Phase II trial of dolastatin-10 in patients with advanced breast cancer

Natural-source payloads used in the conjugated drugs architecture for cancer therapy: Recent advances and future directions

Drug conjugates are obtained from tumor-located vectors connected to cytotoxic agents via linkers, which are designed to deliver hyper-toxic payloads directly to targeted cancer cells. These drug conjugates include antibody-drug conjugates (ADCs), peptide-drug conjugates (PDCs), small molecule-drug conjugates (SMDCs), nucleic acid aptamer-drug conjugates (ApDCs), and virus-like drug conjugate (VDCs), which show great therapeutic value in the clinic. Drug conjugates consist of a targeting carrier, a linker, and a payload. Payloads are key therapy components. Cytotoxic molecules and their derivatives derived from natural products are commonly used in the payload portion of conjugates. The ideal payload should have sufficient toxicity, stability, coupling sites, and the ability to be released under specific conditions to kill tumor cells. Microtubule protein inhibitors, DNA damage agents, and RNA inhibitors are common cytotoxic molecules. Among these conjugates, cytotoxic molecules of natural origin are summarized based on their mechanism of action, conformational relationships, and the discovery of new derivatives. This paper also mentions some cytotoxic molecules that have the potential to be payloads. It also summarizes the latest technologies and novel conjugates developed in recent years to overcome the shortcomings of ADCs, PDCs, SMDCs, ApDCs, and VDCs. In addition, this paper summarizes the clinical trials conducted on conjugates of these cytotoxic molecules over the last five years. It provides a reference for designing and developing safer and more efficient conjugates.

Click Chemistry Selectively Activates an Auristatin Protodrug with either Intratumoral or Systemic Tumor-Targeting Agents

The Click Activated Protodrugs Against Cancer (CAPAC) platform enables the activation of powerful cancer drugs at tumors. CAPAC utilizes a click chemistry reaction between tetrazine and trans-cyclooctene. The reaction between activator, linked to a tumor-targeting agent, and protodrug leads to the targeted activation of the drug. Here, tumor targeting is achieved by intratumoral injection of a tetrazine-modified hyaluronate (SQL70) or by infusion of a tetrazine-modified HER2-targeting antigen-binding fragment (SQT01). Monomethyl auristatin E (a cytotoxin hindered in its clinical use by severe toxicity) was modified with a trans-cyclooctene to form the protodrug SQP22, which reduced its cytotoxicity in vitro and in vivo. Treatment of SQP22 paired with SQL70 demonstrated antitumor effects in Karpas 299 and RENCA murine tumor models, establishing the requirement of click chemistry for protodrug activation. SQP22 paired with SQT01 induced antitumor effects in the HER2-positive NCI-N87 xenograft model, showing that tumor-targeted activation could be accomplished via systemic dosing. Observed toxicities were limited, with transient myelosuppression and moderate body weight loss detected. This study highlights the capabilities of the CAPAC platform by demonstrating the activity of SQP22 with two differentiated targeting approaches and underscores the power of click chemistry to precisely control the activation of drugs at tumors.

Curing childhood cancer the "Natural" Way: Nature as the source of chemotherapy agents

For many centuries, products of natural origin from plants, marine, microbes and soil micro-organisms have been studied by numerous researchers across the world to yield many of the chemotherapeutic agents we use in this modern era. There has been a tremendous gain in knowledge from various screening and separating techniques which led to the discovery of biologically active small molecules from natural products. Preclinical studies testing the antitumor activities of these agents against tumor cell lines and xenograft animal models were the gateway to the clinical trials in humans leading to the approval of these agents that are in clinical use today. This review summarizes how various chemotherapeutic agents were discovered from products of natural origin, their preclinical development, and their indications in both pediatric and adult oncology. Many of these natural products have contributed to the very high cure rates of both pediatric leukemias and solid tumors.

