Synthesis and opioid receptor binding of indium (III) and [111In]-labeled macrocyclic conjugates of diprenorphine: novel ligands designed for imaging studies of peripheral opioid receptors

Abstract

Radiolabeled diprenorphine (DPN) and analogs are widely used ligands for non-invasive brain imaging of opioid receptors. To develop complementary radioligands optimized for studies of the peripheral opioid receptors, we prepared a pair of hydrophilic DPN derivatives, conjugated to the macrocyclic chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), for complexation with trivalent metals. The non-radioactive indium (III) complexes, tethered to the C6-oxygen position of the DPN scaffold by 6- to 9-atom spacers, displayed high affinities for binding to μ, δ and κ opioid receptors in vitro. Use of the 9-atom linker conferred picomolar affinities equipotent to those of the parent ligand DPN. The [111In]-labeled complexes were prepared in good yield (>70%), with high radiochemical purity (~99%) and high specific radioactivity (>4000 mCi/μmol). Their log D7.4 values were −2.21 to −1.66. In comparison, DPN is lipophilic, with a log D7.4 of +2.25. Further study in vivo is warranted to assess the suitability of these [111In]-labeled DPN-DOTA conjugates for imaging trials.

Design, synthesis and evaluation of 111In labeled DOTA-conjugated tetrapeptides having high affinity and selectivity for mu opioid receptors

“…Binding studies 4.7.1. Competition binding-Assays to establish opioid receptor affinities of nonradioactive peptides were performed using membranes prepared from frozen male Hartley guinea pig brains (μ, κ) or fresh CD-1 mouse brains (δ) as previously described [21,23,69]. Protein concentrations were measured by the bicinchoninic acid assay using BSA for calibration.…”

“…Hydrophilic [ 111 In]-labeled DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10tetraacetic acid) conjugates of the small molecule naltrindole (NTI) [21], as well as [ 64 Cu]labeled NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) conjugates of the peptide H-Tyr-Tic-Phe-Phe-NH 2 (TIPP) [22], maintain high affinity for δ sites. Similarly, [ 111 In]labeled conjugates of diprenorphine [23] and [ 99m Tc]-labeled conjugates of naltrexone [24] are under investigation as universal (μ, δ, κ) opioid receptor radioligands.…”

Tactics for preclinical validation of receptor-binding radiotracers

“…Multiple determinations should be made to ensure the findings are reproducible. Representative procedures are given in references [55] and [98,124] for lipophilic and hydrophilic radiotracers, respectively.…”

NMR Analysis of a Series of 6,14‐Ethenomorphinan Derivatives as PET Precursors and Reference Substances**

“…[8] Recently, Lever et al reported the synthesis and opioid receptor binding properties of 111 In-labelled diprenorphine DOTA conjugates for imaging by means of single photon emission computer tomography (SPECT). [9] In the last two decades, comprehensive reviews [8,[10][11][12] have summarized the diversity of radiotracers available in the field of opioid receptor imaging.…”