The purpose of this study is to evaluate the safety, efficacy, and pharmacokinetics of EZN-2279 in patients with ADA-deficient combined immunodeficiency currently being treated with Adagen.

Inclusion Criteria:

1. Diagnosis of ADA-deficient combined immunodeficiency

2. Stable clinical status while receiving therapy with Adagen®. Patients previously receiving gene therapy or undergoing hematopoietic stem cell transplantation who still require Adagen® treatment are eligible. The dose of Adagen® must be stable for at least 6 months prior to study entry.

3. Have both of the following during the Adagen® Lead-in phase of the study prior to EZN-2279 transition:

   1. Trough plasma ADA activity >15 μmol/h/mL while receiving Adagen® and
   2. Total erythrocyte dAXP ≤0.02 μmol/mL from a trough blood sample

4. Patients or parent/guardian must be capable of understanding the protocol requirements and risks and providing written informed assent/consent

Exclusion Criteria:

1. Autoimmunity requiring immunosuppressive treatment
2. Patients with detectable neutralizing anti-Adagen® antibodies at screening evaluation
3. Severe thrombocytopenia (platelet count <50 x 10^9/L)
4. Current participation in other therapeutic protocols for ADA-deficient combined immunodeficiency
5. Current or prior participation in another clinical study with an investigational agent and/or use of an investigational drug in the 30 days before study entry
6. Known planned participation in a gene-therapy study for the planned duration of this study
7. Any condition that, in the opinion of the PI, makes the patient unsuitable for the study
8. Inability or unwillingness to administer Adagen® or EZN-2279 on a one-time-per-week regimen
9. Inability to comply with the study protocol
10. Female patients who are pregnant or lactating
11. Female patients who are breast-feeding
12. Female subjects of childbearing potential who are not using an FDA approved birth control method

As-treated

Through end of EZN-2279 study treatment, up to 203 weeks