1,2,3-Triazole Stabilized Neurotensin-Based Radiopeptidomimetics for Improved Tumor Targeting

Combination of the amide-to-triazole substitution strategy with alternative structural modifications for the metabolic stabilization of tumor-targeting, radiolabeled peptides

Radiolabeled peptides play a key role in nuclear medicine to selectively deliver radionuclides to malignancies for diagnosis (imaging) and therapy. Yet, their efficiency is often compromised by low metabolic stability. The use of 1,4-disubstituted 1,2,3-triazoles (1,4-Tzs) as stable amide bond bioisosteres can increase the half-life of peptides in vivo while maintaining their biological properties. Previously, the amide-to-triazole substitution strategy was used for the stabilization of the pansomatostatin radioligand [111In]In-AT2S, resulting in the mono-triazolo-peptidomimetic [111In]In-XG1, a radiotracer with moderately enhanced stability in vivo and retained ability to bind multiple somatostatin receptor (SSTR) subtypes. However, inclusion of additional 1,4-Tz led to a loss of affinity towards SST2R, the receptor overexpressed by most SSTR-positive cancers. To enhance further the stability of [111In]In-XG1, alternative modifications at the enzymatically labile position Thr10-Phe11 were employed. Three novel 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-peptide conjugates were synthesized with a 1,4-Tz (Asn5-Ψ[Tz]-Phe6) and either a β-amino acid (β-Phe11), reduced amide bond (Thr10-Ψ[NH]-Phe11), or N-methylated amino acid (N-Me-Phe11). Two of the new peptidomimetics were more stable in blood plasma in vitro than [111In]In-XG1. Yet none of them retained high affinity towards SST2R. We demonstrate for the first time the combination of the amide-to-triazole substitution strategy with alternative stabilization methods to improve the metabolic stability of tumor-targeting peptides.

Automated solid-phase synthesis of metabolically stabilized triazolo-peptidomimetics

The use of 1,4-disubstituted 1,2,3-triazoles as trans-amide bond surrogates has become an important tool for the synthesis of metabolically stabilized peptidomimetics. These heterocyclic bioisosters are generally incorporated into the peptide backbone by applying a diazo-transfer reaction followed by CuAAC (click chemistry) with an α-amino alkyne. Even though the manual synthesis of backbone-modified triazolo-peptidomimetics has been reported by us and others, no procedure has yet been described for an automated synthesis using peptide synthesizers. In order to efficiently adapt these reactions to an automated setup, different conditions were explored, putting special emphasis on the required long-term stability of both the diazo-transfer reagent and the Cu(I) catalyst in solution. ISA·HCl is the reagent of choice to accomplish the diazo-transfer reaction; however, it was found instable in DMF, the most commonly used solvent for SPPS. Thus, an aqueous solution of ISA·HCl was used to prevent its degradation over time, and the composition in the final diazo-transfer reaction was adjusted to preserve suitable swelling conditions of the resins applied. The CuAAC reaction was performed without difficulties using [Cu (CH3 CN)4 ]PF6 as a catalyst and TBTA as a stabilizer to prevent oxidation to Cu(II). The optimized automated two-step procedure was applied to the synthesis of structurally diverse triazolo-peptidomimetics to demonstrate the versatility of the developed methodology. Under the optimized conditions, five triazolo-peptidomimetics (8-5 amino acid residues) were synthesized efficiently using two different resins. Analysis of the crude products by HPLC-MS revealed moderate to good purities of the desired triazolo-peptidomimetics (70-85%). The synthesis time ranged between 9 and 12.5 h.

Improved Tumor-Targeting with Peptidomimetic Analogs of Minigastrin 177Lu-PP-F11N

Simple Summary

Several radiolabeled peptides targeting CCK2R-positive types of cancer (such as medullary thyroid cancer and small cell lung cancer) have been reported in the last 25 years, some of which have entered clinical trials. In an effort to improve its tumor-targeting properties, we applied chemical modifications to the backbone of the peptide ¹⁷⁷Lu-PP-F11N, an analog of minigastrin in clinical trials. The generated radiolabeled peptidomimetics showed significantly improved characteristics in mice bearing CCK2R-positive tumor xenografts, such as higher tumor uptake, slower tumor washout, and increased tumor-to-kidney ratios. These properties make the novel compounds promising candidates for the imaging and therapy of CCK2R-positive tumors and metastases.

