Evaluation of three different families of bombesin receptor radioantagonists for targeted imaging and therapy of gastrin releasing peptide receptor (GRP-R) positive tumors

Synthesis, Radiolabeling, and Preclinical Evaluation of 68Ga/177Lu-Labeled Leuprolide Peptide Analog for the Detection of Breast Cancer

Objectives: The expansion of novel and potent tumor receptor binding peptides is a promising approach for the precise targeting of various cancer. Leuprolide is a 9-residue peptide analog of gonadotropin-releasing hormone and is extensively used in the treatment of sex hormone-dependent tumors, including prostate, breast, and ovarian cancer. This preclinical study was undertaken to prepare a new radiolabeled leuprolide peptide for the detection of breast carcinoma. Methods: A 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA)-coupled 9-amino acid leuprolide peptide was synthesized after typical 9-fluorenylmethyl-oxycarbonyl-based solid-phase peptide synthesis and radiolabeled with both ⁶⁸Ga and ¹⁷⁷Lu radionuclides for theranostic use. The systemic pharmacokinetics was done in healthy balb/c mice. The in vitro tumor cell binding affinity was determined on MCF7, T47D, and MDA-MB-231 breast cancer cell lines. In vivo tumor targeting and micro positron-emission tomography imaging was performed on nude mice with MCF7 breast tumor xenografts. Results: The leuprolide peptide was conveniently synthesized by solid-phase synthesis strategy and its identity and purity were validated by mass spectrometry and high-performance liquid chromatography. The peptide radiolabeled efficiently (˃94%) with both diagnostic (⁶⁸Ga) and therapeutic (¹⁷⁷Lu) radionuclides and displayed nanomolar binding potency to all three tested MCF7, T47D, and MDA-MB-231 cell lines. Fast and favorable pharmacokinetics was observed for ⁶⁸Ga/¹⁷⁷Lu-leuprolide in healthy Balb/c mice. In nude mice, ⁶⁸Ga-leuprolide peptide exhibited rapid clearance from the blood circulation with low to moderate (up to 5% ID/g) uptake/retention by the major body organs. The accumulation in the estrogen receptor-positive MCF7 tumor was 2.24% ± 0.62% ID/g at 45 min p.i, with good tumor to blood and muscle uptake ratios. The radiolabeled peptide was excreted primarily through the renal pathway. Conclusion: The encouraging results of this initial study demonstrate that additional testing of this leuprolide peptide seems to be indicated because of its convincing potential to be a new agent for the management of breast carcinoma.

Internal Radiation Therapy

Targeted therapies are applied to increase the efficiency of antitumor treatment by simultaneously decreasing side effects. This can be achieved using carrier molecules which specifically bind to target structures or areas with remodeling activity. These carrier molecules may be coupled to chemotherapeutic drugs or to radioactive isotopes. In most cases, these carrier molecules are antibodies against tumor antigens, peptides, or small molecules which are binders for overexpressed receptors on tumor cells. The paradigm of endoradiotherapy is exemplified by the peptidic tracer DOTATOC which binds to somatostatin receptors and recently also small molecule inhibitors with high affinity for the prostate-specific membrane antigen.

Optimization of the radiolabelling method for improved in vitro and in vivo stability of 90Y-albumin microspheres

Biologically stable ⁹⁰Y-labelled albumin microspheres (AMS) were developed by optimizing the process of their preparation. Three formulations of ⁹⁰Y-AMS were initially prepared with high radiolabelling yield but depending on the step when the radionuclide ⁹⁰Y and DTPA chelator were added, radiolabelled microspheres with different in vitro and in vivo stability were obtained. DTPA was proved as a useful chelating agent that tightly links radionuclide ⁹⁰Y to albumin. Also, AMS radiolabelled via DTPA during preparation and before microspheres stabilization, showed significant in vitro and in vivo stability ready for the potential use in selective internal radiation therapy.

Comparison of the radiolabeled PSMA-inhibitor 111In-PSMA-617 and the radiolabeled GRP-R antagonist 111In-RM2 in primary prostate cancer samples

Purpose

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRP-R) are expressed in prostate cancer and can be targeted with radiolabeled inhibitors and antagonists. Their performances for the initial characterization of prostatic tumors have been barely evaluated but never compared. We aimed to gather comparative preclinical data of the role of PSMA and GRP-R targeting in prostate cancer.

