Development of an oxime bond containing daunorubicin-gonadotropin-releasing hormone-III conjugate as a potential anticancer drug

Doxorubicin conjugates: a practical approach for its cardiotoxicity alleviation

INTRODUCTION

Doxorubicin (DOX) emerges as a cornerstone in the arsenal of potent chemotherapeutic agents. Yet, the clinical deployment of DOX is tarnished by its proclivity to induce severe cardiotoxic effects, culminating in heart failure and other consequential morbidities. In response, a panoply of strategies has undergone rigorous exploration over recent decades, all aimed at attenuating DOX's cardiotoxic impact. The advent of encapsulating DOX within lipidic or polymeric nanocarriers has yielded a dual triumph, augmenting DOX's therapeutic efficacy while mitigating its deleterious side effects.

AREAS COVERED

Recent strides have spotlighted the emergence of DOX conjugates as particularly auspicious avenues for ameliorating DOX-induced cardiotoxicity. These conjugates entail the fusion of DOX through physical or chemical bonds with diminutive natural or synthetic moieties, polymers, biomolecules, and nanoparticles. This spectrum encompasses interventions that impinge upon DOX's cardiotoxic mechanism, modulate cellular uptake and localization, confer antioxidative properties, or refine cellular targeting.

EXPERT OPINION

The endorsement of DOX conjugates as a compelling stratagem to mitigate DOX-induced cardiotoxicity resounds from this exegesis, amplifying safety margins and the therapeutic profile of this venerated chemotherapeutic agent. Within this ambit, DOX conjugates stand as a beacon of promise in the perpetual pursuit of refining chemotherapy-induced cardiac compromise.

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.

Targeting the Melanocortin 1 Receptor in Melanoma: Biological Activity of α-MSH-Peptide Conjugates

Malignant melanoma is one of the most aggressive and resistant tumor types, with high metastatic properties. Because of the lack of suitable chemotherapeutic agents for treatment, the 5-year survival rate of melanoma patients with regional and distant metastases is lower than 10%. Targeted tumor therapy that provides several promising results might be a good option for the treatment of malignant melanomas. Our goal was to develop novel melanoma-specific peptide-drug conjugates for targeted tumor therapy. Melanocortin-1-receptor (MC1R) is a cell surface receptor responsible for melanogenesis and it is overexpressed on the surface of melanoma cells, providing a good target. Its native ligand, α-MSH (α-melanocyte-stimulating hormone) peptide, or its derivatives, might be potential homing devices for this purpose. Therefore, we prepared three α-MSH derivative-daunomycin (Dau) conjugates and their in vitro and in vivo antitumor activities were compared. Dau has an autofluorescence property; therefore, it is suitable for preparing conjugates for in vitro (e.g., cellular uptake) and in vivo experiments. Dau was attached to the peptides via a non-cleavable oxime linkage that was applied efficiently in our previous experiments, resulting in conjugates with high tumor growth inhibition activity. The results indicated that the most promising conjugate was the compound in which Dau was connected to the side chain of Lys (Ac-SYSNleEHFRWGK(Dau=Aoa)PV-NH2). The highest cellular uptake by melanoma cells was demonstrated using the compound, with the highest tumor growth inhibition detected both on mouse (38.6% on B16) and human uveal melanoma (55% on OMC-1) cells. The effect of the compound was more pronounced than that of the free drug.

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.

pH-Responsive Polymer Nanomaterials for Tumor Therapy

The complexity of the tumor microenvironment presents significant challenges to cancer therapy, while providing opportunities for targeted drug delivery. Using characteristic signals of the tumor microenvironment, various stimuli-responsive drug delivery systems can be constructed for targeted drug delivery to tumor sites. Among these, the pH is frequently utilized, owing to the pH of the tumor microenvironment being lower than that of blood and healthy tissues. pH-responsive polymer carriers can improve the efficiency of drug delivery in vivo, allow targeted drug delivery, and reduce adverse drug reactions, enabling multifunctional and personalized treatment. pH-responsive polymers have gained increasing interest due to their advantageous properties and potential for applicability in tumor therapy. In this review, recent advances in, and common applications of, pH-responsive polymer nanomaterials for drug delivery in cancer therapy are summarized, with a focus on the different types of pH-responsive polymers. Moreover, the challenges and future applications in this field are prospected.

Structural Characterization of Daunomycin-Peptide Conjugates by Various Tandem Mass Spectrometric Techniques

The use of peptide-drug conjugates has generated wide interest as targeted antitumor therapeutics. The anthracycline antibiotic, daunomycin, is a widely used anticancer agent and it is often conjugated to different tumor homing peptides. However, comprehensive analytical characterization of these conjugates via tandem mass spectrometry (MS/MS) is challenging due to the lability of the O-glycosidic bond and the appearance of MS/MS fragment ions with little structural information. Therefore, we aimed to investigate the optimal fragmentation conditions that suppress the prevalent dissociation of the anthracycline drug and provide good sequence coverage. In this study, we comprehensively compared the performance of common fragmentation techniques, such as higher energy collisional dissociation (HCD), electron transfer dissociation (ETD), electron-transfer higher energy collisional dissociation (EThcD) and matrix-assisted laser desorption/ionization–tandem time-of-flight (MALDI-TOF/TOF) activation methods for the structural identification of synthetic daunomycin-peptide conjugates by high-resolution tandem mass spectrometry. Our results showed that peptide backbone fragmentation was inhibited by applying electron-based dissociation methods to conjugates, most possibly due to the “electron predator” effect of the daunomycin. We found that efficient HCD fragmentation was largely influenced by several factors, such as amino acid sequences, charge states and HCD energy. High energy HCD and MALDI-TOF/TOF combined with collision induced dissociation (CID) mode are the methods of choice to unambiguously assign the sequence, localize different conjugation sites and differentiate conjugate isomers.

