Part 1: Biomarker evaluation/screening phase
Primary Objectives:
* Evaluate cerebrospinal fluid (CSF) biomarkers in adult Gaucher disease Type 3 (GD3) participants that distinguish GD3 from adult Gaucher disease Type 1 (GD1) participants
* Screen adult GD3 participants who qualify for treatment with venglustat in Parts 2, Part 3, and Part 4 Parts 2 and 3: Combination treatment phases
Primary objectives:
* Evaluate short-term (Part 2) and long-term (Part 3) safety and tolerability of venglustat in combination with Cerezyme in adult GD3 participants
* Evaluate the change in CSF central nervous system (CNS) biomarkers (glucosylceramide \[GL-1\] and lyso-glucosylceramide \[lyso-GL-1\]) from adult GD3 participants receiving venglustat in combination with Cerezyme (Part 2 only) Part 4: Extended treatment phase with monotherapy
Primary objectives:
• Evaluate safety and tolerability of venglustat monotherapy in adult GD3 participants who have remained systemically stable on venglustat in combination with Cerezyme Parts 2 and 3: Combination treatment phases
Secondary Objectives:
* Evaluate the pharmacokinetics (PK) of venglustat in adult GD3 participants
* Evaluate the efficacy of venglustat in combination with Cerezyme in systemic disease in adult GD3 participants by assessing spleen volume, liver volume, hemoglobin level and platelet count
* Evaluate the efficacy of venglustat in combination with Cerezyme on neurological function in adult GD3 participants by assessing Ataxia using the Scale for the Assessment and Rating of Ataxia (SARA)
* Evaluate plasma biomarkers (lyso-GL-1 and GL-1) in adult GD3 participants Part 4: Extended treatment phase with monotherapy
Secondary objectives:
* Evaluate the efficacy of venglustat in systemic disease in adult GD3 participants by assessing spleen volume, liver volume, hemoglobin level and platelet count
* Evaluate the efficacy of venglustat on neurological function in adult GD3 participants by assessing Ataxia using the Scale for the Assessment and Rating of Ataxia (SARA)
* Evaluate plasma biomarkers (lyso-GL-1 and GL-1) in adult GD3 participants