Synthesis and biological evaluation (in vitro and in vivo) of cyclic arginine-glycine-aspartate (RGD) peptidomimetic-paclitaxel conjugates targeting integrin αVβ3

Small molecule- and peptide-drug conjugates addressing integrins: A story of targeted cancer treatment

Targeted cancer treatment should avoid side effects and damage to healthy cells commonly encountered during traditional chemotherapy. By combining small molecule or peptidic ligands as homing devices with cytotoxic drugs connected by a cleavable or non-cleavable linker in peptide-drug conjugates (PDCs) or small molecule-drug conjugates (SMDCs), cancer cells and tumours can be selectively targeted. The development of highly affine, selective peptides and small molecules in recent years has allowed PDCs and SMDCs to increasingly compete with antibody-drug conjugates (ADCs). Integrins represent an excellent target for conjugates because they are overexpressed by most cancer cells and because of the broad knowledge about native binding partners as well as the multitude of small-molecule and peptidic ligands that have been developed over the last 30 years. In particular, integrin αVβ3 has been addressed using a variety of different PDCs and SMDCs over the last two decades, following various strategies. This review summarises and describes integrin-addressing PDCs and SMDCs while highlighting points of great interest.

Small Molecule-Drug Conjugates Emerge as a New Promising Approach for Cancer Treatment

Antibody drug conjugates (ADCs) have emerged as a new promising class of anti- cancer agents. However, limitations such as higher costs and unavoidable immunogenicity due to their relatively large structures cannot be ignored. Therefore, the development of lightweight drugs such as small molecule-drug conjugates (SMDCs) based on the ADC design idea has become a new option for targeted therapy. SMDCs are derived from the coupling of small-molecule targeting ligands with cytotoxic drugs. They are composed of three parts: small-molecule targeting ligands, cytotoxic molecules, and linkers. Compared with ADCs, SMDCs can be more rapidly and evenly dispersed into tumor tissues, with low cost and no immunogenicity. In this article, we will give a comprehensive review of different types of SMDCs currently under clinical trials to provide ideas and inspirations for the development of clinically applicable SMDCs.

Synthesis of Novel Carborane-Containing Derivatives of RGD Peptide

Short peptides containing the Arg-Gly-Asp (RGD) fragment can selectively bind to integrins on the surface of tumor cells and are attractive transport molecules for the targeted delivery of therapeutic and diagnostic agents to tumors (for example, glioblastoma). We have demonstrated the possibility of obtaining the N- and C-protected RGD peptide containing 3-amino-closo-carborane and a glutaric acid residue as a linker fragment. The resulting carboranyl derivatives of the protected RGD peptide are of interest as starting compounds in the synthesis of unprotected or selectively protected peptides, as well as building blocks for preparation of boron-containing derivatives of the RGD peptide of a more complex structure.

Development and Biochemical Characterization of Self-Immolative Linker Containing GnRH-III-Drug Conjugates

The human gonadotropin releasing hormone (GnRH-I) and its sea lamprey analogue GnRH-III specifically bind to GnRH receptors on cancer cells and can be used as targeting moieties for targeted tumor therapy. Considering that the selective release of drugs in cancer cells is of high relevance, we were encouraged to develop cleavable, self-immolative GnRH-III-drug conjugates which consist of a p-aminobenzyloxycarbonlyl (PABC) spacer between a cathepsin B-cleavable dipeptide (Val-Ala, Val-Cit) and the classical anticancer drugs daunorubicin (Dau) and paclitaxel (PTX). Alongside these compounds, non-cleavable GnRH-III-drug conjugates were also synthesized, and all compounds were analyzed for their antiproliferative activity. The cleavable GnRH-III bioconjugates revealed a growth inhibitory effect on GnRH receptor-expressing A2780 ovarian cancer cells, while their activity was reduced on Panc-1 pancreatic cancer cells exhibiting a lower GnRH receptor level. Moreover, the antiproliferative activity of the non-cleavable counterparts was strongly reduced. Additionally, the efficient cleavage of the Val-Ala linker and the subsequent release of the drugs could be verified by lysosomal degradation studies, while radioligand binding studies ensured that the GnRH-III-drug conjugates bound to the GnRH receptor with high affinity. Our results underline the high value of GnRH-III-based homing devices and the application of cathepsin B-cleavable linker systems for the development of small molecule drug conjugates (SMDCs).

The development of peptide-drug conjugates (PDCs) strategies for paclitaxel

INTRODUCTION

Paclitaxel is a powerful and effective anti-tumor drug with wide clinical application. However, there are still some limitations, including poor water solubility, low specificity, and susceptibility to drug resistance. The peptide-drug conjugates (PDCs) represent a rising class of therapeutic drugs, which combines small-molecule chemotherapeutic drugs with highly flexible peptides through a cleavable or non-cleavable linker. When this strategy is applied, the therapeutic effects of paclitaxel can be improved.

AREAS COVERED

In this review, we discuss the application of the PDCs strategy in paclitaxel, including two parts: the tumor targeting peptide-paclitaxel conjugates and the cell penetrating peptide-paclitaxel conjugates.

EXPERT OPINION

Combining drugs with multifunctional peptides covalently is an effective strategy for delivering paclitaxel to tumors. Depending on different functional peptides, conjugates can increase the water solubility of paclitaxel, tumor permeability of paclitaxel, the accumulation of paclitaxel in tumor tissues, and enhance the antitumor effect of paclitaxel. In addition, due to the change of cell entry mechanism, partial conjugates can restore the therapeutic activity of paclitaxel against resistant tumors. Notably, in order to better translate into the clinical field in the future, more research should be conducted to ensure the safety and effectiveness of peptide-paclitaxel conjugates.

