CAMBRIDGE, Mass. (TheStreet) -- Bluebird Bio's (BLUE) gene therapy, administered once to a patient with sickle cell anemia, is increasing the number of normally functioning red blood cells and is just barely at the level believed necessary to have a curative effect on the inherited blood disease.

After four and a half months of follow-up, the sickle cell patient's blood contained 24% "marked" beta globin, a measure of normally functioning hemoglobin, Bluebird announced Thursday. This healthy marked hemoglobin, the oxygen-carrying molecule in red blood cells, is produced by the working gene inserted into patients via Bluebird's gene therapy. Overall, the patient's total anti-sickling hemoglobin was 31.6%, and he has not been re-hospitalized for a sickle-cell related event since being treated with the gene therapy. 

Bluebird believes its gene therapy, known as Lentiglobin, can begin to reduce or eliminate the serious and life-threatening events associated with sickle cell disease when "marked" plus other anti-sickling hemoglobin reaches at least 30%.

Three months after the sickle cell patient was infused with Lentiglobin, his blood contained 9.6% marked hemoglobin, Bluebird said. This suggests the performance of Lentiglobin is improving over time, although perhaps more slowly than what the company showed previously in patients with beta thalassemia, a blood disease similar to sickle cell.

The Bluebird Lentiglobin data released Thursday come from an analysis conducted in February. More current data will be presented at the European Hematology Association meeting in June.

In a statement, Bluebird's Chief Medical Officer David Davidson described the new sickle cell data plus an update from beta thalassemia patients also treated with Lentiglobin as "very encouraging" and a validation of the company's gene therapy approach to curing inherited diseases.

Two beta thalassemia patients also treated with Lentiglobin have been followed for 14 months and 11 months, respectively, as of last February and have not required blood transfusions. An update on the status of these two beta thalassemia patients will also be presented at the European Hematology Association meeting in June.

Shares of Bluebird closed Wednesday at $165.71. By midday Thursday they had risen 6.7%. Bluebird went public in June 2013 at $17 per share. At Wednesday's close, the company was worth $5.4 billion.

The success of Bluebird to date, even with a relatively small amount of clinical data in patients, has led a resurgence in investor interest in gene therapy stocks like UniQure (QURE), Spark Therapeutics (ONCE) and Avalanche Biotechnologies (AAVL). Gene therapy seeks to replace defective or malfunctioning genes with ones that are healthy and functioning. Unlike drugs which only treat the symptoms of disease and must often be taken chronically, gene therapy offers the possibility of treating a patient once and curing the disease.

Sickle cell disease is an inherited disorder in which a mutated gene produces misshapen red blood cells. The disease gets its name because the deformed red blood cells looks like sickles or crescents. Healthy red blood cells -- made up of the protein hemoglobin which carries oxygen throughout the body -- resemble oval donuts without a hole in the center. They're flexible and move easily through the body. Sickled red blood cells contain abnormal hemoglobin which is inflexible and sticky. Sickled red blood cells don't carry oxygen well and clump together in blood vessels, causing extreme pain and organ damage.

Today, sickle cell patients undergo regular transfusions of healthy red blood cells in order to dilute the amount of sickled hemoglobin relative to normal hemoglobin. A single drug, hydroxyurea, is also currently approved to treat sickle cell disease. Neither red blood cell transfusions nor hydroxyurea are a cure for sickle cell, but they do improve the disease's primary clinical symptoms -- anemia and painful vaso-occlusive "crises." 

Bluebird is enrolling patients with beta thalassemia and sickle cell disease in an ongoing study known as HGB-205, being conducted in France. A second study, known as HGB-206 and conducted in the U.S., will enroll sickle cell patients exclusively. All the patients are treated with the Lentiglobin gene therapy once, with the aim of curing their disease.

On Monday, Bluebird reached an accord with regulators in the U.S. and Europe on plans to seek approval of Lentiglobin as a curative treatment for beta thalassemia. 

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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