Lipophilization of somatostatin analog RC-160 with long chain fatty acid improves its antiproliferative and antiangiogenic activity in vitro

Assembly of pH-Responsive Antibody-Drug-Inspired Conjugates

With the advent of chemical strategies that allow the design of smart bioconjugates, peptide- and protein-drug conjugates are emerging as highly efficient therapeutics to overcome limitations of conventional treatment, as exemplified by antibody-drug conjugates (ADCs). While targeting peptides serve similar roles as antibodies to recognize overexpressed receptors on diseased cell surfaces, peptide-drug conjugates suffer from poor stability and bioavailability due to their low molecular weights. Through a combination of a supramolecular protein-based assembly platform and a pH-responsive linker, the authors devise herein the convenient assembly of a trivalent protein-drug conjugate. The conjugate should ideally possess distinct features of ADCs such as 1) recognition sites that recognize cell receptor and are arranged on 2) distinct locations on a high molecular weight protein scaffold, 3) a stimuli-responsive linker, as well as 4) an attached payload such as a drug molecule. These AD-like conjugates target cancer cells that overexpress somatostatin receptors, can enable controlled release in the microenvironment of cancer cells through a new pH-responsive biotin linker, and exhibit stability in biological media.

A two-step strategy to enhance activity of low potency peptides

Novel strategies are needed to expedite the generation and optimization of peptide probes targeting G protein-coupled receptors (GPCRs). We have previously shown that membrane tethered ligands (MTLs), recombinant proteins comprised of a membrane anchor, an extracellular linker, and a peptide ligand can be used to identify targeted receptor modulators. Although MTLs provide a useful tool to identify and/or modify functionally active peptides, a major limitation of this strategy is the reliance on recombinant protein expression. We now report the generation and pharmacological characterization of prototype peptide-linker-lipid conjugates, synthetic membrane anchored ligands (SMALs), which are designed as mimics of corresponding MTLs. In this study, we systematically compare the activity of selected peptides as MTLs versus SMALs. As prototypes, we focused on the precursor proteins of mature Substance P (SubP) and Cholecystokinin 4 (CCK4), specifically non-amidated SubP (SubP-COOH) and glycine extended CCK4 (CCK4-Gly-COOH). As low affinity soluble peptides these ligands each presented a challenging test case for assessment of MTL/SMAL technology. For each ligand, MTLs and corresponding SMALs showed agonist activity and comparable subtype selectivity. In addition, our results illustrate that membrane anchoring increases ligand potency. Furthermore, both MTL and SMAL induced signaling can be blocked by specific non-peptide antagonists suggesting that the anchored constructs may be orthosteric agonists. In conclusion, MTLs offer a streamlined approach for identifying low activity peptides which can be readily converted to higher potency SMALs. The ability to recapitulate MTL activity with SMALs extends the utility of anchored peptides as probes of GPCR function.

Mechanisms of peptide amphiphile internalization by SJSA-1 cells in vitro

Self-assembly of peptide amphiphiles into nanostructures makes them attractive for a variety of applications in drug and peptide delivery. We here report on the interactions of micelles composed of a palmitoylated, pro-apoptotic peptide derived from p53 tumor suppressor protein with a human cancer cell line. Characterization of self-assembly in aqueous buffered solutions revealed formation of elongated rod-like micelles above a critical micelle concentration. Our results however demonstrate that monomers instead of micelles are internalized, a finding that correlates with the dynamic nature of the assemblies and the noncovalent interactions that hold them together. Internalization is shown to occur via adsorption-mediated, energy-dependent pathways, resulting in accumulation of the material in endocytic vesicles. We conclude that palmitoylation of peptides is an efficient way to increase peptide permeability inside SJSA-1 cells and that increased micelle stability would be required for intact micelle internalization.

Hypermethylation of the somatostatin promoter is a common, early event in human esophageal carcinogenesis

BACKGROUND

The promoter of somatostatin (SST), a primary inhibitor of gastrin-stimulated gastric acid secretion, is hypermethylated in 80% of human colon cancers. The aim of the current study was to investigate whether and at what stage promoter hypermethylation of SST is involved in human esophageal carcinogenesis.

