Preclinical evaluation of therapeutic effects of targeted cytotoxic analogs of somatostatin and bombesin on human gastric carcinomas

Somatostatin and Somatostatin Receptors in Tumour Biology

Somatostatin (SST), a growth hormone inhibitory peptide, is expressed in endocrine and non-endocrine tissues, immune cells and the central nervous system (CNS). Post-release from secretory or immune cells, the first most appreciated role that SST exhibits is the antiproliferative effect in target tissue that served as a potential therapeutic intervention in various tumours of different origins. The SST-mediated in vivo and/or in vitro antiproliferative effect in the tumour is considered direct via activation of five different somatostatin receptor subtypes (SSTR1-5), which are well expressed in most tumours and often more than one receptor in a single cell. Second, the indirect effect is associated with the regulation of growth factors. SSTR subtypes are crucial in tumour diagnosis and prognosis. In this review, with the recent development of new SST analogues and receptor-specific agonists with emerging functional consequences of signaling pathways are promising therapeutic avenues in tumours of different origins that are discussed.

An Insight into GPCR and G-Proteins as Cancer Drivers

G-protein-coupled receptors (GPCRs) are the largest family of cell surface signaling receptors known to play a crucial role in various physiological functions, including tumor growth and metastasis. Various molecules such as hormones, lipids, peptides, and neurotransmitters activate GPCRs that enable the coupling of these receptors to highly specialized transducer proteins, called G-proteins, and initiate multiple signaling pathways. Integration of these intricate networks of signaling cascades leads to numerous biochemical responses involved in diverse pathophysiological activities, including cancer development. While several studies indicate the role of GPCRs in controlling various aspects of cancer progression such as tumor growth, invasion, migration, survival, and metastasis through its aberrant overexpression, mutations, or increased release of agonists, the explicit mechanisms of the involvement of GPCRs in cancer progression is still puzzling. This review provides an insight into the various responses mediated by GPCRs in the development of cancers, the molecular mechanisms involved and the novel pharmacological approaches currently preferred for the treatment of cancer. Thus, these findings extend the knowledge of GPCRs in cancer cells and help in the identification of therapeutics for cancer patients.

Exploiting cancer's phenotypic guise against itself: targeting ectopically expressed peptide G-protein coupled receptors for lung cancer therapy

Lung cancer, claiming millions of lives annually, has the highest mortality rate worldwide. This advocates the development of novel cancer therapies that are highly toxic for cancer cells but negligibly toxic for healthy cells. One of the effective treatments is targeting overexpressed surface receptors of cancer cells with receptor-specific drugs. The receptors-in-focus in the current review are the G-protein coupled receptors (GPCRs), which are often overexpressed in various types of tumors. The peptide subfamily of GPCRs is the pivot of the current article owing to the high affinity and specificity to and of their cognate peptide ligands, and the proven efficacy of peptide-based therapeutics. The article summarizes various ectopically expressed peptide GPCRs in lung cancer, namely, Cholecystokinin-B/Gastrin receptor, the Bombesin receptor family, Bradykinin B1 and B2 receptors, Arginine vasopressin receptors 1a, 1b and 2, and the Somatostatin receptor type 2. The autocrine growth and pro-proliferative pathways they mediate, and the distinct tumor-inhibitory effects of somatostatin receptors are then discussed. The next section covers how these pathways may be influenced or 'corrected' through therapeutics (involving agonists and antagonists) targeting the overexpressed peptide GPCRs. The review proceeds on to Nano-scaled delivery platforms, which enclose chemotherapeutic agents and are decorated with peptide ligands on their external surface, as an effective means of targeting cancer cells. We conclude that targeting these overexpressed peptide GPCRs is potentially evolving as a highly promising form of lung cancer therapy.

G protein-coupled receptors as promising cancer targets

G protein-coupled receptors (GPCRs) regulate an array of fundamental biological processes, such as growth, metabolism and homeostasis. Specifically, GPCRs are involved in cancer initiation and progression. However, compared with the involvement of the epidermal growth factor receptor in cancer, that of GPCRs have been largely ignored. Recent findings have implicated many GPCRs in tumorigenesis, tumor progression, invasion and metastasis. Moreover, GPCRs contribute to the establishment and maintenance of a microenvironment which is permissive for tumor formation and growth, including effects upon surrounding blood vessels, signaling molecules and the extracellular matrix. Thus, GPCRs are considered to be among the most useful drug targets against many solid cancers. Development of selective ligands targeting GPCRs may provide novel and effective treatment strategies against cancer and some anticancer compounds are now in clinical trials. Here, we focus on tumor related GPCRs, such as G protein-coupled receptor 30, the lysophosphatidic acid receptor, angiotensin receptors 1 and 2, the sphingosine 1-phosphate receptors and gastrin releasing peptide receptor. We also summarize their tissue distributions, activation and roles in tumorigenesis and discuss the potential use of GPCR agonists and antagonists in cancer therapy.

