Cloning of a receptor for amphibian [Phe13]bombesin distinct from the receptor for gastrin-releasing peptide: identification of a fourth bombesin receptor subtype (BB4)

The gastrin-releasing peptide/bombesin system revisited by a reverse-evolutionary study considering Xenopus

Bombesin is a putative antibacterial peptide isolated from the skin of the frog, Bombina bombina. Two related (bombesin-like) peptides, gastrin-releasing peptide (GRP) and neuromedin B (NMB) have been found in mammals. The history of GRP/bombesin discovery has caused little attention to be paid to the evolutionary relationship of GRP/bombesin and their receptors in vertebrates. We have classified the peptides and their receptors from the phylogenetic viewpoint using a newly established genetic database and bioinformatics. Here we show, by using a clawed frog (Xenopus tropicalis), that GRP is not a mammalian counterpart of bombesin and also that, whereas the GRP system is widely conserved among vertebrates, the NMB/bombesin system has diversified in certain lineages, in particular in frog species. To understand the derivation of GRP system in the ancestor of mammals, we have focused on the GRP system in Xenopus. Gene expression analyses combined with immunohistochemistry and Western blotting experiments demonstrated that GRP peptides and their receptors are distributed in the brain and stomach of Xenopus. We conclude that GRP peptides and their receptors have evolved from ancestral (GRP-like peptide) homologues to play multiple roles in both the gut and the brain as one of the ‘gut-brain peptide’ systems.

Bombesin Receptor Subtype-3 in Human Diseases

This review summarizes the recent findings of the roles of bombesin receptor subtype-3 (BRS-3) in various patho-physiological conditions. Studies have demonstrated that two mammalians bombesin-like peptides, GRP and NMB, exhibit a large range of functions by binding to three receptors. Knockout studies showed that the mice BRS-3 has important effects on tumor growth, energy homeostasis, glucose regulation, satiety, and lung development (1,7). BRS-3 is an orphan receptor whose natural ligand is unknown. However, several agonists and antagonists have been synthesized which facilitate its characterization, (D-Tyr6, β-Ala11, Phe13, Nle14) Bn-(6-14) and MK-5046 are agonists, whereas ML-18 and Bantag-1 are antagonists. With the development of several selective, high-affinity BRS-3 agonists and antagonists, recent studies provided some insights into the biological effects of BRS-3 in several disease states including lung cancer, obesity, diabetes mellitus, asthma, and kidney diseases.

Characterization of GRP as a functional neuropeptide in basal chordate amphioxus

Amphioxus belongs to the subphylum cephalochordata, an extant representative of the most basal chordates, whose regulation of endocrine system remains ambiguous. Here we clearly demonstrated the existence of a functional GRP neuropeptide in amphioxus, which was able to interact with GRP receptor, activate both PKC and PKA pathways, increase gh, igf, and vegf expression. We also showed that the transcription level of amphioxus grp was affected by temperature and light, indicating the role of this gene in the regulation of energy balance and circadian rhythms. In addition, the expression of the amphioxus grp was detected in cerebral vesicle that has been proposed to be the homologous organ of vertebrate brain. These data collectively suggested that a functional GRP neuropeptide had already emerged in amphioxus, which provided insights into the evolutionary origin of GRP in chordate and the functional homology between the cerebral vesicle and vertebrate brain.

Biomedical applications of radioiodinated peptides

The overexpression of peptide receptors in certain tumors as compared to endogeneous expression levels represents the molecular basis for the design of peptide-based tools for targeted nuclear imaging and therapy. Receptor targeting with radiolabelled peptides became a very important imaging and/or therapeutic approach in nuclear medicine and oncology. A great variety of peptides has been radiolabelled with clinical relevant radionuclides, such as radiometals and radiohalogens. However, to the best of our knowledge concise and updated reviews providing information about the biomedical application of radioiodinated peptides are still missing. This review outlines the synthetic efforts in the preparation of radioiodinated peptides highlighting the importance of radioiodine in nuclear medicine, giving an overview of the most relevant radioiodination strategies that have been employed and describes relevant examples of their use in the biomedical field.

A Hylarana latouchii Skin Secretion-Derived Novel Bombesin-Related Pentadecapeptide (Ranatensin-HLa) Evoke Myotropic Effects on the in vitro Rat Smooth Muscles

Amphibians have developed successful defensive strategies for combating predators and invasive microorganisms encountered in their broad range of environments, which involve secretion of complex cocktails of noxious, toxic and diverse bioactive molecules from the skins. In recent years, amphibian skin secretions have been considered as an extraordinary warehouse for the discovery of therapeutic medicines. In this study, through bioactivity screening of the Hylarana latouchii skin secretion-derived fractions, a novel peptide belonging to ranatensin subfamily (ranatensin-HLa) was discovered, and structurally and pharmacologically-characterised. It consists of 15 amino acid residues, pGlu-NGDRAPQWAVGHFM-NH₂, and its synthetic replicate was found to exhibit pharmacological activities on increasing the contraction of the in vitro rat bladder and uterus smooth muscles. Corresponding characteristic sigmoidal dose-response curves with EC₅₀ values of 7.1 nM and 5.5 nM were produced, respectively, in bladder and uterus. Moreover, the precursor of ranatensin-HLa showed a high degree of similarity to those of bombesin-like peptides from Odorrana grahami and Odorrana schmackeri. Hylarana latouchii skin continues to serve as a storehouse with diverse lead compounds for the development of therapeutically effective medicines.

Phase I Trial of Technetium [Leu13] Bombesin as Cancer Seeking Agent: Possible Scintigraphic Guide for Surgery?

Aims and Background

Bombesin-like neuropeptides work as neurotransmitters and growth factors at the same time. Several human cancers show overexpression of three receptors for mammalian counterparts of amphibian bombesins (ABNs), ie gastrin-releasing peptide (GRP), neuromedin B (NMB) and possibly another peptide. ABNs in turn are able to bind to mammalian and human receptors in vitro, and it is therefore interesting to study radioisotope-labeled bombesin (BN) and BN-like peptides as cancer seeking agents.

Methods and Study Design

Starting from the amino acid sequence of [Leu13] ABN, the Demokritos Institute has synthesized and labeled with technetium a new BN-like peptide that has the same biological characteristics as the amphibian peptide; changes were made only in the N-terminal part of this tetradecapeptide. After having obtained satisfactory results with 99mTc BN in a preclinical study, we started a phase I trial involving cancer patients as well as normal volunteers in Tomsk. Three normal volunteers, one patient with small cell lung cancer and one patient with primary prostate cancer were studied after iv injection of 185 MBq, corresponding to 0.7 micrograms of 99mTc BN. Dynamic images of the tumors were acquired for 20 mins, followed by SPET. Total body images were acquired in patients and normal volunteers 1 and 3 h after 99mTc BN acquisition. In addition, 99mTc sestamibi scintigraphy was performed in the patient with small cell lung carcinoma.

