Dual-drug RGD conjugates provide enhanced cytotoxicity to melanoma and non-small lung cancer cells

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.

Drug conjugates for the treatment of lung cancer: from drug discovery to clinical practice

A drug conjugate consists of a cytotoxic drug bound via a linker to a targeted ligand, allowing the targeted delivery of the drug to one or more tumor sites. This approach simultaneously reduces drug toxicity and increases efficacy, with a powerful combination of efficient killing and precise targeting. Antibody‒drug conjugates (ADCs) are the best-known type of drug conjugate, combining the specificity of antibodies with the cytotoxicity of chemotherapeutic drugs to reduce adverse reactions by preferentially targeting the payload to the tumor. The structure of ADCs has also provided inspiration for the development of additional drug conjugates. In recent years, drug conjugates such as ADCs, peptide‒drug conjugates (PDCs) and radionuclide drug conjugates (RDCs) have been approved by the Food and Drug Administration (FDA). The scope and application of drug conjugates have been expanding, including combination therapy and precise drug delivery, and a variety of new conjugation technology concepts have emerged. Additionally, new conjugation technology-based drugs have been developed in industry. In addition to chemotherapy, targeted therapy and immunotherapy, drug conjugate therapy has undergone continuous development and made significant progress in treating lung cancer in recent years, offering a promising strategy for the treatment of this disease. In this review, we discuss recent advances in the use of drug conjugates for lung cancer treatment, including structure-based drug design, mechanisms of action, clinical trials, and side effects. Furthermore, challenges, potential approaches and future prospects are presented.

Click processes orthogonal to CuAAC and SuFEx forge selectively modifiable fluorescent linkers

The appeal of catalytic click chemistry is largely due to the copper-catalysed azide-alkyne cycloaddition (CuAAC) process, which is orthogonal to the more recently introduced sulfur-fluoride exchange (SuFEx). However, the triazole rings generated by CuAAC are not readily modifiable, and SuFEx connectors cannot be selectively functionalized, attributes that would be attractive in a click process. Here we introduce bisphosphine-copper-catalysed phenoxydiazaborinine formation (CuPDF), a link-and-in situ modify strategy for merging a nitrile, an allene, a diborane and a hydrazine. We also present copper- and palladium-catalysed quinoline formation (Cu/PdQNF), which is applicable in aqueous media, involving an aniline as the modifier. CuPDF and Cu/PdQNF are easy to perform and deliver robust, alterable and tunable fluorescent hubs. CuPDF and Cu/PdQNF are orthogonal to SuFEx and CuAAC, despite the latter and CuPDF also being catalysed by an organocopper species. These advantages were applied to protecting group-free syntheses of sequence-defined branched oligomers, a chemoselectively amendable polymer, three drug conjugates and a two-drug conjugate.

Oxime-Linked Peptide-Daunomycin Conjugates as Good Tools for Selection of Suitable Homing Devices in Targeted Tumor Therapy: An Overview

Chemotherapy is still one of the main therapeutic approaches in cancer therapy. Nevertheless, its poor selectivity causes severe toxic side effects that, together with the development of drug resistance in tumor cells, results in a limitation for its application. Tumor-targeted drug delivery is a possible choice to overcome these drawbacks. As well as monoclonal antibodies, peptides are promising targeting moieties for drug delivery. However, the development of peptide-drug conjugates (PDCs) is still a big challenge. The main reason is that the conjugates have to be stable in circulation, but the drug or its active metabolite should be released efficiently in the tumor cells. For this purpose, suitable linker systems are needed that connect the drug molecule with the homing peptide. The applied linker systems are commonly categorized as cleavable and non-cleavable linkers. Both the groups possess advantages and disadvantages that are summarized briefly in this manuscript. Moreover, in this review paper, we highlight the benefit of oxime-linked anthracycline-peptide conjugates in the development of PDCs. For instance, straightforward synthesis as well as a conjugation reaction proceed in excellent yields, and the autofluorescence of anthracyclines provides a good tool to select the appropriate homing peptides. Furthermore, we demonstrate that these conjugates can be used properly in in vivo studies. The results indicate that the oxime-linked PDCs are potential candidates for targeted tumor therapy.

