Validation of a pharmacological imaging challenge using 11C-buprenorphine and 18F-2-fluoro-2-deoxy-D-glucose positron emission tomography to study the effects of buprenorphine to the rat brain

Decreased opioid receptor availability and impaired neurometabolic coupling as signatures of morphine tolerance in male rats: A positron emission tomography study

Translational neuroimaging techniques are needed to address the impact of opioid tolerance on brain function and quantitatively monitor the impaired neuropharmacological response to opioids at the CNS level. A multiparametric PET study was conducted in rats. Rats received morphine daily to induce tolerance (15 mg/kg/day for 5 days), followed by 2-day withdrawal. Then, opioid effects were precipitated using a buprenorphine challenge (0.1 mg/kg, s.c, BUP-challenge), which safely enables full occupancy of available mu-opioid receptors (MOR). The impact of the BUP-challenge on the pain threshold was estimated using the hot-plate test. The corresponding availability of MOR was estimated using [11C]buprenorphine PET imaging (n = 4). The brain glucose metabolism was investigated using [18F]2-fluoro-D-deoxy-glucose ([18F]FDG) PET imaging after the BUP-challenge or saline (n = 5-6). Opioid tolerance was confirmed by the attenuated antinociceptive response to the BUP-challenge in morphine-treated rats compared to saline controls (p < 0.001). In tolerant rats, [11C]buprenorphine binding was decreased in MOR-rich regions (p < 0.01), and the baseline uptake of [18F]FDG was decreased (p < 0.05). The BUP-challenge decreased [18F]FDG uptake to a lower extent in tolerant rats compared with opioid-naive animals (p < 0.05), suggesting impaired neurometabolic coupling. Moreover, the impact of the BUP-challenge on the neurometabolic connectivity across brain regions was disrupted by opioid tolerance. PET imaging enables the study of the decreased availability of MOR and impaired neurometabolic coupling as molecular signatures of opioid tolerance in rats. Combining molecular neuroimaging with a suitable pharmacological challenge may provide a translational and quantitative paradigm to explore opioid tolerance at the CNS level in parallel to pharmacodynamics.

Selective Mu-Opioid Receptor Imaging Using 18F-Labeled Carfentanils

Carfentanil, a highly potent synthetic opioid, paradoxically serves as a crucial positron emission tomography (PET) imaging tool in neurobiological studies of the mu-opioid receptor (MOR) system when labeled with carbon-11 ([11C]CFN). However, its clinical research use is hindered by extreme potency and the limited availability of short-lived carbon-11 (t1/2 = 20.4 min). We present fluorine-18-labeled fluorocarfentanils ([18F]FCFNs), which can be produced at higher molar activity, allowing for lower mass doses and benefiting from the longer half-life of fluorine-18 (t1/2 = 109.8 min), facilitating broader accessibility. Using copper-mediated radiofluorination, we synthesized a small [18F]FCFN library and conducted preclinical imaging evaluations. Two candidates, o-18F-1 and p-18F-2, showed optimal brain uptake, favorable pharmacokinetics, and high MOR-specific binding. Selectivity was confirmed through in vitro binding assays and in vivo PET scans. These [18F]FCFNs are promising for accessible human brain MOR imaging.

Endogenous opiates and behavior: 2023

This paper is the forty-sixth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2023 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug and alcohol abuse (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Brain PET imaging using 11C-flumazenil and 11C-buprenorphine does not support the hypothesis of a mutual interaction between buprenorphine and benzodiazepines at the neuroreceptor level

Among opioids, buprenorphine presents a favorable safety profile with a limited risk of respiratory depression. However, fatalities have been reported when buprenorphine is combined to a benzodiazepine. Potentiation of buprenorphine interaction with opioid receptors (ORs) with benzodiazepines, and/or vice versa, is hypothesized to explain this drug-drug interaction (DDI). The mutual DDI between buprenorphine and benzodiazepines was investigated at the neuroreceptor level in nonhuman primates (n = 4 individuals) using brain PET imaging and kinetic modelling. The binding potential (BPND) of benzodiazepine receptor (BzR) was assessed using 11C-flumazenil PET imaging before and after administration of buprenorphine (0.2 mg, i.v.). Moreover, the brain kinetics and receptor binding of buprenorphine were investigated in the same individuals using 11C-buprenorphine PET imaging before and after administration of diazepam (10 mg, i.v.). Outcome parameters were compared using a two-way ANOVA. Buprenorphine did not impact the plasma nor brain kinetics of 11C-flumazenil. 11C-flumazenil BPND was unchanged following buprenorphine exposure, in any brain region (p > 0.05). Similarly, diazepam did not impact the plasma or brain kinetics of 11C-buprenorphine. 11C-buprenorphine volume of distribution (VT) was unchanged following diazepam exposure, in any brain region (p > 0.05). To conclude, our PET imaging findings do not support a neuropharmacokinetic or neuroreceptor-related mechanism of the buprenorphine/benzodiazepine interaction.