This study is to evaluate the abuse potential of single-doses of istradefylline compared to placebo and phentermine in recreational stimulant users. Subjects will participate in an outpatient medical Screening visit, a 5-day Qualification (Drug Discrimination) Phase, a 6-period Treatment Phase, and an outpatient safety Follow-Up visit. Study will be approximately 25 weeks total.

Within 28 days of the Screening visit, eligible subjects will be admitted to the CRU (Day -1) for the Qualification Phase. During the Qualification Phase, subjects will receive single oral doses of phentermine 60 mg and matching placebo in a randomized, double blind, crossover manner, with each drug administration separated by approximately 48 hours (Day 1 and Day 3), to ensure that they can discriminate and show positive subjective effects of the active controls. Following evaluation of eligibility, subjects may be discharged (those who fail Qualification criteria) or remain in the CRU (those who pass criteria and are eligible) and then proceed directly to the Treatment Phase. The washout interval between last drug administration in the Qualification Phase and first drug administration in the Treatment Phase will be at least 96 hours (4 days).

Following confirmation of eligibility from the Qualification Phase, subjects will be randomized to one of 6 treatment sequences according to a 6x6 Williams square. Subjects will receive single oral doses of each of the 6 treatments in a randomized, double-blind, crossover manner, Istradefylline 40 mg, Istradefylline 80 mg, Istradefylline 160 mg, Phentermine 45 mg, Phentermine 90 mg, Placebo.

Each drug administration will be separated by at least 21 days. Serial pharmacodynamic evaluations will be conducted up to 24 hours after each study drug administration to confirm exposure to istradefylline.

Inclusion Criteria:

• Subjects who will provide written informed consent
• Healthy volunteer, male and female between 18 and 55 years of age, inclusive
• Subjects with a body mass index (BMI) within 18.0 to 33.0 kg/m2, inclusive
• Subjects who are recreational drug user

Exclusion Criteria:

• Subjects who have an alcohol or substance dependence within the 12 months
• Subjects who have ever been in treatment for substance use disorder
• Subjects who consume on average more than 5 servings of caffeinated beverages per day
• Subjects with a history of or presence of any clinically significant cardiovascular disease
• Subject with hyperthyroidism or glaucoma.