EC50;

the concentration of agonist that elicits a half-maximal response 

["intrinsic effiacy" when referring to receptor stimulation]

- in some cases the response to an agonist is NOT linearly proportional to the # of receptors occupied;

- a max effect can be produced by an agonist when occupying only a small proportion of receptors ("super agonist" or "high efficacy agonist')

- some agonist drugs are not capable of stimulating a full biological response, even when occupying 100% of the receptors ("partial agonists")

- drugs that have no intrinsic efficacy are ANTAGONISTS -- they block the response of agonists or endogenous neurotransmitters/hormones 

- absorption & distribution

- metabolism (biotransformation)

- elimination 

- vascular: bloodstream (plasma)

- interstitial: fluid btwn the cells & tissues

- intracellular: within the cells

only unbound form can diffuse through capillary, produce effects, be metabolized, be excreted;

possible drug-drug interaction

-biotransformation;

- mostly occurs in the liver (also lungs, cytochrome P450)

- metabolizes often (not always) more hydrophilic than the parent drug (favors renal excretion);

- usually the metabolites are less active than the parent drug (or inactive) -- exception = "prodrug"

- genetic polymorphism (extensive/rapid & poor/slow metabolizers)

- dose-overload of enzyme capacity 

- age

- disease (dose adjustment common)

- sex

- smoking

- diet

- rate is proportional to concentration (exponential decline)

[drug half-life or half-time]

rate is constant; 

linear decline

achieved through action potentials (APs)

electrical signals propagated down the axon

mediated predominantly by sodium & potassium channels