Cancers 2021, 13, 6329. https://doi.org/10.3390/cancers13246329 www.mdpi.com/journal/cancers Article An Easily Expandable Multi-Drug LC-MS Assay for the Simultaneous Quantification of 57 Oral Antitumor Drugs in Human Plasma Niklas Kehl 1,2 , Katja Schlichtig 1,2 , Pauline Dürr 1,2,3 , Laura Bellut 2,4 , Frank Dörje 2,3 , Rainer Fietkau 2,5 , Marianne Pavel 2,6 , Andreas Mackensen 2,7 , Bernd Wullich 2,4 , Renke Maas 1,2 , Martin F. Fromm 1,2 , Arne Gessner 1,2, * ,† and R. Verena Taudte 1,2, * ,† 1 Institute of Experimental and Clinical Pharmacology and Toxicology, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 91054 Erlangen, Germany; niklas.kehl@fau.de (N.K.); katja.schlichtig@fau.de (K.S.); pauline.duerr@uk-erlangen.de (P.D.); renke.maas@fau.de (R.M.); martin.fromm@fau.de (M.F.F.) 2 Comprehensive Cancer Center Erlangen-EMN (CCC ER-EMN), University Hospital Erlangen, 91054 Erlangen, Germany; laura.bellut@uk-erlangen.de (L.B.); frank.doerje@uk-erlangen.de (F.D.); rainer.fietkau@uk-erlangen.de (R.F.); marianne.pavel@uk-erlangen.de (M.P.); andreas.mackensen@uk-erlangen.de (A.M.); bernd.wullich@uk-erlangen.de (B.W.) 3 Pharmacy Department, University Hospital Erlangen, 91054 Erlangen, Germany 4 Department of Urology, University Hospital Erlangen, 91054 Erlangen, Germany 5 Department of Radiation Oncology, University Hospital Erlangen, 91054 Erlangen, Germany 6 Department of Internal Medicine 1, University Hospital Erlangen, 91054 Erlangen, Germany 7 Department of Internal Medicine 5, Haematology and Oncology, University Hospital Erlangen, 91054 Erlangen, Germany * Correspondence: arne.gessner@fau.de (A.G.); verena.taudte@fau.de (R.V.T.) † These authors contributed equally to this work. Simple Summary: Oral antitumor therapy has significantly improved clinical outcomes in multiple tumor entities. However, following a standard dosing regime, strong interindividual variability in patients’ plasma concentrations can be observed for many oral antitumor drugs. This results in risks of reduced therapeutic effect and increased side effects. Monitoring these variable plasma concentrations is an important tool in evaluating multiple factors influencing drug exposure and, if necessary, adjusting therapeutic doses. Here, we developed a method for the simultaneous measurement of 57 oral antitumor drug plasma concentrations. Detection and quantification were achieved using liquid chromatography coupled to an Orbitrap mass spectrometer, which can be easily expanded to newly approved oral antitumor drugs in the future. Applicability of the method was proven by measuring 71 plasma samples from 39 patients undergoing oral antitumor therapy. In summary, the developed method provides an important tool for exposure measurements of oral antitumor drugs. Abstract: Oral anticancer drugs have led to significant improvements in the treatment of multiple tumor entities. However, in patients undergoing oral antitumor therapy, plasma concentrations are highly variable, resulting in risks of reduced therapeutic effects or an increase in side effects. One important tool to reduce this variability is therapeutic drug monitoring. In this work we describe a method to simultaneously quantify the plasma concentrations of 57 oral antitumor agents. Quantification of these drugs was achieved using liquid chromatography coupled to an Orbitrap mass spectrometer. The method was fully validated according to the FDA guidelines and constitutes a simple and robust way for exposure monitoring of a wide variety of oral anticancer drugs. Applicability to clinical routine was demonstrated by the analysis of 71 plasma samples taken from 39 patients. In summary, this new multi-drug method allows simultaneous quantification of 57 oral antitumor drugs, which can be applied to exposure monitoring in clinical studies, taking into account the broad variety of oral antitumor drugs prescribed in clinical routine. Citation: Kehl, N.; Schlichtig, K.; Dürr, P.; Bellut, L.; Dörje, F.; Fietkau, R.; Pavel, M.; Mackensen, A.; Wullich, B.; Maas, R.; et al. An Easily Expandable Multi-Drug LC-MS Assay for the Simultaneous Quantification of 57 Oral Antitumor Drugs in Human Plasma. Cancers 2021, 13, 6329. https:// doi.org/10.3390/cancers13246329 Academic Editors: Neeltje Steeghs and Axel H. Schönthal Received: 10 November 2021 Accepted: 13 December 2021 Published: 16 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/license s/by/4.0/).