Gene therapies have been in clinical development for decades, a winding, stop-and-start path toward transforming science's understanding of the genome into commercial therapies. Strimvelis, a gene therapy made by the British pharma GlaxoSmithKline and approved in Europe last year, will be a high-profile test of the viability of selling cures to a tiny handful of patients.

A broader pipeline, though, backs up an emerging field that could soon begin delivering new therapies to patients in need. Biotechs like Spark Therapuetics and BioMarin have promising gene treatments in late-stage development, and big pharmas like Pfizer and GlaxoSmithKline are active in exploring market opportunities in the space.

Only two gene therapies for inherited diseases have been approved for commercial sale in the Western world, both in Europe.

UniQures Glybera was the first to market, authorized under a five-year conditional approval for treatment of an ultra-rare genetic disorder known as familial lipoprotein lipase deficiency. Yet, Glyberas $1 million price tag and low demand (only one patient was ever treated) meant the therapy never gained any traction. In April, UniQure decided against seeking renewal of its marketing authorization when it lapses this October.

Gene therapy got some new momentum after GlaxoSmithKline won approval from the European Commission in May 2016 for Strimvelis, an ex-vivo stem cell gene therapy for a disease called ADA-SCID, perhaps better known as the "bubble boy" disease.

GlaxoSmithKline

Strimvelis is considered the first outright cure for a genetic disease, although its not known if the 100% survival rate seen in clinical trials will persist over a lifetime. Median duration of follow-up at the time of approval was seven years.

The cost of Strimvelis.

Cures dont come cheap. GlaxoSmithKline priced Strimvelis at 594,000 ($669,000), which after Glyberas $1 million pricetag and Strimvelis' better efficacy profile, was actually lower than what some had expected.

With more gene therapies in development, pricing will be a critical problem to solve for drugmakers seeking to commercialize genetic fixes particularly against the backdrop of the ongoing debate over how to price and value drugs.

One-time treatments that promise to deliver cures will clearly be priced higher than existing therapeutics. But its not just the high cost that will be an issue. If a drugmaker markets a cure with all costs paid upfront, the insurer and patient will hold the risk if the gene therapy stops working after, say, 10 years.

In addition, when patients in the U.S. are free to move from insurer to insurer as they change jobs and move through life, why would one insurer pay $500,000 or $1 million upfront if that patient is free to change insurance?

Estimated number of patients affected by ADA-SCID in Europe each year.

ADA-SCIDs ultra-rare nature underscores the challenge of successfully marketing a gene therapy. While some rare diseases have a sizable enough patient population to support a product commercially, others like ADA-SCID occur so infrequently that even a cure like Strimvelis might not actually make much money for GlaxoSmithKline.

The shelf life of freshly transduced cells for treatment with Strimvelis.

With Strimvelis, stem cells are taken from a patients bone marrow, inserted with a correct copy of the ADA gene using a viral vector, and then infused back into the patient, who has received low dose chemotherapy to improve engraftment.

Due to the limited window of time for infusion, GlaxoSmithKline only offers Strimvelis at the San Raffaele Hospital in Milan, Italy, which is near a cell processing laboratory.

While production and dosing procedures for other gene therapies will be different, manufacturing and logistical challenges are weightier than with small molecules or even biologics.

Since the drug is a patients own modified cells, chain of custody and supply chain visibility are crucial. Managing treatment at its own center simplifies things, but also makes it more difficult for patients to access treatment.

In 2003, the Food and Drug Administration temporarily halted 27 gene therapy trials amid safety concerns.

While Strimvelis and a full pipeline of gene therapies in testing have buoyed optimism in the field, progress from gene therapies hasnt been a straight line.

The FDA decided to suspend the 27 studies, which represented around 15% of the total number of gene therapy trials then underway, after two children developed leukemia-like symptoms in a French study. The new concern followed the 1999 death of Jesse Gelsinger, who had reacted to a gene therapy he received for a metabolic disorder.

The number of worldwide clinical trials for gene therapy or gene-modified cell therapy, according to numbers cited by the Alliance for Regenerative Medicine (ARM) in an annual report.

Thirty-one are in Phase 3, reflecting the progress in clinical development. Spark Therapeutics, for example, recently completed an application submission for U.S. approval of voretigene neparvovec in patients with a type of inherited retinal disease. The therapy, developed in collaboration with Pfizer, is one of the most high-profile candidates in the U.S.

Other well-known names such as BioMarin and Bluebird Bio are also making progress on treatments for beta thalassemia and hemophilia A, respectively.

Total amount of global financing for gene and & gene modified cell therapy companies raised last year, according to ARM.

That figure includes sums raised by a variety of means such as upfront payments, IPOs, venture rounds and partnerships.

Much of the work developing gene therapies has been driven forward by biotech companies, but big pharmas are starting to notice too.

Last summer, Pfizer put down $150 million upfront to acquire Bamboo Therapeutics, with another $495 million in milestones to shareholders lined up. More recently, the pharma giant paid $70 million upfront with $475 million in milestones to work with Sangamo Therapeutics on its hemophilia A gene therapies.

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Gene therapy by the numbers - BioPharma Dive