  Citation: Funke, K.; Düster, R.; Wilson, P.D.-G.;Arévalo, L.; Geyer, M.; Schorle, H. Transcriptional CDK Inhibitors as Potential Treatment Option for Testicular Germ Cell Tumors. Cancers 2022, 14, 1690. https://doi.org/10.3390/ cancers14071690 Academic Editors: Michael J. Spinella, Sarah J. Freemantle, Zeeshan Fazal and Ratnakar Singh Received: 3 March 2022 Accepted: 24 March 2022 Published: 26 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/licenses/by/ 4.0/). cancers Article Transcriptional CDK Inhibitors as Potential Treatment Option for Testicular Germ Cell Tumors Kai Funke 1 , Robert Düster 2,† , Prince De-Graft Wilson 1 , Lena Arévalo 1 , Matthias Geyer 2 and Hubert Schorle 1, * 1 Department of Developmental Pathology, Institute of Pathology, University Hospital Bonn, 53127 Bonn, Germany; kai.funke@ukbonn.de (K.F.); sharkalemagh@gmail.com (P.D.-G.W.); lena.arevalo@ukbonn.de (L.A.) 2 The Institute of Structural Biology, University of Bonn, 53127 Bonn, Germany; robert.duster@mssm.edu (R.D.); matthias.geyer@uni-bonn.de (M.G.) * Correspondence: schorle@uni-bonn.de † Present address: Department of Oncological Sciences, Icahn School of Medicine at Mount Sinai, New York, NY 10029, USA. Simple Summary: Type II testicular germ cell tumors are a severe type of cancer in young men demanding alternative treatment options to conventional chemotherapy with less side effects. In particular, patients with chemotherapy-resistant tumors face a bad prognosis and low survival rates. In other tumor entities, transcriptional cyclin-dependent kinases (7/8/9/12/13) have been demonstrated to be effective targets. Here, we studied the effects of transcriptional cyclin-dependent kinase inhibitors on a cellular and molecular level. We found several inhibitors to be highly cytotoxic for certain testicular germ cell tumor cell lines while leaving a somatic (fibroblast) control cell line unaffected. This opens up a novel field for effective and specified treatment of type II testicular germ cell tumors. Abstract: Type II testicular germ cell tumors (TGCT) are the most frequently diagnosed solid malig- nancy in young men. Up to 15% of patients with metastatic non-seminomas show cisplatin resistance and a very poor survival rate due to lacking treatment options. Transcriptional cyclin-dependent kinases (CDK) have been shown to be effective targets in the treatment of different types of can- cer. Here, we investigated the effects of the CDK inhibitors dinaciclib, flavopiridol, YKL-5-124, THZ1, NVP2, SY0351 and THZ531. An XTT viability assay revealed a strong cytotoxic impact of CDK7/12/13 inhibitor SY0351 and CDK9 inhibitor NVP2 on the TGCT wild-type cell lines (2102EP, NCCIT, TCam2) and the cisplatin-resistant cell lines (2102EP-R, NCCIT-R). The CDK7 inhibitor YKL-5-124 showed a strong impact on 2102EP, 2102EP-R, NCCIT and NCCIT-R cell lines, leaving the MPAF control cell line mostly unaffected. FACS-based analysis revealed mild effects on the cell cycle of 2102EP and TCam2 cells after SY0351, YKL-5-124 or NVP2 treatment. Molecular analysis showed a cell-line-specific response for SY0351 and NVP2 inhibition while YKL-5-124 induced similar molecular changes in 2102EP, TCam2 and MPAF cells. Thus, after TGCT subtype determination, CDK inhibitors might be a potential alternative for optimized and individualized therapy independent of chemotherapy sensitivity. Keywords: testicular germ cell tumors; transcriptional cyclin-dependent kinase inhibitors; CDK inhibitors; NVP2; SY0351; YKL-5-124 1. Introduction Type II testicular germ cell tumors (TGCT) are the most prevalent malignancies in young men aged 18 to 35 years [1] with rising incidence in Western countries in partic- ular [2]. TGCTs are classified into seminomas and non-seminomas, which are further subdivided into embryonal carcinomas (EC), yolk sac tumors (YST), teratomas (Ter) and Cancers 2022, 14, 1690. https://doi.org/10.3390/cancers14071690 https://www.mdpi.com/journal/cancers