Stimulus-cleavable chemistry in the field of controlled drug delivery

This review comprehensively summarises stimulus-cleavable linkers from various research areas and their cleavage mechanisms, thus provides an insightful guideline to extend their potential applications to controlled drug release from nanomaterials.

Design and characterization of a recombinant immunotoxin for targeted therapy of breast cancer cells: In vitro and in silico analyses

AIMS

GnRH-DFF40 (gonadotropin releasing hormone-DNA fragmentation factor 40) humanized recombinant immunotoxin serves as a prospective candidate for targeted therapy of malignancies with over-expressed gonadotropin releasing hormone receptor (GnRHR). In this study, we attempted to generate a GnRH-based chimeric protein composed of human DFF40 fused with GnRH which encodes an apoptotic nuclease and specifically targets cancer cells displaying GnRH receptor overexpression.

MATERIALS AND METHODS

A codon optimized, synthetic GnRH-DFF40 fusion gene and its single counterpart (DFF40) were constructed in pET28a expression vector. Cytotoxicity of these expressed proteins were evaluated on three breast cancer cell lines (MCF7, MDA-MB231, and SKBR3). The stability and biological activity of the recombinant proteins were investigated in the treated cell line and cell-free system. Also, the ability of this fusion and its single form in inducing apoptosis, and inhibiting metastasis and migration were evaluated by flow cytometry, migration assay and wound healing analysis, respectively. In silico analyses were also done to understand the specific interactions between GnRH and its receptor.

KEY FINDINGS

GnRH-DFF40 fusion protein and DFF40 were successfully expressed. The purified chimeric protein showed dose-dependent cytotoxicity against all three cell lines. The recombinant fusion protein was biologically active with nucleolytic functionality and apoptosis induction ability. Moreover, the fusion could inhibit the invasion property of MDA-MB-231 cells. In silico analysis also showed that four residues from GnRH domain and 11 GnRHR residues had the most interaction sites for specific targeted delivery of the immunotoxin in cancer cells.

SIGNIFICANCE

Fusion construct could be a prospective candidate for targeted therapy of cancers upregulating GnRH receptor.

pH-Sensitive Biomaterials for Drug Delivery

The development of precise and personalized medicine requires novel formulation strategies to deliver the therapeutic payloads to the pathological tissues, producing enhanced therapeutic outcome and reduced side effects. As many diseased tissues are feathered with acidic characteristics microenvironment, pH-sensitive biomaterials for drug delivery present great promise for the purpose, which could protect the therapeutic payloads from metabolism and degradation during in vivo circulation and exhibit responsive release of the therapeutics triggered by the acidic pathological tissues, especially for cancer treatment. In the past decades, many methodologies, such as acidic cleavage linkage, have been applied for fabrication of pH-responsive materials for both in vitro and in vivo applications. In this review, we will summarize some pH-sensitive drug delivery system for medical application, mainly focusing on the pH-sensitive linkage bonds and pH-sensitive biomaterials.

Development of programmable gemcitabine-GnRH pro-drugs bearing linker controllable "click" oxime bond tethers and preclinical evaluation against prostate cancer

Peptide-drug conjugates (PDCs) are gaining considerable attention as anti-neoplastic agents. However, their development is often laborious and time-consuming. Herein, we have developed and preclinically evaluated three PDCs with gemcitabine as the anticancer cytotoxic unit and D-Lys⁶-GnRH (gonadotropin-releasing hormone; GnRH) as the cancer-targeting unit. These units were tethered via acid-labile programmable linkers to guide a differential drug release rate from the PDC through a combination of ester or amide and "click" type oxime ligations. The pro-drugs were designed to enable the selective targeting of malignant tumor cells with linker guided differential drug release rates. We exploited the oxime bond responsiveness against the acidic pH of the tumor microenvironment and the GnRH endocytosis via the GnRH-R GPCR which is overexpressed on cancer cells. The challenging metabolic properties of gemcitabine were addressed during design of the PDCs. We developed a rapid (1 hour) and cost-effective "click" oxime bond ligation platform to assemble in one-pot the 3 desired PDCs that does not require purification, surpassing traditional time-ineffective and low yield methods. The internalization of the tumor-homing peptide unit in cancer cells, overexpressing the GnRH-R, was first validated through confocal laser microscopy and flow cytometry analysis. Subsequently, the three PDCs were evaluated for their in vitro antiproliferative effect in prostate cancer cells. Their stability and the release of gemcitabine over time were monitored in vitro in cell culture and in human plasma using LC-MS/MS. We then assessed the ability of the developed PDCs to internalize in prostate cancer cells and to release gemcitabine. The most potent analog, designated GOXG₁, was used for pharmacokinetic studies in mice. The metabolism of GOXG₁ was examined in liver microsomes, as well as in buffers mimicking the pH of intracellular organelles, resulting in the identification of two metabolites. The major metabolite at low pH emanated from the cleavage of the pH-labile oxime bond, validating our design approach. NMR spectroscopy and in vitro radioligand binding assays were exploited for GOXG₁ to validate that upon conjugating the drug to the peptide, the peptide microenvironment responsible for its GnRH-R binding is not perturbed and to confirm its high binding potency to the GnRH-R. Finally, the binding of GOXG₁ to the GnRH-R and the associated elicitation of testosterone release in mice were also determined. The facile platform established herein for the rapid assembly of PDCs with linker controllable characteristics from aldehyde and aminooxy units through rapid "click" oxime ligation, that does not require purification steps, could pave the way for a new generation of potent cancer therapeutics, diagnostics and theranostics.