METHODS

SST promoter hypermethylation was examined by real-time methylation-specific polymerase chain reaction (PCR) (MSP) in 260 human esophageal tissue specimens. Real-time reverse-transcriptase PCR and MSP were also performed on esophageal cancer cell lines before and after treatment with 5-aza-2'-deoxycytidine (5-Aza-dC).

RESULTS

SST hypermethylation showed highly discriminative receiver-operator characteristic curve profiles, clearly distinguishing esophageal squamous cell carcinomas (ESCC) and esophageal adenocarcinomas (EAC) from normal esophagus (NE) (P < .01). Both SST methylation frequency and normalized methylation value (NMV) were significantly higher in Barrett metaplasia without dysplasia or EAC (BE), low-grade and high-grade (HGD) dysplasia occurring in BE, EAC, and ESCC than in NE (P < .01). SST hypermethylation frequency was significantly lower in NE (9%) than in BE (70%), HGD (71.4%), or EAC (71.6%), whereas 14 (53.8%) of 26 ESCCs exhibited SST hypermethylation. There was a significant relation between SST hypermethylation and BE segment length, a known clinical risk factor for neoplastic progression. Demethylation of KYSE220 ESCC and OE33 EAC cells with 5-Aza-dC reduced SST methylation and increased SST mRNA expression. SST mRNA levels in native unmethylated EACs were significantly higher than in native methylated EACs (P < .05).

CONCLUSIONS

SST promoter hypermethylation is a common event in human esophageal carcinomas and is related to early neoplastic progression in Barrett esophagus.

Myristoyl-based transport of peptides into living cells

Translocation of membrane-impermeant molecules to the interior of living cells is a necessity for many biochemical investigations. Myristoylation was studied as a means to introduce peptides into living cells. Uptake of a myristoylated, fluorescent peptide was efficient in the B lymphocyte cell line BA/F3. In contrast, this cell line was resistant to uptake of a cell-penetrating peptide derived from the TAT protein. In BA/F3 cells, membrane association was shown to be rapid, reaching a maximum within 30 min. Cellular uptake of the peptide lagged the membrane association but occurred within a similar time frame. Experiments performed at 37 versus 4 degrees C demonstrated profound temperature dependence in the cellular uptake of myristoylated cargo. Myristoylated peptides with either positive or negative charge were shown to load efficiently. In contrast to TAT-conjugated cargo, pyrenebutyrate did not enhance cellular uptake of the myristoylated peptide. The myristoylated peptide did not adversely affect cell viability at concentrations up to 100 muM. This assessment of myristoyl-based transport provides fundamental data needed in understanding the intracellular delivery of myristoylated peptide cargoes for cell-based biochemical studies.

A New Prostate Carcinoma Binding Peptide (DUP-1) for Tumor Imaging and Therapy

Purpose: Prostate carcinomas belong to the most widespread tumors, and their number is increasing. Imaging modalities used for diagnosis, such as ultrasound, computed tomography, and positron emission tomography, often produce poor results. Radiolabeled peptides with high sensitivity and specificity for prostate cancer would be a desirable tool for tumor diagnosis and treatment.

Experimental Design: We used phage display and the prostate-specific membrane antigen–negative cell line DU-145 to identify a peptide. The isolated DUP-1 was tested invitro for its binding specificity, kinetics, and affinity. Internalization of the peptide was evaluated with confocal microscopy. The tumor accumulation in a nude mouse model was analyzed with 131I-labeled DUP-1 in PC-3 and DU-145 prostate tumors as well as in the rat prostate tumor model AT-1.

Results: The synthesized peptide showed rapid binding kinetics peaking at 10 minutes. It shows specific binding to prostate carcinoma cells but low binding affinity to nontumor cells. Peptide binding is competed with unlabeled DUP-1, and a time-dependent internalization into DU-145 cells was shown. Biodistribution studies of DUP-1 in nude mice with s.c. transplanted DU-145 and PC-3 tumors showed a tumor accumulation of 5% and 7% injected dose per gram, and bound peptide could not be removed by perfusion. The rat prostate tumor model showed an increase of radioactivity in the prostate tumor up to 300% in comparison with normal prostate tissue.

