New approaches to treatment of various cancers based on cytotoxic analogs of LHRH, somatostatin and bombesin

In-vivo studies of targeted and localized cancer drug release from microporous poly-di-methyl-siloxane (PDMS) devices for the treatment of triple negative breast cancer

Triple-negative breast cancer (TNBC) treatment is challenging and frequently characterized by an aggressive phenotype and low prognosis in comparison to other subtypes. This paper presents fabricated implantable drug-loaded microporous poly-di-methyl-siloxane (PDMS) devices for the delivery of targeted therapeutic agents [Luteinizing Hormone-Releasing Hormone conjugated paclitaxel (PTX-LHRH) and Luteinizing Hormone-Releasing Hormone conjugated prodigiosin (PG-LHRH)] for the treatment and possible prevention of triple-negative cancer recurrence. In vitro assessment using the Alamar blue assay demonstrated a significant reduction (p < 0.05) in percentage of cell growth in a time-dependent manner in the groups treated with PG, PG-LHRH, PTX, and PTX-LHRH. Subcutaneous triple-negative xenograft breast tumors were then induced in athymic female nude mice that were four weeks old. Two weeks later, the tumors were surgically but partially removed, and the device implanted. Mice were observed for tumor regrowth and organ toxicity. The animal study revealed that there was no tumor regrowth, six weeks post-treatment, when the LHRH targeted drugs (LHRH-PTX and LHRH-PGS) were used for the treatment. The possible cytotoxic effects of the released drugs on the liver, kidney, and lung are assessed using quantitative biochemical assay from blood samples of the treatment groups. Ex vivo histopathological results from organ tissues showed that the targeted cancer drugs released from the implantable drug-loaded device did not induce any adverse effect on the liver, kidneys, or lungs, based on the results of qualitative toxicity studies. The implications of the results are discussed for the targeted and localized treatment of triple negative breast cancer.

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.

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

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

Recent Development of Non-Peptide GnRH Antagonists

The decapeptide gonadotropin-releasing hormone, also referred to as luteinizing hormone-releasing hormone with the sequence (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH₂) plays an important role in regulating the reproductive system. It stimulates differential release of the gonadotropins FSH and LH from pituitary tissue. To date, treatment of hormone-dependent diseases targeting the GnRH receptor, including peptide GnRH agonist and antagonists are now available on the market. The inherited issues associate with peptide agonists and antagonists have however, led to significant interest in developing orally active, small molecule, non-peptide antagonists. In this review, we will summarize all developed small molecule GnRH antagonists along with the most recent clinical data and therapeutic applications.

A Phase II Trial of AEZS-108 in Castration- and Taxane-Resistant Prostate Cancer

BACKGROUND

AEZS-108 (zoptarelin doxorubicin) is a cytotoxic hybrid molecule consisting of doxorubicin covalently coupled with a luteinizing hormone-releasing hormone (LHRH) analogue, which selectively targets doxorubicin to tumor cells expressing LHRH receptors. We report the clinical efficacy of AEZS-108 in a phase II trial in men with metastatic castrate-resistant prostate cancer who had disease progression after taxane-based chemotherapy.

PATIENTS AND METHODS

Patients received AEZS-108 210 mg/m² intravenously every 3 weeks. The primary end point was clinical benefit defined as nonprogression at 12 weeks with no dose-limiting toxicities (DLTs) or other toxicities requiring termination of treatment. Secondary end points included response rate, pain response, progression-free survival (PFS), and overall survival (OS). Circulating tumor cells (CTCs) were captured and tested for LHRH receptors, as well as for internalization of AEZS-108 using autofluorescence.

RESULTS

Twenty-five patients were enrolled; 20 patients had at least 1 measurable lesion at baseline. Patients received a median of 5 cycles (range, 1-9) and 44% of patients received at least 6 cycles with 2 patients who completed ≥ 8 cycles. Considering clinical benefits, 13 patients (52%) remained progression-free at 12 weeks with no DLT or other toxicities requiring termination of treatment. For clinical response according to Response Evaluation Criteria in Solid Tumors version 1.1 criteria, 1 patient (4%) experienced a confirmed partial response (PR) within 12 weeks, 14 patients (56%) had stable disease (SD), and 8 patients (32%) had disease progression. For maximal prostate-specific antigen (PSA) response, 1 patient (4%) experienced a confirmed PR within 12 weeks, 21 patients (84%) had SD, and 3 patients (12%) had disease progression as denoted by their best PSA response. Pain improved in 13 (59%) patients. The median PFS was 3.8 months (95% confidence interval [CI], 2.1-4.4), and median OS was 6.0 months (95% CI, 4.2-10.1) with a median follow-up of 16.1 months (range, 3.2-36.1). Baseline CTC enumeration was an independent predictor of OS but not PFS.

CONCLUSION

AEZS-108 showed activity in patients who were pretreated, a subset typically very difficult to treat, and maintained an acceptable safety profile.

Evaluation of the Biological Properties and the Enzymatic Stability of Glycosylated Luteinizing Hormone-Releasing Hormone Analogs

