The U.S. Food and Drug Administration lifted the clinical hold on CRISPR Therapeutics AG and Vertex Pharmaceuticals Inc.'s experimental sickle cell disease treatment CTX001.

Sickle cell disease is a group of blood disorders normally inherited from a person's parents.

The FDA also accepted the investigational new drug application for CTX001, a stem cell therapy being developed to treat severe hemoglobinopathies — a genetic defect that leads to abnormalities in the hemoglobin molecule in blood cells.

Previously, the FDA had placed a clinical hold on the drug to seek resolution of certain questions as part of its review of the application, which was submitted in April to support the planned initiation of a phase 1/2 trial in the U.S.

CRISPR Therapeutics and Vertex Pharmaceuticals, which are jointly developing the drug under a co-development and co-commercialization agreement, said in an Oct. 10 news release that clinical trial applications for CTX001 have been approved in multiple countries outside the U.S. for both beta-thalassemia and sickle cell disease.

The companies are enrolling patients in the phase 1/2 trial of the treatment in Europe for adult patients with transfusion-dependent beta-thalassemia, and expect to start a phase 1/2 clinical study of the drug in sickle cell disease by the end of 2018.