- Explain why nitroglycerine is usually not given orally and why different routes of administration are utilized. What effect do the different routes of administration have on the bioavailability of the drug?
- Identify and describe another drug that is not given orally and explain why. How does the alternate route of administration affect bioavailability?
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- Hesham Alsayyed Ahmad
- Amy Anderson
- Josilyn Brunsworth
- Tiana Conti
- Clyde Custodio
- Nana-Afua Dzifa Dodoo
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Nitroglycerine is usually not given orally due to its poor oral bioavailability. Bioavailability refers to the fraction of a drug that reaches the systemic circulation in an unchanged form after administration, and it is influenced by various factors including the route of administration.
Oral administration of nitroglycerine is avoided because it undergoes extensive first-pass metabolism in the liver. When a drug is taken orally, it is absorbed from the gastrointestinal tract into the portal circulation, which carries it directly to the liver. In the liver, nitroglycerine is rapidly metabolized by enzymes, such as glutathione S-transferase, resulting in a significant reduction in its bioavailability. This extensive first-pass metabolism greatly limits the therapeutic effectiveness of nitroglycerine when given orally.
To overcome this issue, alternative routes of administration are utilized for nitroglycerine. The most common routes include sublingual (under the tongue), transdermal (through the skin), and intravenous (directly into the bloodstream) administration.
- Sublingual administration: Nitroglycerine tablets or sprays are placed under the tongue, where the drug is rapidly absorbed into the bloodstream through the sublingual mucosa. This route bypasses the liver and avoids first-pass metabolism, resulting in higher bioavailability compared to oral administration.
- Transdermal administration: Nitroglycerine can be applied topically as a patch or ointment. The drug is absorbed through the skin and enters the systemic circulation, again bypassing the liver. Transdermal administration provides a controlled and sustained release of nitroglycerine, ensuring a prolonged therapeutic effect.
- Intravenous administration: Nitroglycerine can be administered directly into the bloodstream through intravenous infusion. This route completely bypasses the gastrointestinal tract and liver, resulting in 100% bioavailability. Intravenous administration is typically used in critical situations or in hospital settings where immediate effects are required.
Another drug that is not given orally due to low bioavailability is insulin. Insulin is a peptide hormone used for the treatment of diabetes. The reason insulin is not administered orally is that it gets broken down by the digestive enzymes in the gastrointestinal tract. Insulin is a large protein molecule that cannot be absorbed intact through the intestinal wall.
To overcome this limitation, insulin is typically administered through subcutaneous injections. Subcutaneous administration involves injecting the insulin into the fatty tissue just below the skin. By bypassing the gastrointestinal tract, insulin can directly enter the bloodstream and exert its therapeutic effects.
The alternate route of administration, in this case, affects the bioavailability of insulin. Subcutaneous administration allows for a slower and sustained release of insulin, resulting in a more gradual onset and longer duration of action compared to intravenous administration. The bioavailability of subcutaneously administered insulin is generally high, providing effective glycemic control in diabetic patients.