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Vertex sickle cell drug, based on novel gene-editing technology, wins approval in UK

New therapy, the first using CRISPR/Cas9 tools, will also treat beta thalassemia

Vertex Pharmaceuticals and CRISPR Therapeutics won approval in the United Kingdom for drug to treat sickle cell disease. The drug is based on a novel gene-editing technology.Craig F. Walker/Globe Staff

Britain has approved the first drug based on a novel gene-editing technology, authorizing Boston’s Vertex Pharmaceuticals and CRISPR Therapeutics to sell the medicine, called Casgevy, in the United Kingdom as a one-time treatment for a pair of blood disorders.

Casgevy is the brand name for exa-cel, a therapy using a Nobel prize-winning set of gene-editing tools called CRISPR/Cas9. That technology enables scientists to edit DNA at precise locations, replacing specific strands of genetic code that contain harmful mutations.

Casgevy is expected to be the first of many therapies deploying CRISPR/Cas9, raising the curtain on a new era of gene-editing. But Vertex chief scientific officer David Altshuler said in an interview that it’s just as important as a breakthrough treatment for patients suffering from the inherited blood disorders sickle cell disease and beta thalassemia.


“This is a historic day both for medicine and science,” Altshuler said. “Patients waiting for decades are going to have access to a medicine with the potential to be a one-time treatment for a lifetime of benefit... It’s a demonstration of the specificity and precision and effectiveness” of a gene-editing approach described in a scientific paper just 11 years ago.

The UK’s Medicines and Healthcare Products Regulatory Agency, which has sought to accelerate its reviews of innovative therapies, on Thursday said it gave the green light for doctors to prescribe Casgevy to treat patients over 12 who suffer from sickle cell disease and beta thalassemia.

Sickle cell, which afflicts mostly people of African or Caribbean backgrounds, causes red blood cells to contort into a sickle shape, blocking blood flow and causing pain, fatigue, and infections.

Beta thalassemia, more common in people of South Asian, Southeast Asia, and Middle Eastern backgrounds, reduces the production of hemoglobin, a blood component carrying oxygen to organs and tissues throughout the body. Both are lifelong conditions that can be fatal.


Britain’s go-ahead comes about two weeks before the Food and Drug Administration is set to determine whether Vertex and CRISPR, a Swiss biotech with offices in Boston and San Francisco, can market the same drug in the United States, a much larger market, for sickle cell disease. That illness affects an estimated 100,000 Americans, though only about 20,000 with severe cases would be eligible for the treatment.

In a separate proceeding in March, the FDA is scheduled to rule on whether to approve the new therapy for beta thalassemia in the US. Vertex hopes to use the same commercial name, Casgevy, in the United States, said Stuart Arbuckle, the company’s chief operating officer, but that name has to be okayed by the FDA as part of its approvals.

Only about 2,000 Americans with beta thalassemia could benefit from the new drug. In Europe, by contrast, roughly 5,000 people with sickle cell disease and another 5,000 with thalassemia are eligible.

“This is a transformational therapy,” said Dr. Sharl Azar, medical director of the Complex Sickle Cell Disease Treatment Center at Massachusetts General Hospital, which will be an authorized treatment center for the new therapy once it’s approved in the US.

To administer the treatment, hematologists at hospitals collect stem cells from sickle cell patients and send them to Vertex. The company has designated manufacturing sites in the US and Europe to edit the genetic code. The healthy cells are returned to hospitals where they are transplanted into the patients’ bone marrow. This will replace the donor transplants now used to treat the disease.


Doctors today are limited in their ability to treat sickle cell patients because the national bone marrow registry has a shortage of Black and brown donors, forcing them to rely on family members, Azar said.

“This is an enormous step forward for us,” he said. “It will open up the barriers to bone marrow transplants for many of our patients.”

In a pair of clinical studies, involving 29 sickle cell and 42 thalassemia patients globally, the drug relieved the pain of many with sickle cell disease and the need for thalassemia patients to receive regular blood transfusions to replenish their low levels of hemoglobin. The clinical trials showed no significant safety concerns.

The trials were “found to restore healthy hemoglobin production in the majority of participants with sickle-cell disease and transfusion-dependent beta thalassemia, relieving the symptoms of disease,” Julian Beach, interim executive director of health care quality and access at the British regulatory agency, said in a statement Thursday.

The regulators are continuing to monitor clinical data from the patients who are participating in the ongoing studies, Beach said.

A rival sickle cell gene therapy, developed by Somerville-based Bluebird Bio, is also scheduled to come before the FDA late next month. Bluebird won US approval for a gene therapy to treat thalassemia last year.

Robert Weisman can be reached at