11C-ER176, a Radioligand for 18-kDa Translocator Protein, Has Adequate Sensitivity to Robustly Image All Three Affinity Genotypes in Human Brain

Masamichi Ikawa, Talakad G Lohith, Stal Shrestha, Sanjay Telu, Sami S Zoghbi, Sabrina Castellano, Sabrina Taliani, Federico Da Settimo, Masahiro Fujita, Victor W Pike, Robert B Innis, Biomarkers Consortium Radioligand Project Team, Masamichi Ikawa, Talakad G Lohith, Stal Shrestha, Sanjay Telu, Sami S Zoghbi, Sabrina Castellano, Sabrina Taliani, Federico Da Settimo, Masahiro Fujita, Victor W Pike, Robert B Innis, Biomarkers Consortium Radioligand Project Team

Abstract

For PET imaging of 18-kDa translocator protein (TSPO), a biomarker of neuroinflammation, most second-generation radioligands are sensitive to the single nucleotide polymorphism rs6971; however, this is probably not the case for the prototypical agent 11C-PK11195 (11C-labeled N-butan-2-yl-1-(2-chlorophenyl)-N-methylisoquinoline-3-carboxamide), which has a relatively lower signal-to-noise ratio. We recently found that 11C-ER176 (11C-(R)-N-sec-butyl-4-(2-chlorophenyl)-N-methylquinazoline-2-carboxamide), a new analog of 11C-(R)-PK11195, showed little sensitivity to rs6971 when tested in vitro and had high specific binding in monkey brain. This study sought, first, to determine whether the sensitivity of 11C-ER176 in humans is similar to the low sensitivity measured in vitro and, second, to measure the nondisplaceable binding potential (BPND, or the ratio of specific-to-nondisplaceable uptake) of 11C-ER176 in human brain.

Methods: Nine healthy volunteers-3 high-affinity binders (HABs), 3 mixed-affinity binders (MABs), and 3 low-affinity binders (LABs)-were studied with whole-body 11C-ER176 PET imaging. SUVs from 60 to 120 min after injection derived from each organ were compared between genotypes. Eight separate healthy volunteers-3 HABs, 3 MABs, and 2 LABs-underwent brain PET imaging. The 3 HABs underwent a repeated brain scan after TSPO blockade with XBD173 (N-benzyl-N-ethyl-2-(7-methyl-8-oxo-2-phenylpurin-9-yl)acetamide) to determine nondisplaceable distribution volume (VND) via Lassen occupancy plotting and thereby estimate BPND in brain.

Results: Regional SUV averaged from 60 to 120 min after injection in brain and peripheral organs with high TSPO densities such as lung and spleen were greater in HABs than in LABs. On the basis of VND determined via the occupancy plot, the whole-brain BPND for LABs was estimated to be 1.4 ± 0.8, which was much lower than that for HABs (4.2 ± 1.3) but about the same as that for HABs with 11C-PBR28 ([methyl-11C]N-acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine)) (∼1.2).

Conclusion: Obvious in vivo sensitivity to rs6971 was observed in 11C-ER176 that had not been expected from in vitro studies, suggesting that the future development of any improved radioligand for TSPO should consider the possibility that in vitro properties will not be reflected in vivo. We also found that 11C-ER176 has adequately high BPND for all rs6971 genotypes. Thus, the new radioligand would likely have greater sensitivity in detecting abnormalities in patients.

Keywords: 11C-ER176; 18-kDa translocator protein (TSPO); XBD173; positron emission tomography; rs6971 polymorphism.

© 2017 by the Society of Nuclear Medicine and Molecular Imaging.

