disposition includes: (a) hypersecretion of gastric acid and duodenal secretions which are of small volume, viscous and low in bicarbonate; (b) increased intestinal permeability to some sugars and probe substances; (c) hypergammaglobulinaemia and sometimes hypoalbuminaemia; (d) significant elevation of free fatty palmitoleic acid level and decreased low-density and high-density serum lipoproteins; (e) an average increase by 30 to 45% in plasma volume in patients with cystic fibrosis who have moderately severe pulmonary disease, right ventricle hypertrophy and dilatation, which occurs in 15 to 35% of patients with a Shwachman score of 81 to 100; (f) abnormal bile acid metabolism and enterohepatic recirculation; and (g) enlarged kidneys and glomerulomegaly with increased glomerular filtration rate, tubular clearance and urine flow rate in some patients with cystic fibrosis. Delayed absorption from the gastrointestinal tract has been reported in patients with cystic fibrosis for cloxacillin, epicillin, clindamycin, ciprofloxacin and probably for cephalexin, para-aminobenzoic acid and chloramphenicol. A possible increased absorption was reported for cimetidine. Of 7 drugs studied only theophylline had significantly decreased plasma protein binding. An increased volume of distribution and increased renal clearance reported for several drugs is caused mainly by increases in plasma volume and urine flow rate in many of these patients. Possible increased elimination of some drugs in bile (which probably results from bile acid malabsorption) and in bronchial secretions (which are abundant in some cystic fibrosis patients with acute pulmonary infection) may explain enhanced non-renal elimination of these drugs. The metabolism of cimetidine in cystic fibrosis was reported not to be changed significantly compared to control subjects.

Individualized aminoglycoside dosage regimens in patients with cystic fibrosis

Individualized dosage regimens have recently been recommended for patients treated with aminoglycoside antibiotics. We have developed a calculator-based program for our patients with cystic fibrosis and have studied 93 courses of intravenous aminoglycoside treatment, comparing predicted and measured values in 45 courses. Pharmacokinetic parameters differed notably among subjects: this was reflected by widely variable total daily aminoglycoside dosage requirements. The mean daily dosage requirements (+/- SD) for tobramycin (62 treatment courses) was 13.0 +/- 3.74 mg/kg, and for gentamicin (26 treatment courses) was 11.5 +/- 2.6 mg/kg. The accuracy of the program was evaluated by its ability to predict peak and trough values in individuals: 84 percent of measured peaks were within 2 micrograms/ml of the predicted level. Nephrotoxicity was observed in one patient, ototoxicity in three. This program provides a simple, safe, and effective method of tailoring an aminoglycoside regimen to the patient's needs.

Tobramycin in patients with cystic fibrosis. Adjustment in dosing interval for effective treatment

The efficacy of the dosing regimen of tobramycin was investigated in 28 patients with cystic fibrosis who had an acute exacerbation of chronic pulmonary infection with Pseudomonas aeruginosa. The initial dose of tobramycin was 3.3 mg/kg of body weight three times daily (ie, 10 mg/kg/day). A highly significant relationship was found between the serum concentration of tobramycin before the dose and the change in the forced expiratory volume in one second (FEV1), both measured on the tenth day of treatment (rs = 0.75; p less than 0.001). In nine of the 16 patients who had a six-hour serum concentration of 1 mg/L or less on the tenth day of treatment, the eight-hour dosing interval of tobramycin was shortened to achieve a serum concentration of tobramycin of about 1 mg/L before the dose. In the other seven patients, the dosage of tobramycin was not changed. On the 20th day, seven of the nine patients in whom the dosing interval was shortened exhibited an increase in FEV1 of 20 percent or more. Such an increase was observed only in one of the seven patients in whom the dosing interval was not reduced (p less than 0.05). We conclude that individualizing the dosage of tobramycin in patients with cystic fibrosis results in a better clinical outcome.