Abstract

The cholecystokinin-2 receptor (CCK2R) is an attractive target in nuclear medicine due to its overexpression by different tumors. Several radiolabeled peptidic ligands targeting the CCK2R have been investigated in the past; however, their low stability against proteases can limit their uptake in tumors and metastases. Substitution of single or multiple amide bonds with metabolically stable 1,4-disubstituted 1,2,3-triazoles as amide bond bioisosteres proved a promising strategy for improving the tumor-targeting properties of a truncated analog of minigastrin. In this study, we applied the previously studied structural modifications to improve the pharmacokinetic and pharmacodynamic properties of PP-F11N, a minigastrin analog currently in clinical trials. Novel minigastrins (NMGs) as analogs of PP-F11N with one or two amide bonds substituted by 1,2,3-triazoles were synthesized, radiolabeled with ¹⁷⁷Lu³⁺, and subjected to full evaluation in vitro (cell internalization, receptor affinity, stability in blood plasma) and in vivo (stability, biodistribution, SPECT/CT imaging). NMGs with triazoles inserted between the amino acids DGlu¹⁰-Ala¹¹ and/or Tyr¹²-Gly¹³ showed a significantly increased cellular uptake and affinity toward the CCK2R in vitro. Resistance against the metabolic degradation of the NMGs was comparable to those of the clinical candidate PP-F11N. Imaging by SPECT/CT and biodistribution studies demonstrated a higher uptake in CCK2R-positive tumors but also in the CCK2R-positive stomach. The peptidomimetic compounds showed a slow tumor washout and high tumor-to-kidney ratios. The structural modifications led to the identification of analogs with promising properties for progression to clinical applications in the diagnosis and therapy of CCK2R-positive neoplasms.

1,5-Disubstituted 1,2,3-Triazoles as Amide Bond Isosteres Yield Novel Tumor-Targeting Minigastrin Analogs

1,5-Disubstituted 1,2,3-triazoles (1,5-Tz) are considered bioisosteres of cis-amide bonds. However, their use for enhancing the pharmacological properties of peptides or proteins is not yet well established. Aiming to illustrate their utility, we chose the peptide conjugate [Nle15]MG11 (DOTA-dGlu-Ala-Tyr-Gly-Trp-Nle-Asp-Phe-NH2) as a model compound since it is known that the cholecystokinin-2 receptor (CCK2R) is able to accommodate turn conformations. Analogs of [Nle15]MG11 incorporating 1,5-Tz in the backbone were synthesized and radiolabeled with lutetium-177, and their pharmacological properties (cell internalization, receptor binding affinity and specificity, plasma stability, and biodistribution) were evaluated and compared with [Nle15]MG11 as well as their previously reported analogs bearing 1,4-disubstituted 1,2,3-triazoles. Our investigations led to the discovery of novel triazole-modified analogs of [Nle15]MG11 with nanomolar CCK2R-binding affinity and 2-fold increased tumor uptake. This study illustrates that substitution of amides by 1,5-disubstituted 1,2,3-triazoles is an effective strategy to enhance the pharmacological properties of biologically active peptides.

Potential use of radiolabelled neurotensin in PET imaging and therapy of patients with pancreatic cancer

Pancreatic cancer is the fourth leading cause of cancer-related death in both men and women. Neurotensin receptors are overexpressed in different malignancies, above all pancreatic cancer. On the other hand, neurotensin receptor expression in inflammation is quite low. This fact can probably solve the most important problem of F-FDG PET imaging - distinguishing malignant and inflammatory processes. The first therapeutic injection of radiolabelled neurotensin in human with pancreatic cancer has been successfully performed. Animal experiments are also very close to the first in human injection of radiolabelled neurotensin for diagnostic purposes. The purpose of this article is to provide an overview of radiolabelled neurotensin analogues that can be used in imaging and therapy in patients with pancreatic ductal adenocarcinoma.

