# A Case Report of Severe Thrombocytopenic Purpura during Neoadjuvant Pembrolizumab Administration for Triple-Negative Breast Cancer

## TL;DR

A 51-year-old woman with breast cancer developed severe immune-related thrombocytopenia while receiving pembrolizumab, requiring hospitalization and treatment.

## Contribution

This case report highlights a rare but severe immune-related adverse event associated with pembrolizumab in triple-negative breast cancer treatment.

## Key findings

- The patient experienced severe thrombocytopenia with a bleeding tendency during pembrolizumab-based neoadjuvant therapy.
- Platelet count improved after transfusion and corticosteroid treatment, and bone marrow examination confirmed immune thrombocytopenic purpura.
- The patient achieved a complete pathological response despite incomplete neoadjuvant chemotherapy.

## Abstract

Immune thrombocytopenic purpura (ITP) is one of the immune-related adverse events (irAEs) related to immune checkpoint inhibitors (ICIs). Here, we report a case of a 51-year-old woman with triple-negative breast cancer (TNBC) who experienced severe thrombocytopenic purpura during the neoadjuvant chemotherapy (NAC), including pembrolizumab.

A 51-year-old woman was diagnosed with Stage II B TNBC and underwent NAC using pembrolizumab + paclitaxel + carboplatin. Her blood test on cycle 4, day 15 (C4D15) showed a significant decrease in platelets to <2000/μL accompanied by overt bleeding tendency. She was hospitalized for further investigation and treatment. Her platelet count recovered after platelet concentrate transfusion and corticosteroid administration. Her bone marrow examination showed normal cellularity, and she was judged as ITP. Due to the event and good clinical response to NAC, she underwent a right partial mastectomy and axillary lymph node dissection without completion of the planned NAC. The surgical specimen showed a complete pathological response.

Thrombocytopenia is known as one of the hematologic irAEs; however, severe thrombocytopenia with a bleeding tendency is rarely reported. Sufficient explanations to patients and appropriate referral to other related departments are important for earlier detection and treatment of irAE.