1 Department of Clinical Pharmacology, Pharmacia Corporation, 301 Henrietta Street, Kalamazoo, MI 49007; 2 Kalamazoo Center for Medical Studies, Michigan State University, Kalamazoo, MI, USA
Received 28 March 2002; returned 19 September 2002; revised 18 December 2002; accepted 21 January 2003
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Abstract |
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Methods: In two randomized, double-blind, placebo-controlled, dose-escalating trials, subjects were exposed either to oral (375, 500 or 625 mg) or intravenous (500 or 625 mg) linezolid or placebo twice daily. Serial blood and urine samples were obtained after the first- and multiple-dose administrations for up to 18 days. Non-compartmental pharmacokinetic analyses were used to describe the disposition of linezolid.
Results: Plasma linezolid concentrations and area under the concentrationtime curves increased proportionally with dose irrespective of the route of administration. Plasma linezolid concentrations remained above the MIC90 for susceptible target pathogens (4.0 mg/L) for the majority of the 12 h dosing interval. Mean clearance, half-life and volume of distribution were similar irrespective of dose for both the oral and intravenous routes. Linezolid was well tolerated and the frequency of drug-related adverse events was similar between the linezolid and placebo groups.
Conclusions: Oral and intravenous linezolid exhibit linear pharmacokinetics, with concentrations remaining above the target MIC90 for most of the dosing interval. These results support a twice-daily schedule for linezolid and demonstrate the feasibility of converting from intravenous to oral dosing without a dose adjustment.
Keywords: pharmacokinetics, linezolid, oxazolidinones, volunteers, Gram-positive, clinical trial
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Introduction |
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Materials and methods |
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Subjects
Inclusion and exclusion criteria were similar for both studies. Men and non-pregnant women aged 1855 years were eligible for enrolment. All eligible participants were healthy non-smokers willing to abstain from vigorous exercise and to follow a controlled diet. Subjects were also required to be within 15% of ideal body weight, to have a negative drug screen and negative tests for hepatitis B surface antigen, hepatitis C antibody and HIV. Subjects were excluded for the following: a history of clinically significant cardiovascular, renal, hepatic, pulmonary, gastrointestinal, endocrine, haematological, vascular or collagen diseases; a history of nervous system or muscle disease, seizure disorder or a psychiatric disorder that might hinder compliance with the study; females receiving hormone replacement therapy, those breastfeeding or those who were neither surgically sterile nor post-menopausal; a positive reaction for occult blood in the stools; a history of drug or alcohol abuse during the past year; and receipt of an investigational drug, or donation of blood in the preceding 8 weeks. Subjects who had received a known enzyme-inducing drug during the 30 days preceding the study, any prescription drug within 2 weeks of the study start date or any non-prescription drug within 7 days of study drug administration were ineligible.
Study designs
Two randomized, double-blind, placebo-controlled, sequential dose-escalating trials in parallel groups of subjects were conducted, one evaluating oral administration and the other evaluating intravenous administration of linezolid (Pharmacia). Each subject received either placebo or one dose level of linezolid in a randomized manner. Progression to the next dose level was initiated provided that the lower doses were adequately tolerated. In the oral study, volunteers were exposed to linezolid 375, 500 or 625 mg or placebo. The dose levels in this study were constrained by the tablet strengths available at the time, and the 375 and 625 mg dose levels approximate the currently approved doses of 400 and 600 mg, which are administered twice daily.
Subjects in each dose group were randomized in a 3:1 fashion to receive linezolid or placebo. Study medication (placebo tablets or a combination of 125 and 250 mg linezolid tablets) was administered as a single dose on day 1 at 0800 h (for determination of single-dose pharmacokinetics) then every 12 h beginning on day 2 for 14.5 days (30 doses over 16 days). Doses were administered on an empty stomach (i.e. 1 h before or 3 h after a meal). On days 1 and 16, subjects were required to fast from 2200 h the night before until 4 h after the morning dose.
In the intravenous study, subjects were exposed to linezolid 500 or 625 mg or placebo infused over 30 min. The linezolid solution for infusion comprised linezolid 2 mg/mL. Subjects in each dose group were randomized in a 2:1 fashion to receive linezolid or placebo. Subjects received a single dose of study medication on day 1, then every 12 h beginning on day 2 for 7.5 days (16 doses over 9 days).
