PET imaging of VAP-1
Amine oxidase, copper containing 3 (AOC3), or vascular adhesion protein-1 (VAP-1), is an endothelial glycoprotein which mediates leukocyte trafficking from blood stream to sites of inflammation. VAP-1 is normally stored in intracellular granules within endothelial cells, but in inflamed tissue it is rapidly translocated onto the luminal surface, and its production is upregulated. Increased VAP-1 on endothelial cell surfaces is typical in certain cancers. In addition to the endothelial cells, VAP-1 is also expressed in normal smooth muscle cells and adipocytes.
VAP-1 is unique among the leukocyte homing associated molecules as it is not only an adhesin, but also an enzyme (member of semicarbazide-sensitive amine oxidase, SSAO, family) that catalyses oxidative deamination of primary amines, like monoamine oxidases (MAO-A and MAO-B), and produces H2O2, aldehyde, and ammonium (Salmi & Jalkanen, 2001 and 2005). Deamination products may induce β-amyloid aggregation, which can be seen in atherosclerotic plaques.
VAP-1 also exists as a soluble form (sVAP-1), which is constantly formed by proteolytic shedding, and is a prognostic biomarker (Aalto et al., 2014). sVAP-1 is enzymatically active, and can bind VAP-1 ligands.
Several Ga-68 labeled peptides have been synthesized and tested in inflammation and tumour animal models (Aalto et al., 2011; Autio et al., 2010; Lankinen et al., 2008; Silvola et al., 2010; Ujula et al., 2009; Virtanen et a., 2017; Moisio et al., 2018).
Sialic acid-binding immunoglobulin-like lectins (Siglecs) are involved in inflammatory and immune responses. Siglec-10 is a VAP-1 ligand which is expressed on B cells, monocytes, and eosinophils, but is absent from granulocytes. Siglec-9 is a VAP-1 ligand on granulocytes and macrophages (Aalto et al., 2011; Silvola et al., 2016). Binding of Siglec-10 is completely dependent on the enzymatic activity of VAP-1, while Siglec-9 can bind to enzymatically inactive VAP-1, although less effectively (Kivi et al., 2009; Aalto et al., 2011). [68Ga]DOTA-Siglec-9 has in animal models shown promise to detect catheter-related bacterial infection (Ahtinen et al., 2014), synovitis (Virtanen et al., 2015; Siitonen et al., 2017), pulmonary inflammation (Retamal et al., 2016), atherosclerotic plaques (Silvola et al., 2016). In animal osteomyelitis model, both positive (Jødal et al., 2019) and negative (Afzelius et al., 2020) results have been obtained. Clinical grade radiosynthesis of [68Ga]DOTA-Siglec-9 has been established (Käkelä et al., 2018), and found to be safe in rats with 1000 times higher dose than the dose administered in human studies (Chrusciel et al., 2019).
[18F]AlF-NOTA-Siglec-9 and [68Ga]NOTA-Siglec-9 were evaluated in turpentine-induced sterile skin/muscle inflammation in rats, and both tracers showed clear accumulation in the inflamed tissue (Moisio et al., 2018).
Quantification of VAP-1 using PET
Peptide tracers are metabolized and eliminated quickly from plasma, allowing relatively short PET imaging sessions. Radioactive metabolites of VAP-1 PET tracers have not yet been identified, but they probably do not specifically bind to VAP-1. However, the inflamed or infected tissue may have increased nonspecific uptake of the radioactive metabolites, including 68Ga3+. Reversible two-tissue compartmental model can be used to estimate distribution volume of [68Ga]DOTA-Siglec-9 in porcine infection models (Jødal et al., 2018 and 2019).
VAP-1 tracers bind in plasma to sVAP-1. Currently it is not known whether the sVAP-1 bound tracers are available for binding to endothelial VAP-1 in tissue capillaries. The sVAP-1 levels are increased in some diseases such as diabetes mellitus and autoimmune hepatitis, but not in most other inflammatory diseases (Jalkanen & Salmi, 2008). If the studied inflammatory disease is causing increased sVAP-1 in plasma, then the estimated PET parameters such as SUV will be negatively biased; even the plasma input models may provide biased results of distribution volume or net influx rate, if the input curve includes the plasma protein bound tracer.
