Safety and Pharmacokinetics of AT-007 in Healthy Subjects and in Adult Subjects With Classic Galactosemia

The study is designed to assess the safety and PK of AT-007 in healthy subjects and subjects with Classic Galactosemia as well as the effect of AT-007 on biomarkers of galactose metabolism (galactose, galactitol, and other galactose metabolites) in subjects with Classic Galactosemia. This study consists of 4 parts: * Part A (SAD) in 32 healthy subjects. Once daily oral escalating dose (6 active, 2 placebo). * Part B and C (MAD for 7 days) in 36 healthy subjects. Once daily multiple daily dosing (8 active, 2 placebo per each dose cohort). * Part D (SAD followed by MAD for 27 days) in 18 subjects with Classic Galactosemia. Once daily followed by multiple daily oral dosing (6 active, 2 placebo for each dose cohort).