Subjects abstained from the following for 48 h before, and throughout the studies: vigorous exercise, alcohol, caffeine-containing beverages, grapefruit, grapefruit juice and tyramine-containing foods and beverages. Nicotine and illicit drugs were not permitted at any time. Inpatient meals were free of tyramine-containing foods and beverages. Evaluations conducted at screening included physical examination, medical history, toxicology screen, hepatitis A and B serologies, HIV screen, pregnancy test, haematology, clinical chemistries, urinalysis, faecal occult blood test and a 12-lead electrocardiogram. Evaluations during the treatment and post-treatment periods included pharmacokinetic sampling (blood and urine), sample collection for safety evaluation (blood and urine) that included haematological assays and serum chemistries, 12-lead electrocardiogram and cardiac telemetry, body weight, faecal occult blood test (post-treatment only), vital signs and adverse events.
Sample collection and analysis
Blood samples (7 mL) were drawn by direct venipuncture or indwelling catheter into a K3-EDTA Vacutainer (Becton Dickinson). Serial blood samples were collected on day 1 following single-dose administration in both the oral (0, 15, 30 min and 1, 1.5, 2, 3, 4, 6, 8, 10, 12 and 16 h) and intravenous (0, 15, 30, 35, 45 min and 1, 1.5, 2, 3, 4, 6, 8, 10, 12 and 18 h) studies. Blood samples were also taken during the multiple-dose study periods. In the oral study, serial blood samples were taken on day 2 (0 h), day 3 (0 and 12 h), day 4 (0 h), day 15 (0 and 12 h), day 16 (0, 15, 30 min and 1, 1.5, 2, 3, 4, 6, 8, 10, 12 and 16 h), day 17 (0, 6 and 12 h) and day 18 (0 h). In the intravenous study, serial blood samples were collected on days 28 (0 h), day 9 (0, 15, 30, 35, 45 min and 1, 1.5, 2, 3, 4, 6, 8, 10, 12 and 18 h), day 10 (0 and 12 h) and day 11 (0 h) after the start of the infusion. Urine samples were also collected during the study period. In the oral study, urine samples were collected on day 1 (pre-dose, 04, 48, 812 and 1224 h), day 16 (04, 48, 812 and 1224 h) and day 17 (012 and 1224 h). In the intravenous study, urine samples were collected on day 1 (pre-dose, 06, 612 and 1224 h), day 9 (06, 612 and 1224 h) and day 10 (012 and 1224 h). Blood and urine specimens were stored at 20°C or colder until analysis.
Linezolid was measured in human plasma and urine specimens by AvTech Laboratories, Inc., Kalamazoo, MI, USA, using sensitive and selective high-performance liquid chromatography methods, with ultraviolet detection (251 nm) and PNU-101145 as the internal standard. In brief, plasma (0.5 mL) and urine samples (diluted) were prepared using solid-phase extraction. Each sample was eluted with methanol. Upon evaporation of the organic material, the residue was reconstituted in acetonitrile:water and transferred to injection vials. 0.060 mL aliquots for plasma samples and 0.050 mL aliquots for urine samples were injected onto the chromatography system. The chromatography system used a reverse phase column (Zorbax RX-8, MAC-MOD Analytical, Chadds Ford, PA, USA) and a mobile phase comprising trifluoroacetic acid:tetrahydrofuran:methanol:water (0.1:1.2:25:73.7, v/v/v/v). Detection was by UV absorbance at 251 nm. Retention times of linezolid and the internal standard were 7 and 10 min, respectively. In plasma, mean recoveries for linezolid and internal standard were 95.4% and 95.8%, respectively. In urine, mean recoveries for linezolid and internal standard were 98.3% and 97.7%, respectively. Plasma samples were analysed in eight and four runs in the oral and intravenous studies, respectively. Corresponding urine samples were analysed in five and three runs, respectively. Calibration standard responses were linear over the range 0.010020.0 mg/L, using (1/concentration) weighted least-squares regression, and the lower quantifying limit was 0.0100 mg/L. Correlation coefficients were all equal to 1.0. Coefficients of variation (CV) were
6.9% with mean accuracy between 97.6% and 104% for the intravenous study plasma samples, and
3.9% with mean accuracy between 98% and 102% for the oral study plasma samples. Inter-day accuracy and precision (CV%) for the three quality control standards (0.0500, 5.00 and 15.0 mg/L) were
4.3% for the intravenous study plasma samples and
3.3% for the oral study plasma samples. Corresponding quality control values for the urine samples were 0.600, 20.0 and 75.0 mg/L. Inter-day accuracy and precision for the three quality control samples were
3.4% for the intravenous study urine samples and
4.7% for the oral study urine samples.
