Receptor Occupancy (RO) / Receptor Autoradiogarphy
Receptor Occupancy Autoradiography studies such as in vitro, in vivo and ex vivo receptor occupancy (RO) can be used for CNS drug development and offer a simple yet powerful way to:
- Determine the tissue distributions of target of interest.
- Obtain Ki values by displacement assay on brain slices.
- Monitor the interaction of novel drug candidates with their targets in the brain after peripheral administration to select the best novel lead compounds. A drug candidate has to be potent and possess good ‘drug-like’ properties to achieve significant brain occupancy of its target.
- Better understand the pharmacokinetic and pharmacodynamic relationships of a novel drug candidate.
- Ex vivo RO can be used to better understand the percentage of receptor occupancy required for behavioral efficacy.
in vitro Approach
Cryostat-cut sections of brain or target organ are incubated in a binding buffer containing a radiotracer for the receptor of interest along with various concentrations of the competing unlabeled test drug. Sections are then washed in fresh buffer to remove unbound or loosely bound radiotracer, dried and imaged using phosphor-imaging or beta-imaging.
in vivo Approach
The unlabeled test drug is administered to the live animal. After a set time has elapsed (uptake), receptor occupancy is subsequently measured in vivo by injecting the animal with a receptor specific radiolabeled compound. Immediately following sacrifice of the animal cryostat-cut tissue sections from the brain (or target organ) are then imaged using phosphorimaging or beta-imaging.
ex vivo Approach
The unlabeled test drug is administered to the live animal. After a set time has elapsed (uptake) receptor occupancy is subsequently measured in vitro immediately following sacrifice of the animal using in vitro autoradiography on cryostat-cut tissue sections from the brain (or target organ). Radiotracer binding is reduced by drug bound to receptors in the sections. A short non-equilibrium incubation time in the radiotracer is used to avoid appreciable dissociation of the unlabeled test drug before the termination of the assay.