Bio-generation of stable isotope-labeled internal standards for absolute and relative quantitation of phase II drug metabolites in plasma samples using LC–MS/MS

Bifunctionalized isotopologs as calibrants for standard-free quantitation of drug metabolites in plasma by liquid chromatography coupled with radiometry and mass spectrometry

The reported method involves a novel workflow that eliminates the need for authentic reference standards for the quantitation of drug metabolites in biological samples using a single multi-isotopically labeled compound bearing both radio and stable isotopes. The resulting radio and stable bifunctionalized isotopolog (RADSTIL) of the parent drug is employed as a substrate for in vitro biotransformation to targeted RADSTILs of metabolites as calibrants. Inclusion of a radio label enables both radiometric and mass spectrometric detection. The addition of stable labels ensures the subsequent isotopic interference-free quantitation of unlabeled metabolites in preclinical and clinical samples. This affords a more accurate quantitation workflow compared with the current semi-quantitation method, which utilizes isotopic interfering radio isotopologs of metabolites alone as calibrants. The proof-of-concept is illustrated with (¹⁴ C,¹³ C₂ )-acetaminophen where in vitro biotransformation produced (¹⁴ C,¹³ C₂ )-sulfate and (¹⁴ C,¹³ C₂ )-glucuronide calibrants. Absolute quantitation of the acetaminophen metabolites was then achieved by liquid chromatography coupled with radiometry and mass spectrometry. Quantitative data obtained by this method fell within 82-86% of the values from conventional LC-MS/MS method.

Metabolic Study of Stable Isotope Labeled Indolinone Derivative in Hepatocyte Cell by UPLC/Q TOF MS

The aggregation process of α-synuclein (α-syn) is substantial in the pathogenesis of Parkinson's disease. Indolinone derivatives are inhibitors of α-syn aggregates and can be used as PET-based radiotracers for imaging α-syn fibrils. However, no investigations on the metabolism of indolinone derivatives have been reported until now. In the present research, a ¹³C and ¹⁵N isotope labeling strategy was developed to synthesize compound [¹³C₂,¹⁵N]-(Z)-1-(4-aminobenzyl)-3-((E)-(3-phenyl)allylidene)indolin-2-one (M0'), which was then used in a study of metabolism in hepatocytes. The metabolites were characterized using accurate mass and characteristic ion measurements. In the metabolic system, compound M0' was the main component (accounting for 97.5% of compound-related components) after incubation in hepatocytes for 3 h, which indicated that compound M0' possessed great metabolic stability. Seven metabolites have been successfully verified by UPLC/Q TOF MS in metabolic studies, including hydroxyl M0' (M1'), hydroxyl and methylated M0' (M2'), N-acetylated M0' (M3'), sulfate of hydroxyl M0' (M4'), the glucose conjugate of M0' (M5'), glucuronide conjugate of M0' (M6'), and glucuronide conjugate of hydroxyl M0' (M7'). The study on metabolism provides the important information to develop effective α-syn aggregate inhibitors and new PET-tracer-related indolinone derivatives.

Production and Application of Stable Isotope-Labeled Internal Standards for RNA Modification Analysis

Post-transcriptional RNA modifications have been found to be present in a wide variety of organisms and in different types of RNA. Nucleoside modifications are interesting due to their already known roles in translation fidelity, enzyme recognition, disease progression, and RNA stability. In addition, the abundance of modified nucleosides fluctuates based on growth phase, external stress, or possibly other factors not yet explored. With modifications ever changing, a method to determine absolute quantities for multiple nucleoside modifications is required. Here, we report metabolic isotope labeling to produce isotopically labeled internal standards in bacteria and yeast. These can be used for the quantification of 26 different modified nucleosides. We explain in detail how these internal standards are produced and show their mass spectrometric characterization. We apply our internal standards and quantify the modification content of transfer RNA (tRNA) from bacteria and various eukaryotes. We can show that the origin of the internal standard has no impact on the quantification result. Furthermore, we use our internal standard for the quantification of modified nucleosides in mouse tissue messenger RNA (mRNA), where we find different modification profiles in liver and brain tissue.

Advances in detection and quantification of methylcytosine and its derivatives

Methylation of the fifth carbon atom in cytosine is an epigenetic modification of deoxyribonucleic acid that plays important roles in numerous cellular processes and disease pathogenesis. Three additional states of cytosine, that is, 5-hydroxymethylcytosine, 5-formylcytosine and 5-carboxylcytosine, have been identified and associated with the diagnosis and/or prognosis of diseases. However, accurate measurement of those intermediates is a challenge since their global levels are relatively low. A number of innovative methods have been developed to detect and quantify these compounds in biological samples, such as blood, tissue and urine, etc. This review focuses on recent advancement in detection and quantification of four cytosine modifications, based on which, the development, diagnosis, and prognosis of diseases could be monitored through non-invasive procedures.