A practical process for the preparation of [(32)P]S1P and binding assay for S1P receptor ligands

PET imaging of neuroinflammation: any credible alternatives to TSPO yet?

Over the last decades, the role of neuroinflammation in neuropsychiatric conditions has attracted an exponentially growing interest. A key driver for this trend was the ability to image brain inflammation in vivo using PET radioligands targeting the Translocator Protein 18 kDa (TSPO), which is known to be expressed in activated microglia and astrocytes upon inflammatory events as well as constitutively in endothelial cells. TSPO is a mitochondrial protein that is expressed mostly by microglial cells upon activation but is also expressed by astrocytes in some conditions and constitutively by endothelial cells. Therefore, our current understanding of neuroinflammation dynamics is hampered by the lack of alternative targets available for PET imaging. We performed a systematic search and review on radiotracers developed for neuroinflammation PET imaging apart from TSPO. The following targets of interest were identified through literature screening (including previous narrative reviews): P2Y12R, P2X7R, CSF1R, COX (microglial targets), MAO-B, I2BS (astrocytic targets), CB2R & S1PRs (not specific of a single cell type). We determined the level of development and provided a scoping review for each target. Strikingly, astrocytic biomarker MAO-B has progressed in clinical investigations the furthest, while few radiotracers (notably targeting S1P1Rs, CSF1R) are being implemented in clinical investigations. Other targets such as CB2R and P2X7R have proven disappointing in clinical studies (e.g. poor signal, lack of changes in disease conditions, etc.). While astrocytic targets are promising, development of new biomarkers and tracers specific for microglial activation has proven challenging.

An overview of sphingosine-1-phosphate receptor 2: Structure, biological function, and small-molecule modulators

Over the past decade, there has been a notable increase in research on sphingosine-1-phosphate receptor 2 (S1PR2), which is a type of G-protein-coupled receptor. Upon activation by S1P or other ligands, S1PR2 initiates downstream signaling pathways such as phosphoinositide 3-kinase (PI3K), Mitogen-activated protein kinase (MAPK), Rho/Rho-associated coiled-coil containing kinases (ROCK), and others, contributing to the diverse biological functions of S1PR2 and playing a pivotal role in various physiological processes and disease progressions, such as multiple sclerosis, fibrosis, inflammation, and tumors. Due to the extensive biological functions of S1PR2, many S1PR2 modulators, including agonists and antagonists, have been developed and discovered by pharmaceutical companies (e.g., Novartis and Galapagos NV) and academic medicinal chemists for disease diagnosis and treatment. However, few reviews have been published that comprehensively overview the functions and regulators of S1PR2. Herein, we provide an in-depth review of the advances in the function of S1PR2 and its modulators. We first summarize the structure and biological function of S1PR2 and its pathological role in human diseases. We then focus on the discovery approach, design strategy, development process, and biomedical application of S1PR2 modulators. Additionally, we outline the major challenges and future directions in this field. Our comprehensive review will aid in the discovery and development of more effective and clinically applicable S1PR2 modulators.

Ying and Yang of Ceramide in the Vascular Endothelium

Ceramides, a group of biologically active sphingolipids, have been described as the new cholesterol given strong evidence linking high plasma ceramide with endothelial damage, risk for early adverse cardiovascular events, and development of cardiometabolic disease. This relationship has sparked great interest in investigating therapeutic targets with the goal of suppressing ceramide formation. However, the growing data challenge this paradigm of ceramide as solely eliciting detrimental effects to the cardiovascular system. Studies show that ceramides are necessary for maintaining proper endothelial redox states, mechanosensation, and membrane integrity. Recent work in preclinical models and isolated human microvessels highlights that the loss of ceramide formation can in fact propagate vascular endothelial dysfunction. Here, we delve into these conflicting findings to evaluate how ceramide may be capable of exerting both beneficial and damaging effects within the vascular endothelium. We propose a unifying theory that while basal levels of ceramide in response to physiological stimuli are required for the production of vasoprotective metabolites such as S1P (sphingosine-1-phosphate), the chronic accumulation of ceramide can promote activation of pro-oxidative stress pathways in endothelial cells. Clinically, the evidence discussed here highlights the potential challenges associated with therapeutic suppression of ceramide formation as a means of reducing cardiovascular disease risk.