Current understandings and clinical translation of nanomedicines for breast cancer therapy

Breast cancer is one of the most frequently diagnosed cancers that is threatening women's life. Current clinical treatment regimens for breast cancer often involve neoadjuvant and adjuvant systemic therapies, which somewhat are associated with unfavorable features. Also, the heterogeneous nature of breast cancers requires precision medicine that cannot be fulfilled by a single type of systemically administered drug. Taking advantage of the nanocarriers, nanomedicines emerge as promising therapeutic agents for breast cancer that could resolve the defects of drugs and achieve precise drug delivery to almost all sites of primary and metastatic breast tumors (e.g. tumor vasculature, tumor stroma components, breast cancer cells, and some immune cells). Seven nanomedicines as represented by Doxil® have been approved for breast cancer clinical treatment so far. More nanomedicines including both non-targeting and active targeting nanomedicines are being evaluated in the clinical trials. However, we have to realize that the translation of nanomedicines, particularly the active targeting nanomedicines is not as successful as people have expected. This review provides a comprehensive landscape of the nanomedicines for breast cancer treatment, from laboratory investigations to clinical applications. We also highlight the key advances in the understanding of the biological fate and the targeting strategies of breast cancer nanomedicine and the implications to clinical translation.

Protocol for the preparation of site-specific succinylated histone mimics to investigate the impact on nucleosome dynamics

Lysine succinylation is a recently discovered posttranslational modification that plays critical roles in metabolism, epigenetic signaling, and human diseases. To investigate the effects of site-specific histone lysine succinylation on nucleosome dynamics requires the generation of homogeneously modified histones, which is a significant challenge. Here, we report a protocol for the rapid site-specific installation of a succinyl lysine analog onto histone. We then use a Förster resonance energy transfer approach to characterize the impact on nucleosome dynamics.

For complete details on the use and execution of this protocol, please refer to Jing et al. (2018).

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Strategy for site-specific installation of a succinyl-lysine analog into histones

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Strategy is generally applicable to all histones and other acid-resistant proteins

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A FRET-based biophysical assay reveals that H2BK34succ destabilizes nucleosomes

Synthetic Integrin-Targeting Dextran-Fc Hybrids Efficiently Inhibit Tumor Proliferation In Vitro

Herein, we present the design, synthesis, and biological evaluation of novel integrin-targeting molecular hybrids combining RGD peptides and a potent cytotoxin presented on dextran polysaccharides. Based on an aglycosylated Fc as a centerpiece, endosomal-cleavable cytotoxic agent monomethyl auristatin E (MMAE) and dextran as multimerization site were covalently connected by two bioorthogonal enzyme-mediated reactions site-specifically. Decoration of dextran with cyclic RGD peptides, introduced by copper “click” reaction, resulted in the final constructs with the potential to kill integrin-overexpressing tumor cells. We found that these modifications had little impact on the stability of the Fc scaffold and the RGD-bearing construct showed good binding properties of αvβ3-expressing U87MG cells. Furthermore, the construct showed a remarkable antiproliferative activity. These results demonstrate the general capability of our design to provoke receptor-mediated endocytosis upon binding to the cellular surface, followed by endosomal cleavage of the linkage between Fc-dextran and MMAE and its subsequent release. Our approach opens new avenues to transcribe small molecule binders into tailor-made multimeric molecular hybrids with antitumor potential.

Cyclic Dipeptide: A Privileged Molecular Scaffold to Derive Structural Diversity and Functional Utility

Cyclic dipeptides (CDPs) are the simplest form of cyclic peptides with a wide range of applications from therapeutics to biomaterials. CDP is a versatile molecular platform endowed with unique properties such as conformational rigidity, intermolecular interactions, structural diversification through chemical synthesis, bioavailability and biocompatibility. A variety of natural products with the CDP core exhibit anticancer, antifungal, antibacterial, and antiviral activities. The inherent bioactivities have inspired the development of synthetic analogues as drug candidates and drug delivery systems. CDP plays a crucial role as conformation and molecular assembly directing core in the design of molecular receptors, peptidomimetics and fabrication of functional material architectures. In recent years, CDP has rapidly become a privileged scaffold for the design of advanced drug candidates, drug delivery agents, bioimaging, and biomaterials to mitigate numerous disease conditions. This review describes the structural diversification and multifarious biomedical applications of the CDP scaffold, discusses challenges, and provides future directions for the emerging field.

Cyclic RGD and isoDGR Integrin Ligands Containing cis-2-amino-1-cyclopentanecarboxylic (cis-β-ACPC) Scaffolds

Integrin ligands containing the tripeptide sequences Arg-Gly-Asp (RGD) and iso-Asp-Gly- Arg (isoDGR) were actively investigated as inhibitors of tumor angiogenesis and directing unit in tumor-targeting drug conjugates. Reported herein is the synthesis, of two RGD and one isoDGR cyclic peptidomimetics containing (1S,2R) and (1R,2S) cis-2-amino-1-cyclopentanecarboxylic acid (cis-β-ACPC), using a mixed solid phase/solution phase synthetic protocol. The three ligands were examined in vitro in competitive binding assays to the purified α_vβ₃ and α₅β₁ receptors using biotinylated vitronectin (α_vβ₃) and fibronectin (α₅β₁) as natural displaced ligands. The IC50 values of the ligands ranged from nanomolar (the two RGD ligands) to micromolar (the isoDGR ligand) with a pronounced selectivity for α_vβ₃ over α₅β₁. In vitro cell adhesion assays were also performed using the human skin melanoma cell line WM115 (rich in integrin α_vβ₃). The two RGD ligands showed IC₅₀ values in the same micromolar range as the reference compound (cyclo[RGDfV]), while for the isoDGR derivative an IC₅₀ value could not be measured for the cell adhesion assay. A conformational analysis of the free RGD and isoDGR ligands by NMR (VT-NMR and NOESY experiments) and computational studies (MC/EM and MD), followed by docking simulations performed in the α_Vβ₃ integrin active site, provided a rationale for the behavior of these ligands toward the receptor.

Three cheers for nitrogen: aza-DKPs, the aza analogues of 2,5-diketopiperazines

Nitrogen-containing heterocycles represent a major source of pharmacological probes and drug candidates. To extend their molecular diversity and their potential biological activities, it is of importance to design and synthesize new N-heterocyclic scaffolds. Therefore, aza-diketopiperazines (aza-DKPs), the aza analogues of well-known 2,5-diketopiperazines (DKPs), emerged as a promising new scaffold. Although the first synthesis of an aza-DKP dates from 1951, significant developments have been made during the last decade. This feature article summarizes the different synthetic strategies to access and functionalise aza-DKPs. Their biological properties and potential applications in medicinal chemistry and drug discovery are discussed as well.

