PEG spacers of different length influence the biological profile of bombesin-based radiolabeled antagonists

In vitro and in vivo evaluation of Bombesin-MMAE conjugates for targeted tumour therapy

Targeted tumour therapy has proved to be an efficient alternative to overcome the limitations of conventional chemotherapy. The upregulation of the bombesin receptor 2 (BB2) in several malignancies and the advantages offered by peptide drug conjugates over antibody drug conjugates in terms of production and tumour targeting motivated us to synthesise and test bombesin conjugates armed with the tubulin binder monomethyl auristatin E. The widely used Val-Cit-PABC was initially included as cathepsin cleavable self-immolative linker for the release of the free drug. However, the poor stability of the Val-Cit-conjugates in mouse plasma encouraged us to consider the optimised alternatives Glu-Val-Cit-PABC and Glu-Gly-Cit-PABC. Conjugate BN-EVcM1, featuring Glu-Val-Cit-PABC, combined suitable stability (t(½) in mouse and human plasma: 8.4 h and 4.6 h, respectively), antiproliferative activity in vitro (IC50 = 29.6 nM on the human prostate cancer cell line PC-3) and the full release of the free payload within 24 h. Three conjugates, namely BN-EGcM1, BN-EVcM1 and BN-EVcM2, improved the accumulation of MMAE in PC-3 human prostate cancer xenograft mice models, compared to the administration of the free drug. Among them, BN-EVcM1 also stood out for the significantly extended survival of mice in in vivo acute efficacy studies and for the significant inhibition of the growth of a PC-3 tumour in mice in both acute and chronic efficacy studies.

Development of a Novel Peptide-Based PET Tracer [68Ga]Ga-DOTA-BP1 for BCMA Detection in Multiple Myeloma

B-cell maturation antigen (BCMA) has emerged as a promising tumor marker for the diagnosis and treatment of multiple myeloma. The noninvasive and rapid detection of BCMA expression in vivo provides significant value in screening and evaluating multiple myeloma patients receiving BCMA-targeted therapy. We identified the BCMA-targeting peptide BP1 from a one-bead-one-compound (OBOC) peptide library using a high-throughput microarray strategy. The BCMA-targeting specificity and affinity of BP1 were assessed by surface plasmon resonance imaging (SPRi), flow cytometry, and confocal imaging. BCMA-positive (H929) and BCMA-negative (K562) subcutaneous tumor models were established and labeled with 68Ga for BP1, followed by PET imaging and biodistribution studies. PET imaging demonstrated that 68Ga-labeled BP1 has significant specific uptake in multiple myeloma, enabling rapid identification of BCMA expression and precise delineation of the disease. Thus, BP1 represents an ideal candidate for multiple myeloma imaging.

Radiopharmaceuticals Targeting Gastrin-Releasing Peptide Receptor for Diagnosis and Therapy of Prostate Cancer

The high incidence and heavy disease burden of prostate cancer (PC) require accurate and comprehensive assessment for appropriate disease management. Prostate-specific membrane antigen (PSMA) positron emission tomography (PET) cannot detect PSMA-negative lesions, despite its key role in PC disease management. The overexpression of gastrin-releasing peptide receptor (GRPR) in PC lesions reportedly performs as a complementary target for the diagnosis and therapy of PC. Radiopharmaceuticals derived from the natural ligands of GRPR have been developed. These radiopharmaceuticals enable the visualization and quantification of GRPR within the body, which can be used for disease assessment and therapeutic guidance. Recently developed radiopharmaceuticals exhibit improved pharmacokinetic parameters without deterioration in affinity. Several heterodimers targeting GRPR have been constructed as alternatives because of their potential to detect tumor lesions with a low diagnostic efficiency of single target detection. Moreover, some GRPR-targeted radiopharmaceuticals have entered clinical trials for the initial staging or biochemical recurrence detection of PC to guide disease stratification and therapy, indicating considerable potential in PC disease management. Herein, we comprehensively summarize the progress of radiopharmaceuticals targeting GRPR. In particular, we discuss the impact of ligands, chelators, and linkers on the distribution of radiopharmaceuticals. Furthermore, we summarize a potential design scheme to facilitate the advancement of radiopharmaceuticals and, thus, prompt clinical translation.

Towards the Magic Radioactive Bullet: Improving Targeted Radionuclide Therapy by Reducing the Renal Retention of Radioligands

Targeted radionuclide therapy (TRT) is an emerging field and has the potential to become a major pillar in effective cancer treatment. Several pharmaceuticals are already in routine use for treating cancer, and there is still a high potential for new compounds for this application. But, a major issue for many radiolabeled low-to-moderate-molecular-weight molecules is their clearance via the kidneys and their subsequent reuptake. High renal accumulation of radioactive compounds may lead to nephrotoxicity, and therefore, the kidneys are often the dose-limiting organs in TRT with these radioligands. Over the years, different strategies have been developed aiming for reduced kidney retention and enhanced therapeutic efficacy of radioligands. In this review, we will give an overview of the efforts and achievements of the used strategies, with focus on the therapeutic potential of low-to-moderate-molecular-weight molecules. Among the strategies discussed here is coadministration of compounds that compete for binding to the endocytic receptors in the proximal tubuli. In addition, the influence of altering the molecular design of radiolabeled ligands on pharmacokinetics is discussed, which includes changes in their physicochemical properties and implementation of cleavable linkers or albumin-binding moieties. Furthermore, we discuss the influence of chelator and radionuclide choice on reabsorption of radioligands by the kidneys.