Results

No relevant side effects were observed. Both tumors were well visualized on early 1-2 mins images with planar as well as tomographic imaging. Total body images showed radioactivity in the liver, kidneys and thyroid gland. The stomach and spleen were never imaged. Radioactivity was found in the urinary bladder 4 mins after injection in the patient with prostate cancer. Three-hour total body scans showed radioactivity in the duodenum. In the patient in whom also 99mTc sestamibi scintigraphy was performed, thyroid uptake was much higher with sestamibi than with 99mTc BN, whereas the uptake of small cell lung carcinoma was higher with 99mTc BN than with sestamibi.

Conclusions

99mTc BN is able to clearly image tumors with BN receptor overexpression. Our first impression is that in the future this radiopharmaceutical may serve as a cancer seeking agent and, due to its high tumoral uptake, also as a radiotracer for radioisotope-guided surgery.

Pharmacological Effects of Two Novel Bombesin-Like Peptides from the Skin Secretions of Chinese Piebald Odorous Frog (Odorrana schmackeri) and European Edible Frog (Pelophylax kl. esculentus) on Smooth Muscle

Bombesin-like peptides, which were identified from a diversity of amphibian skin secretions, have been demonstrated to possess several biological functions such as stimulation of smooth muscle contraction and regulation of food intake. Here, we report two novel bombesin-like peptides, bombesin-OS and bombesin-PE, which were isolated from Odorrana schmackeri and Pelophylax kl. esculentus, respectively. The mature peptides were identified and structurally confirmed by high performance Scliquid chromatography (HPLC) and tandem mass spectrometry (MS/MS). Subsequently, the effects of these purified chemically-synthetic peptides on smooth muscle were determined in bladder, uterus, and ileum. The synthetic replications were revealed to have significant pharmacological effects on these tissues. The EC₅₀ values of bombesin-OS for bladder, uterus and ileum, were 10.8 nM, 33.64 nM, and 12.29 nM, respectively. Furthermore, compared with bombesin-OS, bombesin-PE showed similar contractile activity on ileum smooth muscle and uterus smooth muscle, but had a higher potency on bladder smooth muscle. The EC₅₀ value of bombesin-OS for bladder was around 1000-fold less than that of bombesin-PE. This suggests that bombesin-OS and bombesin-PE have unique binding properties to their receptors. The precursor of bombesin-OS was homologous with that of a bombesin-like peptide, odorranain-BLP-5, and bombesin-PE belongs to the ranatensin subfamily. We identified the structure of bombesin-OS and bombesin-PE, two homologues peptides whose actions may provide a further clue in the classification of ranid frogs, also in the provision of new drugs for human health.

Characterization of NMB, GRP and their receptors (BRS3, NMBR and GRPR) in chickens

The two structurally and functionally related peptides, gastrin-releasing peptide (GRP) and neuromedin B (NMB) play critical roles in many physiological/pathological processes in mammals. However, the information regarding the expression and functionality of avian NMB, GRP and their receptors is limited. Here, we characterized cNMB, cGRP and their receptors (cNMBR, cGRPR and cBRS3) in chickens. Our results showed that: (1) cNMBR and cGRPR expressed in CHO cells could be potently activated by cNMB and cGRP, respectively, as monitored by cell-based luciferase reporter assays, indicating that cNMBR and cGRPR are cNMB- and cGRP-specific receptors; strikingly, BRS3 of chickens (/spotted gars), which is orthologous to mouse bombesin receptor subtype-3 (BRS3), could be potently activated by GRP and NMB, demonstrating that both peptides are the endogenous ligands for chicken (/spotted gar) BRS3; (2) quantitative real-time PCR (qPCR) revealed that cGRPR is widely expressed in chicken tissues with abundant expression in the ovary, pancreas, proventriculus, spinal cord and brain, whereas cNMB, cNMBR and cBRS3 are mainly expressed in the brain and testes; (3) interestingly, qPCR, Western blot and immunostaining revealed that cGRP is predominantly expressed in the anterior pituitary and mainly localized to LH-cells, suggesting that cGRP is likely a novel pituitary hormone in chickens. In summary, our data help to uncover the roles of GRP, NMB and their receptors in birds, and provide the first persuasive evidence from an evolutionary prospective that in vertebrates, GRP and NMB are the endogenous ligands for BRS3, an orphan receptor that has puzzled endocrinologists for more than two decades.

Molecular imaging probes derived from natural peptides

Covering: up to the end of 2015.Peptides are naturally occurring compounds that play an important role in all living systems and are responsible for a range of essential functions. Peptide receptors have been implicated in disease states such as oncology, metabolic disorders and cardiovascular disease. Therefore, natural peptides have been exploited as diagnostic and therapeutic agents due to the unique target specificity for their endogenous receptors. This review discusses a variety of natural peptides highlighting their discovery, endogenous receptors, as well as their derivatization to create molecular imaging agents, with an emphasis on the design of radiolabelled peptides. This review also highlights methods for discovering new and novel peptides when knowledge of specific targets and endogenous ligands are not available.

Biological evaluation and molecular docking studies of AA3052, a compound containing a μ-selective opioid peptide agonist DALDA and d-Phe-Phe-d-Phe-Leu-Leu-NH2, a substance P analogue

The design of novel drugs for pain relief with improved analgesic properties and diminished side effect induction profile still remains a challenging pursuit. Tolerance is one of the most burdensome phenomena that may hamper ongoing opioid therapy, especially in chronic pain patients. Therefore, a promising strategy of hybridizing two pharmacophores that target distinct binding sites involved in pain modulation and transmission was established. Previous studies have led to the development of opioid agonist/NK1 agonist hybrids that produce sufficient analgesia and also suppress opioid-induced tolerance development. In our present investigation we assessed the antinociceptive potency of a new AA3052 chimera comprised of a potent MOR selective dermorphin derivative (DALDA) and an NK1 agonist, a stabilized substance P analogue. We have shown that AA3052 significantly prolonged responses to both mechanical and noxious thermal stimuli in rats after intracerebroventricular administration. Additionally, AA3052 did not trigger the development of tolerance in a 6-day daily injection paradigm nor did it produce any sedative effects, as assessed in the rotarod performance test. However, the antinociceptive effect of AA3052 was independent of opioid receptor stimulation by the DALDA pharmacophore as shown in the agonist-stimulated G-protein assay. Altogether the current results confirm the antinociceptive effectiveness of a novel opioid/SP hybrid agonist, AA3052, and more importantly its ability to inhibit the development of tolerance.