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

In this review we analyzed the pharmacokinetic basis for high dose treatment with antibiotics of patients with cystic fibrosis. Both our results and those from other well designed pharmacokinetic studies do not support the view that low blood levels of antibacterials are a common feature of CF. We were unable to detect a decrease in absorption, nor could we find evidence for enhanced elimination of antibacterials in CF. Both these factors have been considered responsible for reducing the plasma (and tissue) levels of antibiotics. Most recent studies on kidney function are in agreement with these findings, since neither inulin nor creatinine clearance differ between CF-patients and healthy volunteers. In contrast to previous discussion, the volume of distribution (Vdss) was not elevated for any compound. The rational of weight correction of volume terms like Vdss or total clearance has never been clearly demonstrated and should therefore not be used without prior proof of relevance. Since the variability of pharmacokinetic parameters of antibiotics in CF-patients may be considerable, we suggest that a dose increase of 20-30% may be justified, but cannot agree with two to fourfold increases in dosage as previously proposed and applied in many CF-centers. Until more findings become available for non-adult CF-patients, these conclusions are only valid for adult CF-patients.

Kinetic parameters of amikacin in cystic fibrosis children

35 kinetic studies have been performed, in nine CF children three to 15 years old; six kinetic studies were performed in four non-CF children, one to 12 years old. The dosage was 5 to 12.5 mg/kg, i.v. during 0.5 to 1.0 h, three to four times per day. Amikacin concentrations were measured in the plasma of all children, and in the sputum of CF-patients, by fluorescent polarization (TDX Abbott). The pharmacokinetic parameters in the plasma did not differ significantly in both groups of patients. In CF children t1/2 = 0.94 h (SD = 0.25 h), Vd (area) = 0.257 l/kg (SD = 0.06 l/kg), total body clearance = 130.7 ml/min/1.73 m2 (SD = 32.4 ml/min/1.73 m2). In non-CF children t1/2 = 0.83 h (SD = 0.15 h), Vd (area) = 0.265 l/kg (SD = 0.04 l/kg) and clearance = 155 ml/min/1.73 m2 (SD = 17.4 ml/min/1.73 m2). The parameters were not affected by the dosage of amikacin. The peak plasma concentrations ranged from 19 to 43.8 mg/l. Amikacin peak level in the sputum of CF children never reached the average MIC (4 mg/l) of Pseudomonas aeruginosa strains isolated in these patients. Amikacin concentration in the sputum reached its highest value about 2 h after the completion of i.v. infusion and was directly related to the peak plasma concentration. According to these parameters, the best dosage regimen appeared to be 7.5 to 8 mg/kg or 225 to 240 mg/m2 administered intravenously in 1.0 h, three times per day.

Direct determination of tobramycin clearance in patients with mild-to-moderate cystic fibrosis

Studies performed in patients with cystic fibrosis (CF) have suggested altered pharmacokinetic parameters for aminoglycosides. Specifically, increased plasma clearance (Cl) of aminoglycosides and increased apparent volume of distribution have been noted. In the present study, tobramycin Cl is determined by both serum concentration data and direct renal clearance (Clren). Tobramycin Clren appeared to be directly correlated to the measured creatinine clearance (Clcr) (r = 0.93, p less than 0.01). The tobramycin Cl, by both methods of determination, was not elevated in comparison to the Clcr or expected values for patients without the disease. These results appear to corroborate a recent study in which the renal and plasma Cl of gentamicin was measured in patients with mild-to-moderate CF and were not noted to be elevated. It is suggested that standard doses of tobramycin be used initially in patients with mild-to-moderate CF with dosage adjustment based on serum concentration data to achieve the desired goals.

