Evaluation of DOTA-chelated neurotensin analogs with spacer-enhanced biological performance for neurotensin-receptor-1-positive tumor targeting

Preclinical evaluation of a new technetium-99m labeled neurotensin analogue for NTSR1 targeted radionuclide imaging

Neurotensin is a regulatory peptide that can act as a growth factor on different types of normal and cancerous cells. Binding of Neurotensin to relevant receptors leads to cell proliferation, survival, migration and invasion by changing intracellular enzyme activity. Therefore, the design of a neurotensin-based radiopeptide plays an important role in targeted imaging or therapy of neurotensin receptor-positive tumors. A [Lys8]-neurotensin (7-13) peptide was synthesized and attached to HYNIC as a chelator via a linker. The labeling procedure was carried out at 100 °C for 10 min using 99mTc as a radionuclide and EDDA/tricine as coligands. Stability of the labeled peptide in human serum was determined using RTLC and HPLC methods. The receptor binding internalization was studied using HT-29 colon carcinoma cells, and tissue biodistribution was evaluated in mice bearing CT-26 tumors. The [99mTc]Tc-Tricine/EDDA/HYNIC-GABA-[Lys8]-neurotensin (7-13) peptide demonstrated a labeling yield of over 98 %, a specific activity of 37.00 GBq/µmol, high stability in human serum, a nanomolar range of Kd, and a tumor uptake of 0.36 ± 0.15 % ID/g at 1-h post-injection. These results suggest that the labeled peptide is a suitable imaging agent for neurotensin receptor-positive tumors.

Rational design of NT-PSMA heterobivalent probes for prostate cancer theranostics

Targeting the prostate-specific membrane antigen (PSMA) with radiopharmaceuticals for imaging and/or therapy has demonstrated significant advancement in the management of prostate cancer patients. However, PSMA targeting remains unsuccessful in prostate cancers with low expression of PSMA, which account for 15% of cases. The neurotensin receptor-1 (NTS1) has been highlighted as a suitable oncotarget for imaging and therapy of PSMA-negative prostate cancer lesions. Therefore, heterobivalent probes targeting both PSMA and NTS1 could improve the prostate cancer management. Herein, we report the development of a branched hybrid probe (JMV 7489) designed to target PSMA and/or NTS1 bearing relevant pharmacophores and DOTA as the chelating agent. The new ligand was synthesized with a hybrid approach, which includes both syntheses in batch and in the solid phase. Saturation binding experiments were next performed on HT-29 and PC3-PIP cells to derive K d and B max values. On the PC3-PIP cells, [68Ga]Ga-JMV 7489 displayed good affinity towards PSMA (K d = 53 ± 17 nM; B max = 1393 ± 29 fmol/106 cells) in the same range as the corresponding reference monomer. A lower affinity value towards NTS1 was depicted (K d = 157 ± 71 nM; B max = 241 ± 42 fmol/106 cells on PC3-PIP cells; K d = 246 ± 1 nM; B max = 151 ± 44 fmol/106 cells on HT-29 cells) and, surprisingly, it was also the case for the corresponding monomer [68Ga]Ga-JMV 7089. These results indicate that the DOTA macrocycle and the linker are critical elements to design heterobivalent probes targeting PSMA and NTS1 with high affinity towards NTS1.

Examination of Charge Modifications of an Endolysosomal Trapping Inhibitor in an Antagonistic NTSR1-Targeted Construct for Colon Cancer

