Detection of Colon Cancer with99mTc-Labeled Bombesin Derivative (99mTc-leu13-BN1)

Investigation of Tumor Cells and Receptor-Ligand Simulation Models for the Development of PET Imaging Probes Targeting PSMA and GRPR and a Possible Crosstalk between the Two Receptors

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) have both been used in nuclear medicine as targets for molecular imaging and therapy of prostate (PCa) and breast cancer (BCa). Three bioconjugate probes, the PSMA specific: [⁶⁸Ga]Ga-1, ((HBED-CC)-Ahx-Lys-NH-CO-NH Glu or PSMA-11), the GRPR specific: [⁶⁸Ga]Ga-2, ((HBED-CC)-4-amino-1-carboxymethyl piperidine-[D-Phe⁶, Sta¹³]BN(6-14), a bombesin (BN) analogue), and 3 (the BN analogue: 4-amino-1-carboxymethyl piperidine-[(R)-Phe⁶, Sta¹³]BN(6-14) connected with the fluorescent dye, BDP-FL), were synthesized and tested in vitro with PCa and BCa cell lines, more specifically, with PCa cells, PC-3 and LNCaP, with BCa cells, T47D, MDA-MB-231, and with the in-house created PSMA-overexpressing PC-3^(PSMA), T47D^(PSMA), and MDA-MB-231^(PSMA). In addition, biomolecular simulations were conducted on the association of 1 and 2 with PSMA and GRPR. The PSMA overexpression resulted in an increase of cell-bound radioligand [⁶⁸Ga]Ga-1 (PSMA) for PCa and BCa cells and also of [⁶⁸Ga]Ga-2 (GRPR), especially in those cell lines already expressing GRPR. The results were confirmed by fluorescence-activated cell sorting with a PE-labeled PSMA-specific antibody and the fluorescence tracer 3. The docking calculations and molecular dynamics simulations showed how 1 enters the PSMA funnel region and how pharmacophore Glu-urea-Lys interacts with the arginine patch, the S1', and S1 subpockets by forming hydrogen and van der Waals bonds. The chelating moiety of 1, that is, HBED-CC, forms additional stabilizing hydrogen bonding and van der Waals interactions in the arene-binding site. Ligand 2 is diving into the GRPR transmembrane (TM) helical cavity, thereby forming hydrogen bonds through its amidated end, water-mediated hydrogen bonds, and π-π interactions. Our results provide valuable information regarding the molecular mechanisms involved in the interactions of 1 and 2 with PSMA and GRPR, which might be useful for the diagnostic imaging and therapy of PCa and BCa.

Peptide-Drug Conjugates and Their Targets in Advanced Cancer Therapies

Cancer became recently the leading cause of death in industrialized countries. Even though standard treatments achieve significant effects in growth inhibition and tumor elimination, they cause severe side effects as most of the applied drugs exhibit only minor selectivity for the malignant tissue. Hence, specific addressing of tumor cells without affecting healthy tissue is currently a major desire in cancer therapy. Cell surface receptors, which bind peptides are frequently overexpressed on cancer cells and can therefore be considered as promising targets for selective tumor therapy. In this review, the benefits of peptides as tumor homing agents are presented and an overview of the most commonly addressed peptide receptors is given. A special focus was set on the bombesin receptor family and the neuropeptide Y receptor family. In the second part, the specific requirements of peptide-drug conjugates (PDC) and intelligent linker structures as an essential component of PDC are outlined. Furthermore, different drug cargos are presented including classical and recent toxic agents as well as radionuclides for diagnostic and therapeutic approaches. In the last part, boron neutron capture therapy as advanced targeted cancer therapy is introduced and past and recent developments are reviewed.

Tumor targeting with 99m Tc radiolabeled peptides: Clinical application and recent development

Targeting overexpressed receptors on the cancer cells with radiolabeled peptides has become very important in nuclear oncology in the recent years. Peptides are small and have easy preparation and easy radiolabeling protocol with no side-effect and toxicity. These properties made them a valuable tool for tumor targeting. Based on the successful imaging of neuroendocrine tumors with ¹¹¹ In-octreotide, other receptor-targeting peptides such as bombesin (BBN), cholecystokinin/gastrin analogues, neurotensin analogues, glucagon-like peptide-1, and RGD peptides are currently under development or undergoing clinical trials. The most frequently used radionuclides for tumor imaging are ^99m Tc and ¹¹¹ In for single-photon emission computed tomography and ⁶⁸ Ga and ¹⁸ F for positron emission tomography imaging. This review presents some of the ^99m Tc-labeled peptides, with regard to their potential for radionuclide imaging of tumors in clinical and preclinical application.

