A validated UPLC-MS/MS method for the quantitation of an unstable peptide, monocyte locomotion inhibitory factor (MLIF) in human plasma and its application to a pharmacokinetic study.

•MLIF, a pentapeptide with strong anti-inflammatory activity, was developed for neural protection in acute ischemic stroke and currently being evaluated in phase I trials.•To this unstable peptide, chemical and enzymatic degradation process was successfully prevented by addition of acidification reagent to biological samples.•Plasma samples were extracted by protein precipitation and the analysis was robust, reliable, and accurate.•Few references are available regarding to MLIF measurement and this was the first method to determine MLIF in human plasma. The method was successfully applied to the first-in-human study of MLIF.