Determination of lekethromycin in plasma and tissues of pneumonia-infected rats by ultra-high performance liquid chromatography-tandem mass spectrometry.

Lekethromycin (LKMS), a novel semi-synthetic macrolide lactone, had the characteristics of high plasma protein binding rate, fast absorption, slow elimination, and wide distribution in rat pharmacokinetics studies. A reliable analytical ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS)-based method was established by using tulathromycin and TLM (CP-60, 300) as internal standards for detection of LKMS and LKMS-HA, respectively. Samples preparation and UPLC-MS/MS conditions were optimized for complete and accurate quantification. Tissue samples were extracted with 1% formic acid in acetonitrile and purified by PCX cartridges. According to FDA and EMA guidelines for bioanalytical method, several rat characteristic tissues were selected for method validation, such as muscle, lung, spleen, liver, kidney, and intestines. The transitions m/z 402.900 > 158.300, m/z 577.372 > 158.309, m/z 404.200 > 158.200, and m/z 577.372 > 116.253 were monitored and quantified for LKMS, LKMS-HA, tulathromycin and TLM, respectively. According to the ratio with IS peak aera, the accuracy and precision of LKMS were 84.31%-112.50% with RSD 0.93%-9.79% and LKMS-HA were 84.62%-103.96% with RSD 0.73%-10.69%, and the method had been established and complied with FDA, EU, and Japanese guidelines. Finally, this method was applied to detect LKMS and LKMS-HA in plasma and tissues of pneumonia-infected rats that were intramuscularly administered and treated with LKMS intramuscular injection of 5 mg/kg BW and 10 mg/kg BW, and the characteristics of pharmacokinetics and tissue distribution were compared with normal rats.