Lentiviral Gene Transfer for Treatment of Children Older Than Two Years of Age With X-Linked Severe Combined Immunodeficiency (XSCID)

This is a Phase I/II non-randomized clinical trial of ex vivo hematopoietic stem cell (HSC) gene transfer treatment for X-linked severe combined immunodeficiency (XSCID, also known as SCID-X1) using a self-inactivating lentiviral vector incorporating additional features to improve safety and performance. The study will treat 35 patients with XSCID who are between 2 and 50 years of age and who have clinically significant impairment of immunity. Patients will receive a total busulfan dose of approximately 6 mg/kg/body weight (target busulfan Area Under Curve is 4500 min\*micromol/L/day) delivered as 3mg/kg body weight on day 1 and dose adjusted on day 2 (if busulfan AUC result is available) to achieve the target dose, to condition their bone marrow, and this will be followed by a single infusion of autologous transduced CD34+HSC. Patients will then be followed to evaluate engraftment, expansion, and function of gene corrected lymphocytes that arise from the transplant; to evaluate improvement in laboratory measures of immune function; to evaluate any clinical benefit that accrues from the treatment; and to evaluate the safety of this treatment. The primary endpoint of the study with respect to these outcomes will be at 2 years, though data relevant to these measures will be collected at intervals throughout the study and during the longer follow-up period of at least 15 years recommended by the Food and Drug Administration (FDA) Guidance "Long Term Follow-Up After Administration of Human Gene Therapy Products" https://www.fda.gov/media/113768/download for patients participating in gene transfer clinical trials. XSCID results from defects in the IL2RGgene encoding the common gamma chain (yc) shared by receptors for Interleukin 2 (IL-2), IL-4, IL-7, IL-9, IL-15 and IL-21. At birth XSCID patients generally lack or have a severe deficiency of T-lymphocytes and NK cells, while their B- lymphocytes are normal in number but are severely deficient in function, failing to ma