Study Design and Patients
Patients with evidence of gram-positive bacteria cultured from sites of infection were enrolled in the study during March 2017 to March 2017, and written informed consent was obtained from all subjects. Exclusion criteria included age ≤ 18 years, recent use of linezolid (within 2 weeks), gastrointestinal bleeding and undergoing hemodialysis or continuous renal replacement therapy.
The patients were randomly assigned to intragastric or intravenous linezolid group. All patients received linezolid (Zyvox, Pfizer, New York, USA) at 600 mg, scheduled every 12 h. The intravenous group received intravenous linezolid for 60 min. The intragastric group received oral or nasogastric administration for 2 min. Two trained research nurses crushed and dissolved linezolid tablets in 10 mL sterile water for the patients who received nasal feeding. Baseline characteristics, APACHE II, serum albumin, aspartate aminotransferase (AST), and creatinine concentrations were captured within the first 24 hours of administration. Glomerular filtration rate (GFR) was estimated using the Cockroft and Gault formula.
Blood Sampling and Drug Assays
A blood sample was collected at the same time as the linezolid adiministration (pre-dose sample), and 0.5, 1, 2, 4, 6, 8, and 12 h (just before the subsequent dose) after the start of infusion of the first and the fifth dose in both intragastric and intravenous groups. Blood samples were centrifuged at 3000 rpm for 10 min at 4 °C. Plasma was then collected and stored at − 80 °C until the assay.
Linezolid concentrations in plasma were measured using a sensitive and selective high-performance liquid chromatography (HPLC; column: Diamonsil ODS, 4.6 × 250 mm, 5 µm; mobile-phase: acetonitrile − 0.01% phosphoric acid water solution (27:73, v/v)) based on previously described researches[12, 13]. The precision and accuracy of the method were evaluated by performing replicate analyses of quality control samples against calibration standards. Analytical methods were linear (r2 > 0.9999) over the calibration range of 0.25–25 mg/L. Intra- and inter-day precision (relative standard deviation, %) were 2.1% and 5.0%, respectively. The accuracy was within the range of 99.45–104.42%.
Antimicrobial Therapy Responses and Safety Assessment
The antimicrobial effect of linezolid was assessed using the body temperature normalized (< 37.3 °C) within 3 days and 28-day mortality. The safety was determined by the assessment of adverse events reported by the participant or medical provider and laboratory tests monitor. The relationship between adverse reactions and linezolid was graded as definitely related, probably related, possibly related, probably not related or not related.
The PK modeling for linezolid was conducted using the Nonparametric Adaptive Grid (NPAG) algorithm within the freely available software Pmetrics package for R (Los Angeles, CA, USA) . One-compartment and two-compartment models were fitted to the linezolid concentration data. PK parameters were estimated based on linezolid concentrations and clearance (CL), central volume of distribution (V) and rate constant for drug distribution from the central to peripheral compartment (Kcp), rate constant for drug distribution from the peripheral to central compartment (Kpc). Patient demographics and pathophysiological factors, which included age, sex, bodyweight, height, BMI, APACHE II, SOFA, serum albumin, AST and GFR, were tested for their association with the identified PK parameters. Model performance was evaluated based on visual fit plots and small Akaike information criterion (AIC).
Monte Carlo Simulation
Monte Carlo simulations (n = 10000) were conducted using Pmetrics to determine the probability of target attainment (PTA) of achieving %T > MIC = 100% for studied regimen. For PD analysis, different MICs at 0.5, 1, 2, and 4 mg/L were tested for methicillin-resistant Staphylococcus aureus (MRSA), methicillin-sensitive Staphylococcus aureus (MSSA), Enterococcus faecalis (E. faecalis), Enterococcus faecium (E. faecium), Viridans Streptococci (V. Streptococci), Staphylococcus hominis (S. hominis) and Streptococcus pyogenes (S. pyogenes) on the basis of microorganism susceptibility data obtained from the European Committee on Antimicrobial Susceptibility Testing (EUCAST) database (www.eucast.org). To generate insight in the PTA, 4 different dosing regimens were simulated: 600 mg every 12 h, 600 mg every 8 h, 900 mg every 12 h, and 900 mg every 8 h.
Fractional Target Attainment Calculation
The MIC data for MRSA, MSSA, E. faecalis, E. faecium, V. Streptococci, S. hominis and S. pyogenes obtained from the EUCAST database were used to determine fractional target attainment (FTA), which identifies the likely success of treatment by comparing the PTA against the MIC distribution. The FTA was calculated using %T > MIC = 100% for various doses: 600 mg every 12 h, 600 mg every 8 h, 900 mg every 12 h, and 900 mg every 8 h. A dosing regimen was considered optimal if the FTA was ≥ 85%.
A statistical analysis was performed using SPSS version 13.0 (SPSS Inc., Chicago, IL). All data are presented as mean ± SD. The continuous variables were analyzed using the Mann-Whitney U test, and the categorical variables were analyzed using the Fisher’s exact test. A P value of < 0.05 was considered statistically significant.