NCMP-32. Uncharted territory: first report of Progressive Multifocal Leukoencephalopathy linked to Zanubrutinib in CNS Mantle Cell Lymphoma

Zanubrutinib is a second-generation Bruton Tyrosine Kinase (BTK) inhibitor, FDA-approved for Mantle Cell Lymphoma (MCL), designed to be more selective and potentially better tolerated than first-generation agents like Ibrutinib. Progressive multifocal leukoencephalopathy (PML), a rare but severe complication, has not previously been reported with Zanubrutinib. We report a 74-year-old woman with a germline CHEK2 mutation and asymptomatic MCL (CD5+, cyclin D1+, Ki67 10–20%) diagnosed in 2016, managed with surveillance. In 2023, she was also diagnosed with localized breast cancer treated with lumpectomy and exemestane. Later that year, she developed new headaches. Brain MRI revealed diffuse bilateral pachymeningeal enhancement, suspicious for lymphomatous spread, without parenchymal involvement. By early 2024, follow-up imaging showed stable pachymeningeal enhancement with new scattered punctate right frontal subcortical lesions, coinciding with systemic MCL progression. Spinal imaging revealed epidural and bone marrow involvement; CSF was negative for malignancy. Zanubrutinib was initiated in March 2024. By August 2024, new bilateral frontal and parietal fuzzy hyperintense and faintly enhancing white matter lesions emerged, while prior punctate lesions regressed. Biopsy confirmed PML with JC virus DNA by in situ hybridization and absence of B-cell clonality. Zanubrutinib was discontinued. Follow-up imaging showed regression of lesions without new enhancement, consistent with immune reconstitution. The initial subcortical punctate lesions remained stable. In 2025, she was diagnosed with localized pancreatic adenocarcinoma, precluding cytotoxic therapy initially due to concern for PML reactivation. However, as of May 2025, neuroimaging remains stable, and patient was started on palliative gemcitabine/paclitaxel. This is the first reported case of PML associated with Zanubrutinib. It underscores the importance of vigilance for PML even in patients without overt immunosuppression, especially those with overlapping malignancies and treatment exposures in the correct clinical context.