Many low-molecular weight targeted radiotherapeutics (TRTs) are capable of rapidly achieving exceptional tumor to non-target ratios shortly after administration. However, the low tumor residence time of many TRTs limits therapeutic dose delivery and has become the Achilles heel to their clinical translation. To combat the tumor efflux of these otherwise promising agents, we have previously presented a strategy of equipping low-molecular weight TRTs with irreversible cysteine cathepsin inhibitors (e.g., E-64 analogues). These inhibitors are capable of forming irreversible adducts with cysteine proteases within the endolysosomal compartments of cells. Using these endolysosomal trapping agents (ETs), the receptor-targeted constructs are able to increase tumor retention and, thus, deliverable therapeutic doses. In this study, we examine this approach in the development of agents targeting the neurotensin receptor subtype 1 (NTSR1), a receptor overexpressed in numerous cancers. Using an antagonistic NTSR1-targeting vector, we explore the impact of charge modification of the ETs on the in vitro and in vivo biological performance of the constructs using HT-29 colon cancer models. Four ETs (based on the epoxysuccinyl peptide E-64) with various charge states were synthesized and incorporated into the structures of the NTSR1-targeted antagonist. These four ¹⁷⁷Lu-labeled, ET-enhanced, NTSR1-targeted agents (¹⁷⁷Lu-NA-ET1-4), along with the structurally analogous ¹⁷⁷Lu-3BP-227, currently in clinical trials, underwent a battery of in vitro assays using HT-29 xenograft colon cancer cells to examine their NTSR1 binding, internalization and efflux, inhibition, and adduct formation properties. The biodistribution profile of these constructs was studied in an HT-29 mouse model. Charge modification of the terminal carboxylic acid and arginine of the ETs had deleterious effects on inhibition kinetics and in vitro adduct formation. Contrastingly, deletion of the arginine resulted in a modest increase in inhibition kinetics. Incorporation of ETs into the NTSR1-targeted agents was well-tolerated with minimal impact on the in vivo NTSR1 targeting but resulted in increased renal uptake. This study demonstrates that the ETs can be successfully incorporated into antagonistic NTSR1-targeted constructs without compromising their adduct formation capabilities. Based on these results, further exploration of the endolysosomal trapping approach is warranted in NTSR1- and other receptor-targeted antagonistic constructs.

Examination of the impact molecular charge has on NTSR1-targeted agents incorporated with cysteine protease inhibitors

Our laboratory has previously reported a strategy of employing cysteine cathepsin (CC) inhibitors as adduct forming, trapping agents to extend the tumor residence time of neurotensin receptor subtype 1 (NTSR1)-targeted radiopharmaceuticals. As a follow-up, we herein report a small library of CC trapping agent (CCTA)-incorporated, NTSR1-targeted conjugates with structural modifications that reduce the number of charged functional groups for both the CCTA and the peptide targeting sequence. These modifications were pursued to reduce the renal uptake and increase the translational potential of the CCTA-incorporated, NTSR1-targeted agents as radiotherapeutics. The biological performance of these constructs was examined using a battery of in vitro and in vivo studies employing the NTSR1-positive HT-29 human colon cancer cell line as our model. In vitro studies confirmed the ability of these constructs to target the NTSR1 and efficiently form intracellular adducts with cysteine proteases. Biodistribution studies using an HT-29 xenograft mouse model revealed that truncation (removal of Lys₆-Pro₇) of the NTSR1-targeted peptide (¹⁷⁷Lu-NE2a) had the greatest (3.7-fold) effect at lowering renal recognition/uptake relative to our previously reported construct. Other charge-reducing modifications to the CCTA resulted in unexpected increases in renal uptake. All of the constructs demonstrated similar levels of in vivo NTSR1-positive tumor targeting with the highest tumor residualization resulting from the construct containing the zwitterionic CCTA (¹⁷⁷Lu-NE2a). In vivo adduct formation of the conjugates was confirmed using autoradiographic SDS-PAGE analysis.

