Absolute Quantitative Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry and Imaging Mass Spectrometry of Pharmaceutical Drugs from Biological Specimens

Abstract

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) allows for the high-throughput analysis of plasma samples for pharmaceutical drugs. A new multiplexing time-of-flight instrument with a continuous raster laser was used to improve the accurate quantitation of drugs in plasma samples using internal standards. This instrument has the capability of isolating and fragmenting multiple analytes of interest from a single laser shot. Quantitative assays of plasma for enalapril using ramipril as the internal standard and for verapamil, enalapril, and promethazine with their respective isotopically labeled internal standards were developed using this instrumentation. Verapamil, enalapril, and ramipril are all drugs used to treat hypertension, while promethazine is an antihistamine. The quantitative accuracies and precision for each analyte improved greatly upon normalization to their respective internal standard.

MALDI imaging mass spectrometry (IMS) allows for the elucidation of the distribution of pharmaceutical drugs within tissue sections. Absolute pixel-to-pixel quantitation by MALDI IMS is challenging because of matrix and tissue heterogeneities. Through careful application of isotopically labeled internal standards, accurate quantitative results can be obtained. A homogenously dosed standard tissue was developed to assess different methods of internal standard application. Internal standards with calibration standards on an adjacent non-dosed section were then used to quantitatively determine the distributions of rifampicin, an antibiotic used to treat tuberculosis, and of promethazine when dosed in animal models. The quantitative distribution of rifampicin was elucidated in rabbit livers, while the quantitative distribution of promethazine was elucidated in mouse livers and kidneys. The MALDI IMS data were compared to conventional quantitation methods using HPLC-MS/MS for further validation. The results from all experiments compared favorably to HPLC-MS/MS (>90% similarities), but quantitative MALDI IMS provided the localization of the drugs within the tissue sections. This information is lost when homogenizing the tissue for HPLC-MS/MS analysis. These studies allow for the direct quantitation of pharmaceutical drugs in relation to histological and anatomical features within tissue sections, providing unparalleled information about drugs and their targets.