The use of peptide analogs for the treatment of gastrointestinal, pancreatic, liver and urinary bladder cancers

Peptide hormones can influence the development and growth of many cancers which are not considered classical hormone-dependent tumors. Analogs of somatostatin, bombesin/gastrin-releasing peptide (GRP), luteinizing hormone-releasing hormone (LH-RH) and growth hormone-releasing hormone (GH-RH) can interfere with receptors on tumor cells or intracellular pathways that are important in cell proliferation and in this way inhibit tumor growth. The first part of this review explains how these peptide hormones and their analogs affect tumors. The second part of this review describes how various hormone analogs can be used for the treatment of gastric, colorectal, pancreatic, liver and urinary bladder cancers. These tumors are major health problems worldwide and their treatment remains a great challenge. Receptors for somatostatin, bombesin/GRP, LH-RH and GH-RH are present in a large percentage of these cancers. We have developed a series of cytotoxic peptides based on doxorubicin or its derivative 2-pyrrolino-doxorubicin coupled to an analog of LH-RH, somatostatin or bombesin. This new class of targeted analogs might provide a more effective therapy for various cancers that express receptors for these carrier peptides, while producing significantly reduced peripheral toxicity. Under experimental conditions, these peptide hormone analogs strongly inhibited the growth of these tumors. Cytotoxic analogs were particularly effective on tumors that express the specific peptide receptors and acted more powerfully than the carrier peptide or the cytotoxic compound alone. Clinical trials on these peptide analogs are in progress.

Effect of somatostatin on expression of VEGF and bFGF in human gastric cancer xenografts in nude mice

AIM: To observe the effect of somatostatin on the expression of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) in subcutaneous xenografts derived from human gastric carcinoma SGC-7901 cells in nude mice.

METHODS: An animal model of human gastric cancer xenograft was established by subcutaneously implanting SGC-7901 cells in nude mice. Twenty-four nude mice were randomly divided into four groups and treated with normal saline (group A), octreotide (group B), high-dose somatostatin (group C) and low-dose somatostatin (group D) for three weeks, respectively. After treatment, the animals were killed to take the tumors. The expression of VEGF and bFGF was examined by immunohistochemistry and Western blot.

RESULTS: Immunohistochemical analysis indicated that the integral optical density (IOD)/area (/pix²) of VEGF in groups A-D were 0.644 ± 0.022, 0.549 ± 0.002, 0.345 ± 0.019 and 0.435 ± 0.018, respectively. The expression of VEGF protein in tumor tissue was significantly higher in group A than in the other three groups (P < 0.001). Compared with group B, the expression of VEGF was reduced more significantly in groups C and D (P < 0.001 for both). The IOD/area (/pix²) of bFGF in groups A-D were 0.723 ± 0.018, 0.558 ± 0.004, 0.288 ± 0.017 and 0.595 ± 0.011, respectively. The expression of bFGF protein in tumor tissue was significantly higher in group A than in the other three groups (P < 0.001). Compared with group B, the expression of bFGF was reduced more significantly only in group C (P < 0.001). Western blot analysis indicated that the relative expression of VEGF in groups A-D were 0.98 ± 0.02, 0.76 ± 0.02, 0.53 ± 0.01 and 0.53 ± 0.01, respectively. The relative expression of bFGF in groups A-D were 0.76 ± 0.02, 0.71 ± 0.02, 0.32 ± 0.01 and 0.51 ± 0.01, respectively. The expression of bFGF and VEGF protein was significantly higher in group A than in the other three groups (P < 0.001). Compared with group B, the expression of VEGF and bFGF was reduced more significantly in groups C and D (P < 0.001).

CONCLUSION: Somatostatin can down-regulate the expression of VEGF and bFGF in subcutaneous xenografts derived from human gastric carcinoma SGC-7901 cells in nude mice, and the effect is more obvious than that of octreotide.

