European regulators recommended approval of GlaxoSmithKline (GSK)'s gene therapy for the treatment of severe combined immunodeficiency (SCID) due to adenosine deaminase (ADA) deficiency (ADA-SCID). Once the European Commission finalizes the decision, the treatment will be sold as Strimvelis and will become the second gene therapy to be approved in Europe. The first was UniQure's alipogene tiparvovec for the treatment of lipoprotein lipase deficiency.

ADA-SCID is a rare metabolic disorder caused by mutations in the enzyme ADA, leading to the aberrant development of T and B cells, immunodeficiency and risk of opportunistic infections. Although some patients can be treated with bone marrow transplants from healthy donors, immune matching from donors is never perfect and immune incompatibility can lead to rejection. With Strimvelis, the patient's own bone marrow is removed and the cells are treated with a viral vector that inserts the ADA gene into cellular DNA. Gene-corrected cells are then re-introduced into the patient. In a trial of the one-off treatment, 11 of 12 patients (92%) hit the primary endpoint of intervention-free survival (they did not require bone marrow transplantation or more than 3 months of enzyme replacement therapy after treatment).

GSK has not yet disclosed how much the gene therapy will cost. But because ADA-SCID is a rare disease (there are only 15 new cases per year in Europe), and a one-off treatment provides long-term efficacy, analysts expect Strimvelis — and all gene therapies — to be very expensive. GSK has said it is considering an amortization model, in which payments are spread over several years.

UniQure's alipogene tiparvovec was approved in Europe in 2012, but product sales only began in the third quarter of 2015, in part owing to pricing discussions. UniQure had also considered an annuity- based pricing plan for its US$1-million drug, but decided against it due to the challenges of measuring outcomes and surrogate markers associated with lipoprotein lipase deficiency.

GSK has not disclosed its regulatory plans for Strimvelis in the United States.