AB0781 [68GA]GA-NOTA-MMR-SDAB FOR PET IMAGING OF MACROPHAGES IN RHEUMATOID ARTHRITIS
Authors: M. M. Steinz, F. Khodadust, J. M. A. Stuijt, E. Mantel, J. P. Van Hamburg, N. Devoogdt, J. A. Van Ginderachter, G. Jansen, W. Beaino, A. D. Windhorst, S. W. Tas, C. J. Van der Laken
Journal: Annals of the Rheumatic Diseases (2024)

<h3>Background:</h3> Rheumatoid arthritis (RA) is a chronic inflammatory joint disease characterized by synovial inflammation and infiltration of various immune cells, including macrophages. Novel whole-body molecular imaging with positron emission tomography (PET) and the use of specific PET tracers can non-invasively detect and quantify the presence of macrophages in the RA inflamed synovium, as read-out of RA disease activity. The mannose receptor (CD206) is a macrophage C-lectin surface receptor suitable for molecular-imaging of macrophages, however its value for PET-imaging in the context of RA remains to be established. Moreover, recently three distinct pathotypes of RA have been identified: fibroid-pauci (FP) immune (fibroblast-rich), diffuse-myeloid (DM, macrophage-rich) and lympho-myeloid (LM, lymphocyte- and macrophage rich) pathotype [1, 2] which may differentially express CD206 as base for PET stratification of RA subgroups <h3>Objectives:</h3> (1) Investigate the expression of CD206 in the three different RA pathotypes on human synovial tissue of RA patients. (2) Establish if a novel nanobody (sdAb) PET tracer targeting CD206, i.e. [<sup>68</sup>Ga]Ga-NOTA-MMR-sdAb, is suitable for PET imaging of macrophages in an animal model of RA. <h3>Methods:</h3> Immunofluorescence (IF) analysis of CD206 expression was performed in ankle or knee biopsies of inflamed synovium from RA patients with a fibroid-pauci immune (N=9), diffuse-myeloid (N=8) and lympho-myeloid pathotype (N=8). The expression in all RA pathotypes was compared to expression of pan-macrophage marker CD68. PET scans (Mediso nanoPET/CT scanner) with 10 μg, ~15 MBq [<sup>68</sup>Ga]Ga-NOTA-MMR-sdAb were performed in antigen-induced arthritic (AIA) rats [3] (N=4). [<sup>68</sup>Ga]Ga-NOTA-MMR-sdAb was synthesized (&gt;95% radiochemical purity and high molar activity) as described before [4]. PET imaging results were validated by immunohistochemistry (IHC) for macrophage receptor CD68 (a pan-macrophage marker) and CD206 in AIA knee tissues (N=8). <h3>Results:</h3> In synovial tissue of RA patients, average expression of CD206 was significantly increased in the DM and LM compared to the FP immunopathotype (respectively 3.4- and 1.7-fold ** p&lt;0.01) (Figure 1A and B). Quantitative CD206 expression was highest in the DM immunopathotype. Dynamic PET scanning in AIA rats, showed that the uptake of [<sup>68</sup>Ga]Ga-NOTA-MMR-sdAb in the arthritic knee was significantly higher (1.7-fold) than in the control knee from 2 min onwards after iv tracer injection (SUV<sub>mean</sub> arthritic 1.04 ± 0.08; SUV<sub>mean</sub> control 0.61 ± 0.06, p&lt;0.001) (Figure 1C–E). IHC of CD68 and CD206 confirmed increased infiltration of macrophages in AIA arthritis-affected knees compared to the control knees (Figure 1F). <h3>Conclusion:</h3> The results of this study support the use of [<sup>68</sup>Ga]Ga-NOTA-MMR-sdAb for macrophage PET-imaging in RA joints, which should be further validated in (pre)clinical studies. Moreover, the human staining results indicate that CD206 may be used as a PET-imaging target to distinguish RA patients with a macrophage-rich pathotype vs. those with low or no macrophage infiltration. This may offer opportunities of baseline stratification of patients that most likely will or will not respond to macrophage targeting therapies. <h3>REFERENCES:</h3> [1] Pitzalis C, Kelly S, Humby F. New learnings on the pathophysiology of RA from synovial biopsies. Curr Opin Rheumatol. 2013;25(3):334-44. [2] Humby F, Lewis M, Ramamoorthi N, Hackney JA, Barnes MR, Bombardieri M, et al. Synovial cellular and molecular signatures stratify clinical response to csDMARD therapy and predict radiographic progression in early rheumatoid arthritis patients. Annals of the rheumatic diseases. 2019;78(6):761-72. [3] Chandrupatla DM, Weijers K, Gent YY, de Greeuw I, Lammertsma AA, Jansen G, et al. Sustained macrophage infiltration upon multiple intra-articular injections: an improved rat model of rheumatoid arthritis for PET guided therapy evaluation. Biomed Res Int. 2015;2015:509295. [4] Xavier C, Blykers A, Laoui D, Bolli E, Vaneyken I, Bridoux J, et al. Clinical Translation of [(68)Ga]Ga-NOTA-anti-MMR-sdAb for PET/CT Imaging of Protumorigenic Macrophages. Mol Imaging Biol. 2019;21(5):898-906. <h3>Acknowledgements:</h3> This work has received funding from the Innovative Medicines Initiative 2 Joint Undertaking (JU) under grant agreement No 831514 (Immune-Image). The JU receives support from the European Union's Horizon 2020 research and innovation programme and EFPIA. <h3>Disclosure of Interests:</h3> <b>None declared.</b>