Loss of somatostatin receptor subtype 2 in prostate cancer is linked to an aggressive cancer phenotype, high tumor cell proliferation and predicts early metastatic and biochemical relapse

Somatostatin receptor subtype 2 (SSTR2) is the most frequently expressed SSTR subtype in normal human tissues. SSTR2 expression is differentially regulated in various tumor types and therapeutic somatostatin analogs binding to SSTR2 are in clinical use. In prostate cancers highly contradictory results in terms of SSTR2 expression and its consequences have been published over the past years. The aim of this study was to clarify prevalence and clinical significance of SSTR2 expression in prostate cancer. Therefore, quantitative immunohistochemistry (IHC) using a tissue microarray containing samples from 3,261 prostate cancer patients with extensive clinical and molecular cancer characteristics and oncological follow-up data was performed. IHC data was compared to publicly available Gene Expression Omnibus datasets of human prostate cancer gene expression arrays. While membranous SSTR2 staining was always seen in normal prostate epithelium, SSTR2 staining was absent in more than half (56.1%) of 2,195 interpretable prostate cancer samples. About 13% of all analyzed prostate cancers showed moderate to strong cytoplasmic and membranous SSTR2 staining. Staining intensities were inversely correlated with high Gleason grade, advanced pT category, high tumor cell proliferation (p<0.0001 each), high pre-operative PSA levels, (p = 0.0011) and positive surgical margins (p = 0.006). In silico analysis confirmed lower SSTR2 gene expression in prostate cancers vs. normal adjacent tissue (p = 0.0424), prostate cancer metastases vs. primary cancers (p = 0.0011) and recurrent vs. non-recurrent prostate cancers (p = 0.0438). PSA-free survival gradually declined with SSTR2 staining intensity (p<0.0001). SSTR2-negative cancers were more likely to develop metastases over time (p<0.05). In conclusion, most prostate cancers are indeed SSTR2-negative and loss of SSTR2 strongly predicts an unfavorable tumor phenotype and poor prognosis. Therefore, SSTR2 expression seems an important factor in the pathogenesis of prostate cancer and re-introduction of the receptor in SSTR2-negative prostate cancers may feature a promising target for novel gene therapy approaches.

PET and PET/CT with 68gallium-labeled somatostatin analogues in Non GEP-NETs Tumors

Somatostatin (SST) is a 28-amino-acid cyclic neuropeptide mainly secreted by neurons and endocrine cells. A major interest for SST receptors (SSTR) as target for in vivo diagnostic and therapeutic purposes was born since a series of stable synthetic SST-analouges PET became available, being the native somatostatin non feasible for clinical use due to the very low metabolic stability. The rationale for the employment of SST-analogues to image cancer is both based on the expression of SSTR by tumor and on the high affinity of these compounds for SSTR. The primary indication of SST-analogues imaging is for neuroendocrine tumors (NETs), which usually express a high density of SSTR, so they can be effectively targeted and visualized with radiolabeled SST-analogues in vivo. Particularly, SST-analogues imaging has been widely employed in gastroenteropancreatic (GEP) NETs. Nevertheless, a variety of tumors other than NETs expresses SSTR thus SST-analogues imaging can also be used in these tumors, particularly if treatment with radiolabeled therapeutic SST-analouges PET is being considered. The aim of this paper is to provide a concise overview of the role of positron emission tomography/computed tomography (PET/CT) with (68)Ga-radiolabeled SST-analouges PET in tumors other than GEP-NETs.

Peptide receptor targeting in cancer: the somatostatin paradigm

Peptide receptors involved in pathophysiological processes represent promising therapeutic targets. Neuropeptide somatostatin (SST) is produced by specialized cells in a large number of human organs and tissues. SST primarily acts as inhibitor of endocrine and exocrine secretion via the activation of five G-protein-coupled receptors, named sst1–5, while in central nervous system, SST acts as a neurotransmitter/neuromodulator, regulating locomotory and cognitive functions. Critical points of SST/SST receptor biology, such as signaling pathways of individual receptor subtypes, homo- and heterodimerization, trafficking, and cross-talk with growth factor receptors, have been extensively studied, although functions associated with several pathological conditions, including cancer, are still not completely unraveled. Importantly, SST exerts antiproliferative and antiangiogenic effects on cancer cells in vitro, and on experimental tumors in vivo. Moreover, SST agonists are clinically effective as antitumor agents for pituitary adenomas and gastro-pancreatic neuroendocrine tumors. However, SST receptors being expressed by tumor cells of various tumor histotypes, their pharmacological use is potentially extendible to other cancer types, although to date no significant results have been obtained. In this paper the most recent findings on the expression and functional roles of SST and SST receptors in tumor cells are discussed.