Co-administration of succinylated gelatine with a (99m)Tc-bombesin analogue, effects on pharmacokinetics and tumor uptake

The bombesin analogue, [(99m)Tc-GGC]-(Ornithine)3-BN(2-14), (99m)Tc-BN-O, targeting gastrin releasing peptide receptors (GRPrs) on the surface of tumors, was pre-clinically investigated as potential imaging agent for single photon emission computed tomography (SPECT). In addition, the improvement of its pharmacokinetic profile (PK) was investigated through the co-administration of a succinylated gelatin plasma expander (Gelofusine), aiming to reduce its kidney accumulation and enhance its tumor-to-normal tissue contrast ratios. Biodistribution data were collected from normal mice and rats, and PC-3 tumor bearing mice, in reference to its PK, metabolism and tumor uptake. Imaging data were also collected from PC-3 tumor bearing mice. Biodistribution and imaging experiments showed that (99m)Tc-BN-O was able to efficiently localize the tumor (5.23 and 7.00% ID/g at 30 and 60min post injection, respectively), while at the same time it was rapidly cleared from the circulation through the kidneys. HPLC analysis of kidney samples, collected at 60min p.i. from normal mice and rats, showed that the majority of radioactivity detected was due to intact peptide i.e. 56% for mice and 73% for rats. Co-administration of (99m)Tc-BN-O with Gelo resulted in the reduction of kidney uptake in both animal models. The integrated area under the curve (AUC30-60 min) from the concentration-time plots of kidneys was decreased in both mice and rats by 25 and 50%, respectively. In PC-3 tumor bearing mice, an increase of tumor uptake (AUCtumor increased by 69%) was also observed with Gelo. An improvement in tumor-to-blood and tumor-to-normal tissue ratios was noted in all cases with the exception of the pancreas, which normally expresses GRPr. The results of this preclinical study may also be extended to other similar peptides, which are utilized in prostate cancer imaging and present similar PK profile.

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].

(68)Ga-labeled bombesin analogs for receptor-mediated imaging

Targeted receptor-mediated imaging techniques have become crucial tools in present targeted diagnosis and radiotherapy as they provide accurate and specific diagnosis of disease information. Peptide-based pharmaceuticals are gaining popularity, and there has been vast interest in developing (68)Ga-labeled bombesin (Bn) analogs. The gastrin-releasing peptide (GRP) family and its Bn analog have been implicated in the biology of several human cancers. The three bombesin receptors GRP, NMB, and BRS-3 receptor are most frequently ectopically expressed by common, important malignancies. The low expression of Bn/GRP receptors in normal tissue and relatively high expression in a variety of human tumors can be of biological importance and form a molecular basis for Bn/GRP receptor-mediated imaging. To develop a Bn-like peptide with favorable tumor targeting and pharmacokinetic characteristics for possible clinical use, several modifications in the Bn-like peptides, such as the use of a variety of chelating agents, i.e., acyclic and macrocyclic agents with different spacer groups and with different metal ions (gallium), have been performed in recent years without significant disturbance of the vital binding scaffold. The favorable physical properties of (68)Ga, i.e., short half-life, and the fast localization of small peptides make this an ideal combination to study receptor-mediated imaging in patients.

Radiolabelled peptides for oncological diagnosis

Radiolabelled receptor-binding peptides targeting receptors (over)expressed on tumour cells are widely under investigation for tumour diagnosis and therapy. The concept of using radiolabelled receptor-binding peptides to target receptor-expressing tissues in vivo has stimulated a large body of research in nuclear medicine. The ¹¹¹In-labelled somatostatin analogue octreotide (OctreoScan™) is the most successful radiopeptide for tumour imaging, and was the first to be approved for diagnostic use. Based on the success of these studies, other receptor-targeting peptides such as cholecystokinin/gastrin analogues, glucagon-like peptide-1, bombesin (BN), chemokine receptor CXCR4 targeting peptides, and RGD peptides are currently under development or undergoing clinical trials. In this review, we discuss some of these peptides and their analogues, with regard to their potential for radionuclide imaging of tumours.

