Tacrolimus is on the list of things for me to try if things continue as they are. It is similar to cyclosporin but in oral form. There have been a number of studies on it but quite recent. It is regularly used to prevent transplant rejection. The main problems, and the reason it is not that popular yet, in treating young people are that it can cause kidney failure - it can cause an (initially) reversible increase in the serum creatinine in approximately 30% of people and also an increase in blood pressure. My gastroenterologist tells me that as ulcerative colitis is "curable" through surgery many are reluctant to prescribe it as the worst thing would be to end up needing a colectomy as well as a renal transplant. However, it can be very effective in resistant cases - one study followed people up for up to around 6 years and many did well atlhough some obviously did come to colectomy.
It can work very quickly I believe. I'd be interested to know what you have tried in the past and how long it took to work and also how long you have been on it. I don't intend to scare you with the side-effects - my gastroenterologist said that if I went on it and he detected even a slight increase in creatinine he would stop it for good as it should be reversible at that point.
Hope you continue to do well on it