Gene Transfer for X-Linked Severe Combined Immunodeficiency in Newly Diagnosed Infants

Bone marrow CD34+ cells will be obtained in the operating room, transduced with the
lentiviral vector that contains a normal copy of the γc gene, and reinfused without any
myeloreductive conditioning. Participants will be monitored for hematopoietic recovery from
busulfan and for severe adverse events for 42 days post gene therapy. The primary endpoint
assessing the efficacy of this approach will be T-cell immune reconstitution 52 weeks (± 4)
weeks after transplantation. Continued and detailed evaluation of all aspects of immune
reconstitution, protocol-related toxicity, and retroviral integration sites will also be
performed. This study will evaluate the first use of a SIN lentiviral vector for the
treatment of SCID-X1 and may lead to a new form of therapy that could be applied to the
majority of newly diagnosed patients.

OBJECTIVES

Assess the safety, feasibility and efficacy of lentiviral gene transfer in newly diagnosed
SCID-X1 patients transplanted with autologous CD34+ cells that have been transduced with a
self-inactivating lentiviral vector (CL20-i4-EF1α-hγc-OPT) expressing a γc gene.

Primary Objective 1: Evaluate the safety and feasibility of infusing at least 1 million
transduced CD34+ cells per kilogram of body weight in SCID-X1 infants.

Primary Objective 2: Evaluate the efficacy of lentiviral gene transfer for inducing
significant T-cell reconstitution 52 weeks (± 4 weeks) after transplantation. Significant
reconstitution of T cells is defined as at least 2 of the following 3 criteria being present:

- The development of T-cell proliferative responses to phytohemagglutinin (PHA) that are ≥
50% the value seen in normal controls

- ≥ 1000 autologous CD3+ T-cells/μl in the peripheral blood

- ≥ 500 autologous CD4+ T-cells/μl in the peripheral blood

- ≥ 200 autologous CD4+ CD45RA T-cells/μl in the peripheral blood

OTHER PRE-SPECIFIED OBJECTIVES:

- Correlate busulfan and its metabolite pharmacokinetics with toxicity, efficacy,
engraftment of vector-transduced cells, and event-free survival and overall survival.

- Evaluate the efficacy of busulfan dose-targeting with busulfan administration every 24
hours for a total of 2 doses in order to achieve a cumulative busulfan area under the
curve of 22 mg*hr/L.

- Evaluate B-cell function during longterm follow-up of protocol patients. Evaluation will
include γc expression in circulating B-cells, measurement of serum IgG, IgA, and IgM
concentration, measurement of antibody responses to vaccination, evaluation of IgG
production after cessation of intravenous gamma globulin therapy in patients with
clinical indications to discontinue IVIG.

- Evaluate NK cell numbers in long term follow-up of protocol patients. Evaluation will
include flow cytometry evaluation of NK cell numbers.

- Determine the vector copy number and the location of vector-integration sites in sorted
blood cells. Sorted T-cells, B-cells, NK cells, granulocytes and monocytes will be
evaluated for vector copy number. Vector copy number in sorted T-cells will be evaluated
as a potential safety measure and will be reported to the FDA if the vector copy number
is greater than 5 copies per T cell in any patient at any time. Studies on sorted cells
will also include deep sequencing with an automated sequencer to characterize insertion
sites, and expression array analysis of T-cell clones to assay for gene expression
alterations within 100 kb of the insertion sites.

- Evaluate the overall, long-term safety of lentiviral gene transfer. This will include
complete clinical evaluation of any AEs resulting from the gene transfer procedure. If
any oncogenic event is seen, this evaluation will include complete molecular
characterization of the tumor clone including insertion site analysis, gene expression
analysis, and evaluation for the LMO2, Cdkn2a, Notch1, Cyclin D2 gene alterations.

Inclusion Criteria:

* Treatment Eligibility Criteria:

- Age <2 years at the time of enrollment.

- No prior therapy with allogeneic stem cell transplantation.

- A clinical diagnosis of SCID-X1 documented in the medical record.

- A proven mutation in the common gamma chain gene as defined by direct sequencing of
patient DNA.

- Age > 2 months to < 1 year of age at the time of busulfan administration.

- Less than 300 CD3+ T-cells by flow cytometry or higher if evidence of maternal
engraftment as supported by peripheral blood FISH analysis for XY and XX.

- Lymphocyte proliferation to phytohemagglutinin (PHA) <10% of the lower limit of normal
for the laboratory.

Treatment Exclusion Criteria:

- Availability of a HLA matched sibling for allogeneic transplantation

- Prior therapy with allogeneic stem cell transplantation

- Positive for HIV infection by genome PCR

- Presence of a medical condition indicating that survival will be less than 16 weeks
such as the requirement for mechanical ventilation, severe failure of a major organ
system, or evidence of a serious, progressive infection that is refractory to medical
therapy.

- The presence of any medical contraindications to general anesthesia and bone marrow
harvest by aspiration

- A social situation indicating that the family may not be able to comply with protocol
procedures and recommended medical care.