Background/Purpose: Neutrophil Extracellular Traps (NETs) contribute to the pathophysiology of multiple immune-mediated inflammatory diseases including Rheumatoid Arthritis (RA), and may be promising therapeutic targets. The first-in-class therapeutic anti-citrullinated histone antibody CIT-013 binds citrullinated histones H2A and H4 in NETs and has demonstrated efficacy in models of NET-mediated inflammation by inhibiting de novo NET formation and by enhancing clearance of existing NETs by Fc receptor-mediated phagocytosis (Chirivi et al 2021; van der Linden et al 2024).In this study, we describe the use of a NET targeting antibody as an in vivo radiotracer to detect and visualize NETs in inflamed tissues.

Methods: Mice with collagen induced arthritic (CIA) mice with no, mild, moderate and established disease (n=10/group) were intravenously injected with indium-111 labeled mouse anti-citrullinated histone antibody (mACHA). Twenty-four hours after injection, whole body Single Photon Emission Computed Tomographic (SPECT) imaging was performed on selected animals. For all animals, tissue distribution of the radiotracer was quantified ex vivo by measuring gamma radiation.

Results: In CIA mice, indium-111 labeled mACHA distributes to inflamed joints and can be detected in animals with mild inflammation with increasing accumulation of the radiotracer in the fore and hind paws correlating with higher arthritis severity. Furthermore, the radiotracer enables a highly sensitive detection of inflammation in joints not apparently inflamed upon visual inspection, such as the knee.

Conclusion: mACHA can be used as a radiotracer to visualize NETs in sites of inflammation. This proof of principle imaging study enables a human whole body PET CT imaging clinical study in RA and hidradenitis suppurativa patients during 2025. This clinical study will lead to further insights in CIT-013’s distribution, target engagement and mechanism of action in patients with immune mediated inflammatory diseases.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/radiolabelled-anti-citrullinated-histone-antibody-cit-013-as-a-tool-to-visualize-net-rich-inflamed-joints-in-a-collagen-induced-arthritic-mouse-model/