The enzymatic stability, antitumor activity, and gonadotropin stimulatory effects of glycosylated luteinizing hormone-releasing hormone (LHRH) analogs were investigated in this study. Conjugation of carbohydrate units, including lactose (Lac), glucose (GS), and galactose (Gal) to LHRH peptide protected the peptide from proteolytic degradation and increased the peptides' half-lives in human plasma, rat kidney membrane enzymes, and liver homogenate markedly. Among all seven modified analogs, compound 1 (Lac-[Q(1)][w(6)]LHRH) and compound 6 (GS(4)-[w(6)]LHRH) were stable in human plasma during 4 h of experiment. The half-lives of compounds 1 and 6 improved significantly in kidney membrane enzymes (from 3 min for LHRH to 68 and 103 min, respectively). The major cleavage sites for most of the glycosylated compounds were found to be at Trp(3)-Ser(4) and Ser(4)-Tyr(5) in compounds 1-5. Compound 6 was hydrolyzed at Ser(4)-Tyr(5) and the sugar conjugation site. The antiproliferative activity of the glycopeptides was evaluated on LHRH receptor-positive prostate cancer cells. The glycosylated LHRH derivatives had a significant growth inhibitory effect on the LNCaP cells after a 48-h treatment. It was demonstrated that compound 1 significantly increased the release of luteinizing hormone (LH) at 5 and 10 nM concentrations and compound 5 (GS-[Q(1)]LHRH) stimulated the release of follicle-stimulating hormone (FSH) at 5 nM concentration in dispersed rat pituitary cells (p < 0.05). In our studies, compound 1-bearing lactose and D-Trp was the most stable and active and is a promising candidate for future preclinical investigations in terms of in vitro biological activity and metabolic stability.

Comparative in vitro biological evaluation of daunorubicin containing GnRH-I and GnRH-II conjugates developed for tumor targeting

Hormone based drug targeting is a promising tool for selective tumor therapy. In this study, synthesis and systematic comparative biological evaluation of novel drug containing analogs of gonadotropin-releasing hormone GnRH-I and GnRH-II is reported demonstrating their suitability for tumor targeting. The cytotoxic conjugates were prepared by the attachment of the chemotherapeutical agent daunorubicin (Dau) to GnRH analogs directly or through an enzyme-labile spacer with oxime linkage. All conjugates were found to be proteolytically stable under circumstances applied in biological assays. Both GnRH-I and GnRH-II were able to bind similarly to high-affinity GnRH-I receptors on human pituitary and human prostate cancer cells. The in vitro long-term cytotoxic effect of the conjugates was comparable with that of the free drug in human breast and colon cancer cell lines. Furthermore, a concentration-dependent cellular uptake profile was observed. The in vitro apoptotic effect of the compounds was evaluated by flow cytometry analysis using annexin-V. Our results show that both the GnRH-I and the GnRH-II based analogs might be applied for targeted tumor therapy.

Phase I, dose-escalation study of the targeted cytotoxic LHRH analog AEZS-108 in patients with castration- and taxane-resistant prostate cancer

PURPOSE

AEZS-108, formerly AN-152, is a cytotoxic hybrid molecule consisting of a luteinizing hormone-releasing hormone (LHRH) agonist moiety covalently coupled to doxorubicin, allowing it to deliver doxorubicin selectively to cells expressing LHRH receptors. LHRH receptors are expressed on the cell membrane of many tumors, including prostate cancer. This phase I study determined the maximum tolerated dose (MTD) of AEZS-108 in men with taxane- and castration-resistant prostate cancer (CRPC) while providing additional information on the safety profile and efficacy of this agent.

EXPERIMENTAL DESIGN

AEZS-108 was administered as an intravenous infusion every 21 days until progression or unacceptable toxicity in cohorts of 3 or 6 patients until the MTD was reached. Blood was collected for capture of circulating tumor cells (CTC) to visualize internalization of AEZS-108, an autofluorescent molecule.

RESULTS

The MTD of AEZS-108 in this cohort was 210 mg/m(2), which was lower than that seen in a phase I study conducted in women with endometrial or ovarian cancers. The dose-limiting toxicity was persistent neutropenia. Three patients had a PSA response with an additional 10 patients maintaining PSA stable disease. Of the 10 patients evaluable by RECIST criteria, 9 achieved stable disease. AEZS-108 internalization in CTCs was routinely visualized using its autofluorescence.

CONCLUSION

These findings show that AEZS-108 has an acceptable safety profile and a signal of efficacy, lowering PSA in heavily pretreated patients with prostate cancer, and that internalization of AEZS-108 in prostate cancer CTCs may be a viable pharmacodynamic marker. A phase II study in men with prostate cancer is ongoing.

Prostate cancer relevant antigens and enzymes for targeted drug delivery

Chemotherapy is one of the most widely used approaches in combating advanced prostate cancer, but its therapeutic efficacy is usually insufficient due to poor specificity and associated toxicity. Lack of targeted delivery to prostate cancer cells is also the primary obstacles in achieving feasible therapeutic effect of other promising agents including peptide, protein, and nucleic acid. Consequently, there remains a critical need for strategies to increase the selectivity of anti-prostate cancer agents. This review will focus on various prostate cancer-relevant antigens and enzymes that could be exploited for prostate cancer targeted drug delivery. Among various targeting strategies, active targeting is the most advanced approach to specifically deliver drugs to their designated cancer cells. In this approach, drug carriers are modified with targeting ligands that can specifically bind to prostate cancer-specific antigens. Moreover, there are several specific enzymes in the tumor microenvironment of prostate cancer that can be exploited for stimulus-responsive drug delivery systems. These systems can specifically release the active drug in the tumor microenvironment of prostate cancer, leading to enhanced tumor penetration efficiency.

Novel LHRH-receptor-targeted cytolytic peptide, EP-100: first-in-human phase I study in patients with advanced LHRH-receptor-expressing solid tumors

Purpose

To conduct a phase I study determining the safety, pharmacokinetics and preliminary efficacy of EP-100, a novel anticancer drug consisting of natural luteinizing-hormone-releasing hormone (LHRH) ligand linked to a cationic membrane-disrupting peptide.

Methods

Patients with advanced, solid tumors, positive for LHRH receptor by immunohistochemistry (IHC), received EP-100 weekly or twice weekly for 3 of 4 weeks in a 28 day cycle. A modified Fibonacci 3 + 3 dose-escalation schema was used. Initial cohorts received EP-100 once weekly (cohorts 1–7, 0.6–7.8 mg/m², n = 21). Later cohorts received doses twice weekly (cohorts 7–11, 7.8–40 mg/m², n = 16).

