This study will test the safety and tolerability (how the body reacts to the drug) of REGN668 and placebo (an inactive substance that contains no medicine) in healthy subjects.

Inclusion Criteria:

1. Male or female 18 to 65 years of age.
2. Weight > 50 kg and < 120 kg.
3. For women of childbearing potential, a negative serum pregnancy test at the screening visit (visit 1) and a negative urine pregnancy test on day -1.
4. Willingness to refrain from the consumption of alcohol for 24 hours prior to each study visit.
5. For men and women of childbearing potential, willingness to utilize adequate contraception and not become pregnant (or have their partner[s] become pregnant) during the full duration of the study.
6. Willing, committed, and able to return for ALL clinic visits and complete all study-related procedures.
7. Able to read, understand and willing to sign the informed consent form.

Exclusion Criteria:

1. Onset of a new exercise routine or major change to a previous exercise routine within 4 weeks prior to screening visit.
2. Pregnant or breast-feeding women.
3. Significant concomitant illness or history of significant illness such as cardiac, renal, neurological, endocrinological, metabolic or lymphatic disease, or any other illness or condition that would adversely affect the subject's participation in this study.
4. Hospitalization for any reason within 60 days of screening (visit 1).
5. Known history of Human Immunodeficiency Virus (HIV), Hepatitis B or Hepatitis C, and/or positive Hepatitis B surface antigen (HBsAg), positive Hepatitis C antibody (HCV).
6. Previous exposure to any therapeutic or investigational biological agent. results.
7. History of alcohol or substance abuse within previous 5 years.