Age and sex effects on the pharmacokinetics of linezolid.

OBJECTIVE: To evaluate the possible effects of age and sex on the pharmacokinetics of linezolid in healthy volunteers. METHODS: A single 600-mg dose of linezolid was administered orally to young (18-40 years) and elderly (> or =65 years) healthy males and females. Blood and urine samples were collected until 48 h after dosing and assayed for linezolid concentrations using a validated high-performance liquid chromatography method. Pharmacokinetic parameters were assessed using noncompartmental methods. Comparisons of the pharmacokinetic parameters for each age and sex group were performed using a two-way analysis of variance model. Pairwise comparisons were done using least-square means analysis. RESULTS: Peak plasma drug concentrations occurred within 1.5 h after linezolid administration for males and females in both age groups. However, the maximum concentration achieved differed significantly between males and females. There was no significant difference between males and females or young and elderly for mean apparent elimination rate constant or half-life. There was no difference in mean apparent oral clearance (CLPO) between the young and elderly; however, there was a significant difference between males and females. Mean CLPO for females was approximately 37% less than mean CLPO for males when not corrected for body weight. Correcting for differences in weight reduced this difference to approximately 20%. Overall, females had a slightly lower volume of distribution than males, but this was not affected by the age of subjects. CONCLUSIONS: A dose adjustment based on age and sex is not warranted due to the wide range of linezolid concentrations that are well tolerated and the relative small difference in linezolid disposition between males and females.

Linezolid absolute bioavailability and the effect of food on oral bioavailability.

Linezolid is a novel oxazolidinone antibiotic that has a spectrum of activity encompassing a variety of Gram-positive bacteria. The objectives of this study were twofold: (1) to compare the absorption of linezolid tablets given immediately following a high-fat meal with the absorption of tablets administered while fasting, and (2) to assess the bioavailability of a 375-mg oral dose given while fasting relative to a 375-mg dose of linezolid sterile solution given intravenously. Venous blood samples were taken over the 48 h following the single dose administration of both the oral and intravenous (IV) treatment. Samples were subsequently frozen for the determination of linezolid concentrations by HPLC. The only statistically significant difference between the fasted and the fed treatment was in peak plasma concentration, with the mean C(max) for fasted subjects being 23% greater than that for subjects after consumption of a high-fat meal. Comparable AUC(0-infinity) values were measured under both conditions, indicating that the overall extent of absorption is the same. Therefore, the difference in C(max), while statistically significant, should not affect the therapeutic efficacy of linezolid when it is administered with food. There were no statistically significant differences in AUC(0-infinity), CL or half-life between the fasted oral treatment and the intravenous treatment. As expected, C(max) was statistically different between the two treatments. However, the mean absolute bioavailability (F) of the tablet, using the IV sterile solution as the reference treatment, was 103% (+/-20%).

Single dose pharmacokinetics of linezolid in infants and children.

BACKGROUND: Linezolid is an oxazolidinone antibiotic with excellent in vitro activity against a number of Gram-positive organisms including antibiotic-resistant isolates. The safety and pharmacokinetics of intravenously administered linezolid were evaluated in children and adolescents to examine the potential for developmental dependence on its disposition characteristics. METHODS: Fifty-eight children (3 months to 16 years old) participated in this study; 44 received a single 1.5-mg/kg dose and 14 received a single 10-mg/kg dose of linezolid administered by intravenous infusion. Repeated blood samples (n = 10 in children > or = 12 months; n = 8 in children 3 to 12 months) were obtained during 24 h after drug administration, and linezolid was quantitated from plasma by high performance liquid chromatography with mass spectrometry detection. Plasma concentration vs. time data were evaluated with a model independent approach. RESULTS: Linezolid was well-tolerated by all subjects. The disposition of linezolid appears to be age-dependent. A significant although weak correlation between age and total body clearance was observed. The mean (+/- SD) values for elimination half-life, total clearance and apparent volume of distribution were 3.0 +/- 1.1 h, 0.34 +/- 0.15 liter/h/kg and 0.73 +/- 0.18 liter/kg, respectively. Estimates of total body clearance and volume of distribution were significantly greater in children than historical values of adult data. As such maximum achievable linezolid plasma concentrations were slightly lower in children, and concentrations 12 h after a single 10-mg/kg dose were below the MIC90 for selected pathogens with in vitro susceptibility to the drug. CONCLUSION: Based on these data a linezolid dose of 10 mg/kg given two to three times daily would appear appropriate for use in pediatric therapeutic clinical trials of this agent.

