# Exceptional Response to Pembrolizumab in Metastatic ER+/HER2− Breast Cancer With Liver Metastases: A Case Report and Literature Review

## TL;DR

A patient with hormone receptor-positive breast cancer showed a strong response to pembrolizumab after developing high PD-L1 expression in liver metastases.

## Contribution

Demonstrates exceptional response to immunotherapy in a typically resistant breast cancer subtype with high PD-L1 expression.

## Key findings

- The patient achieved near-complete remission with pembrolizumab after progressing on other therapies.
- High PD-L1 expression (CPS 95%) was observed in liver metastases, which correlated with treatment response.
- The remission lasted about 10 months before disease progression.

## Abstract

Hormone receptor–positive (HR+) and HER2-negative breast cancer is the most common subtype in women, particularly in the postmenopausal setting. Unlike triple-negative breast cancer, the benefit of immune checkpoint inhibitors (ICIs) in HR+/HER2− disease remains uncertain because of low tumor immunogenicity and limited PD-L1 expression.

We describe a case of a 70-year-old woman who presented with severe anemia and was incidentally found to have a bleeding left breast mass. Biopsy confirmed Grade 3 invasive ductal carcinoma (ER+/PR+ > 95%, HER2−) with nodal involvement but no distant metastases, consistent with Stage IIIc disease. She was treated with neoadjuvant anastrozole, modified radical mastectomy, adjuvant chemotherapy, radiation, and continued endocrine therapy. After 3 years, she developed extensive hepatic metastases. Biopsy revealed ER+/PR−/HER2− disease with striking PD-L1 expression (CPS 95%). The disease progressed on fulvestrant and palbociclib, but switching to carboplatin, gemcitabine, and pembrolizumab led to rapid improvement: liver function normalized and imaging showed near-complete response within 3 months. This remission lasted about 10 months before disease progression and transition to hospice care.

This case explains the potential role of ICIs in HR+/HER2− breast cancer with unusually high PD-L1 expression. It underscores the importance of biomarker-driven treatment and supports expanding PD-L1 testing to better identify patients who may benefit from immunotherapy in this traditionally resistant subtype.