MNTX 1109 is a single center, open label study, composed of normal healthy adult subjects to evaluate the PK of MNTX and its metabolites administered once daily as a subcutaneous (SC) injection for a period of seven days. Blood samples will be obtained at screening and for drug concentration data.

Inclusion Criteria:

1. Body weight ≥40 kg (88 lb)
2. In good health with no clinically significant abnormal findings on the physical examination, medical history or the screening 12-lead ECG
3. Non-smoker
4. Has no known allergies to study drug or other related chemically-related compounds (i.e. naltrexone, naloxone).

Exclusion Criteria:

1. History of clinically significant gastrointestinal, renal, hepatic, neurologic, hematologic, endocrine, oncologic, pulmonary, immunologic, psychiatric, or cardiovascular disease
2. History of clinically significant allergies
3. Positive urine screen for drugs
4. History of positive blood screen for human immunodeficiency virus (HIV), hepatitis B virus (Hep B), or hepatitis C virus (HCV)
5. Prior exposure, allergy or known hypersensitivity to methylnaltrexone
6. Diagnosis of alcohol or substance dependence within the past 12 months.