Cumulative and acute toxicity of repeated high-dose tobramycin treatment in cystic fibrosis

Forty-six patients with cystic fibrosis and chronic bronchopulmonary Pseudomonas aeruginosa infection entered a study of tobramycin-related chronic and acute nephro- and acousticovestibular toxicity. The patients (mean age, 15.7 years) had previously received 2-week courses of tobramycin therapy, for a mean cumulative total of 279 days each. The cumulative tobramycin dose ranged from 632 to 7,644 mg/kg. The patients were studied before and at the end of a 2-week course of treatment with tobramycin (10 to 20 mg/kg per day) to discriminate between acute and chronic toxicity. In patients studied at the beginning of the present course of treatment, the glomerular filtration rate, measured as 24-h creatinine clearance, did not correlate with the cumulative dose of tobramycin received during previous courses. Eighteen patients (39%) had a reduced glomerular filtration rate compared with normal values (mean, 12.5% reduction) but normal serum creatinine values. Two patients (5%) had a high-frequency hearing deficit (above 8 kHz), but only one deficit was possibly related to tobramycin. No chronic vestibular toxicity was observed. During the course of treatment, no patients developed acute nephrotoxicity. After 2 weeks of treatment 32% had a slightly reduced hearing threshold (15 to 30 dB) in two or more high frequencies, and 28% had a fall in vestibular response greater than 25% of the initial value but remained within normal limits. Thus, the acute and chronic toxicity of repeated high-dose tobramycin treatment in cystic fibrosis patients seems to be very mild.

Tobramycin elimination rate change from first to later doses in older cystic fibrosis patients

Adults with cystic fibrosis frequently require larger than usual tobramycin dosages in order to achieve desired serum concentrations. The application of dosing methods utilizing first-dose pharmacokinetics has been advocated as a means for rapidly attaining therapeutic serum concentrations. Review of ten cystic fibrosis patients between ages 13 and 33 years admitted for exacerbation of pulmonary disease caused primarily by Pseudomonas aeruginosa (Staphylococcus aureus in two patients) was conducted. Elimination rate constant (ke, h-1) was calculated from two concentration-time pairs obtained following the first dose. Two concentration-time pairs were again measured between cumulative doses (n) 7 to 19, and ke was calculated. First dose ke varied significantly from nth dose ke (p = 0.018). First-dose pharmacokinetic analysis may not be a reliable predictor of maintenance tobramycin dosage requirements due to apparent changes in ke over time.

Individualizing gentamicin dosage in patients with cystic fibrosis: limitations to pharmacokinetic approach

Gentamicin serum concentrations were measured in 15 children and seven adults with cystic fibrosis and in eight children with other diseases. Potentially toxic trough concentrations occurred in three of the first nine patients studied, in whom the dose and a 4-hour dosing interval were prescribed on the basis of one-compartment pharmacokinetic calculations (Sawchuck-Zaske method). In contrast, final concentrations were within the accepted target ranges for the remaining 13 patients with cystic fibrosis, in whom the dose and interval were adjusted empirically on the basis of a single pair of "peak" and trough values. The mean +/- SD final dosage required to achieve target concentrations was 13.8 +/- 2.9 mg/kg/d for children and 11.8 +/- 1.1 mg/kg/d for adults (P greater than 0.05), generally divided into four doses at 6-hour intervals. Mean half-life and incremental increase in serum concentration from previous trough to subsequent "peak," an indirect measurement of volume of distribution, were not significantly different between children or adults with cystic fibrosis and pediatric control subjects; there was little interpatient variability in these values. Thus the high dosage requirements were related more to the higher target concentrations than to altered pharmacokinetic disposition in patients with cystic fibrosis. We conclude that the initial dose of gentamicin to achieve a peak of 8 to 12 micrograms/mL and a trough of less than 2.0 micrograms/mL in patients with cystic fibrosis should be 3 mg/kg administered every 6 hours in children and every eight hours in adults. Subsequent dosage adjustment should be made on the basis of a pair of peak and trough serum concentration measurements obtained after the fifth dose. Dosing intervals in this patient population generally should be no shorter than every 6 hours, even if the initial trough concentration is less than 1 microgram/mL.

