Bombesin antagonists inhibit in vitro and in vivo growth of human gastric cancer and binding of bombesin to its receptors

RC-3095, a gastrin-releasing peptide receptor antagonist, synergizes with gemcitabine to inhibit the growth of human pancreatic cancer CFPAC-1 in vitro and in vivo

OBJECTIVES

Pancreatic cancer remains a lethal disease. In this study, we investigated the efficacy of a combination of gastrin-releasing peptide receptor antagonist RC-3095 and gemcitabine on pancreatic cancer CFPAC-1.

METHODS

The antiproliferation effects of RC-3095, gemcitabine, or the combination on pancreatic cancer were monitored in vitro. Nude mice bearing xenografts of CFPAC-1 cell received injections of the vehicle (control), RC-3095 (20 μg, subcutaneously, daily), gemcitabine (15 mg/kg, intraperitoneally, every 3 days), or the combination of RC-3095 and gemcitabine for 4 weeks. The histological changes and protein expression were tested using immunohistochemistry and Western blotting.

RESULTS

Treatment with the combination in culture exhibited a powerful inhibition effect on CFPAC-1 cell proliferation. In xenograft mice model, RC-3095 or gemcitabine significantly reduced the volume and weight of tumors after 4 weeks of treatment, as compared with controls. The combination more potently inhibited the tumor growth than either agent used individually. Immunohistochemistry and Western blotting showed gastrin-releasing peptide receptor/bombesin receptor subtype-3 positive cells and protein expression in tumors decreased by treatment with RC-3095 or gemcitabine alone or greater in combination.

CONCLUSIONS

Our data suggested that the combination could be considered for the possible new approaches for treatment of pancreatic cancers.

Targeting gastrin-releasing peptide as a new approach to treat aggressive refractory neuroblastomas

BACKGROUND

The overall survival for neuroblastoma remains dismal, in part due to the emergence of resistance to chemotherapeutic drugs. We have demonstrated that gastrin-releasing peptide (GRP), a gut peptide secreted by neuroblastoma, acts as an autocrine growth factor. We hypothesized that knockdown of GRP will induce apoptosis in neuroblastoma cells and potentiate the cytotoxic effects of chemotherapeutic agents.

METHODS

The human neuroblastoma cell lines (JF, SK-N-SH) were transfected with small interfering (si) RNA targeted at GRP. Apoptosis was assessed by DNA fragmentation assay. Immunoblotting was used to confirm molecular markers of apoptosis, and flow cytometry was performed to determine cell cycle arrest after GRP knockdown.

RESULTS

siGRP resulted in an increase in apoptosis in the absence of chemotherapeutic interventions. A combination of GRP silencing and chemotherapeutic drugs resulted in enhanced apoptosis when compared to either of the treatments alone. GRP silencing led to increased expression of proapoptotic proteins, p53 and p21.

CONCLUSION

Silencing of GRP induces apoptosis in neuroblastoma cells; it acts synergistically with chemotherapeutic effects of etoposide and vincristine. GRP knockdown-mediated apoptosis appears to be associated with upregulation of p53 in neuroblastoma cells. Targeting GRP may be postulated as a potential novel agent for combinational treatment to treat aggressive neuroblastomas.

Gastrin-releasing peptide links stressor to cancer progression

INTRODUCTION

Gastrin-releasing peptide (GRP) plays an important role in cancer growth and metastasis; however, the mechanisms of how GRP affects cancer progression are not well understood. Recent studies revealed that chronic stress is a major risk factor for cancer progression, and this effect may be mediated by GRP. In this review, we will discuss the mechanisms and implications of GRP linking stressor to cancer progression.

MATERIALS AND METHODS

We retrieved the studies of the relationship between GRP, stress and cancers through PubMed using systematic methods to search, select, and evaluate the findings.

RESULTS

The results suggested that GRP can mediate the effects of stress on cancers at systemic, tissue and cellular levels: Stress elicits the secretion of GRP in the brain and GRP in turn activates the stress response pathways resulting in an elevation of stress hormones and GRP in the plasma and tissues. GRP in synergy with stress hormones stimulates the growth and invasion of cancer cells by suppressing the anti-tumor immune function and directly activating the pro-proliferative and pro-migratory signaling pathways in cancer cells.

CONCLUSION

GRP is a multi-functional peptide, which acts as a stress mediator as well as a growth factor linking stressor to cancer progression. GRP and its high-affinity receptor are useful targets for the diagnosis and treatment of cancers.

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.

Gastrin-releasing peptide mediates its morphogenic properties in human colon cancer by upregulating intracellular adhesion protein-1 (ICAM-1) via focal adhesion kinase

Gastrin-releasing peptide (GRP) and its receptor (GRPR) act as morphogens when expressed in colorectal cancer (CRC), promoting the assumption of a better differentiated phenotype by regulating cell motility in the context of remodeling and retarding tumor cell metastasis by enhancing cell-matrix attachment. Although we have shown that these processes are mediated by focal adhesion kinase (FAK), the downstream target(s) of GRP-induced FAK activation are not known. Since osteoblast differentiation is mediated by FAK-initiated upregulation of ICAM-1 (Nakayamada S, Okada Y, Saito K, Tamura M, Tanaka Y. J Biol Chem 278: 45368-45374, 2003), we determined whether GRP-induced activation of FAK alters ICAM-1 expression in CRC and, if so, determined the contribution of ICAM-1 to mediating GRP's morphogenic properties. Caco-2 and HT-29 cells variably express GRP/GRPR. These cells only express ICAM-1 when GRPR are present. In human CRC, GRPR and ICAM-1 are only expressed by better differentiated tumor cells, with ICAM-1 located at the basolateral membrane. ICAM-1 expression was only observed subsequent to GRPR signaling via FAK. To study the effect of ICAM-1 expression on tumor cell motility, CRC cells expressing GRP, GRPR, and ICAM-1 were cultured in the presence and absence of GRPR antagonist or monoclonal antibody to ICAM-1. CRC cells engaged in directed motility in the context of remodeling and were highly adherent to the extracellular matrix, only in the absence of antagonist or ICAM-1 antibody. These data indicate that GRP upregulation of ICAM-1 via FAK promotes tumor cell motility and attachment to the extracellular matrix.

