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

Effect of Polyplex Size on Penetration into Tumor Spheroids

Ovarian cancer is one of the most lethal gynecological cancers in the world. In recent years, nucleic acid (NA)-based formulations have been shown to be promising treatments for ovarian cancer, including tumor nodules. However, gene therapy is not that far advanced in clinical reality due to unfavorable physicochemical properties of the NAs, such as high molecular weight, poor cellular uptake, rapid degradation by nucleases, etc. One of the strategies used to overcome these drawbacks is the complexation of anionic NAs via electrostatic interactions with cationic polymers, resulting in the formation of so-called polyplexes. In this work, the role of the size of pDNA and siRNA polyplexes on their penetration into ovarian-cancer-based tumor spheroids was investigated. For this, a methoxypoly(ethylene glycol) poly(2-(dimethylamino)ethyl methacrylate) (mPEG-pDMAEMA) diblock copolymer was synthesized as a polymeric carrier for NA binding and condensation with either plasmid DNA (pDNA) or short interfering RNA (siRNA). When prepared in HEPES buffer (10 mM, pH 7.4) at a nitrogen/phosphate (N/P) charge ratio of 5 and pDNA polyplexes were formed with a size of 162 ± 11 nm, while siRNA-based polyplexes displayed a size of 25 ± 2 nm. The polyplexes had a slightly positive zeta potential of +7-8 mV in the same buffer. SiRNA and pDNA polyplexes were tracked in vitro into tumor spheroids, resembling in vivo avascular ovarian tumor nodules. For this purpose, reproducible spheroids were obtained by coculturing ovarian carcinoma cells with primary mouse embryonic fibroblasts in different ratios (5:2, 1:1, and 2:5). Penetration studies revealed that after 24 h of incubation, siRNA polyplexes were able to penetrate deeper into the homospheroids (composed of only cancer cells) and heterospheroids (cancer cells cocultured with fibroblasts) compared to pDNA polyplexes which were mainly located in the rim. The penetration of the polyplexes was slowed when increasing the fraction of fibroblasts present in the spheroids. Furthermore, in the presence of serum siRNA polyplexes encoding for luciferase showed a high cellular uptake in 2D cells resulting in ∼50% silencing of luciferase expression. Taken together, these findings show that self-assembled small siRNA polyplexes have good potential as a platform to test ovarian tumor nodulus penetration..

Cancer-targeted delivery systems based on peptides

There is a growing interest for the discovery of new cancer-targeted delivery systems for drug delivery and diagnosis. A synopsis of the bibliographic data will be presented on bombesin, neurotensin, octreotide, Arg-Gly-Asp, luteinizing hormone-releasing hormone and other peptides. Many of them have reached the clinics for therapeutic or diagnostic purposes, and have been utilized as carriers of known cytotoxic agents such as doxorubicin, paclitaxel, cisplatin, methotrexate or dyes and radioisotopes. In our article, recent advances in the development of peptides as carriers of cytotoxic drugs or radiometals will be analyzed.

Targeting GRPR in urological cancers--from basic research to clinical application

Gastrin releasing peptide (GRP) is a regulatory peptide that acts through its receptor (GRPR) to regulate physiological functions in various organs. GRPR is overexpressed in neoplastic cells of most prostate cancers and some renal cell cancers and in the tumoral vessels of urinary tract cancers. Thus, targeting these tumours with specifically designed GRP analogues has potential clinical application. Potent and specific radioactive, cytotoxic or nonradioactive GRP analogues have been designed and tested in various animal tumour models with the aim of receptor targeting for tumour diagnosis or therapy. All three categories of compound were found suitable for tumour targeting in animal models. The cytotoxic and nonradioactive GRP analogues have not yet shown convincing tumour-reducing effects in human trials; however, the first clinical studies of radioactive GRP analogues--both agonists and antagonists--suggest promising opportunities for both diagnostic tumour imaging and radiotherapy of prostate and other GRPR-expressing cancers.