Procedures

We retrospectively studied 20 frozen prostatectomy samples with various metastatic risks of the D’Amico classification. Tissue samples were investigated by tissular microimaging using the radiolabeled PSMA inhibitor ¹¹¹In-PSMA-617 and the radiolabeled GRP-R antagonist ¹¹¹In-RM2. Bindings of the two radiopharmaceuticals were compared to histology and clinico-biological data (Gleason score, PSA values, metastatic risks).

Results

Binding of ¹¹¹In-PSMA-617 was high whatever the metastatic risk (p = 0.665), Gleason score (p = 0.555), or PSA value (p = 0.404) while ¹¹¹In-RM2 exhibited a significantly higher binding in the low metastatic risk group (p = 0.046), in the low PSA value group (p = 0.001), and in samples with Gleason 6 score (p = 0.006).

Conclusion

PSMA and GRP-R based imaging might have complementary performances for the initial characterization of prostatic tumors. Prospective clinical studies comparing the two tracers in this setting are needed.

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.

Bombesin receptor-targeted liposomes for enhanced delivery to lung cancer cells

Background: Gastrin-releasing peptide is a member of the bombesin family of peptides. Its cognate receptor, gastrin releasing peptide receptor (GRPR), is widely expressed in cancers of the lung, pancreas and ovaries. Gastrin releasing peptide (GRP) is an autocrine growth factor in small cell lung cancer, which has very poor patient outcomes. High affinity antagonist peptides have been developed for in vivo cancer imaging. In this report we decorated pegylated liposomes with a GRPR antagonist peptide and studied its interaction with, and accumulation within, lung cancer cells. Results: An N-terminally cysteine modified GRPR antagonist (termed cystabn) was synthesised and shown to inhibit cell growth in vitro. Cystabn was used to prepare a targeted 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino(polyethylene glycol)-2000] (DSPE-PEG2000) lipid conjugate that was formulated into liposomes. The liposomes displayed desirable colloidal properties and good stability under storage conditions. Flow cytometric and microscopic studies showed that fluorescently labelled cystabn-decorated liposomes accumulated more extensively in GRPR over-expressing cells than matched liposomes that contained no cystabn targeting motif. Conclusion: The use of GRPR antagonistic peptides for nanoparticle targeting has potential for enhancing drug accumulation in resistant cancer cells.

New Gastrin Releasing Peptide Receptor-Directed [99mTc]Demobesin 1 Mimics: Synthesis and Comparative Evaluation

We have previously reported on the gastrin releasing peptide receptor (GRPR) antagonist [^99mTc]1, ([^99mTc]demobesin 1, ^99mTc-[N₄'-diglycolate-dPhe⁶,Leu-NHEt¹³]BBN(6-13)). [^99mTc]1 has shown superior biological profile compared to analogous agonist-based ^99mTc-radioligands. We herein present a small library of [^99mTc]1 mimics generated after structural modifications in (a) the linker ([^99mTc]2, [^99mTc]3, [^99mTc]4), (b) the peptide chain ([^99mTc]5, [^99mTc]6), and (c) the C-terminus ([^99mTc]7 or [^99mTc]8). The effects of above modifications on the biological properties of analogs were studied in PC-3 cells and tumor-bearing SCID mice. All analogs showed subnanomolar affinity for the human GRPR, while most receptor-affine 4 and 8 behaved as potent GRPR antagonists in a functional internalization assay. In mice bearing PC-3 tumors, [^99mTc]1-[^99mTc]6 exhibited GRPR-specific tumor uptake, rapidly clearing from normal tissues. [^99mTc]4 displayed the highest tumor uptake (28.8 ± 4.1%ID/g at 1 h pi), which remained high even after 24 h pi (16.3 ± 1.8%ID/g), well surpassing that of [^99mTc]1 (5.4 ± 0.7%ID/g at 24 h pi).

