CDX-301 is a protein that stimulates the growth of bone marrow stem cells and certain immune cells. This study is testing CDX-301 in healthy volunteers who will be monitored for safety and biological response for approximately one month or more.

A total of seven cohorts of healthy volunteers are planned with doses of CDX-301 (rhuFlt3L) ranging between 1 and 75 μg/kg and dosing durations between 5 and 10 days.

Volunteers will be admitted to an inpatient unit during the Treatment Period. Blood samples will be collected periodically to assess the effect of CDX-301. Volunteers will be followed for safety for 28 days following the last dose. Any volunteer who develops anti-CDX-301 antibodies will be followed monthly until the antibody response is below the limit of detection. In general, the total duration of the study will be between approximately 33-38 days.

Inclusion Criteria:

Among other criteria, volunteers must meet the following conditions to be eligible for the study:

1. Ages 18 - 55
2. Body Weight ≤ 120 kg
3. Generally good health and without significant medical conditions
4. Willing to use effective method of contraception
5. Abstinence from alcohol for 72 hours prior to study drug administration and throughout the study
6. Negative screening test for HIV, hepatitis B, and hepatitis C
7. Provide written informed consent

Exclusion Criteria:

Among other criteria, volunteers who meet the following conditions are NOT eligible for the study:

1. Drug or alcohol abuse within 12 months
2. Positive drug screen
3. Receipt of certain types of experimental drugs or other treatments, or certain medications
4. Use of systemic immunosuppressive agents (excluding topical steroids) within 12 months
5. History of certain diseases including syphilis, herpes zoster, primary or secondary immunodeficiency
6. Diagnosis with or family history of significant autoimmunity (ex: type I diabetes, multiple sclerosis, rheumatoid arthritis, scleroderma)
7. Any history of cancer, excluding adequately treated and cured basal or squamous cell carcinoma of the skin, or cervical carcinoma in situ within 2 years
8. History of asthma requiring any use of inhaled or oral medication within 5 years
9. Herpes zoster within 3 months
10. Donation of blood within 8 weeks, and donation of plasma within 2 weeks