Phage Display-Based Homing Peptide-Daunomycin Conjugates for Selective Drug Targeting to PANC-1 Pancreatic Cancer

The Pancreatic Ductal Adenocarcinoma (PDAC) is one of the most aggressive and dangerous cancerous diseases, leading to a high rate of mortality. Therefore, the development of new, more efficient treatment approaches is necessary to cure this illness. Peptide-based drug targeting provides a new tool for this purpose. Previously, a hexapeptide Cys-Lys-Ala-Ala-Lys-Asn (CKAAKN) was applied efficiently as the homing device for drug-loaded nanostructures in PDAC cells. In this research, Cys was replaced by Ser in the sequence and this new SKAAKN targeting moiety was used in conjugates containing daunomycin (Dau). Five different structures were developed and tested. The results indicated that linear versions with one Dau were not effective on PANC-1 cells in vitro; however, branched conjugates with two Dau molecules showed significant antitumor activity. Differences in the antitumor effect of the conjugates could be explained with the different cellular uptake and lysosomal degradation. The most efficient conjugate was Dau=Aoa-GFLG-K(Dau=Aoa)SKAAKN-OH (conjugate 4) that also showed significant tumor growth inhibition on s.c. implanted PANC-1 tumor-bearing mice with negligible side effects. Our novel results suggest that peptide-based drug delivery systems could be a promising tool for the treatment of pancreatic cancers.

Improved In Vivo Anti-Tumor and Anti-Metastatic Effect of GnRH-III-Daunorubicin Analogs on Colorectal and Breast Carcinoma Bearing Mice

Among various homing devices, gonadotropin-releasing hormone-III (GnRH-III) peptide represents a suitable targeting moiety for drug delivery systems. The anti-tumor activity of the previously developed GnRH-III-[4Lys(Bu),8Lys(Dau=Aoa)] conjugate and the novel synthesized GnRH-III-[2ΔHis,3d-Tic,4Lys(Bu),8Lys(Dau=Aoa)] conjugate, containing the anti-cancer drug daunorubicin, were evaluated. Here, we demonstrate that both GnRH-III-Dau conjugates possess an efficient growth inhibitory effect on more than 20 cancer cell lines, whereby the biological activity is strongly connected to the expression of gonadotropin-releasing hormone receptors (GnRH-R). The novel conjugate showed a higher in vitro anti-proliferative activity and a higher uptake capacity. Moreover, the treatment with GnRH-III-Dau conjugates cause a significant in vivo tumor growth and metastases inhibitory effect in three different orthotopic models, including 4T1 mice and MDA-MB-231 human breast carcinoma, as well as HT-29 human colorectal cancer bearing BALB/s and SCID mice, while toxic side-effects were substantially reduced in comparison to the treatment with the free drug. These findings illustrate that our novel lead compound is a highly promising candidate for targeted tumor therapy in both colon cancer and metastatic breast cancer.

Apoptotic Effects of Drug Targeting Conjugates Containing Different GnRH Analogs on Colon Carcinoma Cells

The wide range of cellular target reactions (e.g., antitumor) of gonadotropin-releasing hormone (GnRH) variants provides the possibility to develop multifunctional GnRH conjugates. The aim of our work was to compare the cytotoxic/apoptotic activity of different GnRH-based, daunorubicin (Dau)-linked conjugates with or without butyrated Lys in position 4 (⁴Lys(Bu)) at a molecular level in a human colorectal carcinoma cell line. Cell viability was measured by impedimetry, cellular uptake and apoptosis were studied by flow cytometry, and the expression of apoptosis-related genes was analyzed by qRT-PCR. The modification with ⁴Lys(Bu) resulted in an increased cytotoxic and apoptotic effects and cellular uptake of the GnRH-I and GnRH-III conjugates. Depending on the GnRH isoform and the presence of ⁴Lys(Bu), the conjugates could regulate the expression of several apoptosis-related genes, especially tumor necrosis factor (TNF), tumor protein p53 (TP53) and the members of growth-factor signaling. The stronger cytotoxicity of GnRH-I and GnRH-III conjugates containing ⁴Lys(Bu) was associated with a stronger inhibitory effect on the expression of growth-factor signaling elements in comparison with their ⁴Ser counterparts, in which the upregulation of TP53 and caspases (e.g., CASP9) seemed to play a more important role. We were able to provide further evidence that targeting the GnRH receptor could serve as a successful therapeutic approach in colon cancer, and GnRH-III-[⁴Lys(Bu),⁸Lys(Dau=Aoa)] proved to be the best candidate for this purpose.

Aryl Sulfate is a Useful Motif for Conjugating and Releasing Phenolic Molecules: Sulfur Fluorine Exchange Click Chemistry Enables Discovery of Ortho-Hydroxy-Protected Aryl Sulfate Linker

A new self-immolative linker motif, Ortho Hydroxy-Protected Aryl Sulfate (OHPAS), was devised, and OHPAS-containing antibody drug conjugates (ADC) were tested in vitro and in vivo. Conveniently synthesized using Sulfur Fluorine Exchange (SuFEx) chemistry, it is based structurally on diaryl sulfate, with one aryl acting as a payload and the other as a self-immolative sulfate unit having a latent phenol function at the ortho position. The chemically stable OHPAS linker was stable in plasma samples from 5 different species, yet it can release the payload molecule smoothly upon chemical or biological triggering. The payload release proceeds via intramolecular cyclization, producing a cyclic sulfate coproduct that eventually hydrolyzes to a catechol monosulfate. A set of OHPAS-containing ADCs based on Trastuzumab were prepared with a drug to antibody ratio of ∼2, and were shown to be cytotoxic in 5 different cancer cell lines in vitro and dose-dependently inhibited tumor growth in a NCI-N87 mouse xenograft model. We conclude that OHPAS conjugates will be of considerable use for delivering phenol-containing payloads to tissues targeted for medical intervention.