Conclusions: DUP-1 holds promise as a lead peptide structure applicable in the development of new diagnostic tracers or anticancer agents that specifically target prostate carcinoma.

Somatostatin analogues: multiple roles in cellular proliferation, neoplasia, and angiogenesis

Angiogenesis, the development of new blood vessels is a crucial process both for tumor growth and metastatic dissemination. Additionally, dysregulation in angiogenesis has been implicated in the pathogenesis of cardiovascular disease, proliferative retinopathy, diabetic nephropathy, and rheumatoid arthritis (RA). The neuropeptide somatostatin has been shown to be a powerful inhibitor of neovascularization in several experimental models. Furthermore, somatostatin receptors (sst) are expressed on endothelial cells; particularly, sst2 has been found to be uniquely up-regulated during the angiogenic switch, from quiescent to proliferative endothelium. The present manuscript reviews the anti-angiogenic activity of somatostatin and its analogues in neoplastic and nonneoplastic disease. The role of sst subtypes particularly sst2 in mediating its angioinhibitory activity is described. Somatostatin agonists may also exert their anti-angiogenic activity indirectly by inhibition of growth factors like vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis or through its immunomodulatory effects. However, the therapeutic utility of somatostatin agonists as anti-angiogenic drugs in these diseases remains confusing because of conflicting results from different studies. More basic research, as well as patient-oriented studies, is required to firmly establish the clinical potential of somatostatin agonists in therapeutic angiogenesis. The currently available somatostatin agonists have high affinity of sst2 with lower affinities for sst3 and sst5. The emergence of novel somatostatin agonists especially bispecific analogues (agonists targeting multiple cellular receptors) and conjugates (synthesized by chemically linking somatostatin analogues with other antineoplastic agents) with improved receptor specificity signify a new generation of anti-angiogenics, which may represent novel strategies in the treatment of neovascularization-related diseases.

Somatostatin receptors: from basic science to clinical approach. Unlabeled somatostatin analogues-1: Prostate cancer

Neuroendocrine cells have been found in all the stages of prostate cancer, but their clinical significance is not completely understood. Neuroendocrine cells are androgen receptor- and prostate-specific antigen-negative, do not proliferate, and secrete many neuropeptides, such as chromogranin A. Neuroendocrine differentiation of prostate cancer correlates with an advancing tumour stage, poor prognosis and tumour progression after androgen deprivation. Furthermore, neuroendocrine phenotype is associated with the increased expression of neo-angiogenesis and vascular endothelial growth factor and with an over-expression of survivin, a new anti-apoptosis protein. Chromogranin A is the quantitatively major secretory protein of the vesicles inside neuroendocrine prostate cells and it is the marker most frequently used to detect neuroendocrine features, both in tissues and in general circulation. Tumours displaying neuroendocrine phenotype tend to be more aggressive and resistant to hormone-therapy. Neuroendocrine differentiation seems to be a dynamic phenomenon: in vitro and in vivo data suggest that it can be induced by androgen suppression. Moreover, the differences in the expression of somatostatin receptors between primary and hormone-refractory prostate cancer are likely to be related to the changes in neuroendocrine phenotype during androgen deprivation. Circulating chromogranin A levels seem to be scarcely affected by endocrine- and chemotherapy, while they significantly decreased after treatment with somatostatin analogs.

Pharmacological properties of hydrophilic and lipophilic derivatives of octreotate

