Synthesis and Evaluation of Two Positron-Labeled Nitric Oxide Synthase Inhibitors, S-[¹¹C]Methylisothiourea and S-(2-[¹⁸F]Fluoroethyl)isothiourea, as Potential Positron Emission Tomography Tracers

Abstract

In an effort to develop a tracer for probing inducible nitric oxide synthase (iNOS) levels in vivo utilizing positron emission tomography, we have synthesized and evaluated two positron-emitting iNOS selective inhibitors: S-[11C]methylisothiourea (1b) and S-(2-[18F]fluoroethyl)-isothiourea (3b). Prior to fluorine-18 labeling, the nonradioactive fluoro derivative S-(2-fluoroethyl)isothiourea (3a) was prepared and determined to have a 9-fold higher selectivity for iNOS compared to endothelial NOS (eNOS). Radiochemical synthesis of both compounds, in high radiochemical purity and at high specific activity, was accomplished by the S-alkylation reaction of labeled precursors (11CH3I or 18FCH2CH2OTf) with thiourea. An in vitro model, J774 macrophage cell line, was used to assess the uptake of radiolabeled iNOS inhibitor in response to iNOS induction at the cellular level. Increased cell uptake of these two labeled compounds at stimulated iNOS levels, as well as blocking under controlled in vitro conditions, was observed. Lipophilicity (log P o/w), stability, and tissue biodistribution data of both compounds are reported. Serum stability studies indicate that 3b metabolized much more rapidly compared to the relatively stable 1b in vitro and in vivo. Based on in vitro cell uptake data, both tracers were further evaluated in lipopolysaccharide (LPS)-pretreated rats. LPS has been reported to induce iNOS protein expression in the liver, lung, heart, and kidney and other tissues. The uptake for LPS-pretreated rats (6 h post-treatment) was significantly increased in the liver, kidney, and heart for 3b at 10 min and in the liver and lung for 1b at 30 min. The results suggest that this first generation of radiolabeled inhibitors may be useful for assessing induction of iNOS in vivo with PET.

Design and Synthesis of 2-Amino-4-methylpyridine Analogues as Inhibitors for Inducible Nitric Oxide Synthase and in Vivo Evaluation of [18F]6-(2-Fluoropropyl)-4-methyl-pyridin-2-amine as a Potential PET Tracer for Inducible Nitric Oxide Synthase

“…In the past decade, the development of radiolabeled PET tracers for iNOS has been limited12 compared with the relatively rapid development of novel iNOS inhibitors as pharmaceuticals. In this paper, we describe the synthesis and screening of a series of position-6 substituted 2-amino-4-methylpyridine analogues as potential PET tracers for imaging iNOS, the radiosynthesis of [ 18 F] 9 , and the in vivo evaluation of [18F] 9 in a mouse model of lipopolysaccharide (LPS)-induced iNOS activation.…”

Carbon‐11 Radiolabelling of Organosulfur Compounds: 11C Synthesis of the Progesterone Receptor Agonist Tanaproget

“…Simple thioureas typically serve as precursors to larger organic molecules and heterocyclic structures, [27][28][29][30][31][32][33][34] hence radiolabelling such molecules could provide an ovel route to more complex biological molecules. To date, there has only been one reported synthesis of a 11 C radiolabelled thiourea by 11 C methylation to generate an isothiourea, [35] however,l abelling in the thiocarbonyl position has not been previously reported. Thioureas are typically prepared by the condensation reactiono f amines with isothiocyanates [36] or thiophosgene, [37] however, depending on the reaction conditions, they can also be formed in one-pot by directly treating primary amines with carbon disulfide.…”

Synthesis and biological evaluation ofS-[11C]methylated mercaptoimidazole piperazinyl derivatives as potential radioligands for imaging 5-HT1Areceptors by positron emission tomography (PET)

“…Compounds 9 and 10 were used to identify the radioligands [ 11 C]9 and [ 11 C]10, and they were used in the competitive binding assays to determine their affinity toward the 5-HT 1A receptor. 29 However, to the best of our knowledge, the labelling of 2-mercaptoimidazoles with [ 11 C]methyl iodide has not been applied yet. S-Methylation of labelling precursors 3 and 8 with […”