Purpose : Tumor lysis syndrome (TLS) is a rare but life-threatening phenomenon that occurs mainly in patients with aggressive hematologic or highly chemotherapy sensitive solid tumors such as high-grade neuroendocrine carcinoma or testicular cancer. Tumor lysis syndrome is exceedingly rare in hormone receptor-positive, HER2-negative breast cancer. Furthermore, TLS following treatment with alpelisib, a novel phosphatidylinositol 3-kinase (PI3K) inhibitor used to treat PIK3CA-mutated (gene encoding p110α subunit of PI3K), hormone receptor positive advanced breast cancer, has never been described in patients with nonhematologic malignancies.

Methods : In the following case, we present a patient with hormone receptor-positive, HER2-negative, PIK3CA-mutated metastatic breast cancer who developed TLS 12 days after starting fulvestrant and alpelisib.

Results : Patient was promptly treated with improvement in her renal function to baseline without requiring renal replacement therapy. Alpelisib was resumed at a reduced dose with no further complications.

Conclusion : Through this case, we discuss the potential complications of TLS and the importance of prompt recognition and treatment.