We're using budesonide, another immunosuppressant, on Spencer's IBD. It has fewer side effects than either pred or imuran, as I understand it -- mainly some fur loss on his feet and occasional moderate panting. Spencer also still has flares but they are not at all severe. And he isn't losing weight on budesonide the way he was on pred. So I can't tell you anything about imuran except that my vet finds it scary, and you might ask your vet about budesonide instead. But if they know what type of inflammatory cell(s) Conner has, it may be that imuran is more appropriate.
Can't remember now whether Conner had a biopsy? I've resisted it with Spencer, but his new internist told me something new to me, which is that a biopsy of the intestines can be done by ultrasound-guided needle aspiration! It'll probably be $$, but at least it will be minimally invasive.
Spencer sends puppy kisses to you both! Keep us posted, okay?