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Investigation of in vivo and in vitro Metabolic fate of palbociclib.Sample preparation by protein precipitation and solid phase extraction.Structural elucidation of the metabolites using UHPLC-QTOF-MS/MS.In silico toxicological screening of metabolites by using TOPKAT and DEREK softwares.Palbociclib (PAB) is a CDK4/6 inhibitor and U. S Food and Drug Administration (FDA) granted regular approval for the treatment of hormone receptor (HR) positive, metastatic breast cancer in combination with an aromatase inhibitor in postmenopausal women. Metabolite identification is a crucial aspect during drug discovery and development as the drug metabolites may be pharmacologically active or possess toxicological activity. As there are no reports on the metabolism studies of the PAB, the present study focused on investigation of the in vitro and in vivo metabolic fate of the drug. The in vitro metabolism studies were carried out by using microsomes (HLM and RLM) and S9 fractions (Human and rat). The in vivo metabolism of the drug was studied by administration of the PAB orally to the Sprague–Dawley rats followed by analysis of urine, faeces and plasma samples. The sample preparation includes simple protein precipitation (PP) followed by solid phase extraction (SPE). The extracted samples were analyzed by ultrahigh-performance liquid chromatography–quadruple time-of-flight tandem mass spectrometry (UHPLC/Q-TOF/MS/MS). A total of 14 metabolites were detected in in vivo matrices. The PAB was metabolized via hydroxylation, oxidation, sulphation, N-dealkylation, acetylation and carbonylation pathways. A few of the metabolites were also detected in in vitro samples. Metabolite identification and characterization were performed by using UHPLC/Q-TOF/MS/MS in combination with HRMS data. To identify the toxicity potential of these metabolites, in silico toxicity assessment was carried out using TOPKAT and DEREK softwares.