Mapping human brain fatty acid amide hydrolase activity with PET

Endocannabinoid tone has recently been implicated in a number of prevalent neuropsychiatric conditions. [¹¹C]CURB is the first available positron emission tomography (PET) radiotracer for imaging fatty acid amide hydrolase (FAAH), the enzyme which metabolizes the prominent endocannabinoid anandamide. Here, we sought to determine the most suitable kinetic modeling approach for quantifying [¹¹C]CURB that binds selectively to FAAH. Six healthy volunteers were scanned with arterial blood sampling for 90 minutes. Kinetic parameters were estimated regionally using a one-tissue compartment model (TCM), a 2-TCM with and without irreversible trapping, and an irreversible 3-TCM. The 2-TCM with irreversible trapping provided the best identifiability of PET outcome measures among the approaches studied (coefficient of variation (COV) of the net influx constant K_i and the composite parameter λk₃ (λ=K₁/k₂) <5%, and COV(k₃)<10%). Reducing scan time to 60 minutes did not compromise the identifiability of rate constants. Arterial spin labeling measures of regional cerebral blood flow were only slightly correlated with K_i, but not with k₃ or λk₃. Our data suggest that λk₃ is sensitive to changes in FAAH activity, therefore, optimal for PET quantification of FAAH activities with [¹¹C]CURB. Simulations showed that [¹¹C]CURB binding in healthy subjects is far from a flow-limited uptake.