Progressive Multifocal Leukoencephalopathy Unmasked by Teclistamab in a Refractory Multiple Myeloma Patient.

This case report describes the development of Progressive Multifocal Leukoencephalopathy (PML) in a 72-year-old male with relapsed/refractory multiple myeloma (RRMM), following a single dose of teclistamab amidst a COVID-19 infection. Shortly after starting teclistamab treatment, the patient developed symptoms, including fever, altered mental status, and right-sided paresis. A diagnosis of PML was confirmed through the detection of JC virus PCR in the cerebrospinal fluid. Our report emphasizes the occurrence of PML after only one dose of teclistamab and highlights teclistamab's potential for severe infectious complications, despite its promise in treating RRMM.