Visualising Intestinal Inflammation and Fibrosis using Zirconium-89 Labelled Antibodies in a Preclinical Model of Inflammatory Bowel Disease

Abstract

Inflammatory bowel disease (IBD) is a chronic, relapsing and remitting inflammatory condition of the gastrointestinal tract. The diagnosis and monitoring of IBD is reliant on endoscopic techniques which are invasive and do not provide quantification. Molecular imaging approaches such as immuno-PET have superior sensitivity and provide quantitative information of the entire body. Immuno-PET combines the superior target selectivity provided by antibodies with the sensitivity of PET. This thesis outlines the development of two novel radiolabelled antibody tracers against intestinal inflammation in a preclinical model of IBD and one novel tracer against intestinal fibrosis in a model of chronic murine IBD. Symptom flare in IBD typically corresponds with an increased activation of innate immune pathways. We aimed to compare immuno-PET of the innate immune mediators IL-1β and CD11b against standard 18F-FDG and MRI to detect colonic inflammation. For visualising intestinal inflammation, 89Zr-α-IL-1β and 89Zr-α-CD11b immuno-PET detected colonic inflammation, as did 18F-FDG, and all PET tracers were more sensitive than MRI. While 18F-FDG volumes of interest correlated with colitis severity and a strong trend was observed with 89Zr-α-IL-1β, no correlation was observed for 89Zr-α-CD11b or MRI. 89Zr-α-IL-1β was distributed mainly to the gastrointestinal tract, while 89Zr-α-CD11b was distributed in more tissue types. Intestinal fibrosis is one of the most common complications of IBD, with severe fibrosis leading to stricture and stenosis in approximately 30% of patients. Currently, intestinal fibrosis is diagnosed and monitored using endoscopies and MRI. Fibrosis is characterised by the excessive deposition of extracellular matrix (ECM) as a result of multiple periods of inflammation and subsequent healing, as seen in IBD and murine colitis. Matrix metalloproteinases (MMP) play a large role in fibrogenic pathways by regulating the deposition of ECM during tissue renewal. MMP-9 is found to be elevated in fibrotic tissue resected from IBD patients and in preclinical models of intestinal fibrosis. We aimed to visualise intestinal fibrosis by targeting pro-MMP-9 using f(ab’)2 antibody fragments, radiolabelled with zirconium-89. This was the first immuno-PET study of fibrosis in any tissue in a preclinical or clinical setting. In our preclinical model of chronic IBD, 89Zr-pro-MMP-9- f(ab’)2 successfully detected intestinal fibrosis in the absence of inflammation. Furthermore, immuno-PET and biodistribution studies indicated that the kidneys became fibrotic after multiple rounds of DSS. This was further confirmed by an increase in collagen and pro-MMP- 9 levels in the kidneys, in the absence of elevated immune markers. As the mechanisms underlying fibrosis are similar across all organs, immuno-PET of pro-MMP-9 may be a valuable addition to the detection of fibrosis in all tissues. Additionally, development of these technologies for human subjects will provide a less invasive approach than endoscopy for diagnosing and monitoring IBD.