Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental disorder with life-long consequences that affects young children during critical times in their development. ASD is defined by impairments in social-communication as well as the presence of restricted interests and repetitive behaviors. ASD is frequently associated with co-occurring language delays. Currently the only well-accepted treatment for core ASD symptoms is behavior therapy such as Applied Behavioral Analysis and Early Intensive Behavioral Intervention. There is no US Food and Drug Administration approved medical therapy that addresses core ASD symptoms or the pathophysiological processes that underlie ASD.~The primary aim of this study is to evaluate the effect of a liquid form of leucovorin calcium on social and communication impairments in very young children with ASD. Participants entered into the trial will have delayed language and moderate ASD symptoms. The investigators hypothesize that leucovorin calcium will significantly improve social communication as well as core and associated behavioral symptoms of ASD, and be well-tolerated with no significant adverse effects, in young children with ASD.~To assess whether the liquid form of leucovorin calcium is superior to placebo, the investigators will study 80 children across two sites, between the ages of 2.5 and 5 years, with confirmed ASD and known social and communication delays at baseline. Participants will be randomly assigned to receive active treatment or placebo for 12 weeks under double-blind conditions. At the end of 12 weeks, all participants will receive active treatment for 12 weeks. Language skills and social communication abilities will be measured at screening and after each treatment arm in order to determine if the supplement positively influences social communication. Additionally, the investigators will measure changes in neural pathways using either magnetoencephalography at Phoenix Children's Hospital or functional Near Infrared Spectroscopy at State University of New York, Downstate. While these measures will be considered exploratory, they will be important to begin to elucidate the neuronal mechanisms underlying leucovorin's impact.
The primary objective of this study is to evaluate the cognitive and behavioral effects of liquid leucovorin calcium on young children with autism spectrum disorder (ASD) and determine whether it improves social communication as well as the core and associated symptoms of ASD. The investigators will enroll 80 children across two sites, between the ages of 2.5 and 5 years, with confirmed ASD and known social and communication delays. Participation will last approximately 26 weeks, from screening visit to end of treatment.