Therapeutic approaches targeting the neurotensin receptors

Introduction: Neurotensin is a gut-brain peptide hormone, a 13 amino acid neuropeptide found in the central nervous system and in the GI tract. The neurotensinergic system is implicated in various physiological and pathological processes related to neuropsychiatric and metabolic machineries, cancer growth, food, and drug intake. NT mediates its functions through its two G protein-coupled receptors: neurotensin receptor 1 (NTS1/NTSR1) and neurotensin receptor 2 (NTS2/NTSR2). Over the past decade, the role of NTS3/NTSR3/sortilin has also gained importance in human pathologies. Several approaches have appeared dealing with the discovery of compounds able to modulate the functions of this neuropeptide through its receptors for therapeutic gain.Areas covered: The article provides an overview of over four decades of research and details the drug discovery approaches and patented strategies targeting NTSR in the past decade.Expert opinion: Neurotensin is an important neurotransmitter that enables crosstalk with various neurotransmitter and neuroendocrine systems. While significant efforts have been made that have led to selective agonists and antagonists with promising in vitro and in vivo activities, the therapeutic potential of compounds targeting the neurotensinergic system is still to be fully harnessed for successful clinical translation of compounds for the treatment of several pathologies.

Strategies for improving stability and pharmacokinetic characteristics of radiolabeled peptides for imaging and therapy

Tumor cells overexpress a variety of receptors that are emerging targets in cancer chemotherapy. Radiolabeled peptides with high affinity and selectivity for these overexpressed receptors have been designed for both imaging and therapy purposes. Such peptides display advantages such as high selectivity for tumor cells, rapid tumor tissue penetration, and rapid clearance from non-target tissues and the circulation. However, the very short in vivo half-life of radiolabeled peptides, arising from enzymatic degradation and/or efficient clearance by the kidney, limits their accumulation in tumors. This review presents various strategies that have been applied to extend the half-life extension and improve the pharmacokinetic characteristics of radiolabeled peptides. These include amino acid substitution, modification of the peptide termini, dimerization and multimerization of the peptide, cyclization, conjugation with polymers, sugars and albumin and use of peptidase inhibitors.

1,4-Disubstituted 1,2,3-Triazoles as Amide Bond Surrogates for the Stabilisation of Linear Peptides with Biological Activity

Peptides represent an important class of biologically active molecules with high potential for the development of diagnostic and therapeutic agents due to their structural diversity, favourable pharmacokinetic properties, and synthetic availability. However, the widespread use of peptides and conjugates thereof in clinical applications can be hampered by their low stability in vivo due to rapid degradation by endogenous proteases. A promising approach to circumvent this potential limitation includes the substitution of metabolically labile amide bonds in the peptide backbone by stable isosteric amide bond mimetics. In this review, we focus on the incorporation of 1,4-disubstituted 1,2,3-triazoles as amide bond surrogates in linear peptides with the aim to increase their stability without impacting their biological function(s). We highlight the properties of this heterocycle as a trans-amide bond surrogate and summarise approaches for the synthesis of triazole-containing peptidomimetics via the Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC). The impacts of the incorporation of triazoles in the backbone of diverse peptides on their biological properties such as, e.g., blood serum stability and affinity as well as selectivity towards their respective molecular target(s) are discussed.

Targeted Delivery of Antisense Oligonucleotides Using Neurotensin Peptides

Despite recent advances, targeted delivery of therapeutic oligonucleotide to extra-hepatic tissues continues to be a challenging endeavor and efficient ligand-receptor systems need to be identified. To determine the feasibility of using neurotensin to improve the productive uptake of antisense oligonucleotides (ASO), we synthesized neurotensin-ASO conjugates and evaluated their cellular uptake and activity in cells and in mice. We performed a comprehensive structure-activity relationship study of the conjugates and determined the influence of ASO charge, ASO length, peptide charge, linker chemistry and ligand identity on receptor binding and internalization. We identified a modified neurotensin peptide capable of improving the cellular uptake and activity of gapmer ASOs in sortilin expressing cells (sixfold) and in spinal cord in mice (twofold). Neurotensin conjugation also improved the potency of morpholino ASO designed to correct splicing of survival motor neuron pre-mRNA in the cortex and striatum after intracerebroventricular injection. Neurotensin-mediated targeted delivery represents a possible approach for enhancing the potency of ASOs with diverse nucleic acid modifications.

