18 F‐Labeled Derivatives of Irbesartan for Angiotensin II Receptor PET Imaging

Evaluation of the high affinity [18F]fluoropyridine-candesartan in rats for PET imaging of renal AT1 receptors

INTRODUCTION

Alterations in the expression of the Angiotensin II type 1 receptors (AT1R) have been demonstrated in the development of several heart and renal diseases. The aim of this study was to evaluate the novel compound [18F]fluoropyridine-candesartan as a PET imaging tracer of AT1R in rat kidneys.

METHODS

Competition binding assays were carried out with membranes from CHO-K1 cells expressing human AT1R. Binding to plasma proteins was assessed by ultrafiltration. Radiolabeled metabolites in rat plasma and kidneys of control and pretreated animals (candesartan 10 mg/kg or losartan 30 mg/kg) were analyzed by column-switch HPLC. Dynamic PET/CT images of [18F]fluoropyridine-candesartan in male Sprague-Dawley rats were acquired for 60 min at baseline, pre-treatment with the AT1R antagonist losartan (30 mg/kg) or the AT2R antagonist PD123,319 (5 mg/kg).

RESULTS

Fluoropyridine-candesartan bound with a high affinity for AT1R (Ki = 5.9 ± 1.1 nM), comparable to fluoropyridine-losartan but lower than the parent compound candesartan (Ki = 0.4 ± 0.1 nM). [18F]Fluoropyridine-candesartan bound strongly to plasma proteins (99.3%) and was mainly metabolized to radiolabeled hydrophilic compounds, displaying minimal interference on renal AT1R binding with 82% of unchanged tracer in the kidneys at 20 min post-injection. PET imaging displayed high renal and liver accumulations and slow clearances, with maximum tissue-to-blood ratios of 14 ± 3 and 54 ± 12 in kidney cortex and liver, respectively, at 10 min post-injection. Binding specificity for AT1R was demonstrated with marked reductions in kidney cortex (-84%) and liver (-93%) tissue-to-blood ratios at 20 min post-injection, when blocking with AT1R antagonist losartan (30 mg/kg). No change was observed in kidney cortex of rats pre-treated with AT2R antagonist PD 123,319 (5 mg/kg), confirming binding selectivity for AT1 over AT2 receptors.

CONCLUSION

High kidney-to-blood ratios and binding selectivity to renal AT1R combined with tracer in vivo stability displaying minimal interference from labeled metabolites support further PET imaging studies with [18F]fluoropyridine-candesartan.

Radioiodination and biological evaluation of irbesartan as a tracer for cardiac imaging

Irbesartan was labeled using 125I or 131I with N-bromosuccinimide (NBS) as an oxidizing agent. A lot of operators such as, quantity of substrate, the quantity of oxidizing agent, reaction temperature, reaction time, and pH of reaction medium were studied to optimize high radiochemical yield of [125I]IodoIrbesartan ([125I]Irb.). The preclinical evaluation of IodoIrbesartan in experimental mice indicated high accumulation in target organ of heart with a high heart/blood ratio of 12.85 at 30 min post-injection. This study indicates the suitability of [125I]IodoIrbesartan ([125I]Irb.) for cardiac imaging.

AT1R-Specific Ligand Angiotensin II as a Novel SPECT Agent for Hepatocellular Carcinoma Diagnosis

Hepatocellular carcinoma (HCC) is characterized by a high mortality and early diagnosis and treatment are critically needed. Ang II type 1 receptor (AT1R) has recently emerged as a potential molecular target for cancer diagnosis and intervention. Here, we labeled angiotensin II (Ang II), an AT1R ligand that is overexpressed in various solid cancers, with the near-infrared fluorescent dye, MPA, and radionuclide technetium-99m, and evaluated its capacity for HCC detection. These analyses were done in vitro using HepG2 (AT1R-positive) and BxPC3 (AT1R-negative) cell lines, and in vivo using a subcutaneous and orthotopic xenograft mouse model by fluorescence and SPECT imaging. Both Ang II-PEG₄-MPA- and [^99mTc]Tc-HYNIC-PEG₄-Ang II-bound AT1R exhibited a high affinity in vitro and [^99mTc]Tc-HYNIC-PEG₄-Ang II displayed an acceptable level of in vitro stability in rat plasma and whole blood. In vivo imaging revealed excellent specific tumor-targeting in HepG2 mouse xenografts models. In vitro and in vivo competition experiments revealed specific Ang II-PEG₄-MPA and [^99mTc]Tc-HYNIC-PEG₄-Ang II uptake by HepG2 cells and tumors. Altogether, AT1R-positive tumors were successfully detected via fluorescence and SPECT imaging using Ang II-PEG₄-MPA and [^99mTc]Tc-HYNIC-PEG₄-Ang II, respectively. Given their superior targeting capacity, Ang II-PEG₄-MPA and [^99mTc]Tc-HYNIC-PEG₄-Ang II are promising tools for HCC detection and warrant clinical translation.