Development of Radiogallium-Labeled Peptides for Platelet-Derived Growth Factor Receptor β (PDGFRβ) Imaging: Influence of Different Linkers

The purpose of this study is to develop peptide-based platelet-derived growth factor receptor β (PDGFRβ) imaging probes and examine the effects of several linkers, namely un-natural amino acids (D-alanine and β-alanine) and ethylene-glycol (EG), on the properties of Ga-DOTA-(linker)-IPLPPPRRPFFK peptides. Seven radiotracers, ⁶⁷Ga-DOTA-(linker)-IPLPPPRRPFFK peptides, were designed, synthesized, and evaluated. The stability and cell uptake in PDGFRβ positive peptide cells were evaluated in vitro. The biodistribution of [⁶⁷Ga]Ga-DOTA-EG₂-IPLPPPRRPFFK ([⁶⁷Ga]27) and [⁶⁷Ga]Ga-DOTA-EG₄-IPLPPPRRPFFK ([⁶⁷Ga]28), which were selected based on in vitro stability in murine plasma and cell uptake rates, were determined in BxPC3-luc-bearing nu/nu mice. Seven ⁶⁷Ga-labeled peptides were successfully synthesized with high radiochemical yields (>85%) and purities (>99%). All evaluated radiotracers were stable in PBS (pH 7.4) at 37 °C. However, only [⁶⁷Ga]27 and [⁶⁷Ga]28 remained more than 75% after incubation in murine plasma at 37 °C for 1 h. [⁶⁷Ga]27 exhibited the highest BxPC3-luc cell uptake among the prepared radiolabeled peptides. As regards the results of the biodistribution experiments, the tumor-to-blood ratios of [⁶⁷Ga]27 and [⁶⁷Ga]28 at 1 h post-injection were 2.61 ± 0.75 and 2.05 ± 0.77, respectively. Co-injection of [⁶⁷Ga]27 and an excess amount of IPLPPPRRPFFK peptide as a blocking agent can significantly decrease this ratio. However, tumor accumulation was not considered sufficient. Therefore, further probe modification is required to assess tumor accumulation for in vivo imaging.

Optimizing the Profile of [99mTc]Tc-NT(7-13) Tracers in Pancreatic Cancer Models by Means of Protease Inhibitors

Background: The overexpression of neurotensin subtype 1 receptors (NTS1Rs) in human tumors may be elegantly exploited for directing neurotensin (NT)-based radionuclide carriers specifically to cancer sites for theranostic purposes. We have recently shown that [^99mTc]Tc–DT1 ([^99mTc]Tc–[N₄–Gly⁷]NT(7–13)) and [^99mTc]Tc–DT5 ([^99mTc]Tc–[N₄–βAla⁷,Dab⁹]NT(7–13)) show notably improved uptake in human colon adenocarcinoma WiDr xenografts in mice treated with neprilysin (NEP) inhibitors and/or angiotensin-converting enzyme (ACE) inhibitors compared with untreated controls. Aiming toward translation of this promising approach in NTS1R-positive pancreatic ductal adenocarcinoma (PDAC) patients, we now report on the impact of registered NEP/ACE inhibitors on the performance of [^99mTc]Tc–DT1 and [^99mTc]Tc–DT5 in pancreatic cancer models. Methods: The cellular uptake of [^99mTc]Tc–DT1 and [^99mTc]Tc–DT5 was tested in a panel of pancreatic cell lines, and their stability was assessed in mice treated or not treated with Entresto, lisinopril, or their combinations. Biodistribution was conducted in severe combined immunodeficiency (SCID) mice bearing pancreatic AsPC-1 xenografts. Results: The Entresto + lisinopril combination maximized the metabolic stability of the fast-internalizing [^99mTc]Tc–DT1 in mice, resulting in notably enhanced tumor uptake (7.05 ± 0.80% injected activity (IA)/g vs. 1.25 ± 0.80% IA/g in non-treated controls at 4 h post-injection; p < 0.0001). Conclusions: This study has shown the feasibility of optimizing the uptake of [^99mTc]Tc–DT1 in pancreatic cancer models with the aid of clinically established NEP/ACE inhibitors, in favor of clinical translation prospects.

