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

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.

Renal function in pediatric cystic fibrosis patients in the first decade of life

With increasing life expectancy and the need for lung transplantation in the cystic fibrosis (CF) population, there are increasing reports of chronic kidney disease (CKD). However, values for baseline or longitudinal glomerular filtration rate (GFR) as measured by exogenous clearance markers are lacking in this population. Retrospective cross-sectional study in 2 to 18-year-olds cared for at a single CF center who had a GFR measured by plasma disappearance of Technetium-99 m diethylenetriaminepentaacetic acid (mGFR). The primary outcome was evidence of renal dysfunction as defined by CKD stage II or below (mGFR <90 ml/min/1.73 m(2), persistent abnormalities in urinary sediment, abnormal renal imaging). Of 63 patients evaluated, four had apparent renal dysfunction, one demonstrated decreased mGFR, and three others had persistent microscopic hematuria. The mean mGFR was substantially higher (140 ± 24 ml/min/1.73 m(2)) than expected or previously reported for healthy children. We did not demonstrate the presence of significant renal impairment after limited aminoglycoside exposure in the first decade following diagnosis with CF. However, we did document the presence of glomerular hyperfiltration in a significant proportion of our CF patients.

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.

[Antibiotic therapy in cystic fibrosis. I. Pharmacologic specifics of antibiotics]

Antibiotherapy is one of the main treatment in cystic fibrosis. Antibiotic administration schedules are different from normal patients because of pharmacokinetic and pharmacodynamic particularities. In moderate disease, the digestive resorption of antibiotics is delayed and their half-life is reduced due to an increase in total clearance. In severe disease, the volume of distribution of antibiotics is increased due to the higher proportion of lean mass in these malnourished patients. Other particularities limit the action of antibiotics such as thick sputum, which limits drug penetration; the property of Pseudomonas aeruginosa to be surrounded by a biofilm; alteration of local antibacterial defense; and inhibition of antibiotics by local factors. Systematic prescription of a biotherapy beta-lactam-aminoglycoside and obtaining high antibiotic concentration in situ might limit this antagonism. In spite of particular therapeutic schedules such as single daily dose for aminoglycoside and continuous infusion for beta-lactams, the intervals between administrations must be narrowed for time-dependent antibiotics, and the total daily dose increased by 20 to 30% for concentration-dependent antibiotics.

Physiological basis of cystic fibrosis: a historical perspective

Physiological Basis of Cystic Fibrosis: A Historical Perspective. Physiol. Rev. 79, Suppl.: S3-S22, 1999. - Cystic fibrosis made a relatively late entry into medical physiology, although references to conditions probably reflecting the disease can be traced back well into the Middle Ages. This review begins with the origins of recognition of the symptoms of this genetic disease and proceeds to briefly review the early period of basic research into its cause. It then presents the two apparently distinct faces of cystic fibrosis: 1) as that of a mucus abnormality and 2) as that of defects in electrolyte transport. It considers principal findings of the organ and cell pathophysiology as well as some of the apparent conflicts and enigmas still current in understanding the disease process. It is written from the perspective of the author, whose career spans back to much of the initial endeavors to explain this fatal mutation.

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.

Pharmacokinetics of famotidine in patients with cystic fibrosis

Famotidine pharmacokinetics were studied in 13 patients with severe cystic fibrosis (CF) ranging from 10 to 47 years of age and 25 to 72 kg in weight. Patients were randomized to first receive famotidine either 20 mg intravenously or 40 mg orally. Twelve patients were crossed over to the alternate treatment. Repeated blood samples were obtained over 12 hours after intravenous and oral administration and urine was collected over 24 hours for quantitation of famotidine by means of high-performance liquid chromatography (HPLC). A compartment model-dependent approach was used to characterize the disposition of famotidine. From the intravenous data, the mean +/- standard deviation elimination half-life (t1/2) was 2.11 +/- 0.75 hours, the total clearance (Cl) was 0.79 +/- 0.41 L/kg/hr, the renal clearance was 0.57 +/- 0.26 L/kg/hr, the fraction eliminated unchanged in the urine was 83% +/- 16%, and the apparent volume of distribution (Vdss) was 1.33 +/- 0.53 L/kg. The bioavailability determined from comparison of intravenous and oral area under the curve data was 71% +/- 27%. Results of this study support an initial famotidine dose of 20 mg intravenously or 40 mg orally every 12 hours in patients with CF who are older than 9 years of age.

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.

Immunoreactive calmodulin in cystic fibrosis kidneys

BACKGROUND

Microscopic nephrocalcinosis has been detected at autopsy in 35 of 38 (92%) cystic fibrosis (CF) patients. A primary defect of calcium metabolism in the kidney has been postulated. Intracellular calcium and calmodulin (a calcium binding protein) have been shown to be increased in CF tissues.

