Colony Stimulating Factor‐1 Receptor‐Related Disorder Treated With Ilunazebart

ABSTRACT

Colony stimulating factor‐1 receptor–related disorder (CSF1R‐RD) is a rare, autosomal dominant neurodegenerative disease caused by loss‐of‐function variants in the CSF1R gene, leading to microglial dysfunction and progressive white matter degeneration. Therapeutic strategies targeting microglial pathways, including activation of triggering receptor expressed on myeloid cells 2 (TREM2), have been proposed to compensate for impaired CSF1R signaling. Iluzanebart (VGL101), a monoclonal antibody TREM2 agonist, has shown promise in preclinical models. We report a longitudinal clinical course, neuroimaging findings, genetic analysis, and postmortem neuropathological examination of a 52‐year‐old patient with genetically confirmed CSF1R‐RD (c.2507G>A, p.Ser836Asn). The patient received iluzanebart as part of a Phase 2 clinical trial. Clinical progression, MRI changes, and histopathological features at autopsy were systematically analyzed. Despite treatment, the patient experienced progressive cognitive decline, neuropsychiatric symptoms, motor impairment, and seizures, culminating in death. Serial neuroimaging demonstrated worsening white matter degeneration and brain atrophy without evidence of therapeutic response. Neuropathological examination revealed characteristic features of CSF1R‐RD, including severe myelin loss, axonal spheroids, infiltration of Iba1‐positive cells with macrophage‐like morphology, and cortical neuronal abnormalities. There was no histological evidence of treatment‐related benefit or harm. This case represents the first clinicopathological assessment of iluzanebart in CSF1R‐RD and demonstrates no observable clinical, radiological, or neuropathological improvement. These findings highlight the challenges of targeting microglial dysfunction in advanced disease and suggest that TREM2 activation alone may be insufficient to alter disease progression.