Page 95 - Interactive Theoritical Notes of Bioharmaceutics and pharamcokinetics.docx compressed

PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

i. Gastrointestinal physiological factors affecting drug absorption

1. pH, surface area, blood supply

2. Hepatic metabolism

3. Gastric emptying

4. Intestinal motility and transient time

5. Food and diet

6. Drug stability in GIT

1. pH, surface area

The gastrointestinal pH may influence the absorption of drugs in a variety of

ways. It may influence:

▪ Drug dissolution or its absorption, if the drug is a weak electrolyte.

▪ The chemical stability of the drug in the lumen. Chemical degradation due to

pH-dependent hydrolysis can occur in the GIT. The result of this instability is

incomplete bioavailability, as only a fraction of the administered dose reaches

the systemic circulation in the form of intact drug.

For example,

➢ The gastric instability of penicillin G (benzylpenicillin) precludes its oral use.

➢ The antibiotic erythromycin and proton pump inhibitors (e.g. omeprazole)

degrade rapidly at acidic pH values and therefore have to be formulated as enteric-

coated dosage forms to ensure good bioavailability.

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