Somatostatin receptors in non-neuroendocrine malignancies: the potential role of somatostatin analogs in solid tumors

Somatostatin receptors (sstrs) are G-protein-coupled receptors that mediate various physiological effects when activated by the neuropeptide somatostatin or its synthetic analogs. In addition to the well-documented antisecretory effects of sstr2-preferential somatostatin analogs octreotide and lanreotide, ligand binding to sstr initiates an inhibitory action on tumor growth. This effect may result from both indirect actions (suppression of growth factors and growth-promoting hormones [e.g., GH/IGF-1 axis] and inhibition of angiogenesis) and direct actions (activation of antigrowth activities [e.g., apoptosis]). As solid tumor cells express multiple sstrs, there is a rationale to evaluate the potential antitumor effects of pasireotide (SOM230), a multireceptor-targeted somatostatin analog with high binding affinity for sstr1–3 and sstr5. Pasireotide reduces systemic IGF-1 levels more potently than currently available somatostatin analogs and has been well tolerated in clinical trials.

Evaluation of multi-target and single-target liposomal drugs for the treatment of gastric cancer

We studied the effects of multi- and single-target liposomal drugs on human gastric cancer cell AGS both in vitro and in vivo. The cytotoxic effect of dihydrotanshinone I was significantly enhanced by treatment with octreotide-polyethylene glycol(PEG)-liposome, Arg-Gly-Asp(RGD)-PEG-liposome, and RGD/octreotide-PEG-liposome encapsulated with 0.5 mug/ml of dihydrotanshinone I to AGS cell for 24 h, compared to control. Furthermore, the AGS cell survival rate for multi-target versus single target liposomal drugs was significantly suppressed. Microscopic examination revealed that significant cell death occurred in the multi- and single-target liposomal encapsulated drug groups. Significant suppression of tumor growth in AGS cell xenograft nude mice given octreotide-PEG-liposome, RGD/octreotide-PEG-liposome encapsulated drug, versus those given a free drug was noted after 13 d of experimentation with the multi-targeted liposome: up to 60.75% and 41.2% reduction of tumor volume as compared to dimethylsulfoxide (DMSO) control and the free drug groups respectively. The treated animals showed no gross signs of toxicity. The results have potential clinical application.

Future clinical prospects in somatostatin/cortistatin/somatostatin receptor field

Somatostatin receptors (sst), somatostatin (SS) and cortistatin (CST) are widely expressed in the various systems in the human and rodent organisms and are "responsible" for maintaining homeostasis, which is essential for survival. Because of their broad expression pattern sst, SS and CST interactions may play regulatory roles in both physiology and pathophysiology in mammalian organisms. SS analogue treatment strategies as well as the use of SS analogues for diagnostic purposes have been established in diseases of different origins. This review focuses on the currently determined role for SS analogues in today's clinical practice and the potential clinical prospects for SS, CST and sst interactions in the future, with a focus on neuroendocrine and non-neuroendocrine tumours and immune-mediated diseases. Moreover, the role of new SS analogues and new insights in sst physiology will be discussed.

Modulation of key signal transduction molecules by a novel peptide combination effective for the treatment of gastrointestinal carcinomas

We have reported earlier a novel combination of four structurally designed synthetic neuropeptide analogs of vasoactive intestinal peptide (VIP), bombesin, substance P and somatostatin, code-named DRF 7295 which have anti-tumor efficacy for adenocarcinomas in vitro and in vivo (Jaggi et al., Invest New Drugs, 2008). The discovery, synthesis, in vitro and in vivo efficacy was reported (Jaggi et al., Invest New Drugs, 2008). Gastrointestinal tumor cells of the colon, pancreas and duodenum were found to most sensitive to DRF7295 in vitro and in vivo (Jaggi et al., Invest New Drugs, 2008). We have further investigated and report here the modulation of cellular signaling in gastrointestinal carcinomas by DRF 7295, which may be mediating its observed anticancer activity in these cancer types. DRF 7295 inhibits the binding of specific neuropeptides initiating a cascade of cellular signaling events leading to programmed cell death. It down regulates the second messenger cAMP, epidermal growth factor (EGF) dependent proliferation and the phosphorylated MAP Kinase pERK1/2 in gastrointestinal carcinomas, thus depriving the tumour cells of critical pro-proliferative cellular signals. It triggers bcl2 and Caspase 3 dependent apoptotic cell death and induces p53 tumor suppressor protein in the treated carcinoma cells in vitro. It has significant anti-angiogenic potential as reflected in the inhibition of tube like formation in the endothelial cells and down regulation of VEGF levels. Tumour xenograft studies confirmed the in vivo efficacy of DRF 7295 for gastrointestinal carcinomas (Jaggi et al., Invest New Drugs, 2008). The Phase I clinical trials have shown DRF 7295 to be well tolerated and devoid of systemic toxicities of the conventional cytotoxics (Mukherjee et al., Phase I dose escalating study of DRF7295: a new class of peptide based drugs. "Abstract" ASCO ID:948, 2003). The drug may have a promising role in disease stabilization in colorectal and other cancers. Thus DRF 7295 is a novel targeted drug in the class of signal transduction modulators, with potential for treatment of gastrointestinal carcinomas.

