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Suppository

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1. A suppository is a medicated solid dosage form, intended for use in the rectum, vagina or the urethra.
2. The suppository uses vehicles that either melt at body temperature or dissolve or disintegrate in the small amount of rectal body fluids.
3. Suppository is mainly used in old people, post operative people, bed ridden people or infants or in cases where for some reason the oral route is ruled out, (cases of severe nausea and vomiting or in paralytic ileus).
4. For rectal administration, one half to two or more times the oral dose is given for all but very potent drugs. So much range is there because drug release differs widely from one base to another.
5. The suppository base and the amount of drug must be considered together i.e. the dose of the drug changes with the type of base.
6. The amount of drug to be given in the suppository depends on
   1. the base used and on
   2. the chemical and physical form of the drug.
7. Rectal suppositories for adults usually weigh 2g and are torpedo shaped.
8. Children’s suppositories weigh about 1g.
9. Urethral suppositories for males weigh 4g each and for females they weigh 2g each.
10. Suppositories can exert local or systemic action.
11. The action level depends on
    1. the nature of the drug
    2. the concentration of the drug and
    3. the rate and site of absorption.
12. Emollients, astringents, antibacterial agents, hormones, steroids and local anesthetics are given as suppositories.
13. Suppositories are mainly used to treat constipation or hemorrhoids.
14. By giving a drug in the form of suppositories, we can partly overcome the first pass effect.
15. This is because; the lower homorrhoidal vein surrounding the colon and the rectum enter into the inferior vena cava and thus bypass the liver. The upper homorhoidal vein connects with the portal veins leading to the liver. So the drug absorbed from the lower end of the rectum ( from a suppository placed just in lower end of the rectum) is absorbed by the lower hemorrhoidal vein and it bypasses the liver. The drug released from a suppository placed deep in the rectum is absorbed by the upper hemorrhoidal vein and it does not bypass the liver, hence it suffers from first pass effect. There are also many ‘joinings’ between the two hemorrhoidal veins. In spite of this fact, it was established that, the drug released from the suppository, at least partially, overcomes the first pass effect, especially if it is placed at the lower end of the rectum. So if the suppository is properly placed in the rectum (not too deep) then the drug is absorbed (usually 50 to 70%) by the lower homorhoidal vein and it bypasses the liver and escapes the first pass effect.
16. pH of the rectal mucosa plays an important role in rectal absorption.
17. Rat colon has a pH of about 6.8.
18. Rectal fluids have no buffer capacity.
19. Weaker acids and bases are more readily absorbed than the stronger highly ionized drugs.
20. The barrier separating the colonic lumen from the blood is preferentially permeable to the unionized forms of drugs.
21. Absorption of acidic drugs will be increased in the rectum if we lower the pH.
22. Absorption of salicylic acid rose from 12% at a pH of about 7 to 42% at a pH of 4.
23. For quinine, absorption decreased from 20% at pH 7 to 9% at pH 4.
24. Phenol is a weak acid; it is completely unionized at pH 7 and at pH 4. So there is no change in its absorption when we lower the pH.
25. Drug absorption from suppositories is influenced by
    1. diffusion of the released drug to the site of absorption
    2. nature of the drug
    3. presence of a surfactant
    4. water-lipid partition coefficient of the drug
    5. physiological state of the colon (amount and chemical nature of the fluids and solids present in it)
    6. the state of anorectal membrance (thickness of the mucous wall which can impede the absorption)
26. A drug that can be absorbed from the colon most likely would be completely absorbed in the small intestine before reaching colon.
27. Body temp. is 37OC on average, it may vary from 36OC to 38OC.
28. The affinity between the drug and the base has a bearing on drug release
29. If the drug is more soluble in the base, it will be released very slowly from the base. If it is less soluble in the base, it is released readily.
30. Water soluble, oil insoluble salts are preferred in fat-base suppositories
31. But for water soluble bases, such as glycero gelatin (the suppository dissolves and releases the drug) the water soluble type salt is the one of choice for quicker drug absorption.
32. Ephedrine sulfate and quinine hydrochloride and sodium barbital and sodium salicylate are preferred to their respective bases and acids.
33. The rate limiting step in drug absorption from suppositories is the partitioning of the dissolved drug from the melted base.
34. The rate of release by drug from suppository depends on
    1. The rate of diffusion of the drug to the surface of the suppository
    2. particle size of the drug
    3. Presence of surfactants
35. Surfactants can both increase and decrease drug absorption rate.
36. Ex. Sodium iodide - for this drug the surfactant may reduce the surface tension on the rectal membrane and wash away the mucous blanket and hence enhance the absorption.
37. Ex. Phenol – type drugs. A drug – surfactant complex may form and the absorption may be reduced.

Author

Prof. J. Vijaya Ratna

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