A pharmacokinetic evaluation of concomitant administration of linezolid and aztreonam.

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

Lifibrol: a novel lipid-lowering drug for the therapy of hypercholesterolemia. Lifibrol Study Group.

OBJECTIVE: To determine the effects of lifibrol on serum lipids in adult patients with primary hypercholesterolemia. METHODS: These were two double-blind, randomized placebo-controlled studies. Each patient in each study had an 8-week dietary lead-in period on the American Heart Association Step I diet before administration of lifibrol or placebo. The first study consisted of active dosing of 4 weeks, and the second study had 12 weeks of active dosing. The setting for the study involved outpatients in private or university hospitals in the United States. All patients had primary hypercholesterolemia with low-density lipoprotein (LDL) cholesterol levels of > 160 mg/dl after the dietary lead-in period. There were 155 patients in the 4-week study and 336 patients in the 12-week study. In the first study, patients were randomly assigned to receive either 150, 300, 450, 600, or 900 mg lifibrol as a single daily dose for 4 weeks. In the second study, patients were randomized to receive either 150, 300, or 600 mg lifibrol for 12 weeks. Efficacy was determined by serial measurements of serum lipids either on a weekly or biweekly basis during each study. RESULTS: Compared with baseline, lifibrol reduced LDL cholesterol (> 40%, p < 0.0001) and apolipoprotein B (approximately 40%, p < 0.0001) by 4 weeks in both studies. After 6 weeks, high-density lipoprotein (HDL) cholesterol levels increased in the placebo and 150 and 300 mg lifibrol groups. In the 600 mg lifibrol group, triglycerides (approximately 25%, p < 0.001), lipoprotein (a) (approximately 30%, p < 0.001), and HDL cholesterol (approximately 5%, p < 0.002) decreased. Lifibrol reduced key sterol intermediates (e.g., lanosterol, lathosterol, beta-sitosterol, and campesterol) and increased serum bile acids, but it had no effect on urinary mevalonic acid excretion. The pharmacokinetics of lifibrol are independent of dose and are similar in men and women. Lifibrol was well tolerated. The most frequent medical event in both studies was skin rash. CONCLUSIONS: Lifibrol is a potent lipid-lowering drug in patients with hypercholesterolemia.

Dose-related haemodynamic effects and pharmacokinetics of oral nicorandil in patients evaluated for chest pain.

We studied the acute haemodynamic dose response of nicorandil, a combined nitrate and potassium channel opener, in patients evaluated for chest pain. Single dose oral nicorandil (5, 10, 20, or 30 mg) or placebo was given to 42 right-heart catheterized patients using a randomized block design. Persistent, significant (P < 0.05) haemodynamic changes occurred primarily after 30 mg. Arterial systolic pressure fell significantly after all doses and remained reduced (maximum, 31 mmHg) up to 6 h after 30 mg; heart rate increased significantly up to 1 h. Individual haemodynamic sensitivity varied and three patients (1, 10 mg; 2, 30 mg) developed transient symptomatic hypotension associated with bradycardia. Pulmonary artery systolic pressure (diastolic was unchanged) declined significantly (maximum, 5 mmHg) up to 6 h after 30 mg whereas pulmonary capillary wedge (baseline normal) and mean right atrial pressures decreased transiently. Cardiac index (baseline normal) declined slightly (significantly after 30 mg); however, stroke volume index and stroke work index were significantly and persistently reduced after all doses. Total systemic vascular resistance declined slightly after 30 mg. Individual plasma nicorandil concentrations were variable and systemic bioavailability was reduced compared with values reported in healthy subjects. Nicorandil demonstrated cardiac unloading actions. Variable plasma concentrations, haemodynamic effects, and patient sensitivity warrant low initial doses with individual dose titration, especially if cardiac filling pressures are low.

