Modeling serotonin 1A receptors in baboon using the agonist tracer [11C]CUNBID-101 and positron emission tomography

Background: Several lines of evidence implicate serotonin 1A (5-HT1A) receptors in the pathophysiology of major neuropsychiatric disorders. In vivo quantification of 5-HT1A receptors to date has relied on various 5-HT1A antagonists. We have recently reported [11C]-[O-Methyl-11C] 2- {4- [4- (7- methoxynaphthalen-1-yl) piperazin-1-yl] -butyl} -4-methyl-2H- [1,2,4] triazine-3,5-dione ([11C]MPT) as a promising 5-HT1A receptor agonist radiotracer in baboons. Although [11C]MPT (5-HT1A Ki = 1.4 nM, Emax = 95% and EC50 = 0.05 nM) demonstrated specific binding the rate of decay of radioactivity in all brain regions was relatively slow in baboons making quantification difficult. Structure activity relationship studies suggest that phenyl analogues of MPT are excellent 5-HT1A agonists. The 2-methoxyphenyl analogue MMT possesses a 5-HT1A Ki = 0.7 nM, EC50 = 0.3 nM, and Emax = 95%. However [11C]MMT did not show specific binding in baboons. 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) as a superior agonist PET ligand than [11C]MPT. Here we report the evaluation of in vivo modeling options for [11C]MMP in anesthetized baboons under its new name, [11C]CUNBID-101. Methods: A series of PET scans were performed in two male baboons (Papio anubis) with an ECAT EXACT HR+ scanner. 4.88 ± 0.52 mCi (SA 1.84 ± 0.39 Ci/μmol) of [11C]CUNBID-101 was injected as an i.v. bolus and emission data were collected for 120 min in 3D mode. We evaluated several different models (one- and two-tissue compartment (1TC, 2TC) iterative and non-iterative kinetic models, likelihood estimation in graphical analysis (LEGA; a bias-free alternative to the graphical method), and basis pursuit), using binding potential (BP = Bmax/KD). To assess the performance of each model, we compared Results: using five different metrics (percent difference and within subject mean sum of squares (WSMSS) for reproducibility, variance across subjects, identifiability based on bootstrap resampling of residuals for each method, and time stability analysis to determine minimal required scanning time). Models were also evaluated in scans performed after injections of the 5-HT1A antagonist WAY100635 (0.5 mg/kg, i.v.) and the 5-HT1A agonist 8-OH-DPAT (2 mg/kg, i.v.). Results: and Conclusions: All metabolites are more polar than [11C]CUNBID-101 and no significant change in metabolites was observed in the blocking studies. The free fraction is 59 ± 3%. 100 minutes of scanning time is adequate and for ROI-level analysis, LEGA gives best Results:. Median test-retest percent difference for BP is 9.07% ± 4.33% across all regions; WSMSS = 2.22; variance = 4.42; bootstrap identifiability = 0.59. Pre-administration of WAY100635 and 8-OH-DPAT resulted in 87% ± 5% and 76% ± 4% average reductions in BP values respectively across all ROIs. Based on a measurable free fraction, high affinity, selectivity, blood brain permeability and favorable plasma and brain kinetics, [11C]CUNBID-101 is an excellent candidate for imaging high affinity 5-HT1A receptors.