Multimeric Presentation of RGD Peptidomimetics Enhances Integrin Binding and Tumor Cell Uptake

The use of multimeric ligands is considered as a promising strategy to improve tumor targeting for diagnosis and therapy. Herein, tetrameric RGD (Arg-Gly-Asp) peptidomimetics were designed to target α_v β₃ integrin-expressing tumor cells. These compounds were prepared by an oxime chemoselective assembly of cyclo(DKP-RGD) ligands and a cyclodecapeptide scaffold, which allows a tetrameric presentation. The resulting tetrameric RGD peptidomimetics were shown to improve α_v β₃ integrin binding compared with the monomeric form. Interestingly, these compounds were also able to enhance tumor cell endocytosis in the same way as tetrameric RGD peptides. Altogether, the results show the potential of the tetrameric cyclo(DKP-RGD) ligands for in vivo imaging and drug delivery.

β-Glucuronidase triggers extracellular MMAE release from an integrin-targeted conjugate

A non-internalizing αvβ3 integrin ligand was conjugated to the anticancer drug MMAE through a β-glucuronidase-responsive linker. In the presence of β-glucuronidase, only the conjugate bearing a PEG4 spacer inhibited the proliferation of integrin-expressing cancer cells at low nanomolar concentrations, indicating important structural requirements for the efficacy of these therapeutics.

Synthesis and biological evaluation of RGD conjugated with Ketoprofen/Naproxen and radiolabeled with [99mTc] via N4(GGAG) for αVβ3 integrin-targeted drug delivery

BACKGROUND

Integrins are interesting targets in oncology. RGD sequence has high affinity for α_Vβ₃ integrin receptors. Diagnostic/therapeutic agents can be selectively delivered into cancer cells overexpressing α_Vβ₃ integrin by using RGD as a carrier. Nonsteroidal anti-inflammatory drugs (NSAIDs) have shown anticancer properties in in vitro and in vivo studies. The anti-cancer properties of NSAIDs occur though COX-2 inhibition. Regarding the anti-cancer properties of NSAIDs and overexpression of COX-2 enzyme in cancer cells, targeted delivery of NSAIDs into cancer cells to maximize their efficiency and minimize their side effects may gain increased clinical interest.

OBJECTIVES

In this study, RGD was conjugated to ketoprofen/Naproxen to selectively transfer these non-selective COX inhibitors into cancer cells.

METHODS

Keto/Nap-RGD-N4 peptides were synthesized based on solid phase fmoc peptide synthesis. Radiolabeling with [^99mTc] via N4 (GGAG) ligand was done for biological evaluation. Affinity and specificity of Keto/Nap-RGD-N4 to integrin was determined using A2780, OVCAR-3, SKOV-3 and HT-1080 cell lines. Percentage of Intenalization was measured in A2780 cells. Biodistriburion was studied in normal and tumor model mice.

RESULTS

Radiolabeled compounds showed high affinity to cells expressing α_Vβ₃ integrin in comparison to cells not expressing α_Vβ₃. The affinity to A2780 was significantly higher than OVCAR-3 cells. The %internalization into A2780 cells was quite low. Compounds showed more than 50% inhibition on A2780 and OVCAR-3 cells, less than 10% on MCF-7 and HT-1080 cells and no cytotoxicity on fibroblast cells after 48 h incubation. Although uptake of radiolabeled compounds in tumor was high at 1 h post-injection, the tumor/blood ratio was less than 1.5 which made SPECT imaging impossible.

CONCLUSION

Provided that NSAID drugs are conjugated to RGD, there will be a selective delivery to target tissues as well as synergetic anti-tumor effects which reduce systemic doses and toxicity. Graphical abstract.

iRGD-paclitaxel conjugate nanoparticles for targeted paclitaxel delivery

Paclitaxel (PTX) is a chemotherapeutic agent which shows antitumor activities against a broad spectrum of cancers. Yet, the current formulation of PTX used in clinic may cause a number of adverse reactions, which significantly limit its application. To obtain better clinical use of PTX, we report, for the first time, iRGD-PTX conjugate nanoparticles (NPs) for targeted PTX delivery. iRGD-PTX conjugate was synthesized from thiolated iRGD and 6-maleimidocaproic acid-PTX through Michael addition reaction. iRGD-PTX NPs with hydrodynamic diameter of ~110 nm were self-assembled from iRGD-PTX conjugate in deionized water. The as-prepared iRGD-PTX NPs exhibit good stability in phosphate buffered saline (PBS) buffer and fetal bovine serum containing PBS buffer. iRGD-PTX NPs exhibit sustained drug release behaviors. The in vitro studies show that iRGD-PTX NPs can be internalized by 4T1 cells by integrin αV-mediated endocytosis, resulting in better in vitro antitumor activity as compared to free PTX. The in vivo studies demonstrate that iRGD-PTX NPs exhibit enhanced tumor accumulation. The iRGD-PTX NPs reported here represent a novel PTX nanoplatform to achieve targeted PTX delivery.

Bisubstrate Inhibitors of Nicotinamide N-Methyltransferase (NNMT) with Enhanced Activity

Nicotinamide N-methyltransferase (NNMT) catalyzes the methylation of nicotinamide to form N-methylnicotinamide. Overexpression of NNMT is associated with a variety of diseases, including a number of cancers and metabolic disorders, suggesting a role for NNMT as a potential therapeutic target. By structural modification of a lead NNMT inhibitor previously developed in our group, we prepared a diverse library of inhibitors to probe the different regions of the enzyme's active site. This investigation revealed that incorporation of a naphthalene moiety, intended to bind the hydrophobic nicotinamide binding pocket via π-π stacking interactions, significantly increases the activity of bisubstrate-like NNMT inhibitors (half-maximal inhibitory concentration 1.41 μM). These findings are further supported by isothermal titration calorimetry binding assays as well as modeling studies. The most active NNMT inhibitor identified in the present study demonstrated a dose-dependent inhibitory effect on the cell proliferation of the HSC-2 human oral cancer cell line.