Preclinical PET Imaging of NTSR-1-Positive Tumors with 64Cu- and 68Ga-DOTA-Neurotensin Analogs and Therapy with an 225Ac-DOTA-Neurotensin Analog

Background: The aim of the study was to perform PET imaging and radiotherapy with a novel neurotensin derivative for neurotensin receptor 1 (NTSR-1)-positive tumors in an animal model. Materials and Methods: A di-DOTA analog of NT(6-13) with three unnatural amino acids was synthesized and radiolabeled with either ⁶⁴Cu or ⁶⁸Ga and tested for serum stability and tumor imaging in mice bearing NTSR-1-positive PC3, and HT29 xenografts. A dose-response therapy study was performed with 18.5, 37, and 74 kBq of ²²⁵Ac-di-DOTA-α,ɛ-Lys-NT(6-13). Results: ⁶⁸Ga-di-DOTA-α,ɛ-Lys-NT(6-13) was >99% stable in serum for 48 h, had an IC₅₀ of 5 nM using ¹²⁵I labeled NT(8-13) for binding to HT-29 cells, and high uptake in tumor models expressing NTSR-1. ⁶⁸Ga-di-DOTA-α,ɛ-Lys-NT(6-13) had an average %ID/g (n = 4) at 2 h of 4.0 for tumor, 0.5 for blood, 12.0 for kidney, and <1 for other tissues, resulting in a favorable T/B of 8. Mean survivals of tumor-bearing mice treated with 18.5 or 37 kBq of ²²⁵Ac-di-DOTA-α,ɛ-Lys-NT(6-13) were 81 and 93 d, respectively, versus 53 d for controls. Whole-body toxicity was seen for the 74 kBq dose. Conclusions: Based on the results of the animal model, di-DOTA-α,ɛ-Lys-NT(6-13) is a useful imaging agent for NTSR-1-positive tumors when radiolabeled with ⁶⁸Ga, and when radiolabeled with ²²⁵Ac, a potent therapeutic agent.

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.

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.

Modifications at Arg and Ile Give Neurotensin(8-13) Derivatives with High Stability and Retained NTS1 Receptor Affinity

Due to its expression in various malignant tumors, the neurotensin receptor 1 (NTS₁R) has been suggested and explored as a target for tumor diagnosis and therapy. Animal model-based investigations of various radiolabeled NTS₁R ligands derived from the hexapeptide neurotensin(8-13) (NT(8-13)), e.g. ⁶⁸Ga- and ¹⁸F-labeled compounds for PET diagnostics, give rise to optimize such radiotracers for clinical use. As NT(8-13) is rapidly degraded in vivo; structural modifications are required in terms of increased metabolic stability. In this study, the stabilization of the peptide backbone of NT(8-13) against enzymatic degradation was systematically explored by performing an N-methyl scan, replacing Ile¹² by tert-butylglycine¹² (Tle¹²) and N-terminal acylation. N-Methylation of either arginine, Arg⁸, or Arg⁹, combined with the Ile¹²/Tle¹² exchange, proved to be most favorable with respect to NTS₁R affinity (K _i < 2 nM) and stability in human plasma (t _1/2 > 48 h), a valuable result regarding the development of radiopharmaceuticals derived from NT(8-13).

Enhanced tumor retention of NTSR1-targeted agents by employing a hydrophilic cysteine cathepsin inhibitor