Renal function in cystic fibrosis: proteinuria and enzymuria before and after tobramycin therapy

Proteinuria and enzymuria were measured in 27 patients with Cystic Fibrosis before and after tobramycin therapy. Prior to treatment, kidney function was normal in 23 patients. Four patients showed a pathological proteinuria and two haematuria. Renal biopsy in one patient showed segmental basement membrane alterations on electron microscopy; there were no immunoglobulin deposits. During intravenous therapy with tobramycin (10 mg/kg per day) and azlocillin (100 mg/kg per day) mean urinary N-acetyl-beta-D-glucosaminidase (NAG) excretion rose six-fold and mean urinary alaninaminopeptidase excretion increased ten-fold. After cessation of therapy, enzymuria rapidly returned to pretreatment values in all 14 patients. Aerosol tobramycin therapy in four patients did not affect urinary excretion of NAG. It can be concluded that tobramycin did not cause persistent renal damage in our patients, whether given intravenously or as an aerosol.

Introduction to pharmacokinetics: aminoglycosides in cystic fibrosis as a prototype

Pharmacokinetics is the study of the time course of drug absorption, distribution, metabolism, and excretion, allowing examination of the potential relationships between drug disposition and pharmacologic or toxicologic effects. The pharmacokinetics of the aminoglycosides and certain other drugs are different in patients with cystic fibrosis (CF), but this topic is controversial. Differences in disease severity between study subjects and in the methods used might explain the disparities. Understanding the fundamental principles of pharmacokinetics is necessary for the clinician to evaluate drug disposition data in patients with CF. To determine whether observed pharmacokinetic differences are attributable to CF, the investigator must consider a number of factors in the design and conduct of pharmacokinetic studies: analytical methods, study population selection, techniques for drug administration, method used to collect biologic specimens, evaluation of parallel rates and routes of drug excretion, and selection of pharmacokinetic and statistical techniques. Pharmacokinetic investigation in patients with CF should permit evaluation of the complete disposition profile for a drug, allow comparison between the experimental data and factors that characterize the disease state, and be rigorous enough to provide explanations for any observed variability in pharmacokinetics.

Cephalosporin therapeutics in cystic fibrosis

Cephalosporins modified at the C-3 and C-7 positions of the cephem-nucleus have high antimicrobial activity and are safe. With evolution through first, second, and third generations, they have gained increasing gram-negative activity, but often at the expense of potency against gram-positive organisms. All third-generation cephalosporins have some intrinsic anti-Pseudomonas activity, indicating their potential benefit in the treatment of acute pulmonary exacerbations in patients with cystic fibrosis. Rational therapy in this clinical setting requires recognition of the pharmacodynamic and pharmacokinetic idiosyncrasies intrinsic to this patient population. When these priorities are recognized, only two of the available agents, cefsulodin and ceftazidime, appear to be of any therapeutic value. Both agents have been evaluated extensively in the treatment of acute pulmonary exacerbation in cystic fibrosis, and both have been found to be safe and effective.

Pharmacokinetics of amikacin in cystic fibrosis: a study of bronchial diffusion

36 pharmacokinetic studies of amikacin were performed to evaluate the bronchial diffusion of amikacin in 9 children with cystic fibrosis, 3 to 15 years old. Amikacin was administered i.v. according to a variable dosage regimen. Four children without cystic fibrosis were enrolled as controls. The mean half life was 1.1, the volume of distribution averaged 0.26 l/kg, and the mean plasma clearance was 131 ml/min/1.73 m2, which no differed from that of the controls. The mean peak plasma concentration was always above the MIC but its level depended on the unit dose: 18.5 mg/l, 25,95 mg/l and 31,46 mg/l for doses of 5, 7.5 and 12.5 mg/kg, respectively. Between consecutive amikacin infusions, the plasma level was above the MIC for 21% and 46% of the time after the 5 and 7.5 mg/kg doses. The maximum concentration in sputum between H1 and H2 was always below the MIC, except after 15 mg/kg. The ratio AUC sputum/AUC plasma was between 0.028 and 0.61, and it increased from the beginning to the end of the course of treatment. No side effects were observed on hearing, or vestibular and renal function. The results are used to suggest more appropriate dosing regimens.

