This is a randomized, double-blind, placebo-controlled within-participant crossover study in healthy male participants 18-55 years of age to assess pain tolerance during a cold pressor test.

The study will be conducted at a single center in New Zealand.

Inclusion Criteria:

1. Male participants aged 18 to 55 years, inclusive, at the time of signing the informed consent.
2. Overtly healthy with no clinically relevant abnormalities based on the medical history, physical examinations, clinical laboratory evaluations, and 12-lead ECG that, in the opinion of the investigator, would affect participant safety.
3. Body mass index (BMI) within the range of 18-32 kg/m2 (inclusive).
4. Male participants are eligible to participate if they agree to refrain from donating semen. Plus, agree to use a male condom when having sexual intercourse with woman of childbearing potential or women who are currently pregnant.
5. Capable of giving signed informed consent.
6. Willing to undergo Pain Tolerability Assessments using the cold pressor procedure.

Exclusion Criteria:

1. Inability to take oral medications or gastrointestinal abnormalities potentially impacting absorption.
2. Clinically significant cardiovascular, hematological, renal, hepatic, pulmonary, endocrine, gastrointestinal, immunological, dermatological, neurological, or psychiatric disease which could interfere with, or the treatment for which might interfere with, the conduct of the study or which would, in the opinion of the investigator, unacceptably increase the participant's risk by participating in the study.
3. Alanine aminotransferase (ALT) or aspartate aminotransferase (AST), >1.5 x upper limit of normal (ULN) at Screening.
4. Total bilirubin > 1.5 x ULN (for participants with known Gilbert's syndrome, a total bilirubin >1.5 x ULN is allowed provided the direct bilirubin is ≤1.5 x ULN)) at Screening.
5. Estimated glomerular filtration rate (eGFR) <80 mL/min based on serum creatinine levels using the 2021 chronic kidney disease epidemiology (CKD-EPI) creatinine equation at Screening. The CKD-EPI 2009 equation may be used per site standard practice.
6. Creatinine phosphokinase is ≥ 2 × ULN at Screening.
7. Any of the protocol defined abnormalities on 12-lead ECG or blood pressure (BP) at Screening, confirmed by repeat.
8. Use of prescription drugs ≤ 14 days (or 5 half-lives of the drug, whichever is longer) prior to Day 1, use of over-the-counter drugs, herbal medications, or vitamin supplements ≤ 7 days (or 5 half-lives of the drug, whichever is longer) prior to Day 1 or use of antibiotics and systemic steroids ≤ 28 days prior Day 1. The Sponsor may allow exceptions only if the medication's administration is deemed unlikely to confound safety or efficacy.
9. Any vaccination ≤ 14 days prior to Day 1 or anticipated vaccination while participating in the study.
10. Receipt of an investigational product or device, or participation in a drug research study ≤ 28 days (or 5 half-lives of the drug, whichever is longer) prior to Day 1.
11. Receipt of an investigational biologic / monoclonal antibody within 180 days (or 5 half-lives, whichever is longer) prior to Day 1.
12. Positive for Coronavirus Disease 2019 (COVID-19) suggesting active infection at Screening or on Day 1.
13. Positive human immunodeficiency virus (HIV) at Screening.
14. Presence of any condition for which administration of a sodium channel blocker is contraindicated.
15. Smoking or vaping of tobacco or nicotine, or any other use of tobacco or any products containing nicotine ≤ 14 days prior to Day 1.
16. Consumption of alcohol within 48 hours prior to Day 1 and/or food and beverages containing methylxanthines (caffeinated coffee, caffeinated tea, caffeinated soda, and chocolate) within 48 hours of Day 1, and grapefruit, grapefruit juice, Seville oranges, or Seville orange juice ≤ 14 days prior to Day 1.
17. Positive urine drug screen, urine cotinine test, or alcohol breath test at Screening or on Day 1.
18. Past or current history or evidence of 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 illicit drugs (such as cocaine, phencyclidine [PCP], and crack) within 6 months of Screening, and/or a positive screen for substances of abuse or alcohol at Screening or on Day 1.
19. Past or current history or evidence of dependence on recreational drug use (eg, marijuana), use of recreational drugs ≤ 28 days prior to Day 1, or positive test result at Screening or on Day 1.
20. Donation or loss of over 500 mL blood ≤ 3 months prior to Day 1.
21. Use of moderate or strong CYP 3A4 inhibitors or inducers.
22. Has known psychiatric disorders that would interfere with the cooperation with the requirements of the study.
23. Participant is under legal custodianship.
24. Pain conditions that may require analgesic treatment during the study or history of opioid medication use
25. Hand/arm/skin conditions impacting ability to participate in cold pressor assessment.
26. Recent serious injury, surgical procedure, or medical condition which, in the opinion of the investigator, unacceptably increases the participant's risk by being included in the study.
27. Meets protocol defined Pain Tolerance Threshold (PTT) in any of the 5 predose cold pressor tests
28. Any relative or absolute contraindication to suzetrigine as outlined in the prescribing information for Journavx.