A rapid sensitive and selective MRM based method for the determination of Cremophor EL (CrEL) in rat plasma was developed using liquid chromatography/tandem mass spectrometry (LC-MS/MS). CrEL and polypropylene glycol (internal standard) were extracted from rat plasma with acetonitrile and analysed on C18 column (XBridge, 50 × 4.6？mm, 3.5？μm). The most abundant molecular ions corresponding to PEG oligomers at m/z 828, 872, 916 and 960 with daughter ion at m/z 89 were selected for multiple reaction monitoring (MRM) in electrospray mode of ionisation. Plasma concentrations of CrEL were quantified after administration through oral and intravenous routes in male sprague dawley rats at a dose of 0.26？g/kg. The standard curve was linear (0.9972) over the concentration range of 1.00 to 200？μg/mL. The lower limit of quantitation for CrEL was 1.00？μg/mL using 50？μL plasma. The coefficient of variation and relative error for inter and intra assay at three QC levels were 0.69 to 9.21 and ？7.60 to 4.74 respectively. A novel proposal was conveyed to the scientific community, where formulation excipient can be analysed as qualifier in the analysis of NCEs to address the spiky plasma concentration profiles. 1. Introduction In early stages of drug discovery, pharmacokinetic and drug metabolism and disposition studies were conducted in rodents, as they were relatively inexpensive and can be easily acquired and handled [1]. In a typical pharmacokinetic (PK) study, new chemical entities (NCEs) are administered to rats via intravenous and oral routes. Serial blood samples are collected and analysed by liquid chromatography/tandem mass spectrometry (LC-MS/MS). Few NCEs have spiky plasma concentration profiles and various reasons for such profiles could be due to enterohepatic circulation or discrepancies in sample collection/sample processing. Spiky profiles in elimination phase will lead to inaccurate quantification of PK parameters. Extensive studies needed to be carried to characterize enterohepatic circulation behavior of test compounds. Drugs that undergo enterohepatic cycling to a significant extent include colchicine, phenytoin, leflunomide, and tetracycline antibiotics. As formulation excipients have fixed plasma concentration profiles irrespective of NCEs dosed, monitoring the plasma concentration levels of excipient along with NCEs will help to take a decision on the spiky plasma concentration profiles of NCEs. A thoroughly developed and validated bioanalytical method is required to fix the plasma concentration profile and understand the pharmacokinetic disposition
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