BRISBANE, Calif. ( TheStreet) -- U.S. drug regulators kept the long knives in their scabbards when reviewing InterMune's ( ITMN) lung drug pirfenidone, raising the odds for the drug's approval and sending the company's shares sharply higher Friday. Anxious investors were relieved that the U.S. Food and Drug Administration's review wasn't as critical as expected, which sent InterMune shares surging 56% to $22.82 in a significant relief rally. Earlier this morning, InterMune shares were up 80%. FDA staff, in a review posted to the agency's web site, questioned whether one positive and one negative study of pirfenidone in patients idiopathic pulmonary fibrosis (IPF) is enough to prove "substantial evidence" of efficacy enough to support the drug's approval. Yet the FDA review didn't raise any new concerns with pirfenidone's efficacy or safety unanticipated by investors and acknowledged that IPF is a fatal disease with no effective treatments. Pirfenidone will be reviewed next Tuesday by a panel of outside experts convened by the FDA. InterMune is hoping that pirfenidone will become the first drug approved for IPF by the FDA and its counterparts in Europe. With well over 200,000 IPF patients in the U.S. and Europe, InterMune could easily transform pirfenidone into a highly profitable drug with more than $1 billion in peak sales. IPF is a disease marked by scarring of the lungs caused by the gradual buildup of fibrous tissue. As the fibrotic scar tissue accumulates, air sacs in the lungs lose their ability to transfer oxygen into the bloodstream. There are no effective treatments for IPF, and unless patients can undergo a successful lung transplant, they eventually die of the disease. Pirefenidone is approved as an IPF treatment in Japan, where the drug is sold by Japanese drug maker Shionogi & Co. Other companies with drugs in clinical trials for IPF include Gilead Sciences ( GILD), United Therapeutics ( UTHR) and Celgene ( CELG). -- Reported by Adam Feuerstein in Boston. Follow Adam Feuerstein on Twitter.