Arena Pharmaceuticals, Inc. is an American biopharmaceutical company founded in 1997 and headquartered in San Diego, California. The company has small molecule drugs in development for possible clinical utility in multiple therapeutic areas. The company's most advanced investigational clinical programs are ralinepag in testing for pulmonary arterial hypertension (PAH), etrasimod in evaluation for inflammatory indications and APD371 for the treatment of pain associated with Crohn's disease. In 2016, the company downsized and shifted its focus to developing new drugs currently being tested in clinical trials. In December 2021, Pfizer announced that it had agreed to acquire Arena for $6.7 billion in cash. In March 2022, it was announced the acquisition by Pfizer had been concluded.