Peptide tracers, or the labelled high-mass metabolites, are not assumed to enter to intracellular space (including VAP-1 storage granules) during the PET study, but to bind only to endothelial VAP-1. Therefore the binding kinetics can be assumed to be fast. However, the dual nature of VAP-1 binding mechanism of Siglec-9 may lead to both reversible and irreversible components in the kinetics of Siglec-9 ligands. In prominent inflammation the leaky blood vessel walls will allow large molecules to distribute into the intracellular space, causing increased tracer uptake that is non-specific to the target but still related to the inflammation.
Aalto K, Autio A, Kiss EA, Elima K, Nymalm Y, Veres TZ, Marttila-Ichihara F, Elovaara H, Saanijoki T, Crocker PR, Maksimow M, Bligt E, Salminen TA, Salmi M, Roivainen A, Jalkanen S. Siglec-9 is a novel leukocyte ligand for vascular adhesion protein-1 and can be used in PET imaging of inflammation and cancer. Blood 2011; 118(13): 3725-3733. doi: 10.1182/blood-2010-09-311076.
Aalto K, Havulinna AS, Jalkanen S, Salomaa V, Salmi M. Soluble vascular adhesion protein-1 predicts incident major adverse cardiovascular events and improves reclassification in a finnish prospective cohort study. Circ Cardiovasc Genet. 2014; 7(4): 529-535. doi: 10.1161/CIRCGENETICS.113.000543.
Ahtinen H, Kulkova J, Lindholm L, Eerola E, Hakanen AJ, Moritz N, Söderström M, Saanijoki T, Jalkanen S, Roivainen A, Aro HT. 68Ga-DOTA-Siglec-9 PET/CT imaging of peri-implant tissue responses and staphylococcal infections. EJNMMI Res. 2014; 4:45. doi: 10.1186/s13550-014-0045-3.
Autio A, Ujula T, Luoto P, Salomäki S, Jalkanen S, Roivainen A. PET imaging of inflammation and adenocarcinoma xenografts using vascular adhesion protein 1 targeting peptide 68Ga-DOTAVAP-P1: comparison with 18F-FDG. Eur J Nucl Med Mol Imaging. 2010; 37(10):1918-1925. doi: 10.1007/s00259-010-1497-y.
Autio A, Henttinen T, Sipilä HJ, Jalkanen S, Roivainen A. Mini-PEG spacering of VAP-1-targeting 68Ga-DOTAVAP-P1 peptide improves PET imaging of inflammation. EJNMMI Res. 2011; 1(1): 10. doi: 10.1186/2191-219X-1-10.
Autio A, Jalkanen S, Roivainen A. Nuclear imaging of inflammation: homing-associated molecules as targets. EJNMMI Res. 2013a; 3(1): 1. doi: 10.1186/2191-219X-3-1.
Autio A, Vainio PJ, Suilamo S, Mali A, Vainio J, Saanijoki T, Noponen T, Ahtinen H, Luoto P, Teräs M, Jalkanen S, Roivainen A. Preclinical evaluation of a radioiodinated fully human antibody for in vivo imaging of vascular adhesion protein-1-positive vasculature in inflammation. J Nucl Med. 2013b; 54(8): 1315-1319. doi: 10.2967/jnumed.113.120295.
Jalkanen S, Salmi M. VAP-1 and CD73, endothelial cell surface enzymes in leukocyte extravasation. Arterioscler Thromb Vasc Biol. 2008; 28(1): 18-26. doi: 10.1161/ATVBAHA.107.153130.
Kivi E, Elima K, Aalto K, Nymalm Y, Auvinen K, Koivunen E, Otto DM, Crocker PR, Salminen TA, Salmi M, Jalkanen S. Human Siglec-10 can bind to vascular adhesion protein-1 and serves as its substrate. Blood 2009; 114(26): 5385-5392. doi: 10.1182/blood-2009-04-219253.
