Quantification of P2X7 receptors with [11C]SMW139
Purinergic receptors (purinoceptors) include the P1 purinoceptors (adenosine receptors), and P2 purinoceptors which are preferably activated by ATP and other substrates than adenosine. P2X7R is structurally and functionally distinct from the other P2X receptors. The affinity of ATP for P2X7R is low, and micromolar extracellular concentrations of ATP do not activate it. Intense stimulation of P2X7Rs results in the formation of a large transmembrane pore that further increases the extracellular [ATP], and can lead to activation of caspases and cell death.
P2X7R is expressed in many cell types, including sympathetic neurons, astrocytes, oligodendrocytes, microglial cells, probably also CNS neurons, skeletal muscle cells, osteoblasts and osteoclasts, keratinocytes, fibroblasts, epithelial cells, white blood cells, erythrocytes, and endo- and exocrine cells. In the brain, P2X7R levels are highest in microglia, and the receptor is important in regulation of immune responses, including the neuroinflammatory cascades that precede and promote many neurodegenerative brain diseases such as Parkinson’s disease. The gene of the P2X7R is polymorphic, including many splice variations and single nucleotide polymorphisms, associated with diseases including multiple sclerosis, bipolar disorder, and increased risk for bone fractures.
Various inflammatory mediators can up-regulate P2X7R expression on macrophages and other cell types, while anti-inflammatory mediators can down-regulate the expression (Bartlett et al., 2014).
Several PET tracers targeting P2X7R have been synthesized, including [11C]JNJ-54173717 (Ory et al., 2016), [18F]JNJ-64413739 (Koole et al., 2019), and [11C]GSK1482160 (Territo et al., 2017; Han et al., 2017). Since P2X7Rs are widely distributed, a reference region cannot be found, and quantification is dependent on arterial plasma time-activity curve (PTAC) as the input function. With no reference region, volume of distribution has been used to represent the receptor binding, and Lassen plot has been used to estimate receptor occupancies.
[11C]SMW139 is a recently developed tracer (Wilkinson et al., 2017; Janssen et al., 2018) aimed for P2X7R imaging in the brain. From several adamantanyl benzamides, [11C]SMW139 (“trifluorinated benzamide 34”) had the best properties, including metabolic stability, and its inhibitory activity was not markedly affected by the six functionally characterized hP2X7R SNPs, suggesting that P2X7R polymorphisms would not confound the [11C]SMW139 binding (Wilkinson et al., 2017).
In a rat model, where over-expression of P2X7R was induced by injection of adeno-associated viral vector, a clear increase in [11C]SMW139 uptake was seen in the affected site (Janssen et al., 2018). Kinetics in the rat brain is fast, suggesting that [11C]SMW139 concentration is in equilibrium with plasma, and that volume of distribution could be used for analysis.
In postmortem human brain studies no significant binding difference was found between AD and control subjects (Janssen et al., 2018), but there was a slight trend towards lower binding especially in white matter of AD subjects. Binding was higher in white matter than in grey matter in both groups (Janssen et al., 2018).
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Janssen B, Vugts DJ, Wilkinson SM, Ory D, Chalon S, Hoozemans JJM, Schuit RC, Beaiono W, Kooijman EJM, van den Hoek J, Chishty M, Doméné A, Van der Perren A, Villa A, Maggi A, Molenaar GT, Funke U, Shevchenko RV, Baekelandt V, Bormans G, Lammertsma AA, Kassiou M, Windhorst AD. Identification of the allosteric P2X7 receptor antagonist [11C]SMW139 as a PET tracer of microglial activation. Sci Rep. 2018; 8(1): 6580. doi: 10.1038/s41598-018-24814-0.
Wilkinson SM, Barron ML, O’Brien-Brown J, Janssen B, Stokes L, Werry EL, Chishty M, Skarratt KK, Ong JA, Hibbs DE, Vugts DJ, Fuller S, Windhorst AD, Kassiou M. Pharmacological evaluation of novel bioisosteres of an adamantanyl benzamide P2X7 receptor antagonist. ACS Chem Neurosci. 2017; 8: 2374-2380. doi: 10.1021/acschemneuro.7b00272.
Updated at: 2019-05-05
Created at: 2018-03-29
Written by: Vesa Oikonen, Päivi Marjamäki