Pfizer Inc. is buying Arena Pharmaceuticals, Inc for $6.7 billion, adding a potential follow-up to its inflammatory diseases pill Xeljanz (tofacitinib) to its pipeline plus several other potential medicines targeting gastroenterology, dermatology and cardiology.

It’s the fifth largest biopharma merger of the year and will see Pfizer acquire all the outstanding shares of San Diego, Calif.-based Arena for $100 per share in an all-cash transaction for a total equity value of approximately $6.7 billion.

The acquisition comes after a relatively dry period for biopharma M&A at the end of 2021, which has been busy by historical standards but fallen short of the records set in 2020.

At the heart of the deal is Arena’s oral, selective sphingosine 1-phosphate (S1P) receptor etrasimod, which is being lined up as a successor to Pfizer’s own Xeljanz, a Janus kinase (JAK) inhibitor, approved as an oral therapy for ulcerative colitis and other inflammatory diseases. But Xeljanz is held back by a black box warning for uncommon but serious adverse events such as malignancy and opportunistic infections.

Arena’s development program for etrasimod includes two phase III studies in ulcerative colitis (UC), a phase II/III program in Crohn’s disease, a planned phase III program in atopic dermatitis and ongoing phase II studies in eosinophilic esophagitis and alopecia areata.

New York-based Pfizer cited the results of the phase II OASIS study, a placebo-controlled dose-ranging study testing etrasimod in moderate-to-severe UC patients over 12 weeks versus placebo. In the study, most patients who achieved clinical response, clinical remission, or endoscopic improvement at week 12 experienced sustained or improved effects up to week 46 with etrasimod 2 mg in the open-label extension.

Along with a “favorable” risk/benefit profile, Pfizer said the findings were “encouraging” and pointed to the drug’s potential as an oral therapy for UC.

There are several drugs approved that work along this pathway, with Johnson & Johnson’s Ponvory (ponesimod) approved by the FDA this year for multiple sclerosis.

Bristol Myers Squibb Co.’s Zeposia (ozanimod) works along both the S1P1 and S1P15 pathways and is approved in ulcerative colitis and relapsing forms of multiple sclerosis.

Arena’s pipeline also includes two development-stage cardiovascular assets, temanogrel and APD418. Temanogrel is currently in phase II for microvascular obstruction and Raynaud's phenomenon secondary to systemic sclerosis. APD418 is currently in phase II for acute heart failure.

The boards of both companies have unanimously approved the deal, which will add to Pfizer’s pipeline several potential drugs including etrasimod, modulator currently in development for several immuno-inflammatory diseases including gastrointestinal and dermatological diseases.

Analyst Joseph Schwartz, of SVB Leering Research, said that although the transaction value is below a price target of $112, Arena shareholders “are being rewarded fairly given the poor performance of many biotechs in 2021.”

Pfizer is a “logical partner” for Arena given its clout as a major pharma company and Schwartz added that etrasimod is “partially de-risked” following the approval of Zeposia and phase II data in UC.

Arena’s shares (NASDAQ:ARNA) were up nearly 87% in pre-market trading, while shares in Pfizer (NYSE:PFE) ticked up following the announcement.