© 2021 The Author(s). Published by S. Karger AG, Basel Case Report Case Rep Oncol 2021;14:931–937 Durable Response to PD1 Inhibitor Pembrolizumab in a Metastatic, Metaplastic Breast Cancer Elan Gorshein a Kant Matsuda b Gregory Riedlinger b Levi Sokol c Lorna Rodriguez-Rodriguez d Firas Eladoumikdachi e Miral Grandhi e Shridar Ganesan a Deborah L. Toppmeyer a Lindsay Potdevin e Kathleen Toomey f Kim M. Hirshfield a Nancy Chan a a Division of Medical Oncology, Department of Medicine, Rutgers Robert Wood Johnson Medical School, Rutgers Cancer Institute of New Jersey, New Brunswick, NJ, USA; b Department of Pathology and Laboratory Medicine, Rutgers Robert Wood Johnson Medical School, Rutgers Cancer Institute of New Jersey, New Brunswick, NJ, USA; c Department of Radiology, Rutgers Robert Wood Johnson Medical School, New Brunswick, NJ, USA; d Department of Gynecologic Oncology, Rutgers Robert Wood Johnson Medical School, Rutgers Cancer Institute of New Jersey, New Brunswick, NJ, USA; e Division of Surgical Oncology, Department of Surgery, Rutgers Robert Wood Johnson Medical School, Rutgers Cancer Institute of New Jersey, New Brunswick, NJ, USA; f Division of Medical Oncology, Department of Medicine, Rutgers Robert Wood Johnson Medical School, Steeplechase Cancer Center, Somerville, NJ, USA Keywords Metaplastic breast cancer · Exceptional response · Immunotherapy · Program death ligand-1 Abstract Metaplastic breast cancer (MBC) is a rare and aggressive subtype of breast cancer. Tumor characteristics typically feature estrogen receptor, progesterone receptor, and HER2-nega- tive, triple-negative breast cancer (TNBC), with a poorer prognosis relative to pure invasive ductal or lobular disease. Resistance to chemotherapy often leads to local recurrence and distant metastasis. Genomic profiling has identified multiple molecular abnormalities that may translate to targetable therapies in MBC. These tumors are known to display higher PD- L1 expressivity than other subtypes of breast cancer, and disease control with pembrolizumab and chemotherapy has been documented. We identify a patient with metastatic, metaplastic Received: February 4, 2021 Accepted: February 26, 2021 Published online: June 18, 2021 Correspondence to: Nancy Chan, nancy.chan @cinj.rutgers.edu www.karger.com/cro This article is licensed under the Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC) (http://www.karger.com/Services/OpenAccessLicense). Usage and distribution for commercial purposes requires written permission. DOI: 10.1159/000515510 Elan Gorshein, Lorna Rodriguez-Rodriguez, and Kim M. Hirshfield: Contribution during his/her time at Rutgers Cancer Institute of New Jersey.