Pharmacokinetics

Drug absorption

Passage of a drug from its site of administration into plasma, must cross cell membrane

Drug absorption - passive diffusion

Diffusion of fat soluble substances across cell membrane

Drug absorption - filtration

Filtration of small molecules across cell membrane

Drug absorption - active transport

Diffusion of water soluble substances across cell membrane

Drug absorption - carrier-mediated transport

Diffusion via protein across cell membrane

Factors determining drug absorption

Nature of absorbing surface - number of cell membranes, surface area, blood supply, chemical structure of drug - lipid or water soluble, degree of ionisation, molecular weight, pH of aqueous phase, pH of area for drug admin, dissolution rate

Drug distribution

Transfer of drugs between body locations, once in plasma many drugs exist bound to plasma proteins such as albumin

Drug protein complex

Always in equilibrium, free drug loss = break up of a drug protein complex, only free drug is available to exert pharmacological effect

Factors determining drug distribution

Hypoalbuminemia - low levels of albumin, affinity of drug for plasma protein - likeliness of drug to bind to plasma protein, drugs binding to tissues, barriers to distribution - blood brain barrier or placenta

Drug metabolism

Enzyme modification of drug molecules, turn lipid soluble substances into water soluble substances - cannot cross cell membrane and more likely to be excreted, start of drug clearance and occurs in liver

Process of metabolism

Oxidation, reduction and hydrolysis, conjugation = joining lipid soluble substances to a polar substance to prevent it crossing cell membrane 

Pro-drugs

Drugs that are made more metabolically active by the liver

Hepatic first pass effect

Variable proportion of an orally administrated drug that is metabolised by the liver before exerting an effect

High hepatic first pass effect

Liver metabolises large proportion of drug

Low hepatic first pass effect

Liver metabolises small proportion of drug

Drug Excretion

Irreversible loss of drug from body, routes include renal, biliary, pulmonary, sweat, saliva and breastmilk

Drug clearance

Volume of blood cleared irreversibly of drug per unit of time, helps determine dose required