Preclinical Evaluation of a Novel High-Affinity Radioligand [99mTc]Tc-BQ0413 Targeting Prostate-Specific Membrane Antigen (PSMA)

Radionuclide imaging using radiolabeled inhibitors of prostate-specific membrane antigen (PSMA) can be used for the staging of prostate cancer. Previously, we optimized the Glu-urea-Lys binding moiety using a linker structure containing 2-napththyl-L-alanine and L-tyrosine. We have now designed a molecule that contains mercaptoacetyl-triglutamate chelator for labeling with Tc-99m (designated as BQ0413). The purpose of this study was to evaluate the imaging properties of [99mTc]Tc-BQ0413. PSMA-transfected PC3-pip cells were used to evaluate the specificity and affinity of [99mTc]Tc-BQ0413 binding in vitro. PC3-pip tumor-bearing BALB/C nu/nu mice were used as an in vivo model. [99mTc]Tc-BQ0413 bound specifically to PC3-pip cells with an affinity of 33 ± 15 pM. In tumor-bearing mice, the tumor uptake of [99mTc]Tc-BQ0413 (38 ± 6 %IA/g in PC3-pip 3 h after the injection of 40 pmol) was dependent on PSMA expression (3 ± 2 %IA/g and 0.9 ± 0.3 %IA/g in PSMA-negative PC-3 and SKOV-3 tumors, respectively). We show that both unlabeled BQ0413 and the commonly used binder PSMA-11 enable the blocking of [99mTc]Tc-BQ0413 uptake in normal PSMA-expressing tissues without blocking the uptake in tumors. This resulted in an appreciable increase in tumor-to-organ ratios. At the same injected mass (5 nmol), the use of BQ0413 was more efficient in suppressing renal uptake than the use of PSMA-11. In conclusion, [99mTc]Tc-BQ0413 is a promising probe for the visualization of PSMA-positive lesions using single-photon emission computed tomography (SPECT).

Targeting the Gastrin-Releasing Peptide Receptor (GRP-R) in Cancer Therapy: Development of Bombesin-Based Peptide-Drug Conjugates

Targeted tumour therapy has proved to be an efficient alternative to overcome the limitations of conventional chemotherapy. Among several receptors upregulated in cancer cells, the gastrin-releasing peptide receptor (GRP-R) has recently emerged as a promising target for cancer imaging, diagnosing and treatment due to its overexpression on cancerous tissues such as breast, prostate, pancreatic and small-cell lung cancer. Herein, we report on the in vitro and in vivo selective delivery of the cytotoxic drug daunorubicin to prostate and breast cancer, by targeting GRP-R. Exploiting many bombesin analogues as homing peptides, including a newly developed peptide, we produced eleven daunorubicin-containing peptide-drug conjugates (PDCs), acting as drug delivery systems to safely reach the tumour environment. Two of our bioconjugates revealed remarkable anti-proliferative activity, an efficient uptake by all three tested human breast and prostate cancer cell lines, high stability in plasma and a prompt release of the drug-containing metabolite by lysosomal enzymes. Moreover, they revealed a safe profile and a consistent reduction of the tumour volume in vivo. In conclusion, we highlight the importance of GRP-R binding PDCs in targeted cancer therapy, with the possibility of further tailoring and optimisation.

Homomultimer strategy for improvement of radiolabeled peptides and antibody fragments in tumor targeting

A homomultimeric radioligand is composed of multiple identical ligands connected to the linker and radionuclide to detect a variety of overexpressed receptors on cancer cells. Multimer strategy holds great potential for introducing new radiotracers based on peptide and monoclonal antibody (mAb) derivatives in molecular imaging and therapy. It offers a reliable procedure for the preparation of biological-based targeting with diverse affinities and pharmacokinetics. In this context, we provide a useful summary and interpretation of the main results by a comprehensive look at multimeric radiopharmaceuticals in nuclear oncology. Therefore, there will be explanations for the strategy mechanisms and the main variables affecting the biodistribution results. The discussion is followed by highlights of recent work in the targeting of various types of receptors. The consequences are expressed based on comparing some parameters between monomer and multimer counterparts in each relevant section.

Impact of REDV peptide density and its linker structure on the capture, movement, and adhesion of flowing endothelial progenitor cells in microfluidic devices

Ligand-immobilization to stents and vascular grafts is expected to promote endothelialization by capturing flowing endothelial progenitor cells (EPCs). However, the optimized ligand density and linker structure have not been fully elucidated. Here, we report that flowing EPCs were selectively captured by the REDV peptide conjugated with a short linker. The microchannel surface was modified with the REDV peptide via Gly-Gly-Gly (G3), (Gly-Gly-Gly)₃ (G9), and diethylene glycol (diEG) linkers, and the moving velocity and captured ratio were evaluated. On the unmodified microchannels, the moving velocity of the cells exhibited a unimodal distribution similar to the liquid flow. The velocity of the endothelial cells and EPCs on the peptide-immobilized surface indicated a bimodal distribution, and approximately 20 to 30% of cells moved slower than the liquid flow, suggesting that the cells were captured and rolled on the surface. When the immobilized ligand density was lower than 1 molecule/nm², selective cell capture was observed only in REDV with G3 and diEG linkers, but not in G9 linkers. An in silico study revealed that the G9 linker tends to form a bent structure, and the REDV peptide is oriented to the substrate side. These results indicated that REDV captured the flowing EPC in a sequence-specific manner, and that the short linker was more adequate.