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].

Preclinical studies of SPECT and PET tracers for NET

Radiolabeled somatostatin analogues are routinely used for the detection of neuroendocrine tumors (NETs), exploiting the expression of somatostatin-receptor subtypes on the cell membrane. Because of the heterogeneity of the origin of these tumors, the performance of radiolabeled somatostatin analogues in certain types of NETs is limited due to the low incidence or low levels of receptor expression. In this review, the most recent developments and in vitro and in vivo characterization of these radiolabeled peptide analogues are discussed.

177Lu-labeled RGD-BBN heterodimeric peptide for targeting prostate carcinoma

INTRODUCTION

Radiolabeled Arg-Gly-Asp (RGD) and bombesin (BBN) heterodimers have been investigated for dual targeting of tumor integrin αvβ3 receptors and gastrin-releasing peptide receptors. The goal of this study was to evaluate the potential use of a Lu-labeled RGD-BBN heterodimer for targeted prostate cancer therapy.

MATERIALS AND METHODS

A 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid-conjugated RGD-BBN peptide (DO3A-RGD-BBN) was radiolabeled with Lu and purified by high-performance liquid chromatography. The in-vivo biodistribution study of Lu-DO3A-RGD-BBN was carried out in mice bearing human prostate cancer PC3 xenografts. The receptor-targeting specificity of the radiolabeled peptide was assayed by injecting the tracer with the unlabeled RGD-BBN peptide. Radiation absorbed doses in adult male patients, based on biodistribution data from mice, were also calculated.

RESULTS

DO3A-RGD-BBN peptides were successfully labeled with Lu, and high radiochemical purity (>95%) could be achieved after high-performance liquid chromatography purification. In human PC3 xenograft-bearing mice, the tumor accumulation of Lu-DO3A-RGD-BBN was 5.88±1.12, 2.77±0.30, 2.04±0.19, and 1.18±0.19%ID/g at 0.5, 2, 24, and 48 h, respectively. With rapid clearance from normal tissues, the radiolabeled probe displayed high tumor-to-blood and tumor-to-muscle ratios. On calculating the radiation absorbed doses for Lu-DO3A-RGD-BBN, we found that the prostate tumor and the pancreas were the organs receiving the highest radiation absorbed doses.

CONCLUSION

Dual integrin αvβ3 and GPRP-targeted agent Lu-DO3A-RGD-BBN shows excellent prostate cancer-targeting ability, and it is worthy of further evaluation for prostate cancer-targeted therapy.

Bombesin peptide antagonist for target-selective delivery of liposomal doxorubicin on cancer cells

PURPOSE

This study addresses novel peptide modified liposomal doxorubicin to specifically target tissues overexpressing bombesin (BN) receptors.

METHODS

DOTA-(AEEA)₂-peptides containing the [7-14]bombesin and the new BN-AA1 sequence have been synthesized to compare their binding properties and in serum stabilities. The amphiphilic peptide derivative (MonY-BN-AA1) containing BN-AA1, a hydrophobic moiety, polyethylenglycole (PEG), and diethylenetriaminepentaacetate (DTPA), has been synthesized. Liposomes have been obtained by mixing of MonY-BN-AA1 with 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC).

RESULTS

Both ¹¹¹In labeled peptide derivatives present nanomolar Kd to PC-3 cells. ¹⁷⁷Lu labeled peptide DOTA-(AEEA)₂-BN-AA1 is very stable (half-life 414.1 h), while DOTA-(AEEA)₂-BN, shows a half-life of 15.5 h. In vivo studies on the therapeutic efficacy of DSPC/MonY-BN-AA1/Dox in comparison to DSPC/MonY-BN/Dox, were performed in PC-3 xenograft bearing mice. Both formulations showed similar tumor growth inhibition (TGI) compared to control animals treated with non-targeted DSPC/Dox liposomes or saline solution. For DSPC/MonY-BN-AA1/Dox the maximum effect was observed 19 days after treatment.

CONCLUSIONS

DSPC/MonY-BN-AA1/Dox nanovectors confirm the ability to selectively target and provide therapeutic efficacy in mice. The lack of receptor activation and possible acute biological side effects provided by using the AA1 antagonist bombesin sequence should provide safe working conditions for further development of this class of drug delivery vehicles.

The molecular basis for high affinity of a universal ligand for human bombesin receptor (BnR) family members

There is increased interest in the Bn-receptor family because they are frequently over/ectopically expressed by tumors and thus useful as targets for imaging or receptor-targeted-cytotoxicity. The synthetic Bn-analog, [D-Tyr(6), β-Ala(11), Phe(13), Nle(14)]Bn(6-14) [Univ.Lig] has the unique property of having high affinity for all three human BNRs (GRPR, NMBR, BRS-3), and thus could be especially useful for this approach. However, the molecular basis of this property is unclear and is the subject of this study. To accomplish this, site-directed mutagenesis was used after identifying potentially important amino acids using sequence homology analysis of all BnRs with high affinity for Univ.Lig compared to the Cholecystokinin-receptor (CCK(A)R), which has low affinity. Using various criteria 74 amino acids were identified and 101 mutations made in GRPR by changing each to those of CCK(A)R or to alanine. 22 GRPR mutations showed a significant decrease in affinity for Univ.Lig (>2-fold) with 2 in EC2[D97N, G112V], 1 in UTM6[Y284A], 2 in EC4[R287N, H300S] showing >10-fold decrease in Univ.Lig affinity. Additional mutations were made to explore the molecular basis for these changes. Our results show that high affinity for Univ.Lig by human Bn-receptors requires positively charged amino acids in extracellular (EC)-domain 4 and to a lesser extent EC2 and EC3 suggesting charge-charge interactions may be particularly important for determining the general high affinity of this ligand. Furthermore, transmembrane amino acids particularly in UTM6 are important contributing both charge-charge interactions as well as interaction with a tyrosine residue in close proximity suggesting possible receptor-peptide cation-π or H-bonding interactions are also important for determining its high affinity.

Preparation and evaluation of bombesin peptide derivatives as potential tumor imaging agents: effects of structure and composition of amino acid sequence on in vitro and in vivo characteristics

OBJECTIVES

Among the many clinically relevant peptide receptor systems, bombesin (BN) receptors have attracted enormous attraction due to their overexpression in various frequently occurring human tumors including breast and prostate, thus making such receptors promising targets with radiolabeled BN analogs. The present study describes the preparation and evaluation of a series of new BN derivatives as potential tumor imaging agents.