Kiss and Run: Promoting Effective and Targeted Cellular Uptake of a Drug Delivery Vehicle Composed of an Integrin-Targeting Diketopiperazine Peptidomimetic and a Cell-Penetrating Peptide

Cell-penetrating peptides (CPPs) have emerged as powerful tools in terms of drug delivery. Those short, often cationic peptides are characterized by their usually low toxicity and their ability to transport diverse cargos inside almost any kinds of cells. Still, one major drawback is their nonselective uptake making their application in targeted cancer therapies questionable. In this work, we aimed to combine the power of a CPP (sC18) with an integrin-targeting unit (c[DKP-f3-RGD]). The latter is composed of the Arg-Gly-Asp peptide sequence cyclized via a diketopiperazine scaffold and is characterized by its high selectivity toward integrin α_vβ₃. The two parts were linked via copper-catalyzed alkyne-azide click reaction (CuAAC), while the CPP was additionally functionalized with either a fluorescent dye or the anticancer drug daunorubicin. Both functionalities allowed a careful biological evaluation of these novel peptide-conjugates regarding their cellular uptake mechanism, as well as cytotoxicity in α_vβ₃ integrin receptor expressing cells versus cells that do not express α_vβ₃. Our results show that the uptake follows a "kiss-and-run"-like model, in which the conjugates first target and recognize the receptor, but translocate mainly by CPP mediation. Thereby, we observed significantly more pronounced toxic effects in α_vβ₃ expressing U87 cells compared to HT-29 and MCF-7 cells, when the cells were exposed to the substances with only very short contact times (15 min). All in all, we present new concepts for the design of cancer selective peptide-drug conjugates.

Conjugates of Cryptophycin and RGD or isoDGR Peptidomimetics for Targeted Drug Delivery

RGD-cryptophycin and isoDGR-cryptophycin conjugates were synthetized by combining peptidomimetic integrin ligands and cryptophycin, a highly potent tubulin-binding antimitotic agent across lysosomally cleavable Val-Ala or uncleavable linkers. The conjugates were able to effectively inhibit binding of biotinylated vitronectin to integrin αvβ3, showing a binding affinity in the same range as that of the free ligands. The antiproliferative activity of the novel conjugates was evaluated on human melanoma cells M21 and M21-L with different expression levels of integrin αvβ3, showing nanomolar potency of all four compounds against both cell lines. Conjugates containing uncleavable linker show reduced activity compared to the corresponding cleavable conjugates, indicating efficient intracellular drug release in the case of cryptophycin-based SMDCs. However, no significant correlation between the in vitro biological activity of the conjugates and the integrin αvβ3 expression level was observed, which is presumably due to a non-integrin-mediated uptake. This reveals the complexity of effective and selective αvβ3 integrin-mediated drug delivery.

Synthesis and Biological Evaluation of RGD and isoDGR-Monomethyl Auristatin Conjugates Targeting Integrin αV β3

This work reports the synthesis of a series of small-molecule-drug conjugates containing the αV β3 -integrin ligand cyclo[DKP-RGD] or cyclo[DKP-isoDGR], a lysosomally cleavable Val-Ala (VA) linker or an "uncleavable" version devoid of this sequence, and monomethyl auristatin E (MMAE) or F (MMAF) as the cytotoxic agent. The conjugates were obtained via a straightforward synthetic scheme taking advantage of a copper-catalyzed azide-alkyne cycloaddition as the key step. The conjugates were tested for their binding affinity for the isolated αv β3 receptor and were shown to retain nanomolar IC50 values, in the same range as those of the free ligands. The cytotoxic activity of the conjugates was evaluated in cell viability assays with αv β3 integrin overexpressing human glioblastoma (U87) and human melanoma (M21) cells. The conjugates possess markedly lower cytotoxic activity than the free drugs, which is consistent with inefficient integrin-mediated internalization. In almost all cases the conjugates featuring isoDGR as integrin ligand exhibited higher potency than their RGD counterparts. In particular, the cyclo[DKP-isoDGR]-VA-MMAE conjugate has low nanomolar IC50 values in cell viability assays with both cancer cell lines tested (U87: 11.50±0.13 nm; M21: 6.94±0.09 nm) and is therefore a promising candidate for in vivo experiments.

Efficacy of a Selective Binder of αVβ3 Integrin Linked to the Tyrosine Kinase Inhibitor Sunitinib in Ovarian Carcinoma Preclinical Models

Ovarian carcinoma, the most lethal gynecological cancer, is characterized by late diagnosis, with drug resistance limiting the efficacy of platinum-based therapy. Since some integrins are upregulated in cancer, including ovarian carcinoma, they represent a potential target for drug delivery. Receptor tyrosine kinases are also deregulated in cancer and their expression has been associated with drug resistance. Here, the antitumor effects of three conjugates possessing a selective binder of the extracellular portion of integrin α_Vβ₃ covalently linked to the tyrosine kinase inhibitor sunitinib were investigated in cisplatin-sensitive and -resistant ovarian carcinoma cells expressing both tyrosine kinase VEGFR2 and α_Vβ₃ at different levels. We found that one of the three compounds was active in inhibiting the growth of both drug-sensitive and -resistant cells in the micromolar range with a slightly increased potency in resistant cells as compared to sunitinib. The same compound markedly impaired cell migratory and invasive abilities and reduced paxillin phosphorylation. Antitumor activity studies in IGROV-1/Pt1 cells xenografted in nude mice revealed a striking activity of this conjugate versus sunitinib. Taken together, our results support the interest of integrin-targeted sunitinib conjugates for the treatment of drug-resistant tumors.

Neutrophil Elastase Promotes Linker Cleavage and Paclitaxel Release from an Integrin-Targeted Conjugate

This work takes advantage of one of the hallmarks of cancer, that is, the presence of tumor infiltrating cells of the immune system and leukocyte-secreted enzymes, to promote the activation of an anticancer drug at the tumor site. The peptidomimetic integrin ligand cyclo(DKP-RGD) was found to accumulate on the surface of αv β3 integrin-expressing human renal cell carcinoma 786-O cells. The ligand was conjugated to the anticancer drug paclitaxel through a Asn-Pro-Val (NPV) tripeptide linker, which is a substrate of neutrophil-secreted elastase. In vitro linker cleavage assays and cell antiproliferative experiments demonstrate the efficacy of this tumor-targeting conjugate, opening the way to potential therapeutic applications.

