3.19 : 薬物吸収に影響を与える要因：薬物の溶解

The drug absorption process from solid oral dosage forms, like tablets or capsules, into the systemic circulation involves sequential events.

Firstly, drug disintegration occurs, breaking the solid drug into smaller particles.

Next, dissolution takes place, causing these smaller drug particles to dissolve in bodily fluids like gastrointestinal fluids.

The dissolved drug then permeates the intestinal membrane, facilitating absorption into the bloodstream.

Notably, the drug absorption process can be either dissolution rate-limited or permeation rate-limited.

Dissolution is a rate-limiting step for lipophilic or poorly aqueous soluble drugs, while it is intentionally controlled in the extended-release formulations. Meanwhile, permeation through lipid membranes is the rate-limiting step for hydrophilic drugs.

The biopharmaceutics classification system categorizes drugs into four classes based on absorption patterns.

Class I drugs are well absorbed orally; class II drugs face limitations due to low solubility; class III drugs exhibit poor permeability, and class IV drugs with both low solubility and permeability experience poor absorption.