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Pharmacokinetic assessments |
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For both the oral and intravenous studies, the peak plasma concentration (Cmax), time to reach peak concentration (Tmax) and minimum plasma concentration (Cmin) were recorded from experimental observations. The apparent terminal elimination rate constants (z) were determined by linear least-squares regression of the data points in the log-linear portion of the plasma linezolid concentrationtime profiles. Elimination half-lives (t1/2) were calculated as (ln2)/
z. Following the single dose, the area under the plasma concentration versus time curve (AUC) was calculated from 024 h (AUC024) by the trapezoidal method, and extrapolated from the last measurable time point to infinity (
) using Ct/
z, where Ct is the predicted concentration at time t. Following the last dose, AUC0
was calculated by the trapezoidal method from time zero to
, where
is the dosing interval of 12 h. Clearance [apparent oral clearance (CLPO) for oral dosing] was calculated as dose/AUC0
for the first dose and dose/AUC0
for the last dose. Renal clearance (CLr) was calculated as Ae/AUC, where Ae is the amount of drug excreted in the urine in the 24 h after the first dose and in the 12 h after the last dose. Non-renal clearance (CLnr) was calculated as CLPO CLr. For oral dosing, volume of distribution at steady state (VD) was determined as CLPO/
z. For intravenous dosing, volume of distribution at steady state (Vss) was determined as CL x (MRTT/2), where MRT is the mean residence time and T is the infusion time (0.5 h).
Safety assessments
Adverse events, laboratory assays (e.g. haematology, including haematocrit, haemoglobin, reticulocytes, platelets and other measures, clinical chemistries and urinalysis), vital signs, 12-lead electrocardiogram and cardiac telemetry were performed for each subject on selected study days to assess the tolerability of linezolid. Adverse event data were obtained voluntarily from subjects, and by daily monitoring and questioning of subjects by study personnel. Adverse events were assessed for seriousness, intensity, potential relation to study medication, clinical outcome and effect on study treatment.
Statistical methods
Analysis of variance (ANOVA) was used to determine differences in pharmacokinetic data between dose levels within the oral or intravenous route of administration. Comparisons between first-dose and steady-state pharmacokinetic parameters were evaluated by paired t-test. Descriptive statistics were used to summarize demographic data and assess safety variables including adverse events, laboratory assays, vital signs and electrocardiogram. Regression analysis of selected parameters versus dose was conducted to assess linearity of the pharmacokinetics of linezolid. Linear regression models were used to examine possible effects of dose escalation on the safety variables. All analyses of pharmacokinetic data and safety variables were performed using Statistical Analytical Software (SAS Institute Inc., Cary, NC, USA). A probability value <0.05 was considered statistically significant.
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Results |
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Mean values for the pharmacokinetic parameters for single- and multiple-dose linezolid administration are given in Tables and
. Following the initial single dose, linezolid was rapidly absorbed and reached maximum concentrations within 12 h after oral administration. As expected, Cmax was observed at 0.5 h (at the end of the infusion) after intravenous administration. Cmax and AUC0
increased proportionally with dose for both oral and intravenous linezolid administration. Oral clearance values following the single oral dose were 108, 125 and 112 mL/min for the 375, 500 and 625 mg dose levels, respectively; average clearance values were 133 and 138 mL/min for the intravenous 500 and 625 mg doses. With both routes of administration there was wide variability in clearance that could be accounted for primarily by variability in non-renal clearance. Renal clearance demonstrated less variability with both routes of administration. The apparent elimination half-life of linezolid was
5 h and was independent of either dose or route of administration.
