Background

[PubMed]

Benzodiazepines are potent psychoactive drugs used for their sedative and anxiolytic properties (1, 2). There are two types of benzodiazepine receptors, which have been designated as central benzodiazepine receptors (CBR) and peripheral benzodiazepine receptors (PBR). PBR is also known as translocator protein (TSPO). The CBR is found exclusively in the central nervous system on the membranes of neurons and is coupled to the γ-aminobutyric acid receptor/chloride channel (3). In contrast, the PBR is a mitochondrial protein found in brain and peripheral tissues (adrenal gland, heart, lung, kidney, and testis) (4). The brain has lower levels of PBR than do the peripheral tissues. Both glial cells and macrophages contain high levels of PBR (5-7). Increased PBR expression after brain injury or neuroinflammation is associated with microglial activation, such as occurs with the neuronal damage that accompanies several neurodegenerative diseases, including Alzheimer’s disease, Wernicke’s encephalopathy, multiple sclerosis, and epilepsy.

PBRs have been studied in vivo by positron emission tomography (PET) using 1-(2-chlorophenyl)-N-[¹¹C]methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide ([¹¹C]PK11195), an isoquinoline carboxamide with specific PBR antagonistic activity (8). [¹¹C]PK11195 has been developed as a PET agent for non-invasive studies of microglia and macrophage activation in the brain, lung, and heart. However, accumulation of this tracer in the brain is limited. N-(2,5-Dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide (DAA1106) was found to be a selective agonist for studying PBRs in the central nervous system (9, 10). DAA1106 was reported to have a higher affinity for PBRs in mitochondrial fractions of rat and monkey brains than PK11195 (9, 10). Therefore, both tracers are able to cross the normal cell membrane to reach the mitochondrial receptor sites. N-(5-Fluoro-2-phenoxyphenyl)-N-(2-[¹⁸F]fluoroethyl-5-methoxybenzyl)acetamide ([¹⁸F]FEDAA1106) and [¹¹C]DAA1106 have been developed as potential PET ligands with highly selective and specific binding to PBR (11, 12). N-[¹⁸F]Fluoroacetyl-N-(2,5-dimethoxybenzyl)-2-phenoxyaniline ([¹⁸F]PBR06), which has an aryloxyanilide structure, has been evaluated for imaging PBR in the brain.

Synthesis

[PubMed]

Imaizumi et al. (13) reported the synthesis of [¹⁸F]PBR06 by nucleophilic displacement of the bromo precursor with [¹⁸F]KF/Kryptofix 2.2.2./K₂CO₃ at 110°C for 10 min and purified by high-performance liquid chromatography. The specific activity was 26.8 ± 17.5 GBq/μmol (0.72 ± 0.47 Ci/μmol) at the time of injection with 10–20% radiochemical yield.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

In vitro [³H]PK11195 PBR-binding studies using monkey brain mitochondrial homogenates showed inhibition constant (K_i) values of 0.30 ± 0.08nM and 3.84 ± 1.26 nM for PBR06 and PK11195, respectively.

Animal Studies

Human Studies

[PubMed]

Fujimura et al. (14) performed PET kinetics studies of the brain for 5 h after injection of 185 MBq (5 mCi) of [¹⁸F]PBR06 in 9 healthy subjects. A 2-tissue compartment model was shown to be better than a 1-tissue compartment model. A brain scan of 120 min exhibited the best estimation of V_T value, which was stable during 60-120 min.

Fujimura et al. (15) later performed 2-dimensional dynamic scans for radiation dosimetry after injection of 185 MBq (5 mCi) of [¹⁸F]PBR06 in 7 healthy subjects. The highest radiation doses were in the gallbladder wall (367.0 µSv/MBq, 1380 mrem/mCi), spleen (64.5 µSv/MBq, 239 mrem/mCi), liver (46.1 µSv/MBq, 170 mrem/mCi), kidneys (39.3 µSv/MBq, 146 mrem/mCi), lung (36.5 µSv/MBq, 135 mrem/mCi), and urinary bladder wall (31.0 µSv/MBq, 115 mrem/mCi). The effective dose was 18.5 µSv/MBq (69 mrem/mCi).

NIH Support

Intramural research program

References

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Imaizumi M., Briard E., Zoghbi S.S., Gourley J.P., Hong J., Musachio J.L., Gladding R., Pike V.W., Innis R.B., Fujita M. Kinetic evaluation in nonhuman primates of two new PET ligands for peripheral benzodiazepine receptors in brain. Synapse. 2007;61(8):595–605. [PubMed: 17455247]

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Fujimura Y., Zoghbi S.S., Simeon F.G., Taku A., Pike V.W., Innis R.B., Fujita M. Quantification of translocator protein (18 kDa) in the human brain with PET and a novel radioligand, (18)F-PBR06. J Nucl Med. 2009;50(7):1047–53. [PMC free article: PMC2786064] [PubMed: 19525468]

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Fujimura Y., Kimura Y., Simeon F.G., Dickstein L.P., Pike V.W., Innis R.B., Fujita M. Biodistribution and radiation dosimetry in humans of a new PET ligand, (18)F-PBR06, to image translocator protein (18 kDa). J Nucl Med. 2010;51(1):145–9. [PMC free article: PMC2997572] [PubMed: 20008980]