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Wolfram syndrome is a rare, progressive neurodegenerative disorder characterized by juvenile-onset diabetes, optic nerve atrophy and mutation of the WFS1 gene. Calcium dyshomeostasis is a key mechanism underlying neuronal and pancreatic β-cell death and dysfunction in this endoplasmic reticulum (ER) disorder. Dantrolene sodium is an FDA-approved muscle relaxant that preserves ER calcium through inhibition of ryanodine receptor (RyR) activity. We therefore hypothesized that dantrolene could have therapeutic potential for Wolfram syndrome. Our preclinical studies showed that dantrolene improves β-cell and neuronal cell survival in mouse and induced pluripotent stem (iPS) cell models of Wolfram syndrome. To translate these findings to humans, we conducted the first clinical trial in 19 Wolfram syndrome patients through a 6-month phase 1b/2 study of dantrolene sodium with an optional extension phase up to 24-months (NCT02829268). The endpoints of this study were to assess: (1) dantrolene safety and tolerability in pediatric and adult Wolfram syndrome patients and (2) its effects on remaining β-cell, visual and neurologic functions and quality of life measures. Among pediatric subjects, dantrolene was well-tolerated at a daily dose of 0.5mg/kg in those < 50kg and 2mg/kg in those > 50kg. Adults tolerated dantrolene well at 50mg - 100mg daily. Common adverse events included mild fatigue, diarrhea, headache and hypoglycemia. A mild increase in 30-minute mixed-meal-stimulated C-peptide was observed in subjects after 6- and 12-months of dantrolene treatment. No significant changes were observed in visual or neurological functions or quality of life measures, although mild improvement in disease severity was noted by the Wolfram Unified Rating Scale. These findings suggest that dantrolene is safe for use in Wolfram syndrome at the specified doses and support further study of dantrolene in a randomized, placebo-controlled trial. Disclosure D. Abreu: None. T.S. Pearson: None. R.C. Bucelli: None. A. Simpson: None. C.M. Brown: None. K. Kries: None. H. Gu: None. S.I. Stone: Research Support; Self; National Institute of Diabetes and Digestive and Kidney Diseases. Speaker’s Bureau; Self; AbbVie Inc. L. Tychsen: None. G. Van Stavern: None. N.H. White: None. B.A. Marshall: None. T. Hershey: Research Support; Spouse/Partner; Sage Pharmaceuticals. F. Urano: Board Member; Self; Healthbeat. Research Support; Self; Aetas, Amylyx, JDRF, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health. Stock/Shareholder; Self; CytRx. Other Relationship; Self; Novus Biologicals. Funding National Institutes of Health (R21DK113487-02)