A low flow ionization technique to integrate quantitative and qualitative small molecule bioanalysis

Abstract

Mass spectrometry-based assays are used in drug discovery and development to detect, characterize and quantify drugs, metabolites, impurities and degradants. Recently, high resolution-based mass spectrometers have begun to emerge as a platform with potential for performing integrated qualitative and quantitative assays in order to streamline the drug discovery and development process. However, the widely different LC–MS response observed for a drug and its metabolites limit the direct use of LC–MS responses for relative quantitative determination of metabolites. This in turn limits the use of conventional LC–ESI-MS methods, in the absence of reference standards, as an integrated technique for detection, characterization and quantification of drugs and metabolites. The goal of this study was to explore the use of LC–captive spray ionization (CSI)-mass spectrometry for detection, characterization and quantification of drugs and metabolites. CSI allows the use of conventional HPLC or uHPLC columns and flow rates of 0.35–0.6 mL/min (before post-column flow splitting) and can be considered as a technique which can function as a nanospray or microspray. Also, in comparison to conventional nanospray ionization (NSI) techniques, setup and maintenance of CSI do not require: (1) X, Y, and Z positioning or cameras to guide the spray positioning, (2) difficult to control splitters to deliver nano-flow ratios and difficult to maintain nanospray nozzles. Evaluations using equimolar mixture of buspirone and four monoxy metabolites present in human plasma show that LC–CSI-MS is a highly sensitive technique that gives a near equimolar response for the compounds used in this example. Comparisons of LC–ESI-MS data with that obtained using LC–CSI-MS show that reasonable quantification of metabolites may be achievable without using reference standards or administration of radiolabeled drugs.