Conditioning SCID Infants Diagnosed Early (CSIDE)

The investigators want to study if lower doses of chemotherapy will help babies with SCID to achieve good immunity with less short and long-term risks of complications after transplantation. This trial identifies babies with types of immune deficiencies that are most likely to succeed with this approach and offers them transplant early in life before they get severe infections or later if their infections are under control. It includes only patients receiving unrelated or mismatched related donor transplants.

The study will test if patients receiving transplant using either a low dose busulfan or a medium dose busulfan will have immune recovery of both T and B cells, measured by the ability to respond to immunizations after transplant. The exact regimen depends on the subtype of SCID the patient has. Donors used for transplant must be unrelated or half-matched related (haploidentical) donors, and peripheral blood stem cells must be used. To minimize the chance of graft-versus-host disease (GVHD), the stem cells will have most, but not all, of the T cells removed, using a newer, experimental approach of a well-established technology. Once the stem cell transplant is completed, patients will be followed for 3 years. Approximately 9-18 months after the transplant, vaccinations will be administered, and a blood test measuring whether your child's body has responded to the vaccine will be collected.

This is a prospective, multicenter, phase II, open-label study of two reduced busulfan dose levels in newborns identified at birth with SCID of appropriate genotype/phenotype and clinical status, undergoing either haploidentical related or well-matched unrelated donor TCRαβ+/CD19+ depleted HCT. Subjects will be enrolled on either of 2 strata according to genotype (defects of cytokine receptor function i.e. IL2RG or JAK3 and defects of receptor recombination i.e. RAG1 or RAG2). Thus up to 32 subjects on each of 2 strata or 64 subjects total would be enrolled over 4 years with 3 years follow-up.

Patients with IL2RG/JAK3 would be randomized to receive busulfan targeted either to cumulative exposure of 25-35 mgh/L or 55-65 mgh/L with Thymoglobulin. Patients with RAG1/2 would be randomized to receive busulfan targeted to cumulative exposure of 25-35 mgh/L or 55-65 mgh/L, in conjunction with fludarabine, thiotepa and Thymoglobulin. Safety/feasibility of the novel TCR αβ+/CD19+ depleted allogeneic HCT strategy will be monitored on an ongoing basis using stopping rules for lack of neutrophil engraftment and other important short-term toxicities.

Donor selection would be determined clinically at the discretion of the treating clinicians at each site. Pharmacokinetic monitoring of busulfan exposure will be performed per local practices at CLIA-certified laboratories. Patients will receive busulfan and pharmacokinetic measurement to individualize dosing. Time-concentration data of the initial dose and subsequent doses will be reviewed centrally (Dr. Janel Long-Boyle) using a cloud-based application (InsightRx) to guide dose adjustment in real-time (Long-Boyle, Chan, Keizer, 2017, ASBMT Tandem abstract accepted). Clinical and laboratory data will be collected at defined time points over 3 years and entered in an electronic data capture system using study-specific case report forms. These data will be used to measure the outcomes including the primary outcome (cAUC of busulfan that promotes humoral immune reconstitution at 2 years post HCT with acceptable regimen-related toxicity at 42 days post HCT) and secondary outcomes (the quality of donor cell engraftment and immune function achieved in B and T cell compartments and survival). Mechanistic studies supporting the exploratory endpoints will be conducted centrally in designated laboratories.

Inclusion Criteria:

1. Infants with SCID, either typical or leaky or Omenn syndrome.

1. Typical SCID is defined as either of the following

   • Absence or very low number of T cells (CD3+ T cells 80% of CD3+ or CD4+ T cells are CD62L negative AND/OR >50% of CD3+ or CD4+ T cells express HLA-DR (