Design, synthesis and evaluation of 18F-labeled bradykinin B1 receptor-targeting small molecules for PET imaging

Research progress of 18F labeled small molecule positron emission tomography (PET) imaging agents

With the development of positron emission tomography (PET) technology, a variety of PET imaging agents labeled with radionuclide ¹⁸F have been developed and widely used in the diagnosis and treatment of various clinical diseases in recent years. For example, they have showed a great value of study in the field of tumor detection, tumor treatment and evaluation of tumor therapy in a non-invasive, qualitative and quantitative way. In this review, we highlight the recent development in chemical synthesis, structure and characterization, imaging characterization, and potential applications of these ¹⁸F labeled small molecule PET imaging agents for the past five years. The development and application of ¹⁸F labeled small molecules will expand our knowledge of the function and distribution of diseases-related molecular targets and shed light on the diagnosis and treatment of various diseases including tumors.

Physicochemical characterisation of novel tetrabutylammonium aryltrifluoroborate ionic liquids

While there have been many studies on the physicochemical characterisation of ILs, little work has previously been reported on the properties unique to the trifluoroborate anion. Here we have characterised the thermal properties, viscosity, liquid nanostructure and intramolecular interactions of 15 novel aryltrifluoroborate ILs. These ILs all contained a tetrabutylammonium cation paired with either meta- or para-substituted aryltrifluoroborate anions, or di-anionic substituted aryltrifluroborate anions. It was found that of the 15 samples analysed, 4 would technically be considered molten salts as they have melting points greater than 100 °C. Overall the structure-property relationship trends of these samples are similar to those previously reported for alkyl and perfluoroalkyltrifluoroborate ILs which contained K+ or Cs+ cations, with the big difference being the ILs in this study having considerably lower melting points.

A Systematic Review of Molecular Imaging Agents Targeting Bradykinin B1 and B2 Receptors

Kinins, bradykinin and kallidin are vasoactive peptides that signal through the bradykinin B1 and B2 receptors (B1R and B2R). B2R is constitutively expressed in healthy tissues and mediates responses such as vasodilation, fluid balance and retention, smooth muscle contraction, and algesia, while B1R is absent in normal tissues and is induced by tissue trauma or inflammation. B2R is activated by kinins, while B1R is activated by kinins that lack the C-terminal arginine residue. Perturbations of the kinin system have been implicated in inflammation, chronic pain, vasculopathy, neuropathy, obesity, diabetes, and cancer. In general, excess activation and signaling of the kinin system lead to a pro-inflammatory state. Depending on the disease context, agonism or antagonism of the bradykinin receptors have been considered as therapeutic options. In this review, we summarize molecular imaging agents targeting these G protein-coupled receptors, including optical and radioactive probes that have been used to interrogate B1R/B2R expression at the cellular and anatomical levels, respectively. Several of these preclinical agents, described herein, have the potential to guide therapeutic interventions for these receptors.

Discovery and development of plasma kallikrein inhibitors for multiple diseases

Plasma kallikrein (PKal) belongs to the family of trypsin-like serine proteases. The expression of PKal is associated with multiple physiological systems or pathways such as coagulation pathway, platelet aggregation process, kallikrein-kinin system, renin-angiotensin system and complement pathway. On the basis of PKal's multiple physiological functions, it has been considered as a potential target for several diseases including hereditary angioedema, microvascular complications of diabetes mellitus and cerebrovascular disease. Up to now, many PKal inhibitors have been identified and a few of them have reached clinical trials or market. This review summarizes the development of small molecule and peptide PKal inhibitors having different scaffolds and discusses their structure-activity relationship and selectivity. We hope this review facilitates a comprehensive understanding of the types of PKal inhibitors developed to tackle different manifestations of PKal-associated diseases.