Radiolabeled NGR-Based Heterodimers for Angiogenesis Imaging: A Review of Preclinical Studies

Since angiogenesis/neoangiogenesis has a major role in tumor development, progression and metastatic spread, the establishment of angiogenesis-targeting imaging and therapeutic vectors is of utmost significance. Aminopeptidase N (APN/CD13) is a pivotal biomarker of angiogenic processes abundantly expressed on the cell surface of active vascular endothelial and various neoplastic cells, constituting a valuable target for cancer diagnostics and therapy. Since the asparagine-glycine-arginine (NGR) sequence has been shown to colocalize with APN/CD13, the research interest in NGR-peptide-mediated vascular targeting is steadily growing. Earlier preclinical experiments have already demonstrated the imaging and therapeutic feasibility of NGR-based probes labeled with different positron emission tomography (PET) and single-photon emission computed tomography (SPECT) radionuclides, including Gallium-68 (68Ga), Copper-64 (64Cu), Technetium-99m (99mTc), Lutetium-177 (177Lu), Rhenium-188 (188Re) or Bismuth-213 (213Bi). To improve the tumor binding affinity and the retention time of single-receptor targeting peptides, NGR motifs containing heterodimers have been introduced to identify multi-receptor overexpressing malignancies. Preclinical studies with various tumor-bearing experimental animals provide useful tools for the investigation of the in vivo imaging behavior of NGR-based heterobivalent ligands. Herein, we review the reported preclinical achievements on NGR heterodimers that could be highly relevant for the development of further target-specific multivalent compounds in diagnostic and therapeutic settings.

A Brief Guide to Preparing a Peptide-Drug Conjugate

Peptide-drug conjugates (PDCs) have recently emerged as interesting hybrid constructs not only for targeted therapy, but also for the early diagnosis of different pathologies. In most cases, the crucial step in the PDC synthesis is the final conjugation step, where a specific drug is bound to a particular peptide-/peptidomimetic-targeting unit. Thus, this concept paper aims to give a short guide to determining the finest conjugation reaction, by considering in particular the reaction conditions, the stability of the linker and the major pros and cons of each reaction. Based on the recent PDCs reported in literature, the most common and efficient conjugation methods will be systematically presented and compared, generating a short guide to consult while planning the synthesis of a novel peptide-drug conjugate.

Research advances in peptide‒drug conjugates

Peptide‒drug conjugates (PDCs) are drug delivery systems consisting of a drug covalently coupled to a multifunctional peptide via a cleavable linker. As an emerging prodrug strategy, PDCs not only preserve the function and bioactivity of the peptides but also release the drugs responsively with the cleavable property of the linkers. Given the ability to significantly improve the circulation stability and targeting of drugs in vivo and reduce the toxic side effects of drugs, PDCs have already been extensively applied in drug delivery. Herein, we review the types and mechanisms of peptides, linkers and drugs used to construct PDCs, and summarize the clinical applications and challenges of PDC drugs.

Peptides as multifunctional players in cancer therapy

Peptides exhibit lower affinity and a shorter half-life in the body than antibodies. Conversely, peptides demonstrate higher efficiency in tissue penetration and cell internalization than antibodies. Regardless of the pros and cons of peptides, they have been used as tumor-homing ligands for delivering carriers (such as nanoparticles, extracellular vesicles, and cells) and cargoes (such as cytotoxic peptides and radioisotopes) to tumors. Additionally, tumor-homing peptides have been conjugated with cargoes such as small-molecule or chemotherapeutic drugs via linkers to synthesize peptide-drug conjugates. In addition, peptides selectively bind to cell surface receptors and proteins, such as immune checkpoints, receptor kinases, and hormone receptors, subsequently blocking their biological activity or serving as hormone analogs. Furthermore, peptides internalized into cells bind to intracellular proteins and interfere with protein-protein interactions. Thus, peptides demonstrate great application potential as multifunctional players in cancer therapy.