Antibody-Drug Conjugates Containing Payloads from Marine Origin

Antibody-drug conjugates (ADCs) are an important class of therapeutics for the treatment of cancer. Structurally, an ADC comprises an antibody, which serves as the delivery system, a payload drug that is a potent cytotoxin that kills cancer cells, and a chemical linker that connects the payload with the antibody. Unlike conventional chemotherapy methods, an ADC couples the selective targeting and pharmacokinetic characteristics related to the antibody with the potent cytotoxicity of the payload. This results in high specificity and potency by reducing off-target toxicities in patients by limiting the exposure of healthy tissues to the cytotoxic drug. As a consequence of these outstanding features, significant research efforts have been devoted to the design, synthesis, and development of ADCs, and several ADCs have been approved for clinical use. The ADC field not only relies upon biology and biochemistry (antibody) but also upon organic chemistry (linker and payload). In the latter, total synthesis of natural and designed cytotoxic compounds, together with the development of novel synthetic strategies, have been key aspects of the consecution of clinical ADCs. In the case of payloads from marine origin, impressive structural architectures and biological properties are observed, thus making them prime targets for chemical synthesis and the development of ADCs. In this review, we explore the molecular and biological diversity of ADCs, with particular emphasis on those containing marine cytotoxic drugs as the payload.

Discovery and Development of Dolastatin 10-Derived Antibody Drug Conjugate Anticancer Drugs

Dolastatin 10 is an extremely potent broad-spectrum antitubulin anticancer pentapeptide isolated from Dolabella auricularia. The two-dimensional structure was elucidated by NMR and mass spectrometric analyses. The absolute configuration was determined by a convergent total synthesis. SAR studies established that modifications at C- and N-terminals were tolerated for cytotoxic activity. Human clinical trials of dolastatin 10 and auristatin PE (a C-terminal analog) showed occasional signs of efficacy but failed due to lack of separation of toxicity and efficacy. Nanomolar cytotoxicity helped transition this class of pentapeptides to the next phase of development as antibody drug conjugates (ADCs) by reducing systemic toxicity. Four ADC drugs (Adcetris, Padcev, Polivy, and Blenrep) carrying monomethyl auristatin E (MMAE, vedotin) and monomethyl auristatin F (MMAF, mafodotin) payloads have been approved for treatment of a number of cancers expressing antibody-specific antigens. More than 36 ADCs carrying a variety of pentapeptide analogues are undergoing preclinical and clinical developments. They are being evaluated in more than 200 human trials. A comprehensive review of the discovery, total synthesis of dolastatin 10 and new amino acids, SAR studies of dolastatin 10 and auristatins, conjugations to antibodies, and preclinical and clinical development of ADCs have been presented.

PI3K/AKT Signaling Tips the Balance of Cytoskeletal Forces for Cancer Progression

The PI3K/AKT signaling pathway plays essential roles in multiple cellular processes, which include cell growth, survival, metabolism, and motility. In response to internal and external stimuli, the PI3K/AKT signaling pathway co-opts other signaling pathways, cellular components, and cytoskeletal proteins to reshape individual cells. The cytoskeletal network comprises three main components, which are namely the microfilaments, microtubules, and intermediate filaments. Collectively, they are essential for many fundamental structures and cellular processes. In cancer, aberrant activation of the PI3K/AKT signaling cascade and alteration of cytoskeletal structures have been observed to be highly prevalent, and eventually contribute to many cancer hallmarks. Due to their critical roles in tumor progression, pharmacological agents targeting PI3K/AKT, along with cytoskeletal components, have been developed for better intervention strategies against cancer. In our review, we first discuss existing evidence in-depth and then build on recent advances to propose new directions for therapeutic intervention.

Antibody drug conjugates in gastrointestinal cancer: From lab to clinical development

The antibody-drug conjugates (ADCs) are one the fastest growing biotherapeutics in oncology and are still in their infancy in gastrointestinal (GI) cancer for clinical applications to improve patient survival. The ADC based approach is developed with tumor specific antigen, antibody carrying cytotoxic agents to precisely target and deliver chemotherapeutics at the tumor site. To date, 11 ADCs have been approved by US-FDA, and more than 80 are in the clinical development phase for different oncological indications. However, The ADCs based therapies in GI cancers are still far from having high-efficient clinical outcomes. The limited success of these ADCs and lessons learned from the past are now being used to develop a newer generation of ADC against GI cancers. In this review, we did a comprehensive assessment of the key components of ADCs, including tumor marker, antibody, cytotoxic payload, and linkage strategy, with a focus on technical improvement and some future trends in the pipeline for clinical translation. The various preclinical and clinical ADCs used in gastrointestinal malignancies, their target, composition and bioconjugation, along with preclinical and clinical outcomes, are discussed. The emphasis is also given to new generation ADCs employing novel mAb, payload, linker, and bioconjugation methods are also included.