Bombesin analogue-mediated delivery preferentially enhances the cytotoxicity of a mitochondria-disrupting peptide in tumor cells

Tumor-homing peptides that recognize specific markers on tumor cells have shown potential as drug carriers for targeted cancer therapy. Bombesin receptors are frequently overexpressed or ectopically expressed in a wide range of human tumors. Bombesin and its analogues have been widely used as drug carriers for tumor imaging and tumor therapy. However, the cargos used in previous studies, including radioactive and chemotherapeutic agents, are usually small molecules. Mitochondrial-disrupting peptides depolarize the mitochondria and trigger apoptosis after entering tumor cells. We are interested in whether the bombesin analogue, Bn(6–14), which contains a bombesin receptor-binding motif, can specifically deliver the mitochondria-disrupting peptide, B28, to tumor cells. To this end, we created a chimeric peptide, B28Bn(6–14), by conjugating B28 to Bn(6–14) at its N-terminus. The cytotoxicity of B28Bn(6–14) in tumor cells was much stronger than unconjugated B28. The IC₅₀ values of B28Bn(6–14) in tumor cells (1.7–3.5 µM) were approximately 10 times lower than B28. However, conjugation of B28 to Bn(2–7), which lacks the bombesin receptor-binding motif, did not increase its cytotoxicity. In addition, the IC₅₀ values of B28Bn(6–14) in tumor cells (1.7–3.5 µM) was 3–10 times lower than in normal cells (10.8–16.8 µM). We found that selective binding of B28Bn(6–14) to tumor cells is Bn(6–14)-dependent. Upon entering the tumor cell, B28Bn(6–14) accumulated in the mitochondria and triggered caspase-dependent apoptosis. Intratumoral and intraperitoneal administration of B28Bn(6–14) substantially suppressed the growth of DU145 tumor xenografts in mice. These results demonstrate that Bn(6–14) is able to deliver the mitochondria-disrupting peptide to tumor cells, and B28Bn(6–14) should be further developed as novel anti-cancer agent.

Paclitaxel inhibits ovarian tumor growth by inducing epithelial cancer cells to benign fibroblast-like cells

Paclitaxel is commonly used to treat multiple human malignancies, but its mechanism of action is still poorly defined. Human ovarian cancer SKOV3 cells (parental SKOV3) were treated with paclitaxel (1μM) for 2days, and the morphologic changes in the cells were monitored for more than 4months. Parental SKOV3 underwent a markedly morphologic transition from the epithelial to fibroblast-like phenotype following treatment with paclitaxel; the resulting cells were designated as SKOV3-P. The SKOV3-P cells' proliferative ability was assessed via a 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) assay. The molecular characteristics of these cells were assessed via immunocytochemical staining and Western blot analysis. Their invasiveness and tumor formation ability was evaluated via wound-scratch and colony formation assays. The tumorigenicity of SKOV3-P cells was assessed in vivo after subcutaneous injection of tumor cells between injections of parental and paclitaxel-treated cells in nude mice. SKOV3-P cells have decreased the proliferation and invasion ability, decreased colony-forming ability when cultured in Matrigel and lost their tumor formation as compared with parental SKOV3 cells when injected in nude mice. SKOV3-P cells have decreased expression of E-cadherin, cytokeratin, Snail, PI3K, and P-Akt-Ser473, and increased expression of fibronectin, vimentin, Slug, P27, and PTEN. These results demonstrated that paclitaxel can inhibit tumor growth by inducing ovarian cancer epithelial cells toward a benign fibroblast-like phenotype through dysregulation of previously known pathways involved in the regulation of epithelial to mesenchymal transition (EMT), which may represent a novel mechanism for paclitaxel-induced tumor suppression.

Enhancement of cytotoxicity of antimicrobial peptide magainin II in tumor cells by bombesin-targeted delivery

AIM

To investigate whether the conjugation of magainin II (MG2), an antimicrobial peptides (AMPs), to the tumor-homing peptide bombesin could enhance its cytotoxicity in tumor cells.

METHODS

A magainin II-bombesin conjugate (MG2B) was constructed by attaching magainin II (MG2) to bombesin at its N-terminus. The peptides were synthesized using Fmoc-chemistry. The in vitro cytotoxicity of the peptide in cancer cells was quantitatively determined using the CCK-8 cell counting kit. Moreover, the in vivo antitumor effect of the peptide was determined in tumor xenograft models.

RESULTS

The IC(50) of MG2B for cancer cells (10-15 μmol/L) was at least 10 times lower than the IC(50) of unconjugated MG2 (125 μmol/L). Moreover, the binding affinity of MG2B for cancer cells was higher than that of unconjugated MG2. In contrast, conjugation to a bombesin analog lacking the receptor-binding domain failed to increase the cytotoxicity of MG2, suggesting that bombesin conjugation enhances the cytotoxicity of MG2 in cancer cells through improved binding. Indeed, MG2B selectively induced cell death in cancer cells in vitro with the IC(50) ranging from 10 to 15 μmol/L, which was about 6-10 times lower than the IC(50) for normal cells. MG2B (20 mg/kg per day, intratumorally injected for 5 d) also exhibited antitumor effects in mice bearing MCF-7 tumor grafts. The mean weights of tumor grafts in MG2B- and PBS-treated mice were 0.21±0.05 g and 0.59±0.12 g, respectively.

