Synthesis and evaluation of PET probes for the imaging of I2 imidazoline receptors in peripheral tissues

PET imaging of neuroinflammation: any credible alternatives to TSPO yet?

Over the last decades, the role of neuroinflammation in neuropsychiatric conditions has attracted an exponentially growing interest. A key driver for this trend was the ability to image brain inflammation in vivo using PET radioligands targeting the Translocator Protein 18 kDa (TSPO), which is known to be expressed in activated microglia and astrocytes upon inflammatory events as well as constitutively in endothelial cells. TSPO is a mitochondrial protein that is expressed mostly by microglial cells upon activation but is also expressed by astrocytes in some conditions and constitutively by endothelial cells. Therefore, our current understanding of neuroinflammation dynamics is hampered by the lack of alternative targets available for PET imaging. We performed a systematic search and review on radiotracers developed for neuroinflammation PET imaging apart from TSPO. The following targets of interest were identified through literature screening (including previous narrative reviews): P2Y12R, P2X7R, CSF1R, COX (microglial targets), MAO-B, I2BS (astrocytic targets), CB2R & S1PRs (not specific of a single cell type). We determined the level of development and provided a scoping review for each target. Strikingly, astrocytic biomarker MAO-B has progressed in clinical investigations the furthest, while few radiotracers (notably targeting S1P1Rs, CSF1R) are being implemented in clinical investigations. Other targets such as CB2R and P2X7R have proven disappointing in clinical studies (e.g. poor signal, lack of changes in disease conditions, etc.). While astrocytic targets are promising, development of new biomarkers and tracers specific for microglial activation has proven challenging.

The development status of PET radiotracers for evaluating neuroinflammation

Neuroinflammation is associated with the pathophysiologies of neurodegenerative and psychiatric disorders. Evaluating neuroinflammation using positron emission tomography (PET) plays an important role in the early diagnosis and determination of proper treatment of brain diseases. To quantify neuroinflammatory responses in vivo, many PET tracers have been developed using translocator proteins, imidazole-2 binding site, cyclooxygenase, monoamine oxidase-B, adenosine, cannabinoid, purinergic P2X7, and CSF-1 receptors as biomarkers. In this review, we introduce the latest developments in PET tracers that can image neuroinflammation, focusing on clinical trials, and further consider their current implications.

Novel synthesis of 11C-labeled imidazolines via Pd(0)-mediated 11C-carbomethoxylation using [11C]CO and arylborons

A labeling technique was developed for the imidazoline I2 receptor ligand 2-(3-fluoro-tolyl)-4, 5-dihydro-1H-imidazole (FTIMD) using Pd(0)-mediated 11C-carbomethoxylation with [11C]CO, followed by imidazoline ring formation with ethylenediamine-trimethylaluminium (EDA-AlMe3). To achieve this, [11C]CO was passed through a methanol (MeOH) solution containing 3-fluoro-4-methylphenylboronic acid (1), palladium (II) acetate (Pd [OAc]2), triphenylphosphine (PPh3), and p-benzoquinone (PBQ). The mixture was then heated at 65°C for 5 min. EDA was introduced into the reaction mixture, and MeOH was completely evaporated at temperatures exceeding 100°C. The dried reaction mixture was combined with an EDA-AlMe (1:1) toluene solution and heated at 145°C for 10 min. Portions of the reaction mixture were analyzed through high-performance liquid chromatography, resulting in [11C]FTIMD with 26% (n = 2) decay-corrected radiochemical yield (RCY). This method could be utilized for various arylborons to produce [2-11C]imidazolines 4a-h with RCYs ranging from low to moderate. Notably, [2-11C]benazoline was obtained with a moderate RCY of 65%. The proposed technique serves as an alternative to the Grignard method, which uses [11C]CO to generate a [2-11C]-labeled imidazoline ring.

Imidazoline-I2 PET Tracers in Neuroimaging

Targeting neuroinflammation, and in particular, microglial activation and astrocytosis, is a current area of the focus of new treatment interventions for a number of neurodegenerative disorders. Probing the roles of microglia and astrocytes in human disease requires the development of useful tools, such as PET imaging tools that are specific for the cell type(s) of interest. This review concentrates on the recent advances in the development of Imidazoline₂ binding site (I₂BS) PET tracers, which are purported to target astrocytes, and hence could represent key clinical imaging tools for targeting astrocytes in neurodegenerative disease. Five PET tracers for the I₂BS are described in this review, with only one (¹¹C-BU99008) being currently validated to GMP for clinical use, and data reported from healthy volunteers, Alzheimer's disease patients, and Parkinson's disease patients. The clinical data utilising ¹¹C-BU99008 have revealed the potential early involvement of astrogliosis in neurodegeneration that might precede the activation of microglia, which, if confirmed, could provide a vital new means for potentially targeting neurodegeneration earlier in the disease course.