Results

LHRH-receptor expression was confirmed by IHC in 52 of 89 consented patients; 37 patients received at least 1 dose. Cohorts receiving doses of 5.2 mg/m² and above achieved therapeutic levels from in vitro studies Clearance was rapid (mean half-life 7.1 ± 3.8 to 15.9 ± 3.6 min). The maximum-tolerated dose was not reached at the highest dose evaluated (40 mg/m² twice weekly). Grade 2 increase in alanine aminotransferase/serum aspartate aminotransferase in one patient resolved, did not recur upon re-treatment, and was not observed in other patients. The only drug-related adverse event was transient infusion-related dermatologic reactions (10 patients). No complete or partial tumor responses were observed; seven patients had stable disease of 16 weeks.

Conclusions

EP-100 was well tolerated in patients with advanced, LHRH-receptor-expressing solid tumors. The recommended phase 2 dose is 40 mg/m² twice weekly for 3 of 4 weeks per cycle.

Electronic supplementary material

The online version of this article (doi:10.1007/s00280-014-2424-x) contains supplementary material, which is available to authorized users.

Targeted cancer therapy with gonadotropin-releasing hormone chimeric proteins

Tumor-associated antigens (TAAs) have been identified mainly to determine cancer prognosis. In the past few years, TAAs have been used in the development of treatment modalities such as tumor vaccination. This review describes an additional application of TAAs: as a target for specific antitumor treatment. Since TAAs are overexpressed on the tumor cell surface, they can be targeted to deliver drugs directly to cancer cells. One such delivery system exploits chimeric proteins. Chimeric proteins are a class of targeted molecules designed to recognize and specifically destroy cells that overexpress specific receptors. These molecules, designed and constructed by gene fusion techniques, comprise both cell-targeting and cell-killing moieties. The authors' laboratory has developed a number of chimeric proteins using gonadotropin-releasing hormone (GnRH) as the targeting moiety. These chimeras recognize a GnRH binding site that is expressed on adenocarcinoma cells. GnRH was fused to a large number of killing moieties, including bacterial and human proapoptotic proteins. All GnRH-based chimeric proteins selectively killed adenocarcinoma cells both in vitro and in vivo. Utilizing chimeric proteins for targeted therapy represents a new and exciting therapeutic modality for the treatment of cancer in humans.

Suppression of the proliferation of human U-87 MG glioblastoma cells by new antagonists of growth hormone-releasing hormone in vivo and in vitro

Five-year survival of patients afflicted with glioblastoma multiforme (GBM) is rare, making this cancer one of the most feared malignancies. Previously, we reported that growth hormone-releasing hormone (GHRH) is a potent growth factor in cancers. The present work evaluated the effects of two antagonistic analogs of GHRH (MIA-604 and MIA-690) on the proliferation of U-87 MG GBM tumors, in vivo as well as in vitro. Both analogs were administered subcutaneously and dose-dependently inhibited the growth of tumors transplanted into nude mice (127 animals in seven groups). The analogs also inhibited cell proliferation in vitro, decreased cell size, and promoted apoptotic and autophagic processes. Both antagonists stimulated contact inhibition, as indicated by the expression of the E-cadherin-β-catenin complex and integrins, and decreased the release of humoral regulators of glial growth such as FGF, PDGFβ, and TGFβ, as revealed by genomic or proteomic detection methods. The GHRH analogs downregulated other tumor markers (Jun-proto-oncogene, mitogen-activated protein kinase-1, and melanoma cell adhesion molecule), upregulated tumor suppressors (p53, metastasis suppressor-1, nexin, TNF receptor 1A, BCL-2-associated agonist of cell death, and ifκBα), and inhibited the expression of the regulators of angiogenesis and invasion (angiopoetin-1, VEGF, matrix metallopeptidase-1, S100 calcium binding protein A4, and synuclein-γ). Our findings indicate that GHRH antagonists inhibit growth of GBMs by multiple mechanisms and decrease both tumor cell size and number.

AEZS-108 : a targeted cytotoxic analog of LHRH for the treatment of cancers positive for LHRH receptors

INTRODUCTION

Receptors for the luteinizing hormone-releasing hormone [LHRH, also known as gonadotropin-releasing hormone (GnRH)] can be regarded as an ideal target for a personalized medicine approach in cancer therapy. LHRH receptors are expressed in about 80% of human endometrial and ovarian cancers, 86% of prostate cancer, and about 50% of breast cancers including triple-negative breast cancer, as well as bladder, colorectal, and pancreatic cancers, sarcomas, lymphomas, melanomas, and renal cell carcinomas. Apart from the pituitary and reproductive organs, other organs and hematopoietic stem cells express LHRH receptors. Thus, a targeted cytotoxic LHRH analog such as AEZS-108 (formerly known as AN-152), in which doxorubin is linked to the LHRH agonist [D-Lys(6)]LHRH, appears to be a suitable drug for targeted chemotherapy of cancers expressing receptors for LHRH, which would be more efficacious and less toxic than standard systemic chemotherapy.

AREAS COVERED

This review discusses the development of AEZS-108, its targeting mechanism, preclinical studies, and clinical trials in patients with endometrial, ovarian, prostatic, and bladder cancers. We emphasize its development as a personalized medicine approach. The studies reviewed demonstrate the effects of the cytotoxic LHRH analog, AEZS-108, mediated by LHRH receptors, in in vivo models of LHRH-receptor-positive human endometrial, ovarian, breast, prostatic, colorectal, pancreatic, and bladder cancers xenografted into nude mice. Intravenous administration of AEZS 108 inhibits the growth of LHRH-receptor-positive tumors better than equimolar doses of the cytotoxic agent doxorubicin and is far less toxic. AEZS 108 has no antitumor activity in cancers negative to LHRH receptor. This strongly supports the concept of targeting cytotoxic chemotherapy to tumor cells expressing LHRH receptors. Early clinical trials have demonstrated the efficacy of AEZS-108. A Phase I trial assessed the maximum tolerated dose and pharmacokinetics and pharmacodynamics of AEZS-108 given once every 3 weeks in patients with gynecological cancers. Two Phase II studies in heavily pretreated ovarian and recurrent endometrial cancers showed good clinical activity after a maximum of six courses of AEZS-108 as a single agent. Ongoing clinical studies with AEZS-108 in men with castration-resistant prostate cancer and patients with chemotherapy refractory bladder cancer had shown early signs of clinical efficacy. Side effects are moderate and easily manageable. In particular, no pituitary or cardiac toxicity is observed.

