The continuous rise in opioid overdoses in the United States is predominantly driven by very potent synthetic opioids, mostly fentanyl and its derivatives (fentanyls). Although naloxone (NLX) has been shown to effectively reverse overdoses by conventional opioids, there may be a need for higher or repeated doses of NLX to revert overdoses from highly potent fentanyls. Here, we used positron emission tomography (PET) to assess NLX’s dose-dependence on both its rate of displacement of [11C]carfentanil ([11C]CFN) binding and its duration of mu opioid receptor (MOR) occupancy in the male rat brain. We showed that clinically relevant doses of intravenously (IV) administered NLX (0.035 mg/kg, Human Equivalent Dose (HED) 0.4 mg; 0.17 mg/kg, HED 2 mg) rapidly displaced the specific binding of [11C]CFN in the thalamus in a dose-dependent manner. Brain MOR occupancy by IV NLX was greater than 90% at 5 min after NLX administration for both doses, but at 27.3 min after 0.035 mg/kg dose and at 85 min after 0.17 mg/kg NLX, only 50% occupancy remained. This indicates that the duration of NLX occupancy at MORs is short-lived. Overall, these results show that clinically relevant doses of IV NLX can promptly displace fentanyls at brain MORs, but repeated or higher NLX doses may be required to prevent re-narcotization following overdoses with long-acting fentanyls.

Adenosine receptor (AR) radiotracers for positron emission tomography (PET) have provided knowledge on the in vivo biodistribution of ARs in the central nervous system (CNS), which is of therapeutic interest for various neuropsychiatric disorders. Additionally, radioligands that can image changes in endogenous adenosine levels in different physiological and pathological conditions are still lacking. The binding of known antagonist adenosine A1 receptor (A1R) radiotracer, [11C]­MDPX, failed to be inhibited by elevated endogenous adenosine in a rodent PET study. Since most of the known AR PET radiotracers were antagonists, we propose that an A1R agonist radioligand may possess higher sensitivity to measure changes in endogenous adenosine concentration. Herein, we report our latest findings toward the development of a full agonist adenosine A1 radioligand for PET. Based on a 3,5-dicyanopyridine template, 16 new derivatives were designed and synthesized to optimize both binding affinity and functional activity, resulting in two full agonists (compounds 27 and 29) with single-digit nanomolar affinities and good subtype selectivity (A1/A2A selectivity of ∼1000-fold for compound 27 and 29-fold for compound 29). Rapid O-[11C]­methylation provided [11C]27 and [11C]29 in high radiochemical yields and radiochemical purity. However, subsequent brain PET imaging in rodents showed poor brain permeability for both radioligands. An in vivo PET study using knockout mice for MDR 1a/a, BCRP, and MRP1 indicated that these compounds might be substrates for brain efflux pumps. In addition, in silico evaluation using multiparameter optimization identified high molecular weight and high polar surface area as the main molecular descriptors responsible for low brain penetration. These results will provide further insight toward development of full agonist adenosine A1 radioligands and also highly potent CNS A1AR drugs.

The continuous rise in opioid overdoses in the United States is predominantly driven by very potent synthetic opioids, mostly fentanyl and its derivatives (fentanyls). Although naloxone (NLX) has been shown to effectively reverse overdoses by conventional opioids, there may be a need for higher or repeated doses of NLX to revert overdoses from highly potent fentanyls. Here, we used positron emission tomography (PET) to assess NLX’s dose-dependence on both its rate of displacement of [11C]carfentanil ([11C]CFN) binding and its duration of mu opioid receptor (MOR) occupancy in the male rat brain. We showed that clinically relevant doses of intravenously (IV) administered NLX (0.035mg/kg, Human Equivalent Dose(HED) 0.4mg; 0.17mg/kg, HED 2mg) rapidly displaced the specific binding of [11C]CFN in thalamus in a dose-dependent manner. Brain MOR occupancy by IV NLX was greater than 90% at 5 minutes after NLX administration for both doses, but only 50% occupancy remained at 27.3 min and at 85 min after 0.035mg/Kg and 0.17 mg/Kg NLX, respectively. This indicates that the duration of NLX occupancy at MORs is short-lived. Overall, these results show that clinically relevant doses of IV NLX can promptly displace fentanyls at brain MORs, but that repeated or higher NLX doses may be required to prevent re-narcotization following overdoses with long-acting fentanyls.