Peptide-based radiopharmaceuticals and cytotoxic conjugates: potential tools against cancer

A hope for the diagnosis and treatment of cancer is the development of new tumor-specific peptide-based radiopharmaceuticals. The overexpression of many peptide receptors on human tumors makes such receptors an attractive potential target for diagnostic imaging and radiotherapy with specifically designed radiolabeled peptides. The use of solid-phase peptide synthesis, and the availability of a wide range of bifunctional chelating agents for the convenient radiolabeling of bioactive peptides with different radionuclides have produced a wide variety of medicinally useful peptide radiopharmaceuticals. A few of these peptides, such as somatostatin, bombesin, cholecystokinin/gastrin, neurotensin and vasoactive intestinal peptide are currently under investigation for their possible clinical applications in nuclear oncology. This article presents the recent development in radiolabeled small peptides, with major emphasis on somatostatin and bombesin analogs.

Effects of bombesin and somatostatin on proliferation of human gastric cancer cell line BGC-823 and their postreceptor signal transduction

AIM: To observe the effects of bombesin and somatostatin on the proliferation of human lowly-differentiated gastric cancer cell line BGC-823, and to determine the intracellular concentration of cyclic adenosine monophos-phate (cAMP), the activity of proteinkinase C (PKC) and the expression of PKC isoforms α, β₁, β₂ and ε.

METHODS: BGC-823 cells were cultured in RPMI 1640 medium containing 100 mL/L FCS in the atmosphere of 50 mL/L carbon dioxide at 37℃, and then bombesin or/and somatostatin with different concentrations was added. The viability and proliferation were determined by Mosmann's method (MTT assay); the intracellular cAMP concentrations were detected by radioimmunoassay (RIA); the activity of PKC was determined by incorporation of [γ-³²p]-ATP with exogenous substrate; and the expression of PKC isoforms was analyzed by Western blot.

RESULTS: Bombesin promoted the growth of BGC-823 cells in a dose-dependent manner (r = 0.878, P < 0.05), which was inhibited significantly by its antagonist; it also increased the level of intracellular cAMP (r = 0.68, P < 0.01) and the activity of PKC in a dose-dependent manner. Western blot showed overexpression of PKCα, while the expression of PKCβ1, β2 and ε were not detected. However, somatostatin inhibited the growth of BGC-823 cells in a dose-dependent manner (r = -0.831, P < 0.01), and it also inhibited the growth-promoting effect of bombesin without obvious dose-dependent ways and the activity of PKC in a dose-dependent way (r = -0.74, P < 0.01).

CONCLUSION: Bombesin can promote the growth of BGC-823 cells by the mediation of specific receptor, it can increase the intracellular cAMP level and PKC activity, illuminating that cAMP-PKA and DG-PKC system may be involved in the signal transduction and PKCα may play an important role. Somatostatin can significantly inhibit the growth of BGC-823 cells and the growth-promoting effects of bombesin through down-regulating the activity of PKC.

Rationale for the use of somatostatin analogs as antitumor agents

BACKGROUND

There is a need for novel antitumor agents that demonstrate efficacy in currently refractory tumors without adding to the toxicity of therapy. The somatostatin analogs, which have demonstrated antineoplastic activities in experimental tumor models, and good tolerability and safety profiles are attractive candidates.

MATERIALS AND METHODS

Data from preclinical studies provide evidence for direct and indirect mechanisms by which somatostatin analogs exert antitumor effects.

