Integrated Strategy for Use of Positron Emission Tomography in Nonhuman Primates to Confirm Multitarget Occupancy of Novel Psychotropic Drugs: An Example with AZD3676

Abstract

Positron emission tomography (PET) is widely applied in central nervous system (CNS) drug development for assessment of target engagement in vivo. As the majority of PET investigations have addressed drug interaction at a single binding site, findings of multitarget engagement have been less frequently reported and have often been inconsistent with results obtained in vitro. AZD3676 [N,N-dimethyl-7-(4-(2-(pyridin-2-yl)ethyl)piperazin-1-yl) benzofuran-2-carboxamide] is a novel combined serotonin (5-hydroxytryptamine) 5-HT 1A and 5-HT 1B receptor antagonist that was developed for the treatment of cognitive impairment in Alzheimer's disease. Here, we evaluated the properties of AZD3676 as a CNS drug by combining in vitro and ex vivo radioligand binding techniques, behavioral pharmacology in rodents, and PET imaging in nonhuman primates. Target engagement in the nonhuman primate brain was assessed in PET studies by determination of drug-induced occupancy using receptorselective radioligands. AZD3676 showed preclinical properties consistent with CNS drug potential, including nanomolar receptor affinity and efficacy in rodent models of learning and memory. In PET studies of the monkey brain, AZD3676 inhibited radioligand binding in a dose-dependent manner with similar affinity at both receptors. The equally high affinity at 5-HT 1A and 5-HT 1B receptors as determined in vivo was not predicted from corresponding estimates obtained in vitro, suggesting more than 10-fold selectivity for 5-HT 1A versus 5-HT 1B receptors. These findings support the further integrated use of PET for confirmation of multitarget occupancy of CNS drugs. Importantly, earlier introduction of PET studies in nonhuman primates may reduce future development costs and the requirement for animal experiments in preclinical CNS drug development programs.

In Vivo Receptor Visualization and Evaluation of Receptor Occupancy with Positron Emission Tomography

“…A human PET experiment using flesinoxan and ziprasidone as test compounds confirmed competitive binding with respect to 2–12.1 in the brain, but there was a discrepancy compared with the results in rats, possibly due to differences in the distributions of 5-HT receptors in the brain between humans and rats. Other PET experiments using 2–12.1 confirmed competition at 5-HT receptors in the brain, − supporting the usefulness of this tracer.…”

“…[ 11 C]­WAY100635 ( 2–12.1 ) is widely used as a PET tracer targeting 5-HT 1A . − This tracer has high binding affinity for subtype 1A (IC 50 = 8.9 nM vs [ 3 H]­prazosin), and receptor occupancy of many test compounds has been evaluated in animals and humans . In addition, NAD298, a candidate for the treatment of depression targeting 5-HT 1A , was confirmed to exhibit competitive binding in monkey brain with ED 50 = 4–5 nM.…”

Aligning physiology with psychology: Translational neuroscience in neuropsychiatric drug discovery

“…Pharmacokinetic measures such as determination of the plasma concentrations of the experimental compound provides valuable, but indirect measures of potential target engagement in the brain, as well as providing dose-equivalence measures between animals and humans. However, neuroimaging techniques such as PET is an important tool with which in vivo target engagement of novel molecules can be determined directly in human subjects and animals (Varnäs et al, 2016).…”

Analysis of PK11195 concentrations in rodent whole blood and tissue samples by rapid and reproducible chromatographic method to support target-occupancy PET studies

“…In human clinical trials, the percentage TO quantification is supported by pharmacokinetic/pharmacodynamic (PK/PD) analysis of arterial or venous blood for each individual in order to determine plasma or whole blood levels of the blocking/displacement agent at a given time point [3,4]. Also, this approach has been extensively and directly applied to PET TO studies in large animals [5,6]. Unfortunately, individual PK/PD analysis of drugs used in PET TO studies of small animals, specifically, rodents, is less standard [7,8] and at best the impact of using population-based assessments in rodents is poorly described.…”