This study will employ a single-center, randomized, double-blind parallel-group design for MNTX and placebo, with a positive control (moxifloxacin) dosed single-blind to confirm methodology, in normal healthy male and female volunteers. Multiple ECGs will be obtained after single clinical and supratherapeutic doses of MNTX to assess ECG effects compared to placebo.

Inclusion Criteria:

1. Healthy and free of acute active illnesses
2. Males and females between the ages of 18 and 45 years, inclusive
3. Body mass index between 18-30, inclusive, and weight between 50-110 kgs
4. ECG within normal limits (including PR <220, QRS <110, and QTc <450 ms.)

Exclusion Criteria:

1. Previous MNTX exposure
2. Currently pregnant or nursing
3. History or presence of cardiovascular, respiratory, hepatic, renal, gastrointestinal, endocrine, neurological, or other medically significant disorders
4. Consumption of alcoholic beverages within 7 days prior to study confinement
5. Any evidence of congenital or familial long-QT syndrome
6. History of drug abuse of positive findings on urine drug screen
7. Known allergy or hypersensitivity to MNTX or its excipients, moxifloxacin, opioids, or related drugs or a history or relevant adverse drug reactions of any origin.