Phase I/II trial of lentiviral vector mediated gene therapy for Adenosine Deaminase deficiency

Adenosine deaminase deficiency(ADA) causes a severe problem with the development of the white cells in the blood and leads to a disease called severe combined immunodeficiency (SCID - also sometimes termed 'bubble babies'). Affected children are unable to fight infection and without treatment will die in the first year of life. The options for treatment are very limited. A bone marrow transplant is effective if a good donor can be found but in cases where an unrelated or mismatched donor has to be used, the procedure can be dangerous and 3-5 out of 10 children die from the effects of the transplant. Another treatment is regular injections of the ADA enzyme but this does not fully correct the white cells problems and children still remain vulnerable to infection.

Gene therapy is way of introducing a working copy of the ADA gene into patients cells so that they can grow a new immune system. This has some advantages over a transplant because it uses the child's own cells. We and others have treated ADA patients in this way previously and 7 out of 10 children have been able to grow a new immune system. However, to carry the gene into the patient cells we and others used a disabled virus called a gammaretroviral vector (GRV). Unfortunately, in gene therapy treatments for other diseases, GRVs caused the development of leukaemia (a blood cancer) and trials had to be stopped. For these reasons, we want to develop a safer way of introducing the gene into patient cells. We are planning to use a different vector called a lentiviral vector (LV). We have tested LVs in the laboratory and shown that they are much safer than GRVs but also that they are equally good at correcting the immune system.

We now want to see if they can treat patients effectively and safely. In this study, we plan to treat 10 patients with ADA SCID who do not have a good donor for transplant by gene therapy. We will introduce a working copy of the ADA gene into their bone marrow cells using a LV vector that has been prepared for use in patients. We will follow patients for 3 years to see if they can grow a new immune system and also to see if this treatment is safe. If we see a good response to the treatment in these 10 patients, then we hope that this will become a standard treatment for patients with ADA deficiency.

Patients will be selected based on the following inclusion and exclusion criteria:

Inclusion criteria
1. Diagnosis of ADA-SCID confirmed by DNA sequencing OR by confirmed absence of <3% of ADA enzymatic activity in peripheral blood or (for neonates) in umbilical cord blood erythrocytes and/or leucocytes or in cultured fetal cells derived from either chorionic villus biopsy or amniocentesis, prior to institution of PEG-ADA replacement therapy
2. Patients who lack a fully HLA-matched family donor
3. Patients < 5yrs of age
4. Parental/guardian signed informed consent

Exclusion criteria
1. Cytogenetic abnormalities on peripheral blood
2. Evidence of infection with HIV-1&2, hepatitis B, HCV
3. Evidence of active malignant disease
4. Known sensitivity to busulfan

PEG-ADA therapy will be stopped 1-2 weeks prior to the gene therapy procedure. For the transduction protocol, CD34+ cells will be purified either from autologous bone marrow or from G-CSF mobilised PBMCs recovered by leukapheresis.

Patient conditioning will be initiated immediately after bone marrow or PBMC collection and will consist of Busulfan i.v. (total dosage 4mg/kg). Patients will not proceed to conditioning if the harvest is <1 x 106 CD34+ cells/kg. In these patients, PEG-ADA will be restarted and patients will be withdrawn from the study.

After purification, CD34+ cells will be transduced with the lentiviral vector in established protocols, then evaluated for sterility, cell count, and viability, and immediately infused into patients. Other tests for safety including testing for RCLs will be performed on reinfused cells and results will be recorded retrospectively.

If after 180 days there is no evidence of the transgene in PBMCs by qPCR or there is no evidence of T cell recovery, then patients will re-start PEG-ADA and will be withdrawn from the study.

Patients will be followed-up on study at 1 month, 6 weeks, 3, 6, 9, 12, 18, 24, 30 and 36 months post gene therapy.

If successful, the research will have impact upon

1) Patients and their families - who will have an safe and effective treatment option for their disease
2) Clinicians dealing with the disease - who will now be able to have a further treatment option
3) The NHS - which will benefit from a treatment option that is economically more beneficial than current standard options such as enzyme replacement therapy and safer than allogeneic BMT
4) Biotech companies interested in vector manufacture - who may see learn from the techniques used to generate a high quality clinical grade lentiviral vector for clinical use
5) Industrial partners - who may see the vector as a possible commercial opportunity