Investigation of [¹¹C]carfentanil for mu opioid receptor quantification in the rat brain

[¹¹C]Carfentanil ([¹¹C]CFN) is the only selective carbon-11 labeled radiotracer currently available for positron emission tomography (PET) imaging of mu opioid receptors (MORs). Though used extensively in clinical research, [¹¹C]CFN has not been thoroughly characterized as a tool for preclinical PET imaging. As we were occasionally observing severe vital sign instability in rat [¹¹C]CFN studies, we set out to investigate physiological effects of CFN mass and to explore its influence on MOR quantification. In anesthetized rats (n = 15), significant dose-dependent PCO₂ increases and heart rate decreases were observed at a conventional tracer dose range (IV, > 100 ng/kg). Next, we conducted baseline and retest [¹¹C]CFN PET scans over a wide range of molar activities. Baseline [¹¹C]CFN PET studies (n = 27) found that nondisplaceable binding potential (BP_ND) in the thalamus was positively correlated to CFN injected mass, demonstrating increase of MOR availability at higher injected CFN mass. Consistently, when CFN injected mass was constrained < 40 ng/kg (~ 10% MOR occupancy in rats), baseline MOR availability was significantly decreased. For test-retest variability (TRTV), better reproducibility was achieved by controlling CFN injected mass to limit the difference between scans. Taken together, we report significant cardiorespiratory depression and a paradoxical influence on baseline MOR availability at conventional tracer doses in rats. Our findings might reflect changes in cerebral blood flow, changes in receptor affinity, or receptor internalization, and merits further mechanistic investigation. In conclusion, rat [¹¹C]CFN PET requires stringent quality assurance of radiotracer synthesis and mass injected to avoid pharmacological effects and limit potential influences on MOR quantification and reproducibility.