RESULTS

Direct antitumor activities, mediated through somatostatin receptors (sst(1)-sst(5)) expressed in tumor cells, include blockade of autocrine/paracrine growth-promoting hormone and growth factor production, inhibition of growth factor-mediated mitogenic signals and induction of apoptosis. Indirect antitumor effects include inhibition of growth-promoting hormone and growth factor secretion, and antiangiogenic actions. Many human tumors express more than one somatostatin receptor subtype, with sst(2) being predominant. Somatostatin analogs such as octreotide and lanreotide, which present a high affinity for sst(2), are in current clinical use to alleviate symptoms in patients with endocrine tumors, and radiolabeled somatostatin analogs have been developed for diagnosis and radiotherapy.

CONCLUSIONS

While the rationale exists for the use of somatostatin analogs as antitumor agents, studies are ongoing to identify analogs with activity across the range of receptor subtypes to maximize the potential of such treatment.

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.

Targeted chemotherapy with cytotoxic bombesin analogue AN-215 inhibits growth of experimental human prostate cancers

We developed a powerful cytotoxic analogue of bombesin AN-215, in which the bombesin (BN)-like carrier peptide is conjugated to 2-pyrrolino doxorubicin (AN-201). Human prostate cancers express high levels of receptors for BN/gastrin releasing peptide (GRP) that can be used for targeted chemotherapy. The effects of targeted chemotherapy with cytotoxic BN analogue AN-215 were evaluated in nude mice bearing subcutaneous xenografts of DU-145, LuCaP-35, MDA-PCa-2b and intraosseous implants of C4-2 human prostate cancers. Intraosseous growth of C4-2 tumors was monitored by serum PSA. BN/GRP receptors were evaluated by 125I-[Tyr4]BN binding assays and RT-PCR. The effects of AN-215 on apoptosis and cell proliferation were followed by histology, and the expression of Bcl-2 and Bax protein was determined by Western blot analysis. Targeted analog AN-215 significantly inhibited growth of subcutaneously implanted DU-145, LuCaP-35 and MDA-PCa-2b prostate cancers by 81% to 91% compared to controls, while cytotoxic radical AN-201 was less effective and more toxic. Serum PSA levels of mice bearing intraosseous C4-2 prostate tumors were significantly reduced. In LuCaP-35 tumors administration of BN antagonist RC-3095 prior to AN-215 blocked the receptors for BN/GRP and inhibited the effects of AN-215. High affinity receptors for BN/GRP and their m-RNA were detected on membranes of all 4 tumor models. Therapy with AN-215, but not with AN-201, decreased the ratio of Bcl-2/Bax in DU-145 and the expression of antiapoptotic Bcl-2 in LuCaP-35 tumors. The presence of BN/GRP receptors on primary and metastatic prostate cancers makes possible targeted chemotherapy with AN-215 for the treatment of this malignancy.

Targeted chemotherapy with cytotoxic bombesin analogue AN-215 can overcome chemoresistance in experimental renal cell carcinomas

BACKGROUND

Multidrug resistance (MDR) mediated by membrane transporters, such as P-glycoprotein (MDR-1) and MDR-associated protein (MRP), remains a challenge in the therapy of renal cell carcinoma (RCC). Chemotherapy targeted to hormone receptors may provide a new approach to overcome chemoresistance. The cytotoxic analogue of bombesin/gastrin-releasing peptide (GRP), AN-215, consists of a superactive derivative of doxorubicin, AN-201, which is linked to a bombesin analogue carrier: RC-3094.

METHODS

The authors examined the expression of bombesin/GRP receptors in 3 human RCC cell lines (A-498, ACHN. and 786-0) by using reverse-transcriptase-polymerase chain reaction (RT-PCR) analysis and radioligand-binding assays. They also evaluated the effects of AN-215 and its cytotoxic radical AN-201 in the same RCC models in vivo, and they studied the effects of AN-215 and AN-201 on the expression levels of MDR-1 and subtype 1 of MRP (MRP-1) by using real-time PCR.

RESULTS

A N-215 significantly (P < 0.05) inhibited the growth of A-498, ACHN, and 786-0 RCC xenografted into nude mice by 59.2-67.6%, whereas the cytotoxic radical AN-201 alone had no significant antitumor effects. The efficacy of AN-215 was independent of the expression patterns of MDR-1 and MRP-1 in these RCC cell lines. The induction of MDR-1 by AN-215 was similar (Experiment 2) or weaker (Experiment 1) compared with AN-201. Both AN-215 and AN-201 caused only a minor induction of MRP-1.