Change in the Binding of [11C]BU99008 to Imidazoline I2 Receptor Using Brain PET in Zucker Rats

PURPOSE

The imdazoline I₂ receptor (I₂R) has been found in the feeding centers of the brain, such as the hypothalamus, and certain I₂R ligands have been reported to stimulate food intake. Thus, it has been proposed that I₂R may play a role in feeding control. [¹¹C]BU99008 was developed as a positron emission tomography (PET) tracer for imaging of I₂R. [¹¹C]BU99008 displayed relatively high brain penetration and specific binding by brain PET studies in preclinical studies. Here, we evaluated a pathological condition caused by obesity related to I₂R function by quantitative PET study using [¹¹C]BU99008.

PROCEDURES

PET scans were acquired in the Zucker (ZUC) lean and fatty rats, radioactivity and metabolites of plasma were measured, and the kinetic parameters were estimated.

RESULTS

Radioactivity levels after the injection of [¹¹C]BU99008 in the hypothalamus of both ZUC lean and fatty rats were highly accumulated, and then gradually decreased until 60 min after the injection. The accumulated radioactivity from 30 to 60 min after the injection in the hypothalamus of the ZUC fatty rats was 1.3 times greater than that of lean rats. The volume of distribution (V_T) estimated by Logan graphical analysis in the hypothalamus of the ZUC fatty rats was 1.8 times greater than that in the ZUC lean rats. In metabolite analysis, the percentages of the unchanged form in the plasma of the ZUC fatty rats at 60 min after the injection (5.0 %) was significantly lower than that of lean rats (9.1 %).

CONCLUSIONS

By PET imaging using [¹¹C]BU99008, we demonstrated that the accumulated radioactivity and estimated V_T value in the feeding center of ZUC lean rats was lower than that in fatty rats. PET studies using [¹¹C]BU99008 may contribute to elucidate a pathological condition caused by obesity related to I₂R function.

A useful PET probe [11C]BU99008 with ultra-high specific radioactivity for small animal PET imaging of I2-imidazoline receptors in the hypothalamus

INTRODUCTION

A positron emission tomography (PET) probe with ultra-high specific radioactivity (SA) enables measuring high receptor specific binding in brain regions by avoiding mass effect of the PET probe itself. It has been reported that PET probe with ultra-high SA can detect small change caused by endogenous or exogenous ligand. Recently, Kealey et al. developed [¹¹C]BU99008, a more potent PET probe for I₂-imidazoline receptors (I₂Rs) imaging, with a conventional SA (mean 76GBq/μmol) showed higher specific binding in the brain. Here, to detect small change of specific binding for I₂Rs caused by endogenous or exogenous ligand in an extremely small region, such as hypothalamus in the brain, we synthesized and evaluated [¹¹C]BU99008 with ultra-high SA as a useful PET probe for small-animal PET imaging of I₂Rs.

METHODS

[¹¹C]BU99008 was prepared by [¹¹C]methylation of N-desmethyl precursor with [¹¹C]methyl iodide. Biodistribution, metabolite analysis, and brain PET studies were conducted in rats.

RESULTS

[¹¹C]BU99008 with ultra-high SA in the range of 5400-16,600GBq/μmol were successfully synthesized (n=7), and had appropriate radioactivity for in vivo study. In the biodistribution study, the mean radioactivity levels in all investigated tissues except for the kidney did not show significant difference between [¹¹C]BU99008 with ultra-high SA and that with conventional SA. In the metabolite analysis, the percentage of unchanged [¹¹C]BU99008 at 30min after the injection of probes with ultra-high and conventional SA was similar in rat brain and plasma. In the PET study of rats' brain, radioactivity level (AUC_{30-60 min}) in the hypothalamus of rats injected with [¹¹C]BU99008 with ultra-high SA (64 [SUV ∙ min]) was significantly higher than that observed for that with conventional SA (50 [SUV ∙ min]). The specific binding of [¹¹C]BU99008 with ultra-high SA (86% of total binding) for I₂R was higher than that of conventional SA (76% of total binding).

CONCLUSION

A PET study using [¹¹C]BU99008 with ultra-high SA would thus contribute to the detection of small changes in or small regions with I₂R expression and hence may be useful in elucidating new functions of I₂R.

Small Molecule Radiopharmaceuticals - A Review of Current Approaches

Radiopharmaceuticals are an integral component of nuclear medicine and are widely applied in diagnostics and therapy. Though widely applied, the development of an “ideal” radiopharmaceutical can be challenging. Issues such as specificity, selectivity, sensitivity, and feasible chemistry challenge the design and synthesis of radiopharmaceuticals. Over time, strategies to address the issues have evolved by making use of new technological advances in the fields of biology and chemistry. This review presents the application of few advances in design and synthesis of radiopharmaceuticals. The topics covered are bivalent ligand approach and lipidization as part of design modifications for enhanced selectivity and sensitivity and novel synthetic strategies for optimized chemistry and radiolabeling of radiopharmaceuticals.