Imaging Imidazoline-I₂ Binding Sites in Porcine Brain Using ¹¹C-BU99008

Abstract

Changes in the density of imidazoline-I 2 binding sites have been observed in a range of neurologic disorders including Alzheimer's disease, Huntington's chorea, and glial tumor; however, the precise function of these sites remains unclear. A PET probe for I 2 binding sites would further our understanding of the target and may find application as a biomarker for early disease diagnosis. Compound BU99008 has previously been identified as a promising I 2 ligand from autoradiography studies, displaying high affinity and good selectivity toward the target. In this study, BU99008 was radiolabeled with 11 C in order to image the I 2 binding sites in vivo using PET. Methods: 11 C-BU99008 was radiolabeled by N-alkylation of the desmethyl precursor using 11 C-methyl iodide. A series of PET experiments was performed to investigate the binding of 11 C-BU99008 in porcine brains, in the presence or absence of a nonradiolabeled, competing I 2 ligand, BU224. Results: 11 C-BU99008 was obtained in good yield and specific activity. In vivo, 11 C-BU99008 displayed good brain penetration and gave a heterogeneous distribution with high uptake in the thalamus and low uptake in the cortex and cerebellum. 11 C-BU99008 brain kinetics were well described by the 1-tissue-compartment model, which was used to provide estimates for the total volume of distribution (V T ) across brain regions of interest. Baseline V T values were ranked in the following order: thalamus . striatum . hippocampus . frontal cortex $ cerebellum, consistent with the known distribution and concentration of I 2 binding sites. Administration of a selective I 2 binding site ligand, BU224, reduced the V T to near-homogeneous levels in all brain regions. Conclusion: 11 C-BU99008 appears to be a suitable PET radioligand for imaging the I 2 binding sites in vivo.

Evaluation of 11C-BU99008, a PET Ligand for the Imidazoline2 Binding Sites in Rhesus Brain

“…cortex . cerebellum, consistent with the known I 2 BS densities and results from human tissue-section autoradiography (18) and porcine PET imaging experiments (14). Although the cerebellum showed the lowest brain uptake, there was still a decrease in V T values after BU224 blockade (Fig.…”

“…cortex . cerebellum, Table 2) is consistent with reported I 2 BS densities and distribution determined by tissue-section autoradiography in humans (18) and in vivo pig PET (14). Furthermore, the mean rhesus V T values were significantly correlated (r 2 5 0.72; P , 0.05) with the mean V T values from our previous in vivo porcine 11 C-BU99008 PET data (inset, Fig.…”

“…In view of these in vitro data, and combined with a successful radiolabeling feasibility assessment, we progressed the development of BU99008 as a PET ligand via evaluation in porcine brain, in vivo (14). In that study, 11 C-BU99008 demonstrated reversible kinetics and a brain distribution consistent with the known binding site densities and localization for the I 2 BS protein and a dosedependent decrease in V T after administration of the selective I 2 BS blocker BU224.…”

“…We have recently reported the synthesis, in vitro and in vivo evaluation, and radiosynthesis of a PET radioligand for the I 2 BS, 11 C-BU99008 (13,14). In this article, we report the preclinical in vivo evaluation of 11 C-BU99008, for imaging I 2 BS, in the rhesus monkey brain.…”

Molecular aspects of monoamine oxidase B

“…New ligands enable detection and imaging studies for distribution and occupancy of I 2 sites (Tyacke et al, 2012, Kealey et al, 2013, Keller and Garcia-Sevilla, 2015. Further development will be aided by computational methods, both for structure-function relationships and for drug-like properties to ensure brain permeation (Moraes and de Azevedo, 2012, Nikolic and Agbaba, 2012, Vucicevic et al, 2015.…”