Modeling considerations for¹¹C-CUMI-101, an agonist radiotracer for imaging serotonin 1A receptor in vivo with PET

Several lines of evidence demonstrate involvement of serotonin 1A receptors (5-HT_1ARs) in the pathophysiology of neuropsychiatric disorders such as depression, suicidal behavior, schizophrenia, and Alzheimer's disease. We recently published the synthesis and initial evaluation of [O-methyl-11C]2-(4- (4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5 (2H,4H)dione (¹¹C-MMP), a 5-HT_1AR agonist. Here we determine the optimal modeling parameters for ¹¹C-MMP under its new name, ¹¹C-CUMI-101, in Papio anubis. Methods: PET scans were performed on 2 adult male P. anubis; 166.5 MBq ± 43.0 (4.50 ± 1.16 mCi) of ¹¹C-CUMI-101 were injected as an intravenous bolus, and emission data were collected for 120 min in 3-dimensional mode. We evaluated 4 different models (1- and 2-tissue compartment iterative and noniterative kinetic models, basis pursuit, and likelihood estimation in graphical analysis [LEGA]), using binding potential (BP_F = B_max/K_d) (B_max = maximum number of binding sites; K_d = dissociation constant) as the outcome measure. Arterial blood sampling and metabolite-corrected arterial input function were used for full quantification of BP_F. To assess the performance of each model, we compared results using 6 different metrics (percentage difference, within subject mean sum of squares [WSMSS] for reproducibility; variance across subjects, intraclass correlation coefficient [ICC] for reliability; identifiability based on bootstrap resampling of residuals; and time stability analysis to determine minimal required scanning time) at each of 6 different scanning durations. Models were also evaluated on scans acquired after injecting the 5-HT _1A antagonist [N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2- pyridinyl)cyclohexanecarboxamide] [WAY100635]0.5mg/kg, intravenous) and the 5-HT_1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin) [8-OH-DPAT] 2 mg/kg, intravenous). Results: Allmetabolites aremore polar than ¹¹C-CUMI-101, and no significant change in metaboliteswas observed in the blocking studies. The free fraction is 59% ± 3%. We determined that 100 min of scanning time is adequate and that for the region-of-interest (ROI)-level analysis, the LEGA model gives the best results. The median test-retest percentage difference for BP_F is 11.15% ± 4.82% across all regions, WSMSS = 2.66, variance 56.07, ICC50.43, and bootstrap identifiability 50.59. Preadministration of WAY100635 and 8-OH-DPAT resulted in 87% and 76% average reductions in BPF values, respectively, across ROIs. Conclusion: On the basis of the measurable free fraction, high affinity and selectivity, adequate blood-brain permeability, and favorable plasma and brain kinetics, ¹¹C-CUMI-101 is an excellent candidate for imaging high-affinity 5-HT_1ARs in humans.