Pfizer Inc. said on Monday it would buy drug developer Arena Pharmaceuticals for $6.7 billion in cash, to add a promising treatment candidate that targets diseases affecting the stomach and intestine. The $100 per share offer is double the last closing price of Arena’s shares, which surged 92 percent to $95.90 in premarket trading. This is the latest deal Pfizer has struck this year to expand treatment pipeline. The company last month acquired immuno-oncology company Trillium Therapeutics Inc. for about $2.22 billion to strengthen its arsenal of blood cancer therapies. Arena is developing several treatments for gastroenterology, dermatology, and cardiology. Its lead candidate, etrasimod, is being tested in a late-stage study in ulcerative colitis, as well as a mid-to-late stage study in Crohn’s disease, both types of inflammatory bowel diseases that cause ulcers in the digestive tract. Pfizer is also developing a treatment for ulcerative colitis, a chronic and inflammatory …