Acute effects of increasing doses of nicorandil on renal function in man.

The effects of nicorandil, a nicotinamide derived vasodilator combining nitrate and potassium channel opener actions, on kidney function have not been determined. This study investigated changes in renal blood flow and glomerular filtration rate as estimated using simultaneous 131I-iodohippurate and 125I-iothalamate plasma clearances. Forty-two healthy subjects in sodium balance received placebo and 2.5 mg (n = 8), 5 mg (n = 9), 10 mg (n = 8), 20 mg (n = 8) or 30 mg (n = 9) nicorandil orally. Peak nicorandil plasma concentrations occurred in the first hour. Nicorandil produced dose related decreases in blood pressure with maximum reductions (mean +/- standard error of the mean) after 30 mg of -6 +/- 1 mmHg systolic and -8 +/- 2 mmHg diastolic. Renal blood flow averaged 655 +/- 28 ml/minute/1.73 m2 after placebo. Renal blood flow changed 10 +/- 11% after 2.5 mg, -6 +/- 8% after 5 mg, -12 +/- 11% after 10 mg, -11 +/- 5% after 20 mg, and 8 +/- 6% after 30 mg, however, these changes did not reach statistical significance. Glomerular filtration rate averaged 113 +/- 3 ml/minute/1.73 m2 and was unaltered after nicorandil. Nicorandil had no effect on filtration fraction but fractional excretion of sodium tended to decrease with dose. These dose-related effects of nicorandil are consistent with other mixed vasodilators. At therapeutic doses, renal perfusion and function are preserved despite reductions in systemic blood pressure by nicorandil.

Clinical pharmacology of nicorandil in patients with congestive heart failure.

Nicorandil is a nicotinamide derivative with a potential role in human therapeutics because of its potent vasodilating properties. The pharmacokinetics of oral nicorandil administration and the relationships between plasma nicorandil concentration and hemodynamic responses were examined in 25 patients with moderate to severe congestive heart failure. The dose range from 10 to 60 mg was studied. Elimination half-life for this dose range was substantially longer than that previously reported in normal volunteers. Total area under the curve increased in a curvilinear fashion with progressive dose increments, indicating a disproportionate increase in systemically available drug at higher doses. Hemodynamic responses generally correlated well with plasma nicorandil concentration, with rapid loss of cardiovascular activity corresponding to the efficient clearance of nicorandil.

Pharmacokinetics of methylprednisolone sodium succinate and methylprednisolone in patients undergoing cardiopulmonary bypass.

The pharmacokinetics of methylprednisolone sodium succinate (MPHS) and methylprednisolone (MP) were determined in six patients undergoing open heart surgery with cardiopulmonary bypass. Plasma concentrations of both compounds were measured by high-performance liquid chromatography after doses of MPHS of 1.7-2.4 g. The prodrug ester MPHS yields MP with an average formation rate constant of 0.70 +/- 0.29 hr-1. Peak concentrations of MP occur around 1-2 hours after loading and additional administration of MPHS. The pharmacokinetic values of the two drugs in patients having cardiopulmonary bypass were compared to those in younger, healthy subjects. The volume of distribution of MPHS was lower in the patients, and that of MP was similar to the value in controls. Total clearances of both agents were reduced by about 5 and 2 times. The elimination half-life of MPHS was increased slightly, whereas that of MP increased more than twice in the patients. Significant alterations in clearances occurred in patients, but concentrations of MP were appreciable and prolonged MP due to the extensive formation of MP from MPHS and reduced clearance of MP.

Ion-paired reversed-phase high-performance liquid chromatography assay for determination of ceftriaxone in human plasma and urine.