Enhanced In Vitro Antitumor Activity of GnRH-III-Daunorubicin Bioconjugates Influenced by Sequence Modification

Receptors for gonadotropin releasing hormone (GnRH) are highly expressed in various human cancers including breast, ovarian, endometrial, prostate and colorectal cancer. Ligands like human GnRH-I or the sea lamprey analogue GnRH-III represent a promising approach for the development of efficient drug delivery systems for targeted tumor therapy. Here, we report on the synthesis and cytostatic effect of 14 oxime bond-linked daunorubicin GnRH-III conjugates containing a variety of unnatural amino acids within the peptide sequence. All compounds demonstrated a reduced cell viability in vitro on estrogen receptor α (ERα) positive and ERα negative cancer cells. The best candidate revealed an increased cancer cell growth inhibitory effect compared to our lead-compound GnRH-III-[⁴Lys(Bu),⁸Lys(Dau=Aoa)]. Flow cytometry and fluorescence microscopy studies showed that the cellular uptake of the novel conjugate is substantially improved leading to an accelerated delivery of the drug to its site of action. However, the release of the active drug-metabolite by lysosomal enzymes was not negatively affected by amino acid substitution, while the compound provided a high stability in human blood plasma. Receptor binding studies were carried out to ensure a high binding affinity of the new compound for the GnRH-receptor. It was demonstrated that GnRH-III-[²ΔHis,³d-Tic,⁴Lys(Bu),⁸Lys(Dau=Aoa)] is a highly potent and promising anticancer drug delivery system for targeted tumor therapy.

Comparative cell biological study of in vitro antitumor and antimetastatic activity on melanoma cells of GnRH-III-containing conjugates modified with short-chain fatty acids

Background: Peptide hormone-based targeted tumor therapy is an approved strategy to selectively block the tumor growth and spreading. The gonadotropin-releasing hormone receptors (GnRH-R) overexpressed on different tumors (e.g., melanoma) could be utilized for drug-targeting by application of a GnRH analog as a carrier to deliver a covalently linked chemotherapeutic drug directly to the tumor cells. In this study our aim was (i) to analyze the effects of GnRH-drug conjugates on melanoma cell proliferation, adhesion and migration, (ii) to study the mechanisms of tumor cell responses, and (iii) to compare the activities of conjugates with the free drug.

Results: In the tested conjugates, daunorubicin (Dau) was coupled to ⁸Lys of GnRH-III (GnRH-III(Dau=Aoa)) or its derivatives modified with ⁴Lys acylated with short-chain fatty acids (acetyl group in [⁴Lys(Ac)]-GnRH-III(Dau=Aoa) and butyryl group in [⁴Lys(Bu)]-GnRH-III(Dau=Aoa)). The uptake of conjugates by A2058 melanoma model cells proved to be time dependent. Impedance-based proliferation measurements with xCELLigence SP system showed that all conjugates elicited irreversible tumor growth inhibitory effects mediated via a phosphoinositide 3-kinase-dependent signaling. GnRH-III(Dau=Aoa) and [⁴Lys(Ac)]-GnRH-III(Dau=Aoa) were shown to be blockers of the cell cycle in the G2/M phase, while [⁴Lys(Bu)]-GnRH-III(Dau=Aoa) rather induced apoptosis. In short-term, the melanoma cell adhesion was significantly increased by all the tested conjugates. The modification of the GnRH-III in position 4 was accompanied by an increased cellular uptake, higher cytotoxic and cell adhesion inducer activity. By studying the cell movement of A2058 cells with a holographic microscope, it was found that the migratory behavior of melanoma cells was increased by [⁴Lys(Ac)]-GnRH-III(Dau=Aoa), while the GnRH-III(Dau=Aoa) and [⁴Lys(Bu)]-GnRH-III(Dau=Aoa) decreased this activity.

Conclusion: Internalization and cytotoxicity of the conjugates showed that GnRH-III peptides could guard Dau to melanoma cells and promote antitumor activity. [⁴Lys(Bu)]-GnRH-III(Dau=Aoa) possessing the butyryl side chain acting as a “second drug” proved to be the best candidate for targeted tumor therapy due to its cytotoxicity and immobilizing effect on tumor cell spreading. The applicability of impedimetry and holographic phase imaging for characterizing cancer cell behavior and effects of targeted chemotherapeutics with small structural differences (e.g., length of the side chain in ⁴Lys) was also clearly suggested.

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.

Drug targeting to decrease cardiotoxicity - determination of the cytotoxic effect of GnRH-based conjugates containing doxorubicin, daunorubicin and methotrexate on human cardiomyocytes and endothelial cells

Background: Cardiomyopathy induced by the chemotherapeutic agents doxorubicin and daunorubicin is a major limiting factor for their application in cancer therapy. Chemotactic drug targeting potentially increases the tumor selectivity of drugs and decreases their cardiotoxicity. Increased expression of gonadotropin-releasing hormone (GnRH) receptors on the surface of tumor cells has been reported. Thus, the attachment of the aforementioned chemotherapeutic drugs to GnRH-based peptides may result in compounds with increased therapeutic efficacy. The objective of the present study was to examine the cytotoxic effect of anticancer drug–GnRH-conjugates against two essential cardiovascular cell types, such as cardiomyocytes and endothelial cells. Sixteen different previously developed GnRH-conjugates containing doxorubicin, daunorubicin and methotrexate were investigated in this study. Their cytotoxicity was determined on primary human cardiac myocytes (HCM) and human umbilical vein endothelial cells (HUVEC) using the xCELLigence SP system, which measures impedance changes caused by adhering cells on golden electrode arrays placed at the bottom of the wells. Slopes of impedance–time curves were calculated and for the quantitative determination of cytotoxicity, the difference to the control was analysed.

