agonists and antagonists

agonists

drugs that activate receptors

In a system with "spare receptors", what is the most likely relationship between the EC₅₀ and K_D for a full agonist?

• EC50 will be lower than Kd

how is non specific binding determined in a saturation binding assay

• parallel set of tubes

• with tissue, radioligand and a very high conc of non radioactive drug

EXCESS OF UNLABELLED LIGAND

why is it difficult to obtain Kd by paper method

• difficult to distinguish bmax

• unless very high conc of radioliagnd have been used

antagonists

drugs that block receptor activation

allosertic modulator

drug that binds to site distinct from the agonist site and changes receptor behaviour

functional assays examples

• cell based biochemical assay

• isolated organ assay

• cell based electrophysiological assay

• whole animal assay/human trials

cell based biochemical assay use

• cells grown in lab

• used for GPCR

• measure conc of second messengers such as cAMP

electrophysiology

• measure how the membrane potential of a cell changes in response to drugs

• measures activity of ion channels activated by receptors

whole organism assay

• measure the changes a drug produces in physiology, behaviour or disease state

• many ethical considerations

organ bath assay

• organ is placed in tissue bath supplied with nutrients

• measures how the physiological function changes in response to drugs

• eg force and rate of contraction of heart

EC50

measures potency

• concentration that gives 50% of the maximum effect of the drug

is Kd and Ec50 the same?

• no it is rare

• because ratio of protein to receptors, number of proteins activated from one receptor, ratio of protein to second messengers etc

efficacy

how well a drug activates receptor once its bound

full agonists

• drugs that have the same maximum effect as the natural ligand

partial agonist

• lower effect than natural ligand

• lower efficacy

two features of competitive antagonism

1. The maximal effect Emax of the agonist is not changed

2. In the presence of the antagonist, you will need to use more agonist to bring about a particular effect than you would if the antagonist were not present.

features of concentration effect curve with antagonist

• curve shifts to the right

• same shape

• reaches same emax

• ‘apparent EC50’

reversible competitive antagonism/ orthosteric antagonism

• antagonist binds reversibly binds to the same site as agonist

• overcome by inc conc of agonist

non competitive antagonism

• negative allosteric modulator

• insurmountable

• does not produce same emax

irreversible competitive antagonists

• drugs bind covalently to the receptor

• insurmountable antagonism

tone

• underlying activity of system

• eg vagal tone keeps heart at 70bpm