Quantitative μ-Opioid Receptor PET with [¹⁸F]FCFN in Nonhuman Primates: Naloxone Blockade and Within-Scan...

Background/Objectives: Quantitative PET imaging of μ-opioid receptor (MOR) availability and pharmacologic engagement can advance in vivo studies of opioid receptor biology. We previously evaluated a series of ¹⁸F-labeled carfentanil analogs and prioritized the regioisomer [¹⁸F]fluorocarfentanil ([¹⁸F]FCFN) based on favorable brain kinetics, regional distribution, and naloxone-sensitive binding. Here, we provide proof-of-concept evaluation of [¹⁸F]FCFN in rhesus macaques to assess its potential for quantitative, pharmacologically responsive MOR PET. Methods: [¹⁸F]FCFN was produced by automated radiosynthesis. Dynamic [¹⁸F]FCFN PET scans were performed in two rhesus macaques under baseline conditions, after low- (0.005 mg/kg) and high-dose (0.2 mg/kg) naloxone pretreatment, and during within-scan intravenous or intranasal naloxone challenge. For arterial input scans, two-tissue compartment modeling with metabolite-corrected arterial input functions was used to estimate total distribution volume (V_T). Nondisplaceable binding potential (BP_ND) was estimated using multilinear reference tissue model 2 with the cerebellum as the candidate reference region. Results: Automated radiosynthesis produced [¹⁸F]FCFN in high radiochemical yield and molar activity, enabling low-mass PET injections (0.09-0.32 μg). [¹⁸F]FCFN showed rapid brain uptake and MOR-consistent regional distribution, with lowest uptake in the cerebellum. Regional BP_ND correlated positively with V_T at baseline (R² = 0.983, p < 0.0001) and after high-dose blockade (R² = 0.522, p < 0.004). Naloxone pretreatment reduced regional BP_ND in a dose-dependent manner. Within-scan intravenous and intranasal naloxone challenges also produced marked reductions in BP_ND. Conclusions: [¹⁸F]FCFN enables quantitative, pharmacologically sensitive MOR PET imaging in nonhuman primates at low injected masses. Concordant plasma-input and reference-tissue measures, dose-dependent blockade, and proof-of-concept within-scan displacement support its use for measuring MOR availability and dynamic antagonist engagement across administration routes.