Versatile Synthetic Approach for Selective Diversification of Bicyclic Aza-Diketopiperazines

Herein, we report a convenient synthesis of unprecedented aza-diketopiperazines (aza-DKPs). The strategy is based on selective diversification of bicyclic aza-DKP scaffolds by click reaction, N-acylation, and/or N-alkylation. These scaffolds containing either azido or amino groups were obtained by a key Rh(I)-catalyzed hydroformylative cyclohydrocarbonylation reaction of allyl-substituted aza-DKP. The methodology is readily amenable to the parallel synthesis of original aza-DKPs to enlarge the chemical diversity of aza-heterocycles.

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.

Crosstalk between integrin αvβ3 and ERα contributes to thyroid hormone-induced proliferation of ovarian cancer cells

Ovarian cancer is the leading cause of death in gynecological diseases. Thyroid hormone promotes proliferation of ovarian cancer cells via cell surface receptor integrin αvβ3 that activates extracellular regulated kinase (ERK1/2). However, the mechanisms are still not fully understood. Thyroxine (T₄) at a physiologic total hormone concentration (10⁻⁷ M) significantly increased proliferating cell nuclear antigen (PCNA) abundance in these cell lines, as did 3, 5, 3′-triiodo-L-thyronine (T₃) at a supraphysiologic concentration. Thyroid hormone (T₄ and T₃) treatment of human ovarian cancer cells resulted in enhanced activation of the Ras/MAPK(ERK1/2) signal transduction pathway. An MEK inhibitor (PD98059) blocked hormone-induced cell proliferation but not ER phosphorylation. Knock-down of either integrin αv or β3 by RNAi blocked thyroid hormone-induced phosphorylation of ERK1/2. We also found that thyroid hormone causes elevated phosphorylation and nuclear enrichment of estrogen receptor α (ERα). Confocal microscopy indicated that both T4 and estradiol (E₂) caused nuclear translocation of integrin αv and phosphorylation of ERα. The specific ERα antagonist (ICI 182,780; fulvestrant) blocked T₄-induced ERK1/2 activation, ERα phosphorylation, PCNA expression and proliferation. The nuclear co-localization of integrin αv and phosphorylated ERα was inhibited by ICI. ICI time-course studies indicated that mechanisms involved in T₄- and E₂-induced nuclear co-localization of phosphorylated ERα and integrin αv are dissimilar. Chromatin immunoprecipitation results showed that T₄-induced binding of integrin αv monomer to ERα promoter and this was reduced by ICI. In summary, thyroid hormone stimulates proliferation of ovarian cancer cells via crosstalk between integrin αv and ERα, mimicking functions of E₂.

Synthesis and biological evaluation of RGD and isoDGR peptidomimetic-α-amanitin conjugates for tumor-targeting

RGD-α-amanitin and isoDGR-α-amanitin conjugates were synthesized by joining integrin ligands to α-amanitin via various linkers and spacers. The conjugates were evaluated for their ability to inhibit biotinylated vitronectin binding to the purified α_Vβ₃ receptor, retaining good binding affinity, in the same nanomolar range as the free ligands. The antiproliferative activity of the conjugates was evaluated in three cell lines possessing different levels of α_Vβ₃ integrin expression: human glioblastoma U87 (α_Vβ₃+), human lung carcinoma A549 (α_Vβ₃−) and breast adenocarcinoma MDA-MB-468 (α_Vβ₃−). In the U87, in the MDA-MB-468, and partly in the A549 cancer cell lines, the cyclo[DKP-isoDGR]-α-amanitin conjugates bearing the lysosomally cleavable Val-Ala linker were found to be slightly more potent than α-amanitin. Apparently, for all these α-amanitin conjugates there is no correlation between the cytotoxicity and the expression of α_Vβ₃ integrin. To determine whether the increased cytotoxicity of the cyclo[DKP-isoDGR]-α-amanitin conjugates is governed by an integrin-mediated binding and internalization process, competition experiments were carried out in which the conjugates were tested with U87 (α_Vβ₃+, α_Vβ₅+, α_Vβ₆−, α₅β₁+) and MDA-MB-468 (α_Vβ₃−, α_Vβ₅+, α_Vβ₆+, α₅β₁−) cells in the presence of excess cilengitide, with the aim of blocking integrins on the cell surface. Using the MDA-MB-468 cell line, a fivefold increase of the IC₅₀ was observed for the conjugates in the presence of excess cilengitide, which is known to strongly bind not only α_Vβ₃, but also α_Vβ₅, α_Vβ₆, and α₅β₁. These data indicate that in this case the cyclo[DKP-isoDGR]-α-amanitin conjugates are possibly internalized by a process mediated by integrins different from α_Vβ₃ (e.g., α_Vβ₅).

αvβ3 and α5β1 integrin-specific ligands: From tumor angiogenesis inhibitors to vascularization promoters in regenerative medicine?

Integrins are cell adhesion receptors predominantly important during normal and tumor angiogenesis. A sequence present on several extracellular matrix proteins composed of Arg-Gly-Asp (RGD) has attracted attention due to its role in cell adhesion mediated by integrins. The development of ligands that can bind to integrins involved in tumor angiogenesis and brake disease progression has resulted in new investigational drug entities reaching the clinical trial phase in humans. The use of integrin-specific ligands can be useful for the vascularization of regenerative medicine constructs, which remains a major limitation for translation into clinical practice. In order to enhance vascularization, immobilization of integrin-specific RGD peptidomimetics within constructs is a recommended approach, due to their high specificity and selectivity towards certain desired integrins. This review endeavours to address the potential of peptidomimetic-coated biomaterials as vascular network promoters for regenerative medicine purposes. Clinical studies involving molecules tracking active integrins in cancer angiogenesis and reasons for their failure are also addressed.

