Bluebird’s tweaked sickle cell gene therapy clears early test

Sickle cell smear
A sickle cell smear.

Bluebird Bio has posted early clinical data suggesting work to rescue its sickle cell gene therapy may pay off. The tweaked gene therapy triggered a jump in the proportion of vector-positive cells in the two patients treated so far, raising hopes Bluebird has overcome its shortcomings. 

Cambridge, Massachusetts-based Bluebird was rocked two years ago by its failure to translate the results seen in one patient who received its sickle cell gene therapy into other subjects. That setback drove the biotech to reevaluate its protocol and manufacturing before returning with an approach intended to resolve the engraftment problem that limited the efficacy of its original gene therapy.

New data presented by Bluebird link the modified gene therapy to a greater percentage of cells transduced. The proportion of cells corrected by the gene therapy is now up above 80%, compared to below 50% in patients treated under the original protocol and process. A higher proportion of corrected cells should translate into more healthy red blood cells.  


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Bluebird has also seen a jump in drug product vector copy numbers (VCN), as well as VCN in the patients’ peripheral blood. Collectively, the data have encouraged Bluebird that it is on the right track. 

“The new data in sickle cell disease suggest that the changes made to the HGB-206 protocol and to our manufacturing process are having a favorable impact on the engraftment of the gene-modified stem cells,” Bluebird CMO Dave Davidson said in a statement. “The two patients treated in Group B have consistently higher [drug product] VCN and in vivo VCN than Group A patients.”

Shares in Bluebird ticked up 5% premarket on the back of the sickle cell news and updates on other parts of its pipeline, including the anti-BCMA CAR-T it is developing with Celgene. The CAR-T data reinforce the positive impression made by the asset to date by adding durability to the efficacy already demonstrated in heavily pretreated multiple myeloma patients. 

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