A phase I trial of the bombesin/gastrin-releasing peptide (BN/GRP) antagonist RC3095 in patients with advanced solid malignancies

Bombesin/gastrin-releasing peptides (BN/GRP) were shown to bind selectively to cell surface receptors, stimulating the growth of various types of malignancies in murine and human models. The novel BN/GRP synthetic receptor antagonist, RC-3095, was able to produce long-lasting tumor regressions in murine and human tumor models in vitro and in vivo. Animal toxicology studies showed no detectable organ toxicity apart from local irritation at the injection site. The purpose of this study was to determine the safety and feasibility of the administration of RC-3095 by daily subcutaneous injections in patients with advanced and refractory solid malignancies. Twenty-five patients received RC-3095 once or twice-daily at doses ranging from 8 to 96 ug/kg. Dose was escalated in groups of 3-5 patients per dose level. The only toxicity observed was local discomfort in the injection site at the highest doses. A single dose administration of RC-3095 at the highest dose level (96 ug/kg) was tested in a clearly hypergastrinemic individual with the Zollingen-Ellison syndrome and produced a decrease in plasma gastrin down to 50% of basal levels in 6 h. There was no objective tumor responses in patients included in the study. A short-lasting minor tumor response was observed in a patient with a GRP-expressing progressive medullary carcinoma of the thyroid. Due to problems with the analytical method, plasma pharmacokinetic data was obtained only from two patients included at the highest dose level. In these patients, RC-3095 reached plasma concentrations >100 ng/mL for about 8 h, which were within therapeutic levels on the basis of prior data obtained in mice and rats. The plasma elimination half-life was between 8.6-10.9 h. Due to the occurrence of local toxicity at the injection site, the dose escalation procedure could not be fully evaluated up to a maximum tolerated dose. Thus, a recommended dose of RC-3095 for Phase II trials could not be clearly established. Considering the novelty of its mechanism of action and impressive preclinical anti-tumor activity, further studies exploiting new formulations of RC-3095 for human use, such as slow-release preparations, and analogues with a more favorable pharmacokinetics are warranted.

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.

Recent advances in tumor-targeting anticancer drug conjugates

Traditional cancer chemotherapy relies on the premise that rapidly proliferating cancer cells are more likely to be a killed by cytotoxic agent. In reality, however, cytotoxic agents have very little or no specificity, which leads to systemic toxicity, causing severe undesirable side effects. Therefore, various drug delivery protocols and systems have been explored in the last three decades. Tumor cells overexpress many receptors and biomarkers, which can be used as targets to deliver cytotoxic agents into tumors. In general, a tumor-targeting drug delivery system consists of a tumor recognition moiety and a cytotoxic warhead connected directly or through a suitable linker to form a conjugate. The conjugate, which can be regarded as 'prodrug', should be systemically non-toxic. This means that the linker must be stable in circulation. Upon internalization into the cancer cell the conjugate should be readily cleaved to regenerate the active cytotoxic agent. Tumor-targeting conjugates bearing cytotoxic agents can be classified into several groups based on the type of cancer recognition moieties. This review describes recent advances in tumor-targeting drug conjugates including monoclonal antibodies, polyunsaturated fatty acids, folic acid, hyaluronic acid, and oligopeptides as tumor-targeting moieties.

Role of gastrointestinal hormones in the proliferation of normal and neoplastic tissues

Gastrointestinal (GI) hormones are chemical messengers that regulate the physiological functions of the intestine and pancreas, including secretion, motility, absorption, and digestion. In addition to these well-defined physiological effects, GI hormones can stimulate proliferation of the nonneoplastic intestinal mucosa and pancreas. Furthermore, in an analogous fashion to breast and prostate cancer, certain GI cancers possess receptors for GI hormones; growth can be altered by administration of these hormones or by blocking their respective receptors. The GI hormones that affect proliferation, either stimulatory or inhibitory, include gastrin, cholecystokinin, gastrin-releasing peptide, neurotensin, peptide YY, glucagon-like peptide-2, and somatostatin. The effects of these peptides on normal and neoplastic GI tissues will be described. Also, future perspectives and potential therapeutic implications will be discussed.

Expression of gastrin-releasing peptide receptors in endometrial cancer

BACKGROUND

The Bombesin (BBS)-related peptide, gastrin-releasing peptide, and its cognate receptor are ectopically expressed by many cancers, in which they regulate tumor proliferation and metastasis. But, their role in endometrial cancers is unknown. The purpose of this study was to determine whether endometrial cancer cell lines express functional BBS receptors and to determine whether they were coupled to the regulation of vascular endothelial growth factor (VEGF-A) expression.

STUDY DESIGN

Endometrial cancer cell lines (HEC-1A, KLE, and AN3CA) were cultured according to the recommendations of the American Tissue Culture Collection. Ishikawa cells were maintained in Dulbecco's modified Eagle's medium plus 10% fetal bovine serum. Before BBS treatment, all cell lines were placed in serum-free, phenol-free media for 24 hours. BBS-stimulated increases in intracellular Ca(2+) ([Ca(2+)]i) were used to assess functional BBS receptor status. VEGF-A mRNA expression was determined by Northern blotting.

RESULTS

BBS (100 nM) stimulated an increase in [Ca(2+)]i in HEC-1A, Ishikawa, and KLE cells, indicating the presence of functional BBS receptors. This increase did not occur in AN3CA cells. BBS stimulated a time-dependent increase in VEGF-A mRNA expression in Ishikawa and KLE cells. Ishikawa cells exhibited a peak of VEGF-A mRNA expression between 8 and 12 hours with a partial decline by 24 hours. KLE cells showed a relatively small increase at 12 hours. In contrast, HEC-1A cells exhibited a high baseline level of VEGF-A mRNA expression and did not show a response to BBS.

CONCLUSIONS

These data demonstrate that endometrial cancer cell lines express functional BBS receptors. In Ishikawa, KLE, and HEC-1A cells, BBS receptors are coupled to the regulation of VEGF-A mRNA expression.

Gastrin-releasing peptide is a growth factor for human neuroblastomas

OBJECTIVE

To evaluate whether gastrin-releasing peptide (GRP) and GRP receptor (GRP-R) expression correlate with tumor behavior and to examine the mitogenic actions of GRP on neuroblastomas.

SUMMARY BACKGROUND DATA

Neuroblastoma is the most common solid tumor of infants and children. Despite recent advances in multimodality treatment regimens, the survival for advanced-stage tumors remains dismal. Neuroblastomas are known to produce GRP; however, the proliferative effects of GRP on neuroblastomas have not been elucidated.

