Pembrolizumab treatment of inflammatory progressive multifocal leukoencephalopathy: a report of two cases

Progressive multifocal leukoencephalopathy (PML) is a rare but devastating neurological disease caused by reactivation of the JC virus in susceptible individuals. The illness has classically been associated with the human immunodeficiency virus (HIV) and multiple sclerosis (MS) patients who are treated with natalizumab. It is also associated with haematological malignancies, organ transplantation, autoimmune disease and immunodeficiency. Aside from natalizumab, a range of other immunomodulators including obinutuzumab and rituximab have been associated with PML. The nature of these associations is unclear due to the overall low incidence of PML associated with these drugs and the fact that most patients will have other confounding risk factors for developing the disease. There is no known effective treatment available for PML in the non-HIV, non-MS cohort. Recent case studies and series have proposed that pembrolizumab, an anti-PD-1 immune checkpoint inhibitor, may be a potentially efficacious option for these patients. We present two cases of non-HIV, non-MS patients with PML who were treated with pembrolizumab with little clinical benefit. The literature surrounding pembrolizumab use in PML is discussed, with a focus on potential indicators of successful outcomes for patients who receive this therapy.