Disclosed herein are sustained-release injectable formulations for releasing a therapeutically effective amount of 7-[4-(4-benzo[b]thiophene-4-yl-piperazine-1-yl)butoxy]-1H-quinoline-2-one (brexpiprazole). In a preferred embodiment the formulation comprises brexpiprazole or a salt thereof, particle binders, and water, the particle binders being sodium chloride and at least one member selected from polyoxyethylene sorbitan fatty acid esters and polyethylene glycols.