This article focuses on current applications of various types of mass spectrometry systems to new drug discovery efforts.
Mass spectrometry (MS) has been an important analytical tool for scientists working in the drug metabolism and pharmacokinetics (DMPK) arena for several decades. Before 1990, the use of MS was restricted, in most cases, to metabolite identification studies (1–3). The early 1990s brought the commercialization of electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Those ionization techniques made MS an essential analytical tool for DMPK scientists (4–7). In the mid-1990s, when commercial, high performance liquid chromatography–tandem mass spectrometry (HPLC–MS-MS) systems became commonplace, HPLC–MS-MS became their premier analytical tool (4,5,8–11).
For DMPK scientists, HPLC–MS-MS is now the analytical tool of choice in all areas of research (4,8,12,13). The goal of discovery DMPK is to screen multiple compounds, assess their inherent DMPK properties, and then perform additional metabolism studies on a subset of compounds likely to proceed to drug development (14,15). This column installment focuses on current applications of various types of MS systems to new drug discovery efforts.Types of Mass Spectrometers
Though scientists outside the field of MS may view all MS systems as more or less the same, the reality is quite different. Not only do mass spectrometers differ by type, but MS-MS systems, which are common, can combine two different types of MS systems into a single "hybrid" MS-MS system.
An extensive discussion of all these systems and how they work transcends the scope of this article. I would therefore direct readers who desire such a comprehensive review to previously published works (16–19).
Mass spectrometers can be differentiated by their mass-resolution capabilities. A low-resolution mass spectrometer can distinguish compounds that differ by at least 1 Da. Thus it can distinguish a compound detected at m/z 523 from compounds that are at least 1 Da higher (m/z 524) or 1 Da lower (m/z 522). The quadrupole (Q) MS and ion-trap (IT) MS systems are examples of low-resolution MS systems. A high-resolution mass spectrometer can measure the exact mass of a compound: That is, the mass to an accuracy of up to four decimal places (for example, 523.4267). High-resolution MS (HRMS) systems can therefore differentiate compounds that differ in mass by as little as 0.01 Da. Examples of such systems include time-of-flight (TOF) and orbital trap (Orbitrap, Thermo Fisher Scientific) systems.
MS-MS systems combine various types of mass spectrometers into a single system. A very common MS-MS system is the triple-quadrupole (QQQ) MS-MS system; that is, it uses three quadrupoles. Hybrid MS-MS systems combine two types of MS systems in one instrument. For example, a QTOF MS-MS system combines a quadrupole system and a TOF system, and a QIT MS-MS system combines a quadrupole system and a linear IT system (QTRAP, AB Sciex) (20–22). I address the utility of these various MS-MS systems below, in a discussion about how MS is used for various drug metabolism assays.