Figures

FIGURE 1.
FIGURE 1.
Chemical structures of 3 PET ligands for TSPO and in vitro affinity ratio of HABs to LABs in human brain tissue. Data are from previous publications (9,10).
FIGURE 2.
FIGURE 2.
Time–activity curves for lung (A) and brain (B) after intravenous injection of 11C-ER176 for each genotype. Error bars denote SD. Conc = concentration.
FIGURE 3.
FIGURE 3.
Brain uptake and plasma radioactivity concentrations (conc) of 11C-ER176 in representative HAB at baseline (○) and after blockade with 90 mg of XBD173 (●). (A) Brain time–activity curves from gray matter with unconstrained 2TCM fitting. (B) Time courses of parent concentration in arterial plasma fitted by multiplying triexponential-fitted total plasma radioactivity and sigmoid-fitted plasma parent fraction.
FIGURE 4.
FIGURE 4.
Time–stability analysis: VT obtained from both baseline scans for each genotype and blocked scans for HABs, as well as its identifiability, is plotted as function of duration of image acquisition. (A) VT was calculated for putamen using unconstrained 2TCM with increasingly truncated acquisition times. Values are normalized as percentage of terminal value attained from 90 min of imaging. (B) Corresponding percentage SE, which is inversely proportional to identifiability, is plotted. Data are mean ± SD.
FIGURE 5.
FIGURE 5.
Lassen plot to determine VND (A) or VND/fP (B) of 11C-ER176 in brain of 3 HABs. Each point represents brain region in individual subject. Pearson r2 is shown for each relationship. P < 0.0001 for all correlations. For these 3 subjects, VND, which corresponds to x-intercept, was 0.64, 0.66, and 0.64 mL·cm−3 and VND/fP was 17.7, 13.7, and 15.2 mL·cm−3.

