Discovery of potent and selective peptide agonists at the GRP-preferring bombesin receptor (BB2)

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

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

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

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

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.

Toward the Optimization of Bombesin-Based Radiotracers for Tumor Targeting

The peptide bombesin (BBN) is a peptide with high affinity for the gastrin-releasing peptide receptor (GRPr), a receptor that is overexpressed by, for example, breast and prostate cancers. Thus, GRPr agonists can be used as cancer-targeting vectors to shuttle diagnostic and therapeutic agents into tumor cells. With the aim of optimizing the tumor targeting properties of a radiolabeled [Nle(14)]BBN(7-14) moiety, novel BBN(7-14)- and BBN(6-14)-based radioconjugates were synthesized, labeled with Lu-177, and fully evaluated in vitro and in vivo. The effect of residue and backbone modification on several parameters such as the internalization of the radiolabeled peptides into PC3 and AR42J tumor cells, their affinity toward the human GRPr, metabolic stability in blood plasma, and biodistribution in mice bearing GRPr-expressing PC3 xenografts was studied. As a result of our investigations, a novel radiolabeled GRPr agonist with a high tumor uptake and a high tumor-to-kidney ratio was identified.

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.

Design, synthesis, and biological evaluation of an antagonist-bombesin analogue as targeting vector

The gastrin releasing peptide receptor (GRP-R) is overexpressed on a number of tumors and cancer cell lines including pancreas, prostate, breast, gastrointestinal, and small cell lung cancer (SCLC). Radiolabeled bombesin (BBN) analogues have exhibited high binding affinity and specificity to the GRP-R. A bombesin analogue with an antagonist targeting vector at the C-terminus, DOTA-aminohexanoyl-[D-Phe(6), Leu-NHCH 2CH 2CH3(13), des Met(14)] BBN[6-14] (1, "Bomproamide"), has been synthesized and displays high binding affinity (IC50 = 1.36 +/- 0.09 nM) against (125)I-Tyr (4)-BBN in in vitro competitive assays using PC-3 cells. Maximum internalization of (111)In-1 reached 14% in PC-3 cells after 45 min of incubation. Rapid (0.25 h PI) and high (12.21 +/- 3.2%ID/g) pancreatic uptake of (111)In-1 was observed in healthy CF-1 mice, and 90% of the activity was blocked by coinjection of 100 mug of BBN. Rapid (0.25 h PI) and high uptake (6.90 +/- 1.06%ID/g) was observed in PC-3 prostate cancer xenografts in SCID mice, as well as visualized clearly in a SPECT/CT study. These results support the use of a bombesin construct with an antagonist C-terminal vector as a candidate of choice for specific in vivo imaging of tumors overexpressing GRP-receptors.

Gastrin-releasing peptide receptor as a molecular target in experimental anticancer therapy

Over the last two decades, several lines of experimental evidence have suggested that the gastrin-releasing peptide (GRP) may act as a growth factor in many types of cancer. For that reason, gastrin-releasing peptide receptor (GRPR) antagonists have been developed as anticancer candidate compounds, exhibiting impressive antitumoral activity both in vitro and in vivo in various murine and human tumors. In this article, the GRPR cell surface expression profile in human malignancies is reviewed aiming at the identification of potential tumor types for future clinical trials with GRP analogues and antagonists. In this review, we summarize the current literature regarding the GRPR status in human malignancies. Source data were obtained by searching all published material available through Medline, PubMed and relevant articles from 1971 to 2006. The data available demonstrated a high expression of GRPRs in a large spectrum of human cancers, demonstrating the potential relevance of this intracellular signaling pathway in various human tumor models. The GRPR may be an interesting target for therapeutic intervention in human malignancies, as carriers for cytotoxins, immunotoxins or radioactive compounds, being also a potential tool for tumor detection.

Identification of bombesin receptor subtype-specific ligands: effect of N-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.

The design of a new potent and selective ligand for the orphan bombesin receptor subtype 3 (BRS3)

Extensive SAR studies on the unselective BRS3 agonist, [H-D-Phe6,beta-Ala11,Phe13,Nle14]-bombesin-(6-14)-nonapeptide amide, have highlighted structural features important for BRS3 activity and have provided guidance as to the design of selective agonists. A radically modified heptapeptide agonist, maintaining only the Trp-Ala moiety of the parent [H-D-Phe6,betaAla11,Phe13,Nle14]-peptide amide, and with a very different carboxyl terminal region, has been produced which was potent at BRS3 and essentially had no NMB or GRP receptor activity. Its structure is Ac-Phe-Trp-Ala-His(tauBzl)-Nip-Gly-Arg-NH2.

Design of selective peptidomimetic agonists for the human orphan receptor BRS-3

New tool substances may help to unravel the physiological role of the human orphan receptor BRS-3 and its possible use as a drug target for the treatment of obesity and cancer. In continuation of our work on BRS-3, the solid- and solution-phase synthesis of a library of low molecular weight peptidomimetic agonists based on the recently developed short peptide agonist 4 is described. Functional potencies of the compounds were determined measuring calcium mobilization in a fluorometric imaging plate reader (FLIPR) assay. Focusing on the N-terminus, the d-Phe-Gln moiety of 4 was modified in a combinatorial SAR-oriented medicinal chemistry approach. With the incorporation of N-arylated glycine and alanine building blocks azaglycine, piperazine, or piperidine and the synthesis of semicarbazides and semicarbazones, a number of highly potent and selective compounds with a reduced number of peptide bonds were obtained, which also should have enhanced metabolic stability.

Systematic optimization of a lead-structure identities for a selective short peptide agonist for the human orphan receptor BRS-3

The orphan receptor, human bombesin receptor subtype 3 (BRS-3) was assigned to the G-protein coupled bombesin receptor family because of its high sequence homology with the neuromedin B receptor (NMB-R) and gastrin-releasing peptide receptor (GRP-R). Since its pharmacology is stiIl unknown, new highly potent and selective tool-substances are needed, that may be able to elucidate its possible role in obesity and cancer. We have performed structure activity relationship studies on the high affinity peptide agonists [D-Phe6,beta-Ala11,Phe13,Nle14]Bn(6-14) and [D-Phe6,Phe13]Bn(6-13)propylamide, using their ability to mobilize intracellular calcium in BRS-3 transfected CHOGa-16 cells combined with receptor binding studies. It was demonstrated that for [D-Phe,beta-Ala11,Phe13,Nle14]Bn(6-14) the side chains of the residues Trp8 and Phe13, and to a smaller extent beta-Ala11, are the important amino acid side chains for receptor activation and binding, however for [D-Phe6,Phe13]Bn(6-13) propylamide His12 seems to be more important than Phe13. C-and N-terminal deletions and amino acid substitutions allowed further understanding. It was demonstrated that substitution of His 12 by Tyr leads to a high selectivity towards GRP-R. Using the acquired information, a small tetrapeptide library was designed with compounds presenting Trp and Phe at varying stereochemistry and distances, which led to the discovery of the lead-structure H-D-Phe-Gln-D-Trp-Phe-NH2. Systematic SAR revealed the important structural features of this peptide, C-terminal optimization resulted in the highly active and selective BRS-3 agonist H-D-Phe-Gln-D-Trp-1-(2-phenylethyl)amide. In summary, the size of the peptide was reduced from 8 or 9 amino acids to a tripeptide for BRS-3.