METHODS

Several new BN derivatives with the common structure MAG(3)-X-BN(1-14 or 6-14), where X=Asp or Asp-Asp, were synthesized by solid-phase peptide synthesis. S-benzoylmercaptoacetic acid was incorporated at the end of synthesis via manual conjugation to yield MAG(3)-BN conjugates. Radiolabeling with (99m)Tc was accomplished by ligand exchange method. The receptor-binding affinity assays were performed in MDA-MB-231, MCF-7, T47-D and PC-3 cancer cell lines. In vivo biodistribution and clearance kinetics were assessed in Balb/c mice, and tumor targeting efficacy was determined in nude mice bearing breast tumor xenografts.

RESULTS

The peptides were prepared conveniently and radiolabeled efficiently with (99m)Tc (up to 95% labeling efficiency). In vitro cell binding assays demonstrated high affinity (values in the nanomolar range) of (99m)Tc peptides towards breast and prostate cancer cell lines. In addition, the radioconjugates displayed significant internalization (values ranged between 19% and 35%) in tumor cells. In vivo biodistribution and biokinetics are characterized by efficient clearance from the blood and variable degrees of excretion through the renal pathway. In vivo tumor targeting studies displayed variable uptake capacity of different BN derivatives, underlining the influence of specific amino acid sequence on tumor targeting profiles. Tumor uptake was always higher than the radioactivity in the blood and muscle, with good tumor retention and good tumor-to-blood and tumor-to-muscle ratios, indicating the potential of these agents for targeting tumors in vivo.

CONCLUSIONS

The combination of favorable in vitro and in vivo properties may render these BN peptides as potential candidates for targeting BN/GRP receptor-positive tumors. They deserve further evaluation to determine their real strength. The present data indeed provide useful information regarding peptide structure-pharmacologic activity relationship, which might be useful in designing and developing new BN-like peptides for efficient targeting of tumors in vivo.

Naposomes: a new class of peptide-derivatized, target-selective multimodal nanoparticles for imaging and therapeutic applications

Modified supramolecular aggregates for selective delivery of contrast agents and/or drugs are examined with a focus on a new class of peptide-derivatized nanoparticles: naposomes. These nanoparticles are based on the co-aggregation of two different amphiphilic monomers that give aggregates of different shapes and sizes (micelles, vesicles and liposomes) with diameters ranging between 10 and 300 nm. Structural properties and in vitro and in vivo behaviors are discussed. For the high relaxitivity values (12–19 mM-1s-1) and to detect for the presence of a surface-exposed peptide, the new peptide-derived supramolecular aggregates are very promising candidates as target-selective MRI contrast agents. The efficiency of surface-exposed peptides in homing these nanovectors to a specific target introduces promising new opportunities for the development of diagnostic and therapeutic agents with high specificity toward the biological target and reduced toxic side effects on nontarget organs.

Androgen-regulated gastrin-releasing peptide receptor expression in androgen-dependent human prostate tumor xenografts

Human prostate cancer (PC) overexpresses the gastrin-releasing peptide receptor (GRPR). Radiolabeled GRPR-targeting analogs of bombesin (BN) have successfully been introduced as potential tracers for visualization and treatment of GRPR-overexpressing tumors. A previous study showed GRPR-mediated binding of radiolabeled BN analogs in androgen-dependent but not in androgen-independent xenografts representing the more advanced stages of PC. We have further investigated the effect of androgen modulation on GRPR-expression in three androgen-dependent human PC-bearing xenografts: PC295, PC310 and PC82 using the androgen-independent PC3-model as a reference. Effects of androgen regulation on GRPR expression were initially studied on tumors obtained from our biorepository of xenograft tissues performing reverse transcriptase polymerase chain reaction (RT-PCR) and autoradiography ((125)I-universal-BN). A prospective biodistribution study ((111)In-MP2653) and subsequent autoradiography ((125)I-GRP and (111)In-MP2248) was than performed in castrated and testosterone resupplemented tumor-bearing mice. For all androgen-dependent xenografts, tumor uptake and binding decreased drastically after 7 days of castration. Resupplementation of testosterone to castrated animals restored GRPR expression extensively. Similar findings were concluded from the initial autoradiography and RT-PCR studies. Results from RT-PCR, for which human specific primers are used, indicate that variations in GRPR expression can be ascribed to mRNA downregulation and not to castration-induced reduction in the epithelial fraction of the xenograft tumor tissue. In conclusion, expression of human GRPR in androgen-dependent PC xenografts is reduced by androgen ablation and is reversed by restoring the hormonal status of the animals. This knowledge suggests that hormonal therapy may affect GRPR expression in PC tissue making GRPR-based imaging and therapy especially suitable for non-hormonally treated PC patients.

Bombesin-modified 6-14 C-terminal fragments adsorption on silver surfaces: influence of a surface substrate

Surface-enhanced Raman scattering (SERS) spectroscopy has been applied to investigate the interaction with a silver colloidal surface of following seven 6-14 fragments of bombesin (BN) C-terminus: cyclo[D-Phe(6),His(7),Leu(14)]BN(6-14), [D-Phe(6),Leu-NHEt(13),des-Met(14)]BN(6-14), [D-Phe(6),Leu(13)-(R)-p-chloro-Phe(14)]BN(6-14), [D-Phe(6),beta-Ala(11),Phe(13),Nle(14)]BN(6-14), [D-Tyr(6),beta-Ala(11),Phe(13),Nle(14)]BN(6-14), [D-Tyr(6),beta-Phe(11),Phe(13),Nle(14)OH]BN(6-14), and [D-Cys(6),Asn(7),D-Ala(11),Cys(14)]BN(6-14), potent r-GRP-R receptor antagonists used in chemotherapy and potential effective drugs in cancer treatment. The adsorption active sites and molecular orientations on the colloidal silver surface have been determined on the basis of SERS "surface selection rules" subsequent to a detailed SERS analysis. In addition, the similarities and differences of these spectra with the SERS spectra of the peptides immobilized on a roughened silver electrode surface have been examined. From the data, suggestion has been made about structural properties of these peptides on the colloidal surface.

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.