Differential Surface Engineering Generates Core-Shell Porous Silicon Nanoparticles for Controlled and Targeted Delivery of an Anticancer Drug

An approach to differentially modify the internal surface of porous silicon nanoparticles (pSiNPs) with hydrophobic dodecene and the external surface with antifouling poly-N-(2-hydroxypropyl) acrylamide (polyHPAm) as well as a cell-targeting peptide was developed. Specifically, to generate these core-shell pSiNPs, the interior surface of a porous silicon (pSi) film was hydrosilylated with 1-dodecene, followed by ultrasonication to create pSiNPs. The new external surfaces were modified by silanization with a polymerization initiator, and surface-initiated atom transfer radical polymerization was performed to introduce polyHPAm brushes. Afterward, a fraction of the polymer side chain hydroxyl groups was activated to conjugate cRGDfK─a peptide with a high affinity and selectivity for the α_νβ₃ integrin receptor that is overexpressed in prostate and melanoma cancers. Finally, camptothecin, a hydrophobic anti-cancer drug, was successfully loaded into the pores. This drug delivery system showed excellent colloidal stability in a cell culture medium, and the in vitro drug release kinetics could be fine-tuned by the combination of internal and external surface modifications. In vitro studies by confocal microscopy and flow cytometry revealed improved cellular association attributed to cRGDfK. Furthermore, the cell viability results showed that the drug-loaded and peptide-functionalized nanoparticles had enhanced cytotoxicity toward a C4-2B prostate carcinoma cell line in both 2D cell culture and a 3D spheroid model.

Targeting Ultrasmall Gold Nanoparticles with cRGD Peptide Increases the Uptake and Efficacy of Cytotoxic Payload

Cyclic arginyl-glycyl-aspartic acid peptide (cRGD) peptides show a high affinity towards αVβ3 integrin, a receptor overexpressed in many cancers. We aimed to combine the versatility of ultrasmall gold nanoparticles (usGNP) with the target selectivity of cRGD peptide for the directed delivery of a cytotoxic payload in a novel design. usGNPs were synthesized with a modified Brust-Schiffrin method and functionalized via amide coupling and ligand exchange and their uptake, intracellular trafficking, and toxicity were characterized. Our cRGD functionalized usGNPs demonstrated increased cellular uptake by αVβ3 integrin expressing cells, are internalized via clathrin-dependent endocytosis, accumulated in the lysosomes, and when loaded with mertansine led to increased cytotoxicity. Targeting via cRGD functionalization provides a mechanism to improve the efficacy, tolerability, and retention of therapeutic GNPs.

Advancement and Strategies for the Development of Peptide-Drug Conjugates: Pharmacokinetic Modulation, Role and Clinical Evidence Against Cancer Management

Currently, many new treatment strategies are being used for the management of cancer. Among them, chemotherapy based on peptides has been of great interest due to the unique features of peptides. This review discusses the role of peptide and peptides analogues in the treatment of cancer, with special emphasis on their pharmacokinetic modulation and research progress. Low molecular weight, targeted drug delivery, enhanced permeability, etc., of the peptide-linked drug conjugates, lead to an increase in the effectiveness of cancer therapy. Various peptides have recently been developed as drugs and vaccines with an altered pharmacokinetic parameter which has subsequently been assessed in different phases of the clinical study. Peptides have made a great impact in the area of cancer therapy and diagnosis. Targeted chemotherapy and drug delivery techniques using peptides are emerging as excellent tools in minimizing problems with conventional chemotherapy. It can be concluded that new advances in using peptides to treat different types of cancer have been shown by different clinical studies indicating that peptides could be used as an ideal therapeutic method in treating cancer due to the novel advantages of peptides. The development of identifying and synthesizing novel peptides could provide a promising choice to patients with cancer.

Discovery of Dolastatinol: A Synthetic Analog of Dolastatin 10 and Low Nanomolar Inhibitor of Tubulin Polymerization

We developed a highly potent anticancer agent, dolastatinol, which is a methylene hydroxyl derivative of dolastatin 10. Dolastatinol is a synthetic analog of dolastatin 10, synthesized by a solid-phase peptide Fmoc chemistry protocol on 2-chlorotrityl chloride resin utilizing a pH-triggering self-immolative monosuccinate linker. The introduction of the C-terminus hydroxyl methylene functionality preserves the anticancer properties of the parent dolastatin 10, including strong suppression of the cell proliferation, migration, high cytotoxicity. Our research establishes a new facile route toward the further development of C-terminus-modified dolastatin-10-based microtubule inhibitors for anticancer treatment.