Results: Doxorubicin and daunorubicin exhibited a cytotoxic effect on both cell types, at the highest concentrations tested. Doxorubicin-based conjugates (AN-152, GnRH-III(Dox-O-glut), GnRH-III(Dox-glut-GFLG) and GnRH-III(Dox=Aoa-GFLG) showed the same cytotoxic effect on cardiomyocytes. Among the daunorubicin-based conjugates, [⁴Lys(Ac)]-GnRH-III(Dau=Aoa), GnRH-III(Dau=Aoa-YRRL), {GnRH-III(Dau=Aoa-YRRL-C)}₂ and {[⁴N-MeSer]-GnRH-III(Dau-C)}₂ had a significant but decreased cytotoxic effect, while the other conjugates – GnRH-III(Dau=Aoa), GnRH-III(Dau=Aoa-K(Dau=Aoa)), [⁴Lys(Dau=Aoa)]-GnRH-III(Dau=Aoa), GnRH-III(Dau=Aoa-GFLG), {GnRH-III(Dau-C)}₂ and [⁴N-MeSer]-GnRH-III(Dau=Aoa) – exerted no cytotoxic effect on cardiomyocytes. Mixed conjugates containing methotrexate and daunorubicin – GnRH-III(Mtx-K(Dau=Aoa)) and [⁴Lys(Mtx)]-GnRH-III(Dau=Aoa) – showed no cytotoxic effect on cardiomyocytes, as well.

Conclusion: Based on these results, anticancer drug–GnRH-based conjugates with no cytotoxic effect on cardiomyocytes were identified. In the future, these compounds could provide a more targeted antitumor therapy with no cardiotoxic adverse effects. Moreover, impedimetric cytotoxicity analysis could be a valuable technique to determine the effect of drugs on cardiomyocytes.

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.

Synthesis and in vitro biochemical evaluation of oxime bond-linked daunorubicin-GnRH-III conjugates developed for targeted drug delivery

Gonadotropin releasing hormone-III (GnRH-III), a native isoform of the human GnRH isolated from sea lamprey, specifically binds to GnRH receptors on cancer cells enabling its application as targeting moieties for anticancer drugs. Recently, we reported on the identification of a novel daunorubicin–GnRH-III conjugate (GnRH-III–[⁴Lys(Bu), ⁸Lys(Dau=Aoa)] with efficient in vitro and in vivo antitumor activity. To get a deeper insight into the mechanism of action of our lead compound, the cellular uptake was followed by confocal laser scanning microscopy. Hereby, the drug daunorubicin could be visualized in different subcellular compartments by following the localization of the drug in a time-dependent manner. Colocalization studies were carried out to prove the presence of the drug in lysosomes (early stage) and on its site of action (nuclei after 10 min). Additional flow cytometry studies demonstrated that the cellular uptake of the bioconjugate was inhibited in the presence of the competitive ligand triptorelin indicating a receptor-mediated pathway. For comparative purpose, six novel daunorubicin–GnRH-III bioconjugates have been synthesized and biochemically characterized in which ⁶Asp was replaced by D-Asp, D-Glu and D-Trp. In addition to the analysis of the in vitro cytostatic effect and cellular uptake, receptor binding studies with ¹²⁵I-triptorelin as radiotracer and degradation of the GnRH-III conjugates in the presence of rat liver lysosomal homogenate have been performed. All derivatives showed high binding affinities to GnRH receptors and displayed in vitro cytostatic effects on HT-29 and MCF-7 cancer cells with IC₅₀ values in a low micromolar range. Moreover, we found that the release of the active drug metabolite and the cellular uptake of the bioconjugates were strongly affected by the amino acid exchange which in turn had an impact on the antitumor activity of the bioconjugates.

NGR-peptide-drug conjugates with dual targeting properties

Peptides containing the asparagine-glycine-arginine (NGR) motif are recognized by CD13/aminopeptidase N (APN) receptor isoforms that are selectively overexpressed in tumor neovasculature. Spontaneous decomposition of NGR peptides can result in isoAsp derivatives, which are recognized by RGD-binding integrins that are essential for tumor metastasis. Peptides binding to CD13 and RGD-binding integrins provide tumor-homing, which can be exploited for dual targeted delivery of anticancer drugs. We synthesized small cyclic NGR peptide-daunomycin conjugates using NGR peptides of varying stability (c[KNGRE]-NH2, Ac-c[CNGRC]-NH2 and the thioether bond containing c[CH2-CO-NGRC]-NH2, c[CH2-CO-KNGRC]-NH2). The cytotoxic effect of the novel cyclic NGR peptide-Dau conjugates were examined in vitro on CD13 positive HT-1080 (human fibrosarcoma) and CD13 negative HT-29 (human colon adenocarcinoma) cell lines. Our results confirm the influence of structure on the antitumor activity and dual acting properties of the conjugates. Attachment of the drug through an enzyme-labile spacer to the C-terminus of cyclic NGR peptide resulted in higher antitumor activity on both CD13 positive and negative cells as compared to the branching versions.

Targeted delivery for regenerative medicines: an untapped opportunity for drug conjugates

Regenerative approaches are promising avenues to effectively cure diseases rather than merely treating symptoms, but are associated with concerns around proliferation in other organs. Given that targeted delivery holds the promise of delivering a drug precisely to its desired site of action, usually with the prospect of increasing the therapeutic index, it can be considered as an essential enabler of regenerative medicines. Although significant progress has been made predominantly in oncology for the delivery of cytotoxic drugs using antibody-drug conjugates (ADCs), the physiological conditions and safety requirements for regenerative medicines are very different. Drug conjugates need to be approached differently and, we herein suggest using a broader range of homing modalities and a specific framework to develop safe linkers.

