The action of each drug in a patient depends on its:

1) Pharmacodynamic properties (biological & therapeutic effects)

2) Pharmacokinetic properties (absorption, distribution and excretion , or the way the individual patient handles the drug)

• the dose of drugs have to be tailored to each individual; no two individuals respond to a drug in an identical manner

• The result of the drug activity depends on its interaction with tissue receptors at the site of action.

• A receptor may be an area on the cell membrane or an enzyme

ABSORPTION

• A drug must be absorbed before it can exert its action

• There are several mechanisms of absorption:

1) Passive Diffusion

- lipid soluble drugs are absorbed more rapidly than water soluble drugs

-Lipid soluble drugs dissolve via the cell membrane while small water-soluble drugs pass through a cell membrane via water channels

- No energy is expended by cell

- When concentration of drug is similar on both sides of the membrane a steady state is reached

2) Active Transport

- Require energy

- Exhibit selectivity

-A ‘carrier’ transforms the substance across the membrane then returns to repeat its work (Na+, K+ & amino acids are carried in this way)

- Many factors influence rate of absorption:

i) Drug factors

- dose form (oral vs IM)

- if oral: particle size, rate of dissolution

ii) Patient Factors

- GIT pH; surface area of epithelium; circulation at absorption site

DISTRIBUTION

• After a drug is absorbed, it is transported to various parts of the body at a rate dependent on: cardiac output; regional blood flow;physiochemical properties of drug

• Lipid soluble drugs which readily cross cell membranes are rapidly distributed to highly perfused parts the body including heart, brain, liver and kidneys

• Drugs which do not readily cross membranes are more restricted in their distribution

• Drugs may accumulate in tissues which then serves as a reservoir to prolong the effect of the drug; binding to plasma proteins has a similar effect

METABOLISM

• Many drugs must be transformed (metabolised) before they can be eliminated from the body

• Lipid soluble drugs are difficult to eliminate as they are resorbed from the renal tubules after filtration; therefore they must be transformed into a more polar or ionised form

• This biotransformation aides in elimination of drug & may inactivate it too

• Many drugs are metabolised by the liver

EXCRETION

• Generally, polar drugs may be eliminated unchanged; nonpolar and lipid soluble drugs may be changed (as metabolites) to be excreted

• Kidney play major role in excretion — the rate of elimination being dependent on the balance between glomerular filtration, active tubular secretion and passive tubular resorption

• Other vehicles for excretion include bile, faeces, sweat, milk