Oral administration of narcotic analgesics in the treatment of postoperative and cancer pain is often limited by nausea and vomiting, or poor patient condition. Studies indicate that rectally administered morphine has a variable bioavailability compared to an intramuscular injection, 30-70% when administered in a starch-containing gel and 40-88% from a hard fatty suppository. Increasing the pH of a rectal morphine microenema from 4.5 to 7.4 significantly increased the extent of absorption137. Hydrogels have also been used to successfully deliver morphine, producing a lower and more sustained plasma concentration than intramuscular morphine given on demand116.
Methadone administered rectally has a similar rate of uptake to an oral solution, but with a lower bioavailability. The use of the solvent glycofurol increased the rate and extent of absorption of methadone from a microenema (Figure 7.13)138.
Rectal absorption of acetylsalicylic acid is strongly dependent upon the type of formulation in which it is administered, with an aqueous microenema (20 ml at pH 4) giving the best results. In contrast, the pH of the paracetamol microenema was unimportant. Paracetamol is well absorbed when administered intrarectally, although absorption is slower than from an oral solution.
Indomethacin is rapidly absorbed when delivered rectally, however the bioavailability is less than from oral capsules and there is great intersubject variation. Naproxen, ibuprofen and ketoprofen are all well absorbed when rectally delivered.
Diflunisal (Figure 7.14) had a bioavailability of 55% as a rectal suspension, but this could be inproved to 70-80% by either using a cosolvent (glycofural) or by buffering the
Diflunisal concn. (mg/1)
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