Use of the beta-imager for rapid ex vivo autoradiography exemplified with central nervous system penetrating neurokinin 3 antagonists

The neurokinin 3 (NK3) receptor antagonists represent a novel class of phar macological agents, which are currently under evaluation for the treatment of psychiatric disorders. An efficient brain penetration is one of the main prerequisites to further evaluate compounds displaying high potency to bin d the NK3 receptor. The present report describes a method for determining t he in vivo occupancy of central NK3 receptors after peripheral administrati on of drugs. An ex vivo measurement of NK3 receptor occupancy by quantitati ve autoradiography employing [H-3]senktide as the radioligand has been deve loped. The speed of the method, which is usually considered low due to the time dedicated to film exposure (from weeks to months), has been considerab ly increased by the use of the beta -imager. The high sensitivity of this n ew radiomager was used to visualize and quantitatively analyze the [H-3]sen ktide binding sites in brain sections within hours. Using this method, we h ave demonstrated that the reference NK3 antagonist SR142801 dose dependentl y occupied the NK3 receptors in the gerbil brain after subcutaneous adminis tration with an ED50 of 0.85 mg/kg. The less active enantiomer SR142806 occ upied the NK3 receptors only by 25% at the highest used dose of 10 mg/kg. T hese values are in accordance with the reported behavioral effects of the c ompounds. Our results indicate that ex vivo receptor occupancy measurements can be dependently used to predict the central activity of NK3 antagonists . More generally, the combination of ex vivo receptor autoradiography with the beta -imager detection constitutes a new and fast method to evaluate th e brain penetration of drug candidates.