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.