Shares of Alnylam Pharmaceuticals Inc. /zigman2/quotes/205655272/composite ALNY +0.26% were up 9% in afternoon trading after the Food and Drug Administration approved Givlaari, its treatment for acute hepatic porphyria, a rare, genetic metabolic disorder. Patients in the clinical trial who took the drug had fewer porphyria attacks than patients taking a placebo. The annual list price for Givlaari is $575,000 and is dependent on a patient's weight. The biopharmaceutical company also said Wednesday that it has reached a value-based agreement in principle with Harvard Pilgrim, in which the health plan will pay full price for the therapy only if the real-world results match the trials. There are about 3,000 potential patients who have been diagnosed with the genetic disease in the U.S. and Europe, making it an ultra-rare orphan disease. (An orphan disease affects fewer than 200,000 people worldwide.) Alnylam stock has jumped 50% year-to-date, while the S&P 500 /zigman2/quotes/210599714/realtime SPX +0.78% has rallied 24%.