EXPERT OPINION

AEZS-108 is a cytotoxic analog designed for receptor-mediated targeted chemotherapy and consists of an LHRH carrier linked to doxorubicin. Preclinical studies demonstrate that the uptake of AEZS-108 is achieved by receptor-mediated endocytosis. Results of Phase I and II clinical trials in patients with gynecological cancers demonstrated anticancer activity without cardiotoxicity even in highly pretreated patients. Phase I/II studies in castration-resistant prostate cancer and chemotherapy refractory bladder cancer are in progress. Targeted chemotherapy with a cytotoxic analog of LHRH, such as AEZS-108, is therefore being considered for Phase III studies in advanced endometrial cancers positive for LHRH receptor. LHRH receptors are also present in human colon cancers, melanomas, lymphomas, and sarcomas, and treatment of these cancers with AEZS-108 should also be undertaken. Before such treatment with AEZS-108 is begun, the status of tumoral LHRH receptors of patients must be determined.

Targeted chemotherapy of endometrial, ovarian and breast cancers with cytotoxic analogs of luteinizing hormone-releasing hormone (LHRH)

Receptors luteinizing hormone-releasing hormone (LHRH) are expressed in about 80 % of human endometrial and ovarian cancers and account for more than 50 % of breast cancers including triple negative breast cancers. Apart from the pituitary and reproductive organs, no other organs or hematopoietic stem cells express LHRH (GnRH) receptors. Thus, these receptors can be regarded as an ideal target for a personalized medicine approach in cancer therapy. AEZS-108 (formerly known as AN-152) in which doxorubin is linked to the LHRH agonist [D: -Lys(6)]LHRH, appears to be the most advanced compound in late stage clinical development. Results of phase I and phase II clinical trials in patients with gynecological cancers demonstrated anticancer activity without any cardiotoxicity even in highly pretreated patients. AEZS-108 is therefore being considered for phase II trials in triple negative breast cancers and phase III studies in advanced endometrial cancers positive for LHRH-receptor. EP-100 is a membrane-disrupting peptide targeted to LHRH receptors, which is undergoing early clinical studies in ovarian cancer patients.

The in vivo antitumor activity of LHRH targeted methotrexate-human serum albumin nanoparticles in 4T1 tumor-bearing Balb/c mice

The use of targeted drug delivery systems is a growing trend in cancer treatment to decrease the adverse effect of anti-cancer drugs. In this study, we sought to conjugate methotrexate-human serum albumin nanoparticles (MTX-HSA NPs) with luteinizing-hormone releasing hormone (LHRH). The LHRH was intended to target LHRH receptors overexpressed on the several types of tumors. The expression of LHRH receptors on the 4T1 breast cancer cells was confirmed by FITC conjugated LHRH receptor antibody using fluorescence microscopy. Female Balb/c mice bearing 4T1 breast cancer tumor were treated with a single i.v. injection of free MTX, non-targeted MTX-HSA NPs and LHRH targeted MTX-HSA NPs. LHRH targeted MTX-HSA nanoparticles showed stronger anti-tumor activity in vivo. By 7 days after treatment, average tumor volume in the LHRH targeted MTX-HSA NPs treated group decreased to 8.67% of the initial tumor volume when the number of attached LHRH molecules on MTX-HSA NPs was the highest, while the average tumor volume in non-targeted MTX-HSA NPs treated mice grew rapidly and reached 250.7% of the initial tumor volume 7 days after the treatment. LHRH targeted MTX-HSA NPs could significantly extend the survival time of tumor bearing mice compared with the non-targeted MTX-HSA NPs and free MTX formulations.

Accumulation of arginine-rich cell-penetrating peptides in tumors and the potential for anticancer drug delivery in vivo

We investigated the biodistribution of arginine-rich cell-penetrating peptides (CPPs) in tumor-xenografted nude mice after intravenous injection of fluorescently labeled CPPs using in vivo imaging. The CPPs used included HIV-1 Tat (48-60), penetratin, and the L- and D-enantiomers of oligoarginines (8, 12, and 16 residues), all of which are reported to have high cell penetration. Among the tested peptides, high accumulation in tumors was observed for the D-form of octaarginine (r8), and glycosaminoglycans played a key role. Injection of an r8-doxorubicin conjugate (4mg doxorubicin/kg) effectively suppressed tumor proliferation, with no significant decrease in mouse weight, whereas administration of doxorubicin itself (6mg/kg), yielding a similar degree of tumor-growth suppression, resulted in significant weight loss. These results suggest the potential of r8 as a prototypic tumor-targeting vector.