Modification of daunorubicin-GnRH-III bioconjugates with oligoethylene glycol derivatives to improve solubility and bioavailability for targeted cancer chemotherapy

Daunorubicin-GnRH-III bioconjugates have recently been developed as drug delivery systems with potential applications in targeted cancer chemotherapy. In order to improve their biochemical properties, several strategies have been pursued: (1) incorporation of an enzymatic cleavable spacer between the anticancer drug and the peptide-based targeting moiety, (2) peptide modification by short chain fatty acids, or (3) attachment of two anticancer drugs to the same GnRH-III derivative. Although these modifications led to more potent bioconjugates, a decrease in their solubility was observed. Here we report on the design, synthesis and biochemical characterization of daunorubicin-GnRH-III bioconjugates with increased solubility, which could be achieved by incorporating oligoethylene glycol-based spacers in their structure. First, we have evaluated the effect of an oligoethylene glycol-based spacer on the solubility, enzymatic stability/degradation, cellular uptake, and in vitro cytostatic effect of a bioconjugate containing only one daunorubicin attached through a GFLG tetrapeptide spacer to the GnRH-III targeting moiety. Thereafter, more complex compounds containing two copies of daunorubicin, GFLG spacers as well as Lys(nBu) in position 4 of GnRH-III were synthesized and biochemically characterized. Our results indicated that all synthesized oligoethylene glycol-containing bioconjugates had higher solubility in cell culture medium than the unmodified analogs. They were degraded in the presence of rat liver lysosomal homogenate leading to the formation of small drug containing metabolites. In the case of bioconjugates containing two copies of daunorubicin, the incorporation of oligoethylene glycol-based spacers led to increased in vitro cytostatic effect on MCF-7 human breast cancer cells.

Improved in vivo antitumor effect of a daunorubicin - GnRH-III bioconjugate modified by apoptosis inducing agent butyric acid on colorectal carcinoma bearing mice

Compared to classical chemotherapy, peptide-based drug targeting is a promising therapeutic approach for cancer, which can provide increased selectivity and decreased side effects to anticancer drugs. Among various homing devices, gonadotropin-releasing hormone-III (GnRH-III) peptide represents a suitable targeting moiety, in particular in the treatment of hormone independent tumors that highly express GnRH receptors (e.g. colon carcinoma). We have previously shown that GnRH-III[⁴Lys(Ac),⁸Lys(Dau = Aoa)] bioconjugate, in which daunorubicin was attached via oxime linkage to the ⁸Lys of a GnRH-III derivative, exerted significant in vivo antitumor effect on subcutaneously developed HT-29 colon tumor. In contrast, results of the study reported here indicated that this compound was not active on an orthotopically developed tumor. However, if Lys in position 4 was acylated with butyric acid instead of acetic acid, the resulting bioconjugate GnRH-III[⁴Lys(Bu),⁸Lys(Dau = Aoa)] had significant tumor growth inhibitory effect. Furthermore, it prevented tumor neovascularization, without detectable side effects. Nevertheless, the development of metastases could not be inhibited by the bioconjugate; therefore, its application in combination with a metastasis preventive agent might be necessary in order to achieve complete tumor remission. In spite of this result, the treatment with GnRH-III[⁴Lys(Bu),⁸Lys(Dau = Aoa)] bioconjugate proved to have significant benefits over the administration of free daunorubicin, which was used at the maximum tolerated dose.

Mechanisms and biomaterials in pH-responsive tumour targeted drug delivery: A review

As the mainstay in the treatment of various cancers, chemotherapy plays a vital role, but still faces many challenges, such as poor tumour selectivity and multidrug resistance (MDR). Targeted drug delivery using nanotechnology has provided a new strategy for addressing the limitations of the conventional chemotherapy. In the last decade, the volume of research published in this area has increased tremendously, especially with functional nano drug delivery systems (nanocarriers). Coupling a specific stimuli-triggered drug release mechanism with these delivery systems is one of the most prevalent approaches for improving therapeutic outcomes. Among the various stimuli, pH triggered delivery is regarded as the most general strategy, targeting the acidic extracellular microenvironment and intracellular organelles of solid tumours. In this review, we discuss recent advances in the development of pH-sensitive nanocarriers for tumour-targeted drug delivery. The review focuses on the chemical design of pH-sensitive biomaterials, which are used to fabricate nanocarriers for extracellular and/or intracellular tumour site-specific drug release. The pH-responsive biomaterials bring forth conformational changes in these nanocarriers through various mechanisms such as protonation, charge reversal or cleavage of a chemical bond, facilitating tumour specific cell uptake or drug release. A greater understanding of these mechanisms will help to design more efficient drug delivery systems to address the challenges encountered in conventional chemotherapy.

Octreotide-Mediated Tumor-Targeted Drug Delivery via a Cleavable Doxorubicin-Peptide Conjugate

Although recent methods for targeted drug delivery have addressed many of the existing problems of cancer therapy associated with undesirable side effects, significant challenges remain that have to be met before they find significant clinical relevance. One such area is the delicate chemical bond that is applied to connect a cytotoxic drug with targeting moieties like antibodies or peptides. Here we describe a novel platform that can be utilized for the preparation of drug-carrier conjugates in a site-specific manner, which provides excellent versatility and enables triggered release inside cancer cells. Its key feature is a cleavable doxorubicin-octreotide bioconjugate that targets overexpressed somatostatin receptors on tumor cells, where the coupling between the two components was achieved through the first cleavable disulfide-intercalating linker. The tumor targeting ability and suppression of adrenocorticotropic hormone secretion in AtT-20 cells by both octreotide and the doxorubicin hybrid were determined via a specific radioimmunoassay. Both substances reduced the hormone secretion to a similar extent, which demonstrated that the tumor homing peptide is able to interact with the relevant cell surface receptors after the attachment of the drug. Effective drug release was quickly accomplished in the presence of the physiological reducing agent glutathione. We also demonstrate the relevance of this scaffold in biological context in cytotoxicity assays with pituitary, pancreatic, and breast cancer cell lines.

