I just did a quick check on webmd for interactions....this is what I found...
Interactions between your drugs
Moderate
mercaptopurine mesalamine
Applies to: mercaptopurine, mesalamine
Using mercaptopurine together with mesalamine may increase the effects of mercaptopurine. Contact your doctor if you experience nausea, vomiting, stomach pain, diarrhea, bleeding, fever, chills, and other signs of infection. If your doctor does prescribe these medications together, you may need a dose adjustment or special test to safely use both medications. It is important to tell your doctor about all other medications you use, including vitamins and herbs. Do not stop using any medications without first talking to your doctor.