Comparison of fusion protein and DC vaccine in inhibition of mouse B16F10 melanoma tumor

Dendritic cell (DC) vaccine and fusion protein vaccine have been put into clinical use in cancer immunotherapy. This study compared DC vaccine and fusion protein vaccine directly in their capability of inducing specific immune response. We used mouse Granulocyte Macrophage-Colony Stimulating Factor (mGM-CSF) fused with gastrin-releasing peptide (GRP) and Gonadotrophin Releasing Hormone (GnRH) respectively to obtain mGM-CSF/GRP6 (mG6) and mGM-CSF/mGGn (mGGn) fusion proteins. We prepared fusion protein vaccine and DC vaccine including mG6 protein vaccine (6P), mGGn protein vaccine (nP), mG6 DC vaccine (6D) and mGGn DC vaccine (nD), then the two proteins were mixed to prepare combination proteins vaccine (6nP) and DC vaccine (6nD). After that, C57BL/6 mice were injected with B16F10 cell line to build melanoma tumor model, and were immunized with vaccines to produce antibodies to inhibit and destruct melanoma tumor cells. The discoveries showed that anti- mGM-CSF-GRP6 and anti- mGM-CSF-mGGn antibody vaccines were successfully created as expected; this was deduced from significant inhibition of melanoma tumor in vivo and significant reduction of tumor weight and volume. The effects of DC groups were better than that of the protein groups and the combination of vaccines were more effective than vaccine given separately. Our results indicate that using combination vaccine provides a new strategy to inhibit melanoma tumor growth but a complete cure of melanoma needs further investigations.

Dual-Modality Imaging of Prostate Cancer with a Fluorescent and Radiogallium-Labeled Gastrin-Releasing Peptide Receptor Antagonist

Gastrin-releasing peptide (GRP) receptors (GRPr) are frequently overexpressed in human prostate cancer, and radiolabeled GRPr affinity ligands have shown promise for in vivo imaging of prostate cancer with PET. The goal of this study was to develop a dual-modality imaging probe that can be used for noninvasive PET imaging and optical imaging of prostate cancer.

METHODS

We designed and synthesized an IRDye 650 and DOTA-conjugated GRPr antagonist, HZ220 (DOTA-Lys(IRDye 650)-PEG₄-[D-Phe⁶, Sta¹³]-BN(6-14)NH₂), by reacting DOTA-Lys-PEG₄-[D-Phe⁶, Sta¹³]-BN(6-14)NH₂ (HZ219) with IRDye 650 N-hydroxysuccinimide (NHS) ester. Receptor-specific binding of gallium-labeled HZ220 was characterized in PC-3 prostate cancer cells (PC-3), and tumor uptake in mice was imaged with PET/CT and fluorescence imaging. Receptor binding affinity, in vivo tumor uptake, and biodistribution were compared with the GRPr antagonists HZ219, DOTA-PEG₄-[D-Phe⁶, Sta¹³]-BN(6-14)NH₂ (DOTA-AR), and DOTA-(4-amino-1-carboxymethyl-piperidine)-[D-Phe⁶, Sta¹³]-BN(6-14)NH₂ (DOTA-RM2).

RESULTS

After hydrophilic-lipophilic balance cartridge purification, ⁶⁸Ga-HZ220 was obtained with a radiochemical yield of 56% ± 8% (non-decay-corrected), and the radiochemical purity was greater than 95%. Ga-HZ220 had a lower affinity for GRPr (inhibitory concentration of 50% [IC₅₀], 21.4 ± 7.4 nM) than Ga-DOTA-AR (IC₅₀, 0.48 ± 0.18 nM) or Ga-HZ219 (IC₅₀, 0.69 ± 0.18 nM). Nevertheless, ⁶⁸Ga-HZ220 had an in vivo tumor accumulation similar to ⁶⁸Ga-DOTA-AR (4.63 ± 0.31 vs. 4.07 ± 0.29 percentage injected activity per mL [%IA/mL] at 1 h after injection) but lower than that of ⁶⁸Ga-DOTA-RM2 (10.4 ± 0.4 %IA/mL). The tumor uptake of ⁶⁸Ga-HZ220 was blocked significantly with an excessive amount of GRP antagonists. IVIS spectrum imaging also visualized PC-3 xenografts in vivo and ex vivo with a high-contrast ratio. Autoradiography and fluorescent-based microscopic imaging with ⁶⁸Ga-HZ220 consistently colocated the expression of GRPr. ⁶⁸Ga-HZ220 displayed a higher kidney uptake than both ⁶⁸Ga-DOTA-AR and ⁶⁸Ga-DOTA-RM2 (16.9 ± 6.5 vs. 4.48 ± 1.63 vs. 5.01 ± 2.29 %IA/mL).

