Most drugs used for mentalDisorders function by bindingto receptors of neurotransmit-ters in brain and blocking orreinforcing their signal tranmission.
Therefore, it is possible to estimateThe effectiveness of drugs byexamining the quantity of binding to receptors.
As can be seen below, by the prescription of a radiolabeledsubstance (ligand) after taking a drug that binds to the same receptors, the drug and radiolabeled substance (ligand)compete for the receptors. Thus, the quantity of ligand binding to receptors decreases in comparison to the case ofnot taking a drug.
This degree of decrease is defined as receptor occupancy, which we use as an indicator of the quantity of binding to receptor.Therefore,we can measure the quantity of binding to specific receptors with PET, and this enables us to estimate the receptor occupancy in a living body.