RESUMEN
Pembrolizumab, a humanised monoclonal antibody and immune checkpoint inhibitor (ICI) that blocks programmed death receptor 1 and its ligands, is an effective immunotherapy for malignancies such as melanoma, lung, head and neck, cancers, and Hodgkin's lymphoma. It has an overall response rate between 73% and 83%, with complete response rate of 27%-30%. It is well tolerated with minor side effects in 70% of cases characterised by fatigue, rash, pruritus and diarrhoea. In rare cases, more serious and life-threatening complications can occur at a rate of 0.3%-1.3%. We report a case of a woman in her 70s with non-small-cell lung cancer treated with ICI. She presented to the emergency department with left-sided ptosis and muscle weakness 3 weeks of her first dose of pembrolizumab infusion as a treatment plan of her cancer. She was diagnosed with myasthenia gravis, myocarditis and myositis as ICI-induced immune-related adverse effects resistant to medical intervention. We wish to raise awareness of the triad of life-threatening complication of ICI therapy that accounts for 30%-50% of fatal complications.