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An influential U.S. drug pricing watchdog raised the price estimate of two experimental gene therapies from Vertex Pharmaceuticals VRTX.O /CRISPR Therapeutics CRSPR.BN and bluebird bio BLUE.O to as much as $2.05 mln, saying the new price can be cost effective to treat sickle cell disease.

A price of up to $2.05 million a year, higher than the prior estimate in April of up to $1.9 million would meet some commonly used thresholds for cost effectiveness, the Institute for Clinical and Economic Review (ICER) said.

Sickle cell disease leads to a shortage of healthy blood cells and restricts blood circulation, causing periodic episodes of pain in patients.

The higher estimated pricing reflects the ICER's assumption of more pain attacks per patient, said David Rind, chief medical officer of ICER.

"The treatment is preventing more attacks and it's worth more," Rind said.

The ICER's draft report in April had assumed four attacks per patient in a year, compared with 5.1 attacks in its final report.

Its price range of $1.35 million to $2.05 million a year for the gene therapies suggests the highest price a manufacturer in the United States should charge for a treatment.

Both the bluebird and Vertex/CRISPR gene therapies are yet to be approved in the United States. Vertex, CRISPR and bluebird bio did not immediately respond to Reuters requests for comment.

Recently approved gene therapies in the U.S. carry a high list price due to their potential to be one-time cures for rare conditions.

CSL's (CSL) Hemgenix for hemophilia B has a list price of $3.5 million, while BioMarin Pharmaceutical's BMRN.O therapy, Roctavian, for severe hemophilia A is priced at $2.9 million.

Rind said the impact of high prices on healthcare budgets could potentially be an issue, but it was too early to assess.

"There's a fair amount of nervousness about the new gene therapies," he said.