Inclusion Criteria:
* Healthy male volunteers aged 18 to 45 years
* Body Mass Index (BMI) between 18 and 32 kg/m² (inclusive)
* Normal resting heart rate (50 to 100 bpm) and blood pressure (systolic 90-130 mmHg, diastolic 60-85 mmHg) at Screening and Day 0
* Willingness to provide written informed consent and HIPAA authorization, and agreement to use a highly effective method of birth control during the study and for 1 month after study drug administration
* Nonsmoker (no smoking within 6 months of Screening Visit 1); no medicinal or recreational marijuana use within 3 months prior to screening or during the study
* Willingness to fast for at least 8 hours overnight and 4 hours post-dose (for Study A)
* Ability to communicate well with the investigator and comply with clinic procedures
Exclusion Criteria:
* History or presence of any clinically significant respiratory, gastrointestinal, renal, hepatic, hematological, neurological (including seizure history), cardiovascular, psychiatric (including known addictive disorders), musculoskeletal, genitourinary, immunological, or dermatological disorders
* History of suicide attempts or current suicidal ideation
* Clinically significant abnormal physical, neurological, or laboratory findings, or abnormal ECG (including average QTc interval ≥ 450 msec or a history of congenitally prolonged QT interval)
* Need for prescription medications within 14 days (or 5 half-lives) prior to dosing
* Acetaminophen use \> 1000 mg (single dose) within 3 days prior to Day 1
* Use of any investigational drug, medical device, or herbal medicine within 3 months prior to or during the study
* Known hypersensitivity or intolerance to drugs with the same mechanism of action as PRX-P4-003 or (-)-fencamfamine
* History of alcohol abuse (\>4 drinks/day) or drug addiction within the past 2 years; positive urine drug or alcohol screen at Screening or Day -1
* Unwillingness to refrain from alcohol or drugs 24 hours prior to Day -1 and during the inpatient stay
* Seropositive for Hepatitis B, Hepatitis C, or known HIV infection
* Donation or loss of \> 500 mL of blood in the 8 weeks prior to dosing, or planned donation during trial participation
* Inability or unwillingness to comply with protocol requirements/appointments, or inability to understand English (unless a certified translation of the informed consent is available)
