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

Enzymological Characterization of 64Cu-Labeled Neprilysin Substrates and Their Application for Modulating the Renal Clearance of Targeted Radiopharmaceuticals

The applicability of radioligands for targeted endoradionuclide therapy is limited due to radiation-induced toxicity to healthy tissues, in particular to the kidneys as primary organs of elimination. The targeting of enzymes of the renal brush border membrane by cleavable linkers that permit the formation of fast eliminating radionuclide-carrying cleavage fragments gains increasing interest. Herein, we synthesized a small library of ⁶⁴Cu-labeled cleavable linkers and quantified their substrate potentials toward neprilysin (NEP), a highly abundant peptidase at the renal brush border membrane. This allowed for the derivation of structure-activity relationships, and selected cleavable linkers were attached to the somatostatin receptor subtype 2 ligand [Tyr³]octreotate. Radiopharmacological characterization revealed that a substrate-based targeting of NEP in the kidneys with small peptides entails their premature cleavage in the blood circulation by soluble and endothelium-derived NEP. However, for a kidney-specific targeting of NEP, the additional targeting of albumin in the blood is highlighted.

Rationally Designed Amanitins Achieve Enhanced Cytotoxicity

For 70 years, α-amanitin, the most cytotoxic peptide in its class, has been without a synthetic rival; through synthesis, we address the structure-activity relationships to inform the design of new amatoxins and disclose analogues that are more cytotoxic than the natural product when evaluated on CHO, HEK293, and HeLa cells, whereas on liver-derived HepG2 cells, the same toxins show diminished cytotoxicity.

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.

Cholecystokinin-2 Receptor Targeting with Radiolabeled Peptides: Current Status and Future Directions

A wide variety of radiolabeled peptide analogs for specific targeting of cholecystokinin- 2 receptors (CCK2R) has been developed in the last decades. Peptide probes based on the natural ligands Minigastrin (MG) and Cholecystokinin (CCK) have a high potential for molecular imaging and targeted radiotherapy of different human tumors, such as Medullary Thyroid Carcinoma (MTC) and Small Cell Lung Cancer (SCLC). MG analogs with high persistent uptake in CCK2R expressing tumors have been preferably used for the development of radiolabeled peptide analogs. The clinical translation of CCK2R targeting has been prevented due to high kidney uptake or low metabolic stability of the different radiopeptides developed. Great efforts in radiopharmaceutical development have been undertaken to overcome these limitations. Various modifications in the linear peptide sequence of MG have been introduced mainly with the aim to reduce kidney retention. Furthermore, improved tumor uptake could be obtained by in situ stabilization of the radiopeptide against enzymatic degradation through coinjection of peptidase inhibitors. Recent developments focusing on the stabilization of the Cterminal receptor binding sequence (Trp-Met-Asp-Phe-NH2) have led to new radiolabeled MG analogs with highly improved tumor uptake and tumor-to-kidney ratio. In this review, all the different aspects in the radiopharmaceutical development of CCK2R targeting peptide probes are covered, giving also an overview on the clinical investigations performed so far. The recent development of radiolabeled MG analogs, which are highly stabilized against enzymatic degradation in vivo, promises to have a high impact on the clinical management of patients with CCK2R expressing tumors in the near future.

Oxidant and Antioxidant Effects of Gentisic Acid in a ¹⁷⁷Lu-Labelled Methionine-Containing Minigastrin Analogue

BACKGROUND

The radiolabelling of receptor-binding peptides for therapy is a challenge since the peptide itself is exposed (during labelling, storage and transport) to radiation-induced damage, directly or indirectly, in aqueous solution. Hence, the use of radiostabilizers seems to be mandatory, especially in peptide molecules that contain radiation-sensitive amino acids.

OBJECTIVE

The aim of this study was to investigate the effect of two stabilizers, gentisic acid and methionine, to delve into how each of them affects the radiolabelling and stability of the minigastrin analogue [177Lu]Lu-DOTA-His-His-Glu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH2 through the analysis of the 22 species distinguished over time by an optimized HPLC system.

METHODS

The stabilizers, in different combinations, were present from the beginning of the labelling process carried out at 96 °C for 15 min. The stability was studied for up to 7 days.

RESULTS

The unexpected selective oxidation of the methionine residue of the radiolabelled peptide, promoted by gentisic acid, led to studying the effect of pH, from 3.5 to 6.0, in the presence of only this stabilizer. A pH-dependent antioxidant behaviour was revealed, showing a decrease in peptide impurities but an increase in the selective oxidation as the pH was increased.

CONCLUSION

The selective oxidation of the methionine residue could be induced by oxidizing species probably produced in the reaction between gentisic acid and free radicals of water, during the protection of the radiolabelled peptide from the attack of these harmful species. Therefore, the addition of methionine becomes necessary to effectively decrease this selective oxidation in the methioninecontaining peptide.