CONCLUSION

⁶⁸Ga-HZ220 is a promising bimodal ligand for noninvasive PET imaging and intraoperative optical imaging of GRPr-expressing malignancies. Bimodal nuclear/fluorescence imaging may not only improve cancer detection and guide surgical resections, but also improve our understanding of the uptake of GRPr ligands on the cellular level.

Preclinical Study on GRPR-Targeted (68)Ga-Probes for PET Imaging of Prostate Cancer

Gastrin-releasing peptide receptor (GRPR) targeted positron emission tomography (PET) is a highly promising approach for imaging of prostate cancer (PCa) in small animal models and patients. Developing a GRPR-targeted PET probe with excellent in vivo performance such as high tumor uptake, high contrast, and optimal pharmacokinetics is still very challenging. Herein, a novel bombesin (BBN) analogue (named SCH1) based on JMV594 peptide modified with an 8-amino octanoic acid spacer (AOC) was thus designed and conjugated with the metal chelator 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA). The resulting NODAGA-SCH1 was then radiolabeled with (68)Ga and evaluated for PET imaging of PCa. Compared with (68)Ga-NODAGA-JMV594 probe, (68)Ga-NODAGA-SCH1 exhibited excellent PET/CT imaging properties on PC-3 tumor-bearing nude mice, such as high tumor uptake (5.80 ± 0.42 vs 3.78 ± 0.28%ID/g, 2 h) and high tumor/muscle contrast (16.6 ± 1.50 vs 8.42 ± 0.61%ID/g, 2 h). Importantly, biodistribution data indicated a relatively similar accumulation of (68)Ga-NODAGA-SCH1 was observed in the liver (4.21 ± 0.42%ID/g) and kidney (3.41 ± 0.46%ID/g) suggesting that the clearance is through both the kidney and the liver. Overall, (68)Ga-NODAGA-SCH1 showed promising in vivo properties and is a promising candidate for translation into clinical PET-imaging of PCa patients.

Gastrin-releasing Peptide Receptor Imaging in Breast Cancer Using the Receptor Antagonist (68)Ga-RM2 And PET

Introduction: The gastrin-releasing peptide receptor (GRPR) is overexpressed in breast cancer. The present study evaluates GRPR imaging as a novel imaging modality in breast cancer by employing positron emission tomography (PET) and the GRPR antagonist ⁶⁸Ga-RM2.

Methods: Fifteen female patients with biopsy confirmed primary breast carcinoma (3 bilateral tumors; median clinical stage IIB) underwent ⁶⁸Ga-RM2-PET/CT for pretreatment staging. In vivo tumor uptake of ⁶⁸Ga-RM2 was correlated with estrogen (ER) and progesterone (PR) receptor expression, HER2/neu status and MIB-1 proliferation index in breast core biopsy specimens.

Results: 13/18 tumors demonstrated strongly increased ⁶⁸Ga-RM2 uptake compared to normal breast tissue (defined as PET-positive). All PET-positive primary tumors were ER- and PR-positive (13/13) in contrast to only 1/5 PET-negative tumors. Mean SUV_MAX of ER-positive tumors was 10.6±6.0 compared to 2.3±1.0 in ER-negative tumors (p=0.016). In a multivariate analysis including ER, PR, HER2/neu and MIB-1, only ER expression predicted ⁶⁸Ga-RM2 uptake (model: r²=0.55, p=0.025). Normal breast tissue showed inter- and intraindividually variable, moderate GRPR binding (SUV_MAX 2.3±1.0), while physiological uptake of other organs was considerably less except pancreas. Of note, ⁶⁸Ga-RM2-PET/CT detected internal mammary lymph nodes with high ⁶⁸Ga-RM2 uptake (n=8), a contralateral axillary lymph node metastasis (verified by biopsy) and bone metastases (n=1; not detected by bone scan and CT).

