Kinetic analysis in healthy humans of a novel positron emission tomography radioligand to image the peripheral benzodiazepine receptor, a potential biomarker for inflammation
Introduction
The peripheral benzodiazepine receptor (PBR) is a mitochondrial protein that is highly expressed in phagocytic inflammatory cells, namely macrophages in the periphery and activated microglia in the central nervous system (Papadopoulos et al., 2006, Zavala et al., 1984). Both in vitro and in vivo imaging of PBRs can localize and quantify inflammation in tissues (Venneti et al., 2006). For the past two decades, [3H]PK 11195 has been used for in vitro studies (e.g., binding to homogenates or sections of tissues), and the PET radioligand [11C]PK 11195 has been used for in vivo imaging (Venneti et al., 2006). [3H]PK 11195 is useful as an in vitro radioligand and has a high ratio of specific to nonspecific binding, in part because much of the nonspecific binding can be washed away from tissue homogenates or sections. Such washing is not possible for in vivo imaging, and [11C]PK 11195 has relatively low ratios of specific to nonspecific binding. For example, by using reference tissue models, Kropholler et al. (2006) have reported that the ratio of specific to nonspecific binding of the active enantiomer (R) of [11C]PK 11195 in human brain is only about 0.2–0.5.
A new class of 11C- and 18F-labeled radioligands with an aryloxyanilide structure has been developed for in vivo imaging of PBR with PET (Okuyama et al., 1999). These radioligands have 4 to 18 times greater affinity for PBRs than PK 11195 and have higher levels of brain uptake (Zhang et al., 2003). Among this class of ligands, [11C]DAA1106 and [18F]FEDAA1106 have been studied in monkeys and demonstrate high brain uptake and high ratios of specific to nonspecific binding (Maeda et al., 2004, Zhang et al., 2004). Studies in human with these ligands show high brain uptake, but displacement studies have not been performed to measure definitively the percentage of specific binding in human brain (Fujimura et al., 2006, Ikoma et al., 2007). Furthermore, [11C]DAA1106 has been directly compared with [11C](R)-PK11195 in rats under baseline conditions and after inflammation had been induced with neurotoxins (Venneti et al., 2007a, Venneti et al., 2007b). The radioligand with an aryloxyanilide structure, [11C]DAA1106, was superior to that with an isoquinoline structure, [11C](R)-PK11195, in terms of higher brain uptake and retention in areas with inflammation.
We recently developed additional 11C- and 18F-labeled analogs with an aryloxyanilide structure, and some showed promising results in animals. One of these compounds is [11C]PBR28 ([O-methyl-11C]N-acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine). The affinity of PBR28 for peripheral benzodiazepine receptors (KI, inhibition constant = 0.7–2.5 nM in rat, monkey and human) is two to fivefold greater than that of PK 11195, and the lipophilicity of PBR28 is ∼ 100-fold lower than that of PK 11195 (Briard et al., in press). The relatively high affinity and low lipophilicity likely contribute to the high in vivo specific signal of [11C]PBR28. For example, more than 90% of uptake into monkey brain can be displaced by nonradioactive PBR ligands, and such displacement is the pharmacological definition of specific binding (Imaizumi et al., in press). Because of the promising imaging results in monkeys, we extended the use of this radioligand to human subjects. Based on whole body biodistribution in healthy subjects, the effective dose of [11C]PBR28 is 6.6 μSv/MBq, similar to that of other 11C-labeled ligands (Brown et al., 2007).
Having confirmed the radiation safety of [11C]PBR28, we sought in the current study to evaluate the ability of this radioligand to quantify PBRs in human brain. Binding in brain was quantified with compartmental modeling using serial brain images and concurrent measurements of unchanged parent radioligand in arterial plasma.
Section snippets
Radiopharmaceutical preparation
[11C]PBR28 was prepared by the 11C-methylation of its desmethyl analogue with [11C]iodomethane, itself prepared from cyclotron-produced [11C]carbon dioxide, and purified with reverse phase HPLC. Preparations were conducted according to our exploratory Investigational New Drug Application #73,935, submitted to the US Food and Drug Administration, and a copy of which is available at: http://pdsp.med.unc.edu/snidd/. The radioligand was obtained in high radiochemical purity (> 99%).
Human subjects
Twelve healthy
Pharmacological effects
Injection of [11C]PBR28 caused no pharmacological effects, based on patient reports, ECG, blood pressure, pulse, and respiration rate after radioligand administration. In addition, no effects were noted in any of the blood and urine tests acquired about 24 h after radioligand injection. The injected mass dose of PBR28 was 4.9 ± 2.6 nmol (n = 13 injections in 12 subjects).
Brain images: binders
After injection of [11C]PBR28, 10 of 12 subjects showed moderate levels of activity in brain that washed out gradually. The peak
Discussion
[11C]PBR28 had generally promising imaging characteristics, including peak concentrations in brain (∼ 200% SUV) that were moderate in healthy subjects who presumably had no inflammation in brain. Brain uptake could be quantified with a two-tissue compartment model as distribution volume, which provided relatively stable values after about 90 min of imaging. Nevertheless, the estimates of distribution continued to increase in the later portion of longer scan durations: ∼ 11% from 90 to 150 min.
Conclusions
Binding of [11C]PBR28 in brain was measured in healthy human subjects with ∼ 90 min of brain imaging combined with serial concentrations of [11C]PBR28 in arterial plasma. Binding in human brain was about 1/20th of that in rhesus monkey and required a two-tissue compartment model. For unknown reason(s), a small percentage (~ 14%) of healthy subjects showed no significant binding of [11C]PBR28 in brain.
Acknowledgments
We thank Janet L. Sangare, MS, C-RNP, Alicja Lerner, MD, PhD, and the staff of the PET Department for successful completion of PET scans; PMOD Technologies (Adliswil, Switzerland) for providing its image analysis and modeling software; John L. Musachio, PhD, and Emmanuelle Briard, PhD, for contributions to the preparation of the exploratory IND; Ed Tuan, BS, for assisting with the radiometabolite analysis, Kohji Abe, PhD, for analyzing data; and Amira K Brown, PhD, for providing results of
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