** Shares of biotech firm Elicio Therapeutics ELTX.O fall 50% to $7.43 premarket

** Company says its experimental vaccine, ELI-002 7P, being tested in pancreatic cancer patients failed to prevent the return of the disease

** ELTX says mid-stage study tested 144 patients after surgery and standard treatment

** Says subgroup with fully removed tumors saw 35% lower risk of cancer returning

** Pancreatic cancer is hard to treat as it is often diagnosed late and responds poorly to existing therapies

** Company says vaccine triggered immune response linked to better outcomes

** Plans late-stage study focused on subgroup and longer treatment - ELTX

** As of last close, stock up ~87% YTD