Achillion Pharmaceuticals, Inc., a clinical-stage biopharmaceutical company, discovers, develops, and commercializes small molecule drug therapies for immune system disorders. Its lead drug candidate is ACH-4471, an inhibitor of factor D that is in Phase II clinical trials for patients with paroxysmal nocturnal hemoglobinuria (PNH) and C3 glomerulopathy/immune complex membranoproliferative glomerulonephritis. The company is also developing ACH-5228, a factor D inhibitor that is in Phase I clinical trials; and ACH-5548, a factor D inhibitor, which is in Phase I clinical trials for the treatment of PNH and other complement-mediated diseases. It has license agreements with Ora, Inc. for the development and commercialization of ACH-702; and GCA Therapeutics, Ltd for elvucitabine, a nucleoside reverse transcriptase inhibitor for the treatment of hepatitis B infection and human immunodeficiency virus infection. The company was incorporated in 1998 and is based in Blue Bell, Pennsylvania. As of January 28, 2020, Achillion Pharmaceuticals, Inc. operates as a subsidiary of Alexion Pharmaceuticals, Inc.