A rapid, sensitive, and specific ion-paired reversed-phase HPLC assay for ceftriaxone in human plasma and urine is described. Small volumes (50 microL) of sample are deproteinized with acetonitrile and are directly injected on a C18 analytical column. The UV absorbance is monitored at 280 nm. The assay is linear between 1 and 125 micrograms/mL of ceftriaxone, with less than 10% coefficient of both intra- and interday variation. Chromatography was specific for ceftriaxone as endogenous compounds and 30 common drugs did not interfere. The assay was used in open heart surgery patients where potential interference from corticosteroids was overcome.

Factors affecting ceftriaxone plasma protein binding during open heart surgery.

Factors most likely contributing to reduced ceftriaxone plasma protein binding in patients undergoing open heart surgery (OHS) were examined. Binding was determined by equilibrium dialysis. It was found that ceftriaxone does not bind significantly to red blood cells, alpha 1-acid glycoprotein, or to protamine, and that the pH of serum did not significantly affect binding. Albumin is the major protein to bind ceftriaxone, and binding decreases with lower albumin concentrations due to fewer binding sites. The binding of ceftriaxone was not affected by the in vitro addition of heparin or methylprednisolone, but high concentrations of methylprednisolone hemisuccinate increased the free fraction of ceftriaxone. Increased concentrations of free fatty acids (FFA) were demonstrated in several patients undergoing OHS. The in vitro addition of palmitic, stearic, linoleic, and oleic acids in high concentrations decreased the binding of ceftriaxone. Ceftriaxone binding in patient samples correlated with the molar ratio of FFA to albumin, but not to either individually. The dual effect of increased FFA and decreased albumin concentrations in OHS patients appears responsible for most of the observed binding alterations.

Dose-dependent pharmacokinetics of mezlocillin in rats.

The pharmacokinetics of mezlocillin were examined in rats following bolus intravenous doses of 20 or 200 mg/kg. Mezlocillin exhibited bi- or triexponential disposition profiles, and the area under the concentration-time curve increased nonproportionally with dose similar to reported findings in humans. Apparent total, renal, and nonrenal clearances and the volume of distribution at steady-state all decreased by 45 to 50% with the higher dose, and the elimination half-life was longer (8 +/- 2 versus 15 +/- 3 min). Mezlocillin exhibits low saturable binding in rat serum, ranging from 20 to 40% bound. Pharmacokinetic parameters based on free drug demonstrated dose-dependent characteristics similar to those with total drug. Use of the volume of distribution from the low dose allowed calculation of the true mean residence time. The linear relationship between dose and mean residence time from free concentrations yielded pooled Michaelis-Menten parameters. These were used as initial estimates in the simultaneous nonlinear fitting of the low- and high-dose mean free concentrations to a three-compartment model with sequential distribution and Michaelis-Menten elimination to describe the nonlinearity of mezlocillin disposition further.

Ceftriaxone disposition in open-heart surgery patients.

The effects of cardiopulmonary bypass (CPB) with hypothermia and systemic heparinization on ceftriaxone disposition were evaluated in seven male patients. A bolus dose of drug (14 mg/kg of body weight) was given, and blood and urine specimens were collected before, during, and after CPB for 96 h. Creatinine, albumin, and total and free ceftriaxone concentrations in plasma were measured. The ceftriaxone free fraction (ff) in vitro was estimated by equilibrium dialysis, and the in vivo ff was obtained by the ratio of renal clearance due to filtration to creatinine clearance. Pharmacokinetic parameters were based on concentrations of total drug and free drug. Albumin decreased from 3.10 +/- 0.29 g/dl presurgery to 1.42 +/- 0.17 g/dl and recovered to 2.46 +/- 0.26 g/dl on postoperative day 4. CPB markedly increased the in vitro ff, which was reversed by protamine post-CPB (ff pre-CPB, 0.15 +/- 0.01; during CPB, 0.53 +/- 0.20; post-CPB, 0.16 +/- 0.02). The in vitro ff exceeded the in vivo ff (0.53 +/- 0.20 versus 0.24 +/- 0.07), probably due to continued free fatty acid release caused by heparin during dialysis. Clearances based on free drug decreased, and the renal clearance due to filtration increased (7.6 +/- 2.8 versus 15.0 +/- 4.5 ml/min) while the creatinine clearance decreased (114 +/- 29 versus 72 +/- 28 ml/min) during CPB. Diminished binding owing to low albumin and free fatty acids explain this behavior. Lower binding also increased the volume of distribution (154 +/- 41 ml/kg) and extended the half-life (15 +/- 6 h). In summary, ceftriaxone disposition was significantly altered by CPB, resulting in marked increases in free drug concentrations, half-life, and volume of distribution and in decreased intrinsic clearance.