Lankinen P, Mäkinen TJ, Pöyhönen TA, Virsu P, Salomäki S, Hakanen AJ, Jalkanen S, Aro HT, Roivainen A. 68Ga-DOTAVAP-P1 PET imaging capable of demonstrating the phase of inflammation in healing bones and the progress of infection in osteomyelitic bones. Eur J Nucl Med Mol Imaging 2008; 35(2): 352-364. doi: 10.1007/s00259-007-0637-5.
Salmi M, Jalkanen S. VAP-1: an adhesin and an enzyme. Trends Immunol. 2001; 22(4): 211-216. doi: 10.1016/S1471-4906(01)01870-1.
Salmi M, Jalkanen S. Cell-surface enzymes in control of leukocyte trafficking. Nat Rev Immunol. 2005; 5(10): 760-771. doi: 10.1038/nri1705.
Salmi M, Jalkanen S. Homing-associated molecules CD73 and VAP-1 as targets to prevent harmful inflammations and cancer spread. FEBS Letters 2011; 585: 1543-1550. doi: 10.1016/j.febslet.2011.04.033.
Silvola J, Autio A, Luoto P, Jalkanen S, Roivainen A. Preliminary evaluation of novel 68Ga-DOTAVAP-PEG-P2 peptide targeting vascular adhesion protein-1. Clin Physiol Funct Imaging 2010; 30(1):75-78. doi: 10.1111/j.1475-097X.2009.00907.x.
Silvola JM, Virtanen H, Siitonen R, Hellberg S, Liljenbäck H, Metsälä O, Ståhle M, Saanijoki T, Käkelä M, Hakovirta H, Ylä-Herttuala S, Saukko P, Jauhiainen M, Veres TZ, Jalkanen S, Knuuti J, Saraste A, Roivainen A. Leukocyte trafficking-associated vascular adhesion protein 1 is expressed and functionally active in atherosclerotic plaques. Sci Rep. 2016; 6:35089. doi: 10.1038/srep35089.
Ujula T, Salomäki S, Virsu P, Lankinen P, Mäkinen TJ, Autio A, Yegutkin GG, Knuuti J, Jalkanen S, Roivainen A. Synthesis, 68Ga labeling and preliminary evaluation of DOTA peptide binding vascular adhesion protein-1: a potential PET imaging agent for diagnosing osteomyelitis. Nucl Med Biol. 2009; 36(6): 631-641. doi: 10.1016/j.nucmedbio.2009.04.008.
Virtanen H, Autio A, Siitonen R, Liljenbäck H, Saanijoki T, Lankinen P, Mäkilä J, Käkelä M, Teuho J, Savisto N, Jaakkola K, Jalkanen S, Roivainen A. 68Ga-DOTA-Siglec-9 - a new imaging tool to detect synovitis. Arthritis Res Ther. 2015; 17:308. doi: 10.1186/s13075-015-0826-8.
Virtanen H, Silvola JMU, Autio A, Li X-G, Liljenbäck H, Hellberg S, Siitonen R, Ståhle M, Käkelä M, Airaksinen AJ, Helariutta K, Tolvanen T, Veres TZ, Saraste A, Knuuti J, Jalkanen S, Roivainen A. Comparison of 68Ga-DOTA-Siglec-9 and 18F-fluorodeoxyribose-Siglec-9: inflammation imaging and radiation dosimetry. Contrast Media Mol Imaging 2017; 7645070. doi: 10.1155/2017/7645070.
Virtanen H. Vascular adhesion protein-1 as in vivo target for imaging of leukocyte transendothelial migration in inflammation. Annales Universitatis Turkuensis, Ser D, 2017. ISBN 978-951-29-6886-2.
Wu C, Li F, Niu G, Chen X. PET imaging of inflammation biomarkers. Theranostics 2013; 3(7): 448-466. doi: 10.7150/thno.6592.
Updated at: 2021-01-20
Created at: 2014-11-07
Written by: Vesa Oikonen, Helena Virtanen, Anne Roivainen