Derivatives of somatostatin (SST) represent the most important peptides for receptor targeting in oncological applications. Whereas the pharmacophor in somatostatin receptor-affine substances has been thoroughly investigated, the influence of modifications at the N-terminal has not yet been systematically studied. In order to investigate the influence of hydrophilic versus lipophilic modifications at the N-terminal end, a series of homologous derivatives of Tyr3-octreotate modified with oligomers of ethylene glycol or fatty acids were synthesized. For this purpose, Tyr3-octreotate was assembled using solid phase peptide synthesis and the fatty acids or oligomers of ethylene glycol were conjugated to the N-terminal end. The oligomers of ethylene glycol were activated by 4-nitrophenylchloroformate to obtain carbamate-linked hydrophilic compounds. The receptor affinities of these compounds were determined by competition experiments with [125I]Tyr3-octreotide on rat cortex membranes. The hydrophilic derivatives and the short chain lipophilic derivatives revealed IC50 values between 0.66 +/- 0.02 nM and 2.16 +/- 0.31 nM respectively. After labeling with (125)I the organ distribution of selected derivatives was investigated in Lewis rats bearing the rat pancreatic tumor CA20948. All of the compounds showed high tumor uptake. The peptides conjugated to oligomers of ethylene glycol showed low uptake into the liver and kidneys. Increasing the length of the fatty acids resulted in a remarkable decrease in kidney uptake. In conclusion, the systematic modifications at the N-terminal result in a low effect on the receptor affinity but allow the modulation of the pharmacokinetic properties of octreotide derivatives.

Dimethylarginine dimethylaminohydrolase activity modulates ADMA levels, VEGF expression, and cell phenotype

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase and is metabolised by dimethylarginine dimethylaminohydrolase (DDAH). Elevated levels of circulating ADMA correlate with various cardiovascular pathologies less is known about the cellular effects of altered DDAH activity. We modified DDAH activity in cells and measured the changes in ADMA levels, morphological phenotypes on Matrigel, and expression of vascular endothelial growth factor (VEGF). DDAH over-expressing ECV304 cells secreted less ADMA and when grown on Matrigel had enhanced tube formation compared to untransfected cells. VEGF mRNA levels were 2.1-fold higher in both ECV304 and murine endothelial cells (sEnd.1) over-expressing DDAH. In addition the DDAH inhibitor, S-2-amino-4(3-methylguanidino)butanoic acid (4124W 1mM), markedly reduced human umbilical vein endothelial cell tube formation in vitro. We have found that upregulating DDAH activity lowers ADMA levels, increases the levels of VEGF mRNA in endothelial cells, and enhances tube formation in an in vitro model, whilst blockade of DDAH reduces tube formation.

Improving pharmacokinetic properties of adrenocorticotropin by site-specific lipid modification

Although many peptides are potentially good therapeutic agents for treating various diseases, only a few have been developed for limited applications. A major shortcoming is that peptides have generally very short serum half lives. In the present study, we use adrenocorticotropin (ACTH) as a model and explore the potential of combining site-specific amino acid substitution and lipid modification to increase the circulating half-lives of peptides. Phe39 of ACTH was substituted by Cys, which has a free sulfhydryl group that can react specifically with iodoacetamide derivatives of lipophilic groups. The biological activities of lipophilized ACTH(F39C)s were higher than native ACTH. Lipophilized ACTH(F39C)s bound more tightly to human serum albumin and cell membranes in vitro and had longer serum half-lives in vivo than native ACTH. These results indicate that the pharmacokinetic properties of peptides can be improved by site-specific substitution with cysteine residues and subsequent conjugation with lipophilic moieties.

Antiangiogenic effects of somatostatin analogues

Inhibition of angiogenesis has become a target for antineoplastic therapy and for treatment of retinal neovascularization. The presence of somatostatin receptors on tumour cells and on the proliferating vascular endothelium has led to several in vitro and in vivo studies to investigate the antiproliferative and antiangiogenic effects of somatostatin analogues. Currently available data suggest that somatostatin analogues might inhibit angiogenesis directly through somatostatin receptors present on endothelial cells and also indirectly through the inhibition of growth factor secretion such as IGF-I and vascular endothelial growth factor (VEGF) and reducing monocyte chemotaxis. However, beneficial effects on inhibition of neovascularization have been questioned by some studies. More work is therefore required to firmly establish the role of somatostatin analogues as potential antiangiogenic therapy. The currently available somatostatin analogues have high affinity for somatostatin receptor subtype 2 (sst2) and, to a lesser extent, sst5 and sst3. However, because vascular endothelial cells express several types of somatostatin receptors, it will be important to investigate somatostatin analogues with different receptor subtype affinities, which might increase the spectrum of available therapy for tumours.