Supplementary Material for: Severe Thrombocytopenia from Trastuzumab and Pertuzumab Combination Therapy in a Patient with HER2-Positive Metastatic Rectal Cancer

Introduction
In recent years, trastuzumab and pertuzumab have been used in treatment protocols for patients with HER2-positive colorectal cancer. Although severe thrombocytopenia is an uncommon side effect of anti-HER2 antibody therapy, we present the first patient with HER2-positive metastatic rectal cancer who developed significant thrombocytopenia after trastuzumab and pertuzumab administration.
Case presentation
The condition was identified as drug-induced immune thrombocytopenia (DITP) associated with trastuzumab and pertuzumab. Despite the discontinuation of anti-HER2 treatment and administration of corticosteroids, and in addition to frequent platelet transfusions, a low platelet count persisted. Consequently, we determined that the patient presented with a condition similar to immune thrombocytopenic purpura (ITP) and selected a treatment approach consisting of eltrombopag, a thrombopoietin receptor agonist. Subsequently, the patient’s platelet count did not decrease further but rather improved.
Conclusion
Although uncommon, anti-HER2 antibodies can cause severe thrombocytopenia. Furthermore, if thrombocytopenia persists after treatment discontinuation and the administration of corticosteroids, exploring treatment options aligned with managing ITP is essential.