METHODS

Sections of paraffin-embedded neuroblastomas from 33 patients were analyzed for GRP and GRP-R protein expression by immunohistochemistry. Functional binding of GRP-R to the Ca2+ signaling pathway was examined. In addition, the proliferative effect of GRP on neuroblastoma cells (SK-N-SH, IMR-32, SH-SY5Y, LAN-1) was determined.

RESULTS

Immunohistochemical analysis showed GRP and GRP-R protein expression in neuroblastomas; an increased expression of GRP-R was noted in a higher percentage of undifferentiated tumors compared with tumors that were benign. GRP-R mRNA was confirmed in neuroblastoma cell lines. GRP treatment resulted in intracellular calcium [Ca2+]i mobilization in two cell lines (SK-N-SH, LAN-1). GRP treatment stimulated growth of all four neuroblastoma cell lines; this effect was inhibited in SK-N-SH cells by pretreatment with GRP antibody.

CONCLUSIONS

These findings show increased GRP-R expression in the more aggressive and undifferentiated neuroblastomas. The synchronous expression of GRP and its receptor, GRP-R, suggests a role for these proteins in tumor growth. Moreover, these findings show enhanced proliferation of neuroblastoma cells in vitro after GRP treatment, suggesting that GRP may act as an autocrine and/or paracrine growth factor for neuroblastomas. Treatment with specific GRP-R antagonists may provide novel adjuvant therapy for neuroblastomas in children.

Inhibition of growth of ES-2 human ovarian cancers by bombesin antagonist RC-3095, and luteinizing hormone-releasing hormone antagonist Cetrorelix

We evaluated the effects of the bombesin/gastrin-releasing peptide (GRP) antagonist RC-3095, and the luteinizing hormone-releasing hormone (LH-RH) antagonist Cetrorelix, administered singly or in combination, on the growth of human ovarian carcinoma cell line ES-2, xenografted into nude mice. RC-3095 at a dose of 20 microg/day and Cetrorelix (100 microg/day), significantly reduced the volume of ES-2 tumors by 63.0% (P<0.01) and 38.0% (P<0.05) respectively, after 44 days of treatment, as compared with controls. The combination of RC-3095 with Cetrorelix inhibited the growth of ES-2 tumors by 66.2% (P<0.01). Serum levels of LH were significantly decreased in the groups treated with Cetrorelix alone and/or in combination with RC-3095. RT-PCR analyses revealed that the expression of mRNA for receptors of GRP (GRPR/BRS-1) and Neuromedin B (NMBR/BRS-2) on tumors was significantly decreased in all the treated groups. The expression of mRNA for epidermal growth factor receptors (EGFR) on tumors was reduced by 36.5 % (P<0.05) in the animals treated with Cetrorelix and by 72.5% (P<0.05) in the group that received the combination of RC-3095 with Cetrorelix. Our results indicate that the bombesin antagonist RC-3095 and the LH-RH antagonist Cetrorelix inhibit effectively the growth of ES-2 ovarian cancers in nude mice. These antagonists and their combination could be considered for the therapy of patients with ovarian cancer.

Is there a role for agonist gastrin-releasing peptide receptor radioligands in tumour imaging?

Gastrin-releasing peptide (GRP) has been shown to be a tumour growth stimulating agent for a number of normal and human cancer cell lines. The tumour growth effect is a direct result of GRP binding to membrane G-protein coupled GRP receptors (GRP-R) on the cell surface. Available data on the role of GRP and GRP-R in human lung, prostate, breast, colorectal and gastric carcinoma are reviewed and it is suggested that radiolabelled agonists are preferable to antagonists for imaging and therapy as they appear to be internalised, yielding a higher target/background ratio. The use of rhenium or indium radiolabels for therapy may provide a new approach to GRP/bombesin expressing tumours.

Inhibition of growth of OV-1063 human epithelial ovarian cancers and c- jun and c- fos oncogene expression by bombesin antagonists

Receptors for bombesin are present on human ovarian cancers and bombesin-like peptides could function as growth factors in this carcinoma. Therefore, we investigated the effects of bombesin/gastrin-releasing peptide (GRP) antagonists RC-3940-II and RC-3095 on the growth of human ovarian carcinoma cell line OV-1063, xenografted into nude mice. Treatment with RC-3940-II at doses of 10 microg and 20 microg per day s.c. decreased tumour volume by 60.9% (P< 0.05) and 73.5% (P< 0.05) respectively, after 25 days, compared to controls. RC-3095 at a dose of 20 microg per day reduced the volume of OV-1063 tumours by 47.7% (P = 0.15). In comparison, luteinizing hormone-releasing hormone (LH-RH) antagonist Cetrorelix at a dose of 100 microg per day caused a 64.2% inhibition (P< 0.05). RT-PCR analysis showed that OV-1063 tumours expressed mRNA for bombesin receptor subtypes BRS-1, BRS-2, and BRS-3. In OV-1063 cells cultured in vitro, GRP(14-27) induced the expression of mRNA for c- jun and c- fos oncogenes in a time-dependent manner. Antagonist RC-3940-II inhibited the stimulatory effect of GRP(14-27) on c- jun and c- fos in vitro. In vivo, the levels of c- jun and c- fos mRNA in OV-1063 tumours were decreased by 43% (P< 0.05) and 45% (P = 0. 05) respectively, after treatment with RC-3940-II at 20 microg per day. Exposure of OV-1063, UCI-107 and ES-2 ovarian carcinoma cells to RC-3940-II at 1 microM concentration for 24 h in vitro, extended the latency period for the development of palpable tumours in nude mice. Our results indicate that antagonists of bombesin/GRP inhibit the growth of OV-1063 ovarian cancers by mechanisms that probably involve the downregulation of c- jun and c- fos proto-oncogenes.

