Reaching out for Sensitive Evaluation of the Mu Opioid Receptor in Vivo: Positron Emission Tomography Imaging of the Agonist [¹¹C]AH7921

Abstract

Imaging of the mu opioid receptor (MOR) availability with positron emission tomography (PET) is a pertinent challenge in Neuroscience. Both, regulation of receptor expression and occupancy by endogeneous opioids play into cognitive and behavioral phenotypes of healthy function and disease. Receptor expression in the active and inactive states can be measured using high affinity radioagonist and radioantagonist PET tracers, respectively. Occupancy assessment requires radioligands showing competitive and reversible binding with moderate affinity to the MOR, which may lead to physical extinction of the receptor specific signal in vivo. We investigated a moderately potent, selective MOR agonist in rat to test if a radiotracer design paradigm tailored to competition with endogeneous opioids leads to viable imaging results. The benzamide 3,4-dichlorobenzenecarboxylic acid (dimethylamino)cyclohexyl)methyl amide (AH-7921, 1) was synthesized and characterized in rat brain using autoradiography and positron emission tomography. Compound 1 was found to activate with low nanomolar potency the MOR and to a lesser extent KOR as a full agonist. Concentration dependent binding studies with agonist and antagonist radioligands were conducted to assess competition behavior and obtain inhibition constants. Kinetic analysis of 3,4-dichlorobenzene[C]carboxylic acid (dimethylamino)cyclohexyl)methyl amide binding in rat brain resulted in low but reproducible binding potential in the thalamus (0.8 ± 0.1). A radioactive metabolite was detected in brain (17%, after 15 min). Nonetheless, we conclude that quantitative imaging of MOR availability is possible when using a moderate affinity radiotracer.

Pharmacological Characterization of Low-to-Moderate Affinity Opioid Receptor Agonists and Brain Imaging with 18F-Labeled Derivatives in Rats

“…The parent 3a has a K d of 1.8 nM, whereas carfentanil lies in the range of 0.03–0.08 nM. 6 − 8 , 15 …”

“…PET ligands derived from these scaffolds retain the lack of discrimination between different OR subtypes showing near identical affinities in all receptors, thereby confounding studies of single-receptor subtypes in normal function and disease. 14 , 15 …”

“…Herein, the structure is utilized as a template for the development of fluorinated analogues. 8 , 15 …”

[18F]Fluorophenylazocarboxylates: Design and Synthesis of Potential Radioligands for Dopamine D3 and μ-Opioid Receptor

“…Moreover, [ 18 F]fluorophenylazocarboxylic- tert -butyl esters could also be useful to obtain a series of potential μ-opioid receptor-selective radioligands based on the structure of AH-7921 ( Figure 1 A). 36 − 38 …”

Phosgene‐Free Carbonyl Insertion: Hot Advances in 11C‐Chemistry

“…The final RCY of 12 % exceeds the previously reported yield using Grignard reagents. [12] This method is advantageous over classical approaches to amide synthesis with metal organyls as these reagents are typically very sensitive to the atmosphere and must be handled with care to ensure final products have high molar activities. Overall, 11 Clabelled asymmetric amides were labelled using a facile transition metal catalyzed reaction with mild conditions.…”