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Multiple-dose linezolid was well tolerated in these healthy volunteers at doses up to 625 mg and administered twice daily by either oral or intravenous routes. There were no early withdrawals from the study and no serious adverse events after either oral or intravenous drug administration. In the oral study, 13 of 18 subjects (72%) receiving linezolid and five of six subjects (83%) receiving placebo experienced drug-related adverse events. In the intravenous study, 10 of 12 linezolid-treated subjects (83%) and six of six placebo-treated subjects (100%) had treatment-related adverse events. The majority (>97%) of individual drug-related adverse events associated with linezolid were considered mild or moderate in intensity and required no intervention. The most commonly reported treatment-related adverse events were related to the gastrointestinal tract and the skin. Tongue discolouration was the most frequently reported event, occurring in eight of 24 subjects (all linezolid-treated) in the oral study and in 10 of 18 subjects (eight linezolid- and two placebo-treated) in the intravenous study. The brown colour was not accompanied by an unusual or disagreeable taste. Cultures taken from subjects with discoloured tongues were negative for fungi and yeast. The discoloration was of cosmetic concern and disappeared with discontinuation of linezolid. In patients receiving oral linezolid, other common events were associated with gastrointestinal disturbance, including diarrhoea (three of 18 linezolid- and three of six placebo-treated) and flatulence (three of 18 linezolid- and one of 12 placebo-treated). In the intravenous study, a moderate number of subjects reported appetite loss (two of 12 linezolid; one of six placebo), nausea (one of 12 linezolid; one of six placebo), diarrhoea (three of 12 linezolid; two of six placebo), flatulence (0 of 12 linezolid; one of six placebo) and upset stomach (0 of 12 linezolid; one of six placebo). The most commonly reported skin reaction was rash (four of 12 linezolid; three of six placebo). These events were evenly distributed between linezolid- and placebo-treated subjects. There were no clinically important changes in physical examination, vital signs, electrocardiography (including QTc intervals) or telemetry findings among the dose groups after oral or intravenous linezolid. There were no clinically important changes from baseline in laboratory values (haematology, clinical chemistry, urinalysis) that were indicative of organ dysfunction attributable to study medication. Specifically regarding haematology, there were no changes in red or white cell parameters, including haemoglobin, haematocrit and red blood cell count. However, after 1014 days of dosing, some subjects in the highest dose group had decreased platelet counts and percent reticulocytes (Table ).
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Discussion |
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There was a large degree of variability in clearance between subjects, which was mostly accounted for by non-renal clearance. The variability in non-renal clearance is probably related to the non-enzymic metabolism of linezolid, and the values for renal and non-renal clearance in these studies are similar to those reported elsewhere.19 The variability observed in clearance values between subjects should not influence the dosing regimen for the treatment of target pathogens, as it would be expected that concentrations in the subjects with the highest clearance would remain above the target MIC90 for at least 4050% of the dosing interval. This is believed to be important for the efficacy of linezolid based on an in vivo animal model.20 Steady-state elimination half-lives of oral and intravenous linezolid were similar (5.5 and 4.5 h, respectively) and were independent of dose.
Oral and intravenous linezolid were well tolerated in these healthy volunteers at dosages up to 625 mg administered twice daily. Adverse events were primarily mild in intensity and occurred at the same frequency in the linezolid and placebo treatment groups. One exception was the occurrence of tongue discolouration; however, this event was not associated with taste complaints or other distressing symptoms or conditions. In pooled tolerability data from Phase II trials, tongue discolouration was reported in 2.5% of patients.21
Linezolid has been associated with mild, reversible, time-dependent myelosuppression.22 In a pooled analysis of seven comparator-controlled trials, decreases in haemoglobin (linezolid 6.6%; comparators 6.4%) and absolute neutrophil count (linezolid 3.3%; comparators 3.4%) were similar for patients treated with linezolid or comparator antimicrobials. Decreases in platelets were more likely in patients treated with linezolid versus comparator drugs (2.9% versus 1.6%, respectively).22 Of note in the current studies is the fact that, by chance, the highest dose level group had the lowest baseline value for platelets (although within the normal range of values), and low values prior to linezolid treatment are possibly a risk factor for developing thrombocytopenia with long-term dosing of linezolid.
In summary, the pharmacokinetic parameters of single and multiple doses of linezolid are similar and independent of the route of administration. Multiple dosing of linezolid was well tolerated. These data support twice-daily dosing, as well as simple and convenient conversion between oral and intravenous formulations without the need for a change in dose or administration schedule.
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Acknowledgements |
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Footnotes |
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References |
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