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 (CBRs) and peripheral benzodiazepine receptors (PBRs). CBRs are found exclusively in the central nervous system on the membranes of neurons and are coupled to the γ-aminobutyric acid receptor/chloride channel (3). In contrast, PBRs are mitochondrial proteins found in the brain and peripheral tissues (adrenal gland, heart, lung, kidney, and testis) (4); the brain has lower levels of PBR than do the peripheral tissues, and 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 with 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. [¹¹C]PK11195 has been developed as a PET agent for non-invasive studies of microglia and macrophage activation in the brain, lung, and heart [PuBMed]. However, accumulation of this tracer in the brain is limited. N-(2,5-Dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide (DAA1106) is a selective agonist used to study PBRs in the central nervous system (8, 9). DAA1106 was reported to have a higher affinity for PBRs in mitochondrial fractions of rat and monkey brains than PK11195 (8, 9). 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-labeled DAA1106 ([¹¹C]DAA1106) have been developed as potential PET ligands with highly selective and specific binding to PBR. N-Acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine (PBR28), which has an aryloxyanilide structure, has been shown to have high affinity and selectivity for PBR (10). N-Acetyl-N-(2-[¹¹C]methoxybenzyl)-2-phenoxy-5-pyridinamine ([¹¹C]PBR28) has been developed for imaging PBR in the brain (11). N-Acetyl-N-(2-[¹⁸F]fluoroethoxybenzyl)-2-phenoxy-5-pyridinamine ([¹⁸F]FEPPA), an analog of [¹¹C]PBR28, is being evaluated as a PET imaging agent for PBR in the brain (12).

Synthesis

[PubMed]

Wilson et al. (12) reported the synthesis of [¹⁸F]FEPPA with ¹⁸F-fluorination of N-(2-((n-4-phenoxypyridin-3-yl)acetimido)methyl)phenoxy)ethyl 4-methylbenzenesulphonate in acetonitrile solution of K₂CO₃/Kryptofix222 for 10 min at 90ºC. [¹⁸F]FEPPA was purified with high-performance liquid chromatography (HPLC) with radiochemical yields of 50% to 60% in a total synthesis time of 40–50 min. The specific activity was 44.4–99.9 GBq/μmol (1.2–2.7 Ci/μmol) at the end of synthesis with radiochemical purities of >99%.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

In vitro [³H]PK11195 PBR-binding studies showed that FEPPA and PBR28 had K_i values of 0.07 and 0.22 nM, respectively (12). FEPPA has a lipophilicity value (Log D, measured) of 2.99 and little activity against various neurotransmitters and transporters.

Animal Studies

Human Studies

[Pub Med]

No publications are currently available.

References

1.

Mohler H. , Okada T. Benzodiazepine receptor: demonstration in the central nervous system. Science. 1977; 198 (4319):849–51. [PubMed: 918669]

2.

Hunkeler W. , Mohler H. , Pieri L. , Polc P. , Bonetti E.P. , Cumin R. , Schaffner R. , Haefely W. Selective antagonists of benzodiazepines. Nature. 1981; 290 (5806):514–6. [PubMed: 6261143]

3.

Olsen R.W. , Tobin A.J. Molecular biology of GABAA receptors. Faseb J. 1990; 4 (5):1469–80. [PubMed: 2155149]

4.

Anholt R.R. , Pedersen P.L. , De Souza E.B. , Snyder S.H. The peripheral-type benzodiazepine receptor. Localization to the mitochondrial outer membrane. J Biol Chem. 1986; 261 (2):576–83. [PubMed: 3001071]

5.

Jones H.A. , Valind S.O. , Clark I.C. , Bolden G.E. , Krausz T. , Schofield J.B. , Boobis A.R. , Haslett C. Kinetics of lung macrophages monitored in vivo following particulate challenge in rabbits. Toxicol Appl Pharmacol. 2002; 183 (1):46–54. [PubMed: 12217641]

6.

Kuhlmann A.C. , Guilarte T.R. Cellular and subcellular localization of peripheral benzodiazepine receptors after trimethyltin neurotoxicity. J Neurochem. 2000; 74 (4):1694–704. [PubMed: 10737628]

7.

Zavala F. , Lenfant M. Benzodiazepines and PK 11195 exert immunomodulating activities by binding on a specific receptor on macrophages. Ann N Y Acad Sci. 1987; 496 :240–9. [PubMed: 2886095]

8.

Okuyama S. , Chaki S. , Yoshikawa R. , Ogawa S. , Suzuki Y. , Okubo T. , Nakazato A. , Nagamine M. , Tomisawa K. Neuropharmacological profile of peripheral benzodiazepine receptor agonists, DAA1097 and DAA1106. Life Sci. 1999; 64 (16):1455–64. [PubMed: 10321725]

9.

Chaki S. , Funakoshi T. , Yoshikawa R. , Okuyama S. , Okubo T. , Nakazato A. , Nagamine M. , Tomisawa K. Binding characteristics of [3H]DAA1106, a novel and selective ligand for peripheral benzodiazepine receptors. Eur J Pharmacol. 1999; 371 (2-3):197–204. [PubMed: 10357257]

10.

Imaizumi M. , Kim H.J. , Zoghbi S.S. , Briard E. , Hong J. , Musachio J.L. , Ruetzler C. , Chuang D.M. , Pike V.W. , Innis R.B. , Fujita M. PET imaging with [11C]PBR28 can localize and quantify upregulated peripheral benzodiazepine receptors associated with cerebral ischemia in rat. Neurosci Lett. 2007; 411 (3):200–5. [PubMed: 17127001]

11. Fujita, M., M. Imaizumi, S.S. Zoghbi, Y. Fujimura, A.G. Farris, T. Suhara, J. Hong, V.W. Pike, and R.B. Innis, Kinetic analysis in healthy humans of a novel positron emission tomography radioligand to image the peripheral benzodiazepine receptor, a potential biomarker for inflammation. Neuroimage, 2007. [PMC free article: PMC2265774] [PubMed: 18093844]

12.

Wilson A.A. , Garcia A. , Parkes J. , McCormick P. , Stephenson K.A. , Houle S. , Vasdev N. Radiosynthesis and initial evaluation of [(18)F]-FEPPA for PET imaging of peripheral benzodiazepine receptors. Nucl Med Biol. 2008; 35 (3):305–14. [PubMed: 18355686]