** Shares of Agios Pharmaceuticals rise ~3% to $50.29; Fulcrum Therapeutics up ~20% at $4.15

** Larger rival Pfizer withdraws its sickle cell disease drug, Oxbryta, from all approved markets, citing risks of a painful complication and death

** Sickle cell disease is an inherited blood disorder that affects the hemoglobin in red blood cells

** Analysts expect the withdrawal to help foster enrollment for trials of rival sickle cell drugs, such as Agios' mitapivat and Fulcrum's pociredir

** "We expect this will increase attention and urgency for the clinical development of mitapivat," according to Piper Sandler analysts

** Leerink Partners analysts say Oxbryta's withdrawal could have a positive influence on the enrollment of FULC's early-stage study, especially among patients who are on or have been on Oxbryta alone

** Including session's gains, AGIO shares more than doubled and FULC stock is down YTD

(Reporting by Mariam Sunny in Bengaluru)

((Mariam.ESunny@thomsonreuters.com;))