A Phase 1, single-center, randomized, double-blind, placebo-controlled study to evaluate the safety, tolerabiltiy, and pharmacodynamics of different multiple dose regimens of RDX5791 in healthy volunteers.

The starting doses for this study are based on the results from the multiple ascending-dose study performed on RDX5791 (Protocol RDX5791-101). Subjects will be screened within 3 weeks of check in to the CPU. Each cohort of 15 subjects (12 RDX5791, 3 placebo) will check in to the CPU on Day -5 before dinner. Each subject will receive a diet standardized for Na+ content while in the CPU. Subjects will receive doses of RDX5791 with approximately 240 mL of non-carbonated water on Days 1 to 7 (just prior to meals, depending on QD, bid, or tid regimens). Subjects will be provided standardized meals within 10 minutes after dosing. Doses can be administered with breakfast, lunch, or dinner.

Inclusion Criteria:

• Body mass index between 18 and 29.9 kg/m², inclusive
• No clinically significant abnormalities in medical history
• Females must be non-pregnant, non-lactating, and either postmenopausal for at least 12 months or have agreed to use an effective form of contraception from the time of signing the informed consent until 45 days after end of study
• Men must be either sterile, abstinent, or agreed to use an approved method of contraception from check-in until 45 days after final study visit

Exclusion Criteria:

• Diagnosis or treatment of any clinically symptomatic biochemical or structural abnormality of the GI tract
• Any surgery on the small intestine or colon, excluding appendectomy
• Loose stools (Bristol Stool Form Score of 6 or 7) ≥2 days in the past 7 days
• Hepatic dysfunction ([ALT] or [AST]) >1.5 times the upper limit of normal or renal impairment
• Any evidence of or treatment of malignancy, excluding non-melanomatous malignancies of the skin
• Use of diuretic medications, medications that are known to affect stool consistency and/or GI motility
• Use of an investigational agent within 30 days prior to Day -2
• Positive virology, alcohol, or drugs of abuse test during screening
• Use of any prescription medication within 7 days before admission to the CPU
• Have had significant blood loss (>450 mL) or have donated 1 or more units of blood or plasma within 8 weeks prior to study entry