Elsevier

Bioorganic & Medicinal Chemistry

Volume 25, Issue 20, 15 October 2017, Pages 5576-5585
Bioorganic & Medicinal Chemistry

Synthesis and evaluation of radioiodinated 1-{2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinolin-8-yl}piperidin-4-amine derivatives for platelet-derived growth factor receptor β (PDGFRβ) imaging

https://doi.org/10.1016/j.bmc.2017.08.025Get rights and content

Abstract

Platelet-derived growth factor receptor β (PDGFRβ) is a transmembrane tyrosine kinase receptor and it is upregulated in various malignant tumors. Radiolabeled PDGFRβ inhibitors can be a convenient tool for the imaging of tumors overexpressing PDGFRβ. In this study, [125I]-1-{5-iodo-2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinoline-8-yl}piperidin-4-amine ([125I]IIQP) and [125I]-N-3-iodobenzoyl-1-{2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinolin-8-yl}-piperidin-4-amine ([125I]IB-IQP) were designed and synthesized, and their potential as PDGFRβ imaging agents was evaluated. In cellular uptake experiments, [125I]IIQP and [125I]IB-IQP showed higher uptake by PDGFRβ-positive cells than by PDGFRβ-negative cells, and the uptake in PDGFRβ-positive cells was inhibited by co-culture with PDGFRβ ligands. The biodistribution of both radiotracers in normal mice exhibited hepatobiliary excretion as the main route. In mice inoculated with BxPC3-luc (PDGFRβ-positive), the tumor uptake of radioactivity at 1 h after the injection of [125I]IIQP was significantly higher than that after the injection of [125I]IB-IQP. These results indicated that [125I]IIQP can be a suitable PDGFRβ imaging agent. However, further modification of its structure will be required to obtain a more appropriate PDGFRβ-targeted imaging agent with a higher signal/noise ratio.

Introduction

PDGFRβ belongs to a subfamily of receptor tyrosine kinases (RTKs). It possesses an outer membrane with a PDGF ligand binding site and an inner membrane tyrosine kinase (TK) domain with an adenosine triphosphate (ATP) binding site.1 Binding of PDGF ligands to the extracellular binding domain triggers PDGFRβ dimerization, thus inducing phosphorylation of its intracellular regions due to an activated ATP-binding domain. This is followed by the activation of signaling pathways that regulate important cellular functions.2 The expression of PDGFRβ is highly restricted in most normal cells; however, upregulation of PDGFRβ in numerous human tumors and its relationship with tumor progression features such as cell migration, metastasis, angiogenesis, and proliferation, have been reported.3, 4, 5 Therefore, PDGFRβ can be an attractive target not only for cancer therapy but also for developing tumor-imaging agents.6, 7, 8

Nuclear medicine has been a revolutionary invention in the field of diagnostic imaging because it is noninvasive and facilitates continuous monitoring of disease progression or treatment response.9, 10 Moreover, the imaging modalities in nuclear medicine, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) can quantify receptor density and molecular target expression in patients, thus becoming a useful tool in drug development.10, 11, 12 Previous studies have reported imaging agents, including radiolabeled protein,13, 14 aptamer,15 affibody molecules,16, 17 and peptide18, 19 that target the extracellular parts of PDGFRβ. Although the imaging of intracellular parts of PDGFRβ, particularly its ATP-binding site, has hardly been reported,8 the ATP-binding is very important for following signal transductions and growth of cancer cells. Therefore, the ATP-binding site can be a target for TK inhibitors (TKIs). Meanwhile, mutations of the ATP-binding site have frequently been found.20, 21 The mutation status often affects therapeutic effects of TKIs. Thus, the information of the ATP-binding site, containing the mutation status, is useful for the prediction of therapeutic effects of TKIs. Other radiolabeled TK inhibitors (TKIs)22, 23, 24, 25, 26 and their potential advantages, such as assessment of drug sensitivity,20 mutation status,21 and usefulness in drug screening11, 27, 28 have been demonstrated.

