The European Commission granted a marketing authorisation valid throughout the European Union on 26 May 2016 for Strimvelis, a gene therapy product, used to treat severe combined immunodeficiency due to adenosine deaminase deficiency (ADA-SCID). ADA-SCID is a rare inherited condition in which there is a mutation in the gene needed to make an enzyme called adenosine deaminase (ADA). As a result, patients lack the ADA enzyme. Because ADA is essential for maintaining healthy lymphocytes the immune system of patients with ADA-SCID does not work properly and without effective treatment they rarely survive more than 2 years. Strimvelis is used in patients with ADA-SCID who cannot be treated by a bone-marrow transplant because they do not have a suitable, matched, related donor.

Strimvelis, an orphan medicine, contains cells derived from the patient’s own bone marrow. The CD34+ cells have been genetically modified to contain a working gene for ADA.

To make Strimvelis, a sample of the patient’s bone marrow is collected. Then, CD34+ cells are extracted from the bone marrow cells. A working gene for ADA is inserted into the CD34+ cells using a type of virus called a retrovirus, which has been altered genetically so that it can carry the ADA gene into cells and does not cause viral disease in humans.

Once given back to the patient into a vein, Strimvelis is transported in the bloodstream to the bone marrow where the CD34+ cells start to grow and make normal lymphocytes that can produce ADA. These lymphocytes improve the patient’s ability to fight infection, and so overcome the symptoms of the condition related to the immune system. The effects are expected to last for the patient’s lifetime.

The benefits of Strimvelis have been shown in one main study involving 12 patients from 6 months to around 6 years old with ADA-SCID. Patients in the study had no appropriate bone marrow donor and alternative treatment had not worked or was not available. All patients were treated with Strimvelis and were still alive 3 years after treatment. The rate of severe infections declined after treatment and continued to decline with longer-term follow-up beyond 3 years.

The most common side effect with Strimvelis is pyrexia. Serious side effects with Strimvelis may include effects linked to autoimmunity as haemolytic anaemia, aplastic anaemia, hepatitis, thrombocytopenia and Guillain-Barré syndrome.