Design, synthesis, and biological evaluation of multiple F-18 S1PR1 radiotracers in rodent and nonhuman primate

Here we report our design and synthesis of 28 new fluorine-containing compounds as potential F-18 radiotracers for CNS imaging of sphingosine-1-phosphate receptor 1 (S1PR1), and determination of their in vitro binding potency and selectivity toward S1PR1 over other S1PR subtypes. Nine potent and selective compounds, 7c&d, 9a&c, 12b, 15b, and 18a-c with IC50 values ranging from 0.6-12.3 nM for S1PR1 and weak binding toward S1PR2, 3, 4, and 5, were further 18F-radiolabeled to produce [18F]7c&d, [18F]9a&c, [18F]12b, [18F]15b, and [18F]18a-c. Multi-step F-18 radiochemistry procedures were investigated for radiosynthesis of [18F]7c&d and [18F]9a&c, and the presumed intermediates were synthesized and authenticated by analytic HPLC. We then performed nonhuman primate (NHP) PET brain imaging studies for eight radiotracers: [18F]7c&d, [18F]9a, [18F]12b, [18F]15b, and [18F]18a-c. Three radiotracers, [18F]7c, [18F]7d, and [18F]15b, had high NHP brain uptake with standardized uptake values (SUVs) at 2 h post-injection of 2.42, 2.84, and 2.00, respectively, and good brain retention. Our ex vivo biodistribution study in rats confirmed [18F]7d had a high brain uptake with no in vivo defluorination. Radiometabolic analysis of [18F]7c and [18F]7d in rat plasma and brain samples found that [18F]7c has a more favorable metabolic profile than [18F]7d. However, the trend of increased brain uptake precludes [18F]7c as a suitable PET radiotracer for imaging S1PR1 in the brain. Further structural optmization is warranted to identify a highly S1PR1-specific radiotracer with rapid brain uptake kinetics.

Discovery of a Promising Fluorine-18 Positron Emission Tomography Radiotracer for Imaging Sphingosine-1-Phosphate Receptor 1 in the Brain

Sphingosine-1-phosphate receptor 1 (S1PR1) is recognized as a novel therapeutic and diagnostic target in neurological disorders. We recently transferred the S1PR1 radioligand [11C]CS1P1 into clinical investigation for multiple sclerosis. Herein, we reported the design, synthesis and evaluation of novel F-18 S1PR1 radioligands. We combined the structural advantages of our two lead S1PR1 radioligands and synthesized 14 new S1PR1 compounds, then performed F-18 radiochemistry on the most promising compounds. Compound 6h is potent (IC50 = 8.7 nM) and selective for S1PR1. [18F]6h exhibited a high uptake in macaque brain (SUV > 3.0) and favorable brain washout pharmacokinetics in positron emission tomography (PET) study. PET blocking and displacement studies confirmed the specificity of [18F]6h in vivo. Radiometabolite analysis confirmed no radiometabolite of [18F]6h entered into the brain to confound the PET measurement. In summary, [18F]6h is a promising radioligand to image S1PR1 and worth translational clinical investigation for humans with brain disorders.

Synthesis and evaluation of highly selective quinazoline-2,4-dione ligands for sphingosine-1-phosphate receptor 2

A series of twenty-nine new quinazoline-2,4-dione compounds were synthesized and their IC50 values for binding toward sphingosine-1-phosphate receptor 2 (S1PR2) were determined using a [32P]S1P binding assay. Seven compounds 2a, 2g, 2h, 2i, 2j, 2k, and 5h exhibit high S1PR2 binding potencies (IC50 values < 50 nM) and four of these new compounds 2g, 2i, 2j, and 2k have IC50 values (<10 nM) of 6.3, 5.7, 4.8, and 2.6 nM, and are highly selective for S1PR2 over other S1PR subtypes, S1PR1, 3, 4, and 5. Compounds 2a and 2i were chosen for C-11 radiosynthesis through O-[11C]methylation of precursors 13 and 2k with good radiochemical yields (35-40%), high chemical and radiochemical purity (>98%), and high molar activity (153-222 GBq μmol-1, at the end of bombardment). [11C]2a and [11C]2i were further evaluated by the ex vivo biodistribution study. The results showed that both tracers have low brain uptake, preventing their potential for neuroimaging application. Further explorations of this class of S1PR2 PET tracers in peripheral tissue diseases are underway.