Investigation of cancer cell lines for peptide receptor-targeted drug development

Many tumors highly express specific populations of G-protein-coupled receptors (GPCRs) that could be utilized for receptor-targeted therapy. We confirmed significant quantities of mRNAs specific for certain somatostatin (SST), vasoactive intestinal peptide (VIP), and bombesin (BN) receptors in various commercially available tumor cell lines. Very few of the tumor cell lines examined displayed the high receptor-binding affinity despite exhibiting the expression of appropriate mRNAs and proteins of the cognate receptors. However, binding assays establish that some tumor cell lines, such as pancreatic cancer CFPAC-1, prostate cancer DU-145, and pancreatic carcinoid BON, demonstrate high BN receptor binding. BON cells also demonstrate high somatostatin receptor (SSTR) affinity binding. We also found that tumor cell lines, such as BON and host cells expressing SST receptor subtypes 1 or 2 (CHO-R1 or CHO-R2), underwent a decrease in cell surface receptor density in multiple passages. BON and CHO-R2 cells also rapidly internalize a significant proportion of cell surface ligand-receptor complexes. The tumor cells CFPAC-1, DU-145, and BON with high receptor binding could be useful for peptide drug studies. BON cells were further applied to test SST/BN analogs and cytotoxic conjugates. Furthermore, the in vivo antitumor assay showed that the cytotoxic conjugate CPT-SST targeting all SSTR subtypes displayed a potent tumor-suppressive ability to BON tumors expressing multiple SSTR subtypes.

Enhanced anti-tumoral activity of methotrexate-human serum albumin conjugated nanoparticles by targeting with Luteinizing Hormone-Releasing Hormone (LHRH) peptide

Active targeting could increase the efficacy of anticancer drugs. Methotrexate-human serum albumin (MTX-HSA) conjugates, functionalized by luteinizing hormone-releasing hormone (LHRH) as targeting moieties, with the aim of specifically targeting the cancer cells, were prepared. Owing to the high expression of LHRH receptors in many cancer cells as compared to normal cells, LHRH was used as the targeting ligand in this study. LHRH was conjugated to MTX-HSA nanoparticles via a cross-linker. Three types of LHRH targeted nanoparticles with a mean particle size between 120–138 nm were prepared. The cytotoxicity of LHRH targeted and non-targeted nanoparticles were determined on the LHRH positive and negative cell lines. The internalization of the targeted and non-targeted nanoparticles in LHRH receptor positive and negative cells was investigated using flow cytometry analysis and fluorescence microscopy. The cytotoxicity of the LHRH targeted nanoparticles on the LHRH receptor positive cells were significantly more than non-targeted nanoparticles. LHRH targeted nanoparticles were also internalized by LHRH receptor positive cells significantly more than non-targeted nanoparticles. There were no significant differences between the uptake of targeted and non-targeted nanoparticles to the LHRH receptor negative cells. The active targeting procedure using LHRH targeted MTX-HSA nanoparticles could increase the anti-tumoral activity of MTX.

Prodrug-based intracellular delivery of anticancer agents

There are numerous anticancer agents based on a prodrug approach. However, no attempt has been made to review the ample available literature with a specific focus on the altered cell uptake pathways enabled by the conjugation and on the intracellular drug-release mechanisms. This article focuses on the cellular interactions of a broad selection of parenterally administered anticancer prodrugs based on synthetic polymers, proteins or lipids. The report also aims to highlight the prodrug design issues, which are key points to obtain an efficient intracellular drug delivery. The chemical basis of these molecular concepts is put into perspective with the uptake and intracellular activation mechanisms, the in vitro and in vivo proofs of concepts and the clinical results. Several active targeting strategies and stimuli-responsive architectures are discussed throughout the article.

Application of human pancreatic carcinoid BON cells for receptor-targeted drug development

In our previous study, we found that several tumor cell lines displayed high receptor-specific binding affinity, one of which, the human pancreatic carcinoid BON cell line, demonstrates high affinity binding of the bombesin (BN) and somatostatin (SST) receptor-specific ligands. In the present study, BON cells, as a representative model, were further applied to evaluate various peptide analogs and cytotoxic receptor-targeted peptide conjugates. We observed quick ligand-receptor internalization in BON cells as well as high binding affinity. Furthermore, BON cells have high expression of multidrug resistance-associated genes (MDR1) and show camptothecin (CPT) resistance. Various receptor-specific cytotoxic conjugates were synthesized and evaluated in the BON cell model via in vitro and in vivo studies. We found that all the tested conjugates displayed potent antitumor ability in xenografts. Especially, the CPT conjugates, CPT-SST, and CPT-BN, are most likely to increase sensitivity to CPT-resistant BON cells. Our findings suggest that appropriately defined tumor cell lines may provide physiologically relevant cell-based evaluations of novel peptide analogs and receptor-targeted chemotherapeutics.

In vitro degradation and antitumor activity of oxime bond-linked daunorubicin-GnRH-III bioconjugates and DNA-binding properties of daunorubicin-amino acid metabolites

Bioconjugates with receptor-mediated tumor-targeting functions and carrying cytotoxic agents should enable the specific delivery of chemotherapeutics to malignant tissues, thus increasing their local efficacy while limiting the peripheral toxicity. In the present study, gonadotropin-releasing hormone III (GnRH-III; Glp-His-Trp-Ser-His-Asp-Trp-Lys-Pro-Gly-NH(2)) was employed as a targeting moiety to which daunorubicin was attached via oxime bond, either directly or by insertion of a GFLG or YRRL tetrapeptide spacer. The in vitro antitumor activity of the bioconjugates was determined on MCF-7 human breast and HT-29 human colon cancer cells by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Their degradation/stability (1) in human serum, (2) in the presence of cathepsin B and (3) in rat liver lysosomal homogenate was analyzed by liquid chromatography in combination with mass spectrometry. The results show that (1) all synthesized bioconjugates have in vitro antitumor effect, (2) they are stable in human serum at least for 24 h, except for the compound containing an YRRL spacer and (3) they are hydrolyzed by cathepsin B and in the lysosomal homogenate. To investigate the relationship between the in vitro antitumor activity and the structure of the bioconjugates, the smallest metabolites produced in the lysosomal homogenate were synthesized and their binding to DNA was assessed by fluorescence spectroscopy. Our data indicate that the incorporation of a peptide spacer in the structure of oxime bond-linked daunorubicin-GnRH-III bioconjugates is not required for their antitumor activity. Moreover, the antitumor activity is influenced by the structure of the metabolites (daunorubicin-amino acid derivatives) and their DNA-binding properties.

