Pfizer has snapped up the remaining stake in privately-held, US gene therapy group Bamboo Therapeutics in a deal that could be worth as much $645 million.

The US drugs giant already owned around 22 percent of the firm's fully diluted equity, and has now acquired the remainder for an upfront payment of $150 million.

Under the deal, Bamboo's selling shareholders will also be eligible for potential milestone payments of up to $495 million contingent upon the progression of key assets through development, regulatory approval and commercialisation.

The move significantly ramps up Pfizer's expertise in gene therapy, with the addition of clinical and several preclinical assets that complement its own rare disease portfolio, an advanced recombinant Adeno-Associated Virus (rAAV) vector design and production technology, and a fully functional Phase I/II gene therapy manufacturing facility.

"The field of gene therapy research has made tremendous strides in recent years, and we are pleased to be able to further enhance our leadership position in this area through this transaction with Bamboo," said Mikael Dolsten, president of Pfizer Worldwide R&D.

"We believe that gene therapy may hold the promise of bringing true disease modification for patients suffering from devastating diseases, and we hope to see this promise come to fruition - through new and existing in-house capabilities and potential partnership opportunities - in the years to come."

Rare diseases impact 350 million patients worldwide and, although more than 7,000 known have been identified, only five percent have an approved medication.