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<p>Comprehensive genomic profiling can identify actionable mutations and guide therapy selection in metastatic breast cancer. Tumor mutation burden (TMB) may predict response to immune checkpoint inhibitor (ICI) therapy, but its clinical utility in estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer remains unclear. In this case report, we describe a 63-year-old woman with ER-positive, HER2-negative metastatic breast cancer who had exhausted standard treatment options. Comprehensive genomic profiling of the pancreatic metastatic tissue revealed a TMB of 33 mutations per megabase (mut/Mb), consistent with TMB-high status. Based on this result, pembrolizumab therapy was initiated in October 2022. Subsequent imaging demonstrated a complete response, with disappearance of FDG uptake in the pancreatic, renal, and lung metastases on positron emission tomography/computed tomography (PET/CT). Tumor marker levels also normalized, and this response has been maintained as of January 2026. This case highlights the potential utility of TMB-high status as a predictive biomarker for ICI therapy in ER-positive, HER2-negative metastatic breast cancer. Although this subtype is typically considered less immunogenic, TMB-high may identify patients who could benefit from immune checkpoint blockade. Comprehensive genomic profiling should be considered when standard treatments are limited.</p>

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A case of ER-positive, HER2-negative metastatic breast cancer with high tumor mutation burden achieving complete response to pembrolizumab monotherapy

  • 期刊论文

  • 发表日期：

    2026年5月22日

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  • Akihiro Umejima,
  • Nobuko Kawaguchi-Sakita,
  • Yosuke Yamada,
  • Kosuke Kawaguchi,
  • Masahiro Kawashima,
  • Masahiro Takada,
  • Chiaki Suzuki,
  • Yukiko Mori,
  • Masahiro Yoshioka,
  • Masashi Kanai,
  • Manabu Muto,
  • Masakazu Toi

摘要

Comprehensive genomic profiling can identify actionable mutations and guide therapy selection in metastatic breast cancer. Tumor mutation burden (TMB) may predict response to immune checkpoint inhibitor (ICI) therapy, but its clinical utility in estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer remains unclear. In this case report, we describe a 63-year-old woman with ER-positive, HER2-negative metastatic breast cancer who had exhausted standard treatment options. Comprehensive genomic profiling of the pancreatic metastatic tissue revealed a TMB of 33 mutations per megabase (mut/Mb), consistent with TMB-high status. Based on this result, pembrolizumab therapy was initiated in October 2022. Subsequent imaging demonstrated a complete response, with disappearance of FDG uptake in the pancreatic, renal, and lung metastases on positron emission tomography/computed tomography (PET/CT). Tumor marker levels also normalized, and this response has been maintained as of January 2026. This case highlights the potential utility of TMB-high status as a predictive biomarker for ICI therapy in ER-positive, HER2-negative metastatic breast cancer. Although this subtype is typically considered less immunogenic, TMB-high may identify patients who could benefit from immune checkpoint blockade. Comprehensive genomic profiling should be considered when standard treatments are limited.