AIMS

To determine whether CF kidneys obtained at autopsy contain normal amounts of calmodulin and whether the presence of immunoreactive calmodulin correlates with nephrocalcinosis.

METHODS

An immunoperoxidase method was used to stain kidneys obtained from six CF individuals aged three to 27.5 years (mean 15.75 +/- 10.2 years) and five individuals who died of chronic debilitating disease aged eight to 17 years (mean 13.6 +/- 3.9 years). Calcium staining was undertaken using the von Kossa (silver precipitation) and Alizarin red methods.

RESULTS

No quantifiable difference in immunoreactive calmodulin staining was demonstrated in autopsy renal specimens between six CF patients and five chronically diseased controls. Three of six CF patients aged 22.8 +/- four years and two of five chronically diseased controls aged eight to 16 years respectively had detectable immunoreactive calmodulin staining. Within the CF group the presence of microscopic nephrocalcinosis did not correlate with immunoreactive calmodulin staining.

CONCLUSION

This study does not support the premise that a calmodulin-dependent defect of intracellular calcium handling contributes to the microscopic nephrocalcinosis seen in CF.

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.

Angiotensin converting enzyme inhibitor therapy to decrease microalbuminuria in normotensive children with insulin-dependent diabetes mellitus

It has been proposed that lowering glomerular pressure in children with insulin-dependent diabetes mellitus will reduce microalbuminuria and that this reduction may preserve renal function. We therefore conducted a double-blind, placebo-controlled, crossover trial to compare 3 months of treatment with the angiotensin converting enzyme inhibitor captopril (0.9 mg/kg/day), and 3 months of placebo administration to 12 normotensive adolescents with insulin-dependent diabetes mellitus, 11 with microalbuminuria (albumin excretion rate of 15 to 200 micrograms/min) and one with early overt nephropathy. Mean age (+/- SD) was 14.4 +/- 1.7 years, and disease duration was 5.1 +/- 2.5 years. Albumin excretion rate decreased significantly during captopril therapy (baseline 78 +/- 114 micrograms/min; mean of monthly measurements 38 +/- 55 micrograms/min vs placebo 78 +/- 140 micrograms/min; p less than 0.001). During captopril therapy, albumin excretion was reduced by 41 +/- 44% and decreased in 10 of 12 subjects, but was unchanged in two, one with a borderline albumin excretion rate (16.3 micrograms/min) and one with diabetes of short duration (2.9 years). Plasma renin activity rose significantly during captopril therapy, and mean arterial pressure decreased slightly (placebo 81 +/- 7 mm Hg; captopril 76 +/- 5 mm Hg; p = 0.004). After 3 months of captopril treatment, glomerular filtration rate and renal plasma flow did not change significantly. Hemoglobin Alc values remained stable during the study. The only side effect of captopril was diarrhea in one patient. We conclude that, in the short term, captopril is effective in decreasing albumin excretion rate in normotensive children with insulin-dependent diabetes mellitus and microalbuminuria, without significant side effects. Longer trials are indicated in an attempt to delay or prevent overt nephropathy.

Increased renal clearance of cefsulodin due to higher glomerular filtration rate in cystic fibrosis

The steady-state renal clearance of cefsulodin was studied in 6 patients with cystic fibrosis and 8 healthy controls. The drug was administered by constant rate infusion to obtain 2 values of plasma concentration, 2 and 30 mg/L. As an estimate of the glomerular filtration rate, the renal clearance of inulin was measured simultaneously. The results showed the figures for inulin clearance to be approximately 30% higher in cystic fibrosis patients than in healthy controls at both concentrations, and a corresponding increase in the renal clearance of cefsulodin was seen in patients over controls. The ratio between the renal clearances of the 2 substances was on average 0.9 in both groups. The correlation found between the 2 renal clearances (r = 0.75; p less than 0.001) indicates that glomerular filtration rate has considerable influence on the renal elimination of cefsulodin. This finding emphasises the importance of glomerular filtration rate for the renal clearance of drugs in cystic fibrosis.

Pharmacokinetics of drugs in cystic fibrosis

Considerable work has been conducted on patients with CF to clearly demonstrate enhanced clearances of many, but not all drugs. The precise mechanisms for enhanced drug clearance in CF remain to be elucidated, and it will be important to examine the two major organs of drug elimination, the kidney and liver, for answers to these questions. Although it has not been studied to date, the role of the lung as a contributor to enhanced drug clearance in CF needs to be considered as well. In the liver it appears that both hepatic biotransformation as well as biliary excretion may be enhanced. In the kidney, there is enhanced CL of acidic drugs, but variable findings for cationic and zwitterionic drugs. The latter finding can be explained on the basis of the pKa of the drugs and the urine pH. It would be most appealing if enhanced renal and nonrenal clearance of drugs in CF could be attributed to a common single defect. At this time, there is no obvious candidate, although altered transmembrane transport would represent a place to start.

