RECTAL ABSORPTION OF OMEPRAZOLE FROM SUPPOSITORIES IN RABBITS

Rectal absorption of omeprazole, a proton pump inhibitor, from suppositories was studied in rabbits. The suppositories were prepared by the conventional melting method with two types of bases, water-soluble polyethylene glycol (PEG) 4000 and oil-soluble Witepsol H15 bases, and administered intrarectally (ir) to rabbits at a dose of 10 mg omeprazole/kg. The plasma omeprazole concentration-time profiles of the two suppositories were compared with those following intravenous (iv) administration of the same dose. There were no significant differences between the two suppositories in bioavailabilities and peak plasma concentrations (C-max). Bioavailabilities and C-max of PEG- and Witepsol suppositories were 30.3 and 33.9%, and 7.0 and 5.6 mu g/ml, respectively. However, PEG suppository showed significantly (P<0.05) shorter time to reach peak plasma concentration (T-max), mean absorption time (MAT) and mean residence time in the plasma (MRT) than Witepsol suppository. The T-max, MRT and MAT were 25.0, 83.0 and 38.5 min for PEG suppository, but were 90.0, 122.5 and 78.0 min for Witepsol suppository, respectively. These differences between the two suppositories could be explained by the difference in the in vitro dissolution rates between the suppositories. The dissolution of omeprazole from PEG suppository was reportedly much faster than that from Witepsol suppository. It suggests that plasma profiles of omeprazole, especially C-max, MAT and MRT, could be controlled by modifying the in vitro dissolution rate of the drug from the suppositories. Above results suggest that rectal suppository is worth developing as an alternative dosage form of omeprazole to the conventional oral preparations which need sophisticated treatments, such as enteric coating, to prevent acid degradation of the drug in the stomach fluid.