CONCLUSIONS

The current findings indicated that targeted chemotherapy with cytotoxic bombesin/GRP analogue AN-215 can inhibit the growth of RCC, providing a new treatment modality for patients with advanced RCC.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Targeted therapy with a cytotoxic somatostatin analog, AN-238, inhibits growth of human experimental endometrial carcinomas expressing multidrug resistance protein MDR-1

BACKGROUND

Chemoresistance mediated by membrane transporters such as multidrug resistance (MDR-1) glycoprotein remains a challenge in the chemotherapy treatment of advanced or recurrent endometrial carcinoma. Targeted chemotherapy might overcome this resistance. The cytotoxic somatostatin (SST) analog, AN-238, consists of a superactive derivative of doxorubicin (DOX), 2-pyrrolino-DOX (AN-201), linked to the SST analog carrier, RC-121. This conjugate binds strongly to SST receptor subtypes (sst) 2a (sst2(a)) and 5 (sst(5)) and can be targeted to tumors that express these receptors.

METHODS

The presence of sst2(a) and sst(5) was determined in 3 human endometrial carcinoma cell lines (HEC-1A, RL-95-2, and AN3CA). Nude mice bearing xenografts of these cancers were treated with AN-238 and its radical, AN-201. The antitumor effects and toxicity were compared. The authors studied the effects of AN-238 and AN-201 on the expression levels of MDR-1, multidrug resistance related protein (MRP-1), and breast carcinoma resistance protein (BCRP) by real-time polymerase chain reaction.

RESULTS

The authors demonstrated the presence of mRNA and receptor protein for sst(2a) and sst(5) on HEC-1A, RL-95-2, and AN3CA tumors. AN-238 significantly (P < 0.05) inhibited the growth of these tumors, whereas AN-201 had no effect. Blockade of SST receptors nullified the effects of AN-238. In all 3 endometrial carcinoma lines, AN-238 caused a weaker induction of MDR-1 than AN-201. No major induction of MRP-1 and BCRP occurred after treatment with AN-238 or AN-201.

CONCLUSIONS

Targeted chemotherapy with the cytotoxic SST analog, AN-238, inhibited powerfully the growth of endometrial carcinoma, which express SST receptors, regardless of their expression level of MDR-1.

Endocrine tumours of the gastrointestinal tract. Somatostatin receptors as tools for diagnosis and therapy: molecular aspects

Somatostatin is a neuropeptide that acts as an endogenous inhibitor of various cellular functions including endocrine and exocrine secretions and the proliferation of normal and tumour cells. Its action is mediated by a family of G-protein-coupled receptors (sst1-sst5) that are widely distributed in normal and tumour cells. Gastroenteropancreatic endocrine tumours express multiple somatostatin receptors, sst2 being clearly predominant. These receptors represent the molecular basis for the clinical use of somatostatin analogues in the treatment of endocrine tumours and their in vivo localisation. This review covers current knowledge in somatostatin receptor biology and signalling.

Effective Inhibition of Experimental Human Ovarian Cancers with a Targeted Cytotoxic Bombesin Analogue AN-215

PURPOSE

To determine whether the cytotoxic analogue of bombesin/gastrin-releasing peptide (GRP) AN-215 can inhibit the in vivo growth of four human ovarian cancer cell lines. AN-215 consists of 2-pyrrolinodoxorubicin (AN-201), a superactive derivative of doxorubicin linked to a bombesin antagonist carrier des-D-Tpi-RC-3095. This conjugate binds strongly to receptors for bombesin/GRP and can be targeted to tumors that express these receptors. Bombesin/GRP receptors are found in 77% of human ovarian cancer specimens.

EXPERIMENTAL DESIGN

Nude mice bearing xenografts of ES-2, SKOV-3, OV-1063, and UCI-107 human ovarian carcinomas were treated with AN-215. The antitumor effects and the toxicity were determined. The expression of bombesin receptor subtypes was measured by reverse-transcriptase PCR analysis, and the presence of bombesin/GRP receptors was determined by radioligand binding assays.

RESULTS

AN-215 significantly (P < 0.05) inhibited growth of ES-2, OV-1063, and UCI-107 tumors, prevented the metastatic spread of ES-2 cancers, and prolonged the survival of nude mice bearing i.p. ES-2 xenografts. Cytotoxic radical AN-201, the unconjugated mixture of bombesin antagonist RC-3095 and AN-201 or RC-3095 alone had no significant effects. Blockade of bombesin/GRP receptors abolished the effect of AN-215. The expression of bombesin/GRP receptors was not changed after repeated treatment with AN-215.

CONCLUSIONS

Our findings indicate that targeted chemotherapy with cytotoxic bombesin/GRP analogue AN-215 can inhibit ovarian tumors, which express bombesin/GRP receptors. AN-215 might provide a new treatment modality for women with advanced ovarian carcinoma.

