Inclusion Criteria:  Confirmed diagnosis of NPC1 Annual Severity Increment Score between 0.5 and 2.0 using the 17-domain NPC Severity Scale Treated or Not Treated with Miglustat (patients must be on a stable dose for at least 3 months prior to the Screening Visit, or have discontinued Miglustat for at least 3 months prior to Screening Visit). Body weight greater than 4.5 kg and less than or equal to 125 kg Presenting at least 1 neurological symptom of the disease Written informed consent Willing and capable to participate in all aspects of trial design Ability to travel to the trial site at scheduled times Contraception requirements per protocol Caregiver consent as appropriate to participate in all protocol-specified assessments for duration of trial Inclusion criteria for Open Label Extension are 1) Received double-blind treatment for at least 48 weeks with CGI-S deterioration by at least 2 levels for 2 consecutive assessment visits 12 weeks apart, or 2) completion of double-blind treatment and completed all assessments through week 96, or 3) Discontinued early from double-blind treatment but completed all assessments through week 96 Inclusion criteria for patients age 0 to 3 years in open-label sub-study in countries following EMA guidance only: Confirmed diagnosis of NPC1; treated or not with Miglustat per main study; body weight greater than 4.5kg; patient may be asymptomatic; written assent for child to participate in safety assessments; caregiver consent to participate in caregiver assessments; ability to travel to the trial site for all scheduled visits.  Exclusion Criteria:  Recipient of liver transplant or planned liver transplantation Patients with active liver disease from any cause other than NPC1 Clinical evidence of acute liver disease including symptoms of jaundice or right upper quadrant pain or international normalized ratio > 1.8 Stage 3 chronic kidney disease or worse as indicated by an estimated glomerular filtration rate <60ml/min/1.73m2. Use of curcumin or fish oil within 12 weeks prior to enrollment Known or suspected allergy or intolerance to the study treatment In the opinion of the Investigator, the patient's clinical condition does not allow for the blood collection required as per protocol specific procedures. Treatment with any investigational drug during the 3 months prior to entering the study, including leucine in a clinical trial; however, leucine as a nutraceutical is allowed Treatment with HPBCD prior to entering the study Treatment with any other investigational drug during the study Pregnancy or breastfeeding Current participation in another trial is not permitted unless it is a noninterventional study and the sole purpose of the trial is for long-term follow up describing clinical features or survival data (registry) Patients with uncontrolled, severe epileptic seizure periods (at least 3 consecutive severe epileptic seizures that required medication) within 2 months prior to completion of informed consent or assent, as applicable. Neurologically asymptomatic patients Inability to participate in the primary study assessment (4D-NPC-SS or 5D-NPC-SS, depending on jurisdiction) as determined by the Investigator  Exclusion criteria for patients age 0 to 3 years in open-label sub-study in countries following EMA guidance only are similar to the main study with the addition of exclusion criterion of history of fetal hydrops or fetal ascites  -