Progressive multifocal leukoencephalopathy and anti-CD20 monoclonal antibodies: What do we know after 20 years of rituximab

In 1997, rituximab was the first monoclonal antibody clinically approved for the treatment of cancer. Ten years later, progressive multifocal leukoencephalopathy (PML), until that time a rare opportunistic infection mostly seen in AIDS patients, was added as a black box warning after retrospective case-control studies showed an increased incidence in both B-cell lymphoproliferative disorders and autoimmune diseases. Despite more than 5 million worldwide exposures to date (and about 500 000 new exposures per year), insufficient data collection has hampered identification of risk minimization strategies, and concerns have been raised about a class effect extending to the newer anti-CD20 monoclonal antibodies (ofatumumab, obinutuzumab, and ocrelizumab). Here, we report current PML case counts registered in the FAERS and EudraVigilance databases and comment on severe CD4(+) T lymphopenia as a plausible common mechanism of action for anti-CD20 antibodies in causation of PML.