We explored the approach of using an analog of E-64, a well-known and hydrophilic cysteine cathepsin (CC) inhibitor, as a potent cysteine cathepsin-trapping agent (CCTA) to improve the tumor retention of low-molecular-weight, receptor-targeted radiopharmaceuticals. The synthesized hydrophilic CCTA-incorporated, NTSR1-targeted agents demonstrated a substantial increase in cellular retention upon uptake into the NTRS1-positive HT-29 human colon cancer cell line. Similarly, biodistribution studies using HT-29 xenograft mice revealed a significant and substantial increase in tumor retention for the CCTA-incorporated, NTSR1-targeted agent. The intracellular trapping mechanism of the CCTA-incorporated agents by macromolecular adduct formation was confirmed using multiple in vitro and in vivo techniques. Furthermore, utilization of the more hydrophilic CCTA greatly increased the hydrophilicity of the resulting NTSR1-targeted constructs leading to substantial decreases in most non-target tissues in contrast to our previously reported dipeptidyl acyloxymethyl ketone (AOMK) constructs. This work further confirms that the CCTA trapping approach can make significant improvements in the clinical potential of NTSR1-and other receptor-targeted radiopharmaceuticals.

Increasing time on target: utilization of inhibitors of cysteine cathepsins to enhance the tumor retention of receptor-targeted agents

We report a strategy of utilizing irreversible cysteine cathepsin inhibitor as trapping agent to increase the tumor residence time of receptor-targeted agents. The targeted constructs incorporating these cysteine cathepsin trapping agents were able to form high molecular weight adducts with intracellular cysteine cathepsins, thus achieving superior retention in tumor tissues.

New extracellular factors in glioblastoma multiforme development: neurotensin, growth differentiation factor-15, sphingosine-1-phosphate and cytomegalovirus infection

Recent years have seen considerable progress in understanding the biochemistry of cancer. For example, more significance is now assigned to the tumor microenvironment, especially with regard to intercellular signaling in the tumor niche which depends on many factors secreted by tumor cells. In addition, great progress has been made in understanding the influence of factors such as neurotensin, growth differentiation factor-15 (GDF-15), sphingosine-1-phosphate (S1P), and infection with cytomegalovirus (CMV) on the 'hallmarks of cancer' in glioblastoma multiforme. Therefore, in the present work we describe the influence of these factors on the proliferation and apoptosis of neoplastic cells, cancer stem cells, angiogenesis, migration and invasion, and cancer immune evasion in a glioblastoma multiforme tumor. In particular, we discuss the effect of neurotensin, GDF-15, S1P (including the drug FTY720), and infection with CMV on tumor-associated macrophages (TAM), microglial cells, neutrophil and regulatory T cells (T_reg), on the tumor microenvironment. In order to better understand the role of the aforementioned factors in tumoral processes, we outline the latest models of intratumoral heterogeneity in glioblastoma multiforme. Based on the most recent reports, we discuss the problems of multi-drug therapy in treating glioblastoma multiforme.

New Insights in the Design of Bioactive Peptides and Chelating Agents for Imaging and Therapy in Oncology

Many synthetic peptides have been developed for diagnosis and therapy of human cancers based on their ability to target specific receptors on cancer cell surface or to penetrate the cell membrane. Chemical modifications of amino acid chains have significantly improved the biological activity, the stability and efficacy of peptide analogues currently employed as anticancer drugs or as molecular imaging tracers. The stability of somatostatin, integrins and bombesin analogues in the human body have been significantly increased by cyclization and/or insertion of non-natural amino acids in the peptide sequences. Moreover, the overall pharmacokinetic properties of such analogues and others (including cholecystokinin, vasoactive intestinal peptide and neurotensin analogues) have been improved by PEGylation and glycosylation. Furthermore, conjugation of those peptide analogues to new linkers and bifunctional chelators (such as AAZTA, TETA, TRAP, NOPO etc.), produced radiolabeled moieties with increased half life and higher binding affinity to the cognate receptors. This review describes the most important and recent chemical modifications introduced in the amino acid sequences as well as linkers and new bifunctional chelators which have significantly improved the specificity and sensitivity of peptides used in oncologic diagnosis and therapy.