Drug Combination in Cancer Treatment-From Cocktails to Conjugated Combinations

It is well recognized today that anticancer drugs often are most effective when used in combination. However, the establishment of chemotherapy as key modality in clinical oncology began with sporadic discoveries of chemicals that showed antiproliferative properties and which as a first attempt were used as single agents. In this review we describe the development of chemotherapy from its origins as a single drug treatment with cytotoxic agents to polydrug therapy that includes targeted drugs. We discuss the limitations of the first chemotherapeutic drugs as a motivation for the establishment of combined drug treatment as standard practice in spite of concerns about frequent severe, dose limiting toxicities. Next, we introduce the development of targeted treatment as a concept for advancement within the broader field of small-molecule drug combination therapy in cancer and its accelerating progress that was boosted by recent scientific and technological progresses. Finally, we describe an alternative strategy of drug combinations using drug-conjugates for selective delivery of cytotoxic drugs to tumor cells that potentiates future improvement of drug combinations in cancer treatment. Overall, in this review we outline the development of chemotherapy from a pharmacological perspective, from its early stages to modern concepts of using targeted therapies for combinational treatment.

Peptide-Drug Conjugates and Their Targets in Advanced Cancer Therapies

Cancer became recently the leading cause of death in industrialized countries. Even though standard treatments achieve significant effects in growth inhibition and tumor elimination, they cause severe side effects as most of the applied drugs exhibit only minor selectivity for the malignant tissue. Hence, specific addressing of tumor cells without affecting healthy tissue is currently a major desire in cancer therapy. Cell surface receptors, which bind peptides are frequently overexpressed on cancer cells and can therefore be considered as promising targets for selective tumor therapy. In this review, the benefits of peptides as tumor homing agents are presented and an overview of the most commonly addressed peptide receptors is given. A special focus was set on the bombesin receptor family and the neuropeptide Y receptor family. In the second part, the specific requirements of peptide-drug conjugates (PDC) and intelligent linker structures as an essential component of PDC are outlined. Furthermore, different drug cargos are presented including classical and recent toxic agents as well as radionuclides for diagnostic and therapeutic approaches. In the last part, boron neutron capture therapy as advanced targeted cancer therapy is introduced and past and recent developments are reviewed.

The discovery of antioxidants in marine microorganisms and their protective effects on the hepatic cells from chemical-induced oxidative stress

The marine environment is emerging as a biodiversity resource for the discovery of natural molecules or chemical scaffolds with pharmaceutical activity. Marine microbes have a tremendous ability to sense and respond to their surroundings to survive in a stressful environment by producing different molecules. As oxidative stress is directly or indirectly involved in various pathological conditions in humans, we believe that marine-derived antioxidant molecules will have a great prospect as a novel antioxidant molecule. We, in this work, explored the marine microbial resources from the Gulf of Mannar, Bay of Bengal, India. High-throughput screening of antioxidant molecule producing marine microbes has revealed that extract from Kocuria marina CDMP 10, can effectively reduce the DPPH free radical. Methanolic crude extract obtained by the freeze-thawing was fractionated and purified by using activity guided purification with the help of reverse phase HPLC and analysed through UPLC-MS. Chemical analysis, as well as MS-spectra, indicated that marine bacteria K. marina CDMP 10 derived antioxidant fraction contains the short peptides. The antioxidant activity of the three highly hydrophobic peptides, (Ser-Ser-Gln, Phe-Glu, Asp-Ile and Leu-Glu) was confirmed by in vitro as well as a cell-based assay. These small peptide molecules are noncytotoxic and can prevent the human cells from chemical-induced oxidative stress. Ser-Ser-Gln peptide demonstrated a potent free radical scavenging activity in Hepatocellular carcinoma cell lines. This study suggests that these short peptides from K. marina CDMP 10 may serve as a potential pharmaceutical candidate with antioxidant activity.

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.

