This study aims to figure out how much etrasimod, a medication, ends up in breast milk after taking it for several days. To do this, the researchers will work with at least 8 healthy women who are breastfeeding. These women will take a 2 mg dose of etrasimod every day for 7 days while staying at the testing site. During the study, they won't be allowed to breastfeed their babies to keep the babies safe. Instead, they need to have another plan for feeding their babies during this time. The study will look at how the medication is absorbed and how it moves through the body, checking things like the highest level in the blood and how long it takes to reach that level. The researchers will also see how much of the drug is in the breast milk compared to the blood and whether it's safe for the women to take. After the last dose, the women will stay at the site for at least another 24 hours before going home, and follow-up safety calls will be made about 14 and 28 to 35 days later. The whole process from start to finish will take about 10 weeks

Inclusion

1. Healthy (as determined by medical evaluation including medical history, physical examination, laboratory tests, vital signs and 12-lead ECGs) lactating women who are actively breastfeeding or expressing breast milk, who are at least 12 weeks post partum and not currently pregnant (must have a negative pregnancy test), and must be 18 to 55 years of age, inclusive, at the time of signing the informed consent document (ICD).
2. Body mass index (BMI) of 16-35 kg/m2; and a total body weight >45 kg (99 lb).
3. Participants must be willing to temporarily discontinue breastfeeding their infants for a total of 21 days, ie, from the evening of the day before Day 1 through to 14 days after the last dose (approximately 8 AM the morning of Day 21). Participants must be willing to regularly pump breasts throughout the study and express breast milk according to a schedule designed to maintain lactation until the completion of breast milk collection

Exclusion

1. Evidence or history of clinically significant hematological, renal, endocrine, pulmonary (such as moderate or severe chronic pulmonary disorders like asthma or chronic obstructive pulmonary disease [COPD]), gastrointestinal, cardiovascular, hepatic, neurological/psychiatric, anaphylactic, ophthalmologic disorders (such as macular edema, uveitis, retinopathy), or allergic disease (including drug allergies, but excluding untreated, asymptomatic, seasonal allergies at the time of dosing).
2. Participants with history or presence of second-degree or third-degree atrioventricular (AV) block, sick sinus syndrome, or sinoatrial block.
3. Resting HR <50 bpm at Screening or pre-randomization on Day 1. Measurement can be repeated up to 3 times to confirm the finding. Mean values will be used if repeated.
4. Recurrent symptomatic bradycardia or recurrent cardiogenic syncope
5. Any condition possibly affecting drug absorption (eg, gastrectomy, cholecystectomy).
6. Known immunodeficiency disorder, including positive serology for human immunodeficiency virus (HIV), or a first degree relative with a hereditary immunodeficiency, and history of organ transplant (except corneal transplant).
7. History or evidence of hepatitis B or hepatitis C viruses. Hepatitis B vaccination is allowed.89.
8. Participants with any of the acute or chronic infections or infection history

Female participants

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.