Ciprofloxacin pharmacokinetics in patients with cystic fibrosis

The pharmacokinetics of ciprofloxacin were studied in cystic fibrosis patients and healthy volunteers following oral administration of 500 mg and 1000 mg. Serum kinetics as well as urinary recovery were monitored. As the body weights of cystic fibrosis patients and the healthy volunteers differed significantly, kinetic parameters were calculated on the basis of a constant relative dose in mg/kg body weight. Neither serum kinetics nor urinary recovery differed significantly between the two groups, as indicated by the serum concentration versus time curves ranging from 1.1 to 1.4 mg X h/l, the elimination half-life of 4.4 to 5.1 h and the 24 h urinary recovery which amounted to 35% to 41% of the dose administered. Serum concentrations were linearly proportional to the doses administered. Sputum concentrations were monitored in cystic fibrosis patients. Again, ciprofloxacin sputum levels were linearly proportional to the doses and were within the same range as serum concentrations. Thus, ciprofloxacin kinetics are not altered in cystic fibrosis patients as compared to healthy volunteers.

Pharmacokinetics and dosage requirements of netilmicin in cystic fibrosis patients

The pharmacokinetics of netilmicin were determined in 10 patients with cystic fibrosis. Mean (+/- standard error of the mean) values for total body clearance and volume of distribution were 2.62 (+/- 0.18) ml/min per kg of body weight and 0.38 (+/- 0.01) liter/kg, respectively, and were considerably larger than the same parameters reported for patients without cystic fibrosis.

Evaluation of high-dose tobramycin-carbenicillin therapy in pseudomonal infections in cystic fibrosis

Fourteen episodes of acute pseudomonal pulmonary exacerbations of cystic fibrosis were treated with high-dose tobramycin (10.1-17.1 mg/kg/d) and carbenicillin (600 mg/kg/d). The Sawchuck-Zaske method of dosing tobramycin was used and resulted in good agreement between the desired and measured peak (8.1 +/- 0.5 vs. 8.2 +/- 1.5 micrograms/ml) and trough (0.5 +/- 0.2 vs. 1.0 +/- 0.4 micrograms/ml) concentrations. Ninety-three percent of cases improved clinically. Forced expiratory volume in one second and forced vital capacity increased significantly (39.3 +/- 24.8, p less than 0.001 and 24.1 +/- 22.8 percent, p less than 0.05, respectively) after treatment. Pseudomonas aeruginosa was eradicated from the sputa of 69 percent of the cases, with recolonization occurring within three months. Significant nephrotoxicity and ototoxicity were not seen. Liver enzymes, however, were elevated in 29 percent of those treated. Combination high-dose carbenicillin and tobramycin satisfies the treatment goals of bacteriological and clinical cure with a minimal degree of toxicity.

Increased dosage requirements of tobramycin and gentamicin for treating Pseudomonas pneumonia in patients with cystic fibrosis

