The purpose of this study is to compare the multiple dose pharmacokinetics (PK) of three gefapixant (AF-219) formulations; to assess the effect of Omeprazole on the multiple dose PK of three gefapixant formulations; and, to assess the safety and tolerability of gefapixant.

Inclusion Criteria:

• Be informed of the nature of the study and have provided written informed voluntary consent
• Be able to speak, read, and understand English;
• Be healthy males or females, of any race, between 18 and 55 years of age, inclusive;
• Have a body mass index (BMI) >18.5 and <32.0 kg/m2 and weigh 50 - 100 kg;
• Be in good general health with no clinically relevant abnormalities based on the medical history, physical examination, clinical laboratory evaluations (hematology, clinical chemistry, and urinalysis), and 12-lead electrocardiogram;
• Be only non-smokers or intermittent (social) smokers for at least 5 years, and able to refrain from smoking (or using nicotine) while in confinement;
• If a female of child-bearing potential or not post-menopausal, agree to use 2 forms of acceptable birth control;
• Be able to communicate effectively with the Investigator and other study center personnel and agree to comply with the study procedures and restrictions

Exclusion Criteria:

• Have previously received gefapixant;
• Any disease or condition that might affect drug absorption, metabolism, or excretion or clinically significant cardiovascular, hematological, renal, hepatic, pulmonary, endocrine, gastrointestinal, immunological, dermatological, neurological, or psychiatric disease;
• Clinically significant illness or clinically significant surgery within 4 weeks before the administration of study medication;
• Any past sinus surgery, upper respiratory tract infection within 2 weeks before dosing, or history of hay-fever during the time of the year that dosing will be taking place;
• History of GERD, heartburn, or nausea more than once a month, or any similar symptoms requiring the regular use of antacids, or any use of H2-histamine blockers or proton pump inhibitors within 12 months of Screening;
• QTcB >450 msec in males or >470 msec in females;
• Known or suspected hypersensitivity or allergic reaction to any of the components of gefapixant or omeprazole capsules;
• If female, is pregnant or breast feeding, or has a positive pregnancy test pre-dose;
• Blood loss or blood donation of >550 mL within 90 days or plasma donation >500 mL within 14 days before administration of the first dose of study drug;
• Chronic use of any systemic medications (other than allowable oral and implanted contraceptives and with the exception of vitamins taken at standard supplement doses); use of a drug therapy (including herbal preparations, e.g., St. John's wort) known to induce or inhibit hepatic drug metabolism within 30 days before the first dose of study medication; or use of any medications [prescription or over-the-counter (OTC)], including antacids, high-dose multivitamins, nutritional supplements, and herbal preparations, within 14 days before the first dose of study drug, unless deemed acceptable by the Investigator and Sponsor;
• Past or current history or evidence of drug or alcohol abuse, regular use of more than 2 units of alcohol per day (1 unit of alcohol = 150 mL of wine, 360 mL of beer, or 45 mL of alcohol 40%), use of any recreational soft drugs (e.g., marijuana) within 3 months of screening, use of any hard drugs (such as cocaine, phencyclidine (PCP), and crack) within 1 year of screening, and/or a positive screen for substances of abuse or alcohol at screening or pre-dose;
• Ingestion of grapefruit or grapefruit juice within 48 hours before dose administration;
• Positive screen for hepatitis B surface antigen (HBsAg), hepatitis C antibody, or human immunodeficiency virus (HIV) antibody;
• Receipt of an investigational product or device, or participation in a drug research study within a period of 30 days (or 5 half lives of the drug, whichever is longer) before the first dose of study medication;
• Receipt of an investigational immunomodulator or monoclonal antibody within 180 days (or 5 half lives, whichever is longer) before the first dose of study medication.