Test tubes are seen in front of a displayed AstraZeneca logo in this illustration taken, May 21, 2021. (PHOTO / REUTERS)

AstraZeneca said on Friday its unit Alexion has agreed to buy US drugmaker Pfizer's early-stage rare disease gene therapy portfolio for up to $1 billion, plus royalties on sales.

AstraZeneca bought Alexion in 2021. It focuses on rare diseases and plans to close the deal in the third quarter, the British drugmaker said.

The deal will bring a number of novel adeno-associated virus (AAV) capsids to Alexion and help build on Alexion and AstraZeneca's capabilities in genomic medicine, it said.

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AAV capsids have been shown to be an effective mechanism for delivering therapeutic gene cargos for gene therapy and gene editing, the company said.