# Pneumocystis Pneumonia in Locally Advanced Breast Cancer Despite Prophylactic Use of Trimethoprim-Sulfamethoxazole During Prednisolone Treatment for a Pembrolizumab-Induced Immune-Related Adverse Event: A Case Report

## TL;DR

A breast cancer patient developed Pneumocystis pneumonia despite taking a preventive medication, highlighting the need for careful monitoring and possible adjustments in treatment.

## Contribution

Reports a rare case of PCP in a cancer patient on TMP-SMX prophylaxis during immunotherapy-related treatment.

## Key findings

- PCP occurred in a patient receiving TMP-SMX prophylaxis during prednisolone treatment for pembrolizumab-induced SJS.
- The case suggests that TMP-SMX prophylaxis may not always prevent PCP in high-risk patients.
- Dosage and duration of prophylaxis should be tailored to individual patient factors.

## Abstract

Pneumocystis pneumonia (PCP) primarily affects immunosuppressed patients, with trimethoprim-sulfamethoxazole (TMP-SMX) commonly used for prophylaxis. However, there is insufficient information on PCP occurrence despite TMP-SMX prophylaxis. We encountered a 57-year-old woman with locally advanced breast cancer developing PCP despite prophylactic intake of TMP-SMX, during treatment with prednisolone for Stevens-Johnson syndrome (SJS) induced by pembrolizumab. This case underscores the need to pay attention to the possibility of PCP development even during TMP-SMX prophylaxis. Dosage and duration adjustments according to the patient's condition and weight may be required.