Application of the PET ligand [11C]ORM-13070 to examine receptor occupancy by the α2C-adrenoceptor antagonist ORM-12741: translational validation of target engagement in rat and human brain

Abstract

Background: Availability of the α2C-adrenoceptor (α2C-AR) positron emission tomography (PET) tracer, [11C]ORM-13070, and the α2C-AR antagonist ORM-12741 allows probing of the roles of this G-protein coupled receptor subtype in brain function, both in healthy humans and in patients with various brain disorders. This translational study employed [11C]ORM-13070 autoradiography and PET to determine α2C-AR occupancy by ORM-12741 in rat and human brain, respectively.

Results: ORM-12741 has high affinity (Ki: 0.08 nM) and potent antagonist activity (Kb: 0.04 nM) as well as selectivity (Ki estimates for the human α2A-AR and α2B-AR were 8.3 nM and 0.8 nM, respectively) for the human α2C-AR subtype. [11C]ORM-13070 had highest uptake in the basal ganglia of rat and human brain. Pretreatment with ORM-12741 inhibited [11C]ORM-13070 binding in rat striatum in a time- and dose-dependent manner at 10 and 50 µg/kg (s.c.) with an EC50 estimate of 1.42 ng/mL in rat plasma, corresponding to protein-free drug concentration of 0.23 nM. In the living human brain, time- and dose-related α2C-AR occupancy was detected with EC50 estimates of 24 ng/mL and 31 ng/mL for the caudate nucleus and putamen, respectively, corresponding to protein-free concentrations in plasma of 0.07 nM and 0.1 nM. Modelling-based maximum α2C-AR occupancy estimates were 63% and 52% in the caudate nucleus and the putamen, respectively.

Conclusions: ORM-12741 is a selective α2C-AR antagonist which penetrates the rat and human brain to occupy α2C-ARs in a manner consistent with its receptor pharmacology. Trial registration number and date of registration: ClinicalTrial.cov NCT00829907. Registered 11 December 2008. https://clinicaltrials.gov/ .

Keywords: Brain α2C-adrenoceptors; ORM-12741; Receptor occupancy; [11C]ORM-13070 PET; α2C-Adrenoceptor antagonists.

Conflict of interest statement

JR and KK are employees of Orion Pharma. MSh and JS were employees of Orion Pharma at the time of the study conduct. JOR, MSc, JV, PM, OS and EA were engaged in contract research for Orion Pharma in the context of the current study.

Figures

Fig. 1

a Representative 40 µm coronal brain sections from rats pre-treated with vehicle or ORM-12741 (2, 10, 50 µg/kg subcutaneously) at 10 min post [11C]ORM-13070 intravenous injection. Sections are from the level of the caudate-putamen, olfactory tubercle and frontal cortex. The colour pseudo-autoradiograms show the regional distribution of the [11C]-label, red highest, blue lowest uptake. b The relationship between ORM-12741 (10–1000 µg/kg s.c.) plasma levels and α2C-adrenoceptor occupancy in rat striatum. The blue line represents a binding hyperbole derived from nonlinear regression analysis with a sigmoidal maximum possible effect model. The red lines represent 95% confidence intervals

Fig. 2

Pharmacokinetic profile of ORM-12741 showing mean (± standard error of mean) plasma concentrations following oral administration of four single doses to fasted healthy male volunteers

Fig. 3

A representative set of human brain PET SUV images from one healthy male volunteer at baseline and 1 h, 3.5 h and 6.5 h after 60 mg of ORM-12741

Fig. 4

The relationship between ORM-12741 plasma levels and α2C-adrenoceptor occupancy in human a caudate nucleus and b in putamen. The blue line represents binding hyperboles derived from nonlinear regression analysis with a sigmoidal maximum possible effect model. The red lines represent 95% confidence intervals. Concentrations of ORM-12741 in plasma represent mean values of samples collected at the start and end of the PET scan. Each data point represents an individual occupancy value at one time point