"Merck  Cos blockbuster drug Keytruda met the main goal of a latestage trial testing the treatment in patients with cancers of the digestive tract the U.S. drugmaker said on Wednesday.  Keytruda among a class of medicines called PD inhibitors is Mercks top selling drug and has already been approved to treat several forms of cancer including skin and lung cancer.  PD or PDL inhibitors work by blocking a mechanism of tumors that allows them to evade detection by cancerfighting cells.  Keytruda when compared to chemotherapy enabled certain patients with esophageal cancer to live longer helping the drug meet the main goal of the latestage study Merck said.  This marks the sixth tumor type where Keytruda has demonstrated a survival benefit and represents the first time an antiPD therapy has achieved overall survival for this patient population said Roy Baynes chief medical officer of Merck Research Laboratories.  Esophageal cancer is the seventh most commonly diagnosed cancer in the world the company said.  Merck said trial results would be presented at an upcoming medical meeting."