#5531 MOLECULAR IMAGING OF PDGFR-β IN KIDNEY FIBROSIS

Abstract

Abstract Background and Aims Chronic kidney disease affects >10% of the world's population and is associated with high mortality and morbidity. The best predictor of CKD progression is the extent of fibrosis, i.e., pathological deposition of extracellular matrix (ECM) and loss of functional kidney parenchyme. Currently, there are no specific treatment options and invasive biopsies remain the gold standard for diagnosis. Multiple signaling pathways are involved in fibrosis, including platelet-derived growth factor (PDGF) signaling. Mesenchymal stromal cells express both PDGF receptors PDGFR-α and PDGFR-β, whose activation drives proliferation, migration and production of extracellular matrix, i.e., key processes involved in fibrosis initiation and progression. Based on these notions, we set out to study the bicyclic PDGFR-β-binding peptide (BiPPB) for diagnosis, quantitative imaging and treatment monitoring of kidney fibrosis. Method The biodistribution and kidney accumulation of Cy7-labeled BiPPB and a scrambled peptide control were visualized and quantified using in vivo computed tomography - fluorescence molecular tomography (CT-FMT), ex vivo fluorescence reflectance imaging (FRI) and microscopy. This was done in three different mouse models of kidney fibrosis: the unilateral ischemia-reperfusion injury (I/R), the adenine-induced nephropathy (Adenine) and a transgenic model with constitutive PDGFR-β activation specifically in renal mesenchymal cells leading to increased PDGFR-β expression and expansion of PDGFR-β+ cells (Mutant). In the latter model, we also monitored pharmacological PDGFR-β inhibition with imatinib. Kidney fibrosis and therapeutic efficacy findings were verified by immunohistochemistry (IHC). Results In vivo 3D CT-FMT imaging and ex vivo 2D FRI showed strong accumulation in fibrotic kidneys for Cy7-labeled BiPPB, whereas only moderate amounts accumulated in the contralateral (I/R) and healthy (Adenine) kidney over 48 hours after i.v. injection. The accumulation of the scrambled BiPPB was also significantly lower compared to the specific BiPPB. In transgenic Mutant mice with specific activation of mesenchymal cells and primary renal scaring significantly more Cy7-BiPPB accumulated in the kidneys in comparison to healthy wildtype littermates. In combination with imatinib treatment for three weeks (Mutant imatinib) the accumulation of Cy7-BiPPB was decreased (Fig. 1a-c). This was confirmed ex vivo by significantly increased probe accumulation (2D FRI; Fig. 1d) and PDGFR-β expression (immunohistochemistry (IHC) Fig. 1e) in fibrotic kidneys, which was diminished by imatinib treatment. Conclusion Our findings demonstrate the potential of PDGFR-β-based imaging agents for non-invasive and quantitative monitoring of renal fibrosis progression and therapy response.