CONCLUSION

The results suggest that conjugation of AMPs to bombesin might be an alternative approach for targeted cancer therapy.

The galanin system in cancer

Many tumours of neuroendocrine origin, and also an increasing number of non-neuroendocrine cancers, have been shown to express neuropeptides and/or their corresponding receptors. These peptides and receptors represent the molecular basis for in vivo diagnostic or therapeutic targeting of cancer with radiolabelled or cytotoxic peptide analogues. Galanin is a classical neuropeptide that functions in diverse physiological processes such as food intake, nociception, and blood pressure regulation, and it can also act as a growth factor for neurons. Expression of galanin peptide has been detected in pheochromocytoma, pituitary adenoma, neuroblastic tumours, gastrointestinal cancer, squamous cell carcinoma, brain tumours, melanoma, breast cancer and embryonal carcinoma. In several cancers and tumour cell lines expression of galanin receptors--three are known (GalR1, 2 and 3)--has been shown as well. Expression of peptide or receptors has been correlated with tumour stage or subtypes of pituitary adenoma, neuroblastic tumours, colon carcinoma and squamous cell carcinoma. Galanin treatment has tumour-reducing effects in murine models of gastrointestinal cancer, whereas in animal experiments on adenoma formation, galanin seems to act as a growth factor, promoting both proliferation and tumour formation. In cell culture experiments on tumour cell lines, galanin has shown growth promoting or inhibiting effects. Activation of GalR1 is generally anti-proliferative, whereas activation of GalR2 can have pro- or anti-proliferative effects. Therefore, galanin and its receptors are promising targets for diagnosis and treatment of several types of tumours.

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

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

Synthesis and biological evaluation of a novel pentagastrin-toxin conjugate designed for a targeted prodrug mono-therapy of cancer

A novel carbamate prodrug 2 containing a pentagastrin moiety was synthesized. 2 was designed as a detoxified analogue of the highly cytotoxic natural antibiotic duocarmycin SA (1) for the use in a targeted prodrug monotherapy of cancers expressing cholecystokinin (CCK-B)/gastrin receptors. The synthesis of prodrug 2 was performed using a palladium-catalyzed carbonylation of bromide 6, followed by a radical cyclisation to give the pharmacophoric unit 10, coupling of 10 to the DNA-binding subunit 15 and transformation of the resulting seco-drug 3b into the carbamate 2 via addition of a pentagastrin moiety.

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.

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

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

Therapy of ovarian cancers with targeted cytotoxic analogs of bombesin, somatostatin, and luteinizing hormone-releasing hormone and their combinations

The aim of this study was to investigate the effect of treatment of experimental ovarian cancers with targeted cytotoxic analogs as single compounds and in combination. Targeted cytotoxic analogs of bombesin (AN-215), somatostatin (AN-238), and luteinizing hormone-releasing hormone (AN-207) consisted of 2-pyrrolinodoxorubicin (AN-201) linked to the respective peptide carrier. AN-238 at 200 nmol/kg significantly inhibited growth of UCI-107, ES-2 and OV-1063 ovarian cancers. AN-215 alone at 200 nmol/kg and its combination with AN-238 at one-half of the dose were also able to inhibit the growth of UCI-107 tumors. A combination of AN-238 with AN-207at 50% of the dose strongly suppressed the proliferation of ES-2 and OV-1063 ovarian tumors. Cytotoxic radical AN-201 was toxic and had no significant effect on tumor growth. In contrast, the toxicity of the conjugated peptide analogs was low. Because ovarian cancers tend to acquire chemoresistance, we used real-time PCR to measure the mRNA expression of multidrug resistance protein 1, multidrug resistance-related protein 1, and breast cancer resistance protein after treatment. Low or no induction of multidrug resistance protein 1, multidrug resistance-related protein, and breast cancer resistance protein occurred after treatment with AN-238, AN-215, and the combination of AN-238 with AN-207 or AN-215. These results demonstrate that a therapy with cytotoxic analogs such as single agents and combinations is effective and nontoxic. Our work suggests that cytotoxic peptide analogs of luteinizing hormone-releasing hormone, somatostatin, and bombesin could be used for the therapy of ovarian cancers, considering the lack of induction of chemoresistance.

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.