Pembrolizumab-induced thrombotic thrombocytopenic purpura

Pembrolizumab is a humanised monoclonal antibody targeting the receptor programmed cell death protein-1 (PD-1), with anti-tumour activity demonstrated for many malignancies. Such immune checkpoint inhibitors are associated with many immune-related adverse events including rash, colitis, hepatitis, pneumonitis, endocrinopathy and, rarely, haematological adverse events, including immune-related thrombocytopenia.

We report a 60-year-old female with metastatic non-small cell lung cancer treated with pembrolizumab every three weeks. Following her fifth cycle, she presented to our hospital with community-acquired pneumonia. Thrombocytopenia developed the next day and, after detailed investigations, thrombotic thrombocytopenic purpura was diagnosed.

Pembrolizumab was immediately ceased and plasma exchange commenced along with IV methylprednisolone 250 mg daily for three days followed by oral prednisolone. After five days of plasma exchange, platelet counts normalised and haemolytic anaemia resolved.

Acquired thrombotic thrombocytopenic purpura is an autoimmune disorder caused by an inhibitory autoantibody against ADAMTS-13. While most cases of acquired thrombotic thrombocytopenic purpura are idiopathic, certain conditions (e.g. bacterial infection, autoimmune disorders, malignancies) and medications are associated with thrombotic thrombocytopenic purpura. Other potential causes were eliminated in our patient. As acquired thrombotic thrombocytopenic purpura is an autoimmune disorder, pembrolizumab, given its unique mechanism of action and association with immune-related adverse events, is believed to be implicated in the development of thrombotic thrombocytopenic purpura. This case is one of only two linking anti-PD-1 therapy to thrombotic thrombocytopenic purpura development (the other occurring in a patient on nivolumab plus ipilimumab). Thrombotic thrombocytopenic purpura is life-threatening and clinicians are advised to be aware of its possible occurrence in immune checkpoint inhibitor-treated patients.