PET Imaging of Innate Immune Activation Using 11C Radiotracers Targeting GPR84, 2023, Kalita, James et al.

SNT Gatchaman

SNT Gatchaman

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PET Imaging of Innate Immune Activation Using 11C Radiotracers Targeting GPR84
Mausam Kalita; Jun Hyung Park; Renesmee Chenting Kuo; Samira Hayee; Sara Marsango; Valentina Straniero; Israt S Alam; Angelie Rivera-Rodriguez; Mallesh Pandrala; Mackenzie L Carlson; Samantha T Reyes; Isaac M Jackson; Lorenzo Suigo; Audrey Luo; Sydney C Nagy; Ermanno Valoti; Graeme Milligan; Frezghi Habte; Bin Shen; Michelle L James

Chronic innate immune activation is a key hallmark of many neurological diseases and is known to result in the upregulation of GPR84 in myeloid cells (macrophages, microglia, and monocytes). As such, GPR84 can potentially serve as a sensor of proinflammatory innate immune responses. To assess the utility of GPR84 as an imaging biomarker, we synthesized 11C-MGX-10S and 11C-MGX-11S via carbon-11 alkylation for use as positron emission tomography (PET) tracers targeting this receptor.

In vitro experiments demonstrated significantly higher binding of both radiotracers to hGPR84-HEK293 cells than that of parental control HEK293 cells. Co-incubation with the GPR84 antagonist GLPG1205 reduced the binding of both radiotracers by >90%, demonstrating their high specificity for GPR84 in vitro. In vivo assessment of each radiotracer via PET imaging of healthy mice illustrated the superior brain uptake and pharmacokinetics of 11C-MGX-10S compared to 11C-MGX-11S. Subsequent use of 11C-MGX-10S to image a well-established mouse model of systemic and neuro-inflammation revealed a high PET signal in affected tissues, including the brain, liver, lung, and spleen.

In vivo specificity of 11C-MGX-10S for GPR84 was confirmed by the administration of GLPG1205 followed by radiotracer injection. When compared with 11C-DPA-713─an existing radiotracer used to image innate immune activation in clinical research studies─11C-MGX-10S has multiple advantages, including its higher binding signal in inflamed tissues in the CNS and periphery and low background signal in healthy saline-treated subjects.

The pronounced uptake of 11C-MGX-10S during inflammation, its high specificity for GPR84, and suitable pharmacokinetics strongly support further investigation of 11C-MGX-10S for imaging GPR84-positive myeloid cells associated with innate immune activation in animal models of inflammatory diseases and human neuropathology.

Web | PDF | JACS Au | Open Access
 
See also this paywalled abstract from 2024 —

Development and comparison of two novel PET tracers for imaging pro-inflammatory receptor GPR84 in human cells and tissues
Sydney Nagy; Mausam Kalita; Isaac Jackson; Samantha Reyes; Renesmee Kuo; Noeen Malik; Mallesh Pandrala; Bo Zhang; Sara Marsango; Valentina Straniero; Lorenzo Suigo; Ermanno Valoti; Israt Alam; Graeme Milligan; Michelle James

INTRODUCTION
Inflammation is an essential biological process in response to injury or infection; however, its failure to resolve can lead to cellular damage and is a common feature of many diseases, including fibrotic conditions, sepsis, and ulcerative colitis (UC). Unfortunately, the exact role and dynamics of unresolved inflammation are not fully understood, and accurate methods to track such phenomena are limited. We identified G-protein-coupled receptor 84 (GPR84) as a promising biomarker upregulated during pro-inflammatory immune reactions that is predominately expressed on myeloid cells, a subset of innate immune cells involved in driving toxic inflammation. Here, we report the development and assessment of two novel GPR84-targeting positron emission tomography (PET) tracers, [18F]MGX-110S and [11C]GLPG38. We test the specificity of these tracers in human cells and colon tissue from healthy subjects versus colitis patients.

METHODS
To investigate alterations in GPR84 mRNA expression under pro-inflammatory conditions, quantitative polymerase-chain reaction was performed using mouse brain and human monocyte-derived macrophage cells (hMDMs) treated with lipopolysaccharides (LPS) 24hr prior to RNA extraction. [11C]GLPG38 and [18F]MGX-110S were made via [11C]methylation of the desmethyl precursor and by copper mediated [18F]fluorination of Bpin precursor respectively (Fig 1A-B). Binding specificities of both tracers were assessed using human GPR84-stably-expressing human embryonic kidney cells (HEK-293) versus control HEK-293 cells. [11C]GLPG38 binding was also assessed in hMDMs +/-treatment with LPS. Dynamic PET/CT imaging of healthy mice was commenced just prior to injection of [11C]GLPG38 (100-150µCi) or [18F]MGX-110S (60-120µCi) to determine biodistribution and kinetics. In vitro autoradiography (ARG) of human colon sections (5µm) was conducted to test the ability of each tracer to detect innate immune activation in active UC. These sections subsequently underwent H&E staining.

RESULTS
GPR84 mRNA expression was more robustly upregulated in both LPS mouse brain tissue and human cells treated with LPS compared to translocator protein (18kDa) (TSPO), a widely-evaluated PET biomarker of inflammation (Fig 1C). [11C]GLPG38 was synthesized in 8.13%±3.30% radiochemical yield (n=3) and >95% radiochemical purity (RCP) (Suppl 1A). [18F]MGX-110S was generated in 22.21±11.99% yield (n=3) and >99% RCP (Suppl 1B). Cell binding studies showed 13.5-fold higher binding of [11C]GLPG38 and 33.4-fold higher [18F]MGX-110S binding to hGPR84-HEK-293 cells vs control cells (Fig 1D, p<0.0001, n=4). Co-incubation with GPR84 antagonist (GLPG1205, 1000x by mass) reduced tracer binding in hGPR84-HEK293 cells by >90% (Fig 1D, p<0.0001, n=4) proving high specificity of these tracers in cells. Notably, hMDMs challenged with LPS (100ng/mL) stimulus were found to display significantly higher binding of [11C]GPLG38 compared to unstimulated cells, and this signal was reduced to baseline following co-incubation with GLPG1205 (Suppl 1C). Whole-body PET images of healthy mice depict relatively low background signal in most peripheral tissues, highlighting their potential for imaging inflammation throughout the body (Fig 1E). The highest signal (though lower for [18F]MGX-110S) was observed in liver and gallbladder indicating biliary excretion. In the context of human inflammatory disease, in vitro ARG of [11C]GLPG38 and [18F]MGX-110S in active UC colon tissue samples demonstrated marked increase of binding for both tracers compared to age-, sex-, and ethnicity-matched healthy samples, and this signal was markedly attenuated by pre-blocking with 1000x GLPG1205 by mass (Fig 1F). Regions of high tracer binding corresponded with immune infiltrates shown by H&E staining.

CONCLUSIONS
Both tracers enable specific detection of GPR84 and innate immune activation in cells and UC tissue. [18F]MGX-110S shows high potential for translation given its favorable half-life, in vivo distribution, and specificity for GPR84.

Web | Journal of Nuclear Medicine | Paywall
 
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