Signaling mechanisms regulating bombesin-mediated AP-1 gene induction in the human gastric cancer SIIA

The hormone bombesin (BBS) and its mammalian equivalent gastrin-releasing peptide (GRP) act through specific GRP receptors (GRP-R) to affect multiple cellular functions in the gastrointestinal tract; the intracellular signaling pathways leading to these effects are not clearly defined. Previously, we demonstrated that the human gastric cancer SIIA possesses GRP-R and that BBS stimulates activator protein-1 (AP-1) gene expression. The purpose of our present study was to determine the signaling pathways leading to AP-1 induction in SIIA cells. A rapid induction of c-jun and jun-B gene expression was noted after BBS treatment; this effect was blocked by specific GRP-R antagonists, indicating that BBS is acting through the GRP-R. The signaling pathways leading to increased AP-1 gene expression were delineated using phorbol 12-myristate 13-acetate (PMA), which stimulates protein kinase C (PKC)-dependent pathways, by forskolin (FSK), which stimulates protein kinase A (PKA)-dependent pathways, and by the use of various protein kinase inhibitors. Treatment with PMA stimulated AP-1 gene expression and DNA binding activity similar to the effects noted with BBS; FSK stimulated jun-B expression but produced only minimal increases of c-jun mRNA and AP-1 binding activity. Pretreatment of SIIA cells with either H-7 or H-8 (primarily PKC inhibitors) inhibited the induction of c-jun and jun-B mRNAs in response to BBS, whereas H-89 (PKA inhibitor) exhibited only minimal effects. Pretreatment with tyrphostin-25, a protein tyrosine kinase (PTK) inhibitor, attenuated the BBS-mediated induction of c-jun and jun-B, but the effect was not as pronounced as with H-7. Collectively, our results demonstrate that BBS acts through its receptor to produce a rapid induction of both c-jun and jun-B mRNA and AP-1 DNA binding activity in the SIIA human gastric cancer. Moreover, this induction of AP-1, in response to BBS, is mediated through both PKC- and PTK-dependent signal transduction pathways with only minimal involvement of PKA.

Presence of receptors for bombesin/gastrin-releasing peptide and mRNA for three receptor subtypes in human prostate cancers

BACKGROUND

Bombesin-like peptides can function as autocrine or paracrine growth factors and stimulate the growth of some cancer cells, including human prostate cancer. Three bombesin receptor subtypes, termed gastrin-releasing peptide receptor (GRPR), neuromedin B receptor (NMBR), and bombesin receptor subtype 3 (BRS-3), have been identified in rodents and humans.

METHODS

We investigated the presence and characteristics of the functional receptors for bombesin/GRP in human prostate adenocarcinoma specimens by radio-receptor assay and the mRNA expression of the three bombesin receptor subtypes by RT-PCR.

RESULTS

Of the 80 specimens of primary prostate cancer examined by receptor binding assays, 50 ( approximately 63%) showed high-affinity, low-capacity binding sites for bombesin/GRP, and 12 of these 50 receptor-positive specimens also showed a second binding site. Of the 22 prostate cancer specimens analyzed by RT-PCR, 20 (91%) expressed GRPR mRNA, 3 (14%) showed NMBR mRNA, and 2 ( approximately 9%) revealed BRS-3 mRNA. No correlation was observed between receptor expression and patients' age or pathological data.

CONCLUSIONS

The detection of a wide distribution of bombesin/GRP receptors in human prostate carcinomas supports the view that they may be involved in modulation of tumor progression and suggests that approaches based on binding of bombesin receptor antagonists or new targeted cytotoxic bombesin analogs to prostate cancers could be considered for the therapy.

Pre-clinical evaluation of [(111)In-DTPA-Pro(1), Tyr(4)]bombesin, a new radioligand for bombesin-receptor scintigraphy

Bombesin (BN) is a 14-amino-acid neuropeptide with a high affinity for the gastrin-releasing peptide receptor. This receptor has been found to be expressed in a variety of tumours, including lung, breast, prostate and pancreas. A newly synthesized BN analogue, [DTPA-Pro(1),Tyr(4)]BN, was shown to be a high-affinity BN-receptor (BNR) agonist, stimulating prolactin secretion from 7315b cells with an IC(50) of 8 nM. The (111)In-labelled analogue was found to bind with high affinity to rat BNR in vitro and in vivo. The radioligand is internalized by BNR-expressing cells, in contrast to DTPA-conjugated BN antagonists. Therefore, we further studied the biodistribution of i.v. injected [(111)In-DTPA-Pro(1),Tyr(4)]BN in rats. High and specific uptake was found in tissues of the gastrointestinal tract, notably pancreas. Uptake of radioactivity was blocked by pre- or co-injection of 100 microgram [Tyr(4)]BN, but not when this was administered 30 min after the radioligand. This suggests BNR-mediated internalization of the radioligand within 30 min. The percentage injected dose (ID) taken up by BNR-positive tissues was a bell-shaped function of the amount (0.01-0.1 microgram) of injected ligand. Next to the pancreas, highest uptake was observed in the kidneys, which was not blocked by excess [Tyr(4)]BN. Dynamic gamma camera studies showed rapid clearance of radioactivity from the blood compartment. Urinary excretion amounted to about 35% ID after 1 hr and to 70% ID after 24 hr, with a total body retention of 10% ID. Specific uptake was found in the BNR-positive CA20948 pancreas tumour and CC531 colon carcinoma in tumour-bearing rats. The CA20948 tumour, inoculated in the hindleg, was also visualized scintigraphically. [(111)In-DTPA-Pro(1), Tyr(4)]BN appears to be a promising radioligand for scintigraphy of BNR-expressing tumours.

Inhibition of growth of human malignant glioblastoma in nude mice by antagonists of bombesin/gastrin-releasing peptide

The effects of antagonists of bombesin/gastrin-releasing peptide (GRP) on the growth of human malignant glioblastoma cell line U-87MG xenografted into nude mice were evaluated. Nude mice bearing s.c. implanted U-87MG tumors were treated with bombesin/GRP antagonists RC-3095 and RC-3940-II. RC-3095 and RC-3940-II administered s.c. at a dose of 20 micrograms/day for 4 weeks decreased the volume of U-87MG xenografts by 60 and 74%, respectively, compared with controls. RT-PCR analysis showed that U-87MG xenografts expressed mRNA for bombesin receptor subtype (BRS)-1 (GRP receptor) and BRS-2 (neuromedin-B receptor), but the mRNA for GRP ligand was not detected in U-87MG cells suggesting that GRP may stimulate the growth of U-87MG glioblastomas by a paracrine mechanism. The levels of mRNA for c-fos oncogene were decreased by 30-40% in U-87MG tumors treated with RC-3095 or RC-3940-II. In U-373MG glioblastoma cells, which also express BRS-1, and U-87MG cells, cultured in vitro, GRP(14-27) induced the expression of c-fos mRNA, and some c-jun mRNA, in a time-dependent manner with the maximal effect occurring 2 h after the stimulation and a return to basal levels after 8 h. Antagonist RC-3940-II inhibited the stimulation of c-fos by GRP(14-27). Our results indicate that antagonists of bombesin/GRP inhibit the growth of U-87MG glioblastomas by a mechanism that may involve the downregulation of c-fos oncogene.