Clickable Radiocomplexes With Trivalent Radiometals for Cancer Theranostics: In vitro and in vivo Studies

Pre-targeting approaches based on the inverse-electron-demand Diels-Alder (iEDDA) reaction between strained trans-cyclooctenes (TCO) and electron-deficient tetrazines (Tz) have emerged in recent years as valid alternatives to classic targeted strategies to improve the diagnostic and therapeutic properties of radioactive probes. To explore these pre-targeting strategies based on in vivo click chemistry, a small family of clickable chelators was synthesized and radiolabelled with medically relevant trivalent radiometals. The structure of the clickable chelators was diversified to modulate the pharmacokinetics of the resulting [111In]In-radiocomplexes, as assessed upon injection in healthy mice. The derivative DOTA-Tz was chosen to pursue the studies upon radiolabelling with 90Y, yielding a radiocomplex with high specific activity, high radiochemical yields and suitable in vitro stability. The [90Y]Y-DOTA-Tz complex was evaluated in a prostate cancer PC3 xenograft by ex-vivo biodistribution studies and Cerenkov luminescence imaging (CLI). The results highlighted a quick elimination through the renal system and no relevant accumulation in non-target organs or non-specific tumor uptake. Furthermore, a clickable bombesin antagonist was injected in PC3 tumor-bearing mice followed by the radiocomplex [90Y]Y-DOTA-Tz, and the mice imaged by CLI at different post-injection times (p.i.). Analysis of the images 15 min and 1 h p.i. pointed out an encouraging quick tumor uptake with a fast washout, providing a preliminary proof of concept of the usefulness of the designed clickable complexes for pre-targeting strategies. To the best of our knowledge, the use of peptide antagonists for this purpose was not explored before. Further investigations are needed to optimize the pre-targeting approach based on this type of biomolecules and evaluate its eventual advantages.

The Influence of a Polyethylene Glycol Linker on the Metabolism and Pharmacokinetics of a 89Zr-Radiolabeled Antibody

Most experimental work in the space of bioconjugation chemistry focuses on using new methods to construct covalent bonds between a cargo molecule and a protein of interest such as a monoclonal antibody (mAb). Bond formation is important for generating new diagnostic tools, yet when these compounds advance to preclinical in vitro and in vivo studies, and later for translation to the clinic, understanding the fate of potential metabolites that arise from chemical or enzymatic degradation of the construct is important to obtain a full picture of the pharmacokinetic performance of a new compound. In the context of designing new bioconjugate methods for labeling antibodies with the positron-emitting radionuclide ⁸⁹Zr, we previously developed a photochemical process for making ⁸⁹Zr-mAbs. Experimental studies on [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAb constructs revealed that incorporation of the tris-polyethylene glycol (PEG₃) linker improved the aqueous phase solubility and radiochemical conversion. However, the use of a PEG₃ linker also has an impact on the whole-body residence time of the construct, leading to a more rapid excretion of the ⁸⁹Zr activity when compared with radiotracers that lack the PEG₃ chain. In this work, we investigated the metabolic fate of eight possible metabolites that arise from the logical disconnection of [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAb at bonds which are susceptible to chemical or enzymatic cleavage. Synthesis combined with ⁸⁹Zr-radiolabeling, small-animal positron emission tomography imaging at multiple time points from 0 to 20 h, and measurements of the effective half-life for whole-body excretion are reported. The conclusions are that the use of a PEG₃ linker is non-innocent in terms of its impact on enhancing the metabolism of [⁸⁹Zr]ZrDFO-PEG₃-azepin-mAbs. In most cases, degradation can produce metabolites that are rapidly eliminated from the body, thereby enhancing image contrast by reducing nonspecific accumulation and retention of ⁸⁹Zr in background organs such as the liver, spleen, kidney, and bone.

Development of biotin-tagged near-infrared fluorescence probes for tumor-specific imaging

Near-infrared (NIR) probes are applicable for tumor imaging due to deep tissue penetration and low background signal. And cyanine dyes with long emission wavelength are excellent fluorophores to develop NIR probes. However, the aggregation of cyanine dyes in aqueous solution reduces the utilization of light. To solve this problem, polyethylene glycol (PEG) was introduced into the probes to reduce their aggregation. In our work, two new NIR probes G₁ and G₂ were designed and prepared by conjugating the cyanine dye G₀ with Biotin-PEG5-Azide. The conjugated biotin could enhance the target specificity of probes. And the photophysical and photochemical parameters demonstrated that G₁ and G₂ had a reduced aggregation tendency. In vitro fluorescence imaging proved that these two probes could be specifically taken up by HeLa cells, and in vivo imaging demonstrated that these two probes could specifically target tumors with large tumor-to-muscle (T/M) ratios. All these results indicated that G₁ and G₂ are promising NIR fluorescent contrast agents for tumor-specific imaging.

Preclinical Evaluation of 99mTc-Labeled GRPR Antagonists maSSS/SES-PEG2-RM26 for Imaging of Prostate Cancer

BACKGROUND

Gastrin-releasing peptide receptor (GRPR) is an important target for imaging of prostate cancer. The wide availability of single-photon emission computed tomography/computed tomography (SPECT/CT) and the generator-produced ^99mTc can be utilized to facilitate the use of GRPR-targeting radiotracers for diagnostics of prostate cancers.