1-{2-[5-(2-Methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinolin-8-yl}piperidin-4-amine (CP-673451, IQP) (Fig. 1) is a highly selective benzimidazole inhibitor of PDGFRβ (IC50 = 1 nM) over PDGFRα (IC50 = 10 nM). Importantly, it is 1000 times selective compared with many other RTKs. It competitively inhibits ATP-binding to the ATP-binding site of PDGFRβ and blocks phosphorylation.29, 30 High binding affinity and high selectivity to the molecular target are important for a lead compound of imaging probes. IQP can be an attractive compound in an attempt to image the ATP binding site of PDGFRβ. Therefore, we speculated that radiolabeled IQP derivatives that bind the ATP-binding site may be useful imaging probes.

To develop novel radiolabeled molecular probes for PDGFRβ imaging, we chose radioiodine as a radionuclide in this study because 123I (t1/2 = 13.2 h) and 124I (t1/2 = 4.2 d) are useful radionuclides for SPECT and PET, respectively. We did not have sufficient information about the introduction of radioiodine into IQP structure to retain high affinity for PDGFRβ. To evaluate the feasibility of IQP derivatives as PDGFRβ imaging probes, we designed and synthesized two different types of iodine labeled IQP, 1-{5-iodo-2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]quinolin-8-yl}piperidin-4-amine (IIQP) and N-3-iodobenzoyl-1-{2-[5-(2-methoxyethoxy)-1H-benzo[d]imidazol-1-yl]-quinolin-8-yl}-piperidin-4-amine (IB-IQP). In IIQP, iodine was directly introduced to C-5 of the quinoline group of IQP because it is a reactive site for halogenation of IQP. In IB-IQP, iodine was indirectly introduced by conjugation of 3-iodo benzoyl group using SIB (N-succinimidyl-3-iodobenzoate) with the primary amine group of IQP. We then examined the binding affinities of two derivatives for PDGFRβs. Subsequently, radiotracers [125I]IIQP and [125I]IB-IQP (Fig. 1) were synthesized and evaluated in vitro and in vivo to investigate their usefulness as PDGFRβ imaging agents. Although we are interested in developing imaging probes for SPECT and PET, 125I was used as an alternative radionuclide in these initial studies because of its long half-life (t1/2 = 59.4 d).

Section snippets

General

Commercial reagents and solvents were purchased from Sigma-Aldrich (St. Louis, MO, USA), Wako Pure Chemical Industries (Osaka, Japan), Nacalai Tesque, Inc., (Kyoto, Japan), Tokyo Chemical Industry, Co., Ltd., (Tokyo, Japan) and Kanto Chemical, Co., Inc. (Tokyo, Japan) and used without further purification unless otherwise stated.

[125I]Sodium iodide (644 GBq/mg) was purchased from Perkin Elmer (Waltham, MA, USA). The radioactivity was measured by an Auto Gamma System ARC-7010B (Hitachi, Ltd.,

Synthesis of reference compounds and precursors

Lead compound 12 (Scheme 1) and the non-radioactive iodinated reference compounds, 13 and 18, were easily obtained. Iodine was incorporated to the C-5 of quinoline core of IQP using mixture reagents of NCS and NaI with stirring at 50 °C for 18 h in acetic acid to obtain compound 13 (Scheme 2). On the other hand, compound 18 was synthesized by stirring the mixture of synthesized SIB and IQP at 50 °C for 2 h in a basic solution (Scheme 3). Retention time for compounds 13 and 18 on RP-HPLC were 11.7

Discussion

To develop probes for PDGFRβ imaging, the affinity of newly synthesized IQP analogs, which contain iodine molecules, for PDGFRβ, was evaluated using WST-8 assay. WST-8 assay determines cell viability; therefore, it cannot directly be used for affinity studies. A negative correlation is speculated between the PDGFRβ-analog affinity and TR-PCT1 cell viability because TR-PCT1 cells express PDGFRβ, and they die when the receptor signaling pathway is interrupted.41, 42 Therefore, WST-8 assay was

Conclusion

In this study, we synthesized the radiolabeled compounds [125I]IIQP and [125I]IB-IQP using an iododestannylation reaction under non-carrier added condition with high radiochemical yields and high purities. This method can also be used to synthesize other radioiodinated compounds containing quinoline-benzimidazole derivatives. The present study showed that radioiodinated IIQP could be a promising imaging agent compared with radioiodinated IB-IQP. However, modification of its structure may be

Conflict of interest

The authors have declared that no conflict of interest.

Acknowledgment

This work was supported in part by Grants-in-Aid for Scientific Research (16H01332, 16KT0192) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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