Gold Nanoparticles in Diagnostics and Therapeutics for Human Cancer

The application of nanotechnology for the treatment of cancer is mostly based on early tumor detection and diagnosis by nanodevices capable of selective targeting and delivery of chemotherapeutic drugs to the specific tumor site. Due to the remarkable properties of gold nanoparticles, they have long been considered as a potential tool for diagnosis of various cancers and for drug delivery applications. These properties include high surface area to volume ratio, surface plasmon resonance, surface chemistry and multi-functionalization, facile synthesis, and stable nature. Moreover, the non-toxic and non-immunogenic nature of gold nanoparticles and the high permeability and retention effect provide additional benefits by enabling easy penetration and accumulation of drugs at the tumor sites. Various innovative approaches with gold nanoparticles are under development. In this review, we provide an overview of recent progress made in the application of gold nanoparticles in the treatment of cancer by tumor detection, drug delivery, imaging, photothermal and photodynamic therapy and their current limitations in terms of bioavailability and the fate of the nanoparticles.

On the design principles of peptide-drug conjugates for targeted drug delivery to the malignant tumor site

Cancer is the second leading cause of death affecting nearly one in two people, and the appearance of new cases is projected to rise by >70% by 2030. To effectively combat the menace of cancer, a variety of strategies have been exploited. Among them, the development of peptide–drug conjugates (PDCs) is considered as an inextricable part of this armamentarium and is continuously explored as a viable approach to target malignant tumors. The general architecture of PDCs consists of three building blocks: the tumor-homing peptide, the cytotoxic agent and the biodegradable connecting linker. The aim of the current review is to provide a spherical perspective on the basic principles governing PDCs, as also the methodology to construct them. We aim to offer basic and integral knowledge on the rational design towards the construction of PDCs through analyzing each building block, as also to highlight the overall progress of this rapidly growing field. Therefore, we focus on several intriguing examples from the recent literature, including important PDCs that have progressed to phase III clinical trials. Last, we address possible difficulties that may emerge during the synthesis of PDCs, as also report ways to overcome them.

Gold nanoparticles stabilize peptide-drug-conjugates for sustained targeted drug delivery to cancer cells

Background

Peptide-drug-conjugates (PDCs) are being developed as an effective strategy to specifically deliver cytotoxic drugs to cancer cells. However one of the challenges to their successful application is the relatively low stability of peptides in the blood, liver and kidneys. Since AuNPs seem to have a longer plasma half-life than PDCs, one approach to overcoming this problem would be to conjugate the PDCs to gold nanoparticles (AuNPs), as these have demonstrated favorable physico-chemical and safety properties for drug delivery systems. We set out to test whether PEG coated-AuNPs could provide a suitable platform for the non-covalent loading of pre-formed PDCs and whether this modification would affect the bioavailability of the PDCs and their cytotoxicity toward target cancer cells.

Methods

Peptides specifically internalized by A20 murine lymphoma cells were isolated from a phage library displaying 7mer linear peptides. Peptide specificity was validated by flow cytometry and confocal microscopy. PDCs were synthesized containing a selected peptide (P4) and either chlorambucil (Chlor), melphalan (Melph) or bendamustine (Bend). Gold nanoparticles were sequentially coated with citrate, PEG-6000 and then PDC (PDC-PEG-AuNP). The physico-chemical properties of the coated particles were analyzed by electrophoresis, TEM, UV–VIS and FTIR. Stability of free and PDC-coated AuNP was determined.

Results

Biopanning of the phage library resulted in discovery of several novel peptides that internalized into A20 cells. One of these (P4) was used to synthesize PDCs containing either Chlor, Melph or Bend. All three PDCs specifically killed A20 target cells, however they had short half-lives ranging from 10.6 to 15.4 min. When coated to PEG-AuNPs, the half-lives were extended to 21.0–22.3 h. The PDC-PEG-AuNPs retained cytotoxicity towards the target cells. Moreover, whereas pre-incubation for 24 h of free PDCs almost completely abolished their cytotoxic activity, the PDC-PEG-AuNPs were still active even after 72 h pre-incubation.

Conclusions

Peptide-drug-conjugates hold potential for improving the target efficacy of chemotherapeutic drugs, however their short half-lives may limit their application. This hurdle can be overcome by easily conjugating them to gold nanoparticles. This conjugation also opens up the possibility of developing slow release formulations of targeted drug delivery systems containing PDCs.

Electronic supplementary material

The online version of this article (10.1186/s12951-018-0362-1) contains supplementary material, which is available to authorized users.

