LONDON (Reuters) - AstraZeneca (>> AstraZeneca plc) is getting a $450 million (297.42 million pounds) windfall by letting Celgene (>> Celgene Corporation) develop a prized immunotherapy drug for blood cancers but its chief executive insists the deal is about strategy, not cash.

Some investors are worried about the drugmaker's reliance on such "externalisation" deals to fill a short-term revenue gap. Deutsche Bank analyst Richard Parkers said income from these agreements was of "questionable sustainability".

Chief Executive Pascal Soriot, however, said he had been looking for a partner in blood cancers since last year and by teaming up with U.S.-based Celgene, a leader in haematology, AstraZeneca would accelerate its drug's path to market.

AstraZeneca will effectively give away around half the sales of its drug MEDI4736 in blood cancers under the Celgene agreement, but Soriot said the deal would unlock value that the British company could not have achieved on its own.

"We could do it ourselves but with a partner in haematology we will do much, much better," he told reporters in a conference call. "It's more strategically driven than financially driven, though of course the financial aspects also help."

AstraZeneca believes blood cancers could represent 40 percent of the total cancer market, yet Soriot said it did not include this in a $45 billion 2023 sales forecast issued last year as it was not clear then how it would tap the market.

AstraZeneca has extensive experience in solid tumours, but not blood cancers, which is a highly specialised field involving different groups of doctors.

Citi analyst Andrew Baum said the Celgene collaboration made sense given AstraZeneca's absence of expertise and Celgene's desire to expand its presence in immuno-oncology (I-O).

AstraZeneca is up against several I-O rivals, such as Bristol-Myers Squibb (>> Bristol-Myers Squibb Co), Merck (>> Merck & Co., Inc.) and Roche (>> Roche Holding Ltd.), and Soriot argued that by getting into bed with Celgene it would be at a competitive advantage in the blood cancer space.

Under the deal, AstraZeneca will book sales of MEDI4736 in blood cancers and will pay Celgene an initial royalty of 70 percent, which will decrease to approximately half of sales over a period of four years. (This version of the story was refiled to correct royalty details in final paragraph)

(Editing by Keith Weir)

By Ben Hirschler