Sept 29 (Reuters) - AstraZeneca (AZN.L) will take full control of Caelum Biosciences in a deal worth up to $500 million, the drugmaker said on Wednesday, sharpening its focus on rare-disease drugs following its purchase of Alexion Pharmaceuticals. The deal gives AstraZeneca access to another potentially lucrative rare disease drug that is undergoing late-stage trials and has a "fast track" status for regulatory review in the United States. New Jersey-based Caelum will become part of AstraZeneca"s Alexion division, the rare diseases specialist it bought for $39 billion earlier this year. Alexion will pay $150 million to buy the remaining stake it does not already own in Caelum and make future payments of up to $350 million, depending on milestones achieved. Alexion took a minority stake in Caelum in 2019. Caelum"s drug candidate aims to treat AL amyloidosis, a rare, life-threatening disease that damages the heart and kidneys and affects an estimated 20,000 people across six Western countries. The disease causes abnormal proteins called amyloids to build up in human organs and disrupt their normal function. Caelum"s drug is a type of monoclonal antibody that binds to amyloid deposits to reduce or remove them and improve organ function. "With a median survival time of less than 18 months following diagnosis, there is an urgent need for new treatments for this devastating disease," Alexion CEO Marc Dunoyer said in a statement. Since buying Alexion, AstraZeneca has had mixed fortunes with rare disease drugs under development. While it reported promising results from trials of a Wilson"s disease drug, it abandoned trials for an amyotrophic lateral sclerosis drug after it failed to show efficacy. Shares in AstraZeneca were up 2.2% by 0900 GMT on the London Stock Exchange, in line with the broader market. Reporting by Yadarisa Shabong and Sachin Ravikumar in Bengaluru Editing by Ramakrishnan M. and Mark Potter Our Standards: The Thomson Reuters Trust Principles.