Investigation of the Biological Impact of Charge Distribution on a NTR1-Targeted Peptide

The neurotensin receptor 1 (NTR1) has been shown to be a promising target, due to its increased level of expression relative to normal tissue, for pancreatic and colon cancers. This has prompted the development of a variety of NTR1-targeted radiopharmaceuticals, based on the neurotensin (NT) peptide, for diagnostic and radiotherapeutic applications. A major obstacle for the clinical translation of NTR1-targeted radiotherapeutics would likely be nephrotoxicity due to the high levels of kidney retention. It is well-known that for many peptide-based agents, renal uptake is influenced by the overall molecular charge. Herein, we investigated the effect of charge distribution on receptor binding and kidney retention. Using the [(N-α-Me)Arg⁸,Dmt¹¹,Tle¹²]NT(6-13) targeting vector, three peptides (¹⁷⁷Lu-K2, ¹⁷⁷Lu-K4, and ¹⁷⁷Lu-K6), with the Lys moved closer (K6) or further away (K2) from the pharmacophore, were synthesized. In vitro competitive binding, internalization and efflux, and confocal microscopy studies were conducted using the NTR1-positive HT-29, human colon cancer cell line. The ^177/natLu-K6 demonstrated the highest binding affinity (21.8 ± 1.2 nM) and the highest level of internalization (4.06% ± 0.20% of the total added amount). In vivo biodistribution, autoradiography, and metabolic studies of ¹⁷⁷Lu-radiolabeled K2, K4, and K6 were examined using CF-1 mice. ¹⁷⁷Lu-K4 and ¹⁷⁷Lu-K6 gave the highest levels of in vivo uptake in NTR1-positive tissues, whereas ¹⁷⁷Lu-K2 yielded nearly 2-fold higher renal uptake relative to the other radioconjugates. In conclusion, the position of the Lys (positively charged amino acid) influences the receptor binding, internalization, in vivo NTR1-targeting efficacy, and kidney retention profile of the radioconjugates. In addition, we have found that hydrophobicity likely play a role in the unique biodistribution profiles of these agents.

Analysis of argentinated peptide complexes using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: Peptide = oxytocin, arg(8) -vasopressin, bradykinin, bombesin, somatostatin, neurotensin

RATIONALE

The increased use of silver nanoparticles (AgNPs) for various biological applications, and over-expression of various peptide receptors in different tumors/cancer cells, necessitate the need for dedicated investigations on the intrinsic binding ability of Ag with various biologically important peptides for better understanding of AgNPs-peptide interactions and for the future development of contrasting agents as well as drugs for imaging/biomedical applications.

METHODS

The [M+(Ag)n ](+) complexes are prepared and characterized using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS).

RESULTS

Silver complexes of the peptides [M+(Ag)n ](+) , where M = oxytocin, arg(8) -vasopressin, bradykinin, bombesin, somatostatin, and neurotensin, have been investigated for their intrinsic Ag(+) -binding ability. Unusual binding of up to seven Ag(+) with these small peptides is observed. The mass spectra show n = 1-5 for bombesin and somatostatin, n = 1-6 for bradykinin and arg(8) -vasopressin, and n = 1-7 for oxytocin and neurotensin. In addition, oxytocin and arg(8) -vasopressin show the formation of dimers and their complexes [M2 +(Ag)n ](+) with n = 1-8 and n = 1-5, respectively. The possible amino acid residues responsible for Ag(+) binding in each peptide have been identified on the basis of density functional theory (DFT)-calculated binding energy values of Ag(+) towards individual amino acids.

CONCLUSIONS

Mass spectrometric evidence indicates that the peptides, viz., oxytocin, arg(8) -vasopressin, bradykinin, bombesin, somatostatin, and neurotensin, show greater affinity for Ag(+) . Hence, they may be used as carriers for AgNPs in targeted drug delivery as well as an alternative for iodinated contrasting agents in dual energy X-ray imaging techniques. Radio-labeled Ag with these peptides can also be used in radio-pharmaceuticals for diagnostic and therapeutic applications. Copyright © 2016 John Wiley & Sons, Ltd.