Depleted dopamine in gastric cancer tissues: dopamine treatment retards growth of gastric cancer by inhibiting angiogenesis

PURPOSE

It has been recently shown that the catecholamine neurotransmitter dopamine (DA) strongly and selectively inhibits vascular permeability factor/vascular endothelial growth factor (VPF/VEGF)-induced angiogenesis. Gastric cancer is highly angiogenic and is dependent on VEGF for its growth and progression. Because substantial amounts of DA present in normal stomach tissues has been implicated in several gastric functions, we therefore investigated the role, if any, of this neurotransmitter in the growth and progression of gastric cancer.

EXPERIMENTAL DESIGN

Initially, the status of DA and tyrosine hydroxylase, the rate-limiting enzyme required for DA synthesis, were determined in human gastric cancer tissues and in N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced gastric cancer tissues of rats. On the basis of our observation of inverse correlation between stomach DA and gastric cancer growth, we determined the effect of pharmacological dose of DA on the angiogenesis and growth of MNNG induced gastric cancer in rats and Hs746T human gastric cancer in nude mice.

RESULTS

DA and tyrosine hydroxylase were absent in both human and rat gastric cancer tissues. On the contrary, a low nontoxic pharmacological dose of DA significantly retarded tumor angiogenesis by inhibiting VEGFR-2 phosphorylation in tumor endothelial cells, which expressed DA D(2) receptors. This action of DA was associated with the growth inhibition of both MNNG-induced rat malignant gastric tumors and xenotransplanted human gastric cancer in nude mice.

CONCLUSIONS

Our study demonstrates that there is an inverse correlation between endogenous stomach DA and gastric cancer and indicates that DA already in clinical use for other purposes might have a role as an antiangiogenic agent in the treatment of gastric cancer.

Chemotherapy targeted to cancers through tumoral hormone receptors

Work on cytotoxic analogs of luteinizing hormone-releasing hormone (LH-RH), somatostatin and bombesin, designed for targeting chemotherapy to peptide receptors on various cancers, is reviewed here as the project is at advanced stages of development and clinical trials are pending. Cytotoxic analogs of LH-RH, AN-152 and AN-207, containing doxorubicin (DOX) or 2-pyrrolino-DOX (AN-201), respectively, target LH-RH receptors and can be used for the treatment of prostatic, breast, ovarian and endometrial cancers and melanomas. AN-201 was also incorporated into the cytotoxic analog of somatostatin, AN-238, which can be targeted to receptors for somatostatin in prostatic, renal, mammary, ovarian, gastric, colorectal and pancreatic cancers as well as glioblastomas and lung cancers, suppressing the growth of these tumors and their metastases. A cytotoxic analog of bombesin AN-215, containing 2-pyrrolino-DOX, was likewise synthesized and successfully tested in experimental models of prostate cancer, small cell lung carcinoma, gastrointestinal cancers and brain tumors expressing receptors for bombesin/gastrin-releasing peptide. This new class of targeted cytotoxic peptide analogs might provide a more effective therapy for various cancers.

Future aspects of somatostatin-receptor-mediated therapy

More than 30 years have passed since somatostatin was discovered and its hormonal function defined. The wide range of anatomical distribution and actions of somatostatin and its receptors have stimulated intense scientific and clinical interest. The development of somatostatin analogues helped define its usefulness in the treatment of endocrine diseases and cancer. The molecular cloning of five distinct subtypes of somatostatin receptors in the 1980s has significantly increased our insight into the biology of somatostatin and its receptor subtypes and has led to the design and development of subtype-selective peptides and nonpeptide agonists and antagonists. In the future, the development of somatostatin-receptor-mediated treatment will go along different lines. Tumor-targeted radioactive treatment based on somatostatin analogues will be further developed and improved. New somatostatin analogues will come into clinical practice, both receptor subtype-specific analogues, but also pan-receptor analogues. One is currently in clinical trial--SOM230--which is a cyclo-hexapeptide binding with high affinity to receptor type 1, 2, 3 and 5, but not 4. It has already shown activity both in acromegaly and in neuroendocrine gastrointestinal tumors. Preclinical studies on somatostatin analogues, coupled to cytotoxic agents, have shown rather promising results and will hopefully be further developed in clinical trials. Another interesting area is treatment of neuroendocrine gut tumors with ultra-high doses of somatostatin analogues, which has demonstrated significant clinical effects in patients resistant to standard-dose treatment with the same somatostatin analogue.