A Multinational, Randomized, Double-blind, Placebo-controlled Study to Assess the Efficacy, Pharmacodynamics, Pharmacokinetics, and Safety of Venglustat in Late-onset GM2

Primary Objectives:

Primary population (adult participants with late-onset GM2 gangliosidosis): To assess the efficacy and pharmacodynamics (PD) of daily oral dosing of venglustat when administered over a 104-week period

Secondary population (participants with juvenile/adolescent late-onset GM2 gangliosidosis, GM1 gangliosidosis, saposin C deficiency, sialidosis type 1 or juvenile/adult galactosialidosis): To assess PD response (plasma and CSF GL-1 biomarker and disease specific biomarkers) of venglustat when administered once daily over a 104-week period

Secondary Objectives:

Primary population:

* To assess the PD of daily oral dosing of venglustat and the effect of venglustat on selected performance test and scale over a 104-week period
* To determine the safety and tolerability of venglustat when administered orally once daily over a 104-week period
* To assess the pharmacokinetics (PK) of venglustat in plasma and cerebrospinal fluid (CSF)

Secondary population:

* To assess the effect of venglustat on selected performance tests and scale over a 104-week period
* To determine the safety and tolerability of venglustat when administered once daily over a 104-week period
* To assess the PK of venglustat in plasma and CSF
* To assess the acceptability and palatability of the venglustat tablet

The total duration is up to approximately 223 weeks, including a 60-day screening period, a 104-week primary analysis treatment period, a 104-week open-label extension treatment period and a 6-week post-treatment safety observation period.

Eligibility Criteria:
Inclusion criteria :

* Primary population and adult secondary population: age ≥ 18 years
* Juvenile/adolescent secondary population: 2 ≥ age \< 18 years with weight ≥ 10 kg
* Participants with a diagnosis of late onset GM2 gangliosidosis (Tay-Sachs disease and Sandhoff disease) caused by genetic β-hexosaminidase deficiency resulting from mutations in the HEXA or HEXB genes (primary population only); a secondary population will enroll patients with diagnosis of juvenile/adolescent GM2 gangliosidosis, GM1 gangliosidosis, saposin C deficiency, sialidosis type 1 or juvenile adult galactosialidosis
* For primary population, the participant has the ability to perform the 9-HPT at the screening visit in \< = 240 seconds for the 2 consecutive trials of the dominant hand and the 2 consecutive trials of the nondominant hand.
* Participants with a history of seizures well controlled by medication other than strong or moderate inducer or inhibitor of CYP3A4
* Participant is cooperative, able to ingest oral medication, willing to travel to a study site (if applicable), and able to comply with all aspects of the study, including all assessments, according to the Investigator's judgement
* Signed written informed assent/consent
* Contraception for sexually active male participants or female patient; not pregnant or breastfeeding; no sperm donating for male participant

Exclusion criteria:

* Participant has clinical features of Tay-Sachs or Sandhoff disease, not caused by β-hexosaminidase deficiency resulting from mutations in the HEXA or HEXB genes and/or is without clinical features
* For primary population and participants with juvenile/adolescent late onset GM2 gangliosidosis and GM1 gangliosidosis, the participant cannot understand and perform all age-appropriate study assessments with the exception of 25FWT and PROs.
* Relevant medical disorders that would compromise his/her safety
* Documented diagnosis of hepatitis B, C, human immunodeficiency virus 1 or 2
* World Health Organization (WHO) grade \>= 2 cortical cataract or a grade \>= 2 posterior subcapsular cataract; patients with nuclear cataracts will be accepted
* Participant who requires invasive ventilatory support
* Current treatment by anticoagulants, cataractogenic medications or any medications that may worsen the vision of patient with cataract
* Previous treatment with substrate reduction therapy (SRT) within 3 months prior to study enrollment, strong or moderate inducers or inhibitors of CYP3A4 within 14 days or 5 half-lives prior to enrollment. This also includes the consumption of grapefruit, grapefruit juice or grapefruit products within 72hrs prior to starting investigational medicinal product (IMP) administration.
* Current participation in another study
* Use of investigational medicinal product (IMP) within 3 months or 5 half-lives, whichever is longer, before study enrollment (for N-acetyl-leucine, within 5 half-lives before study enrollment).
* Liver enzymes (alanine aminotransferase \[ALT\]/aspartate aminotransferase \[AST\]) or total bilirubin \> 2 x the upper limite of normal (ULN) at the time of screening unless the participant has the diagnosis of Gilbert syndrome and maintains a level of bilirubin \< 5 mg/dl and direct bilirubin \< 20% (1 mg/dl) of total bilirubin level
* Renal insufficiency is defined by estimated glomerular filtration rate (eGFR) \< 30 mL/min/1.73m2 at the screening visit

The above information is not intended to contain all considerations relevant to a patient's potential participation in a clinical trial.