Drug absorption

Absorption - Process by which unchanged drug enters the circulation Distribution: Reversible transfer of drug from one location in the body to another Metabolism: Transformation of a drug into daughter compound Excretion: Removal of drugs/metabolites from the body

Giving insight into the time course of the different stages of the drug as it is handled by the body

R-COOH and R-NH2 are both unionised and more likely to go into lipids. The unionised drug moves into the membrane easier. At acidic pH< R-COOH drug can move into the membrane.

• Passive transcellular diffusion • Facilitated/carrier-mediated diffusion through cells • Active transport through cells

Drug is orally administered, travels to the stomach and the small intestine where very thin highly vascularised moist villi help in the absorption of the drug.

e.g. Sodium-dependent glucose Amino acid transporter transports beta-lactam, ACE inhibitors (Cardiovascular disease), zidovudine (HIV)

Size of the drug molecule and its configuration Chemical structure (lipid solubility, ionisation) of the molecule Surface area of absorption site Cell membrane is lipid - diffusion occurs more readily if the drug is lipid-soluble (lipophilic); propine (lipophilic prodrug) -> Adrenaline

Rate of diffusion = SA x Concentration difference x permeability

All cells have low affinity for choline, cholinergic nerves have a high affinity uptake system for choline. They synthesise ACh

Blocks high affinity active transport system that gets choline into nerves. This gradually depletes ACh levels, and nerve function is impaired - decreased release of ACh

Action potentials pass through excitable cells