Investigating the Interaction of Cyclic RGD Peptidomimetics with αVβ₆ Integrin by Biochemical and Molecular Docking Studies

The interaction of a small library of cyclic RGD (Arg-Gly-Asp) peptidomimetics with α_Vβ₆ integrin has been investigated by means of competitive solid phase binding assays to the isolated receptor and docking calculations in the crystal structure of the α_Vβ₆ binding site. To this aim, a rigid receptor-flexible ligand docking protocol has been set up and then applied to predict the binding mode of the cyclic RGD peptidomimetics to α_Vβ₆ integrin. Although the RGD interaction with α_Vβ₆ recapitulates the RGD binding mode observed in α_Vβ₃, differences between the integrin binding pockets can strongly affect the ligand binding ability. In general, the peptidomimetics exhibited IC₅₀ values for integrin α_Vβ₆ (i.e., the concentration of compound required for 50% inhibition of biotinylated fibronectin binding to isolated α_Vβ₆ integrin) in the nanomolar range (77–345 nM), about 10–100 times higher than those for the related α_Vβ₃ receptor, with a single notable ligand displaying a low nanomolar IC₅₀ value (2.3 nM). Insights from the properties of the binding pocket combined with the analysis of the docking poses provided a rationale for ligand recognition and selectivity.

Multivalency Increases the Binding Strength of RGD Peptidomimetic-Paclitaxel Conjugates to Integrin αV β3

This work reports the synthesis of three multimeric RGD peptidomimetic‐paclitaxel conjugates featuring a number of α_Vβ₃ integrin ligands ranging from 2 to 4. These constructs were assembled by conjugation of the integrin α_Vβ₃ ligand cyclo[DKP‐RGD]‐CH₂NH₂ with paclitaxel via a 2′‐carbamate with a self‐immolative spacer, the lysosomally cleavable Val‐Ala dipeptide linker, a multimeric scaffold, a triazole linkage, and finally a PEG spacer. Two monomeric conjugates were also synthesized as reference compounds. Remarkably, the new multimeric conjugates showed a binding affinity for the purified integrin α_Vβ₃ receptor that increased with the number of integrin ligands (reaching a minimum IC₅₀ value of 1.2 nm for the trimeric), thus demonstrating that multivalency is an effective strategy to strengthen the ligand–target interactions.

Tumor Targeting with an isoDGR-Drug Conjugate

Herein we report the first example of an isoDGR-drug conjugate (2), designed to release paclitaxel selectively within cancer cells expressing integrin αV β3 . Conjugate 2 was synthesized by connecting the isoDGR peptidomimetic 5 with paclitaxel via the lysosomally cleavable Val-Ala dipeptide linker. Conjugate 2 displayed a low nanomolar affinity for the purified integrin αV β3 receptor (IC50 =11.0 nm). The tumor targeting ability of conjugate 2 was assessed in vitro in anti-proliferative assays on two isogenic cancer cell lines characterized by different integrin αV β3 expression: human glioblastoma U87 (αV β3 +) and U87 β3 -KO (αV β3 -). The isoDGR-PTX conjugate 2 displayed a remarkable targeting index (TI=9.9), especially when compared to the strictly related RGD-PTX conjugate 4 (TI=2.4).

RGD-mediated delivery of small-molecule drugs

Conjugates of cytotoxic agents with RGD peptides (Arg-Gly-Asp) addressed to α_νβ₃, α₅β₁ and α_νβ₆ integrin receptors overexpressed by cancer cells, have recently gained attention as potential selective anticancer chemotherapeutics. In this review, the design and the development of RGD conjugates coupled to different small molecules including known cytotoxic drugs and natural products will be discussed.

Optimized syntheses of Fmoc azido amino acids for the preparation of azidopeptides

The rise of CuI‐catalyzed click chemistry has initiated an increased demand for azido and alkyne derivatives of amino acid as precursors for the synthesis of clicked peptides. However, the use of azido and alkyne amino acids in peptide chemistry is complicated by their high cost. For this reason, we investigated the possibility of the in‐house preparation of a set of five Fmoc azido amino acids: β‐azido l‐alanine and d‐alanine, γ‐azido l‐homoalanine, δ‐azido l‐ornithine and ω‐azido l‐lysine. We investigated several reaction pathways described in the literature, suggested several improvements and proposed several alternative routes for the synthesis of these compounds in high purity. Here, we demonstrate that multigram quantities of these Fmoc azido amino acids can be prepared within a week or two and at user‐friendly costs. We also incorporated these azido amino acids into several model tripeptides, and we observed the formation of a new elimination product of the azido moiety upon conditions of prolonged couplings with 2‐(1H‐benzotriazol‐1‐yl)‐1,1,3,3‐tetramethyluronium hexafluorophosphate/DIPEA. We hope that our detailed synthetic protocols will inspire some peptide chemists to prepare these Fmoc azido acids in their laboratories and will assist them in avoiding the too extensive costs of azidopeptide syntheses.

Experimental procedures and/or analytical data for compounds 3–5, 20, 25, 26, 30 and 43–47 are provided in the supporting information. © 2017 The Authors Journal of Peptide Science published by European Peptide Society and John Wiley & Sons Ltd.

Guidance of Signaling Activations by Cadherins and Integrins in Epithelial Ovarian Cancer Cells

Epithelial ovarian cancer (EOC) is the deadliest tumor among gynecological cancer in the industrialized countries. The EOC incidence and mortality have remained unchanged over the last 30 years, despite the progress in diagnosis and treatment. In order to develop novel and more effective therapeutic approaches, the molecular mechanisms involved in EOC progression have been thoroughly investigated in the last few decades. At the late stage, peritoneal metastases originate from the attachment of small clusters of cancer cells that shed from the primary site and carried by the ascites adhere to the abdominal peritoneum or omentum. This behavior suggests that cell–cell or cell–matrix adhesion mechanisms regulate EOC growth and dissemination. Complex downstream signalings, which might be influenced by functional cross-talk between adhesion molecules and co-expressed and activated signaling proteins, can affect the proliferation/survival and the migration/invasion of EOC cells. This review aimed to define the impact of the mechanisms of cell–cell, through cadherins, and cell–extracellular matrix adhesion, through integrins, on the signaling cascades induced by membrane receptors and cytoplasmic proteins known to have a role in the proliferation, migration and invasion of EOC cells. Finally, some novel approaches using peptidomimetic ligands to cadherin and integrins are summarized.