Surface-enhanced Raman difference between bombesin and its modified analogues on the colloidal and electrochemically roughen silver surfaces

In this article, surface-enhanced Raman scattering (SERS) spectra of bombesin (BN) and its six modified analogues ([D-Phe(12)]BN, [Tyr(4)]BN, [Tyr(4),D-Phe(12)]BN, [D-Phe(12),Leu(14)]BN, [Leu(13)-(R)-Leu(14)]BN, and [Lys(3)]BN) on a colloidal silver surface are reported and compared with SERS spectra of these species immobilized onto an ellectrochemically roughen silver electrode. Changes in enhancement and wavenumber of proper bands upon adsorption on different silver surfaces are consistent with BN and its analogues adsorption primarily through Trp(8). Slightly different adsorption states of these molecules are observed depending upon natural amino acids substitution. For example, the indole ring in all the peptides interacts with silver nanoparticles in a edge-on orientation. It is additionally coordinated to the silver through the N(1)--H bond for all the peptides, except [Phe(12)]BN. This is in contrary to the results obtained for the silver roughen electrode that show direct but not strong N(1)--H/Ag interaction for all peptides except [D-Phe(12),Leu(14)]BN and [Leu(13)-(R)-Leu(14)]BN. For BN only C==O is not involved in the chemical coordination with the colloidal surface. [Lys(3)]BN and BN also adsorb with the C--N bond of NH(2) group normal and horizontal, respectively, to the colloidal surface, whereas C--NH(2) in other peptides is tilted to this surface. Also, the Trp(8) --CH(2)-- moiety of only [Tyr(4)]BN, [Lys(3)]BN, and [Tyr(4),D-Phe(12)]BN coordinates to Ag, whereas the Phe(12) ring of [Phe(12)]BN, [Tyr(4),D-Phe(12)]BN, and [D-Phe(12),Leu(14)]BN assists in the peptides binding only on the colloidal silver.

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.

Phylogenetic analysis of the sequences of gastrin-releasing peptide and its receptors: Biological implications

The many biological activities of the hormone gastrin-releasing peptide (GRP), including stimulation of acid secretion and of tumour growth, are mediated by the gastrin-releasing peptide receptor (GRP-R). Here sequence comparisons are utilised to investigate the likely bioactive regions of the 125 amino acid GRP precursor and of GRP-R. Comparison of the sequences of the GRP precursor from 21 species revealed homology not only in the GRP region between amino acids 1 and 30, but also in C-terminal regions from amino acids 43 to 97. This observation is consistent with recent reports that peptides derived from the C-terminal region are biologically active. Comparison of the GRP-R sequence with the related receptors NMB-R and BRS-3 revealed that the family could be distinguished from other G-protein coupled receptors by the presence of the motif GVSVFTLTALS at the cytoplasmic end of transmembrane helix 3. Comparison of the sequences of the GRP-R from 21 species revealed that the most highly conserved regions occurred in transmembrane helices 2, 3, 5, 6 and 7, and in the third intracellular loop. These results will be important in guiding future structure-function studies of the GRP precursor and of GRP receptors.

Novel 111In-labelled bombesin analogues for molecular imaging of prostate tumours

PURPOSE

It has been shown that some primary human tumours and their metastases, including prostate and breast tumours, overexpress gastrin-releasing peptide (GRP) receptors. Bombesin (BN) is a neuropeptide with a high affinity for these GRP receptors. We demonstrated successful scintigraphic visualisation of BN receptor-positive tumours in preclinical studies using the radiolabelled BN analogue [(111)In-DTPA-Pro(1),Tyr(4)]BN. However, the receptor affinity as well as the serum stability of this analogue leave room for improvement. Therefore new (111)In-labelled BN analogues were synthesised and evaluated in vitro and in vivo.

METHODS AND RESULTS

The receptor affinity of the new BN analogues was tested on human GRP receptor-expressing prostate tumour xenografts and rat colon sections. Analogues with high receptor affinity (low nM range) were selected for further evaluation. Incubation in vitro of GRP receptor-expressing rat CA20948 and human PC3 tumour cells with the (111)In-labelled analogues resulted in rapid receptor-mediated uptake and internalisation. The BN analogue with the best receptor affinity and in vitro internalisation characteristics, Cmp 3 ([(111)In-DTPA-ACMpip(5),Tha(6),betaAla(11),Tha(13),Nle(14)]BN(5-14)), was tested in vivo in biodistribution studies using rats bearing GRP receptor-expressing CA20948 tumours, and nude mice bearing human PC3 xenografts. Injection of (111)In-labelled Cmp 3 in these animals showed high, receptor-mediated uptake in receptor-positive organs and tumours which could be visualised using planar gamma camera and microSPECT/CT imaging.

CONCLUSION

With their enhanced receptor affinity and their rapid receptor-mediated internalisation in vitro and in vivo, the new BN analogues, and especially Cmp 3, are promising candidates for use in diagnostic molecular imaging and targeted radionuclide therapy of GRP receptor-expressing cancers.

Role of gastrin-releasing peptide and neuromedin B in anxiety and fear-related behavior

Bombesin (BB)-like peptides have been implicated in the mediation and/or modulation of the stress response. However, the impact of manipulating this peptidergic system has only been assessed in a limited number of anxiety and fear paradigms. Given that different behavioral paradigms reflect different aspects of anxiety, the objective of the present investigation was to assess the effects of two mammalian BB-related peptides, namely gastrin-releasing peptide (GRP) and neuromedin B (NMB), in paradigms thought to reflect fear and anxiety-related behaviors. To this end, the effects of central (3rd ventricular; i.c.v.) administration of GRP (0.30 nmol), GRP receptor (BB(2)) antagonist, [Leu(13)-(CH(2)NH)Leu(14)]-BN (1.26 nmol), NMB-30 (0.29 nmol), NMB (BB(1)) receptor antagonist, BIM 23127 (1.70 nmol) and a mixed BB(1)/BB(2) receptor antagonist, PD 176252 (0.621 nmol) were assessed in the elevated plus maze (EPM) and in a fear potentiated startle paradigm (a model thought to reflect conditioned fear). The BB(1) receptor antagonist and the mixed BB(1)/BB(2) receptor antagonist elicited anxiolytic effects in the EPM, whereas, the BB(2) receptor antagonist was without effect. In the fear potentiated startle paradigm, pretreatment with either the BB(1) receptor antagonist or the BB(2) receptor agonist attenuated the fear potentiated startle response, without affecting basal startle amplitude. These data suggest that NMB and GRP do affect the stress response. However, whereas NMB manipulations affected both anxiety and fear responses, GRP alterations selectively affected fear-related responses.

Immunohistochemical detection of bombesin receptor subtypes GRP-R and BRS-3 in human tumors using novel antipeptide antibodies

Bombesin (BN)-like peptides can stimulate cancer cell growth through binding to their specific G protein-coupled receptors. It is well established that BN receptors are being overexpressed in a subset of human tumors; however, little is known about the cellular and subcellular localization of individual BN receptor subtypes in these tissues. In this study, we developed and characterized novel antipeptide antibodies to the carboxy terminal regions of the gastrin-releasing peptide-preferring bombesin receptor (GRP-R) and the bombesin receptor subtype-3 (BRS-3). Specificity of the antisera was demonstrated by (1) detection of broad bands migrating at Mr 50,000-70,000 in Western blots of membranes from receptor-expressing tissues; (2) cell surface staining of transfected cells; (3) translocation of GRP-R receptor immunostaining after BN exposure; and (4) abolition of tissue immunostaining by preadsorbtion of the antibodies with their immunizing peptides. The distribution of BN receptors was investigated in 74 formalin-fixed, paraffin-embedded human tumors. GRP-R receptors were most frequently detected in breast and prostate carcinomas. BRS-3 receptors were often detected in prostate and pancreatic carcinomas and in pituitary adenomas. Immunoreactive GRP-R and BRS-3 receptors were in many cases predominantly confined to the plasma membrane and uniformly present on nearly all tumor cells. The development of these novel antipeptide antibodies will facilitate the identification of those tumors, which may be targets for diagnostic or radiotherapeutic application of subtype-selective BN analogs.