References

    1. Papadopoulos V, Baraldi M, Guilarte TR, et al. Translocator protein (18kDa): new nomenclature for the peripheral-type benzodiazepine receptor based on its structure and molecular function. Trends Pharmacol Sci. 2006;27:402–409.
    1. Cagnin A, Kassiou M, Meikle SR, Banati RB. Positron emission tomography imaging of neuroinflammation. Neurotherapeutics. 2007;4:443–452.
    1. Venneti S, Lopresti BJ, Wiley CA. The peripheral benzodiazepine receptor (translocator protein 18kDa) in microglia: from pathology to imaging. Prog Neurobiol. 2006;80:308–322.
    1. Kropholler MA, Boellaard R, Schuitemaker A, Folkersma H, van Berckel BN, Lammertsma AA. Evaluation of reference tissue models for the analysis of [11C](R)-PK11195 studies. J Cereb Blood Flow Metab. 2006;26:1431–1441.
    1. Turkheimer FE, Edison P, Pavese N, et al. Reference and target region modeling of [11C]-(R)-PK11195 brain studies. J Nucl Med. 2007;48:158–167.
    1. Owen DR, Guo Q, Kalk NJ, et al. Determination of [11C]PBR28 binding potential in vivo: a first human TSPO blocking study. J Cereb Blood Flow Metab. 2014;34:989–994.
    1. Kreisl WC, Fujita M, Fujimura Y, et al. Comparison of [11C]-(R)-PK 11195 and [11C]PBR28, two radioligands for translocator protein (18 kDa) in human and monkey: implications for positron emission tomographic imaging of this inflammation biomarker. Neuroimage. 2010;49:2924–2932.
    1. Kreisl WC, Jenko KJ, Hines CS, et al. A genetic polymorphism for translocator protein 18 kDa affects both in vitro and in vivo radioligand binding in human brain to this putative biomarker of neuroinflammation. J Cereb Blood Flow Metab. 2013;33:53–58.
    1. Owen DR, Howell OW, Tang SP, et al. Two binding sites for [3H]PBR28 in human brain: implications for TSPO PET imaging of neuroinflammation. J Cereb Blood Flow Metab. 2010;30:1608–1618.
    1. Zanotti-Fregonara P, Zhang Y, Jenko KJ, et al. Synthesis and evaluation of translocator 18 kDa protein (TSPO) positron emission tomography (PET) radioligands with low binding sensitivity to human single nucleotide polymorphism rs6971. ACS Chem Neurosci. 2014;5:963–971.
    1. Rupprecht R, Rammes G, Eser D, et al. Translocator protein (18 kD) as target for anxiolytics without benzodiazepine-like side effects. Science. 2009;325:490–493.
    1. Castellano S, Taliani S, Milite C, et al. Synthesis and biological evaluation of 4-phenylquinazoline-2-carboxamides designed as a novel class of potent ligands of the translocator protein. J Med Chem. 2012;55:4506–4510.
    1. Kreisl WC, Lyoo CH, McGwier M, et al. In vivo radioligand binding to translocator protein correlates with severity of Alzheimer’s disease. Brain. 2013;136:2228–2238.
    1. Zoghbi SS, Shetty HU, Ichise M, et al. PET imaging of the dopamine transporter with 18F-FECNT: a polar radiometabolite confounds brain radioligand measurements. J Nucl Med. 2006;47:520–527.
    1. Gandelman MS, Baldwin RM, Zoghbi SS, Zea-Ponce Y, Innis RB. Evaluation of ultrafiltration for the free-fraction determination of single photon emission computed tomography (SPECT) radiotracers: beta-CIT, IBF, and iomazenil. J Pharm Sci. 1994;83:1014–1019.
    1. Brown AK, Fujita M, Fujimura Y, et al. Radiation dosimetry and biodistribution in monkey and man of 11C-PBR28: a PET radioligand to image inflammation. J Nucl Med. 2007;48:2072–2079.
    1. Stabin MG, Sparks RB, Crowe E. OLINDA/EXM: the second-generation personal computer software for internal dose assessment in nuclear medicine. J Nucl Med. 2005;46:1023–1027.
    1. Hammers A, Allom R, Koepp MJ, et al. Three-dimensional maximum probability atlas of the human brain, with particular reference to the temporal lobe. Hum Brain Mapp. 2003;19:224–247.
    1. Innis RB, Cunningham VJ, Delforge J, et al. Consensus nomenclature for in vivo imaging of reversibly binding radioligands. J Cereb Blood Flow Metab. 2007;27:1533–1539.
    1. Cunningham VJ, Rabiner EA, Slifstein M, Laruelle M, Gunn RN. Measuring drug occupancy in the absence of a reference region: the Lassen plot re-visited. J Cereb Blood Flow Metab. 2010;30:46–50.
    1. Guo Q, Colasanti A, Owen DR, et al. Quantification of the specific translocator protein signal of 18F-PBR111 in healthy humans: a genetic polymorphism effect on in vivo binding. J Nucl Med. 2013;54:1915–1923.
    1. Fujita M, Seibyl JP, Verhoeff NP, et al. Kinetic and equilibrium analyses of [123I]epidepride binding to striatal and extrastriatal dopamine D2 receptors. Synapse. 1999;34:290–304.
    1. Zanotti-Fregonara P, Innis RB. Suggested pathway to assess radiation safety of 11C-labeled PET tracers for first-in-human studies. Eur J Nucl Med Mol Imaging. 2012;39:544–547.
    1. Thompson MD, Cole DE, Jose PA, Chidiac P. G protein-coupled receptor accessory proteins and signaling: pharmacogenomic insights. Methods Mol Biol. 2014;1175:121–152.
    1. Azarashvili T, Grachev D, Krestinina O, et al. The peripheral-type benzodiazepine receptor is involved in control of Ca2+-induced permeability transition pore opening in rat brain mitochondria. Cell Calcium. 2007;42:27–39.
    1. Rone MB, Fan J, Papadopoulos V. Cholesterol transport in steroid biosynthesis: role of protein-protein interactions and implications in disease states. Biochim Biophys Acta. 2009;1791:646–658.
    1. Li F, Liu J, Zheng Y, Garavito RM, Ferguson-Miller S. Protein structure: crystal structures of translocator protein (TSPO) and mutant mimic of a human polymorphism. Science. 2015;347:555–558.

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