Prodrug Strategies for Paclitaxel

Paclitaxel is an anti-tumor agent with remarkable anti-tumor activity and wide clinical uses. However, it is also faced with various challenges especially for its poor water solubility and low selectivity for the target. To overcome these disadvantages of paclitaxel, approaches using small molecule modifications and macromolecule modifications have been developed by many research groups from all over the world. In this review, we discuss the different strategies especially prodrug strategies that are currently used to make paclitaxel more effective.

Design, synthesis and biological evaluation of novel dimeric and tetrameric cRGD-paclitaxel conjugates for integrin-assisted drug delivery

Integrins are associated with tumour cell survival and progression, and their expression has been shown to be increased in tumours. Thus, four novel conjugates of the tripeptide integrin ligand Arg-Gly-Asp (RGD) and the cytotoxic agent paclitaxel (cRGD-PTX) were prepared to investigate the potential of the multivalent presentation of the RGD moiety in improving the antitumor efficacy of PTX by tumour targeting. PTX was conjugated to two or four integrin recognizing ligands. The influence of multivalent presentation on in vitro αvβ3-receptor affinity was confirmed. For all the conjugates compared to the previously synthesized monovalent counterparts, an enhancement of the binding strength was observed; this behaviour was more pronounced when considering the tetravalent presented RGD-conjugate. Cell growth inhibition assays on a panel of human tumour cell lines showed remarkable cytotoxic activity for all conjugates with IC50 values in a nanomolar range. Among the four conjugates, the bivalent derivative 3b was selected for in vivo studies in an ovarian carcinoma cell model xenografted in immunodeficient mice. A marked antitumor activity was observed, similar to that of PTX, but with a much more favourable toxicity profile. Overall, the novel cRGD-PTX conjugates disclosed here represent promising candidates for further advancement in the domain of targeted anti-tumour therapy.

Small Molecules for Active Targeting in Cancer

For the purpose of this review, active targeting in cancer research encompasses strategies wherein a ligand for a cell surface receptor expressed on tumor cells is used to deliver a cytotoxic or imaging cargo. This area of research is more than two decades old, but in those 20 and more years, how many receptors have been studied extensively? What kinds of the ligands are used for active targeting? Are they mostly naturally occurring molecules such as folic acid, or synthetic substances developed in campaigns for medicinal chemistry efforts? This review outlines the most important receptor or ligand combinations that have been used in active targeting to answer these questions, and therefore to address the most important one of all: is research in active targeting affording diminishing returns, or is this an area for which the potential far exceeds progress made so far?

A review of the ligands and related targeting strategies for active targeting of paclitaxel to tumours

It has been 30 years since the discovery of the anti-tumour property of paclitaxel (PTX), which has been successfully applied in clinic for the treatment of carcinomas of the lungs, breast and ovarian. However, PTX is poorly soluble in water and has no targeting and selectivity to tumour tissue. Recent advances in active tumour targeting of PTX delivery vehicles have addressed some of the issues related to lack of solubility in water and non-specific toxicities associated with PTX. These PTX delivery vehicles are designed for active targeting to specific cancer cells by the addition of ligands for recognition by specific receptors/antigens on cancer cells. This article will focus on various ligands and related targeting strategies serving as potential tools for active targeting of PTX to tumour tissues, illustrating their use in different tumour models. This review also highlights the need of further studies on the discovery of receptors in different cells of specific organ and ligands with binding efficiency to these specific receptors.

Advancement in integrin facilitated drug delivery

The research of integrin-targeted anticancer agents has recorded important advancements in ingenious design of delivery systems, based either on the prodrug approach, or on nanoparticle carriers, but for now, none of these has reached a clinical stage of development. Past work in this area has been extensively reviewed by us and others. Thus, the purpose and scope of the present review is to survey the advancement reported in the last 3years, with focus on innovative delivery systems that appear to afford openings for future developments. These systems exploit the labelling with conventional and novel integrin ligands for targeting the interface of cancer cells and of endothelial cells involved in cancer angiogenesis, with the proteins of the extracellular matrix, in the circulation, in tissues, and in tumour stroma, as the site of progression and metastatic evolution of the disease. Furthermore, these systems implement the expertise in the development of nanomedicines to the purpose of achieving preferential biodistribution and uptake in cancer tissues, internalisation in cancer cells, and release of the transported drugs at intracellular sites. The assessment of the value of controlling these factors, and their combination, for future developments requires support of biological testing in appropriate mechanistic models, but also imperatively demand confirmation in therapeutically relevant in vivo models for biodistribution, efficacy, and lack of off-target effects. Thus, among many studies, we have tried to point out the results supported by relevant in vivo studies, and we have emphasised in specific sections those addressing the medical needs of drug delivery to brain tumours, as well as the delivery of oligonucleotides modulating gene-dependent pathological mechanism. The latter could constitute the basis of a promising third branch in the therapeutic armamentarium against cancer, in addition to antibody-based agents and to cytotoxic agents.

Integrin Ligands with α/β-Hybrid Peptide Structure: Design, Bioactivity, and Conformational Aspects

Integrins are cell surface receptors for proteins of the extracellular matrix and plasma-borne adhesive proteins. Their involvement in diverse pathologies prompted medicinal chemists to develop small-molecule antagonists, and very often such molecules are peptidomimetics designed on the basis of the short native ligand-integrin recognition motifs. This review deals with peptidomimetic integrin ligands composed of α- and β-amino acids. The roles exerted by the β-amino acid components are discussed in terms of biological activity, bioavailability, and selectivity. Special attention is paid to the synthetic accessibility and efficiency of conformationally constrained heterocyclic scaffolds incorporating α/β-amino acid span.