METHODS

Synthetically produced mercaptoacetyl-Ser-Ser-Ser (maSSS)-PEG₂-RM26 and mercaptoacetyl-Ser-Glu-Ser (maSES)-PEG₂-RM26 (RM26 = d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂) were radiolabeled with ^99mTc and characterized in vitro using PC-3 cells and in vivo, using NMRI or PC-3 tumor bearing mice. SPECT/CT imaging and dosimetry calculations were performed for [^99mTc]Tc-maSSS-PEG₂-RM26.

RESULTS

Peptides were radiolabeled with high yields (>98%), demonstrating GRPR specific binding and slow internalization in PC-3 cells. [^99mTc]Tc-maSSS-PEG₂-RM26 outperformed [^99mTc]Tc-maSES-PEG_2-RM26 in terms of GRPR affinity, with a lower dissociation constant (61 pM vs 849 pM) and demonstrating higher tumor uptake. [^99mTc]Tc-maSSS-PEG₂-RM26 had tumor-to-blood, tumor-to-muscle, and tumor-to-bone ratios of 97 ± 56, 188 ± 32, and 177 ± 79, respectively. SPECT/CT images of [^99mTc]Tc-maSSS-PEG₂-RM26 clearly visualized the GRPR-overexpressing tumors. The dosimetry estimated for [^99mTc]Tc-maSSS-PEG₂-RM26 showed the highest absorbed dose in the small intestine (1.65 × 10^-3 mGy/MBq), and the effective dose is 3.49 × 10^-3 mSv/MBq.

CONCLUSION

The GRPR antagonist maSSS-PEG₂-RM26 is a promising GRPR-targeting agent that can be radiolabeled through a single-step with the generator-produced ^99mTc and used for imaging of GRPR-expressing prostate cancer.

Radiolabelled CCK2 R Antagonists Containing PEG Linkers: Design, Synthesis and Evaluation

The cholecystokinin-2/gastrin receptor (CCK₂ R) is considered a suitable target for the development of radiolabelled antagonists, due to its overexpression in various tumours, but no such compounds are available in clinical use. Therefore, we designed novel 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-conjugated ligands based on CCK₂ R antagonist Z360/nastorazepide. As a proof of concept that CCK₂ R antagonistic activity can be retained by extending the Z360/nastorazepide structure using suitable linker, we present herein three compounds containing various PEG linkers synthesised on solid phase and in solution. The antagonistic properties were measured in a functional assay in the A431-CCK₂ R cell line (in the presence of agonist G17), with IC₅₀ values of 3.31, 4.11 and 10.4 nM for compounds containing PEG₄ , PEG₆ and PEG₁₂ , respectively. All compounds were successfully radiolabelled with indium-111, lutetium-177 and gallium-68 (incorporation of radiometal >95 %). The gallium-68-labelled compounds were stable for up to 2 h (PBS, 37 °C). log D_7.4 values were determined for indium-111- and gallium-68-labelled compounds, showing improved hydrophilicity compared to the reference compound.

[99mTc]Tc-DB1 Mimics with Different-Length PEG Spacers: Preclinical Comparison in GRPR-Positive Models

Background: The frequent overexpression of gastrin-releasing peptide receptors (GRPRs) in human cancers provides the rationale for delivering clinically useful radionuclides to tumor sites using peptide carriers. Radiolabeled GRPR antagonists, besides being safer for human use, have often shown higher tumor uptake and faster background clearance than agonists. We herein compared the biological profiles of the GRPR-antagonist-based radiotracers [^99mTc]Tc-[N₄-PEGx-DPhe⁶,Leu-NHEt¹³]BBN(6-13) (N₄: 6-(carboxy)-1,4,8,11-tetraazaundecane; PEG: polyethyleneglycol): (i) [^99mTc]Tc-DB7 (x = 2), (ii) [^99mTc]Tc-DB13 (x = 3), and (iii) [^99mTc]Tc-DB14 (x = 4), in GRPR-positive cells and animal models. The impact of in situ neprilysin (NEP)-inhibition on in vivo stability and tumor uptake was also assessed by treatment of mice with phosphoramidon (PA). Methods: The GRPR affinity of DB7/DB13/DB14 was determined in PC-3 cell membranes, and cell binding of the respective [^99mTc]Tc-radioligands was assessed in PC-3 cells. Each of [^99mTc]Tc-DB7, [^99mTc]Tc-DB13, and [^99mTc]Tc-DB14 was injected into mice without or with PA coinjection and 5 min blood samples were analyzed by HPLC. Biodistribution was conducted at 4 h postinjection (pi) in severe combined immunodeficiency disease (SCID) mice bearing PC-3 xenografts without or with PA coinjection. Results: DB7, -13, and -14 displayed single-digit nanomolar affinities for GRPR. The uptake rates of [^99mTc]Tc-DB7, [^99mTc]Tc-DB13, and [^99mTc]Tc-DB14 in PC-3 cells was comparable and consistent with a radioantagonist profile. The radiotracers were found to be ≈70% intact in mouse blood and >94% intact after coinjection of PA. Treatment of mice with PA enhanced tumor uptake. Conclusions: The present study showed that increase of PEG-spacer length in the [^99mTc]Tc-DB7-[^99mTc]Tc-DB13-[^99mTc]Tc-DB14 series had little effect on GRPR affinity, specific uptake in PC-3 cells, in vivo stability, or tumor uptake. A significant change in in vivo stability and tumor uptake was observed only after treatment of mice with PA, without compromising the favorably low background radioactivity levels.

