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Ei Ei Win,1 Khaing Win Htun,2 Pramote Tragulpiankit,1 Suwida Tangtrakultham,1 Preecha Montakantikul1 1Division of Clinical Pharmacy, Department of Pharmacy, Faculty of Pharmacy, Mahidol University, Bangkok, 10400, Thailand; 2Right Laboratory and Health Screen, Naypyitaw, MyanmarCorrespondence: Preecha Montakantikul, Division of Clinical Pharmacy, Department of Pharmacy, Faculty of Pharmacy, Mahidol University, Bangkok, 10400, Thailand, Tel +66-26448694, Email preecha.mon@mahidol.ac.thPurpose: To evaluate the optimal dosing regimens of meropenem against extended-spectrum beta-lactamase-producing Escherichia coli (ESBL E. coli) in critically ill patients with varying degrees of renal function using Monte Carlo simulation (MCS).Methods: The MCS was performed using the minimum inhibitory concentration (MIC) data from Right Laboratory and Health Screen in Naypyitaw, Myanmar, as well as reported meropenem pharmacokinetic parameters in the target population and the pharmacokinetic-pharmacodynamic index. For each dosing regimen, 10,000 virtual patients were generated to assess the probability of target attainment (PTA) and the cumulative fraction of response (CFR). The most effective dosage regimens were determined using PTA and a CFR of 90%.Results: ESBL E. coli made up 93 of the 396 clinical E. coli isolates, and they are all multidrug-resistant, with resistance to at least five antibiotic classes. The MIC50 and MIC90 were determined to be 0.25 μg/mL. The PTA was affected by five factors: creatinine clearance (CLcr), vasopressor usage, MIC, infusion time, and dosage fractionation. In patients who did not receive vasopressors, the current regimens (US-FDA and EMA) were ineffective in all renal function for MIC > 0.25μg/mL. In the subset group of CLcr > 80 mL/min for MIC 2μg/mL, the maximum total daily dose of 6g/day (2g q 8hr; 3hr infusion) was still ineffective, but 4g/day (1g q 6hr; 3hr infusion) achieved 98.96% PTA. Almost majority of the simulated regimens pro