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

Radiosynthesis of ¹⁸F-LW223. Data are shown as mean ± SEM.

LW223 binding to TSPO is not affected by human rs6971 genetic polymorphism. (A) Chemical structures of established (PK11195, PBR28) and novel (AB5186, LW223) TSPO ligands investigated. (B) Competition binding assays using these TSPO ligands in human brain homogenates genotyped and grouped as HAB, MAB, and LAB (PK11195: HAB = 6, MAB = 8, LAB = 4; PBR28: HAB = 4, MAB [2-site fitting] = 5, LAB = 4; AB5186: HAB = 6, MAB [2-site fitting] = 6, LAB = 5; LW223: HAB = 5, MAB = 5, LAB = 4). Only PK11195 and LW223 were not affected by polymorphism, as is also demonstrated in human heart homogenates (C) (PK11195: HAB = 4, MAB = 5, LAB = 4; PBR28: HAB = 4, MAB = 5, LAB = 4; AB5186: HAB = 4, MAB = 5 [2-site fitting], LAB = 4; LW223: HAB = 5, MAB = 5, LAB = 4).

¹⁸F-LW223, assessed in naïve rats, has favorable metabolic profile and specifically targets TSPO in vivo. (A) Maximum-intensity projection of ¹⁸F-LW223 PET scan in naïve rat showing major uptake organs (left); metabolism of ¹⁸F-LW223 within rat blood showing high percentage of parent radiotracer within plasma up to 2 h after injection (middle, n = 3 per time point); and percentage of parent radiotracer within different tissues demonstrating low level of contaminating radiometabolites (right, n = 3). (B) SUV images of rat heart (left), lung (middle), and brain (right) at baseline (left side of each panel) and under TSPO blockade using prototypical TSPO ligand PK11195 (1 mg/kg, right side of each panel) demonstrating specificity of ¹⁸F-LW223 for this target. SUV PET images were filtered using gaussian 1 × 1 × 1 mm filter. (C) Total V_T within heart (left), lung (middle), and brain (right) of naïve and TSPO-blocked rats. Results represent mean ± SEM (n = 6 for naïve; n = 3 for blocked). P values were obtained using unpaired t test. A = adrenal glands; B = brain; G = gut; H = heart; K = kidney; L = lung. *P < 0.05. **P ≤ 0.01. ***P ≤ 0.001.

¹⁸F-LW223 PET with BP_TC quantification detects macrophage-driven inflammation within heart 7 d after MI without need for additional perfusion scan. (A) Long-axis representative SUV image of heart in naïve and MI rats showing increased global uptake and lack of signal within LV anterior wall due to MI-mediated reduction in perfusion (left) (images were filtered using gaussian 1 × 1 × 1 mm filter)); non–perfusion-corrected V_T within global heart and LV anterior wall, which was site of infarct (middle); and K₁ acting as surrogate marker of perfusion, being reduced within LV anterior wall (right). (B) Representative K₁ (left) and BP_TC images (middle) of LV of MI rat demonstrating true TSPO signal across heart; BP_TC values across global heart and LV anterior wall demonstrating that most TSPO is expressed within infarct (right). n = 6 for natïve and n = 9 for MI. (C) Representative histology examples of hearts from naïve (top) and MI (bottom) rat. Hematoxylin and eosin (H&E) overview (scale bar = 1,000 μm) contains box that indicates position of CD68 examples (macrophage marker) and TSPO examples (scale bar = 50 μm), demonstrating specificity of TSPO for macrophages, which are mostly present within infarct. (D) Quantification of TSPO immunofluorescent stain indicating that most signal is present within LV anterior wall (left), as is also true for CD68 quantification (middle); comparison of TSPO and CD68 indicates significant correlation within heart (right). n = 3 for naïve and n = 5 for MI. All results represent mean ± SEM. P values were obtained using 2-way ANOVA with post hoc Sidak for naïve vs. MI, apart from correlation analysis, which used Pearson correlation. A = anterior; P = posterior. **P ≤ 0.01. ***P ≤ 0.001. ****P ≤ 0.0001.