Characterization of gastrin-releasing peptide receptors aberrantly expressed by non-antral gastric adenocarcinomas

Epithelial cells lining the GI tract except in the gastric antrum do not normally express gastrin-releasing peptide receptors (GRP-R). Because GRP-R activation causes the proliferation of many GI cancer cell lines, aberrant expression has been presumed to negatively influence patient survival. We therefore determined the incidence and quality of GRP-R aberrantly expressed by non-antral gastric adenocarcinomas, and evaluated the impact of receptor expression on patient survival. We studied RNA isolated from 20 consecutive non-antral gastric adenocarcinomas, and determined that 8 (40%) aberrantly expressed GRP-R. Of these, 6 (75%) were found to be mutated. Pharmacologically, the effect of these mutations ranged from rendering the GRP-R non-functional to constitutively active. Contrary to expectations, however, survival of patients whose tumor expressed functional GRP-R (18.5 +/- 9.8 months) was not statistically different from those that did not (8.3 +/- 1.8 months; p = 0.24). Thus our data indicate that mutated isoforms of GRP-R are commonly expressed by non-antral gastric adenocarcinomas. However, expression of functional GRP-R does not alter patient survival, suggesting that this receptor may not be clinically important to the growth of gastric cancers.

Bombesin/gastrin-releasing peptide antagonists RC-3095 and RC-3940-II inhibit tumor growth and decrease the levels and mRNA expression of epidermal growth factor receptors in H-69 small cell lung carcinoma

BACKGROUND

Antagonists of bombesin/gastrin-releasing peptide (BN/GRP) have been developed to block the autocrine stimulatory effect of BN/GRP on tumors such as small cell lung carcinoma (SCLC). Although several studies have addressed the intracellular events that follow the formation of the receptor-ligand complex, the mechanism of action of BN/GRP antagonists remains unclear.

METHODS

In this study the authors investigated the effect of synthetic BN/GRP antagonists RC-3095 and RC-3940-II on tumor growth and the expression of epidermal growth factor receptors (EGF-R) in H-69 SCLC. Athymic nude mice xenografted with H-69 SCLC were treated subcutaneously for 5 weeks with RC-3095 and RC-3940-II at the dose of 10 microg/animal/day.

RESULTS

RC-3095 decreased tumor volume by approximately 50% (P < 0.05) and RC-3940-II by 70-60% (P < 0.01). Tumor burden also was significantly decreased in the groups treated with RC-3095 and RC-3940-II. Receptor analyses demonstrated high affinity binding sites for BN/GRP and EGF on the untreated H-69 SCLC tumors. After treatment with RC-3095 and RC-3940-II, the concentration of receptors for BN/GRP was decreased by 29.0% and 36.5%, respectively (both, P < 0.01) compared with controls, and EGF-R levels were reduced by 62.3% and 63.0%, respectively (both, P < 0.01). Reverse transcriptase-polymerase chain reaction and Southern blot analyses revealed that the levels of mRNA for EGF-R in tumors were lowered by 31% (P < 0.05) and 43% (P < 0.01), respectively, after treatment with RC-3095 and RC-3940-II.

CONCLUSIONS

This study indicates that the inhibition of growth of H-69 SCLC by BN/GRP antagonists RC-3095 and RC-3940-II is accompanied by a marked decrease in the levels and mRNA expression of EGF-R.

Inhibition of growth of MDA-MB-231 human breast cancer xenografts in nude mice by bombesin/gastrin-releasing peptide (GRP) antagonists RC-3940-II and RC-3095

Bombesin or gastrin-releasing peptide (GRP) may act as autocrine growth factors and play a role in the initiation and progression of breast cancer. We investigated the effect of bombesin/GRP antagonists RC-3095 and RC-3940-II on the growth of the MDA-MB-231 oestrogen-independent human breast cancer cell line xenografted into female nude mice. Bombesin/GRP antagonists, RC-3095 and RC-3940-II, were administered subcutaneously twice daily at a dose of 10 micrograms for 5 weeks. The growth of MDA-MB-231 tumours was inhibited during the treatment, as shown by a reduction in tumour volume. RC-3940-II and RC-3095 significantly decreased the final tumour volume by 72.4% and 57.7%, respectively, and greatly reduced tumour weights. RC-3940-II also significantly increased tumour doubling time and appeared to be more effective than RC-3095 in inhibiting the growth of MDA-MB-231 breast cancers. Serum gastrin and insulin-like growth factor-I (IGF-I) levels in animals treated with RC-3095 or RC-3940-II showed no significant changes as compared with controls. There was a significant decrease in the number of binding sites for epidermal growth factor (EGF), as well as bombesin, in tumour cells after chronic treatment with RC-3095 or RC-3940-II, which might be related to inhibition of tumour growth. Reverse transcription polymerase chain reaction, followed by Southern blot analysis, also showed a reduction in the expression of mRNA for EGF receptors in the group treated with RC-3940-II. Our findings suggest that bombesin/GRP antagonists such as RC-3095 or RC-3940-II could be considered for endocrine therapy for oestrogen-independent breast cancers, but further investigations are necessary.

Constitutive activation of the gastrin-releasing peptide receptor expressed by the nonmalignant human colon epithelial cell line NCM460

Gastrin-releasing peptide (GRP) causes multiple effects in humans by activating a specific heptaspanning receptor. Within the gastrointestinal tract, GRP receptors (GRP-R) are not normally expressed by mucosal epithelial cells except for those lining the gastric antrum. In contrast, recent studies have shown that up to 40% of resected colon cancers aberrantly express this receptor. This is important because the GRP-R can cause the proliferation of many, but not all, tissues in which it is expressed. Since GRP and other agonists are not known to exist in the colonic lumen, it has not been clear how or even if GRP-R expression in colon cancer contributes to cell proliferation. To evaluate the functional consequence of GRP-R expression on colonic epithelium, we transfected the recently isolated nonmalignant human colon epithelial cell line NCM460 with the cDNA for this receptor. All NCM460 cell lines expressing varying numbers of GRP-R bound selected agonists and antagonists indistinguishably from receptors expressed by other human tissues. Furthermore GRP-R-expressing transfected cell lines, but not wild-type NCM460 cells, proliferated independently of serum or other growth factors. Further evaluation revealed that GRP-R in these cells tonically stimulated G alpha q/11, resulting in increased phospholipase C activation. Since transfected cells do not secrete GRP, nor is their growth influenced by exposure to receptor-specific antagonists, these data indicate that GRP-R ectopically expressed by NCM460 cells are constitutively active. This report provides the first evidence of mutation-independent heptaspanning receptor constitutive activation resulting in cell proliferation, and identifies a potential mechanism whereby the GRP-R may act as an oncogene in human colon cancer.