Synthesis, Characterization, and Biological Evaluation of a Dual-Action Ligand Targeting αvβ3 Integrin and VEGF Receptors

A dual-action ligand targeting both integrin αVβ3 and vascular endothelial growth factor receptors (VEGFRs), was synthesized via conjugation of a cyclic peptidomimetic αVβ3 Arg-Gly-Asp (RGD) ligand with a decapentapeptide. The latter was obtained from a known VEGFR antagonist by acetylation at the Lys13 side chain. Functionalization of the precursor ligands was carried out in solution and in the solid phase, affording two fragments: an alkyne VEGFR ligand and the azide integrin αVβ3 ligand, which were conjugated by click chemistry. Circular dichroism studies confirmed that both the RGD and VEGFR ligand portions of the dual-action compound substantially adopt the biologically active conformation. In vitro binding assays on isolated integrin αVβ3 and VEGFR-1 showed that the dual-action conjugate retains a good level of affinity for both its target receptors, although with one order of magnitude (10/20 times) decrease in potency. The dual-action ligand strongly inhibited the VEGF-induced morphogenesis in Human Umbilical Vein Endothelial Cells (HUVECs). Remarkably, its efficiency in preventing the formation of new blood vessels was similar to that of the original individual ligands, despite the worse affinity towards integrin αVβ3 and VEGFR-1.

Integrin-targeted delivery into cancer cells of a Pt(IV) pro-drug through conjugation to RGD-containing peptides

Conjugates of a Pt(IV) derivative of picoplatin with monomeric (Pt-c(RGDfK), 5) and tetrameric (Pt-RAFT-{c(RGDfK)}4, 6) RGD-containing peptides were synthesized with the aim of exploiting their selectivity and high affinity for αVβ3 and αVβ5 integrins for targeted delivery of this anticancer metallodrug to tumor cells overexpressing these receptors. Solid- and solution-phase approaches in combination with click chemistry were used for the preparation of the conjugates, which were characterized by high resolution ESI MS and NMR. αVβ3 and αVβ5 integrin expression was evaluated in a broad panel of human cancer and non-malignant cells. SK-MEL-28 melanoma cells were selected based on the high expression levels of both integrins, while CAPAN-1 pancreatic cancer cells and 1BR3G fibroblasts were selected as the negative control. Internalization experiments revealed a good correlation between integrin expression and the cellular uptake of the corresponding fluorescein-labeled peptides and that the internalization capacity of the tetrameric RGD-containing peptide was considerably higher than that of the monomeric one. Cytotoxic experiments indicated that the antitumor activity of picoplatin in melanoma cells was increased by 2.6-fold when its Pt(IV) derivative was conjugated to c(RGDfK) (IC50 = 12.8 ± 2.1 μM) and by 20-fold when conjugated to RAFT-{c(RGDfK)}4 (IC50 = 1.7 ± 0.6 μM). In contrast, the cytotoxicity of the conjugates was inhibited in control cells lacking αVβ3 and αVβ5 integrin expression. Finally, cellular uptake studies by ICP-MS confirmed a good correlation between the levels of expression of integrins, intracellular platinum accumulation and antitumor activity. Indeed, accumulation and cytotoxicity were much higher in SK-MEL-28 cells than in CAPAN-1, being particularly higher in the case of the tetrameric conjugate. The overall results highlight that the great ability of RAFT-{c(RGDfK)}4 to bind to and to be internalized by integrins overexpressed in SK-MEL-28 cells results in higher accumulation of the Pt(IV) complex, leading to a high antitumor activity. These studies provide new insights into the potential of targeting αVβ3 and αVβ5 integrins with Pt(IV) anticancer pro-drugs conjugated to tumor-targeting devices based on RGD-containing peptides, particularly on how multivalency can improve both the selectivity and potency of such metallodrugs by increasing cellular accumulation in tumor tissues.

The thyroid hormone-αvβ3 integrin axis in ovarian cancer: regulation of gene transcription and MAPK-dependent proliferation

Ovarian carcinoma is the fifth common cause of cancer death in women, despite advanced therapeutic approaches. αvβ3 integrin, a plasma membrane receptor, binds thyroid hormones (L-thyroxine, T4; 3,5,3'-triiodo-L-thyronine, T3) and is overexpressed in ovarian cancer. We have demonstrated selective binding of fluorescently labeled hormones to αvβ3-positive ovarian cancer cells but not to integrin-negative cells. Physiologically relevant T3 (1 nM) and T4 (100 nM) concentrations in OVCAR-3 (high αvβ3) and A2780 (low αvβ3) cells promoted αv and β3 transcription in association with basal integrin levels. This transcription was effectively blocked by RGD (Arg-Gly-Asp) peptide and neutralizing αvβ3 antibodies, excluding T3-induced β3 messenger RNA, suggesting subspecialization of T3 and T4 binding to the integrin receptor pocket. We have provided support for extracellular regulated kinase (ERK)-mediated transcriptional regulation of the αv monomer by T3 and of β3 monomer by both hormones and documented a rapid (30-120 min) and dose-dependent (0.1-1000 nM) ERK activation. OVCAR-3 cells and αvβ3-deficient HEK293 cells treated with αvβ3 blockers confirmed the requirement for an intact thyroid hormone-integrin interaction in ERK activation. In addition, novel data indicated that T4, but not T3, controls integrin's outside-in signaling by phosphorylating tyrosine 759 in the β3 subunit. Both hormones induced cell proliferation (cell counts), survival (Annexin-PI), viability (WST-1) and significantly reduced the expression of genes that inhibit cell cycle (p21, p16), promote mitochondrial apoptosis (Nix, PUMA) and tumor suppression (GDF-15, IGFBP-6), particularly in cells with high integrin expression. At last, we have confirmed that hypothyroid environment attenuated ovarian cancer growth using a novel experimental platform that exploited paired euthyroid and severe hypothyroid serum samples from human subjects. To conclude, our data define a critical role for thyroid hormones as potent αvβ3-ligands, driving ovarian cancer cell proliferation and suggest that disruption of this axis may present a novel treatment strategy in this aggressive disease.

Designed cell penetrating peptide dendrimers efficiently internalize cargo into cells

Redesigning linear cell penetrating peptides (CPPs) into a multi-branched topology with short dipeptide branches gave cell penetrating peptide dendrimers (CPPDs) with higher cell penetration, lower toxicity and hemolysis and higher serum stability than linear CPPs. Their use is demonstrated by delivering a cytotoxic peptide and paclitaxel into cells.