Identification of Bombesin Receptor Subtype-Specific Ligands: Effect ofN-Methyl Scanning, Truncation, Substitution, and Evaluation of Putative Reported Selective Ligands

Mammalian bombesin (Bn) receptors include the gastrin-releasing peptide receptor, neuromedin B receptor, and bombesin receptor subtype 3 (BRS-3). These receptors are involved in a variety of physiological/pathologic processes, including thermoregulation, secretion, motility, chemotaxis, and mitogenic effects on both normal and malignant cells. Tumors frequently overexpress these receptors, and their presence is now used for imaging and receptor-mediated cytotoxicity. For these reasons, there is an increased need to develop synthetic, selective receptor subtype-specific ligands, especially agonists for these receptors. In this study, we used a number of strategies to identify useful receptor subtype-selective ligands, including synthesizing new analogs (N-methyl-substituted constrained analogs, truncations, and substitutions) in [d-Tyr(6),betaAla(11),Phe(13),Nle(14)]Bn(6-14), which has high affinity for all Bn receptors and is metabolically stable, as well as completely pharmacologically characterized analogs recently reported to be selective for these receptors in [Ca(2+)](i) assays. Affinities and potencies of each analog were determined for each human Bn receptor subtype. N-Methyl substitutions in positions 14, 12, 11, 10, 9, and 8 did not result in selective analogs, with the exception of position 11, which markedly decreased affinity/potency. N-Terminal truncations or position 12 substitutions did not increase selectivity as previously reported by others. Of the four shortened analogs of [d-Phe(6),betaAla(11),Phe(13),Nle(14)]Bn(6-14) reported to be potent selective BRS-3 ligands on [Ca(2+)](i) assays, only AcPhe,Trp,Ala,His(tauBzl),Nip,Gly,Arg-NH(2) had moderate selectivity for hBRS-3; however, it was less selective than previously reported Apa(11) analogs, demonstrating these are still the most selective BRS-3 analogs available. However, both of these analogs should be useful templates to develop more selective BRS-3 ligands.

Signal transduction of bombesin-induced circular smooth muscle cell contraction in cat esophagus

AIM: To investigate the mechanism of bombesin-induced circular smooth muscle cell contraction in cat esophagus.

METHODS: Specific G protein or phospholipase C involved in cat esophagus contraction was identified, muscle cells were permeabilized with saponin. After permeabilization of muscle cells, the Gi3 antibody inhibited bombesin-induced smooth muscle cell contraction.

RESULTS: Incubation of permeabilized circular muscle cells with PLC-β3 antibody could inhibit bombesin-induced contraction. H-7, chelerythrine (PKC inhibitor) and genistein (protein tyrosine kinase inhibitor) inhibited bombesin-induced contraction, but DAG kinase inhibitor, R59949, could not inhibit it. To examine which mitogen-activated protein kinase (MAPK) was involved in bombesin-induced contraction, the specific MAPK inhibitors (MEK inhibitor, PD98059 and p38 MAPK inhibitor, SB202190) were used. Preincubation of PD98059 blocked the contraction induced by bombesin in a concentration-dependent manner. However, SB202190 had no effects on contraction.

CONCLUSION: Bombesin-induced circular muscle cell contraction in cat esophagus is madiated via a PKC or a PTK-dependent pathway or p44/p42 MAPK pathway.

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.

Radiolabeled peptide conjugates for targeting of the bombesin receptor superfamily subtypes

Research laboratories around the world are currently focusing their efforts toward the development of radiometallated, site-directed, diagnostic/therapeutic agents based upon small peptides such as octreotide, neurotensin, alpha-melanocyte stimulating hormone, vasointestinal peptide and others. Bombesin (BBN) or derivatives of bombesin are also of significant interest. Bombesin is a 14-amino-acid peptide with very high affinity for the BB2 or gastrin-releasing peptide receptor (GRPr). Over-expression of the GRPr on a variety of human cancers (i.e., breast, prostate, pancreatic, small cell lung, etc.) provides potential efficacy toward development of radiometallated BBN derivatives for targeting and, hence, diagnosis/treatment of these specific diseases. New derivatives are being developed that are also capable of targeting the BB1 and BB3 receptor subtypes that are over-expressed on cancer cells. This review highlights some of the more recent developments toward design of BBN receptor-specific radiopharmaceuticals that have taken place over the past 2 years.

Gastrin-releasing peptide receptors in normal and neoplastic human uterus: involvement of multiple tissue compartments

CONTEXT

Bombesin-like neuropeptides including gastrin-releasing peptide (GRP) and their corresponding receptors, mediate multiple physiological actions and have biological significance in cancer. However, information about the function of these neuropeptides and the incidence, distribution, density, and subtype of their receptors in human uterine tissues is scarce.

OBJECTIVE

The objective of the study was to investigate normal and neoplastic human uterine tissues for their bombesin receptor status.

DESIGN

In vitro subtype-specific bombesin receptor autoradiography was used in this study.

PATIENTS

The following tissue samples were taken immediately after surgery: myometrium (n = 41), endometrium (n = 29), leiomyomas (n = 26), leiomyosarcomas (n = 6), endometrial adenocarcinomas (n = 28), and carcinosarcoma (n = 1).

RESULTS

Normal uterine tissues expressed GRP receptors (GRP-Rs) in the myometrium, in subsets of secretory endometrial glands, and in subsets of endometrial blood vessels of the late proliferative and the secretory phase. Most leiomyomas (20 of 26) expressed GRP-R but not the leiomyosarcomas. GRP-Rs were also detected in 10 of 28 adenocarcinomas, one of one carcinosarcoma, and in blood vessels surrounding the adenocarcinomas. No other bombesin receptor subtypes (neuromedin B receptors and bb3) were detected.

