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DRUG ABSORPTION

Principles of absorption

· drug that has a desired systemic effect must enter bloodstream to exert that effect

· once administered, drug must pass through series of membranes before reaching bloodstream eg. gut wall. Rate of absorption depends on drug lipid solubility + membrane area Rate expressed as time taken for peak plasma concentration to be reached

· when administered by any route other than I.V, the absorption of most drugs is less than 100%. Several reasons why absorption is incomplete, which particularly apply to drugs administered orally:

1) drug may be inactivated within gut lumen by gastric acid/digestive enzymes/bacteria

2) absorption of drug may be incomplete

3) proportion of drug dose may be metabolized by the gut wall/liver before reaching systemic circulation. Called presystemic or “first-pass” metabolism. If hepatic metabolism quick, then hepatic extraction ratio can be nearly 100% ® low bioavailability of drug. Bioavailability represents percentage of drug dose that reaches bloodstream in active form. Bioavailability = area under [plasma]-time curve for a particular admin route/area under curve for I.V. admin of same drug

Prodrugs

Method of delivering drug to inaccessible tissue. Prodrug more easily absorbed than drug ® better absorption ® active drug liberated after absorption.

eg. carbimazole ® methimazole

Different drug preparations

- important to realize that rate of absorption and bioavailability will differ between individuals, due to differences in factors such as prescribed dose, GI disease, age, blood flow etc. However, factors involved in drug manufacture play a major role. Tablet contains many other ingredients apart from drug, for example:

· lubricants

· dyes

· disintegrating agents

- proportion of these other ingredients can seriously alter tablet’s performance eg.change of manufacturing process of Lanoxin (digoxin preparation) ® ¯potency ® changes in patients’ blood levels. Raises issue of whether generic or proprietary/brand name should be prescribed (cost also an issue: generic = cheaper)

- sometimes useful to coat tablets ® slow release mechanism (enteric coating)

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