Pharmacology 2060A/B Lecture Notes - Lecture 8: Drug Tolerance, Opioid, Irreversible Antagonist

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Pharmacodynamics- Drug Receptor Interactions
Theories of Drug Receptor Interactions
In the last module we learned that most drugs work by binding to receptors
Is simply occupying the receptor enough to have a response?
What if two drugs occupied the same number of the same receptor, would the response be the same? For example, if
one drug bound 10 dopamine receptors, would it give the same response as another drug that bound 10 dopamine
receptors?
There are two receptor theories that we use to describe drug receptor interactions:
1) The simple occupancy theory
2) The modified occupancy theory
The Simple Occupancy Theory
1) The intensity of a drug’s response is proportional to the number of receptors occupied
2) The maximal response occurs when all the receptors are occupied
This implies that two drugs that act at the same receptor should produce the same effect. This is not true! We know
thisbecause there are many drugs that act at the same receptor yet have different efficacies
The Modified Occupancy Theory
The modified occupancy theory identifies that some characteristics of drug receptor interactions cannot be
accounted for by the simple occupancy theory
The modified occupancy theory states:
1) The intensity of a drug’s response is proportional to the number of receptors occupied
2) Two drugs occupying the same receptor can have different binding strengths (i.e. affinity)
3) Two drugs occupying the same receptor can have different abilities to activate the receptor (i.e. intrinsic
activity)
In summary, in addition to accounting for the number of receptors occupied, the modified occupancy theory takes
into account the affinity of the drug for the receptor and the ability of the drug to activate the receptor
Modified Occupancy Theory- Affinity
Affinity is the attraction that a drug has for its receptor
Drugs with a high affinity are highly attracted to their receptor and therefore bind to the receptor effectively
even at low concentrations
Drugs with low affinity are weakly attracted to their receptor and therefore bind ineffectively to the
receptor even at high concentration
The affinity of a drug is the primary determinant of a drug’s potency. Drugs with high affinity for their
receptor have high potency whereas drugs with low affinity for their receptor have low potency
Modified Occupancy Theory- Intrinsic Activity
Intrinsic activity is the ability of a drug to activate the receptor
Drugs that have high intrinsic activity cause intense receptor activation
Drugs with low intrinsic activity only minimally activate the receptor
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