Structural modifications of ⁹⁹mTc-labelled bombesin-like peptides for optimizing pharmacokinetics in prostate tumor targeting

PURPOSE

The main goal of the present study was to investigate the importance of the addition of a positively charged aa in the naturally occurring bombesin (BN) peptide for its utilization as radiodiagnostic agent, taking into consideration the biodistribution profile, the pharmacokinetic characteristics and the tumor targeting ability.

METHODS

Two BN-derivatives of the general structure [M-chelator]-(spacer)-BN(2-14)-NH(2), where M: (99m)Tc or (185/187)Re, chelator: Gly-Gly-Cys-, spacer: -(arginine)(3)-, M-BN-A; spacer: -(ornithine)(3)-, M-BN-O; have been prepared and evaluated as tumor imaging agents.

RESULTS

The peptides under study presented high radiolabelling efficiency (>98%), significant stability in human plasma (>60% intact radiolabelled peptide after 1h incubation) and comparable receptor binding affinity with the standard [(125)I-Tyr(4)]-BN. Their internalization rates in the prostate cancer PC-3 cells differed, although the amount of internalized peptide was the same. The biodistribution and the dynamic γ-camera imaging studies in normal and PC-3 tumor-bearing SCID mice have shown significant tumor uptake, combined with fast blood clearance, through the urinary pathway.

CONCLUSION

The addition of the charged aa spacer in the BN structure was advantageous for biodistribution, pharmacokinetics and tumor targeting ability, because it reduced the upper abdominal radioactivity levels and increased tumor/normal tissue contrast ratios.

Exploring new frontiers in molecular imaging: Emergence of 68Ga PET/CT

Since US Food and Drug Administration approval of 18-fluorodeoxyglucose as a positron tracer, and the development of hybrid positron emission tomography/computed tomography machines, there has been a great increase in clinical application and progress in the field of nuclear molecular imaging. However, not underestimating the value of ¹⁸F, there are known limitations in the use of this cyclotron-produced positron tracer. We hence turn our focus to an emerging positron tracer, ⁶⁸Ga, and examine the advantages, current clinical uses and potential future applications of this radioisotope.

In vitro binding evaluation of 177Lu-AMBA, a novel 177Lu-labeled GRP-R agonist for systemic radiotherapy in human tissues

Members of the gastrin-releasing peptide (GRP) family and its analogs bombesin (BBN) have been implicated in the biology of several human cancers including prostate, breast, colon and lung. To date, three mammalian GRP/BBN receptor subtypes have been cloned and characterized: the neuromedin B receptor (NMBR), the GRP receptor (GRPR) and the BBN-receptor subtype 3 (BB(3)). The fourth BBN receptor subtype, BB(4), has only been identified in amphibian and at present no mammalian equivalent of this receptor has been described. GRPR analogs have been used as carriers to deliver drugs, radionuclides and cytotoxins to target various cancer types that are GRPR positive. We investigated the in vitro binding properties of (177)Lu-AMBA, a novel radiolabelled BBN analog currently undergoing clinical trial as systemic radiotherapy for hormone refractory prostate cancer (HRPC) patients. Pharmacological analyses of the (177)Lu-AMBA was determined using in vitro binding studies using membrane target system containing specific receptor subtypes. We investigated the distribution of binding sites for (177)Lu-AMBA by receptor autoradiography on human neoplastic and non-neoplastic tissues. Pharmacological characterizations of (177)Lu-AMBA shows, high affinity towards NMB and GRP receptors, while little or no affinity towards BB(3) receptor. Among the 40 different types of non-neoplastic tissues tested seven of them showed limited but specific binding of (177)Lu-AMBA. Fourteen of 17 primary prostate cancers, six of 13 primary breast cancers expressed binding sites for (177)Lu-AMBA. Furthermore, no apparent differences in (177)Lu-AMBA-binding sites expression were observed between matched pairs (primary vs. secondary) of prostate and breast cancer tissues. These data represent the molecular basis for clinical applications of (177)Lu-AMBA for diagnosis and treatment of GRP-R and NMB-R positive tumors.

