The purpose of the study is to assess the extent that radioactive dose of BCX4161 taken by mouth ends up in the urine, feces and expired air. If there are metabolites of BCX4161 made by the body, the chemical composition of these metabolites and their profile over time in blood and urine will be determined.

Inclusion Criteria:

1. Healthy males
2. Age 30 to 65 years of age (inclusive)
3. Body mass index of 18.0 to 32.0 kg/m2
4. Must be willing and able to communicate and participate in the whole study
5. Must provide written informed consent
6. A history of regular bowel movements
7. Must agree to use an adequate method of contraception

Key Exclusion Criteria:

1. Participation in a clinical research study within the previous 3 months
2. Current or history of any drug or alcohol abuse in the past 2 years or positive drugs of abuse screen
3. Current smokers
4. Radiation exposure, including that from the present study, exceeding 5 mSv in the last 12 months or 10 mSv in the last 5 years
5. Clinically significant medical history, current medical or psychiatric condition, ECG finding, or laboratory/urinalysis abnormality
6. Activated partial thromboplastin time or PT outside of normal laboratory limits