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. It is normally stored in intracellular granules within endothelial cells, but in inflamed tissue it is rapidly translocated onto the luminal surface, and its expression is upregulated. Also in certain cancers VAP-1 is found on endothelial cell surfaces. 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 catalyzes 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.
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), and atherosclerotic plaques (Silvola et al., 2016). Clinical grade radiosynthesis of [68Ga]DOTA-Siglec-9 has been established (Käkelä et al., 2018).
[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 model (Jødal et al., 2018).
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 labeled 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.
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Created at: 2014-11-07
Updated at: 2018-08-29
Written by: Vesa Oikonen, Helena Virtanen, Anne Roivainen