Asacol and other the pills are systemically absorbed(enter the blood stream/pass through the liver and kidneys) at a rate of 20-30% depending on the particular medication.
In Rectal medications it is 17-77%.
The amount absorbed especially in rectal meds varies quite a lot person to person.
Action is believed to be topical but that doesn't mean the body doesn't absorb it.
Keep in mind that studies of maintenance level are limited to one year. High doses to induce remission are limited to 6-8 weeks. There are no studies using both high dosage(like 4.8gr asacol) and rectal medications. Lots of doctors are prescribing the higher dosages. Animal studies show that very high dosages are required to cause organ damage or death. It is a relatively "safe" drug.
The kidney problems are much more common than liver but both are rare. You have to look at the alternative risks of not using these drugs which is riskier drugs or severe disease. This is probably why some doctors are hesitant to prescribe pills, suppositories and enemas and let patients dose themselves up good.
The RXlist has Clinical Pharmacology on all medications.
www.rxlist.com/lialda-drug.htm