Preclinical Evaluation of NHS-Activated Gold Nanoparticles Functionalized with Bombesin or Neurotensin-Like Peptides for Targeting Colon and Prostate Tumours

Recent advances and large-scale use of hybrid imaging modalities like PET-CT have led to the necessity of improving nano-drug carriers that can facilitate both functional and metabolic screening in nuclear medicine applications. In this study, we focused on the evaluation of four potential imaging nanoparticle structures labelled with the 68Ga positron emitter. For this purpose, we functionalized NHS-activated PEG-gold nanoparticles with 68Ga-DOTA-Neuromedin B, 68Ga-DOTA-PEG(4)-BBN(7-14), 68Ga-DOTA-NT and 68Ga-DOTA-Neuromedin N. In vitro binding kinetics and specific binding to human HT-29 colon carcinoma cells and DU-145 prostate carcinoma cells respectively were assessed, over 75% retention being obtained in the case of 68Ga-DOTA-PEG(4)-BBN(7-14)-AuNP in prostate tumour cells and over 50% in colon carcinoma cells. Biodistribution in NU/J mice highlighted a three-fold uptake increase in tumours at 30 min post-injection of 68Ga-DOTA-NT-AuNP and 68Ga-DOTA-PEG(4)-BBN(7-14)-AuNP compared to 68Ga-DOTA-NT and 68Ga-DOTA-PEG(4)-BBN(7-14) respectively, therewith fast distribution in prostate and colon tumours and minimum accumulation in non-targeted tissues.

Methods to Enhance the Metabolic Stability of Peptide-Based PET Radiopharmaceuticals

The high affinity and specificity of peptides towards biological targets, in addition to their favorable pharmacological properties, has encouraged the development of many peptide-based pharmaceuticals, including peptide-based positron emission tomography (PET) radiopharmaceuticals. However, the poor in vivo stability of unmodified peptides against proteolysis is a major challenge that must be overcome, as it can result in an impractically short in vivo biological half-life and a subsequently poor bioavailability when used in imaging and therapeutic applications. Consequently, many biologically and pharmacologically interesting peptide-based drugs may never see application. A potential way to overcome this is using peptide analogues designed to mimic the pharmacophore of a native peptide while also containing unnatural modifications that act to maintain or improve the pharmacological properties. This review explores strategies that have been developed to increase the metabolic stability of peptide-based pharmaceuticals. It includes modifications of the C- and/or N-termini, introduction of d- or other unnatural amino acids, backbone modification, PEGylation and alkyl chain incorporation, cyclization and peptide bond substitution, and where those strategies have been, or could be, applied to PET peptide-based radiopharmaceuticals.

The Influence of Different Spacers on Biological Profile of Peptide Radiopharmaceuticals for Diagnosis and Therapy of Human Cancers

BACKGROUND

Cancer is the leading cause of death worldwide. Early detection can reduce the disadvantageous effects of diseases and the mortality in cancer. Nuclear medicine is a powerful tool that has the ability to diagnose malignancy without harming normal tissues. In recent years, radiolabeled peptides have been investigated as potent agents for cancer detection. Therefore, it is necessary to modify radiopeptides in order to achieve more effective agents.

OBJECTIVE

This review describes modifications in the structure of radioconjugates with spacers who have improved the specificity and sensitivity of the peptides that are used in oncologic diagnosis and therapy.

METHODS

To improve the biological activity, researchers have conjugated these peptide analogs to different spacers and bifunctional chelators. Many spacers of different kinds, such as hydrocarbon chain, amino acid sequence, and poly (ethyleneglycol) were introduced in order to modify the pharmacokinetic properties of these biomolecules.

RESULTS

Different spacers have been applied to develop radiolabeled peptides as potential tracers in nuclear medicine. Spacers with different charge and hydrophilicity affect the characteristics of peptide conjugate. For example, the complex with uncharged and hydrophobic spacers leads to increased liver uptake, while the composition with positively charged spacers results in high kidney retention. Therefore, the pharmacokinetics of radio complexes correlates to the structure and total charge of the conjugates.

CONCLUSION

Radio imaging technology has been successfully applied to detect a tumor in the earliest stage. For this purpose, the assessment of useful agents to diagnose the lesion is necessary. Developing peptide radiopharmaceuticals using spacers can improve in vitro and in vivo behavior of radiotracers leading to better noninvasive detection and monitoring of tumor growth.

HYNIC and DOMA conjugated radiolabeled bombesin analogs as receptor-targeted probes for scintigraphic detection of breast tumor

Background

Among the many peptide receptor systems, gastrin-releasing-peptide (GRP) receptors, the mammalian equivalent of bombesin (BN) receptors, are potential targets for diagnosis and therapy of breast tumors due to their overexpression in various frequently occurring human cancers. The aim of this study was to synthesize and comparative evaluation of ^99mTc-labeled new BN peptide analogs. Four new BN analogs, HYNIC-Asp[PheNle]BN(7-14)NH₂, BN1; HYNIC-Pro-Asp[TyrMet]BN(7-14)NH₂, BN2; HYNIC-Asp-Asn[Lys-CHAla-Nle]BN(7-14)NH₂, BN3; and DOMA-GABA[Pro-Tyr-Nle]BN(7-14)NH₂, BN4 were synthesized and biologically evaluated for targeted imaging of GRP receptor-positive breast-tumors.