A Novel Radioimmune 99mTc-Labeled Tracer for Imaging Sphingosine 1-Phosphate Receptor 1 in Tumor Xenografts: An In Vitro and In Vivo Study

Sphingosine-1-phosphate (S1P) is a phospholipid that regulates pleiotropic biological activities and exerts extracellular functions by binding to five specific G-protein-coupled receptors, S1P receptors (S1PR) 1-5. When activated by S1P, S1PR promote the proliferation and invasion of tumor cells by inducing the formation of new blood vessels. We developed and assessed a new monoclonal antibody imaging probe ^99mTc-HYNIC-S1PR1mAb, to explore the feasibility of targeting the S1PR1 in vitro and in vivo. S1PR1mAb was prepared and followed by technetium-99m labeling with succinimidyl 6-hydraziniumnicotinate hydrochloride. Cell uptake and blocking studies were performed to investigate the binding specificity of ^99mTc-HYNIC-S1PR1mAb in vitro. ^99mTc-HYNIC-S1P1mAb was also tested in vivo in mice xenografted with SK-HEP-1 (high-expression of S1PR1) and MCF-7 (low-expression of S1PR1) using single-photon emission-computed tomography (SPECT). Ex vivo gamma counting of tissues from tumor-bearing mice was used to evaluate ^99mTc-HYNIC-S1PR1mAb biodistribution. The biodistribution study results showed significantly higher uptake in SK-HEP-1 tumors than in MCF-7 tumors (P < 0.001). Reduced uptake of ^99mTc-HYNIC-S1PR1mAb in SK-HEP-1 was observed in tumor-bearing nude mice pretreated with fingolimod, which binds competitively to the receptors, especially S1PR1. ^99mTc-HYNIC-S1PR1mAb can be synthesized and specifically targeted to S1PR1 in vitro and in vivo, allowing S1PR1 expression assessment with SPECT imaging.

Physiological and pathological functions of sphingolipids in pregnancy

Signaling by the bioactive sphingolipid, sphingosine 1-phosphate (S1P), and its precursors are emerging areas in pregnancy research. S1P and ceramide levels increase towards end of gestation, suggesting a physiological role in parturition. However, high levels of circulating S1P and ceramide are correlated with pregnancy disorders such as preeclampsia, gestational diabetes mellitus and intrauterine growth restriction. Expression of placental and decidual enzymes that metabolize S1P and S1P receptors are also dysregulated during pregnancy complications. In this review, we provide an in-depth examination of the signaling mechanism of S1P and ceramide in various reproductive tissues during gestation. These factors determine implantation and early pregnancy success by modulating corpus luteum function from progesterone production to luteolysis through to apoptosis. We also highlight the role of S1P through receptor signaling in inducing decidualization and angiogenesis in the decidua, as well as regulating extravillous trophoblast migration to anchor the placenta into the uterine wall. Recent advances on the role of the S1P:ceramide rheostat in controlling the fate of villous trophoblasts and the role of S1P as a negative regulator of trophoblast syncytialization to a multinucleated placental barrier are discussed. This review also explores the role of S1P in anti-inflammatory and pro-inflammatory signaling, its role as a vasoconstrictor, and the effects of S1P metabolizing enzymes and receptors in pregnancy.

Synthesis and characterization of [125I]TZ6544, a promising radioligand for investigating sphingosine-1-phosphate receptor 2

INTRODUCTION

Sphingosine-1-phosphate receptor 2 (S1PR2) activation exerts a critical role in biological abnormalities and diseases. A suitable radiotracer will advance our understanding of S1PR2 pathophysiology of diseases. The objective of this study is to evaluate the potential of iodine-125 labeled [¹²⁵I]TZ6544 to be used for screening new compounds binding toward S1PR2, and assessing the changes of S1PR2 expression in the kidney of streptozotocin-induced diabetic rats.

METHODS

[¹²⁵I]TZ6544 was synthesized from borate precursor by copper (II)-catalyzed iodization reaction with [¹²⁵I]NaI. [¹²⁵I]TZ6544 was characterized using human recombinant S1PR2 cell membrane and biodistribution studies of [¹²⁵]TZ6544 were performed on Wistar rats that were euthanized at 5 and 30 min post-injection. A rat model of diabetes was induced by IV injection of streptozotocin (55 mg/kg). In vitro autoradiography studies, immunostaining, and enzyme-linked immunosorbent assay (ELISA) analysis were performed in both diabetic and control rats.