CONCLUSIONS

These findings may be of physiological and pathophysiological significance. The expression of GRP-R in glands and vessels during specific phases of the cycle suggests a timely precise physiological action of GRP in these targets; in certain uterine neoplasms, the GRP-R overexpression may contribute to tumor development because GRP is a potent growth factor. Furthermore, these findings may be diagnostically and therapeutically relevant. The expression of GRP-R in leiomyomas may allow distinguishing them from receptor-negative leiomyosarcomas; GRP-R in leiomyomas, in a subset of endometrial adenocarcinomas, carcinosarcomas, and in peritumoral vessels may be candidates for receptor targeting in vivo.

Bombesin receptor subtype-3 is expressed by the enteric nervous system and by interstitial cells of Cajal in the rat gastrointestinal tract

Bombesin receptor subtype-3 (BRS-3), a G-protein-coupled orphan receptor, shares 47% and 55% homology with other known mammalian bombesin receptors. Despite the molecular characterization of BRS-3, its function remains unclear as a consequence of its low affinity for bombesin and the absence of an identified natural ligand. Although the other mammalian bombesin receptors are widely distributed in the gut and central nervous system, expression of BRS-3 in the gastrointestinal tract has not been previously described. We report the expression of BRS-3 mRNA and protein in the tunica muscularis of the rat gastrointestinal tract. The mRNA expression pattern was studied by reverse transcription followed by quantitative polymerase chain reaction. To identify the cellular sites of expression of BRS-3, we performed immunocytochemistry by using a N-terminus-specific affinity-purified antiserum. BRS-3 was found to be widely expressed in the rat gastrointestinal tract at both the mRNA and protein levels. BRS-3-like immunoreactivity (BRS-3-LI) was localized in neurons of the myenteric and submucosal ganglia, being primarily concentrated near the neuronal plasma membrane, and in fibers distributed in the longitudinal and circular muscle layers. In addition, BRS-3-LI was observed in the cell bodies and processes of c-kit+ interstitial cells of Cajal. These data have functional applications for the effects mediated by the activation of BRS-3 on gut motility through distinct neuronal and non-neuronal pathways.

Identification of key amino acids in the gastrin-releasing peptide receptor (GRPR) responsible for high affinity binding of gastrin-releasing peptide (GRP)

The bombesin (Bn) receptor family includes the gastrin-releasing peptide (GRPR) and neuromedin B (NMBR) receptors, Bn receptor subtype 3 (BRS-3) and Bn receptor subtype 4 (BB(4)). They share 50% homology, yet their affinities for gastrin-releasing peptide (GRP) differ. The determinants of GRP high affinity for GRPR and BB(4), and low affinity for BRS-3 are largely unknown. To address this question we made an analysis of structural homologies in Bn receptor members correlated with their affinities for GRP to develop criteria to identify amino acids important for GRP selectivity. Fourteen differences were identified and each was mutated singly in GRPR to that found in hBRS-3. Eleven mutants had a loss of GRP affinity. Furthermore, three of four amino acids in the GRPR selected used a similar approach and previously reported to be important for high affinity Bn binding, were important for GRP affinity. Some GRPR mutants containing combinations of these mutations had greater decreases in GRP affinity than any single mutation. Particularly important for GRP selectivity were K101, Q121, A198, P199, S293, R288, T297 in GRPR. These results were confirmed by making the reverse mutations in BRS-3 to make GRP gain of affinity mutants. Modeling studies demonstrated a number of the important amino acids had side-chains oriented inward and within 6A of the binding pocket. These results demonstrated this approach could identify amino acids needed for GRP affinity and complemented results from chimera/mutagenesis studies by identifying which differences in the extracellular domains of Bn receptors were important for GRP affinity.

Mechanisms underlying anorexia after microinjection of bombesin into the lateral cerebroventricle

Intracerebroventricular (i.c.v.) injections of bombesin (BN) and gastrin-releasing peptide (GRP) dose-dependently decreased food intake in male Wistar rats fasted for 17 h. Neuromedin B (NMB) did not show any effect on food intake. After BN administration, locomotor activity did not significantly change, compared with a vehicle-injected group. The anorexia induced by BN (0.3 microg) was perfectly inhibited by pretreatment with a GRP-receptor antagonist, [D-Tyr(6)]BN(6-13) methyl ester (10 microg), an NO synthase inhibitor, L-nitro-arginine (30 microg), and a PKG inhibitor, H-9 (2 microg). The cGMP concentration in the hypothalamus increased 1 h after administration when compared with the vehicle-injected group. On the other hand, an NMB-receptor antagonist, BIM23127 (10 microg), and the protein kinase (PK) C inhibitors, chelerythrine (2 microg) and Go6983 (2 microg), inhibited only the late phase of the anorexia. A PKC activator, phorbol 12, 13-dibutyrate (3 microg), injected into the ventricle decreased food intake. These findings suggest that BN suppresses food intake mainly mediated through the GRP receptor and NO-cGMP-PKG pathway, and NMB receptor and PKC is partly involved in the late phase of the anorexia.

Synthesis and Evaluation of Bombesin Derivatives on the Basis of Pan-Bombesin Peptides Labeled with Indium-111, Lutetium-177, and Yttrium-90 for Targeting Bombesin Receptor-Expressing Tumors

Bombesin receptors are overexpressed on a variety of human tumors like prostate, breast, and lung cancer. The aim of this study was to develop radiolabeled (Indium-111, Lutetium-177, and Yttrium-90) bombesin analogues with affinity to the three bombesin receptor subtypes for targeted radiotherapy. The following structures were synthesized: diethylenetriaminepentaacetic acid-gamma-aminobutyric acid-[D-Tyr6, beta-Ala11, Thi13, Nle14] bombesin (6-14) (BZH1) and 1,4,7,10-tetraazacyclododecane-N,N',N",N"' -tetraacetic acid-gamma-aminobutyric acid-[D-Tyr6, beta-Ala11, Thi13, Nle14] bombesin (6-14) (BZH2). [111In]-BZH1 and in particular [90Y]-BZH2 were shown to have high affinity to all three human bombesin receptor subtypes with binding affinities in the nanomolar range. In human serum metabolic cleavage was found between beta-Ala11 and His12 with an approximate half-life of 2 hours. The metabolic breakdown was inhibited by EDTA and beta-Ala11-His12 (carnosine) indicating that carnosinase is the active enzyme. Both 111In-labeled peptides were shown to internalize into gastrin-releasing peptide-receptor-positive AR4-2J and PC-3 cells with similar high rates, which were independent of the radiometal. The biodistribution studies of [111In]-BZH1 and [111In]-BZH2 ([177Lu]-BZH2) in AR4-2J tumor-bearing rats showed specific and high uptake in gastrin-releasing peptide-receptor-positive organs and in the AR4-2J tumor. A fast clearance from blood and all of the nontarget organs except the kidneys was found. These radiopeptides were composed of the first pan-bombesin radioligands, which show great promise for the early diagnosis of tumors bearing not only gastrin-releasing peptide-receptors but also the other two bombesin receptor subtypes and may be of use in targeted radiotherapy of these tumors.