The pharmacokinetic behavior of tobramycin and gentamicin was evaluated in 27 patients who had cystic fibrosis (CF). A previously studied, age-matched group of 334 patients who had been treated with gentamicin and who did not have CF served as controls. The CF patients, who ranged in age from 2 to 32 years and who had normal renal function, received 36 treatment courses with either tobramycin (19) or gentamicin (17) to treat Pseudomonas pneumonia. Serum concentrations were determined after a 1.5-mg/kg dose to compute half-life (t 1/2), elimination rate constant (k), and apparent volume of distribution (V). From these values, doses were calculated to produce steady-state peak concentrations of 8.0 micrograms/ml with a dosing interval of every six hours. For tobramycin the mean (+/- SD) t1/2 was 1.0 (0.4) hours, V was 0.18 (0.06) l/kg, total body clearance (TBC) was 2.19 (0.71) ml/min/kg, and the calculated dose was 8.2 (2.1) mg/kg/day. For gentamicin t1/2 was 1.1 (0.5) hours, V was 0.20 (0.06) l/kg, TBC was 2.28 (0.89) ml/min/kg, and the calculated dose was 8.8 (2.4) mg/kg/day. The pharmacokinetic parameters were not statistically different between the two drugs, but the mean values of t1/2 and TBC of CF patients differed significantly from those of the control group. The calculated doses were larger than the manufacturer's maximum recommended dose of 7.5 mg/kg/day for 63% of tobramycin and 71% of gentamicin treatment courses. A dosing interval change to every four hours would have been appropriate in 28 of the 36 treatment courses (78%).(ABSTRACT TRUNCATED AT 250 WORDS)

Ceftazidime therapy in patients with cystic fibrosis and multiply-drug-resistant pseudomonas

The in vitro activity of ceftazidime against Pseudomonas aeruginosa and P. cepacia isolates from patients with cystic fibrosis was compared with that of other antipseudomonal drugs. Ceftazidime was as potent as imipenem against P. aeruginosa and the only drug effective against P. cepacia. An evaluation of the elimination kinetics of ceftazidime in 20 cystic fibrosis patients revealed an elimination half-life of 1.76 hours, an apparent distribution volume of 0.27 liters/kg, and a serum clearance rate of 133.9 ml/minute/1.73m2. Urinary recovery of ceftazidime was 87 percent within the first 24 hours after administration of the drug, with 65 percent recovered in the first two-hour fraction. Probenecid administration had no effect on the elimination kinetics of ceftazidime. Forty-three patients who had either shown no response to conventional therapy or had sputum Pseudomonas isolates that were susceptible only to ceftazidime received 75 courses of therapy. In 67 percent of these patients, the clinical response, when evaluated using an objective clinical efficacy scoring system, was considered favorable. Clinical failures were not associated with the development of drug resistance. Thus, ceftazidime can be recommended for the treatment of acute pulmonary exacerbations in patients with cystic fibrosis.

Pharmacokinetics of tobramycin in patients with cystic fibrosis. Implications for the dosing interval

The pharmacokinetics of tobramycin were evaluated in 15 patients (8 to 22 years of age) with cystic fibrosis (CF). A dose of 3.0 to 3.3 mg/kg of body weight was given intravenously over 20 minutes, and concentrations in serum were followed up to eight hours after initiation of the infusion. In the calculation of pharmacokinetic parameters, a two-compartment open model was used. The elimination half-life of the drug was highly inversely correlated with age (p less than 0.0004), and body weight (p less than 0.00002). Total body clearance (TBC), and volume of distribution at steady state (VDSS) were directly correlated with age and body weight. However, when TBC and VDSS were corrected for BSA, no correlation could be demonstrated. The mean one-hour and eight-hour serum concentrations of tobramycin were 5.40 and 0.45 microgram X ml-1, respectively. Between patients, considerable differences were found in the time after administration at which the serum concentration decreased below 1 microgram X ml-1. This interpatient variation has clinical implications for tobramycin therapy in CF, in particular for the dosing interval.

Assessment of glomerular filtration rate and effective renal plasma flow in cystic fibrosis

A study was conducted to examine renal function in 10 healthy control subjects and eight patients with cystic fibrosis in stable condition. Sequential bolus injections of 99mTc-DTPA and 125I-OIH were administered to assess glomerular filtration rate and effective renal plasma flow, respectively. Blood was subsequently collected for 3 hours, and urine for 24 hours. Renal clearances of both radioisotope markers were virtually identical in patients and controls. Inasmuch as neither glomerular filtration rate nor effective renal plasma flow was enhanced in patients with cystic fibrosis, increased clearance of drugs in these patients is unlikely to be the result of enhanced glomerular filtration or tubular secretion.