Key-Protease Inhibition Regimens Promote Tumor Targeting of Neurotensin Radioligands

Neurotensin subtype 1 receptors (NTS1R) represent attractive molecular targets for directing radiolabeled neurotensin (NT) analogs to tumor lesions for diagnostic and therapeutic purposes. This approach has been largely undermined by the rapid in vivo degradation of linear NT-based radioligands. Herein, we aim to increase the tumor targeting of three ^99mTc-labeled NT analogs by the in-situ inhibition of two key proteases involved in their catabolism. DT1 ([N₄-Gly⁷]NT(7-13)), DT5 ([N₄-βAla⁷,Dab⁹]NT(7-13)), and DT6 ([N₄-βAla⁷,Dab⁹,Tle¹²]]NT(7-13)) were labeled with ^99mTc. Their profiles were investigated in NTS1R-positive colon adenocarcinoma WiDr cells and mice treated or not with the neprilysin (NEP)-inhibitor phosphoramidon (PA) and/or the angiotensin converting enzyme (ACE)-inhibitor lisinopril (Lis). Structural modifications led to the partial stabilization of ^99mTc-DT6 in peripheral mice blood (55.1 ± 3.9% intact), whereas ^99mTc-DT1 and ^99mTc-DT5 were totally degraded within 5 min. Coinjection of PA and/or Lis significantly stabilized all three analogs, leading to a remarkable enhancement of tumor uptake for ^99mTc-DT1 and ^99mTc-DT5, but was less effective in the case of poorly internalizing ^99mTc-DT6. In conclusion, NEP and/or ACE inhibition represents a powerful tool to improve tumor targeting and the overall pharmacokinetics of NT-based radioligands, and warrants further validation in the field of NTS1R-targeted tumor imaging and therapy.

Design of Radiolabeled Analogs of Minigastrin by Multiple Amide-to-Triazole Substitutions

The insertion of single 1,4-disubstituted 1,2,3-triazoles as metabolically stable bioisosteres of trans-amide bonds (triazole scan) was recently applied to the ¹⁷⁷Lu-labeled tumor-targeting analog of minigastrin, [Nle¹⁵]MG11. The reported novel mono-triazolo-peptidomimetics of [Nle¹⁵]MG11 showed either improved resistance against enzymatic degradation or a significantly increased affinity toward the target receptor but never both. To enhance further the tumor-targeting properties of the minigastrin analogs, we studied conjugates with multiple amide-to-triazole substitutions for additive or synergistic effects. Promising candidates were identified by modification of two or three amide bonds, which yielded both improved stability and increased receptor affinity of the peptidomimetics in vitro. Biodistribution studies of radiolabeled multi-triazolo-peptidomimetics in mice bearing receptor-positive tumor xenografts revealed up to 4-fold increased tumor uptake in comparison to the all-amide reference compound [Nle¹⁵]MG11. In addition, we report here for the first time a linear peptidomimetic with three triazole insertions in its backbone and maintained biological activity.

Triazolo-Peptidomimetics: Novel Radiolabeled Minigastrin Analogs for Improved Tumor Targeting

MG11 is a truncated analog of minigastrin, a peptide with high affinity and specificity toward the cholecystokinin-2 receptor (CCK2R), which is overexpressed by different tumors. Thus, radiolabeled MG11 derivatives have great potential for use in cancer diagnosis and therapy. A drawback of MG11 is its fast degradation by proteases, leading to moderate tumor uptake in vivo. We introduced 1,4-disubstituted 1,2,3-triazoles as metabolically stable bioisosteres to replace labile amide bonds of the peptide. The "triazole scan" yielded peptidomimetics with improved resistance to enzymatic degradation and/or enhanced affinity toward the CCK2R. Remarkably, our lead compound achieved a 10-fold increase in receptor affinity, resulting in a 2.6-fold improved tumor uptake in vivo. Modeling of the ligand-CCK2R complex suggests that an additional cation-π interaction of the aromatic triazole moiety with the Arg³⁵⁶ residue of the receptor is accountable for these observations. We show for the first time that the amide-to-triazole substitution strategy offers new opportunities in drug development that go beyond the metabolic stabilization of bioactive peptides.

Synthesis of carboxy-polyethylene glycol-amine (CA (PEG)n ) and [1-14 C]-CA (PEG)n via oxa-Michael addition of amino-polyethylene glycols to propiolates vs to acrylates

Synthesis of carboxy-polyethylene glycol-amine (CA (PEG)n ) via oxa-Michael addition of amino-polyethylene glycols to either acrylates or propiolates was investigated. Compared with the oxa-Michael addition to acrylates, the corresponding addition to propiolates was found to proceed under mild reaction conditions and afford the adducts in high yields from a broad scope of substrates. A two-step efficient and convenient synthesis of benzyl [1-14 C]-propiolate from 14 CO2 was therefore developed and utilized as a common synthon to afford practical and high yielding access to [1-14 C]-CA (PEG)n .