Regulation of HER expression and transactivation in human prostate cancer cells by a targeted cytotoxic bombesin analog (AN-215) and a bombesin antagonist (RC-3095)

Bombesin (BN) and gastrin-releasing peptide (GRP) have been shown to stimulate the growth of human prostate cancer in vivo and in vitro by mechanisms initiated by binding of the peptide to BN/GRP receptor (GRPR). GRPR is overexpressed in a variety of human cancers, including human prostatic carcinoma. This led us to evaluate the effectiveness of blocking GRPR and of chemotherapy targeted to GRPR in androgen-dependent (LNCaP) and androgen-independent (PC-3) prostate cancer cells, which exhibit different features of disease progression. Thus, we used a cytotoxic BN/GRP analog, AN-215, consisting of 2-pyrrolinodoxorubicin (AN-201) linked to BN-like carrier peptide, and a BN/GRP receptor antagonist, RC-3095. Semiquantitative RT-PCR and Western blotting revealed that mRNA and protein levels for GRPR increased in prostate cancer cells as compared with nonneoplastic RWPE-1 cells. Immunofluorocytochemistry and Western blot assays revealed that AN-215 was the most effective analog decreasing both the expression of epidermal growth factor receptor family members and the activation of epidermal growth factor receptor and HER-2, which are associated to a poor prognosis. Furthermore, analogs targeted to BN/GRP receptors, AN-215 and RC-3095, blocked the effect of BN on cell growth in RWPE-1, LNCaP and PC-3 cells. These findings shed light on the mechanisms of action of these analogs and support the view that the use of AN-215 and RC-3095 for blocking BN/GRP receptors for targeted therapy may be of benefit for treatment of advanced prostate cancer.

Peptidic tumor targeting agents: the road from phage display peptide selections to clinical applications

Cancer has become the number one cause of death amongst Americans, killing approximately 1,600 people per day. Novel methods for early detection and the development of effective treatments are an eminent priority in medicine. For this reason, isolation of tumor-specific ligands is a growing area of research. Tumor-specific binding agents can be used to probe the tumor cell surface phenotype and to customize treatment accordingly by conjugating the appropriate cell-targeting ligand to an anticancer drug. This refines the molecular diagnosis of the tumor and creates guided drugs that can target the tumor while sparing healthy tissues. Additionally, these targeting agents can be used as in vivo imaging agents that allow for earlier detection of tumors and micrometastasis. Phage display is a powerful technique for the isolation of peptides that bind to a particular target with high affinity and specificity. The biopanning of intact cancer cells or tumors in animals can be used as the bait to isolate peptides that bind to cancer-specific cell surface biomarkers. Over the past 10 years, unbiased biopanning of phage-displayed peptide libraries has generated a suite of cancer targeting peptidic ligands. This review discusses the recent advances in the isolation of cancer-targeting peptides by unbiased biopanning methods and highlights the use of the isolated peptides in clinical applications.

Receptor-mediated tumor targeting based on peptide hormones

IMPORTANCE OF THE FIELD

Tumor targeting with peptides is based on the discovery that receptors for many regulatory peptides are overexpressed in tumor cells, compared with their expression in normal tissues. Consequently, these peptides and their analogues can be used as carriers/targeting moieties for the preparation of diagnostic and therapeutic agents that have increased selectivity and decreased peripheral toxicity.

AREAS COVERED IN THIS REVIEW

Here an overview is given of the most relevant gonadotropin-releasing hormone (GnRH) and somatostatin derivatives, as well as of their applications in cancer diagnosis and therapy. For this purpose, recently published data in these areas (mostly articles published from 2000 to 2009) were reviewed.

WHAT THE READER WILL GAIN

In contrast to other regulatory peptides that stimulate the tumor growth, GnRH and somatostatin derivatives have inhibitory effect; therefore, they were used primarily for the preparation of various conjugates to be used in targeted chemotherapy, targeted radiotherapy, photodynamic therapy, boron neutron capture therapy and cancer diagnosis. Some of these conjugates have already found clinical applications, whereas others are now in preclinical and clinical trials.

TAKE HOME MESSAGE

Tumor targeting with hormone peptides provides a basis for the development of new diagnostic and therapeutic approaches for cancer.

Luteinizing hormone-releasing hormone antagonists

BACKGROUND

Luteinizing hormone-releasing hormone (LH-RH) plays a central role in the vertebrate reproduction by regulating gonadal activity. Based on its binding to pituitary LH-RH receptors, as well as to LH-RH receptors expressed on cancer cells, LH-RH agonists and antagonists have been developed for different therapeutic applications.

OBJECTIVE/METHOD

Here we give an overview of the most relevant LH-RH antagonists and their therapeutic applications. Recently patented compounds as well as drug formulations and dosage are presented.

CONCLUSION

LH-RH antagonists have found clinical applications in in vitro fertilization, benign prostatic hyperplasia, endometriosis and in the treatment of hormone-dependent tumors. Work in progress is focused on further development of both peptidic and orally active non-peptidic LH-RH antagonists.