Gastrin releasing peptide (GRP) receptor targeted radiopharmaceuticals: a concise update

The gastrin releasing peptide (GRP) receptor is becoming an increasingly attractive target for development of new radiolabeled peptides with diagnostic and therapeutic potential. The attractiveness of the GRP receptor as a target is based upon the functional expression of GRP receptors in several tumors of neuroendocrine origin including prostate, breast, and small cell lung cancer. This concise review outlines some of the efforts currently underway to develop new GRP receptor specific radiopharmaceuticals by employing a variety of radiometal chelation systems.

Characterization of the bombesin-like peptide receptor family in primates

In mammals, bombesin-like peptides mediate a broad range of physiological functions through binding to three highly conserved G-protein-coupled receptors: the neuromedin B-preferring, the gastrin-releasing peptide-preferring, and the bombesin-receptor subtype 3. Selective modulation of these receptors presents opportunities for the development of novel therapeutics. To ascertain if rhesus monkey could serve as a surrogate animal model for the development of modulators of bombesin-like receptor function, we undertook a search for additional receptor family members and studied the expression profiles of the three known bombesin-related receptors. We found no evidence for additional receptor family members in mammals, suggesting that the expression of the previously described bombesin-receptor subtype 4 is limited to amphibians. We studied the distribution of the three receptors in a broad array of human and rhesus monkey tissues. Based on the similarity between the human and the rhesus expression profiles, we conclude that the rhesus monkey may be a suitable animal model to evaluate the clinical efficacy and potential side effects of bombesin-like peptide ligands.

Identification and optimization of novel partial agonists of Neuromedin B receptor using parallel synthesis

The design and parallel synthesis of potent, small molecule partial agonists of Neuromedin B receptor based on the 3-amino-2,3,4,9-tetrahydro-1H-carbazole-3-carboxylic acid amide core is described.

Development of Bombesin Analogs with Conformationally Restricted Amino Acid Substitutions with Enhanced Selectivity for the Orphan Receptor Human Bombesin Receptor Subtype 3

The human bombesin receptor subtype 3 (hBRS-3) orphan receptor, which has a high homology to bombesin (Bn) receptors [gastrin-releasing peptide (GRP) and neuromedin B (NMB) receptors], is widely distributed in the rat central nervous system. Its natural ligand or role in physiology is unknown due to lack of selective ligands. Its target disruption leads to obesity, diabetes, and hypertension. A synthetic high-affinity agonist, [D-Tyr6,beta-Ala11,Phe13,Nle14]Bn(6-14), has been described, but it is nonselective for hBRS-3 over other Bn receptors; however, substitution of (R)- or (S)-amino-3-phenylpropionic acid (Apa) for beta-Ala11 resulted in a modestly selective ligand. In the present study, we have attempted to develop a more selective hBRS-3 ligand by using two strategies: substitutions on phenyl ring of Apa11 and the substitution of other conformationally restricted amino acids into position 11 of [D-Tyr6,beta-Ala11,Phe13,Nle14]Bn(6-14). Fifteen analogs were synthesized and affinities were determined for hBRS-3 and Bn receptors (hGRP-R and hNMB-R). Selective analogs were tested for their ability to activate each receptor by stimulating phospholipase C. One analog, [D-Tyr6,Apa-4Cl,Phe13,Nle14]Bn(6-14), retained high affinity for the hBRS-3 (Ki=8 nM) and had enhanced selectivity (>230-fold) for hBRS-3 over hGRP-R or hNMB-R. This analog specifically interacted with hBRS-3, fully activated hBRS-3 receptors, and was a potent agonist at the hBRS-3 receptor. This enhanced selectivity should allow this analog to be useful for investigating the possible role of hBRS-3 in physiological or pathological processes.

Gastrin-Releasing Peptide (GRP) Analogues for Cancer Imaging

Small neuropeptides, labeled with gamma- and/or beta-emitting radionuclides, are currently being investigated for their ability to bind to cell-surface receptors, overexpressed in a wide variety of malignant tissues being, thus, potentially useful for radionuclide detection and/or therapy for tumors. Particular attention has been focused on the amphibian peptide, bombesin (BN), and the molecularly related gastrin-releasing peptide (GRP). These peptides act as neurotransmitters and endocrine cancer cell-growth factors on normal tissues as well as on neoplastic cells of various origin. In recent investigations, modification of the native peptide structure has been attempted in order to obtain derivatives, which might easily be labeled with radionuclides. Thus, iodinated (I-125) BN derivatives, as well as Indium (In-111) labeled BN analogs are currently being investigated, presenting satisfactory tumor localization. Also, some new BN analogs containing a 6-carbon linker have been prepared and labeled with Rhenium-188, resulting in positive in vitro binding to prostate cancer cells. More recent studies refer to the Technetium-99m labeling of BN, performed either directly, after attaching proper technetium-chelating groups onto the BN sequence, or indirectly, by coupling BN to a preformed 99mTc-tagging ligand. Both types of conjugates were found to have a high in vitro affinity for cells with BN receptors, also presenting satisfactory in vivo uptake in experimental tumor models. Pilot clinical studies of a new BN-derived, 99mTc-labeled pentadecapeptide indicated significant uptake by breast cancer and invaded lymph nodes, as well as by prostate cancer, small-cell lung carcinoma, gastro-entero-pancreatic tumors, and others, Further studies of this new GRP derivative, as well as of other new BN-like peptides, are intensively performed internationally today.

Lung cancer-derived bombesin-like peptides down-regulate the generation and function of human dendritic cells

Development of tumors is regulated by tumor-derived neuroendocrine factors, including bombesin-like peptides (BLP). We have evaluated neuroendocrine regulation of dendritic cell (DC) maturation and function by both tumor-derived and purified bombesin (BOM), neuromedin B (NMB), gastrin-releasing peptide (GRP), and a BOM antagonist D-Phe-bombesin (DPB). BOM, NMB and GRP dose-dependently inhibited maturation of DC assessed as down-regulation of CD40, CD80 and CD86 expression on DC. BOM and GRP also inhibited interleukin-12 (IL-12) production by DC and their ability to activate T cells. DPB partly abrogated immunosuppressive effect of tumor cells on DC. These data are a first evidence for the role of BLP in the regulation of DC maturation and function, demonstrating that BLP inhibit DC maturation and longevity in the lung cancer microenvironment. This suggests a new mechanism of tumor escape and provides new targets for the immunopharmacological correction of immune effectors in cancer.