Luteinizing hormone-releasing hormone antagonist Cetrorelix (SB-75) and bombesin antagonist RC-3940-II inhibit the growth of androgen-independent PC-3 prostate cancer in nude mice

BACKGROUND

Hormones like bombesin (BN)/gastrin-releasing peptide (GRP) and luteinizing hormone-releasing hormone (LH-RH) and growth factors such as epidermal growth factor (EGF) might be involved in the relapse of prostate cancer under androgen ablation therapy. Interference with receptors for BN/GRP, LH-RH, or EGF might provide a therapeutic approach to inhibit tumor growth of androgen-independent prostate cancer.

METHODS

LH-RH antagonist Cetrorelix (SB-75) and the BN/GRP antagonist RC-3940-II were tested for their effects on the growth of the androgen-independent PC-3 human prostate cancer cell line xenografted into nude mice. Tumor growth, serum hormone levels, and receptor concentrations for BN/GRP and EGF were measured.

RESULTS

When the treatment was started, tumor volume in all groups was 70-80 mm3. After 4 weeks, tumor volume in the control animals injected with saline was 871 +/- 233 mm3 and that of animals treated with Cetrorelix only 197 +/- 61 mm3. The BN/GRP antagonist RC-3940-II also significantly reduced PC-3 tumor volume in nude mice to 122 +/- 20 mm3. The combination of Cetrorelix and RC-3940-II produced no additional inhibition. High-affinity receptors for EGF were detected in the tumor membranes and their number was significantly decreased after administration of Cetrorelix or RC-3940-II.

CONCLUSIONS

These findings demonstrate that LH-RH antagonists and BN/GRP antagonists inhibit the growth of the androgen-independent prostate cancer cell line PC-3 in vivo. Both analogs may exert a direct inhibitory effect on tumor growth through a down-regulation of EGF receptors.

Reduction in receptors for bombesin and epidermal growth factor in xenografts of human small-cell lung cancer after treatment with bombesin antagonist RC-3095

Antagonists of bombesin/gastrin-releasing peptide (BN/GRP) have been developed to inhibit the stimulatory effects of BN/GRP on the mitogenesis of tumor cells such as human small-cell lung carcinoma (SCLC). The mode of action of these antagonists is not completely understood. In this study, we evaluated the effect of BN/GRP antagonist RC-3095 on receptors for BN/GRP and epidermal growth factor (EGF) in H-128 human SCLC line xenografted into nude mice. Treatment with RC-3095, administered s.c. at a dose of 20 microg/day per animal for 4 weeks caused a 70% reduction in tumor volume and weight. Membrane receptors for BN/GRP and EGF were characterized in untreated and treated animals. In the control group, [125I-Tyr4]BN was bound to a single class of specific, high affinity binding sites with a dissociation constant (Kd) = 6.55 +/- 0.93 nM and maximal binding capacity (Bmax) = 512.8 +/- 34.8 fmol/mg membrane protein. Therapy with RC-3095 decreased the concentration of BN/GRP receptors on H-128 SCLC tumor membranes. Specific, high affinity binding sites for EGF with Kd = 1.78 +/- 0.26 nM and Bmax = 216.8 +/- 19.6 fmol/mg membrane protein were also found on the untreated H-128 SCLC tumors. Treatment with RC-3095 significantly decreased Bmax of receptors for EGF. Our results indicate that the suppression of growth of H-128 SCLC by BN antagonist RC-3095 is accompanied by a decrease in the number of receptors for both BN/GRP and EGF. These observations are in agreement with the results obtained in other experimental cancers. The findings on antagonist RC-3095 reinforce the view that both BN/GRP and EGF receptors participate in a cascade of events involved in the growth of SCLC and other cancers. Although the complete mechanisms of action of antagonist RC-3095 remain to be elucidated, the antitumor effect could be the result of the fall in the EGF receptor number, which might lead to a decrease in EGF receptor autophosphorylation.

Design, synthesis, and in vitro evaluation of cytotoxic analogs of bombesin-like peptides containing doxorubicin or its intensely potent derivative, 2-pyrrolinodoxorubicin

Five peptide fragments, based on the C-terminal sequence of bombesin (BN)-(6-14) or BN-(7-14), were selected as carriers for radicals doxorubicin (DOX) and 2-pyrrolino-DOX to create hybrid cytotoxic analogs. All these compounds had a reduced peptide bond (CH2-NH or CH2-N) between positions 13 (Phe or Leu) and 14 (Phe, Leu, or Tac) (Tac = thiazolidine-4-carboxylic acid). Three pseudononapeptide carriers contained N-terminal D-Phe or D-Tpi at position 6 (Tpi = 2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylic acid). Two pseudooctapeptides had Gln7 at the N terminus. The conjugation of N-(9-fluorenylmethoxycarbonyl) doxorubicin (N-Fmoc-DOX)-14-O-hemiglutarate to the peptide carriers at the N terminus resulted in cytotoxic hybrids of BN-like peptides containing DOX. These hybrids could then be converted to analogs with 2-pyrrolino-DOX by a reaction with 4-iodobutyraldehyde. The ability of the carriers and the conjugates to inhibit the binding of 125I-labeled [Tyr4]BN to receptors for BN/gastrin releasing peptide (GRP) on Swiss 3T3 cells was determined. Cytotoxic conjugates of pseudooctapeptide carrier analogs displayed the highest binding affinity (KD approximately 1 nM). The cytotoxic BN analogs and their corresponding cytotoxic radicals exerted similar inhibitory effects on the in vitro growth of CFPAC-1 human pancreatic cancer, DMS-53 human lung cancer, PC-3 human prostate cancer, and MKN-45 human gastric cancer cell lines that have receptors for BN/GRP. In DMS-53 cells, the activity of 2-pyrrolino-DOX and its conjugates was approximately 2500 times higher than that of DOX and its hybrids. These highly potent cytotoxic analogs of BN have been designed as targeted anti-tumor agents for the treatment of various cancers that possess receptors for BN/GRP.