Dosage of tobramycin in cystic fibrosis--a short report

A 15 kg, 7-year-old girl with cystic fibrosis was admitted with a chest infection. It was found that a tobramycin, 12 mg/kg/day (standard recommended dose 6-7.5 mg/kg/day), was required to obtain acceptable plasma levels and that the volume of distribution varied in line with the patient's weight changes.

Ticarcillin/clavulanic acid pharmacokinetics in children and young adults with cystic fibrosis

The single-dose pharmacokinetics of ticarcillin and clavulanic acid (Timentin) were evaluated in children and young adults with cystic fibrosis after a 0.5-hour intravenous infusion of both a 3.1 and a 3.2 gm formulation (representing 3.0 gm ticarcillin combined with 100 mg and 200 mg clavulanic acid, respectively) in a crossover design. A 75 mg/kg dose of the ticarcillin component was used. Model-dependent and noncompartmental pharmacokinetic parameters were congruous. The disposition of ticarcillin and clavulanic acid was characterized adequately by a one-compartment open model. The elimination half-life, apparent steady-state volume of distribution, and total body clearance of ticarcillin from serum were 1.19 hours, 0.231 L/kg, and 0.150 L/hr/kg, respectively, for the 3.1 gm formulation and 1.21 hours, 0.211 L/kg, and 0.123 L/hr/kg, respectively, for the 3.2 gm formulation. For ticarcillin, 86% and 93% of the dose of the 3.1 and 3.2 gm formulations, respectively, were excreted unchanged in urine during the first 6 hours after infusion. Concomitant renal clearance values were 0.120 and 0.112 L/hr/kg for the 3.1 and 3.2 gm formulations, respectively. Approximately 50% of a clavulanic acid dose was excreted unchanged in urine during the 6-hour postinfusion period for both formulations. For ticarcillin, no significant differences were observed between the 3.1 and 3.2 gm formulations. For clavulanic acid, a significant difference between the two formulations was observed in comparison of the area under the serum concentration vs time curve and dose size (P less than 0.01). Linear inverse relationships were identified between demographic factors (e.g., age, weight, height, body surface area) and both the apparent volume of distribution and total body clearance of ticarcillin and clavulanic acid for both formulations. The ticarcillin/clavulanic acid combination in either the 3.1 or 3.2 gm formulation is suitable for microbiologic and clinical evaluation in patients with cystic fibrosis.

Cloxacillin absorption and disposition in cystic fibrosis

Because of reports of lowered antibiotic serum concentrations in patients with cystic fibrosis (CF), a bioavailability and pharmacokinetic study of cloxacillin was conducted in 12 control and 16 patients with CF after intravenously and orally administered doses of cloxacillin 25 mg/kg. The patients had mild to moderate CF and were in stable condition. Significantly lower serum concentrations in CF were a result of a 78% increase in total body clearance (P less than 0.005) and a 38% increase in the apparent volume of distribution (P less than 0.025). The bioavailability in CF (0.50) was not significantly different than in controls (0.38), but more variability was seen in the group with CF. After the intravenously given dose the fraction of cloxacillin excreted in the urine unchanged was similar in controls (0.644) and patients with CF (0.547). Compared with that in the control subjects, the mean renal clearance in patients with CF was 30% greater (P less than 0.10) and the nonrenal clearance was 144% greater (P less than 0.07). Enhanced nonrenal clearance explains most of the demonstrated difference between serum concentrations in controls and patients with CF after identical weight-adjusted doses. The data suggest enhanced cloxacillin biotransformation in CF.

