[Suspected S-1-Induced Rhabdomyolysis during Adjuvant Chemotherapy after Breast Cancer Surgery-A Case Report]

Area of Science:

• Oncology
• Pharmacology
• Clinical Medicine

Background:

• S-1 is an oral fluoropyrimidine utilized in the adjuvant treatment of various cancers, including hormone receptor-positive and HER2-negative breast cancer, based on POTENT trial findings.
• While generally well-tolerated, S-1 has been associated with rare but severe adverse events, notably rhabdomyolysis.

Purpose of the Study:

• To report a case of suspected S-1-induced rhabdomyolysis in a breast cancer patient undergoing adjuvant therapy.
• To raise awareness among clinicians regarding this rare but serious potential side effect of S-1.

Main Methods:

• A case study of a 56-year-old woman with a history of hypertension and dyslipidemia receiving adjuvant therapy with letrozole and S-1 post-breast cancer surgery.
• Monitoring of serum creatine kinase (CK) levels and clinical symptoms, with subsequent discontinuation of S-1 to assess causality.

Main Results:

• The patient experienced a significant increase in serum CK levels, peaking at 4,419 U/L, accompanied by myalgia during S-1 treatment.
• Serum CK levels normalized after S-1 discontinuation, while other medications were continued, suggesting S-1 as the likely cause.
• No alternative causes for rhabdomyolysis were identified.

Conclusions:

• S-1 is a potential cause of rhabdomyolysis, although extremely rare.
• Clinicians should maintain awareness of this potential adverse effect and monitor patients for muscle-related symptoms during S-1 treatment.