6. Characteristics of Drug-Receptor Interactions:
- Follow Mass-Action Law: rate of reaction is DIRECTLY proportional to the concentration of the reactants, however, there is a limit. This limit is called the point of saturation
- There are only so many receptors on each cell to which a drug can bind. Point of saturation refers to the point at which every receptor is bound.
- Selectivity: every drug has a preferred receptor; however, it may bind to others with the same or lesser affinity than its preferred receptor. Side effects of drugs are a direct result of low selectivity because one drug can bind multiple receptors producing undesired biological responses.
- Not to be confused with “specificity” meaning drug can only bind to one type of receptor, regardless of the drug dose: drugs that are “specific” are unheard of at this point
- Response is Proportional to Drug Dose [Concentration] up to the point of saturation
- Binding Changes the Receptor: when a drug binds a receptor it results in one of two changes as outlined below:
- AGONIST: results in an active receptor conformation to produce desired biological effect, i.e. intracellular signaling or changes in organ function
- ANTAGONIST: a simple occupancy by the antagonist of the receptor to hinder access of an agonist to its binding site on the receptor, therefore, obliterating the response to the agonist.