RESULTS

Radiosynthesis of [¹²⁵I]TZ6544 was achieved successfully with good radiochemical yields of ~47% and high radiochemical purity of >99%. [¹²⁵I]TZ6544 is a potent ligand in vitro for S1PR2 with K_d value of 4.31 nM. [¹²⁵I]TZ6544 and [³²P]-labeled endogenous S1P provided comparable IC₅₀ values in radioactive competitive binding assays against known S1PR2 ligands. Compared to control, the kidney of diabetic rats had increased uptake of [¹²⁵I]TZ6544, which could be reduced by a S1PR2 antagonist, JTE-013. Immunostaining and ELISA analysis confirmed that the diabetic rat had increased S1PR2 expression in the kidney.

CONCLUSIONS

[¹²⁵I]TZ6544 was synthesized successfully in high yields, and in vitro evaluation suggested [¹²⁵I]TZ6544 has high potential to be used for screening new S1PR2 compounds and investigating the pathophysiology of S1PR2 functions. The availability of [¹²⁵I]TZ6544 may facilitate the development of therapeutics and imaging agents targeting S1PR2. ADVANCES IN KNOWLEDGE: [¹²⁵I]TZ6544 showed increased expression of S1PR2 in diabetic rat kidney and can be used to determine binding potency of S1PR2 compounds.

Evaluating the antitumor activity of sphingosine-1-phosphate against human triple-negative breast cancer cells with basal-like morphology

Sphingosine-1-phosphate (S1P) is an important sphingolipid metabolite that regulates a wide range of physiological and pathophysiological processes. Our previous studies show that S1P selectively induces cell apoptosis in human breast cancer luminal A subtype cell line MCF7. In addition, S1P exhibits synergistic effects with chemotherapy drugs against both MCF7 and luminal B subtype cell line MDA-MB-361 at concentration in the high nM to low μM range. In the current study, we evaluated the effect of S1P on proliferation, apoptosis and cytotoxicity towards a panel of nine triple-negative breast cancer with basal-like morphology (TNBC-BL) cell lines (HCC1599, HCC1937, HCC1143, MDA-MB-468, HCC38, HCC70, HCC1806, HCC1187 and DU4475) in the same concentration range. S1P exhibited mild to moderate effects (<20% increase comparted to control) towards the TNBC-BL cell lines except HCC38, HCC70 and HCC1806. Furthermore, it increased cell apoptosis by ~15-20% in all the cell lines compared to the control, and elicited moderate to strong cytotoxic effect towards all cell lines except MDA-MB-468 and HCC1806. However, no synergistic/additive effect was observed between S1P and chemotherapy drug docetaxel for any TNBC-BL cell line.

Design, synthesis, and in vitro bioactivity evaluation of fluorine-containing analogues for sphingosine-1-phosphate 2 receptor

Twenty eight new aryloxybenzene analogues were synthesized and their in vitro binding potencies toward S1PR2 were determined using a [³²P]S1P competitive binding assay. Out of these new analogues, three compounds, 28c (IC₅₀ = 29.9 ± 3.9 nM), 28e (IC₅₀ = 14.6 ± 1.5 nM), and 28g (IC₅₀ = 38.5 ± 6.3 nM) exhibited high binding potency toward S1PR2 and high selectivity over the other four receptor subtypes (S1PR1, 3, 4, and 5; IC₅₀ > 1000 nM). Each of the three potent compounds 28c, 28e, and 28g contains a fluorine atom that will allow to develop F-18 labeled PET radiotracers for imaging S1PR2.

