Fulminant Myocarditis in a Filipino Female with Hormonal Receptor Discordant Primary Breast Cancer with Bone Metastases: Fatal Immune-related Adverse Event

BACKGROUND Although immune-checkpoint inhibitor-related myocarditis is uncommon,1-5 it is potentially fatal with a steep mortality of 25% to 50% compared with other immune-related adverse events. CASE REPORT A 63-year-old hypertensive, diabetic Filipino female known to have triple-negative breast cancer with bone metastases came in because of dyspnea and desaturation. In November 2005, the patient was diagnosed with biopsy-confirmed invasive ductal carcinoma, estrogen- (ER) and progesterone receptor (PR) positive, HER2/neu negative. Docetaxel and epirubicin with radiation therapy (RT) were administered thereafter, she was maintained on anastrozole for five years, then she underwent remission. Nine years later, tumoral spread to the frontal bone, T5 vertebra and left adrenal gland was noted and biopsy revealed triple-hormone-negative mucinous carcinoma favoring metastasis. Genetic sequencing exhibited high tumor mutational burden, pembrolizumab was started. She tolerated the treatment until days after cycle 10, she developed dyspnea at rest, anterior chest discomfort and desaturation. TTE showed segmental hypokinesis with preserved ejection fraction with decreased GLS. Hypotension soon ensued, necessitating vasopressors. Repeat laboratories showed increasing hsTnI, NT-proBNP and D-dimer. ECG demonstrated a new bifascicular block with prolonged QTcF. Satisfying the clinical criteria for fulminant ICI-related myocarditis, hydrocortisone was shifted to methylprednisolone. CMR confirmed prior myocarditis. After three days of methylprednisolone, oral prednisone was started and guideline-directed medical therapy for heart failure was slowly resumed. The patient was eventually discharged improved. LEARNING POINTS It is imperative to recognize that while immunotherapy represents a significant advancement in oncologic treatment—offering improved survival rates and enhanced quality of life for patients—it is not without risk. Rare yet potentially fatal complications may arise, necessitating vigilant monitoring and a high index of suspicion for immune-related adverse events.