BackgroundThis study examined the 52-week safety and efficacy of AST-001 syrup for the core symptoms of autism spectrum disorder (ASD) in children aged 2–11 years.MethodsThis multicenter long-term follow-up study enrolled children with ASD who were identified as responders in an antecedent phase II randomized controlled trial. A total of 61 participants were given the same dose (high or low) they received during the extension phase of the phase-II trial for 52 weeks. Safety and tolerability were evaluated by reporting adverse events. Efficacy was assessed using the Ohio State University Autism Clinical Global Impression (CGI)-Severity and Improvement scale.ResultsThe proportion of participants experiencing any treatment-emergent adverse events was 40.98%, most commonly due to COVID-19 (11/61 participants, 18.03%), but none were related to AST-001. Two participants experienced adverse drug reactions (2/61 participants, 3.28%), including decreased appetite and enteritis. The incidence of serious adverse events was 4.92% (3/61 participants), which were dental caries, otitis media, and septic shock, but none were related to medication. The mean CGI-S score decreased slightly from 4.25 ± 0.87 at baseline to 4.10 ± 0.79 at week 52 of the long-term follow-up study. Among the 48 participants who were CGI-I responders at week 24 of phase-II trial, 45 continued to meet the responder criteria at week 52.ConclusionsAST-001 was well tolerated over the 52-week period, and clinical improvement was maintained among participants who had previously responded to treatment, suggesting continued benefit on ASD core symptoms. These findings provide preliminary long-term data on safety, tolerability, and clinical outcomes in a field where pharmacologic treatment options for the core symptoms of ASD remain limited. However, given the uncontrolled long-term follow-up design in prior responders, the results should be interpreted with caution and do not constitute confirmatory evidence of sustained efficacy. Further large-scale, well-controlled studies are warranted.Clinical Trial Registrationhttps://cris.nih.go.kr/cris/search/detailSearch.do?seq=26492&search_page=L&search_lang=&class_yn=, identifier KCT0007519.