A Phase 3, Randomized, Multi-Center, Multi-National, Open-Label, Active Comparator Study to Evaluate the Efficacy and Safety of Genz 112638 in Patients with Gaucher Disease Type 1 who have been Stabilized with Cerezyme

1. The patient is willing and able to provide signed informed consent prior to any study-related procedures to be performed.
2. The patient is 18 to 65 years of age at the time of randomization.
3. The patient*s Tanner Stage should be * 4 prior to randomization.
4. The patient has a diagnosis of Gaucher disease type 1 confirmed by a documented deficiency of acid *-glucosidase activity by enzyme assay.
5. The patient consents to provide a blood sample for genotyping for Gaucher disease (unless the patient*s Gaucher genotype is already available), chitotriosidase, and for genotyping of cytochrome P450 2D6 (CYP2D6) to categorize the patient*s predicted rate of metabolism.
6. The patient has received treatment with Cerezyme for at least 3 years at a prescribed dose of * 20 U/kg to * 60 U/kg (± 5 U/kg) q2w during the last year of treatment and has not had a dose reduction, regimen change, or treatment interruption for greater than 6 consecutive months prior to randomization.
7. The patient has clinically stable Gaucher disease prior to randomization. Stable Gaucher disease is defined as a patient with all of the following:
A. No bone crisis and free of symptomatic bone disease such as bone pain attributable to osteonecrosis and/or pathological fractures within the last year, and no documentation of acute pathological bone involvement by imaging (e.g., osteonecrosis, pathological fractures, etc.) as determined in review with a central bone reviewer.
B. Mean hemoglobin level of * 11 g/dL if female and * 12 g/dL if male at the time of screening.
C. Mean platelet count * 100,000/mm3 at the time of screening.
8. Spleen volume < 10 times Normal or total splenectomy (provided the splenectomy occurred > 3 years prior to randomization).
9. Liver volume < 1.5 times Normal.
10. Female patients of childbearing potential must have a documented negative pregnancy test prior to randomization. In addition, all female patients of childbearing potential must use a medically accepted form of contraception throughout the study (either a barrier method or hormonal contraceptive with ethinyl estradiol and norethindrone or similar active components).
11. The patient is willing to abstain from consumption of grapefruit or grapefruit juice for 72 hours prior to administration of the first dose of Genz 112638 and throughout the duration of the study.

1. The patient received pharmacological chaperone or substrate reduction therapies within 6 months prior to randomization.
2. The patient has had a partial or total splenectomy within 3 years prior to randomization.
3. The patient has any evidence of neurologic (e.g., peripheral neuropathy, tremor, seizures, Parkinsonism or cognitive impairment) or pulmonary involvement (e.g., pulmonary hypertension) as related to Gaucher disease.
4. The patient is transfusion-dependent.
5. The patient has a documented deficiency of iron, vitamin B-12, or folate that requires treatment not yet initiated or not yet stable under treatment for at least 3 months prior to randomization.
6. The patient has documented prior esophageal varices or liver infarction or current liver enzymes (alanine transaminase [ALT]/aspartate aminotransferase [AST]) or Total Bilirubin > 2 times the upper limit of normal (ULN), unless the patient has a diagnosis of Gilbert Syndrome.
7. The patient has any clinically significant disease, other than Gaucher disease, including cardiovascular, renal, hepatic, gastrointestinal, pulmonary, neurologic, endocrine, metabolic (e.g. hypokalemia, hypomagnesemia), or psychiatric disease, other medical conditions, or serious intercurrent illnesses that, in the opinion of the Investigator, may preclude participation in the study.
8. The patient is known to have any of the following: Clinically significant coronary artery disease including history of myocardial infarction [MI] or ongoing signs or symptoms consistent with cardiac ischemia or heart failure; or clinically significant arrhythmias or conduction defect such as 2nd or 3rd degree atrioventricular (AV) block, complete bundle branch block, prolonged QTc interval, or sustained ventricular tachycardia (VT).
9. The patient has tested positive for the human immunodeficiency virus (HIV) antibody, Hepatitis C antibody, or Hepatitis B surface antigen.
10. The patient has received an investigational product within 30 days prior to randomization.
11. The patient is scheduled for in-patient hospitalization, including elective surgery, during the study.
12. The patient has a history of cancer within 5 years of randomization, with the exception of basal cell carcinoma.
13. The patient is pregnant or lactating.
14. The patient has received any medication that may cause QTc interval prolongation or any medication that is a mechanistic inhibitor of PGP within 30 days prior to randomization.
15. The patient has received any medication that may induce or inhibit CYP2D6, apart from premedications for Cerezyme infusion, within 30 days prior to randomization. Premedications for Cerezyme infusion are allowed up to 7 days prior to the first dose of Genz-112638.
16. The patient has received for the first time (i.e., the patient is not already chronically using), medications known to induce or inhibit CYP3A4, or medications known to be competitive inhibitors or inducers of PGP, 30 days prior to randomization.
17. The patient is a CYP2D6 poor metabolizer who is chronically receiving inhibitors of CYP3A4 and for whom no reasonable alternative medication exists.