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Factors Influencing Drug Absorption Through the Gastrointestinal Tract (GIT)

  • The absorption of drugs through the GIT is affected by multiple factors, which can be broadly classified into the following categories:

Factors Influencing Drug Absorption Through the Gastrointestinal Tract (GIT)

Physicochemical Properties of the Drug

The inherent chemical and physical properties of a drug significantly influence its absorption.

A) Solubility

  • A drug must be in solution to be absorbed. Poorly soluble drugs may have limited absorption.

B) Particle Size

  • Smaller particles dissolve faster and are absorbed more readily than larger ones.

C) Ionization and pH–Partition Hypothesis

  • The extent of drug ionization (determined by its pKa and the pH of the GIT) affects absorption. Non-ionized (lipid-soluble) forms diffuse across membranes more efficiently.

D) Lipophilicity

  • Lipid-soluble drugs cross biological membranes more easily via passive diffusion.

Formulation and Dosage Form Factors

  • The design and composition of the drug product can impact its dissolution and absorption.

A) Disintegration and Dissolution

  • Tablets/capsules must disintegrate and dissolve before absorption can occur.

B) Excipients and Additives

  • Certain ingredients like binders, fillers, and surfactants can enhance or hinder dissolution and absorption.

C) Coatings

  • Enteric coatings protect drugs from gastric acid, delaying absorption until reaching the intestine.

  • Modified-release formulations (e.g., sustained-release tablets) can alter the rate and extent of absorption.

Anatomical and Physiological Factors

Various structural and functional characteristics of the GIT impact drug absorption.

A) Surface Area

  • The small intestine (duodenum) has a large surface area (due to villi and microvilli), which enhances absorption.

B) Blood Flow to GIT

  • Higher blood perfusion increases drug uptake and transport into systemic circulation.

C) Gastric Emptying and Intestinal Transit Time

  • Delayed gastric emptying (e.g., due to food or certain drugs) can slow the rate of absorption.

  • Rapid intestinal transit may reduce absorption if the drug has limited time for dissolution and membrane permeation.

D) pH of the GIT

  • The stomach is acidic (pH ~1-3), favoring absorption of weak acids.

  • The small intestine is alkaline (pH ~5-7), favoring absorption of weak bases.

E) Presence of Food

  • Food can delay gastric emptying and alter GIT pH.

  • Certain foods interact with drugs, e.g., calcium (Ca²⁺) and iron (Fe³⁺) binding to some drugs, reducing absorption.

Biological and Pathological Conditions

Factors related to the patient’s biological state or disease conditions can also impact drug absorption.

A) Age

  • Neonates and elderly patients may have altered gastric pH, enzyme activity, and GI motility, affecting absorption.

B) Diseases

  • Conditions such as Crohn’s disease, celiac disease, or gastric bypass surgery can reduce the absorption surface area or alter drug transport mechanisms.

C) Drug Interactions

  • Some drugs alter gastric pH (e.g., proton pump inhibitors reducing acid production).

  • Others affect gastric motility (e.g., anticholinergics slowing gastric emptying).


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