Pharmacokinetics

Dr. Farhan Ahmad

• the magnitude of response depends on concentration of the drug at the site of action

PHARMACOKINETICSWhat the body does to drug

• ABSORPTION

• DISTRIBUTION

• METABOLISM

• EXCRETION

Absorption

The movement of a drug from its point of entry into the body into

systemic circulation

• I.V- completely (100%) absorbed

• Oral- delayed and incomplete.

• Rate: how rapidly does the drug get from its site of administration, to the general circulation?

• Extent: How much of the administered dose enters the general circulation (per cent bioavailability = F)?

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Absorption of Drugs

1. Sites of absorption through the GI tract2. Factors that modify absorption in the GI tract3. Bioavailability4. Other sites of drug administration/absorption

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Absorption of Drugs

1. Sites of absorption through the GI tract

1) Mouth2) Stomach3) Small intestine 4) Large intestine

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2. Factors that modify absorption in the GI tract1) Drug solubilization

2) Ph of the medium

3) Lipid solubility of the drug

4) Blood flow at the absorption site

5) Surface area of absorption

6) Route of administration (First Pass Effect in Oral Route)

7) Gastric emptying

8) Food

9) Intestinal motility

10) Metabolism of drug by GI tract

Absorption of Drugs

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Factors that modify absorption in the GI tract Drug solubilization – breaking drugs into smaller, more

absorbable particles

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DISINTEGRATION (solid )

DISSOLUTION (solution)

ABSORPTION

SYSTEMIC CIRCULATION (% bioavailability)

Absorption of Drugs

Ph of the medium

stomach (pH 1~3)

in favour of weak acid absorption

duodenum (pH 5~7)

in favour of weak base absorption

ileum (pH 7~8)in favour of weak base absorption

pH and ionization

Acidic drugs are best absorbed in Acidic medium

Basic drugs are best absorbed in Basic medium

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Gastric emptying

small intestine – primary site of drug absorption

Anything that delays/accelerates gastric emptying will decrease/increase drug absorption.

Food

High fat food – delay gastric emptying – slow absorption

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Bioavailability Bioavailability is defined as the fraction of unchanged drug reaching the systemic circulation following administration by any route .

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Bioavailability of a drug

• The bioavailability of a drug (by a route other than intravenous) is given by:

AUC (extra vascular)• F = ----------------------------- AUC (intravenous)