Synthesis, 18F-labeling, and in vitro and in vivo studies of bombesin peptides modified with silicon-based building blocks

The gastrin-releasing peptide receptor (GRPr) is overexpressed on various human tumors. The goal of our study was the synthesis of new 18F-labeled bombesin analogues for the PET imaging of GRPr expression in prostate tumor using a silicon-based one-step n. c. a. radiolabeling method. The silicon-containing building blocks were efficiently coupled to the N-terminus of the peptides via solid-phase synthesis. Radiolabeling of the obtained peptide precursors proceeded smoothly under acidic conditions (34-85% conversion). Using the di-tert-butyl silyl building block as labeling moiety, products containing a hydrolytically stable 18F-label were obtained. In in vitro receptor binding experiments 2-(4-(di-tert-butylfluorosilyl)phenyl)acetyl-Arg-Ava-Gln-Trp-Ala-Val-NMeGly-His-Sta-Leu-NH 2 ( 4b, IC50 = 22.9 nM) displayed a 12-fold higher binding affinity than 2-(4-(di-tert-butylfluorosilyl)phenyl)acetyl-Arg-Ava-Gln-Trp-Ala-Val-Gly-His(3Me)-Sta-Leu-NH2 ( 3b, IC50 = 276.6 nM), and 4b was therefore chosen for further evaluation. In vitro and ex vivo metabolite studies of [18F]4b showed no significant degradation. In biodistribution experiments, tumor uptake of [18F]4b was low and unspecific, whereas the GRPr-rich pancreas revealed a high and specific accumulation of the radiotracer. This study demonstrates the applicability of our silicon-based one-step n. c. a. radiolabeling method for the synthesis of new 18F-labeled bombesin derivatives. This innovative approach represents a general, straightforward access to radiolabeled peptides as PET imaging probes.

International Union of Pharmacology. LXVIII. Mammalian bombesin receptors: nomenclature, distribution, pharmacology, signaling, and functions in normal and disease states

The mammalian bombesin receptor family comprises three G protein-coupled heptahelical receptors: the neuromedin B (NMB) receptor (BB(1)), the gastrin-releasing peptide (GRP) receptor (BB(2)), and the orphan receptor bombesin receptor subtype 3 (BRS-3) (BB(3)). Each receptor is widely distributed, especially in the gastrointestinal (GI) tract and central nervous system (CNS), and the receptors have a large range of effects in both normal physiology and pathophysiological conditions. The mammalian bombesin peptides, GRP and NMB, demonstrate a broad spectrum of pharmacological/biological responses. GRP stimulates smooth muscle contraction and GI motility, release of numerous GI hormones/neurotransmitters, and secretion and/or hormone release from the pancreas, stomach, colon, and numerous endocrine organs and has potent effects on immune cells, potent growth effects on both normal tissues and tumors, potent CNS effects, including regulation of circadian rhythm, thermoregulation; anxiety/fear responses, food intake, and numerous CNS effects on the GI tract as well as the spinal transmission of chronic pruritus. NMB causes contraction of smooth muscle, has growth effects in various tissues, has CNS effects, including effects on feeding and thermoregulation, regulates thyroid-stimulating hormone release, stimulates various CNS neurons, has behavioral effects, and has effects on spinal sensory transmission. GRP, and to a lesser extent NMB, affects growth and/or differentiation of various human tumors, including colon, prostate, lung, and some gynecologic cancers. Knockout studies show that BB(3) has important effects in energy balance, glucose homeostasis, control of body weight, lung development and response to injury, tumor growth, and perhaps GI motility. This review summarizes advances in our understanding of the biology/pharmacology of these receptors, including their classification, structure, pharmacology, physiology, and role in pathophysiological conditions.

Bombesin-related peptides and their receptors: recent advances in their role in physiology and disease states

PURPOSE OF REVIEW

Mammalian bombesin-related peptides, gastrin-releasing peptide and neuromedin B actions are mediated by two receptors (BB1-receptor, BB2-receptor), which are closely related to the orphan receptor BRS-3 (BB3-receptor). The purpose of this review is to highlight advances in the understanding of these peptides in physiology/disease states.

RECENT FINDINGS

Pharmacologic/receptor-knockout studies show involvement of these receptors in a number of new processes/diseases. Neuromedin B/BB1-receptor is an important physiological regulator of pituitary-thyroid function; in mediating behavior, especially feas/anxiety; in mediating satiety through different cascades than gastrin-releasing peptide/BB2 receptors and for its autocrine tumor-growth effects. Gastrin-releasing peptide/BB2-receptor plays important roles in mediating signals for pruritus, lung development/injury, small intestinal mucosal defense, and central nervous system processes such as learning/memory. The signaling mechanisms of its potent growth effects are being elucidated and their possible therapeutic targets identified. BB3-receptor knockout mice provided insights for their obesity/glucose intolerance and demonstrated that this receptor may be important in the lung response to injury, tumor growth and gastrointestinal motility. Each receptor is frequently overexpressed in human tumors and has potent growth effects. This effect is being explored to develop new antitumor treatments, such as bombesin-receptor ligands conjugated to cytotoxic agents.