Location and characterization of the human GRP receptor expressed by gastrointestinal epithelial cells

The exact location of normal gastrin-releasing peptide (GRP) receptor expression by epithelial cells lining the human gastrointestinal (GI) tract is not known; yet this receptor is found on upwards of 50% of GI cancers. Furthermore, the pharmacology reported for GRP receptors expressed by GI cancers varies considerably. Therefore, the purpose of this study was to determine the normal distribution of GRP receptor expression by cells lining the human GI tract, and then determine the normal pharmacology of the human receptor when ectopically expressed by the nonmalignant human colon epithelial cell line NCM460. We obtained endoscopic pinch biopsies of, and extracted the RNA from, epithelial cells lining the esophagus, stomach, jejunum, ileum, and proximal and descending colon, RT-PCR demonstrated that GRP-R expression is limited to cells lining the gastric antrum, indicating that this receptor is aberrantly expressed by GI cancers. To determine the normal pharmacology of this receptor when expressed by nonmalignant human tissues for the first time, we transfected NCM460 cells with the cDNA for the human GRP receptor. By studying three stable NCM460 cell lines expressing varying numbers of receptors, we demonstrate that agonist and antagonist binding affinity, binding kinetics, and G-protein coupling are all independent of receptor number. Finally, by comparing GRP receptors expressed by GI cancers with those on NCM460-transfected cells, we show that the pharmacology of the aberrantly expressed receptors is significantly altered. Thus, these data demonstrate that GI cancers aberrantly express GRP receptors that then behave abnormally.

Pharmacological profiles of two bombesin analogues in cells transfected with human neuromedin B receptors

We examined the effect of two des-Met-bombesin analogues, [(CH3)2CHCO-His-Trp-Ala-Val-D-Ala-His-Leu-NHCH3] (ICI 216140) and [D-Phe6,des-Met14]bombesin(6-14) ethylamide (DPDM-bombesin ethylamide), on neuromedin B-induced Ca2+ and [3H]arachidonate release in BALB 3T3 cells transfected with human neuromedin B receptors. ICI 216140 and DPDM-bombesin ethylamide both stimulated Ca2+ mobilisation in a concentration-dependent manner but were less potent and efficacious than neuromedin B. BIM 23042 [D-Nal-Cys-Tyr-D-Trp-Lys-Val-Cys-Nal-NH2], a selective neuromedin B antagonist and [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P, a broad-spectrum peptide receptor antagonist inhibited neuromedin B-, ICI 216140 and DPDM-bombesin ethylamide-induced Ca2+ release. Pretreatment of cells with either des-Met-bombesin analogue attenuated neuromedin B-induced Ca2+ elevations, suggesting similar agonist-sensitive Ca2+ pools. The pharmacological profiles revealed from the [3H]arachidonate assay were similar, although ICI 216140 was less potent and efficacious than DPDM-bombesin ethylamide. The data suggest that ICI 216140 and DPDM-bombesin ethylamide behave as agonists at the neuromedin B receptor, perhaps as a consequence of neuromedin B receptor overexpression.

Correlation between the effects of bombesin antagonists on cell proliferation and intracellular calcium concentration in Swiss 3T3 and HT-29 cell lines

Bombesin (BN) acts as an autocrine mitogen in various human cancers. Several pseudononapeptide BN-(6-14) analogs with a reduced peptide bond between positions 13 and 14 have been shown to suppress the mitogenic activity of BN or gastrin-releasing peptide (GRP) when assessed by radioreceptor or proliferation assays and may have significant clinical applications. The search for potent and safe BN antagonists requires the evaluation of a large series of analogs in radioreceptor and proliferation assays. In this paper, we report that the ability of BN analogs to inhibit BN-induced calcium transients in Swiss 3T3 cells shows a high correlation with their inhibitory potency as evaluated by classical proliferation tests. The assay of calcium transients allows a rapid characterization of new BN analogs (in terms of minutes rather than days) and can be adapted as a labor and cost-effective screening step in the selection of potentially relevant BN antagonists for further characterization in cell proliferation systems. We also observed that results from the assay of calcium transients in Swiss 3T3 cells can be correlated with the results of the proliferative response in HT-29 cells, a cell line that does not seem to use the same early transmembrane ionic signal system. This result suggests that the calcium pathway is not mandatory for triggering cell division by the BN receptor.

Antagonists of bombesin/gastrin-releasing peptide inhibit growth of SW-1990 human pancreatic adenocarcinoma and production of cyclic AMP

We investigated the effects of bombesin/GRP antagonists RC-3095 and RC-3940-II on the growth of SW-1990 human pancreatic adenocarcinoma cells xenografted into nude mice or cultured in vitro. Nude mice implanted with SW-1990 tumors received s.c. injections of RC-3095 and RC-3940-II or the vehicle (control) for 28 days. Chronic administration of RC-3940-II inhibited the growth of SW-1990 tumors, as shown by a reduction in tumor volume during the treatment and a significant increase in tumor doubling time. RC-3940-II decreased final tumor volume by 57.7% and tumor growth rate by 65%. Final tumor weights in mice treated with RC-3940-II were 75% lower than in controls. Treatment with RC-3095 induced smaller, and not significant, decreases in tumor volume and weight. In cell cultures, both RC-3095 and RC-3940-II effectively inhibited the proliferation of SW-1990 cells, inducing a dose- and time-dependent decrease in the number of cells. RC-3940-II again suppressed in vitro growth of SW-1990 cells more effectively than RC-3095. After 72 hr of culture, RC-3940-II and RC-3095 at 1 microM concentrations decreased cell numbers by 45.7% and 27.7%, respectively. The estimated EC50 value for RC-3940-II was 1 nM. When SW-1990 cells were cultured in the presence of 1 nM and 10 nM RC-3095 for 72 hr, cAMP levels in the incubation medium were decreased to 77.3% and 26.9% of the control value. Our results indicate that bombesin/GRP antagonist RC-3940-II can inhibit the proliferation of SW-1990 human pancreatic adenocarcinoma cells in vivo and in vitro. Our findings also suggest that this effect may involve the intracellular cAMP pathway.