Faraday-Cage-Type Electrochemiluminescence Immunoassay: A Rise of Advanced Biosensing Strategy

Electrochemiluminescence immunoassays are usually carried out through "on-electrode" strategy, i.e., sandwich-type immunoassay format, the sensitivity of which is restricted by two key bottlenecks: (1) the number of signal labels is limited and (2) only a part of signal labels could participate in the electrode reaction. In this Perspective, we discuss the development of an "in-electrode" Faraday-cage-type concept-based immunocomplex immobilization strategy. The biggest difference from the traditional sandwich-type one is that the designed "in-electrode" Faraday-cage-type immunoassay uses a conductive two-dimensional (2-D) nanomaterial simultaneously coated with signal labels and a recognition component as the detection unit, which could directly overlap on the electrode surface. In such a case, electrons could flow freely from the electrode to the detection unit, the outer Helmholtz plane (OHP) of the electrode is extended, and thousands of signal labels coated on the 2-D nanomaterial are all electrochemically "effective." Thus, then, the above-mentioned bottlenecks obstructing the improvement of the sensitivity in sandwich-type immunoassay are eliminated, and as a result a much higher sensitivity of the Faraday-cage-type immunoassay can be obtained. And, the applications of the proposed versatile "in-electrode" Faraday-cage-type immunoassay have been explored in the detection of target polypeptide, protein, pathogen, and microRNA, with the detection sensitivity improved tens to hundreds of times. Finally, the outlook and challenges in the field are summarized. The rise of Faraday-cage-type electrochemiluminescence immunoassay (FCT-ECLIA)-based biosensing strategies opens new horizons for a wide range of early clinical identification and diagnostic applications.

Positron Emission Tomography Imaging of Prostate Cancer with Ga-68-Labeled Gastrin-Releasing Peptide Receptor Agonist BBN7-14 and Antagonist RM26

Radiolabeled bombesin (BBN) analogs have long been used for developing gastrin-releasing peptide receptor (GRPR) targeted imaging probes, and tracers with excellent in vivo performance including high tumor uptake, high contrast, and favorable pharmacokinetics are highly desired. In this study, we compared the ⁶⁸Ga-labeled GRPR agonist (Gln–Trp–Ala–Val–Gly–His–Leu–Met–NH₂, BBN_7–14) and antagonist (d-Phe–Gln–Trp–Ala–Val–Gly–His–Sta–Leu–NH₂, RM26) for the positron emission tomography (PET) imaging of prostate cancer. The in vitro stabilities, receptor binding, cell uptake, internalization, and efflux properties of the probes ⁶⁸Ga–1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA)–Aca–BBN_7–14 and ⁶⁸Ga–NOTA–poly(ethylene glycol)₃ (PEG₃)–RM26 were studied in PC-3 cells, and the in vivo GRPR targeting abilities and kinetics were investigated using PC-3 tumor xenografted mice. BBN_7–14, PEG₃-RM26, NOTA–Aca–BBN_7–14, and NOTA–PEG₃–RM26 showed similar binding affinity to GRPR. In PC-3 tumor-bearing mice, the tumor uptake of ⁶⁸Ga–NOTA–PEG₃–RM26 remained at around 3.00 percentage of injected dose per gram of tissue within 1 h after injection, in contrast with ⁶⁸Ga–NOTA–Aca–BBN_7–14, which demonstrated rapid elimination and high background signal. Additionally, the majority of the ⁶⁸Ga–NOTA–PEG₃–RM26 remained intact in mouse serum at 5 min after injection, while almost all of the ⁶⁸Ga–NOTA–Aca–BBN_7–14 was degraded under the same conditions, demonstrating more-favorable in vivo pharmacokinetic properties and metabolic stabilities of the antagonist probe relative to its agonist counterpart. Overall, the antagonistic GRPR targeted probe ⁶⁸Ga–NOTA–PEG₃–RM26 is a more-promising candidate than the agonist ⁶⁸Ga–NOTA–Aca–BBN_7–14 for the PET imaging of prostate cancer patients.