Syntheses and in vitro biological evaluation of S1PR1 ligands and PET studies of four F-18 labeled radiotracers in the brain of nonhuman primates

A series of seventeen hydroxyl-containing sphingosine 1-phosphate receptor 1 (S1PR1) ligands were designed and synthesized. Their in vitro binding potencies were determined using [32P]S1P competitive binding assays. Compounds 10a, 17a, 17b, and 24 exhibited high S1PR1 binding potencies with IC50 values ranging from 3.9 to 15.4 nM and also displayed high selectivity for S1PR1 over other S1P receptor subtypes (IC50 > 1000 nM for S1PR2-5). The most potent compounds 10a, 17a, 17b, and 24 were subsequently radiolabeled with F-18 in high yields and purities. MicroPET studies in cynomolgus macaque showed that [18F]10a, [18F]17a, and [18F]17b but not [18F]24 crossed the blood brain barrier and had high initial brain uptake. Further validation of [18F]10a, [18F]17a, and [18F]17b in preclinical models of neuroinflammation is warranted to identify a suitable PET radioligand to quantify S1PR1 expression in vivo as a metric of an inflammatory response.

Syntheses and in vitro evaluation of new S1PR1 compounds and initial evaluation of a lead F-18 radiotracer in rodents

Thirteen new sphingosine-1-phosphate receptor 1 (S1PR1) ligands were designed and synthesized by replacing azetidine-3-carboxylic acid moiety of compound 4 with new polar groups. The in vitro binding potency of these new analogs toward S1PR1 was determined. Out of 13 new compounds, four compounds 9a, 10c, 12b, and 16b displayed high S1PR1 binding potency with IC50 values of 13.2 ± 3.2, 14.7 ± 1.7, 9.7 ± 1.6, and 6.3 ± 1.3 nM, respectively; further binding studies of these four ligands toward S1PR2-5 suggested they are highly selective for S1PR1 over other S1PRs. The radiosynthesis of the lead radiotracer [18F]12b was achieved with good radiochemical yield (∼14.1%), high radiochemical purity (>98%), and good specific activity (∼54.1 GBq/μmol, decay corrected to the end of synthesis, EOS). Ex vivo autoradiography and initial biodistribution studies in rodents were performed, suggesting that [18F]12b was able to penetrate the blood-brain barrier (BBB) with high brain uptake (0.71% ID/g at 60 min post-injection) and no defluorination was observed. In vitro autoradiography study in brain slices of lipopolysaccharides (LPS)-induced neuroinflammation mice indicated that SEW2871, a specific S1PR1 ligand was able to reduce the uptake of [18F]12b, suggesting [18F]12b has S1PR1 specific binding. These initial results suggested that [18F]12b has potential to be an F-18 labeled radiotracer for imaging S1PR1 in the brain of the animal in vivo.

Design and synthesis of pyrazolopyridine derivatives as sphingosine 1-phosphate receptor 2 ligands

Eleven new sphingosine 1-phosphate receptor 2 (S1PR2) ligands were synthesized by modifying lead compound N-(2,6-dichloropyridin-4-yl)-2-(4-isopropyl-1,3-dimethyl-1H-pyrazolo[3,4-b]pyridin-6-yl)hydrazine-1-carboxamide (JTE-013) and their binding affinities toward S1PRs were determined in vitro using [³²P]S1P and cell membranes expressing recombinant human S1PRs. Among these ligands, 35a (IC₅₀ = 29.1 ± 2.6 nM) and 35b (IC₅₀ = 56.5 ± 4.0 nM) exhibit binding potency toward S1PR2 comparable to JTE-013 (IC₅₀ = 58.4 ± 7.4 nM) with good selectivity for S1PR2 over the other S1PRs (IC₅₀ > 1000 nM). Further optimization of these analogues may identify additional and more potent and selective compounds targeting S1PR2.

A promising carbon-11-labeled sphingosine-1-phosphate receptor 1-specific PET tracer for imaging vascular injury

BACKGROUND

Sphingosine-1-phosphate receptor 1 (S1PR1) is highly expressed in vascular smooth muscle cells from intimal lesions. PET imaging using S1PR1 as a biomarker would increase our understanding of its role in vascular pathologies including in-stent restenosis.

METHODS

The S1PR1 compound TZ3321 was synthesized for in vitro characterization and labeled with Carbon-11 for in vivo studies. The biodistribution of [11C]TZ3321 was evaluated in normal mice; microPET and immunohistochemistry (IHC) studies were performed using a murine femoral artery wire-injury model of restenosis.

