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Background: Intravenous colistin is difficult to use because plasma concentrations for antibacterial effect overlap those causing nephrotoxicity, and there is large inter-patient variability in pharmacokinetics. The aim was to develop dosing algorithms for achievement of a clinically desirable average steady-state plasma colistin concentration (C ss,avg ) of 2mg/L., Methods: Plasma concentration-time data from 214 adult critically-ill patients (creatinine clearance 0-236mL/min; 29 receiving renal replacement therapy (RRT)) were subjected to population pharmacokinetic analysis. Development of an algorithm for patients not receiving RRT was based upon the relationship between the dose of colistimethate that would be needed to achieve a desired C ss,avg and creatinine clearance. The increase in colistin clearance when patients were on RRT was determined from the population analysis and guided the supplemental dosing needed. To balance potential antibacterial benefit against risk of nephrotoxicity the algorithms were designed to achieve target attainment rates of >80% for C ss,avg ≥2 and <30% for C ss,avg ≥4mg/L., Results: When algorithm doses were applied back to individual patients not on RRT (including patients prescribed intermittent dialysis on a non-dialysis day), >80% of patients with creatinine clearance <80mL/min achieved C ss,avg ≥2mg/L; but for patients with creatinine clearance ≥80mL/min target attainment was <40%, even with the maximum allowed daily dose of 360mg colistin base activity. For patients receiving RRT, target attainment rates were >80% with the proposed supplemental dosing. In all categories of patients, <30% of patients attained C ss,avg ≥4mg/L., Conclusions: The project has generated clinician-friendly dosing algorithms and pointed to circumstances where intravenous monotherapy may be inadequate., (© The Author 2016. Published by Oxford University Press for the Infectious Diseases Society of America. All rights reserved. For permissions, e-mail: journals.permissions@oup.com.)

Background: The US Food and Drug Administration (FDA) and European Medicines Agency (EMA) have approved updated dose recommendations for intravenous colistin in patients with various degrees of renal function. We assessed the recommendations in relation to their ability to achieve clinically relevant plasma colistin concentrations., Methods: Pharmacokinetic data from 162 adult critically ill patients (creatinine clearance range, 5.4-211 mL/min) were used to determine the average steady-state plasma colistin concentration (Css,avg) that would be achieved if each patient received the FDA or EMA dose. Target attainment rates for FDA- and EMA-approved daily doses to achieve colistin Css,avg of ≥0.5, ≥1, ≥2, and ≥4 mg/L were determined for each creatinine clearance category (≥80 mL/min, 50 to <80 mL/min, 30 to <50 mL/min, and <30 mL/min)., Results: For creatinine clearance <30 mL/min, 100% of patients receiving the EMA dose achieved a colistin Css,avg ≥1 mg/L, but the attainment rate was as low as 53.1% for patients receiving the FDA-approved dose. For colistin Css,avg ≥2 mg/L, the attainment rates were 87.5% with the EMA dose but only 6.3%-34.4% in patients receiving the FDA dose. Differences in attainment rates for a colistin Css,avg of ≥2 mg/L and ≥4 mg/L extended to patients with creatinine clearance 30 to <50 mL/min. For patients with creatinine clearance ≥80 mL/min, only approximately 65%-75% of patients achieved a colistin Css,avg of ≥1 mg/L with either set of recommendations., Conclusions: The study highlights important differences between the FDA- and EMA-approved dose recommendations and informs the setting of clinical breakpoints., Clinical Trials Registration: NCT00235690., (© The Author 2015. Published by Oxford University Press for the Infectious Diseases Society of America. All rights reserved. For permissions, e-mail journals.permissions@oup.com.)