Conclusion: Our study demonstrates that ⁶⁸Ga-RM2-PET/CT is a promising imaging method in ER-positive breast cancer. In vivo GRPR binding assessed by ⁶⁸Ga-RM2-PET/CT correlated with ER expression in primary tumors of untreated patients.

Bombesin related peptides/receptors and their promising therapeutic roles in cancer imaging, targeting and treatment

INTRODUCTION

Despite remarkable advances in tumor treatment, many patients still die from common tumors (breast, prostate, lung, CNS, colon, and pancreas), and thus, new approaches are needed. Many of these tumors synthesize bombesin (Bn)-related peptides and over-express their receptors (BnRs), hence functioning as autocrine-growth-factors. Recent studies support the conclusion that Bn-peptides/BnRs are well-positioned for numerous novel antitumor treatments, including interrupting autocrine-growth and the use of over-expressed receptors for imaging and targeting cytotoxic-compounds, either by direct-coupling or combined with nanoparticle-technology.

AREAS COVERED

The unique ability of common neoplasms to synthesize, secrete, and show a growth/proliferative/differentiating response due to BnR over-expression, is reviewed, both in general and with regard to the most frequently investigated neoplasms (breast, prostate, lung, and CNS). Particular attention is paid to advances in the recent years. Also considered are the possible therapeutic approaches to the growth/differentiation effect of Bn-peptides, as well as the therapeutic implication of the frequent BnR over-expression for tumor-imaging and/or targeted-delivery.

EXPERT OPINION

Given that Bn-related-peptides/BnRs are so frequently ectopically-expressed by common tumors, which are often malignant and become refractory to conventional treatments, therapeutic interventions using novel approaches to Bn-peptides and receptors are being explored. Of particular interest is the potential of reproducing with BnRs in common tumors the recent success of utilizing overexpression of somatostatin-receptors by neuroendocrine-tumors to provide the most sensitive imaging methods and targeted delivery of cytotoxic-compounds.

Insights into bombesin receptors and ligands: Highlighting recent advances

This following article is written for Prof. Abba Kastin's Festschrift, to add to the tribute to his important role in the advancement of the role of peptides in physiological, as well as pathophysiological processes. There have been many advances during the 35 years of his prominent role in the Peptide field, not only as editor of the journal Peptides, but also as a scientific investigator and editor of two volumes of the Handbook of Biological Active Peptides [146,147]. Similar to the advances with many different peptides, during this 35 year period, there have been much progress made in the understanding of the pharmacology, cell biology and the role of (bombesin) Bn receptors and their ligands in various disease states, since the original isolation of bombesin from skin of the European frog Bombina bombina in 1970 [76]. This paper will briefly review some of these advances over the time period of Prof. Kastin 35 years in the peptide field concentrating on the advances since 2007 when many of the results from earlier studies were summarized [128,129]. It is appropriate to do this because there have been 280 articles published in Peptides during this time on bombesin-related peptides and it accounts for almost 5% of all publications. Furthermore, 22 Bn publications we have been involved in have been published in either Peptides [14,39,55,58,81,92,93,119,152,216,225,226,231,280,302,309,355,361,362] or in Prof. Kastin's Handbook of Biological Active Peptides [137,138,331].

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.

Gastrin-releasing peptide expression and its effect on the calcification of developing mouse incisor

Gastrin-releasing peptide (GRP) is considered to be one of the cancer growth factors. This peptide's receptor (GRPR) is known as a G protein-coupled receptor, regulating intracellular calcium storage and releasing signals. This study is the first to investigate the function of GRP during mouse incisor development. We hypothesized that GRP is one of the factors that affects the regulation of calcification during tooth development. To verify the expression pattern of GRP, in situ hybridization was processed during incisor development. GRP was expressed at the late bell stage and hard tissue formation stage in the epithelial tissue. To identify the genuine function of GRP during incisor development, a gain-of-function analysis was performed. After GRP overexpression in culture, the phenotype of ameloblasts, odontoblasts and predentin was altered compared to control group. Moreover, enamel and dentin thickness was increased after renal capsule transplantation of GRP-overexpressed incisors. With these results, we suggest that GRP plays a significant role in the formation of enamel and dentin by regulating ameloblasts and predentin formation, respectively. Thus, GRP signaling is strongly related to calcium acquisition and secretion during mouse incisor development.