SUMMARY

This receptor family is involved in an increasing number of central nervous system/peripheral processes physiologically and in disease states, and increased understanding of its role may lead to novel treatments.

Targeting prostate cancer with radiolabelled bombesins

The fact that a number of common human tumours, including those of breast and prostate, express increased levels of the gastrin-releasing peptide receptor (GRP-R) means that this receptor is a potential target for peptide receptor mediated scintigraphy and targeted radionuclide therapy. Although clinical application is yet in its infancy, there is a considerable literature on preclinical studies aimed at developing suitable radioligands for potential clinical application. This brief review provides an overview of this research and also describes some of the limited clinical studies that have been published.

Issues related to targeted delivery of proteins and peptides

While modern genomic and proteomic technology enables rapid screening of novel proteins and peptides as potential drug candidates, design of delivery systems for these biologics remains challenging especially to achieve site-specific pharmacological actions. This article discusses the issues associated with targeted delivery of protein and peptide drugs at physiochemical, physiological, and intracellular levels with a special focus on cancer therapy.

Gastrin-releasing peptide and cancer

Over the past 20 years, abundant evidence has been collected to suggest that gastrin-releasing peptide (GRP) and its receptors play an important role in the development of a variety of cancers. In fact, the detection of GRP and the GRP receptor in small cell lung carcinoma (SCLC), and the demonstration that anti-GRP antibodies inhibited proliferation in SCLC cell lines, established GRP as the prototypical autocrine growth factor. All forms of GRP are generated by processing of a 125-amino acid prohormone; recent studies indicate that C-terminal amidation of GRP18-27 is not essential for bioactivity, and that peptides derived from residues 31 to 125 of the prohormone are present in normal tissue and in tumors. GRP receptors can be divided into four classes, all of which belong to the 7 transmembrane domain family and bind GRP and/or GRP analogues with affinities in the nM range. Over-expression of GRP and its receptors has been demonstrated at both the mRNA and protein level in many types of tumors including lung, prostate, breast, stomach, pancreas and colon. GRP has also been shown to act as a potent mitogen for cancer cells of diverse origin both in vitro and in animal models of carcinogenesis. Other actions of GRP relevant to carcinogenesis include effects on morphogenesis, angiogenesis, cell migration and cell adhesion. Future prospects for the use of radiolabelled and cytotoxic GRP analogues and antagonists for cancer diagnosis and therapy appear promising.

Inhibition of growth of experimental human and hamster pancreatic cancers in vivo by a targeted cytotoxic bombesin analog

OBJECTIVES

Targeting anticancer agents to receptors for peptide hormones such as bombesin/gastrin-releasing peptide (GRP) on tumor cells increases the efficacy and lowers the toxicity of cancer therapy. We studied the expression of bombesin/GRP receptors in 6 experimental pancreatic cancers and evaluated tumor inhibition in vivo produced by targeted chemotherapy with the cytotoxic bombesin analog AN-215.

METHODS

Nude mice with xenografts of Panc-1, CFPAC-1, Capan-1, Capan-2, MiaPaCa-2, and SW-1990 human ductal pancreatic cancers, as well as hamsters with nitrosamine-induced pancreatic cancers, were treated with AN-215 or its cytotoxic radical 2-pyrrolinodoxorubicin (AN-201) for 7 to 12 weeks. Tumor growth reduction and survival were analyzed, and cell proliferation rate and apoptosis were examined by histologic methods. Bombesin/GRP receptors on the tumors were studied by ligand-binding assays and their mRNA expression was studied by reverse transcriptase-polymerase chain reaction.

RESULTS

All tumors expressed mRNA for subtype 1 bombesin/GRP receptor, but MiaPaCa-2, and in one experiment, SW-1990 tumors did not show binding sites for bombesin. AN-215 powerfully inhibited the growth of all pancreatic cancers that expressed functional receptors for bombesin/GRP. AN-201 was less effective on most tumors and somewhat more toxic than AN-215.

CONCLUSIONS

Bombesin/GRP receptors are expressed on most ductal pancreatic carcinoma cell lines and can be used for targeted chemotherapy with the cytotoxic bombesin analog AN-215.