Pfizer (NYSE:PFE) plans to spend almost $7 billion to acquire Arena Pharmaceuticals (NSDQ:ARNA). The purchase would bolster its pipeline of immuno-inflammatory drugs.  ARNA shares surged 80% to $90.08 after the announcement. PFE shares ticked up to $55.20 to 4.57%.  The transaction was valued at $100 per Arena share in cash.  The proposal has won the unanimous…