Molecular imaging probes derived from natural peptides

Covering: up to the end of 2015.Peptides are naturally occurring compounds that play an important role in all living systems and are responsible for a range of essential functions. Peptide receptors have been implicated in disease states such as oncology, metabolic disorders and cardiovascular disease. Therefore, natural peptides have been exploited as diagnostic and therapeutic agents due to the unique target specificity for their endogenous receptors. This review discusses a variety of natural peptides highlighting their discovery, endogenous receptors, as well as their derivatization to create molecular imaging agents, with an emphasis on the design of radiolabelled peptides. This review also highlights methods for discovering new and novel peptides when knowledge of specific targets and endogenous ligands are not available.

In-electrode vs. on-electrode: ultrasensitive Faraday cage-type electrochemiluminescence immunoassay

A new-concept of an "in-electrode" Faraday cage-type electrochemiluminescence immunoassay (ECLIA) method for the ultrasensitive detection of neurotensin (NT) was reported with capture antibody (Ab1)-nanoFe3O4@graphene (GO) and detector antibody (Ab2)&N-(4-aminobutyl)-N-ethylisoluminol (ABEI)@GO, which led to about 1000-fold improvement in sensitivity by extending the Helmholtz plane (OHP) of the proposed electrode assembly effectively.

Methoxinine - an alternative stable amino acid substitute for oxidation-sensitive methionine in radiolabelled peptide conjugates

Radiolabelled peptides with high specificity and affinity towards receptors that are overexpressed by tumour cells are used in nuclear medicine for the diagnosis (imaging) and therapy of cancer. In some cases, the sequences of peptides under investigations contain methionine (Met), an amino acid prone to oxidation during radiolabelling procedures. The formation of oxidative side products can affect the purity of the final radiopharmaceutical product and/or impair its specificity and affinity towards the corresponding receptor. The replacement of Met with oxidation resistant amino acid analogues, for example, norleucine (Nle), can provide a solution. While this approach has been applied successfully to different radiolabelled peptides, a Met → Nle switch only preserves the length of the amino acid side chain important for hydrophobic interactions but not its hydrogen-bonding properties. We report here the use of methoxinine (Mox), a non-canonical amino acid that resembles more closely the electronic properties of Met in comparison to Nle. Specifically, we replaced Met¹⁵ by Mox¹⁵ and Nle¹⁵ in the binding sequence of a radiometal-labelled human gastrin derivative [d-Glu¹⁰ ]HG(10-17), named MG11 (d-Glu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH₂ ). A comparison of the physicochemical properties of ¹⁷⁷ Lu-DOTA[X¹⁵ ]MG11 (X = Met, Nle, Mox) in vitro (cell internalization/externalization properties, receptor affinity (IC₅₀ ), blood plasma stability and logD) showed that Mox indeed represents a suitable, oxidation-stable amino acid substitute of Met in radiolabelled peptide conjugates. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Glycated 99m Tc-Tricarbonyl-Labeled Peptide Conjugates for Tumor Targeting by "Click-to-Chelate"

Attaching polar pharmacological modifiers to molecular imaging probes is a common strategy to modulate their pharmacokinetic profiles to improve such parameters as the clearance rate of radiotracers and/or metabolites, and to enhance signal-to-background ratios. We combined the tumor-targeting peptide sequence of bombesin (BBN) with glucuronic acid and the single-photon emission computed tomography (SPECT) radionuclide ^99m Tc by the "click-to-chelate" methodology. The ^99m Tc-tricarbonyl-labeled glucuronated BBN conjugate was compared with a reference compound lacking the carbohydrate. The radiolabeled conjugates displayed similar characteristics in vitro (cell internalization, receptor affinity), but the hydrophilicity of the glycated version was significantly increased. While the tumor uptake of the two radioconjugates in xenografted mice was similar, the glycated peptide exhibited unexpected higher uptake in organs of the hepatobiliary excretion pathway than the more lipophilic reference compound. Control experiments suggest that this may be the result of unspecific accumulation of metabolites in which the glucuronic acid moiety does not act as an innocent pharmacological modifier.

