Review ArticlesImmune-Checkpoint-Inhibitor Therapy—Principles and Relevance of Biomarkers for Pathologists and OncologistsDarr, Christopher MD*,†; Hilser, Thomas MD†,‡; Kesch, Claudia MD*,†; Isgandarov, Aykhan MD*,†; Reis, Henning MD§,∥; Wahl, Milan MD*,†; Kasper-Virchow, Isabel MD†,‡; Hadaschik, Boris A. MD*,†; Grünwald, Viktor MD*,†,‡ Author Information *Department of Urology †German Cancer Consortium (DKTK) ‡Department of Medical Oncology, West German Cancer Center, University Hospital Essen §Institute of Pathology, University Hospital Essen, Essen ∥Dr. Senckenberg Institute of Pathology, University Hospital Frankfurt, Goethe University, Frankfurt, Germany C.K. reports consulting fees Apogepha, research funding Advanced Accelerator Applications (Novartis). B.A.H. reports personal fees from ABX, Bayer, LightPoint Medical Inc., Janssen R&D, Bristol-Myers-Squibb and Astellas; research funding from Profound Medical, German Cancer Aid, German Research Foundation, Janssen R&D, Bristol-Myers-Squibb, MSD, Pfizer and Astellas; and travel fees from AstraZeneca, Janssen R&D and Astellas. V.G. reports fees from Astellas, AstraZeneca, BMS, EISAI, Ipsen, Janssen-Cilag, Merck Serono, MSD, Novartis, Pfizer, Roche, and research grants from BMS, Ipsen, MSD and Pfizer. The remaining authors have no funding or conflicts of interest to disclose. Reprints: Christopher Darr, MD, Department of Urology, University Hospital Essen, Hufelandstr. 55, 45147 Essen (e-mail: [email protected]). Advances In Anatomic Pathology 30(3):p 160-166, May 2023. | DOI: 10.1097/PAP.0000000000000373 Buy Metrics Abstract Immune-checkpoint-inhibitor (ICI) therapy has been one of the major advances in the treatment of a variety of advanced or metastatic tumors in recent years. Therefore, ICI-therapy is already approved in first-line therapy for multiple tumors, either as monotherapy or as combination therapy. However, there are relevant differences in approval among different tumor entities, especially with respect to PD-L1 testing. Different response to ICI-therapy has been observed in the pivotal trials, so PD-L1 diagnostic testing is used for patient selection. In addition to PD-L1 testing of tumor tissue, liquid biopsy provides a noninvasive way to monitor disease in cancer patients and identify those who would benefit most from ICI-therapy. This overview focuses on the use of ICI-therapy and how it relates to common and potential future biomarkers for patient-directed treatment planning. Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.