AstraZeneca is buying the remaining stake in Caelum Biosciences for $500 million. The company is developing drugs against rare diseases such as AL amyloidosis, which damages the heart and kidneys. AstraZeneca will pay $150 million and about $350 million more when Caelum reaches certain development milestones. AstraZeneca owns a minority stake in Caelum through Alexion Pharmaceuticals, which it acquired earlier this year.