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

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

Oxime bond-linked daunorubicin-GnRH-III bioconjugates exert antitumor activity in castration-resistant prostate cancer cells via the type I GnRH receptor

It is well established that gonadotropin-releasing hormone receptors (GnRH-R) are expressed in different types of cancers, including castration-resistant prostate cancer (CRPC) and mediate the antiproliferative effect of GnRH analogs. Thus, these compounds are employed as targeting moieties to selectively deliver chemotherapeutic agents to cancer cells. GnRH-III, the decapeptide isolated from the sea lamprey brain, has lower potency than GnRH in stimulating gonadotropin secretion, but it exerts antiproliferative effects on many tumors expressing the GnRH-R. GnRH-III-based peptides are considered promising targeting moieties for the preparation of anticancer drug delivery systems. These studies were aimed at i) evaluating the antitumor activity of two cytotoxic oxime bond-linked daunorubicin (Dau)-GnRH-III derivative bioconjugates (Dau-GnRH-III, in which daunorubicin was coupled to the 8Lys in the native form of GnRH-III, and Dau-[4Lys(Ac)]-GnRH-III, in which daunorubicin was attached to the 8Lys of a GnRH-III derivative where 4Ser was replaced by an acetylated lysine) on CRPC cells; and ii) to elucidate the involvement of the classical GnRH-R (type I GnRH-R) in this antitumor activity. Our results demonstrated that both Dau-GnRH-III and Dau-[4Lys(Ac)]-GnRH-III were rapidly internalized into DU145 prostate cancer cells and exerted a significant cytostatic effect. Both bioconjugates increased the levels of the active form of caspase-3, indicating the involvement of apoptosis in their antitumor activity. The antiproliferative effect of both Dau-GnRH-III and Dau-[4Lys(Ac)]-GnRH-III was counteracted by the simultaneous treatment of the cells with Antide, an antagonist of the GnRH-R. Moreover, after silencing the type I GnRH-R the antitumor activity of both bioconjugates was completely abolished. These data demonstrate that in CRPC cells, daunorubicin-GnRH-III derivative bioconjugates: i) inhibit tumor cell proliferation, by triggering the apoptosis process; ii) exert their antitumor effect through the activation of the type I GnRH-R expressed on these cells. Cytotoxic-GnRH-III derivative may represent promising targeted chemotherapeutics for the treatment of CRPC patients.

Synthesis of Hydrophilic Aminooxy Linkers and Multivalent Cores for Chemoselective Aldehyde/Ketone Conjugation

A series of three linear and two trivalent aminooxy-containing hydrophilic linkers and cores were synthesized. The five molecules contain from one to three aminooxy groups, and all but one contain an ether for enhanced aqueous solubility. These unique and versatile molecules can be utilized in the chemoselective conjugation of aldehyde/ketone-containing molecules, including reducing sugars, under mild aqueous conditions, and give rise to oxime-containing conjugates useful in a wide variety of applications and studies. The value of these aminooxy-based molecules and the ease and speed of preparation of both monovalent and multivalent oxime-linked molecules is demonstrated in two examples using the disaccharide cellobiose; one with a linear linker, and the second with a trivalent core.

Synthesis, enzymatic stability and in vitro cytostatic effect of Daunorubicin-GnRH-III derivative dimers

Bioconjugates containing chemotherapeutic agents attached to peptide hormones, such as gonadotropin-releasing hormone (GnRH), are developed as drug delivery systems for targeted cancer chemotherapy. We report here the synthesis and biochemical characterization of disulfide bond-linked dimeric bioconjugates in which daunorubicin was coupled via an oxime linkage to aminooxyacetylated GnRH-III ([Glp-His-Trp-Ser-His-Asp-Trp-Lys(DauAoa-Cys)-Pro-Gly-NH2]2; where Glp is pyroglutamic acid and Aoa is aminooxyacetyl) and its derivatives modified in position four by N-Me-Ser and Lys(Ac). The in vitro stability/degradation of the bioconjugates was determined in human serum, as well as in the presence of rat liver lysosomal homogenate and digestive enzymes. All compounds were stable at least for 24h in human serum and in the presence of pepsin and trypsin, while they were degraded by lysosomal enzymes. The daunorubicin-GnRH-III derivative dimers were partly digested by α-chymotrypsin; however, they had increased stability compared to the corresponding monomers, making them potential candidates for oral administration. The in vitro cytostatic effect of the compounds was determined on MCF-7 human breast cancer cells by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. All daunorubicin-GnRH-III derivative dimers exerted slightly increased in vitro cytostatic effect (IC50 values in low μM range) than the corresponding monomeric bioconjugates.

