An Open-label Study to Evaluate the Pharmacokinetics of Oral Tenapanor in Breast Milk of Lactating Females

Inclusion Criteria: * Subjects must satisfy all of the following criteria at the Screening visit unless otherwise stated: 1. Females greater than or equal to 18 years of age. 2. Body mass index between 18.0 and 35.0 kg/m2, inclusive. 3. In good health with no clinically significant findings from medical history, physical examination, 12-lead ECG, vital signs measurements, and clinical laboratory as determined by the Investigator. 4. Females must be non-pregnant. 5. Has been breastfeeding or actively pumping for at least 4 weeks. 6. Willing to continue regular pumping in order to maintain milk supply for the duration of the study. 7. Will not feed infant breast milk while taking tenapanor and for 72 hours post the last dose of tenapanor. 8. Weaning must not be underway, to ensure an adequate milk supply. 9. Able to comprehend and willing to sign an ICF and to abide by the study restrictions Exclusion Criteria: Subjects will be excluded from the study if they satisfy any of the following criteria at the Screening visit unless otherwise stated: 1. Significant medical or psychiatric disorder, as determined by the Investigator that would interfere with participation. 2. Use or intend to use any prescription or non-prescription medications 4 days before first dose of tenapanor until Day 6 of the study, unless deemed acceptable by the Investigator and/or Sponsor. 3. History of stomach or intestinal surgery or resection that would potentially alter absorption and/or excretion of orally administered drugs (uncomplicated appendectomy and hernia repair will be allowed). 4. Has mastitis or other condition that would prevent the collection of milk from one or both breasts. 5. Loose stools (Type 6 or 7 on the Bristol Stool Form Scale) ≥2 days in the past 7 days 6. Participation in a clinical study involving administration of an investigational drug (new chemical entity) in the past 30 days prior to dosing or 5 half-lives prior to dosing. 7. Subjects who, in the opinion of the Investigator (or designee), should not participate in this study