Quantification of Macrophage-Driven Inflammation During Myocardial Infarction with ¹⁸F-LW223, a Novel TSPO Radiotracer with Binding Independent of the rs6971 Human Polymorphism

Abstract

Rationale: Myocardial infarction (MI) is one of the leading causes of death worldwide and inflammation is central to the tissue response and patient outcomes. The 18kDa translocator protein (TSPO) has been utilized in positron emission tomography (PET) as an inflammatory biomarker. The aims of this study were to: 1) screen novel, fluorinated, TSPO radiotracers for susceptibility to the rs6971 genetic polymorphism using in vitro competition binding assays in human brain and heart, 2) assess whether the in vivo characteristics of our lead radiotracer, 18 F-LW223, are suitable for clinical translation and 3) validate whether 18 F-LW223 can detect macrophage driven inflammation in a rat myocardial infarction model. Methods: Fifty-one human brain and twenty-nine human heart tissue samples were screened for the rs6971 polymorphism. Competition binding assays were conducted with 3 H-PK11195 and the following ligands: PK11195, PBR28 and our novel compounds (AB5186 and LW223). Naive rats and mice were used for in vivo PET kinetic studies, radiometabolite studies and dosimetry experiments. Rats underwent permanent coronary artery ligation and were scanned using PET/CT with invasive input function at 7 days following MI. For quantification of PET signal in the hypoperfused myocardium, K 1 was used as a surrogate marker of perfusion to correct the binding potential for impaired radiotracer transfer from plasma to tissue (BP TC). Results: LW223 binding to TSPO was not susceptible to the rs6971 genetic polymorphism in human brain and heart samples. In rodents, 18 F-LW223 displayed a specific uptake consistent with TSPO expression, a slow metabolism in blood (62% of parent at 120 min), a high plasma free fraction of 38.5% and a suitable dosimetry profile Brain Tissue for Binding Assays Heart Tissue for Binding Assays

PET Imaging of Neuroinflammation in Alzheimer’s Disease

“…Several third generations of TSPO tracers [ 18 F]GE-180, (R,S)-[ 18 F]GE-387, [ 11 C]ER176, [ 11 C]CB184, [ 11 C]CB190, [ 11 C]N′-MPB, and [ 18 F]LW223 have been developed ( 75 , 103 , 105 – 109 ). [ 18 F]GE-180 (flutriciclamide), (S)-[ 18 F]GE-387, and [ 11 C]ER176 resolve the problem of ligand-dependent attenuation of affinity ( 90 , 97 , 197 ) in in vitro binding assay where these tracers are insensitive to TSPO rs6971 polymorphisms ( 104 ).…”

Have (R)-[11C]PK11195 challengers fulfilled the promise? A scoping review of clinical TSPO PET studies

“…One straightforward reason is that (R)-[ 11 C]PK11195 is unsensitive to the TSPO polymorphism. However this situation is expected to change in the near future as some of the newly developed (R)-[ 11 C] PK11195 challengers, sometimes called third-generation tracers, currently being evaluated in animal models and in healthy human subjects, were successfully tested as insensitive or less sensitive to TSPO polymorphism in humans: [ 18 F]LW223 [239]; [ 18 F]CB251 [240]; [ 11 C]ER176 [241]. The latter was suggested to have very favourable properties in healthy subjects, being sensitive enough to detect specific binding in low-affinity binders, with little influence of radiometabolites [242].…”

Preclinical evaluation of (S)-[18F]GE387, a novel 18-kDa translocator protein (TSPO) PET radioligand with low binding sensitivity to human polymorphism rs6971

“…[ 11 C]ER176, which has the lowest LAB to HAB ratio of the clinically tested TSPO radiotracer, is hindered by being radiolabelled with the short-lived radioisotope carbon-11 and the in vitro prediction of low sensitivity to polymorphism was not replicated in human in vivo studies [25]. Recently, novel TSPO radioligands that are at the preclinical stages of evaluation have been reported with low LAB to HAB binding ratio in human thrombocyte membranes ((R)-[ 18 F]NEBIFQUINIDE, LAB to HAB ratio 1.1 [13]) and in human brain tissue ([ 18 F]LW223, LAB to HAB ratio 1 [14]). Both radioligands appear to have good in vitro and preclinical in vivo properties.…”

“…Male Wistar rats (Charles River Laboratories, UK) were housed in Techniplast 2000P IVC cages on a layer of Aspen 1 [14] bedding in a room with constant temperature (21 ± 2 °C) and fixed 12 h light-dark regime (lights on at 7:00 am). Food and water were available ad libitum.…”