The purpose of this study is to determine the daily maximum tolerated dose of propoxyphene napsylate in healthy subjects.

Inclusion Criteria:

• Body mass index ≥ 18 and ≤ 30 (kg/m2)
• Medically healthy with normal screening results, or in the case of results that are abnormal; the abnormal results are clinically insignificant (eg, medical history, physical examination, neurological assessment, vital signs, oxygen saturation, laboratory profiles)
• 12-lead ECGs (calculations by the electrocardiograph) which have no clinically significant findings
• Vital signs which are within normal range
• No tobacco/nicotine-containing product use for a minimum of 6 months
• If female, must be able to adhere to acceptable methods of contraception or be postmenopausal or surgically sterile

Exclusion Criteria:

• History or presence of significant cardiovascular, hepatic, renal, hematological, gastrointestinal, endocrine, immunologic, dermatologic, or neurological disease
• History or presence of any degree of chronic obstructive pulmonary disease
• History of suicidal ideations or depression requiring professional intervention including counseling or antidepressant medication
• Any history of drug or alcohol abuse
• Positive drug (urine)/alcohol (breath) testing at screening or check-in
• Positive test for human immunodeficiency virus (HIV), hepatitis B surface antigen (HBsAg), or hepatitis C antibodies (HCV)
• History of hypersensitivity or allergy to propoxyphene or any opioid compound, naloxone, naltrexone, palonosetron (Aloxi®), glycerin, senna, or bisacodyl (Dulcolax®)
• Use of any prescription medication (with the exception of hormonal contraceptives for females) within 2 weeks of enrollment
• Use of any over-the-counter medication, including herbal products, within 1 week of enrollment
• Use of any drugs known to significantly inhibit or induce liver enzymes involved in drug metabolism [CYP P450]) within 30 days of enrollment
• Blood donation or significant blood loss within 30 days of enrollment
• Plasma donation within 7 days of enrollment
• Participation in another clinical trial within 30 days of enrollment
• Females who are pregnant or lactating
• Any other condition or prior therapy, which, in the opinion of the PI, would make the subject unsuitable for this study