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Supportive Oligonucleotide Therapy (SOT, also branded as Q-REstrain) is a non-FDA approved intravenous antisense oligonucleotide therapy with limited clinical evidence for safety and efficacy, that is offered by some naturopathic clinics for the treatment of various cancers or chronic viral infections. Here, we report a case of anti-Yo-mediated cerebellar paraneoplastic degeneration developing shortly after SOT administration in a patient with treated HER2+ breast cancer.

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A 71-year-old woman presented with subacute onset headache, diplopia, vertigo and progressive gait disturbance 3 days following experimental infusion of SOT (March 2025). She has a notable history of left breast cancer diagnosed in May 2024 (grade 3, ER/PR-negative, HER2-positive), for which she underwent definitive treatment with complete resection and adjuvant chemotherapy with no evidence of recurrence to date. Brain MRI showed no acute abnormalities or cerebellar atrophy. Cerebrospinal fluid (CSF) demonstrated a marked lymphocytic pleocytosis (WBC 121, 91% lymphocytes), unique CSF oligoclonal bands, and CSF antibodies to Yo-PCA-1 (1:1024) and GFAP (1:128), leading to a diagnosis of anti-Yo paraneoplastic cerebellar degeneration. She was treated with IV steroids, IVIG, and plasma exchange with mild improvement in her profound cerebellar symptoms. Repeated screening with CT and PET showed no evidence of breast cancer recurrence or other malignancy. The patient declined escalation of immunotherapy and favored conservative management with physical and occupational therapy with relative stability to date.

To our knowledge, this is the first report of paraneoplastic or any provoked autoimmune neurologic disease occurring shortly after administration of SOT. Although the timing could be coincidental, onset of cerebellar symptoms only three days after treatment suggests SOT or its adjuvant components as a provoking factor for this inflammatory process. This case supports that further clinical studies and increased regulatory monitoring are needed to ensure the safety and efficacy of SOT moving forward.