Development of a radioimmunoassay for a pseudononapeptide bombesin/GRP antagonist with antitumor activity

Bombesin-like and GRP-like peptides may act as autocrine growth factors in the proliferation of some cancers. A pseudononapeptide bombesin antagonist, [D-Tpi6,Leu13 psi(CH2NH)-Leu14]bombesin(6-14), and related analogs synthesized in our laboratory significantly inhibit tumor growth in various cancer models. A radio-immunoassay (RIA), suitable for determination of RC-3095 and its congeners in unextracted serum, was developed in order to facilitate further experimental and clinical evaluation of this bombesin/GRP receptor antagonist for the treatment of various tumors. Antibodies were generated against RC-3095 and Des-Tpi1-RC-3095, conjugated to bovine serum albumin with glutaraldehyde. Antiserum JH-631b was selected for further experiments based on the antibody characterization. At an antiserum dilution of 1:189,000, this antibody bound approximately 50% of 7 fmol of added radiolabeled Tyr1-RC-3095. The antibody crossreacted with C-terminal fragments of RC-3095. Fragments without the C-terminus and naturally existing peptides of the bombesin family or structurally unrelated peptides did not cross-react. The minimum detectable dose of RC-3095 was 0.4 pg/tube. Intra- and interassay coefficients of variation ranged from 3.2 to 4.4% and from 5.6 to 12.8%, respectively. The RIA is suitable for direct determination of RC-3095 in serum. The RIA should be of value for monitoring levels of this analog in serum during long-term therapy.

Potent bombesin antagonists with C-terminal Leu-psi(CH2-N)-Tac-NH2 or its derivatives

Various pseudononapeptide bombesin (BN)-(6-14) antagonists with a reduced peptide bond (CH2-NH) between positions 13 and 14 can suppress the mitogenic activity of BN or gastrin-releasing peptide in 3T3 fibroblast cells and small cell lung carcinoma. In the search for more potent BN antagonists, 10 modified nonapeptide BN antagonists containing N-terminal D-Phe, D-Cpa, and D- or L-Tpi and C-terminal Leu-psi(CH2-N)-Tac-NH2, Leu-psi(CH2-N)-MeTac-NH2, or Leu-psi(CH2-N)-Me2Tac-NH2 have been synthesized by incubating [13 psi 14,CH2-NH,Cys14]BN-(6-14) or [13 psi 14-CH2-NH,Pen14]BN-(6-14) with formaldehyde or acetaldehyde (Cpa = 4-chlorophenylalanine, Tac = thiazolidine-4-carboxylic acid, Tpi = 2,3,4,9-tetrahydro-1H- pyrido[3,4-b]indol-3-carboxylic acid, and Pen = penicillamine). The biological activities of these compounds were then evaluated. [D-Phe6,13 psi 14,CH2-N,Tac14]BN-(6-14) (RC-3950-II) and [D-Phe6,13 psi 14,CH2-N,Me2Tac14]BN-(6-14) (RC-3985-II) exhibited greater potency in inhibition of 125I-labeled [Tyr4]BN binding to Swiss 3T3 cells than their parent compounds [D-Phe6,13 psi 14,CH2-NH,Cys14]BN-(6-14) (RC-3950-I) and [D-Phe6,13 psi 14,CH2-NH,Pen14]BN-(6-14) (RC-3985-I). The order of binding affinities of these compounds was as follows: [13 psi 14,CH2-N,Tac14]BN-(6-14) > [13 psi 14,CH2-N,Me2Tac14]BN-(6-14) > [13 psi 14,CH2-N,MeTac14]BN-(6-14). In most cases, the analogs with C-terminal Leu-psi(CH2-N)-Tac-NH2 were also more potent growth inhibitors of 3T3 cells than compounds containing C-terminal Leu-psi(CH2-N)-Me2Tac-NH2 or Leu-psi(CH2-N)-MeTac-NH2. The best BN antagonists of this series, RC-3950-II and [D-Cpa6,13 psi 14,CH2-N,Tac14]BN- (6-14) (RC-3925-II), inhibited gastrin-releasing peptide-stimulated growth of Swiss 3T3 cells with IC50 values of 1 nM and 6 nM, respectively. Since antagonists of this class inhibit growth of various tumors in animal cancer models, some of them may have clinical applications.

Characterization of bombesin/gastrin-releasing peptide receptors in membranes of MKN45 human gastric cancer

Binding of the radiolabeled bombesin analog [125I-Tyr4]bombesin to crude cell membranes of MKN45 human gastric cancer grown in nude mice was investigated in vitro. Scatchard analyses of multipoint binding data, performed by complete displacement method demonstrated the presence of two classes of [Tyr4]bombesin binding sites. The high-affinity binding sites had a mean dissociation constant (Kd1) of 2.75 nM with a mean maximal binding capacity (Bmax1) of 492 fmol/mg membrane protein, while the low-affinity binding sites showed a mean dissociation constant (Kd2) of 0.41 microM with a mean maximal binding capacity (Bmax2) of 41.4 pmol/mg membrane protein. Binding of [125(1)-Tyr4]bombesin was specific, reversible and linearly related to the protein concentration of tumor membrane. In displacement studies, the binding of radiolabeled [Tyr4]bombesin was inhibited in a dose-dependent manner by gastrin releasing peptide (GRP)(14-27) and two synthetic antagonists of bombesin/GRP, RC-3095 and RC-3950-II. Both antagonists exhibited high affinity in nearly the same concentration range as GRP(14-27). The presence of receptors for bombesin/GRP on human gastric cancer membranes suggests that bombesin-like peptides may play a role in growth of gastric cancer.

Inhibitory effect of bombesin/gastrin-releasing peptide (GRP) antagonists RC-3950-II and RC-3095 on MCF-7 MIII human breast cancer xenografts in nude mice

Bombesin/gastrin-releasing peptide (GRP) may be involved in the growth of human breast cancers. Nude mice bearing xenografts of MCF-7 MIII human breast cancer cell line were treated for 7 weeks with bombesin/GRP antagonists RC-3950-II and RC-3095. RC-3950-II, administered sc twice daily at a dose of 10 micrograms, produced significant inhibitory effects on tumor growth after 2 weeks of administration. RC-3095 acetate (D 22213), injected sc twice daily at the same dose of 10 micrograms, suppressed tumor growth after 4 weeks. Both RC-3950-II and RC-3095 significantly decreased the final tumor volume and tumor weights. RC-3950-II appeared to be somewhat more efficacious than RC-3095 in inhibiting the growth of MCF-7 MIII breast cancers. Chronic treatment with either bombesin/GRP antagonist caused down-regulation of receptors for epidermal growth factor (EGF) in tumor cell membranes, which might be related to inhibition of tumor growth. These findings suggest that bombesin/GRP antagonists should be considered for a new endocrine therapy of breast cancer.