Characteristics of drug absorption via the rectal route.

The characteristics of drug absorption from the colorectal area are summarized as follows: colorectal absorption is a simple diffusion process through the lipoidal membrane in which carrier-mediated mechanisms play no role; most of the rates of absorption increase with increasing solubility and release of drugs by vehicles or adjuvants, although drug-adjuvant interaction is related to the decreased absorption rate; membrane-active adjuvants promoting absorption are more effective in the colorectal area than in the upper gastrointestinal tract; the promotive effect on absorption by fatty acids or their monoglycerides in micellar state is stronger in unsaturated long-chain compounds than in saturated ones, and their action is transient; their promotive absorption is applicable not only to small molecules but also large molecules, probably up to molecular weights of 500,000; and the threshold molecular weight of the blood-lymph permselectivity of dextrans is approximately 17,500-39,000 in the small intestine and 10,500-17,500 in the large intestine.