Antibiotic use in cystic fibrosis

Chronic pulmonary infections contribute significantly to the morbidity and mortality of patients with CF. The primary pathogens are Pseudomonas aeruginosa (PA) and Staphylococcus aureus. Hemophilus influenzae has been isolated from a significant number of patients also. A number of the beta-lactam and aminoglycoside antibiotics reportedly have altered pharmacokinetic variables in CF. Therapy of acute pulmonary deterioration consists of intravenous antibiotics for two weeks. Antibiotic selection is based on culture and sensitivity results. Currently, the combination of a broad-spectrum penicillin and an aminoglycoside seems to provide the best results. Prophylactic antibiotics are effective if the primary isolates are sensitive to the agents used. Chronic PA infections are problematic because effective oral agents are not available. Aerosolized antibiotics do not improve results over adequate systemic therapy for acute exacerbations. Questions regarding optimal dosages, frequency, and duration of therapy remain.

Disposition of tobramycin in patients with cystic fibrosis: a prospective controlled study

The pharmacokinetics of tobramycin in adolescents or young adults with cystic fibrosis and in age-matched controls were prospectively compared. Patients with CF had a higher tobramycin total body clearance (121.2 +/- 14.2 ml/min/1.73 m2) than did controls (102.2 +/- 18.9 ml/min/1.73 m2, P less than 0.05). This was not associated with a higher glomerular filtration rate (iothalamate total body clearance 147.5 +/- 29.2 ml/min/1.73 m2 in patients vs 142.9 +/- 33.3 ml/min/1.73 m2 in controls) or a lower binding of gentamicin to serum proteins (14.3% +/- 2.6% in patients vs 17.4% +/- 3.8% in controls). Tobramycin renal clearance was not significantly different in the two groups (89.5 +/- 17.9 ml/min/1.73 m2 in patients vs 81.0 +/- 15.8 ml/min/1.73 m2 in controls). In the control group, tobramycin total body and renal clearances were highly correlated with iothalamate total body clearance (r = +0.95 and +0.88, P less than 0.01). In patients with cystic fibrosis, the correlation was not significant (r = +0.56, P greater than 0.05 for total body clearance, and r = 0.32, P greater than 0.1 for renal clearance). There was no significant difference in volume of distribution normalized to body surface area or in half-life of elimination. The higher tobramycin total body clearance without an increase in renal clearance, and the lower correlation with glomerular filtration rate indicate that an extrarenal clearance pathway might play a significant role in the elimination of tobramycin from the serum of patients with cystic fibrosis.

Single-dose pharmacokinetics of cefsulodin in patients with cystic fibrosis

The single-dose pharmacokinetics of cefsulodin were evaluated in 12 patients with cystic fibrosis. Each patient received 3 g of cefsulodin intravenously over 30 min. Multiple plasma and urine samples were obtained during the 6-h study period for the determination of cefsulodin. Pharmacokinetic parameters were determined by model-independent methods. Mean values for t1/2, Vss, and CLp were 1.53 h, 0.242 liters/kg, and 117.3 ml/min per 1.73 m2, respectively. Six-hour urine recovery revealed 73.2% of the administered dose with a corresponding cefsulodin urinary clearance of 75.1 ml/min. These pharmacokinetic data in patients with cystic fibrosis appear consistent with data reported for unaffected individuals.

Azlocillin pharmacokinetics in patients with cystic fibrosis

The pharmacokinetics of azlocillin were studied in 10 cystic fibrosis patients, ranging in age from 11 to 28 years. The patients received a 9- to 23-day course of 350 mg of azlocillin per kg in four or six divided daily doses in combination with am aminoglycoside. Blood and urine samples were collected at specified times after the last dose of the course of azlocillin therapy and then assayed for azlocillin content. Pharmacokinetic parameters were determined by noncompartmental analysis. Mean values for serum half-life (1.74 h), disposition constant (0.41 h-1), total body clearance (123 ml/kg per h), and renal clearance (58 ml/kg per h) were determined. All patients exhibited improvement with respect to clinical and laboratory parameters and displayed no adverse reactions. The pharmacokinetic analysis offers further evidence of the dose-dependent nature of azlocillin elimination, but elimination did not appear to be altered in cystic fibrosis patients.