Non-invasive imaging of Kupffer cell status using radiolabelled mannosylated albumin

Objectives Kupffer cells (KCs) play a key role in maintaining liver homeostasis, hepatotoxicity and in liver pathology, but their functional status cannot be directly assessed in vivo (1-5). We report a PET tracer for noninvasive translational imaging of KCs. Methods A mannosylated human serum albumin (18mHSA), that binds to CD206 receptor [on KCs and M2-macrophages(6-7)], was synthesized, labelled by conjugation with N-succinimidyl 4-[18F]fluorobenzoate ([18F]FB). Pharmacological properties of [18F]FB18mHSA were investigated by ex vivo biodistribution and blocking studies at 30, 60 min (n=5). A 60min dynamic PET imaging studies with arterial blood sampling were performed in rats after injection of ~15 MBq of [18F]FB18mHSA (n=5). Results [18F]FB18mHSA was stable rat plasma in vitro, but showed significant metabolism in vivo (16% parent at 60 min). Radioactivity in blood decreased from the first time-point (10 sec) onward. Ex vivo biodistribution showed hepatic uptake was high (SUV 11.4±2.4 (mean±SD) at 30 min; SUV 8.9±3.3 at 60 min), as was accumulation in the kidney (SUV 24±7), due to metabolism in liver and rapid clearance of radioactivity from the blood pool via renal-urinary route. Blocking with a 20 fold excess of the unlabelled 18-mHSA, significantly decreased the uptake in liver (SUV 0.8±1.2 at 30 min, p<0.0005; SUV 4.5±0.7 at 60 min; p<0.05) and organs with immune function like bone-marrow (84%, p<0.0005) and spleen (90%, p<0.0005). PET data was analysed by Logan and Patlak graphical analysis using the metabolite-corrected plasma curve. Data was well described by Logan model, but not by Patlak model, indicating reversible binding kinetics. Conclusions [18F]FB18mHSA allows quantitative noninvasive PET imaging of the KCs and this novel method might be useful to investigate liver toxicity and fibrosis.