Bicyclic RGD peptides with high integrin α v β 3 and α 5 β 1 affinity promote cell adhesion on elastin-like recombinamers

Biomaterial design in tissue engineering aims to identify appropriate cellular microenvironments in which cells can grow and guide new tissue formation. Despite the large diversity of synthetic polymers available for regenerative medicine, most of them fail to fully match the functional properties of their native counterparts. In contrast, the few biological alternatives employed as biomaterials lack the versatility that chemical synthesis can offer. Herein, we studied the HUVEC adhesion and proliferation properties of elastin-like recombinamers (ELRs) that were covalently functionalized with each three high-affinity and selectivity α _v β ₃- and α ₅ β ₁-binding bicyclic RGD peptides. Next to the bicycles, ELRs were also functionalized with various integrin-binding benchmark peptides, i.e. knottin-RGD, cyclo-[KRGDf] and GRGDS, allowing for better classification of the obtained results. Covalent functionalization with the RGD peptides, as validated by MALDI-TOF analysis, guarantees flexibility and minimal steric hindrance for interactions with cellular integrins. In addition to the covalently modified RGD-ELRs, we also synthesized another benchmark ELR comprising RGD as part of the backbone. HUVEC adhesion and proliferation analysis using the PicoGreen® assay revealed a higher short-term adhesion and proliferative capacity of cells on ELR surfaces functionalized with high affinity, integrin-binding bicyclic RGD-peptides compared with the ELRs containing RGD in the backbone.

Cholecystokinin-2 Receptor Targeting with Novel C-terminally Stabilized HYNIC-Minigastrin Analogs Radiolabeled with Technetium-99m

The high overexpression of cholecystokinin-2 receptors (CCK2R) in tumors, such as medullary thyroid carcinoma, allows for highly specific diagnostic and therapeutic targeting with radiolabeled peptide probes derived from natural ligands for the receptor. Based on the ideal imaging characteristics, high availability and low cost of technetium-99m (^99mTc)-labeled radiopharmaceuticals we have developed two hydrazinonicotinic acid (HYNIC) conjugated minigastrin analogs allowing labeling at high specific activity. The CCK2R targeting peptide conjugates show specific amino acid substitutions in the C-terminal receptor-specific sequence with the aim to increase stability and tumor targeting. The CCK2R affinity and the cell uptake of the new radioligands were analyzed using A431 human epidermoid carcinoma cells stably transfected with human CCK2R and mock transfected cells. Metabolic studies in BALB/c mice revealed a high resistance against enzymatic degradation for both radioligands. Biodistribution studies in tumor-xenografted athymic BALB/c nude mice at 1 h and 4 h p.i. showed that the two ^99mTc-labeled compounds showed varying uptake in receptor expressing organs, stomach and pancreas (1.3–10.4% IA/g), as well as kidneys, the main route of excretion (7.8–19.9% IA/g). The tumor uptake in A431-CCK2R xenografts was 24.75 ± 4.38% IA/g for [^99mTc]Tc-HYNIC-MGS5 and 42.48 ± 6.99% IA/g for [^99mTc]Tc-HYNIC-MGS11 at 4 h p.i., whereas the tumor-to-kidney ratio was comparable (2.6–3.3). On demand availability and potential application for radioguided surgery of a ^99mTc-labeled minigastrin analog support the further evaluation of these highly promising new compounds.

DOTA-MGS5, a New Cholecystokinin-2 Receptor-Targeting Peptide Analog with an Optimized Targeting Profile for Theranostic Use

Molecular imaging and targeted radiotherapy with radiolabeled cholecystokinin-2 receptor (CCK2R) targeting peptide probes holds high promise to improve the clinical management of patients with metastatic medullary thyroid carcinoma and other CCK2R-expressing malignancies. Low stability and suboptimal targeting of currently available radiolabeled peptide analogs has prompted us to seek new stabilization strategies. In this study, we present a new minigastrin analog with site-specific C-terminal modifications showing a highly optimized targeting profile. Methods: DOTA-D-Glu-Ala-Tyr-Gly-Trp-(N-Me)Nle-Asp-1-Nal-NH₂ (DOTA-MGS5) radiolabeled with ¹¹¹In, ⁶⁸Ga, and ¹⁷⁷Lu was evaluated in extensive in vitro stability studies. For ¹⁷⁷Lu-DOTA-MGS5, additional metabolic studies were performed on BALB/c mice. Receptor affinity and cell uptake were studied in A431 human epidermoid carcinoma cells transfected with human CCK2R (A431-CCK2R), as well as the same cell line transfected with the empty vector (A431-mock). A431-CCK2R/A431-mock xenografted athymic BALB/c nude mice were used for biodistribution studies and small-animal SPECT/CT. Results: DOTA-MGS5 radiolabeled with ¹¹¹In and ¹⁷⁷Lu showed a highly increased stability against enzymatic degradation in different media up to 24 h of incubation. Similar results were observed for ⁶⁸Ga-DOTA-MGS5 incubated up to 4 h. In the blood of mice injected with ¹⁷⁷Lu-DOTA-MGS5, at least 70% intact radiopeptide was detected up to 1 h after injection. The unlabeled peptide and the complexes with the natural isotopes showed retained receptor affinity, and the radiopeptides showed unexpectedly high cell uptake in A431-CCK2R cells (>60% at 4 h). Regardless of the radiometal used for labeling, impressively high uptake in A431-CCK2R xenografts was found (∼20% injected activity/g 1-4 h after injection), whereas the uptake in A431-mock xenografts was negligible. Low background activity and favorable tumor-to-kidney ratios (4-6) allowed for high image contrast in small-animal SPECT/CT. Conclusion: The excellent targeting properties of DOTA-MGS5 support future clinical studies evaluating the diagnostic and therapeutic potential in patients with progressive or metastatic medullary thyroid carcinoma, as well as other advanced-stage CCK2R-expressing malignancies.