Renal handling of beta 2-microglobulin in patients with cystic fibrosis

We evaluated the renal handling of beta 2-microglobulin (beta 2-M) and creatinine in healthy outpatients (n = 6), normal children hospitalized for infections treated with antibiotics (not including an aminoglycoside) (n = 4); outpatients with cystic fibrosis (CF; n = 12), and hospitalized patients with CF (n = 6) who received a 10- to 14-day course of antibiotic treatment that included an aminoglycoside. The serum beta 2-M concentrations in the normal outpatients (2020.1 +/- 276.6 micrograms/L) were significantly lower (p less than 0.05) than those observed for outpatients (2833.3 +/- 202.6 micrograms/L) or patients with CF (2861.8 +/- 340.5 micrograms/L. There were no significant differences found for creatinine clearance or fractional excretion of beta 2-M when subjects without CF were compared with those with the disease. Furthermore, no significant differences were observed in hospitalized patients with CF when creatinine clearance and fractional excretion of beta 2-M were compared between the initiation and conclusion of aminoglycoside treatment. Glomerular filtration and proximal tubular reabsorption of beta 2-M were not altered in patients with CF. These findings do not support a global defect in proximal renal tubular reabsorption as the underlying cause for altered aminoglycoside clearance in patients with CF.

Effect on renal function of essential fatty acid supplementation in cystic fibrosis

Changes in renal hemodynamics, sodium homeostasis, renal acidifying capacity, and aldosterone excretion were studied before and after long-term intravenous essential fatty acid supplementation for a period of 3 years in 11 patients with cystic fibrosis. The mean (+/- SD) glomerular filtration rate was high at the start of the study (133 +/- 18 ml/min/1.73 m2 body surface area) and decreased significantly (p less than 0.05) to within normal values after 1 year of essential fatty acid supplementation. The urinary elimination of an oral sodium load initially was very low (3.6 +/- 2.5 mmol/hr/1.73 m2 body surface area vs control subjects' values of 7.9 +/- 2.0; p less than 0.001) and increased during treatment but was not normalized (p less than 0.05 vs control subjects' values). Free water clearance and distal tubular sodium delivery, which were significantly decreased before treatment (p less than 0.01 and p less than 0.001 vs control subjects' values, respectively) did not increase significantly. The mean urinary aldosterone excretion did not significantly differ from that in control subjects before and after treatment. The acidifying capacity was disturbed, indicating a low renal bicarbonate threshold, and was changed during treatment in only 2 of 10 patients. These data indicate that essential fatty acid deficiency may contribute to the renal disturbances in cystic fibrosis.

Safety of aerosol tobramycin administration for 3 months to patients with cystic fibrosis

To determine the potential toxicity of prolonged aerosol tobramycin administration, 22 patients with cystic fibrosis were monitored while receiving inhaled tobramycin three times a day for 12 weeks. Prior to, four times during administration and approximately 6 weeks after discontinuation of treatment, we assessed pulmonary function, weight, height, body temperature, eighth cranial nerve function, serum creatinine, blood urea nitrogen, urinary creatinine clearance, plasma iothalamate clearance, urinary beta-2 microglobulin concentration, and Pseudomonas aeruginosa density in sputum. There was no detectable laboratory evidence of nephrotoxicity. Neither a decrease in auditory acuity (range 250-20,000 Hz) nor vestibular dysfunction was detected. Pulmonary function tests significantly improved during the first month in all subjects (P less than 0.05) but returned to enrollment values by the end of the 12th week of administration of tobramycin aerosol. Sputum P. aeruginosa density initially decreased from a mean of 10(7) cfu/gm to a mean of 10(4) cfu/gm after 2 weeks of aerosol tobramycin administration and remained significantly below the enrollment value throughout. Coincident with the reduced bacterial density, a reduction in cough frequency and sputum production, as well as a weight gain was observed. Seventy-three percent of the patients with sputum P. aeruginosa isolates susceptible to tobramycin on enrollment yielded resistant organisms during aerosol administration. However, 1 year later all sputum P. aeruginosa isolates obtained from patients were susceptible to tobramycin. We conclude that thrice daily aerosol tobramycin administration for 3 months is not associated with detectable eighth cranial nerve or renal toxicity. Transient emergence of tobramycin resistant P. aeruginosa may occur.

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.

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.

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.