Methods

Solid-phase synthesis using Fmoc-chemistry was adopted for the synthesis of peptides. BN1–BN4 analogs were better over the standard Gln-Trp-Ala-Val-Gly-His-Leu-Met-NH₂ (BNS). Lipophilicity, serum stability, internalization, and binding affinity studies were carried out using ^99mTc-labeled analogs. Biodistribution and imaging analyses were performed on MDA-MB-231 cell-induced tumor-bearing mice. BN-analogs induced angiogenesis; tumor formation and GRP-receptor-expression were confirmed by histology and immunohistochemistry analyses of tumor sections and important tissue sections.

Results

All the analogs displayed ≥ 97% purity after the HPLC purification. BN-peptide-conjugates exhibited high serum stability and significant binding affinity to GRP-positive tumor; rapid internalization/externalization in/from MDA-MB-231 cells were noticed for the BN analogs. BN4 and BN3 exhibited higher binding affinity, stability than BN1 and BN2. Highly specific in vivo uptakes to the tumor were clearly visualized by scintigraphy; rapid excretion from non-target tissues via kidneys suggests a higher tumor-to-background ratio. BN4, among all the analogs, stimulates the expression of angiogenic markers to a maximum.

Conclusion

Considering its most improved pharmacological characteristics, BN4 is thus considered as most promising probes for early non-invasive diagnosis of GRP receptor-positive breast tumors.

Electronic supplementary material

The online version of this article (10.1186/s13550-019-0493-x) contains supplementary material, which is available to authorized users.

Methylthiazolyl Tacn Ligands for Copper Complexation and Their Bifunctional Chelating Agent Derivatives for Bioconjugation and Copper-64 Radiolabeling: An Example with Bombesin

We present here the synthesis of two new bifunctionalized azachelators, no2th-EtBzNCS and Hno2th1tha, as bioconjugable analogues of two previously described di- and trimethylthiazolyl 1,4,7-triazacyclononane (tacn) ligands, no2th and no3th, for potential uses in copper-64 (⁶⁴Cu) positron emission tomography imaging. The first one bears an isothiocyanate group on the remaining free nitrogen atom of the tacn framework, while the second one presents an additional carboxylic function on one of the three heterocyclic pendants. Their syntheses required regiospecific N-functionalization of the macrocycles. In order to investigate their suitability for in vivo applications, a complete study of their copper(II) chelation was performed. The acid-base properties of the ligands and their thermodynamic stability constants with copper(II) and zinc(II) cations were determined using potentiometric techniques. Structural studies were conducted in both solution and the solid state, consolidated by theoretical calculations. The kinetic inertness in an acidic medium of both copper(II) complexes was determined by spectrophotometry, while cyclic voltammetry experiments were performed to evaluate the stability at the copper(I) redox state. UV-vis, NMR (of the zinc complexes), electron paramagnetic resonance spectroscopy, and density functional theory studies showed excellent agreement between the solution structures of the complexes and their crystallographic data. These investigations unambiguously prove that these bifunctional derivatives display similar coordination properties as their no2th and no3th counterparts, opening the door to targeted bioapplications. The no2th-EtBzNCS and Hno2th1tha ligands were then conjugated to a bombesin antagonist peptide for targeting the gastrin-releasing peptide receptor (GRPr). To highlight the potential of the two chelators for radiopharmaceutical development, the ⁶⁴Cu-radiolabeling properties, in vitro stability, and binding affinity to GRPr of the corresponding bioconjugates were determined. Altogether, the results of this work warrant the further development of ⁶⁴Cu-based radiopharmaceuticals comprising our novel bifunctional chelators.

Peptide-Based Drug-Delivery Systems in Biotechnological Applications: Recent Advances and Perspectives

Peptides of natural and synthetic sources are compounds operating in a wide range of biological interactions. They play a key role in biotechnological applications as both therapeutic and diagnostic tools. They are easily synthesized thanks to solid-phase peptide devices where the amino acid sequence can be exactly selected at molecular levels, by tuning the basic units. Recently, peptides achieved resounding success in drug delivery and in nanomedicine smart applications. These applications are the most significant challenge of recent decades: they can selectively deliver drugs to only pathological tissues whilst saving the other districts of the body. This specific feature allows a reduction in the drug side effects and increases the drug efficacy. In this context, peptide-based aggregates present many advantages, including biocompatibility, high drug loading capacities, chemical diversity, specific targeting, and stimuli responsive drug delivery. A dual behavior is observed: on the one hand they can fulfill a structural and bioactive role. In this review, we focus on the design and the characterization of drug delivery systems using peptide-based carriers; moreover, we will also highlight the peptide ability to self-assemble and to actively address nanosystems toward specific targets.

