The adaptor associated kinase 1 (AAK1) inhibitor 3-methyloxetan-3-yl-4-(3-(2-methoxypyridin-3-yl)pyrazolo[1,5-a]pyrimidin-5-yl)piperazine-1-carboxylate:and pharmaceutically acceptable salts and solid forms thereof are disclosed. Compositions comprising the compound and methods of their use to treat, manage and/or prevent diseases and disorders mediated by mediated by AAK1 activity are also disclosed.