CTX001 is an investigational therapy being developed by Vertex Pharmaceuticals and CRISPR Therapeutics to treat inherited disorders of hemoglobin such as sickle cell disease and beta-thalassemia.

How CTX001 works

Sickle cell disease is caused by a mutation in the HBB gene that provides instructions to make the protein hemoglobin, a molecule inside red blood cells that is responsible for carrying oxygen. In sickle cell disease, this mutation results in missing or deficient hemoglobin.

CTX001 uses gene-editing technology to make a genetic change to increase the production of fetal hemoglobin in a patient’s red blood cells. Fetal hemoglobin is a form of hemoglobin that naturally exists in newborn babies and is later replaced by the adult form of hemoglobin. However, sometimes fetal hemoglobin persists in adults, providing protection for people with sickle cell disease and β-thalassemia.

For the treatment, a patient’s hematopoietic stem cells, which are cells from the bone marrow that give rise to all the red and white cells that form the blood, are collected and then genetically modified in the laboratory so they are able to produce high levels of fetal hemoglobin. These manipulated cells are then reintroduced into the patient’s body, where they will produce large amounts of red blood cells containing fetal hemoglobin, overcoming the hemoglobin deficiencies caused by the disease.

CTX001 in clinical trials

The results of preclinical experiments with CTX001 were presented in December 2017 at the American Society of Hematology (ASH) Annual Meeting. CTX001 was able to efficiently edit the target gene in more than 90 percent of hematopoietic stem cells to achieve about 40 percent of fetal hemoglobin production, which investigators believe is sufficient to improve a patient’s symptoms.

Study results also showed that CTX001 affects only cells at the target site and that it has no off-target effects on hematopoietic stem cells, thereby appearing to be a safe potential treatment.

These positive results prompted CRISPR to partner with Vertex to further develop CTX001 , with a goal of marketing it as a gene-editing treatment for inherited hemoglobin disorders, including sickle cell disease and β-thalassemia.

A Phase 1/2 clinical trial (NCT03745287) called CLIMB-SCD-121 was initiated in November of 2018 to investigate the use of CTX001 in sickle cell disease. The open-label, multi-site, single-dose trial is recruiting 45 patients, ages 18 to 35, with severe sickle cell disease in the U.S., Canada, Belgium, Germany, and Italy. Participants will be given a single IV infusion of CTX001; safety and effectiveness will be monitored for six months to two years.

Preliminary results for the first patient given the treatment were reported in November 2019. Before treatment, this person averaged seven vaso-occlusive crises (VOCs) a year, and at four months after treatment was free of VOCs and had hemoglobin levels of 11.3 g/dl. The study has an estimated completion date of May 2022.

Other information

CTX001 has received Fast Track designation from the U.S. Food and Drug Administration (FDA). Fast track designation allows for faster development and review of drugs that treat a serious medical issue and fill an unmet need.


Last updated: Dec. 6, 2019


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