Click Chemistry and Radiochemistry: The First 10 Years

The advent of click chemistry has had a profound influence on almost all branches of chemical science. This is particularly true of radiochemistry and the synthesis of agents for positron emission tomography (PET), single photon emission computed tomography (SPECT), and targeted radiotherapy. The selectivity, ease, rapidity, and modularity of click ligations make them nearly ideally suited for the construction of radiotracers, a process that often involves working with biomolecules in aqueous conditions with inexorably decaying radioisotopes. In the following pages, our goal is to provide a broad overview of the first 10 years of research at the intersection of click chemistry and radiochemistry. The discussion will focus on four areas that we believe underscore the critical advantages provided by click chemistry: (i) the use of prosthetic groups for radiolabeling reactions, (ii) the creation of coordination scaffolds for radiometals, (iii) the site-specific radiolabeling of proteins and peptides, and (iv) the development of strategies for in vivo pretargeting. Particular emphasis will be placed on the four most prevalent click reactions-the Cu-catalyzed azide-alkyne cycloaddition (CuAAC), the strain-promoted azide-alkyne cycloaddition (SPAAC), the inverse electron demand Diels-Alder reaction (IEDDA), and the Staudinger ligation-although less well-known click ligations will be discussed as well. Ultimately, it is our hope that this review will not only serve to educate readers but will also act as a springboard, inspiring synthetic chemists and radiochemists alike to harness click chemistry in even more innovative and ambitious ways as we embark upon the second decade of this fruitful collaboration.

Crystal structure of 2-(5-(4-fluorophenyl)-3-p-tolyl-4,5-dihydro-1H-pyrazol-1-yl)-4-(5-methyl-1-p-tolyl-1H-1,2,3-triazol-4-yl)thiazole, C29H25FN6S

C29H25FN6S, triclinic, P1̅, a = 7.5726(8) Å, b = 11.1428(13) Å, c = 16.412(2) Å, α = 91.823(5)°, β = 102.277(5)°, γ = 106.692(4)°, V = 1289.8(3) Å3, Z = 2, Rgt(F) = 0.079, wRref(F2) = 0.205, T = 296 K.

Toward the Optimization of Bombesin-Based Radiotracers for Tumor Targeting

The peptide bombesin (BBN) is a peptide with high affinity for the gastrin-releasing peptide receptor (GRPr), a receptor that is overexpressed by, for example, breast and prostate cancers. Thus, GRPr agonists can be used as cancer-targeting vectors to shuttle diagnostic and therapeutic agents into tumor cells. With the aim of optimizing the tumor targeting properties of a radiolabeled [Nle(14)]BBN(7-14) moiety, novel BBN(7-14)- and BBN(6-14)-based radioconjugates were synthesized, labeled with Lu-177, and fully evaluated in vitro and in vivo. The effect of residue and backbone modification on several parameters such as the internalization of the radiolabeled peptides into PC3 and AR42J tumor cells, their affinity toward the human GRPr, metabolic stability in blood plasma, and biodistribution in mice bearing GRPr-expressing PC3 xenografts was studied. As a result of our investigations, a novel radiolabeled GRPr agonist with a high tumor uptake and a high tumor-to-kidney ratio was identified.

Radiolabeled analogs of neurotensin (8–13) containing multiple 1,2,3-triazoles as stable amide bond mimics in the backbone

Substitution of multiple amide bonds by metabolically stable 1,2,3-triazoles yields novel tumour-targeting neurotensin-based peptidomimetics with interesting biological properties.

Structure-Activity Relationship Studies of Amino Acid Substitutions in Radiolabeled Neurotensin Conjugates

Radiolabeled derivatives of the peptide neurotensin (NT) and its binding sequence NT(8-13) have been studied as potential imaging probes and therapeutics for NT-1-receptor-positive cancer. However, a direct comparison of reported NT analogues, even if radiolabeled with the same radionuclide, is difficult because different techniques and models have been used for preclinical evaluations. In an effort to identify a suitable derivative of NT(8-13) for radiotracer development, we herein report a side-by-side in vitro comparison of radiometallated NT derivatives bearing some of the most commonly reported amino acid substitutions in their sequence. Performed investigations include cell internalization experiments, determinations of receptor affinity, measurements of the distribution coefficient, and blood serum stability studies. Of the [(177)Lu]-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-labeled examples studied, analogues of NT(8-13) containing a short hydrophilic tetraethylene glycol (PEG4 ) spacer between the peptide and the radiometal complex, and a minimum number of substitutions of amino acid residues, exhibited the most promising properties in vitro.