Peptide targeting of platinum anti-cancer drugs

Besides various side effects caused by platinum anticancer drugs, they are not efficiently absorbed by the tumor cells. Two Pt-peptide conjugates; cyclic mPeg-CNGRC-Pt (7) and cyclic mPeg-CNGRC-Pten (8) bearing the Asn-Gly-Arg (NGR) targeting sequence, a malonoyl linker, and low molecular weight miniPEG groups have been synthesized. The platinum ligand was attached to the peptide via the carboxylic end of the malonate group at the end of the peptide. The pegylated peptide is nontoxic and highly soluble in water. Platinum conjugates synthesized using the pegylated peptides are also water-soluble with reduced or eliminated peptide immunogenicity. The choice of carboplatin as our untargeted platinum complex was due to the fact that the malonate linker chelates platinum in a manner similar to that of carboplatin. Cell toxicity assay and competition assay on the PC-3 cells (CD13 positive receptors) revealed selective delivery and destruction of PC-3 cells using targeted Pt-peptide conjugates 7 and 8 significantly more than untargeted carboplatin. Platinum uptake on PC-3 cells was 12-fold more for conjugate 7 and 3-fold more for conjugate 8 compared to that of the untargeted carboplatin, indicating selective activation of the CD13 receptors and delivery of the conjugates to CD13 positive cells. Further analysis on effects of conjugates 7 and 8 on PC-3 cells using caspase-3/7, fluorescence microscopy, and DNA fragmentation confirmed that the cells were dying by apoptosis.

Luteinizing hormone-releasing hormone receptor-targeted deslorelin-docetaxel conjugate enhances efficacy of docetaxel in prostate cancer therapy

Docetaxel, a chemotherapeutic agent currently used for improving survival of prostate cancer patients, suffers from low therapeutic index. The objective of this study was to prepare a new docetaxel derivative conjugated to deslorelin, a luteinizing hormone-releasing hormone (LHRH) superagonist, and to determine whether it enhances docetaxel potency in vitro and in vivo. Because docetaxel is not amenable for conjugation with peptides, we introduced a -COOH group in docetaxel, forming docetaxel-hemiglutarate, and subsequently conjugated this to serine in deslorelin, forming deslorelin-docetaxel. Fourier-transform IR, (1)H-nuclear magnetic resonance, and liquid chromatography-mass spectrometry analyses confirmed deslorelin-docetaxel formation. Antiproliferative efficacy in LNCaP and PC-3 cell lines over 24, 48, and 72 hours exhibited the order deslorelin-docetaxel > docetaxel, whereas deslorelin alone had no effect, with deslorelin-docetaxel potency being 15-fold greater than docetaxel at 72 h. Further, cells pretreated with antisense oligonucleotide against LHRH receptor exhibited decreased deslorelin-docetaxel efficacy, without any change in docetaxel efficacy. Thus, deslorelin-docetaxel efficacy is likely mediated via LHRH receptor. Cell cycle analysis showed that docetaxel treatment led to arrest in G(2)-M phase, whereas deslorelin-docetaxel treatment allowed greater progression to apoptosis in both cell lines, with deslorelin-docetaxel exerting 5-fold greater apoptosis compared with docetaxel in prostate cancer cell lines. Antitumor efficacy studies in PC-3 prostate xenograft-bearing mice indicated the efficacy order deslorelin-docetaxel > docetaxel >> deslorelin > PBS, with deslorelin-docetaxel exerting approximately 5.5-fold greater tumor growth inhibition than docetaxel alone. Thus, deslorelin-docetaxel prepared in this study retains pharmacologic effects of both docetaxel and deslorelin while enhancing the antiproliferative, apoptotic, and antitumor efficacy of docetaxel by several folds in prostate cancer therapy.

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.

Targeting tumors with peptides from natural sources

Peptide-based therapies offer the potential for non-genotoxic, genotype-specific alternatives, or adjuvants, to the current range of traditional cancer treatments. Such a patient-tailored cancer-cell-directed therapeutic approach should have fewer side effects and could well be more effective than the current drug- or combination-based regimens. Here, we review the potential of novel natural anticancer peptides such as necrotic peptides, apoptotic peptides, function-blocking peptides, antiangiogenic peptides and immunostimulatory peptides in the context of their ability to induce tumor regression. We focus on the therapeutic prospects of anticancer peptides and their possible application in tumor therapy.

Targeted Therapy of Breast and Gynecological Cancers with Cytotoxic Analogues of Peptide Hormones

Gynecological cancers such as breast, ovarian, and endometrial carcinoma express receptors for luteinizing hormone-releasing hormone (LHRH), bombesin/gastrin-releasing peptide (BN/GRP), and somatostatin (SST). These tumors are therefore suitable candidates for targeted therapy with cytotoxic hybrid molecules consisting of a cytotoxic radical and a peptide hormone analogue as a carrier. These compounds have been shown to be more active and less toxic in vivo than nontargeted chemotherapy in models of various human cancers which express the respective receptors. The current review summarizes experimental and clinical findings with cytotoxic peptide hormone analogues of LHRH (AN-152 [AEZS 108], AN-207), BN/GRP (AN-215), and SST (AN-238) in breast, ovarian, and endometrial cancers.

Role of radiolabeled octreotide therapy in patients with metastatic neuroendocrine neoplasms

Peptide receptor radionuclide therapy is a new therapeutic modality for patients with nonresectable neuroendocrine tumors. The technique is based on the unique ability of these tumors to express cell membrane-specific peptide receptors that can be targeted with radiolabeled somatostatin analogues. A high level of uptake on somatostatin receptor scintigraphy is a prerequisite for effective treatment. The efficacy of this method has been proven in several clinical trials. In a substantial number of patients, an improvement of life quality has been achieved in addition to a marked morphologic and biochemical tumor response. Serious side effects are rarely observed. Attention must be paid to kidney protection during the treatment. The present review summarizes the clinical experience with the treatment of advanced neuroendocrine tumors with radiolabeled somatostatin analogues and focuses on patient selection and the appropriate timing of the therapy. Finally, it emphasizes treatment-related issues that deserve attention in the future.

