Mogamulizumab for Treatment of Human T-lymphotropic Virus Type 1-Associated Myelopathy/Tropical Spastic Paraparesis: A Single-Center US-based Series

Background: Human T-lymphotropic virus type 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a chronic neurological condition characterized by progressive myelopathic symptoms including spasticity, pain, weakness, and urinary symptoms, without proven treatments. Mogamulizumab (MOG) is a monoclonal antibody that binds CCR4 and leads to the clearance of HTLV-1-infected CCR4+ cells. A phase 1-2a study in Japan evaluated MOG for the treatment of HAM/TSP and reported decreases in HTLV-1 proviral load and neuroinflammatory markers, with clinical improvement in some participants. Methods: We administered MOG 0.1 mg/kg every 8 weeks to individuals with HAM/TSP as a compassionate and palliative treatment. Patients who received MOG had (1) a positive peripheral HTLV-1 antibody, (2) progressive myelopathic symptoms, and (3) a diagnosis of HAM/TSP. Results: Four female patients, ages 45-68, received MOG (range, 2-6 infusions) between 1 November 2019 and 30 November 2022. Two patients with <3 years of symptoms had milder disease, with Osame scores <4. The other 2, with >7 years of symptoms, had Osame scores >5. One patient, with 6 total treatments, received dose-reduced MOG after she developed a rash at the initial dose. The 2 patients with milder baseline disease reported symptomatic improvement and saw reductions in Osame and/or modified Ashworth scale scores during follow-up. The other 2 patients showed no improvement. All 4 developed rashes after receiving MOG - a treatment-limiting event in some cases. Conclusions: Clinical trials are needed including diverse patient populations to assess the potential role of MOG for HAM/TSP. Our findings may help inform the development of these trials.