Multiplexed Analysis of Endogenous Guanidino Compounds via Isotope-Coded Doubly Charged Labeling: Application to Lung Cancer Tissues as a Case

Endogenous guanidino compounds (GCs), nitrogen-containing metabolites, have very important physiological activities and participate in biochemical processes. Therefore, accurately characterizing the distribution of endogenous GCs and monitoring their concentration variations are of great significance. In this work, a new derivatization reagent, 4,4'-bis[3-(dimethylamino)propyl]benzyl (BDMAPB), with isotope-coded reagents was designed and synthesized for doubly charged labeling of GCs. BDMAPB-derivatized GCs not only promote the MS signal but also form multicharged quasimolecular ions and abundant fragment ions. With this reagent, an isotope-coded doubly charged labeling (ICDCL) strategy was developed for endogenous GCs with high-resolution liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF MS). The core of this methodology is a 4-fold multiplexed set of [d0]-/[d4]-/[d8]-/[d12]-BDMAPB that yields isotope-coded derivatized GCs. Following a methodological assessment, good linear responses in the range of 25 nM to 1 μM with correlation coefficients over 0.99 were achieved. The limit of detection and the limit of quantitation were below 5 and 25 nM, respectively. The intra- and interday precisions were less than 18%, and the accuracy was in the range of 77.3-122.0%. The percentage recovery in tissues was in the range of 85.1-113.7%. The results indicate that the developed method facilitates long-term testing and ensures accuracy and reliability. Finally, the method was applied for the simultaneous analysis of endogenous GCs in four types of lung tissues (solid adenocarcinoma, solid squamous-cell carcinoma, ground-glass carcinoma, and paracancerous tissues) for absolute quantification, nontargeted screening, and metabolic difference analysis. It is strongly believed that ICDCL combined with isotope-coded BDMAPB will benefit the analysis and study of endogenous GCs.