-the concentration at which half the receptors have ligand bound

K_d=[L][R]/[LR]

-the response to a drug is proportional to the concentrationof receptors that have drug bound to them

-concentration at which a half-maximal effect is obtained

-a measure of potency

-EC50 may or may not equal Kd

-activation of an opposing physiological mechanism to reduce an undesirable response or state

-ex) vasoconstrict nasal mucosa with psuedoephedrine to contract vasodilation caused by histamine

-compensatory pathway

-binds reversibly to the active site but does not induce a conformational change

-increases EC₅₀ therefore decrease the potency

-no effect on maximal efficacy

-either binds irreversibly to active site or reversibly to an allostertic site

-decrease the efficacy

-EC₅₀ does not change

-binds to agonist binding site, but even when the receptors are fully occupied, they have a lower efficacy

-full and partial agonists bind to the same site but with different efficacies

-therefore, a partial agonist will reduce the effect of a full agonist, appearing as a competitive antagonist

-rather than completely inhibiting the response to a full agonist, one will see a reduction to a level of activity equal to the efficacy of the partial agonist

-used as a competitive antagonist with a built-in safe margin so that an overdose will not completely reduce activity

-in some cases, a receptor can be active in the absence of an agonist

-acts by inhibiting this constitutive activity of the unoccupied receptor

-deactivates receptors that are active in the absence of agonists

-there will be situations where the antagonist will have no effect(bc there is not agonist at the receptor

-inverse agonist will have an effect (bc the receptor is constitutively turned on)

-We assume that the max effect is seen when 100% of receptors are bound (this is not the case)

-the maximal effect is obtained at a lower dose than is required for receptor saturation

-EC₅₀<Kd

-due to the fact that various signal transduction pathways activated by drugs allow for significant amplification of a relatively small signal

TD₅₀/ED₅₀

-the greater the therapeutic windo, the better margin of safety

LD₁/ED₉₉

_{the bigger the better}