Drug-Related Brain Vacuolation as an Unusual Presentation of Neurotoxicity in Sprague Dawley Rats

In a 4-week rat study, a nucleotide prodrug antiviral agent containing a synthetic non-natural amino acid (C-331) induced a dose-related vacuolation in the brain's white matter tracts without concurrent neuronal loss, gliosis, or neurological deficits. Routine safety pharmacology studies showed no effects. Penetration of the drug through the blood-brain barrier was negligible. Rat-specific metabolites were not evident. A follow-up 13-week rat study with a 4-week interim assessment was performed using oral doses of 0, 100, 300, or 1000 mg/kg/day for 4 or 13 weeks. Intermittent, transient clonic convulsions (<1 min) occurred in 1 male rat at 100 mg/kg/day (3 observations) and 5 animals at 1000 mg/kg/day during the dosing phase but were not observed during recovery; their relationship to brain vacuolation remains uncertain. No test article-related neurobehavioral or open-field changes, or ophthalmic abnormalities were noted. In all treated groups, widespread vacuolation predominantly affecting white matter was observed in the cerebellum, thalamus, and striatum. The severity of vacuolation increased over time and persisted after the 4-week recovery period. Based on ultrastructural evaluation, the vacuolation resulted from myelin sheath splitting (intramyelinic edema) without axonal degeneration. The vacuolation was considered to be an adverse effect based on the severity and extent of the changes and the potential for causing oligodendrocyte dysfunction. An NOAEL in the rat was not identified. Because of multiple unknowns, including the lack of consistent neurobehavioral signs, species specificity, low exposure multiples, and the potential human significance, further development of C-331 was terminated. The study describes the unusual presentation of species-specific, drug-induced splitting of myelin sheaths/intramyelinic edema without clear correlative neurofunctional effects.