Officials said the boards of both companies have already approved the acquisition.
Arena is a clinical-stage company that's developing therapies for several immuno-inflammatory diseases and gastroenterology, dermatology and cardiology conditions.
Pfizer CEO Albert Bourla said in a tweet that the Arena acquisition allows the company to address "unmet needs" for varying diseases.
Pfizer has said it expects to generate about $36 billion this year from sales of its coronavirus vaccine. File Photo by Debbie Hill/UPI
One of Arena's newest treatments, Etrasimod, is used to treat ulcerative colitis and Crohn's disease. Pfizer has also been developing a treatment for ulcerative colitis, which affects nearly 3 million people in the United States.
The acquisition comes after Pfizer bought immunology company Trillium Therapeutics last month for $2.2 billion.
For much of the past year, Pfizer has been working with BioNTech on their COVID19 vaccines. A British study said Monday that Pfizer's vaccine appears to be less effective in producing antibodies when facing the Omicron coronavirus variant.
Pfizer has said it expects to generate about $36 billion this year from sales of the vaccine.