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After reading this article you will learn about the pharmaceutic factors affecting drug bioavailability.
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Route of Administration:
Each route of drug application presents special biopharmaceutic consideration in designing a drug product. The bioavailability of active drug can be varied from rapid and complete absorption to a slow, sustained rate of absorption or even virtually no absorption by proper selection of route of drug administration and design of drug product.
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The route of drug administration affects the bioavailability of the drug, thereby affecting the onset and duration of pharmacologic effect.
Following factors must be taken into consideration while designing a drug for a particular route of administration.
(a) Anatomic and physiologic characteristics of the administration site e.g., membrane permeability and blood flow.
(b) Physicochemical properties of the site e.g., pH, osmotic pressure, presence of physiologic fluids and pKa of the drug.
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(c) Interaction of drug and dosage form including alteration at the site of administration, e.g., carbenicillin and gentamicin chemically interact at the site of administration.
Oral Administration:
Various factors affecting the bioavailability of drugs following oral administration, viz. dosage, formulation, species, disease, pH of gut, GI motility, presence of food, blood supply to GI tract. Drugs with large molecular wt. may not be well absorbed when administered orally.
Large drug molecules may be absorbed through the lymphatic’s when formulated with a carrier e.g., drug formulated with a surfactant in oil; oil may stimulate the flow of lymph as well as delay the retention of drug. Oily vehicle delays the passage through GI tract.
Many orally administered drugs irritate the stomach lining which may cause nausea or stomach pain. One way to avoid this irritation to stomach is to administer drug along with food or antacid, the other is to make the drug enteric coated. However, enteric coated drugs may be affected by the presence of food in the stomach. Drugs may also be formulated in the soft gelatin capsule to reduce GI irritation.
The stability of drug in GI tract (stomach and intestine) also affects the bioavailability of drug. Some drugs, such as penicillin G, which is unstable in the acid media of stomach is buffered or enteric coated to protect the drug from degradation at a low pH.
Poor bioavailability may also be due to first pass effect (Pre-systemic elimination/pre-systemic metabolism or first pass loss), e.g., Aspirin, glyceryl trinitrate, imipramine, Levasopa, Morphine, Salbutamol etc. If oral administration is poor due to metabolism by enzymes in the GI tract or in the, liver then a higher dose may be required.