RESULTS

The high potency of TZ3321 for S1PR1 (IC 50 = 2.13 ± 1.63 nM), and high selectivity (>1000 nM) for S1PR1 over S1PR2 and S1PR3 were confirmed. Biodistribution data revealed prolonged retention of [11C]TZ3321 in S1PR1-enriched tissues. MicroPET imaging of [11C]TZ3321 showed higher uptake in the wire-injured arteries of ApoE-/- mice than in injured arteries of wild-type mice (SUV 0.40 ± 0.06 vs 0.28 ± 0.04, n = 6, P < .001); FDG-PET showed no difference (SUV 0.98 ± 0.04 vs 0.94 ± 0.01, n = 6, P > .05). Post-PET autoradiography showed >4-fold higher [11C]TZ3321 retention in the injured artery of ApoE-/- mice than in wild-type mice. Subsequent IHC staining confirmed higher expression of S1PR1 in the neointima of the injured artery of ApoE-/- mice than in wild-type mice.

CONCLUSIONS

This preliminary study supports the potential use of PET for quantification of the S1PR1 expression as a biomarker of neointimal hyperplasia.

Design, Synthesis, and In Vitro and In Vivo Evaluation of an (18)F-Labeled Sphingosine 1-Phosphate Receptor 1 (S1P1) PET Tracer

Sphingosine 1-phosphate receptor 1 (S1P1) plays a pivotal signaling role in inflammatory response; because S1P1 modulation has been identified as a therapeutic target for various diseases, a PET tracer for S1P1 would be a useful tool. Fourteen fluorine-containing analogues of S1P ligands were synthesized and their in vitro binding potency measured; four had high potency and selectivity for S1P1 (S1P1 IC50 < 10 nM, >100-fold selectivity for S1P1 over S1P2 and S1P3). The most potent ligand, 28c (IC50 = 2.63 nM for S1P1) was (18)F-labeled and evaluated in a mouse model of LPS-induced acute liver injury to determine its S1P1-binding specificity. The results from biodistribution, autoradiography, and microPET imaging showed higher [(18)F]28c accumulation in the liver of LPS-treated mice than controls. Increased expression of S1P1 in the LPS model was confirmed by immunohistochemical analysis (IHC). These data suggest that [(18)F]28c is a S1P1 PET tracer with high potential for imaging S1P1 in vivo.

A potent and selective C-11 labeled PET tracer for imaging sphingosine-1-phosphate receptor 2 in the CNS demonstrates sexually dimorphic expression

Sphingosine-1-phosphate receptor 2 (S1PR2) plays an essential role in regulating blood-brain barrier (BBB) function during demyelinating central nervous system (CNS) disease. Increased expression of S1PR2 occurs in disease-susceptible CNS regions of female versus male SJL mice and in female multiple sclerosis (MS) patients. Here we reported a novel sensitive and noninvasive method to quantitatively assess S1PR2 expression using a C-11 labeled positron emission tomography (PET) radioligand [(11)C]5a for in vivo imaging of S1PR2. Compound 5a exhibited promising binding potency with IC50 value of 9.52 ± 0.70 nM for S1PR2 and high selectivity over S1PR1 and S1PR3 (both IC50 > 1000 nM). [(11)C]5a was synthesized in ∼40 min with radiochemistry yield of 20 ± 5% (decayed to the end of bombardment (EOB), n > 10), specific activity of 222-370 GBq μmol(-1) (decayed to EOB). The biodistribution study in female SJL mice showed the cerebellar uptake of radioactivity at 30 min of post-injection of [(11)C]5a was increased by Cyclosporin A (CsA) pretreatment (from 0.84 ± 0.04 ID% per g to 2.21 ± 0.21 ID% per g, n = 4, p < 0.01). MicroPET data revealed that naive female SJL mice exhibited higher cerebellar uptake compared with males following CsA pretreatment (standardized uptake values (SUV) 0.58 ± 0.16 vs. 0.48 ± 0.12 at 30 min of post-injection, n = 4, p < 0.05), which was consistent with the autoradiographic results. This data suggested that [(11)C]5a had the capability in assessing the sexual dimorphism of S1PR2 expression in the cerebellum of the SJL mice. The development of radioligands for S1PR2 to identify a clinical suitable S1PR2 PET radiotracer, may greatly contribute to investigating sex differences in S1PR2 expression that contribute to MS subtype and disease progression and it will be very useful for detecting MS in early state and differentiating MS with other patients with neuroinflammatory diseases, and monitoring the efficacy of treating diseases using S1PR2 antagonism.