Fluorescent and Photosensitizing Conjugates of Cell-Penetrating Peptide TAT(47-57): Design, Microwave-Assisted Synthesis at 60 °C, and Properties

Conjugates based on cell-penetrating peptides (CPPs) are scientifically relevant owing to their structural complexity; their ability to enter cells and deliver drugs, labels, antioxidants, bioactive compounds, or DNA fragments; and, consequently, their potential for application in research and biomedicine. In this study, carboxyamidated fluorescently labeled conjugates FAM-GG-TAT(47-57)-NH2 and FAM-PEG6-TAT(47-57)-NH2 and photosensitizer-labeled conjugate Chk-PEG6-TAT(47-57)-NH2 [where TAT(47-57) is the CPP, 5(6)-carboxyfluorescein is the (FAM) fluorophore, chlorin k (Chk) is the photosensitizer, and the dipeptide glycyl-glycine (GG) or hexaethylene glycol (PEG6) is the spacer] were originally designed, prepared, and fully characterized. Practically, all chemical reactions of the synthetic steps (peptide synthesis, spacer incorporation, and conjugation) were microwave-assisted at 60 °C using optimized protocols to give satisfying yields and high-quality products. Detailed analyses of the conjugates using spectrofluorimetry and singlet oxygen detection showed that they display photophysical properties typical of FAM or Chk. Anticandidal activity assays showed that not only this basic property of TAT(47-57) was preserved in the conjugates but also that the minimal inhibitory concentration was slightly reduced for cells incubated with PS-bearing conjugate Chk-PEG6-TAT(47-57)-NH2. Overall, these results indicated that the synthetic approach on-resin assisted by microwaves at 60 °C is simple, straightforward, selective, metal-free, sufficiently fast, cleaner, and more cost-effective than those previously used for preparing this type of macromolecule. Furthermore, such new data show that microwaves at 60 °C and/or conjugation did not harm the integrity of the conjugates' constituents. Therefore, FAM-GG-TAT(47-57)-NH2, FAM-PEG6-TAT(47-57)-NH2, and Chk-PEG6-TAT(47-57)-NH2 have high potential for practical applications in biochemistry, biophysics, and therapeutics.

Radiolabeled bombesin derivatives for preclinical oncological imaging

Despite efforts, cancer is still one of the leading causes of morbidity and mortality worldwide, with approximately 14 million new cases and 8.2 million cancer-related deaths each year, according to the World Health Organization. Among the strategies to reduce cancer progression and improving its management, implementing early detection technologies is crucial. Based on the fact that several types of cancer cells overexpress surface receptors, small molecule ligands, such as peptides, have been developed to allow tumor identification at earlier stages. Allied with imaging techniques such as PET and SPECT, radiolabeled peptides play a pivotal role in nuclear medicine. Bombesin, a peptide of 14 amino acids, is an amphibian homolog to the mammalian gastrin-releasing peptide (GRP), that has been extensively studied as a targeting ligand for diagnosis and therapy of GRP positive tumors, such as breast, pancreas, lungs and prostate cancers. In this context, herein we provide a review of reported bombesin derivatives radiolabeled with a multitude of radioactive isotopes for diagnostic purposes in the preclinical setting. Moreover, since animal models are highly relevant for assessing the potential of clinical translation of this radiopeptides, a brief report of the currently used GRP-positive tumor-bearing animal models is described.

Approaches to improve metabolic stability of a statine-based GRP receptor antagonist

The bombesin receptor family, in particular the gastrin-releasing peptide receptor (GRPr), is an attractive target in the field of nuclear oncology due to the high density of these receptors on the cell surface of several human tumors. The successful clinical implementation of ⁶⁴Cu-CB-TE2A-AR06, ⁶⁸Ga-RM2 and ⁶⁸Ga-NODAGA-MJ9, prompted us to continue the development of GRPr-antagonists. The aim of the present study was to assess if N-terminal modulations of the statine-based GRPr-antagonist influence the binding affinity, the pharmacokinetic performance and the in vivo metabolic stability.

METHODS

The GRPr-antagonist (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂) was functionalized with the chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) via the spacer 4-amino-1-carboxymethyl-piperidine (Pip) and the amino acid N-Methyl-β-Ala, to obtain NMe-RM2 and labeled with ⁶⁸Ga and ¹⁷⁷Lu. The GRPr affinity of the corresponding metalloconjugates determined using [¹²⁵I-Tyr⁴]-BN as radioligand. In vitro evaluation included internalization studies using PC3 cells. The ⁶⁸Ga-conjugate was evaluated in PC3 xenografts by biodistribution and PET studies, while investigations on the metabolic stability and plasma protein binding were performed.

RESULTS

The half maximum inhibitory concentrations (IC₅₀) of the metalloconjugates, using [¹²⁵I-Tyr⁴]-BN, are in the low nanomolar range. PC3-cell culture binding studies of both metallated NMe-RM2 and RM2 show high GRPr-bound activity and low internalization. Metabolic studies showed that ⁶⁸Ga-NMe-RM2 and ⁶⁸Ga-RM2 are being cleaved in a similar fashion into three metabolites, with a good proportion of about 50% of the remaining blood activity at 15min post injection (p.i.) being represented by the intact radiotracer. ⁶⁸Ga-NMe-RM2 was shown to target specifically PC3 xenografts, with high and sustained tumor uptake of about 13% IA/g within a time frame of 3h. The PET images clearly visualized the tumor.

CONCLUSIONS

The relatively high percentage of the remaining intact radiotracer in blood 15min post injection sufficiently enables in vivo targeting of GRPr positive tumors, finding which has been also shown in clinical trials.

