Improvement of methods for the structural characterisation of drug metabolites based on collisional cross sections

Abstract

Drug metabolism is a pivotal determining factor for the changes in physiological drug concentration and can determine or modify its toxicological or pharmacological pathway (Iyanagi, T., Int. Rev. Cytol., 2007, 260). Understanding of processes, involving a drug in a living organism, is therefore crucial to study and analyse the action of the drug or its metabolites, as reported by Caldwell, J. et al, Toxicol. Pathol., 1995, vol. 23, no. 2. Drug metabolites are typically identified using various techniques, but lately, Ion-Mobility Mass Spectrometry (IM-MS) has become a widely popular tool for small molecule (which are drug metabolites) structural identification due to its high efficiency and a low amount requirement for samples. A combination of this technique along with a computational approach has proved to deliver reliable identification predictions of investigated compounds by comparing experimental and calculated collisional cross sections (CCS) of structures. However, even though a corresponding experimental field has made some valuable developments over the last couple of years, its theoretical counterpart has seen a rather slow improvement. Recently, Reading, E. et al, Anal. Chem., 2016, 88 (4), have developed a computational protocol for collisional cross section calculations. The first part of this work addresses the issue of efficiency of the proposed protocol along with its large-scale applicability. Additionally, special attention has been paid to the reproducibility of the published results and also to the possible ways of improving the agreement within different sets of theoretical results as well as between newly calculated and experimental values. The second part of this manuscript focuses on studying fragmentation mechanisms that occur during Mass Spectrometry (MS) measurements. Electro Spray Ionisation (ESI) along with Tandem MS and Collision Induced Dissociation (CID) build up a powerful experimental approach, able to deliver a deeper understanding of a collision process and its products (Molina, E. R. et al, J. Mass Spectrom., 2015, 50). It is feasible due to extensive fragmentation that takes place in activated ions (metabolites). A corresponding computational approach developed by Hase, W. L. et al, Quantum Chem. Progr. Exch. Bull., 1996, 16, and Hase, W. L. et al, J. Phys. Chem., 1996, 100.20, is used to run Collision Dynamics Simulations (CDS) to obtain reactive trajectories. These trajectories are further utilised for fragmentation analysis that gives insights about structural information of the fragments and possible reaction pathways and also allows to build a theoretical MS spectrum