[Carboxyl-11 C]Labelling of Four High-Affinity cPLA2α Inhibitors and Their Evaluation as Radioligands in Mice by Positron Emission Tomography

Abstract

Cytosolic phospholipase A2α (cPLA2α) may play a critical role in neuropsychiatric and neurodegenerative disorders associated with oxidative stress and neuroinflammation. An effective PET radioligand for imaging cPLA2α in living brain might prove useful for biomedical research, especially on neuroinflammation. We selected four high-affinity (IC₅₀ 2.1-12 nm) indole-5-carboxylic acid-based inhibitors of cPLA2α, namely 3-isobutyryl-1-(2-oxo-3-(4-phenoxyphenoxy)propyl)-1H-indole-5-carboxylic acid (1); 3-acetyl-1-(2-oxo-3-(4-(4-(trifluoromethyl)phenoxy)phenoxy)propyl)-1H-indole-5-carboxylic acid (2); 3-(3-methyl-1,2,4-oxadiazol-5-yl)-1-(2-oxo-3-(4-phenoxyphenoxy)propyl)-1H-indole-5-carboxylic acid (3); and 3-(3-methyl-1,2,4-oxadiazol-5-yl)-1-(3-(4-octylphenoxy)-2-oxopropyl)-1H-indole-5-carboxylic acid (4), for labelling in carboxyl position with carbon-11 (t_1/2 =20.4 min) to provide candidate PET radioligands for imaging brain cPLA2α. Compounds [¹¹ C]1-4 were obtained for intravenous injection in adequate overall yields (1.1-5.5 %) from cyclotron-produced [¹¹ C]carbon dioxide and with moderate molar activities (70-141 GBq μmol^-1 ) through the use of Pd⁰ -mediated [¹¹ C]carbon monoxide insertion on iodo precursors. Measured logD_7.4 values were within a narrow moderate range (1.9-2.4). After intravenous injection of [¹¹ C]1-4 in mice, radioactivity uptakes in brain peaked at low values (≤0.8 SUV) and decreased by about 90 % over 15 min. Pretreatments of the mice with high doses of the corresponding non-radioactive ligands did not alter brain time-activity curves. Brain uptakes of radioactivity after administration of [¹¹ C]1 to wild-type and P-gp/BCRP dual knock-out mice were similar (peak 0.4 vs. 0.5 SUV), indicating that [¹¹ C]1 and others in this structural class, are not substrates for efflux transporters.

Keywords: cPLA2α; carbon-11; carbonylation; neuroinflammation; radiopharmaceuticals.

Figure 1.

Structures of 1–4.

Figure 2.

Syntheses of iodo precursors for radiolabelling 16–19. Conditions: a) acetyl chloride or isobutyryl chloride, AlCl₃, DCM, RT, 8 h. b) (1) pyridine, trichloroacetyl chloride, dioxane, 80 °C, 2.5 h; (2) MeOH, NaOH, 80 °C, 30 min.c) N-hydroxyacetamidine, NaH, THF, RT, 1 h. d) epichlorohydrin, KOH, TBAB, RT, 1 h. e) ArOH, DMAP, DCM, 120 °C, 1 h. f) Dess-Martin periodinane, DCM, RT, 4 h.

Figure 3.

Radiosynthesis of [¹¹C]1–4 through Pd(0)-mediated ¹¹C-carbonylation of corresponding iodo precursors.

Figure 4.

Distribution of [¹¹C]1 in monkey and rat blood cells with or without plasma. Values are mean ± SD for n = 3.

Figure 5.

Brain time-activity curves for [¹¹C]1–4 in wild type mice.

Figure 6.

Brain time-activity curves for [¹¹C]1 in wild type (WT) or efflux transporter knock-out (KO) mice under baseline and self-block conditions.

Figure 7.

Time-activity curves for [¹¹C]2 in different organs of wild type mice under baseline conditions.