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Although extensive research on brown adipose tissue (BAT) has stimulated optimism in the battle against obesity and diabetes, BAT physiology and organ crosstalk are not fully understood. Besides BAT, melanin‐concentrating hormone (MCH) and its receptor (MCHR1) play an important role in energy homeostasis. Because of the link between hypothalamic MCH neurons and sympathetic BAT activation via β‐adrenoceptors, we investigated the expression and physiological role of the MCHR1 in BAT. MCHR1 was detected in rodent and human BAT with RT‐qPCR and western blot analyses. In vivo imaging in rats used the glucose analog [18F]FDG and the MCHR1‐tracer [11C]SNAP‐7941. We found that the β3‐adrenoceptor (ADRB3) agonist CL316,243 increased [11C]SNAP‐7941 uptake in BAT. Additionally, a pharmacological concentration of SNAP‐7941—a low‐affinity ADRB3 ligand—stimulated [18F]FDG uptake, reflecting BAT activation. In cultured human adipocytes, CL316,243 induced MCHR1 expression, further supporting a direct interaction between MCHR1 and ADRB3. These findings characterized MCHR1 expression in rodent and human BAT for the first time, including in vitro and in vivo data demonstrating a link between MCHR1 and the β3‐adrenergic system. The presence of MCHR1 in BAT emphasizes the role of BAT in energy homeostasis and may help uncover treatment approaches for obesity., Our findings here describe the expression of melanin‐concentrating hormone receptor 1 (MCHR1) in rodent and human brown adipose tissue (BAT). In vitro and in vivo data demonstrate a link between MCHR1 and the β3‐adrenergic system. The presence of MCHR1 in BAT emphasizes its role in energy homeostasis and may open new possibilities for treatment of obesity.

Purpose The melanin-concentrating hormone receptor 1 (MCHR1) has become an important pharmacological target, since it may be involved in various diseases, such as diabetes, insulin resistance, and obesity. Hence, a suitable positron emission tomography radiotracer for the in vivo assessment of the MCHR1 pharmacology is imperative. The current paper contrasts the extensive in vitro, in vivo, and ex vivo assessments of the radiotracers [18F]FE@SNAP and [11C]SNAP-7941 and provides comprehensive information about their biological and physicochemical properties. Furthermore, it examines their suitability for first-in-man imaging studies. Procedures Kinetic real-time cell-binding studies with [18F]FE@SNAP and [11C]SNAP-7941 were conducted on adherent Chines hamster ovary (CHO-K1) cells stably expressing the human MCHR1 and MCHR2. Small animal imaging studies on mice and rats were performed under displacement and baseline conditions, as well as after pretreatment with the P-glycoprotein/breast cancer resistant protein inhibitor tariquidar. After the imaging studies, detailed analyses of the ex vivo biodistribution were performed. Ex vivo metabolism was determined in rat blood and brain and analyzed at various time points using a quantitative radio-HPLC assay. Results [11C]SNAP-7941 demonstrates high uptake on CHO-K1-hMCHR1 cells, whereas no uptake was detected for the CHO-K1-hMCHR2 cells. In contrast, [18F]FE@SNAP evinced binding to CHO-K1-hMCHR1 and CHO-K1-hMCHR2 cells. Imaging studies with [18F]FE@SNAP and [11C]SNAP-7941 showed an increased brain uptake after tariquidar pretreatment in mice, as well as in rats, and exhibited a significant difference between the time-activity curves of the baseline and blocking groups. Biodistribution of both tracers demonstrated a decreased uptake after displacement. [11C]SNAP-7941 revealed a high metabolic stability in rats, whereas [18F]FE@SNAP was rapidly metabolized. Conclusions Both radiotracers demonstrate appropriate imaging properties for the MCHR1. However, the pronounced metabolic stability as well as superior selectivity and affinity of [11C]SNAP-7941 underlines the decisive superiority over [18F]FE@SNAP.

[11C]SNAP-7941 and its radiofluorinated, fluoro-ethyl derivative [18F]FE@SNAP have been developed as the first positron emission tomography tracers for melanin-concentrating hormone receptor 1 (MCHR1) imaging. Accumulation of these MCHR1 PET-tracers in rat brown adipose tissue (BAT) in vivo provided first indication of MCHR1 expression in rodent BAT. To rule out off-target binding, affinity of both MCHR1 ligands towards adrenergic beta-3 receptors (ADRB3) was examined. Further, specific binding of [11C]SNAP-7941 to brown adipocytes and effects of MCHR1 ligands on brown adipocyte activation were investigated. SNAP-7941 and FE@SNAP evinced to be highly selective towards MCHR1. [11C]SNAP-7941 binding to brown adipocytes was shown to be mainly MCHR1-specific. This data strongly indicates MCHR1 expression in rodent BAT and moreover, a peripheral, anti-obesity effect of MCHR1 antagonists directly exerted in BAT is proposed. Moreover, MCHR1 expression in murine brown adipocytes was confirmed by protein and mRNA analysis. We conclude that MCHR1 PET imaging contributes to basic research in endocrinology by elucidating the involvement of the MCH system in peripheral tissues, such as BAT.

Molecular imaging probes such as PET-tracers have the potential to improve the accuracy of tumor characterization by directly visualizing the biochemical situation. Thus, molecular changes can be detected early before morphological manifestation. The A3 adenosine receptor (A3AR) is described to be highly expressed in colon cancer cell lines and human colorectal cancer (CRC), suggesting this receptor as a tumor marker. The aim of this preclinical study was the evaluation of F18FE@SUPPY as a PET-tracer for CRC using in vitro imaging and in vivo PET imaging. First, affinity and selectivity of FE@SUPPY and its metabolites were determined, proving the favorable binding profile of FE@SUPPY. The human adenocarcinoma cell line HT-29 was characterized regarding its hA3AR expression and was subsequently chosen as tumor graft. Promising results regarding the potential of F18FE@SUPPY as a PET-tracer for CRC imaging were obtained by autoradiography as ≥2.3-fold higher accumulation of F18FE@SUPPY was found in CRC tissue compared to adjacent healthy colon tissue from the same patient. Nevertheless, first in vivo studies using HT-29 xenografts showed insufficient tumor uptake due to (1) poor conservation of target expression in xenografts and (2) unfavorable pharmacokinetics of F18FE@SUPPY in mice. We therefore conclude that HT-29 xenografts are not adequate to visualize hA3ARs using F18FE@SUPPY.

The MCHR1 is involved in the regulation of energy homeostasis and changes of the expression are linked to a variety of associated diseases, such as diabetes and adiposity. The study aimed at the in vitro and in vivo evaluation of [11C]SNAP-7941 and [18F]FE@SNAP as potential PET-tracers for the MCHR1. Competitive binding studies with non-radioactive derivatives and small-animal PET/CT and MRI brain studies were performed under baseline conditions and tracer displacement with the unlabelled MCHR1 antagonist (±)-SNAP-7941. Binding studies evinced high binding affinity of the non-radioactive derivatives. Small-animal imaging of [11C]SNAP-7941 and [18F]FE@SNAP evinced high tracer uptake in MCHR1-rich regions of the ventricular system. Quantitative analysis depicted a significant tracer reduction after displacement with (±)-SNAP-7941. Due to the high binding affinity of the non-labelled derivatives and the high specific tracer uptake of [11C]SNAP-7941 and [18F]FE@SNAP, there is strong evidence that both radiotracers may serve as highly suitable agents for specific MCHR1 imaging.