How Safe Is Taking Entyvio For Ulcerative Colitis?

I am a 22 year old female and have been living with ulcerative colitis for 7 years now. I am under the care of a G.I. in NYC and have been having a flare-up since November. My doctor wants to change my medicine to Entyvio. Have you ever heard of it? What is your opinion about it. Currently I am on Imuran, Allopurinol and Asacol HD