# A Case of Drug-Induced Pancytopenia due to Tamoxifen

## TL;DR

A 46-year-old woman developed blood cell count reduction after taking tamoxifen for breast cancer, suggesting a rare drug-induced pancytopenia.

## Contribution

This is the first reported case of tamoxifen-induced pancytopenia in Japan, highlighting a potential new side effect.

## Key findings

- Tamoxifen may cause pancytopenia, as evidenced by significant blood cell count reduction in a patient.
- Spontaneous remission occurred after discontinuation of tamoxifen, suggesting a drug-related cause.
- No definitive hematological disorder was diagnosed despite suspicion of myelodysplastic syndrome.

## Abstract

Tamoxifen (TAM) is a well-established treatment for hormone receptor-positive breast cancer with a known side-effect profile that includes hot flashes, genital bleeding, and diarrhea (0.1%–5%). Other notable side effects include liver damage, abnormal vaginal discharge, depression, dizziness, and headaches of unknown frequency. However, blood cell count reduction has not yet been reported as a side effect in Japan.

A 46-year-old female patient was diagnosed with right breast cancer (cT1N0M0). The patient underwent partial right breast resection and sentinel lymph node biopsy. Owing to the positive surgical resection margin, a mastectomy was performed. Pathological analysis of the surgical specimen confirmed invasive ductal carcinoma (estrogen receptor [ER]: 95%, progesterone receptor [PgR]: 85%, HER2: 2+ [fluorescence in situ hybridization, FISH negative]), with macrometastasis in one sentinel lymph node. Postoperative treatment included chemotherapy (dose-dense adriamycin and cyclophosphamide [AC] to dose-dense paclitaxel [PTX]), irradiation, and TAM. While initial blood test results before starting TAM showed mild anemia (Hb: 8.9 g/dL Grade 2), a follow-up blood test 5 months after initiating TAM revealed a significant decrease in blood cell counts (white blood cell [WBC]: 2600/μL Grade 2, neutrophil [neu]: 0.55 × 10³/μL Grade 3, Hb: 7.7 g/dL Grade 2, platelet [PLT]: 13.3 × 10⁴/μL). Considering the onset of symptoms following TAM administration, drug-induced pancytopenia was suspected. TAM and its concomitant medication pregabalin were discontinued. However, the blood cell counts continued to decline, necessitating further investigation. Myelodysplastic syndrome (MDS) was suspected, leading to multiple bone marrow biopsies. However, no definitive hematological disorder was diagnosed. The patient received transfusions and granulocyte colony-stimulating factor (G-CSF) injections based on the blood cell count. Approximately 4 months after the onset of neutropenia, gradual recovery was observed and spontaneous remission occurred. Given the rarity of spontaneous recovery from MDS, TAM is considered a potential causative agent of the observed decline in blood cell counts.

We report a case of suspected drug-induced cytopenia associated with tamoxifen administration.