Toward the development of a novel non-RGD cyclic peptide drug conjugate for treatment of human metastatic melanoma

The newly discovered short (9 amino acid) non-RGD S-S bridged cyclic peptide ALOS-4 (H-cycl(Cys-Ser-Ser-Ala-Gly-Ser-Leu-Phe-Cys)-OH), which binds to integrin α_vβ₃ is investigated as peptide carrier for targeted drug delivery against human metastatic melanoma. ALOS4 binds specifically the α_vβ₃ overexpressing human metastatic melanoma WM-266-4 cell line both in vitro and in ex vivo assays. Coupling ALOS4 to the topoisomerase I inhibitor Camptothecin (ALOS4-CPT) increases the cytotoxicity of CPT against human metastatic melanoma cells while reduces dramatically the cytotoxicity against non-cancerous cells as measured by the levels of γH2A.X, active caspase 3 and cell viability. Moreover, conjugating ALOS4 to CPT even increases the chemo-stability of CPT under physiological pH. Bioinformatic analysis using Rosetta platform revealed potential docking sites of ALOS4 on the α_vβ₃ integrin which are distinct from the RGD binding sites. We propose to use this specific non-RGD cyclic peptide as the therapeutic carrier for conjugation of drugs in order to improve efficacy and reduce toxicity of currently available treatments of human malignant melanoma.

RGD-modified dihydrolipoamide dehydrogenase conjugated to titanium dioxide nanoparticles –switchableintegrin-targeted photodynamic treatment of melanoma cells

The photocytotoxic effect of UVA-excited titanium dioxide (TiO₂), which is caused by the generation of reactive oxygen species (ROS), is often used in medical applications, such as cancer treatment. Photodynamic-therapy (PDT) is applied in several cancer models including cutaneous melanoma (CM), however the lack of selectivity causing damage to surrounding healthy tissues limits its applicability and novel targeted-delivery approaches are required. As cancer cells often overexpress integrin receptors (e.g. αvβ₃) on their cell surface, targeted delivery of TiO₂ nanoparticles (NPs) via an Arg-Gly-Asp (RGD) motif would make PDT more selective. We have recently reported that the mitochondrial enzyme dihydrolipoamide dehydrogenase (DLDH) strongly and specifically conjugates TiO₂via coordinative bonds. In this work we have modified DLDH with RGD moieties (DLDH^RGD), creating a molecular bridge between the integrin-expressing cancer cells and the photo-excitable TiO₂ nanoparticles. Physicochemical assays have indicated that the hybrid-conjugated nanobiocomplex, TiO₂–DLDH^RGD, is producing controlled-release ROS under UVA illumination, with anatase NPs being the most photoreactive TiO₂ form. This drug delivery system exhibited a cytotoxic effect in αvβ₃ integrin-expressing mice melanoma cells (B16F10), but not in normal cells lacking this integrin (HEK293). No cytotoxic effect was observed in the absence of UV illumination. Our results demonstrate the feasibility of combining the high efficiency of TiO₂-based PDT, with an integrin-mediated tumor-targeted drug delivery for nanomedicine.

This work presents a UVA switchable integrin-targeted photodynamic therapy in melanoma, composed of an RGD-modified DLDH conjugated to TiO₂ nanoparticles, with high selectivity towards integrin-expressing cancer cells.

Cryptophycins: cytotoxic cyclodepsipeptides with potential for tumor targeting

Cryptophycins are a class of 16-membered highly cytotoxic macrocyclic depsipeptides isolated from cyanobacteria. The biological activity is based on their ability to interact with tubulin. They interfere with microtubule dynamics and prevent microtubules from forming correct mitotic spindles, which causes cell-cycle arrest and apoptosis. Their strong antiproliferative activities with 100-fold to 1000-fold potency compared with those of paclitaxel and vinblastine have been observed. Cryptophycins are highly promising drug candidates, as their biological activity is not negatively affected by P-glycoprotein, a drug efflux system commonly found in multidrug-resistant cancer cell lines and solid tumors. Cryptophycin-52 had been investigated in phase II clinical trials but failed because of its high neurotoxicity. Recently, cryptophycin conjugates with peptides and antibodies have been developed for targeted delivery in tumor therapy. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

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.

