The thinking is that for combination therapy it is the biologic that is doing the 'heavy lifting" of inducing and maintaining IBD remission, with the immunosuppressant preventing formation of antibodies against the biologic and also providing a little side support in maintaining IBD remission. Thus if you are looking to drop one of the two drugs, the thinking it is better to keep the big hitter and drop the minor one, especially if the minor one has serious side effects - as do the thiopurines.
There is a lot of truth in this. If you look at my response a few posts ago I was more inclined to stop the infliximab and stay on the azathioprine, because that was the advice from one of my doctors, but after researching a bit more myself, you are absolutely right, if you were going to give up one of them it would be better to give up the azathioprine.
Antibodies to infliximab are more likely to form at the start of treatment and when you stop. For this reason it's important to have the azathioprine initially, at least for the first six months, but then if you are on a regular schedule of infliximab every 8 weeks it becomes less important, since your body is less likely to form antibodies to the infliximab once it has settled into a regular routine. I have absolutely no idea why that is and if anyone knows why I'd be very grateful for an explanation.
This study:
http://europepmc.org/articles/pmc5279914 is a good read. An excerpt:
The potential benefit from combining infliximab with a second immunosuppressive agent remains an area of controversy. In 2008, Van Assche et al performed a randomized controlled trial in which adult patients with CD were assigned to either withdrawal of immunomodulators (thiopurines or methotrexate) after 6 months of combination therapy or continuation of combination therapy with immunomodulators and infliximab. Although the clinical remission rates after 2 years were comparable, combination therapy was associated with a higher median infliximab trough level and a lower C-reactive protein (CRP).
So in summary, discontinuing the azathioprine after six months of remission, vs staying on it leads to remission rates after two years which are not that different. Although in the group that did discontinue the azathioprine, their lab results indicated that on average they were slightly closer to the beginnings of losing response to the infliximab.
So in my opinion if you wanted the greatest possible chance of prolonging your remission it would be better to have both drugs, but after 6 months of remission you can probably halve the dose of azathioprine.
This study:
https://academic.oup.com/ecco-jcc/article-abstract/12/5/628/4817389?redirectedFrom=fulltext found that halving the dose of azathioprine after 6 months of remission in combination therapy was just as effective as continuation at full dose.
I have a lot of interest in your question because I am in the same position as you. I am on both drugs and in remission, so have questioned what to do next. I think I will halve the azathioprine after 6 months, or at least try to adjust my dose so that I am only just above the threshold level which was outlined in that study of: 6-TGN > 120 pmol/8×10^8 RBCs.
How long have you been in remission for, and have you had your thiopurine metabolite levels tested?