The preparation of (4H)-imidazol-4-ones and their application in the total synthesis of natural products

(4H)-Imidazol-4-ones are an important scaffold for a variety of applications, including natural products, medicine, agriculture, and other applications. Over the years, there have been a number of preparations published for the synthesis of imidazol-4-ones. This review discusses the progress made on the synthesis of imidazol-4-ones, and their application towards the total synthesis of a range of imidazol-4-one containing natural products. Emphasis is made on areas of the field that still need progress.

Synthesis of the Novel AT1 Receptor Tracer [18F]Fluoropyridine-Candesartan via Click Chemistry

A novel 7-((4-(3-((2-[¹⁸F]fluoropyridin-3-yl)oxy)propyl)-1H-1,2,3-triazol-1-yl)methyl)-1H-benzo[d]imidazole derivative of the angiotensin II type-1 receptor (AT₁R) blocker candesartan, [¹⁸F]fluoropyridine-candesartan, was synthesized via the copper-catalyzed azide-alkyne cycloaddition click reaction between 2-[¹⁸F]fluoro-3-(pent-4-yn-1-yloxy)pyridine ([¹⁸F]FPyKYNE) and the tetrazole-protected azido-candesartan derivative, followed by acid deprotection. This three-step, two-pot, and two-step purification synthesis was done within 2 h. The use of tris[(1-hydroxypropyl-1H-1,2,3-triazol-4-yl)methyl]amine (THPTA) as a Cu(I) stabilizing agent increased the overall radiochemical yield by 4-fold (10 ± 2%, n = 13) compared to the reaction without THPTA (2.4 ± 0.2%, n = 3; decay-corrected from ¹⁸F produced at the end-of-beam). Complete separation of [¹⁸F]FPyKYNE from its nitro precursor and [¹⁸F]fluoropyridine-candesartan from the deprotected azido-candesartan allowed for high molar activities (>380 GBq/μmol) of the tracer. The use of 0.1% trifluoroacetic acid in water for reformulation and the addition of sodium ascorbate to the final formulation (1.6 ± 0.2 GBq/mL, n = 3) prevented tracer radiolysis with >97% radiochemical purity for a period of up to 10 h after the end-of-synthesis. A significant reduction in the uptake (86 ± 3%, n = 8) of the tracer was observed ex vivo in rats (at 20 min postinjection) in the AT₁R-rich kidney cortex following pretreatment with saturating doses of the AT₁R antagonist candesartan or losartan. This specific binding to AT₁R was confirmed in vitro in the rat renal cortex (autoradiography) by a reduction of 26 ± 5% (n = 12) with losartan coincubation (10 μM). These favorable binding properties support further studies to assess the potential of [¹⁸F]fluoropyridine-candesartan as a tracer for the positron emission tomography imaging of renal AT₁R.

Synthesis and Evaluation of [18F]FEtLos and [18F]AMBF3Los as Novel 18F-Labelled Losartan Derivatives for Molecular Imaging of Angiotensin II Type 1 Receptors

Losartan is widely used in clinics to treat cardiovascular related diseases by selectively blocking the angiotensin II type 1 receptors (AT₁Rs), which regulate the renin-angiotensin system (RAS). Therefore, monitoring the physiological and pathological biodistribution of AT₁R using positron emission tomography (PET) might be a valuable tool to assess the functionality of RAS. Herein, we describe the synthesis and characterization of two novel losartan derivatives PET tracers, [¹⁸F]fluoroethyl-losartan ([¹⁸F]FEtLos) and [¹⁸F]ammoniomethyltrifluoroborate-losartan ([¹⁸F]AMBF₃Los). [¹⁸F]FEtLos was radiolabeled by ¹⁸F-fluoroalkylation of losartan potassium using the prosthetic group 2-[¹⁸F]fluoroethyl tosylate; whereas [¹⁸F]AMBF₃Los was prepared following an one-step ¹⁸F-¹⁹F isotopic exchange reaction, in an overall yield of 2.7 ± 0.9% and 11 ± 4%, respectively, with high radiochemical purity (>95%). Binding competition assays in AT₁R-expressing membranes showed that AMBF₃Los presented an almost equivalent binding affinity (K_i 7.9 nM) as the cold reference Losartan (K_i 1.5 nM), unlike FEtLos (K_i 2000 nM). In vitro and in vivo assays showed that [¹⁸F]AMBF₃Los displayed a good binding affinity for AT₁R-overexpressing CHO cells and was able to specifically bind to renal AT₁R. Hence, our data demonstrate [¹⁸F]AMBF₃Los as a new tool for PET imaging of AT₁R with possible applications for the diagnosis of cardiovascular, inflammatory and cancer diseases.

Silver(i)-mediated oxidative C(sp3)–H amination of ethers with azole derivatives under mild conditions

A silver(i)-mediated oxidative N–H/C(sp³)–H coupling of NH-azoles with ethers has been developed.