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

Abstract

18F-Labeled building blocks from the type of [18F]fluorophenylazocarboxylic-tert-butyl esters offer a rapid, mild, and reliable method for the 18F-fluoroarylation of biomolecules. Two series of azocarboxamides were synthesized as potential radioligands for dopamine D3 and the μ-opioid receptor, revealing compounds 3d and 3e with single-digit and sub-nanomolar affinity for the D3 receptor and compound 4c with only micromolar affinity for the μ-opioid receptor, but enhanced selectivity for the μ-subtype in comparison to the lead compound AH-7921. A “minimalist procedure” without the use of a cryptand and base for the preparation of 4-[18F]fluorophenylazocarboxylic-tert-butyl ester [18F]2a was established, together with the radiosynthesis of methyl-, methoxy-, and phenyl-substituted derivatives ([18F]2b–f). With the substituted [18F]fluorophenylazocarbylates in hand, two prototype azocarboxylates radioligands were synthesized by 18F-fluoroarylation, namely the methoxy azocarboxamide [18F]3d as the D3 receptor radioligand and [18F]4a as a prototype structure of the μ-opioid receptor radioligand. By introducing the new series of [18F]fluorophenylazocarboxylic-tert-butyl esters, the method of 18F-fluoroarylation was significantly expanded, thereby demonstrating the versatility of 18F-labeled phenylazocarboxylates for the design of potential radiotracers for positron emission tomography .

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

“…With these criteria in mind, we designed a set of ligands to explore modifications in the chemical space in close proximity to the lead 3a . 18 This narrow frame without major deviations from both weight and structure was used to emphasize effects of fluorination in a structure–activity relationship focused on agonist potency and binding affinity. New derivatives ( 3a–l , 4a–d , and 5a ) were obtained in yields of 56–79% with a purity of >97% and tested for receptor binding, agonist activity, and basic pharmacological properties.…”

“… b K i values represent the mean of two independent experiments ±SD, each performed in triplicate. c K i values ± SEM taken from ref ( 18 ). d K i values represent the mean of three to four independent experiments ±SEM, each performed in triplicate.…”

“…The receptor binding affinities of synthesized compounds toward the DOR, KOR, and MOR subtypes were studied using transiently transfected human embryonic kidney (HEK) 293T cells using [ 3 H]diprenorphine as the radioligand ( Table 1 ), following the procedure as described previously. 18 , 20 DAMGO [MOR, K i = 25 ± 4.4 nM ( n = 9); DOR, K i = 1000 ± 150 nM ( n = 6); KOR, K i = 4200 ± 1200 nM ( n = 6)), Leu-Enk (MOR, K i = 160 ± 30 nM ( n = 9); DOR, K i = 6.8 ± 0.58 nM ( n = 6); KOR, K i = 15,000 ± 1200 nM ( n = 6)] and naloxone [MOR, K i = 3.3 ± 0.33 nM ( n = 18); DOR, K i = 56 ± 4.6 nM ( n = 18); KOR, K i = 16 ± 3.9 nM ( n = 10)] were included as references. The potency of the compounds was determined in Chinese hamster ovary (CHO) cells stably transfected with the DOR, KOR, and MOR by measuring the agonist-mediated inhibition of intracellular cAMP response ( Figure 2 ), using references deltorphin (DOR), U50488 (KOR), and DAMGO (MOR).…”

“…A batch of 5–10 GBq [ 18 F]fluoride was extracted from target water with crypt-222 (10 mg, 27 μmol) and K 2 CO 3 (5 mg, 36 μmol) in MeCN–H 2 O (4:1, 0.6 mL) and dried using two cycles of MeCN (0.6 mL) at 130 °C for a total time period of 20 min. The temperature of the reactor was then decreased to 75 °C, and NO 2 -to- 18 F substitution was achieved by the addition of 3l (3.5 mg, 10 μmol) in anhydrous DMF (0.6 mL) at 75 °C for 3 min. The reaction mixture was diluted with H 2 O (3.5 mL) and passed through the first C 18 cartridge.…”

Aliphatic 18F‐Radiofluorination: Recent Advances in the Labeling of Base‐Sensitive Substrates**

“…It has mainly seen implementation in aromatic labeling of onium salts but has promising application for aliphatic labeling as well (Figure 4). [61][62][63] So far, this approach has not been widely applied, possibly due to the fact that it is not easily compatible with many commercial synthesis modules. [64] Another method to reduce the basicity within the reaction mixture is to use non-basic mesylate anions (OMs À , ) for preconditioning of the AEC and subsequent [ 18 F]F À elution from the cartridge before a fixed amount of base is added to the eluate.…”