Site-specific stabilization of minigastrin analogs against enzymatic degradation for enhanced cholecystokinin-2 receptor targeting

Minigastrin (MG) analogs show high affinity to the cholecystokinin-2 receptor (CCK2R) and have therefore been intensively studied to find a suitable analog for imaging and treatment of CCK2R-expressing tumors. The clinical translation of the radioligands developed thus far has been hampered by high kidney uptake or low enzymatic stability. In this study, we aimed to develop new MG analogs with improved targeting properties stabilized against degradation through site-specific amino acid modifications.

Method: Based on the lead structure of a truncated MG analog, four new MG derivatives with substitutions in the C-terminal part of the peptide (Trp-Met-Asp-Phe-NH₂) were synthesized and derivatized with DOTA at the N-terminus for radiolabeling with trivalent radiometals. The in vitro properties of the new analogs were characterized by analyzing the lipophilicity, the protein binding, and the stability of the Indium-111 (¹¹¹In)-labeled analogs in different media. Two different cell lines, AR42J cells physiologically expressing the rat CCK2R and A431 cells transfected with human CCK2R (A431-CCK2R), were used to study the receptor affinity and cell uptake. For the two most promising MG analogs, metabolic studies in normal BALB/c mice were carried out as well as biodistribution and imaging studies in tumor xenografted athymic BALB/c nude mice.

Results: Two out of four synthesized peptide analogs (DOTA-MGS1 and DOTA-MGS4) showed retained receptor affinity and cell uptake when radiolabeled with ¹¹¹In. These two peptide analogs, however, showed a different stability against enzymatic degradation in vitro and in vivo. When injected to normal BALB/c mice, for ¹¹¹In-DOTA-MGS1 at 10 min post injection (p.i.) no intact radiopeptide was found in the blood, whereas for ¹¹¹In-DOTA-MGS4 more than 75% was still intact. ¹¹¹In-DOTA-MGS4 showed a clear increase in injected activity per gram tissue (IA/g) for A431-CCK2R xenografts (10.40±2.21% IA/g 4 h p.i.) when compared to ¹¹¹In-DOTA-MGS1 (1.23±0.15% IA/g 4 h p.i.). The tumor uptake of ¹¹¹In-DOTA-MGS4 was also combined with a low uptake in stomach and kidney leading to high-contrast NanoSPECT/CT images.

Conclusion: Of the four new MG analogs developed, the best results in terms of enzymatic stability and increased tumor targeting were obtained with ¹¹¹In-DOTA-MGS4 showing two substitutions with N-methylated amino acids. ¹¹¹In-DOTA-MGS4 was also superior to other MG analogs reported thus far and seems therefore an extremely promising targeting molecule for theranostic use with alternative radiometals.

Structural studies on radiopharmaceutical DOTA-minigastrin analogue (CP04) complexes and their interaction with CCK2 receptor

Background

The cholecystokinin receptor subtype 2 (CCK-2R) is an important target for diagnostic imaging and targeted radionuclide therapy (TRNT) due to its overexpression in certain cancers (e.g., medullary thyroid carcinoma (MTC)), thus matching with a theranostic principle. Several peptide conjugates suitable for the TRNT of MTC have been synthesized, including a very promising minigastrin analogue DOTA-(DGlu)₆-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH₂ (CP04). In this contribution, we wanted to see whether CP04 binding affinity for CCK-2R is sensitive to the type of the complexed radiometal, as well as to get insights into the structure of CP04-CCK2R complex by molecular modeling.

Results

In vitro studies demonstrated that there is no significant difference in CCK-2R binding affinity and specific cellular uptake between the CP04 conjugates complexed with [⁶⁸Ga]Ga³⁺ or [¹⁷⁷Lu]Lu³⁺. In order to investigate the background of this observation, we proposed a binding model of CP04 with CCK-2R based on homology modeling and molecular docking. In this model, the C-terminal part of the molecule enters the cavity formed between the receptor helices, while the N-terminus (including DOTA and the metal) is located at the binding site outlet, exposed in large extent to the solvent. The radiometals do not influence the conformation of the molecule except for the direct neighborhood of the chelating moiety.

Conclusions

The model seems to be in agreement with much of structure-activity relationship (SAR) studies reported for cholecystokinin and for CCK-2R-targeting radiopharmaceuticals. It also explains relative insensitivity of CCK-2R affinity for the change of the metal. The proposed model partially fits the reported site-directed mutagenesis data.