Conjugates of Cell Adhesion Peptides for Therapeutics and Diagnostics Against Cancer and Autoimmune Diseases

Overexpressed cell-surface receptors are hallmarks of many disease states and are often used as markers for targeting diseased cells over healthy counterparts. Cell adhesion peptides, which are often derived from interacting regions of these receptor-ligand proteins, mimic surfaces of intact proteins and, thus, have been studied as targeting agents for various payloads to certain cell targets for cancers and autoimmune diseases. Because many cytotoxic agents in the free form are often harmful to healthy cells, the use of cell adhesion peptides in targeting their delivery to diseased cells has been studied to potentially reduce required effective doses and associated harmful side-effects. In this review, multiple cell adhesion peptides from extracellular matrix and ICAM proteins were used to selectively direct drug payloads, signal-inhibitor peptides, and diagnostic molecules, to diseased cells over normal counterparts. RGD constructs have been used to improve the selectivity and efficacy of diagnostic and drug-peptide conjugates against cancer cells. From this precedent, novel conjugates of antigenic and cell adhesion peptides, called Bifunctional Peptide Inhibitors (BPIs), have been designed to selectively regulate immune cells and suppress harmful inflammatory responses in autoimmune diseases. Similar peptide conjugations with imaging agents have delivered promising diagnostic methods in animal models of rheumatoid arthritis. BPIs have also been shown to generate immune tolerance and suppress autoimmune diseases in animal models of type-1 diabetes, rheumatoid arthritis, and multiple sclerosis. Collectively, these studies show the potential of cell adhesion peptides in improving the delivery of drugs and diagnostic agents to diseased cells in clinical settings.

Gene-silencing effects of anti-survivin siRNA delivered by RGDV-functionalized nanodiamond carrier in the breast carcinoma cell line MCF-7

Nanodiamond (ND) is a renowned material in nonviral small interfering RNA (siRNA) carrier field due to its unique physical, chemical, and biological properties. In our previous work, it was proven that ND could deliver siRNA into cells efficiently and downregulate the expression of desired protein. However, synthesizing a high-efficient tumor-targeting carrier using ND is still a challenge. In this study, a novel carrier, NDCONH(CH₂)₂NH-VDGR, was synthesized for siRNA delivery, and its properties were characterized with methods including Fourier transform infrared spectrometry, transmission electron microscopy, scanning electron microscopy, gel retardation assay, differential scanning calorimetry, confocal microscopy, releasing test, real-time polymerase chain reaction (PCR) assay, enzyme-linked immunosorbent assay (ELISA), flow cytometry, cytotoxicity assay, and gene-silencing efficacy assay in vitro and in vivo. The mechanism of NDCONH(CH₂)₂NH-VDGR/survivin-siRNA-induced tumor apoptosis was evaluated via flow cytometer assay using Annexin V–fluorescein isothiocyanate/propidium iodide staining method. The NDCONH(CH₂)₂NH-VDGR/survivin-siRNA nanoparticle with 60–110 nm diameter and 35.65±3.90 mV zeta potential was prepared. For real-time PCR assay, the results showed that the expression of survivin mRNA was reduced to 46.77%±6.3%. The expression of survivin protein was downregulated to 48.49%±2.25%, as evaluated by ELISA assay. MTT assay showed that NDCONH(CH₂)₂NH-VDGR/survivin-siRNA had an inhibitory effect on MCF-7 cell proliferation. According to these results, the survivin-siRNA could be delivered, transported, and released stably, which benefits in increasing the gene-silencing effect. Therefore, as an siRNA carrier, NDCONH(CH₂)₂NH-VDGR was suggested to be used in siRNA delivery system and in cancer treatments.

Recent Innovations in Peptide Based Targeted Drug Delivery to Cancer Cells

Targeted delivery of chemotherapeutics and diagnostic agents conjugated to carrier ligands has made significant progress in recent years, both in regards to the structural design of the conjugates and their biological effectiveness. The goal of targeting specific cell surface receptors through structural compatibility has encouraged the use of peptides as highly specific carriers as short peptides are usually non-antigenic, are structurally simple and synthetically diverse. Recent years have seen many developments in the field of peptide based drug conjugates (PDCs), particularly for cancer therapy, as their use aims to bypass off-target side-effects, reducing the morbidity common to conventional chemotherapy. However, no PDCs have as yet obtained regulatory approval. In this review, we describe the evolution of the peptide-based strategy for targeted delivery of chemotherapeutics and discuss recent innovations in the arena that should lead in the near future to their clinical application.