the mesalamine is supposed to inhibit particular enzymes so its not effective at the very end of the colon.
Furthermore, the intention of mesalamine is to block the activity of cyclooxygenase and lipoxygenase, (enzymes) thereby, reducing the production of prostaglandins,
isnt entirely logical, because prostaglandins have a protective role in the bowel.
what it probably is attempting to control is the lipoxygenase, in creating the peroxides
but I would think you would help solve this by eating less fats. The theory being the lipoxygenase would not be 'asked' to perform its fat enzyme function if there is not fat to be digested.
Mesalamine works in the similar bio area as aspirin, so it can be acidic, hence it mite burn, this also is not so logical.