This study is a 12-month open-label extension (OLE) study of AT-007 in adult subjects with CG who previously participated in Study AT-007-1001 Part D and/or Part D Extension.

The study is designed to assess the long-term safety of AT-007 in subjects with CG as well as the pharmacodynamics (PD) (inhibition of galactitol) and PK of AT-007. The effect of 12-month treatment with AT-007 on the levels of galactose and other galactose metabolites in subjects with CG will also be evaluated.

Primary:

- To evaluate the long-term safety of 12-month oral administration of AT-007 in adult subjects with Classic Galactosemia (CG)

Secondary:

• To evaluate the long-term inhibition of galactitol, a biomarker of aldose reductase (AR) activity, induced by 12-month oral administration of AT-007 to adult subjects with CG
• To evaluate the pharmacokinetic (PK) parameters of 12-month oral administration of AT-007 in adult subjects with CG

Exploratory:

• To evaluate the effect of 12-month oral administration of AT-007 on the levels of galactose and its other metabolites in adult subjects with CG
• To evaluate the burden of illness (BOI) of adult subjects with CG
• To evaluate quality of life (QOL) measures of adult subjects with CG
• To evaluate the effect of 12-month oral administration of AT-007 on the BOI and QOL of adult subjects with CG

Inclusion Criteria:

• Male or non-pregnant, non-lactating female subject with a CG diagnosis
• Previous participation in Study AT-007-1001 Part D and/or Part D Extension
• No other significant health problems which preclude participation

Exclusion Criteria:

• Concomitant use of certain medications or over-the-counter therapies
• Discontinuation from Study AT-007-1001 Part D and/or Part D Extension due an adverse event