Effects of camptothecin on tumor cell proliferation and angiogenesis when coupled to a bombesin analog used as a targeted delivery vector

The camptothecin-bombesin conjugate termed DC-51-43, as a novel targeted drug delivery system, was examined in over 10 human tumor cell lines and shows a potent antiproliferative activity. This conjugate has also demonstrated its antitumor activity in our previous experiments. In our present study, we evaluate this conjugate for its antiangiogenic activity by in-vitro and in-vivo experiments. The camptothecin-bombesin conjugate and free camptothecin show potent in-vitro inhibitory activities of cell adhesion to various extracellular matrix components and integrins alphaVbeta3 and alphaVbeta5, not beta1/alphabeta1. This conjugate displays inhibitory activity to cell migration and invasion at concentrations of 10 micromol/l or above. This conjugate is also effective against in-vitro capillary-like tube formation of endothelial cells (at 40 micromol/l), and in-vivo angiogenesis as seen by blocking the spread of host mice endothelial cells into matrigel plugs. These experimental results support the fact that the camptothecin-bombesin conjugate has therapeutic activities against angiogenesis. By binding to bombesin receptor-expressing sites, this bombesin analog, consisting of 11 amino acids, is potentially a novel delivery vector for nonspecific cytotoxic agents.

A conjugate of camptothecin and a somatostatin analog against prostate cancer cell invasion via a possible signaling pathway involving PI3K/Akt, αVβ3/αVβ5 and MMP-2/-9

Camptothecin (CPT) was conjugated to the N-terminal of a somatostatin analog (SSA) directly via a carbamate group and a basic N-terminal linking motif, D-Lys-D-Tyr-Lys-D-Tyr-D-Lys. This new CPT-SSA conjugate termed JF-10-81 was evaluated as a receptor-specific delivery system for its anti-invasive and anti-angiogenic activities. It was found that, in addition to blocking migration and invasion of highly invasive prostate cancer PC-3 cells, this conjugate also inhibited in vitro capillary-like tube formation of endothelial cells and in vivo angiogenesis in C57B1/6N female mice. JF-10-81 was found to block PC-3 cell attachment to various extracellular matrix components, mainly to vitronectin, the ligand of cell surface receptors integrin alphaVbeta3 and alphaVbeta5. Additionally, JF-10-81 reduced expression of integrins alphaVbeta3 and alphaVbeta5 on PC-3 cell surfaces, without effects on beta1 or any alphabeta1 heterodimers. This conjugate also inactivated phosphorylation of protein kinase B (PKB/Akt), down-regulated the expression of latent matrix metalloproteinase (MMP) -2 and MMP-9, but had little effect on MMP-3/-10. Meanwhile, membrane type-1 matrix metalloproteinase (MT1-MMP) and the tissue inhibitor of matrix metalloproteinase-2 (TIMP-2) were not detectable in PC-3 cells. alphaVbeta3/alphaVbeta5 and MMP-2/-9 are known to be highly expressed in many tumor cells and play an important role in tumor progression. Our results support that this conjugate could possibly inhibit prostate cancer PC-3 cell invasion through a signaling pathway involving PI3K/Akt, alphaVbeta3/alphaVbeta5 and MMP-2/-9, and this SSA could be used as an efficient vector to deliver CPT or other cytotoxic agents to target sites for cancer therapy.

Luteinizing hormone-releasing hormone agonists in the treatment of prostate cancer: A review of their discovery, development, and place in therapy

BACKGROUND

Early identification of the biological activity of luteinizing hormone-releasing hormone (LHRH) paved the way for the synthesis of analogues with enhanced potency and biological properties. Early testing in animal models and humans provided insight into the potential clinical uses of these substances, and, within 10 years, LHRH-agonist therapy had become available for use in patients with advanced prostate cancer (PC). Over time, the role of LHRH-agonist therapy has expanded to include use as part of multimodal treatment regimens throughout the course of the disease.

OBJECTIVES

This article reviews the discovery and development of LHRH agonists and summarizes the clinical evidence for their efficacy in PC.

METHODS

Relevant clinical studies were identified through searches of the English-language literature indexed on MEDLINE through May 2006. The main search terms were prostate cancer and LHRH agonist.

RESULTS

Results of the initial therapeutic trials of sustained-release depot formulations of LHRH agonists in patients with PC were reported in the mid-1980s, indicating that these agents were effective and well tolerated in improving clinical symptoms and producing medical castration. Longer-term studies and subsequent meta-analyses of randomized controlled trials in patients with advanced PC found no significant differences in overall survival when single-therapy androgen suppression was achieved through the use of LHRH-agonist therapy or orchiectomy. Randomized trials have reported significant improvements in disease-free and overall survival in patients with locally advanced or high-grade PC treated with LHRH agonists in addition to radiotherapy. Several prospective randomized trials have reported decreases in rates of positive surgical margins with short-term (6 weeks to 4 months) neoadjuvant LHRH-agonist therapy in patients with stage T1 to T3a PC undergoing prostatectomy. Definitive comparisons of immediate and delayed treatment in patients with biochemical relapse have not been reported. However, the results of several studies suggest that immediate LHRH-agonist therapy (or orchiectomy) may improve the course of disease progression and survival. The risks of long-term treatment (eg, osteoporosis; fracture; anabolic loss of muscle mass, with a tendency toward weight gain) must be considered carefully in patients who are likely to receive chronic LHRH-agonist therapy. Intermittent schedules have been developed to reduce the adverse effects associated with LHRH-agonist therapy; some reports support sparing effects on bone and muscle mass and relative improvements in toxicities during off-therapy periods, whereas others have documented continuing decreases in bone mineral density (BMD), with the rate of bone loss highest during the early cycles of therapy. Bisphosphonate therapy has been shown to increase BMD in patients with PC and may therefore be beneficial when overt symptoms of osteopenia or osteoporosis are present.

CONCLUSIONS

LHRH-agonist therapy has been the mainstay of treatment for advanced PC for >20 years. Clinical evidence supports expanding use of these agents at an earlier stage of disease and as part of multimodal regimens that include radiotherapy. There is a need for further study of the efficacy of adjuvant LHRH-agonist therapy along with prostatectomy, in patients with biochemical failure, in intermittent regimens, and in conjunction with cytotoxic therapies in late-stage disease.