Effect of formula feeding on oral absorption of caffeine in premature infants.

The systemic absorption of orally administered caffeine was evaluated in 16 premature infants receiving therapy for recurrent apnea. An oral bolus dose of caffeine (10 mg/kg) was given during a fasted state (n = 8) or immediately preceding formula (n = 8). The first-order absorption rate constant (ka) was estimated from the rise in the plasma concentration-time curve and extent of absorption was estimated by the area under the concentration-time curve. Therapeutic concentrations of caffeine were achieved rapidly followed by very slow elimination of drug. No difference between the fasted and fed groups was found in the apparent rate or extent of caffeine absorption, suggesting that caffeine can be administered concomitantly with feedings.

Developmental pharmacokinetics of mezlocillin in 4 newborn infants.

The disposition of mezlocillin was evaluated in 4 newborn infants in a sequential two-phase study at postnatal ages of 1 day and 8 or 10 days. Renal function was estimated by creatinine clearance (CLCR) and pharmacokinetic parameters of mezlocillin was were determined from serum concentrations and urinary excretion rates. All weight-normalized mezlocillin clearances (total, renal, and nonrenal) and CLCR were less than adult values, but increased after 8 or 10 days of mezlocillin therapy and postnatal development. The volume of distribution at steady state did not significantly change throughout this period, and approximated the expanded extracellular fluid volume typically found in neonates. The elimination half-lives were substantially shorter by phase II of the study. Both renal and nonrenal elimination processes were enhanced by 1 week of postnatal development even though body weight did not increase. Mezlocillin disposition in neonates is thus affected by body weight, gestational age, and postnatal age.

Pharmacokinetics of ticarcillin and clavulanic acid (timentin) in relation to renal function.

The disposition of coadministered ticarcillin (3 g/1.73 m2) and clavulanic acid (100 mg/1.73 m2) was examined after a 30-min infusion in 24 noninfected subjects with various degrees of renal function. Noncompartmental pharmacokinetic parameters for the individual compounds were determined from plasma concentrations and urinary excretion rates. All clearances (total, renal, and nonrenal) and urinary recoveries of unchanged drug were found to be linearly related to creatinine clearance (CLCR). The steady-state volume of distribution (9.9 and 12.9 liters for ticarcillin and clavulanic acid) approximated the extracellular fluid space and was not related to CLCR. The half-lives increased with reduced renal function and ranged from 56 to 392 min for ticarcillin and 26 to 266 min for clavulanic acid. The clearances of both drugs decreased proportionately with reduction in renal function, facilitating dosing adjustments based on CLCR. Calculations of expected steady-state maximum and minimum concentrations in plasma using constant doses and an extended dosing interval related to CLCR further rationalized use of the 30:1 drug combination ratio for all patients.

Stereoselective binding of disopyramide to human plasma protein.

The binding of racemic disopyramide and its two enantiomers to protein were compared in two samples of human plasma, two samples of freshly drawn serum and in a solution of alpha 1-acid glycoprotein. The binding of S(+)-disopyramide was higher at all concentrations as compared to to R(-)-disopyramide, and the binding of the racemate was intermediate. Differences in binding were due to differences in the association constant.