Structural Biochemistry/Rectal

The human rectum is a channel in the human body through which drugs can be easily administered and absorbed well. Sometimes it is the most preferable to give a drug rectally rather than orally, e.g. in cases of nausea and vomiting. Disadvantages of drug administration through rectum include the interruption of absorption by defaecation and lack of patient acceptability ( it is not comfortable for most patients to have drug administered through their rectum). The mechanism of drug absorption from the rectum is probably no different to that in the upper part of the gastrointestinal tract, despite the fact that the physiological circumstances (e.g. pH, fluid content) differ substantially, absorption from aqueous and alcoholic solutions may occur very rapidly, which has proved to be of considerable therapeutic value in the rapid suppression of acute convulsive attacks by diazepam (e.g. in children), but absorption from suppositories is generally slower and very much dependent on the nature of the suppository base, the use of surfactants or other additives, particle size of the active ingredient, etc. There is some evidence that hepatic first-pass elimination of high clearance drugs is partially avoided after rectal administration, e.g. lignocaine. This can be explained by the rectal venous blood supply: the upper part is connected with the portal system, whereas the lower part is directly connected with the systemic circulation. Plasma concentration data following rectal administration of representatives of several classes of drugs are reviewed: anticonvulsants, non-narcotic analgesics and non-steroidal anti-inflammatory agents, hypnosedatives and anaesthetics, strong analgesics, theophylline and derivatives, corticosteroids, antibacterial agents, thiazinamium, promethazine, hyoscine-N-butyl-bromide, streptokinase, progesterone, ergotamine tartrate and levodopa. Only limited number of cases has it been adequately shown that the rectal route of administration gives plasma concentrations which are comparable to the oral route. Potentially the rectal route offers the same possibilities as the oral route, but the influence of the formulation seems to be very critical. It is also likely that the future novel drug delivery systems with zero order release characteristics will be applied rectally. Interesting preliminary results have already been obtained with theophylline administered by 2ml osmotic pumps.