Fully automated chip-based nanoelectrospray combined with electron transfer dissociation for high throughput top-down proteomics

The conventional protocol for protein identification by electrospray ionization mass spectrometry (MS) is based on enzymatic digestion which renders peptides to be analyzed by liquid chromatography-MS and collision-induced dissociation (CID) multistage MS, in the so-called bottom-up approach. Though this method has brought a significant progress to the field, many limitations, among which, the low throughput and impossibility to characterize in detail posttranslational modifications in terms of site(s) and structure, were reported. Therefore, the research is presently focused on the development of procedures for efficient top-down fragmentation of intact protein ions. In this context, we developed here an approach combining fully automated chip-based-nanoelectrospray ionisation (nanoESI), performed on a NanoMate robot, with electron transfer dissociation (ETD) for peptide and top-down protein sequencing and identification. This advanced analytical platform, integrating robotics, microfluidics technology, ETD and alternate ETD/CID, was tested and found ideally suitable for structural investigation of peptides and modified/functionalized peptides as well as for top-down analysis of medium size proteins by tandem MS experiments of significantly increased throughput and sensitivity. The obtained results indicate that NanoMate-ETD and ETD/CID may represent a viable alternative to the current MS strategies, with potential to develop into a method of routine use for high throughput top-down proteomics.

GnRH receptors in cancer: from cell biology to novel targeted therapeutic strategies

The crucial role of pituitary GnRH receptors (GnRH-R) in the control of reproductive functions is well established. These receptors are the target of GnRH agonists (through receptor desensitization) and antagonists (through receptor blockade) for the treatment of steroid-dependent pathologies, including hormone-dependent tumors. It has also become increasingly clear that GnRH-R are expressed in cancer tissues, either related (i.e. prostate, breast, endometrial, and ovarian cancers) or unrelated (i.e. melanoma, glioblastoma, lung, and pancreatic cancers) to the reproductive system. In hormone-related tumors, GnRH-R appear to be expressed even when the tumor has escaped steroid dependence (such as castration-resistant prostate cancer). These receptors are coupled to a G(αi)-mediated intracellular signaling pathway. Activation of tumor GnRH-R by means of GnRH agonists elicits a strong antiproliferative, antimetastatic, and antiangiogenic (more recently demonstrated) activity. Interestingly, GnRH antagonists have also been shown to elicit a direct antitumor effect; thus, these compounds behave as antagonists of GnRH-R at the pituitary level and as agonists of the same receptors expressed in tumors. According to the ligand-induced selective-signaling theory, GnRH-R might assume various conformations, endowed with different activities for GnRH analogs and with different intracellular signaling pathways, according to the cell context. Based on these consistent experimental observations, tumor GnRH-R are now considered a very interesting candidate for novel molecular, GnRH analog-based, targeted strategies for the treatment of tumors expressing these receptors. These agents include GnRH agonists and antagonists, GnRH analog-based cytotoxic (i.e. doxorubicin) or nutraceutic (i.e. curcumin) hybrids, and GnRH-R-targeted nanoparticles delivering anticancer compounds.

Design, synthesis, in vitro stability and cytostatic effect of multifunctional anticancer drug-bioconjugates containing GnRH-III as a targeting moiety

Bioconjugates containing the GnRH-III hormone decapeptide as a targeting moiety are able to deliver chemotherapeutic agents specifically to cancer cells expressing GnRH receptors, thereby increasing their local efficacy while limiting the peripheral toxicity. However, the number of GnRH receptors on cancer cells is limited and they desensitize under continuous hormone treatment. A possible approach to increase the receptor mediated tumor targeting and consequently the cytostatic effect of the bioconjugates would be the attachment of more than one chemotherapeutic agent to one GnRH-III molecule. Here we report on the design, synthesis and biochemical characterization of multifunctional bioconjugates containing GnRH-III as a targeting moiety and daunorubicin as a chemotherapeutic agent. Two different drug design approaches were pursued. The first one was based on the bifunctional [(4)Lys]-GnRH-III (Glp-His-Trp-Lys-His-Asp-Trp-Lys-Pro-Gly-NH(2)) containing two lysine residues in positions 4 and 8, whose ε-amino groups were used for the coupling of daunorubicin. In the second drug design, the native GnRH-III (Glp-His-Trp-Ser-His-Asp-Trp-Lys-Pro-Gly-NH(2)) was used as a scaffold; an additional lysine residue was coupled to the ϵ-amino group of (8) Lys in order to generate two free amino groups available for conjugation of daunorubicin. The in vitro stability/degradation of all synthesized compounds was investigated in human serum, as well as in the presence of rat liver lysosomal homogenate. Their cellular uptake was determined on human breast cancer cells and the cytostatic effect was evaluated on human breast, colon and prostate cancer cell lines. Compared with a monofunctional compound, both drug design approaches resulted in multifunctional bioconjugates with increased cytostatic effect.

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

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

Peptide-mediated targeted drug delivery

Targeted drug delivery to specific group of cells offers an attractive strategy to minimize the undesirable side effects and achieve the therapeutic effect with a lower dose. Both linear and cyclic peptides have been explored as trafficking moiety due to ease of synthesis, structural simplicity, and low probability of undesirable immunogenicity. Peptides derived from sequence of cell surface proteins, such as intercellular adhesion molecule-1 (ICAM-1), LHRH, Bombesin, and LFA-1, have shown potent binding affinity to the target cell surface receptors. Moreover, peptides derived from ICAM-1 receptor can be internalized by the leukemic T-cells along with the conjugated moiety offering the promise to selectively treat cancers and autoimmune diseases. Systematic analyses have revealed that physicochemical properties of the drug-peptide conjugates and their mechanism of receptor-mediated cellular internalization are important controlling factors for developing a successful targeting system. This review is focused on understanding the factors involved in the development of an effective drug-peptide conjugate with an emphasis on the chemistry and biology of the conjugates. Reported results on several promising drug-peptide conjugates have been critically evaluated. The approaches and results presented here will serve as a guide to systematically approach targeted delivery of cytotoxic drug molecules using peptides for treatment of several diseases.