Synthesis and in Vitro and in Vivo Evaluation of MMP-12 Selective Optical Probes

In designing new tracers consisting of a small peptide conjugated to a reporter of comparable size, particular attention needs to be paid to the selection of the reporter group, which can dictate both the in vitro and the in vivo performances of the whole conjugate. In the case of fluorescent tracers, this is particularly true given the large numbers of available dye moieties differing in their structures and properties. Here, we have investigated the in vitro and in vivo properties of a novel series of MMP-12 selective probes composed of cyanine dyes varying in their structure, net charge, and hydrophilic character, tethered through a linker to a potent and specific MMP-12 phosphinic pseudopeptide inhibitor. The impact of linker length has been also explored. The crystallographic structure of one tracer in complex with MMP-12 has been obtained, providing the first crystal structure of a Cy5.5-derived probe and confirming that the binding of the targeting moiety is unaffected. MMP-12 remains the tracers' privileged target, as attested by their affinity selectivity profile evaluated in solution toward a panel of 12 metalloproteases. In vivo assessment of four selected probes has highlighted not only the impact of the dye structure but also that of the linker length on the probes' blood clearance rates and their biodistributions. These experiments have also provided valuable data on the stability of the dye moieties in vivo. This has permitted the identification of one probe, which combines favorable binding to MMP-12 in solution and on cells with optimized in vivo performance including blood clearance rate suitable for short-time imaging. Through this series of tracers, we have identified various critical factors modulating the tracers' in vivo behavior, which is both useful for the development and optimization of MMP-12 selective radiolabeled tracers and informative for the design of fluorescent probes in general.

Pre-clinical evaluation of eight DOTA coupled gastrin-releasing peptide receptor (GRP-R) ligands for in vivo targeting of receptor-expressing tumors

Background

Overexpression of the gastrin-releasing peptide receptor (GRP-R) has been documented in several human neoplasms such as breast, prostate, and ovarian cancer. There is growing interest in developing radiolabeled peptide-based ligands toward these receptors for the purpose of in vivo imaging and radionuclide therapy of GRP-R-overexpressing tumors. A number of different peptide sequences, isotopes, and labeling methods have been proposed for this purpose. The aim of this work is to perform a direct side-by-side comparison of different GRP-R binding peptides utilizing a single labeling strategy to identify the most suitable peptide sequence.

Methods

Solid-phase synthesis of eight derivatives (BN1-8) designed based on literature analysis was carried out. Peptides were coupled to the DOTA chelator through a PEG4 spacer at the N-terminus. Derivatives were characterized for serum stability, binding affinity on PC-3 human prostate cancer cells, biodistribution in tumor-bearing mice, and gamma camera imaging at 1, 6, and 24 h after injection.

Results

Serum stability was quite variable among the different compounds with half-lives ranging from 16 to 400 min at 37 °C. All compounds tested showed K_d values in the nanomolar range with the exception of BN3 that showed no binding. Biodistribution and imaging studies carried out for compounds BN1, BN4, BN7, and BN8 showed targeting of the GRP-R-positive tumors and the pancreas. The BN8 compound (DOTA-PEG-DPhe-Gln-Trp-Ala-Val-NMeGly-His-Sta-Leu-NH₂) showed high affinity, the longest serum stability, and the highest target-to-background ratios in biodistribution and imaging experiments among the compounds tested.

Conclusions

Our results indicate that the NMeGly for Gly substitution and the Sta-Leu substitution at the C-terminus confer high serum stability while maintaining high receptor affinity, resulting in biodistribution properties that outperform those of the other peptides.

Improving the stability of peptidic radiotracers by the introduction of artificial scaffolds: which structure element is most useful?

Peptidic radiotracers are highly potent substances for the specific in vivo imaging of various biological targets with Single Photon Emission Computed Tomography and Positron Emission Tomography. However, some radiolabeled peptides such as bombesin analogs were shown to exhibit only a limited stability, hampering a successful target visualization. One option to positively influence the stability of radiolabeled peptides is the introduction of certain artificial molecular scaffolds. In order to comparatively assess the influence of different structure elements on the stability of radiolabeled peptides and to identify those structure elements being most useful for peptide radiotracer stabilization, several monomeric and dimeric bombesin derivatives were synthesized, exhibiting differing molecular designs and the chelator NODAGA for (68) Ga-labeling. The radiolabeled peptides were evaluated regarding their in vitro stability in human serum to determine the influence of the introduced molecular scaffolds on the peptides' serum stabilities. The results of the evaluations showed that the introduction of scaffold structures and the overall molecular design have a substantial impact on the stabilities of the resulting peptidic radiotracers. But besides some general trends found using certain scaffold structures, the obtained results point to the necessity to empirically assess their influence on stability for each susceptible peptidic radiotracer individually.

Evaluation and comparison of a new DOTA and DTPA-bombesin agonist in vitro and in vivo in low and high GRPR expressing prostate and breast tumor models

We evaluated and compared a new bombesin analog [Tyr-Gly5, Nle(14)]-BBN(6-14) conjugated to DOTA or DTPA and radiolabeled with In-111 in low and high GRPR expressing tumor models. Both peptides were radiolabeled with high radiochemical purity and specific activity. In vitro assays on T-47D, LNCaP and PC-3 cells showed that the affinity of peptides is similar and a higher binding and internalization of DOTA-peptide to PC-3 cells was observed. Both peptides could target PC-3 and LNCaP tumors in vivo and both tumor types could be visualized by microSPECT/CT.