CRISPR Therapeutics Ltd. (CRSP) and Vertex Pharmaceuticals Inc. (VRTX) have been cleared by the U.S. Food and Drug Administration (FDA) to start testing their genetically engineered treatment for sickle cell disease on U.S. patients.

In a press release, the two companies said the FDA has now lifted its clinical hold, enabling them to begin trialing the experimental therapy. The partners in May were forced to halt their plans to test the drug, CTX001, pending the resolution of "certain issues" in their initial application.

At the time, CRISPR and Vertex claimed that CTX001 was placed on hold in the U.S. because the FDA had additional questions as part of its review of the paperwork that companies submit for permission to start a clinical trial. They did not reveal what these questions were or how they answered them.

News of the breakthrough sent CRISPR’s shares surging 14.14% in pre-market trading. Vertex’s stock was flat heading into Thursday’s trading session. The announcement is significant because it is the first time a company will test the experimental and revolutionary technology known as CRISPR-Cas9 on human stem cells in the U.S. The trial will involve the modification of patients' DNA outside of the body. An oncologist at Sichuan University in Chengdu, China conducted the first human trial in 2016.

In the joint statement, the two companies also provided an update on the progress made trialing CTX001 outside of the U.S. They announced that they have been given regulatory clearance in “multiple countries” to begin tests of their experimental treatment for sickle cell disease and beta-thalassemia, a blood disorder that reduces the production of hemoglobin — protein in red blood cells that carries oxygen throughout the body. A Phase 1/2 clinical study is currently on track to begin in Europe by the end of 2018, they added. Phase 1 and 2 studies are designed to test the safety, side effects and best doses of new treatments.

CRISPR and Vertex formed a drug development alliance in 2015 focused on the use of CRISPR-Cas9. Blood disorder treatments were included in the research pact, although at first they worked on cures for cystic fibrosis, an area of specialization for Vertex. Under the terms of their partnership, the two companies agreed to share research and development expenses, as well as the profits generated from any commercialized therapies. 

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