Radiolabeled R954 Derivatives for Imaging Bradykinin B1 Receptor Expression with Positron Emission Tomography

Peptide receptors have emerged as promising targets for diagnosis and therapy. The aberrant overexpression of these receptors in different cancer subtypes allows for the adoption of new treatment strategies that complement conventional chemotherapies. Bradykinin B1 receptor (B1R) is a G protein-coupled receptor that is overexpressed in many cancers, with limited expression in healthy tissues. Previously, we developed ⁶⁸Ga- and ¹⁸F-labeled derivatives of B1R antagonist peptides B9858 and B9958, and successfully targeted B1R-expressing tumor xenografts in vivo. R954 (Ac-Orn-Arg-Oic-Pro-Gly-αMePhe-Ser-d-2-Nal-Ile), a potent B1R antagonist, is reportedly more stable than B9858 against peptidase degradation. We evaluated two radiolabeled derivatives of R954 (⁶⁸Ga-HTK01083 and ¹⁸F-HTK01146) for B1R PET imaging. Peptides were synthesized via solid phase strategy. Nonradioactive standards were obtain by reacting GaCl₃ with DOTA-dPEG2-R954 and by clicking N-propargyl-N,N-dimethylammoniomethyl-trifluoroborate with azidoacetyl-dPEG2-R954. Binding affinity for B1R was determined by an in vitro competition binding assay. ⁶⁸Ga-HTK01083 was obtained by incubating DOTA-dPEG2-R954 with ⁶⁸GaCl₃ under acidic conditions, while ¹⁸F-HTK01146 was prepared via an ¹⁸F-¹⁹F isotope exchange reaction. Biodistribution and imaging studies were conducted at 1 h postinjection (p.i.) in mice inoculated with B1R-expressing (B1R+) and B1R-nonexpressing (B1R-) cells. HTK01083 and HTK01146 bound B1R with good affinity (K_i = 30.5 and 24.8 nM, respectively). ⁶⁸Ga/¹⁸F-labeled R954 were obtained on average in ≥10% decay-corrected radiochemical yield with >99% radiochemical purity and ≥52 GBq/μmol specific activity. For both tracers, clearance was predominantly renal with minimal involvement of the hepatobiliary system. For PET images, B1R+ tumors, kidneys, and bladder were visible. At 1 h p.i., uptake in B1R+ tumor was comparable between ⁶⁸Ga-HTK01083 (8.46 ± 1.44%ID/g) and ¹⁸F-HTK01146 (9.25 ± 0.69%ID/g). B1R+ tumor-to-blood and B1R+ tumor-to-muscle ratios were 6.32 ± 1.44 and 20.7 ± 3.58 for ⁶⁸Ga-HTK01083, and 7.24 ± 2.56 and 19.5 ± 4.29 for ¹⁸F-HTK01146. Our results indicate R954 is a good lead sequence for optimization of B1R tracers for cancer imaging.

Synthesis and evaluation of a 68Ga-labeled bradykinin B1 receptor agonist for imaging with positron emission tomography

A novel ⁶⁸Ga-labeled bradykinin B1 receptor (B1R) agonist, ⁶⁸Ga-Z01115, was synthesized and evaluated for imaging with positron emission tomography (PET). Z01115 exhibited good binding affinity (K_i=25.4±5.1nM) to hB1R. ⁶⁸Ga-Z01115 was prepared in 74±5 decay-corrected radiochemical yield with >99% radiochemical purity and 155±89GBq/µmol (4.2±2.4Ci/μmol) specific activity. ⁶⁸Ga-Z01115 was stable in vitro in mouse plasma (93% remaining intact after 60min incubation), and relatively stable in vivo (51±5% remaining intact at 5min post-injection). PET imaging and biodistribution studies in mice showed that ⁶⁸Ga-Z01115 cleared rapidly from nontarget tissues/organs, and generated high target-to-nontarget contrast images. The uptake of ⁶⁸Ga-Z01115 in B1R-positive (B1R+) tumor was 5.65±0.59%ID/g at 1h post-injection. Average contrast ratios of B1R+ tumor-to-B1R- tumor, -to-blood and -to-muscle were 24.3, 24.4 and 82.9, respectively. Uptake of ⁶⁸Ga-Z01115 in B1R+ tumors was reduced by ∼90% with co-injection of cold standard, confirming it was mediated by B1R. Our data suggest that ⁶⁸Ga-Z01115 is a promising tracer for imaging the expression of B1R that is overexpressed in a variety of cancers.