Marine peptides in breast cancer: Therapeutic and mechanistic understanding

Breast cancer is the most prevalent invasive form of cancer in females and posing a great challenge for overcoming disease burden. The growth in global cancer deaths mandates the discovery of new efficacious natural anti-tumor treatments. In this regard, aquatic species offer a rich supply of possible drugs. Studies have shown that several marine peptides damage cancer cells by a broad range of pathways, including apoptosis, microtubule balance disturbances, and suppression of angiogenesis. Traditional chemotherapeutic agents are characterized by a plethora of side effects, including immune response suppression. The discovery of novel putative anti-cancer peptides with lesser toxicity is therefore necessary and timely, especially those able to thwart multi drug resistance (MDR). This review addresses marine anti-cancer peptides for the treatment of breast cancer.

Marine Antitumor Peptide Dolastatin 10: Biological Activity, Structural Modification and Synthetic Chemistry

Dolastatin 10 (Dol-10), a leading marine pentapeptide isolated from the Indian Ocean mollusk Dolabella auricularia, contains three unique amino acid residues. Dol-10 can effectively induce apoptosis of lung cancer cells and other tumor cells at nanomolar concentration, and it has been developed into commercial drugs for treating some specific lymphomas, so it has received wide attention in recent years. In vitro experiments showed that Dol-10 and its derivatives were highly lethal to common tumor cells, such as L1210 leukemia cells (IC50 = 0.03 nM), small cell lung cancer NCI-H69 cells (IC50 = 0.059 nM), and human prostate cancer DU-145 cells (IC50 = 0.5 nM), etc. With the rise of antibody-drug conjugates (ADCs), milestone progress was made in clinical research based on Dol-10. A variety of ADCs constructed by combining MMAE or MMAF (Dol-10 derivatives) with a specific antibody not only ensured the antitumor activity of the drugs themself but also improved their tumor targeting and reduced the systemic toxicity. They are currently undergoing clinical trials or have been approved for marketing, such as Adcetris®, which had been approved for the treatment of anaplastic large T-cell systemic malignant lymphoma and Hodgkin lymphoma. Dol-10, as one of the most medically valuable natural compounds discovered up to now, has brought unprecedented hope for tumor treatment. It is particularly noteworthy that, by modifying the chemical structure of Dol-10 and combining with the application of ADCs technology, Dol-10 as a new drug candidate still has great potential for development. In this review, the biological activity and chemical work of Dol-10 in the advance of antitumor drugs in the last 35 years will be summarized, which will provide the support for pharmaceutical researchers interested in leading exploration of antitumor marine peptides.

Anti-tumour drugs of marine origin currently at various stages of clinical trials (review)

Oncological diseases for a long time have remained one of the most significant health problems of modern society, which causes great losses in its labour and vital potential. Contemporary oncology still faces unsolved issues as insufficient efficacy of treatment of progressing and metastatic cancer, chemoresistance, and side-effects of the traditional therapy which lead to disabilities among or death of a high number of patients. Development of new anti-tumour preparations with a broad range of pharmaceutical properties and low toxicity is becoming increasingly relevant every year. The objective of the study was to provide a review of the recent data about anti-tumour preparations of marine origin currently being at various phases of clinical trials in order to present the biological value of marine organisms – producers of cytotoxic compounds, and the perspectives of their use in modern biomedical technologies. Unlike the synthetic oncological preparations, natural compounds are safer, have broader range of cytotoxic activity, can inhibit the processes of tumour development and metastasis, and at the same time have effects on several etiopathogenic links of carcinogenesis. Currently, practical oncology uses 12 anti-tumour preparations of marine origin (Fludarabine, Cytarabine, Midostaurin, Nelarabine, Eribulin mesylate, Brentuximab vedotin, Trabectedin, Plitidepsin, Enfortumab vedotin, Polatuzumab vedotin, Belantamab mafodotin, Lurbinectedin), 27 substances are at different stages of clinical trials. Contemporary approaches to the treatment of oncological diseases are based on targeted methods such as immune and genetic therapies, antibody-drug conjugates, nanoparticles of biopolymers, and metals. All those methods employ bioactive compounds of marine origin. Numerous literature data from recent years indicate heightened attention to the marine pharmacology and the high potential of marine organisms for the biomedicinal and pharmaceutic industries.

Use of anticancer peptides as an alternative approach for targeted therapy in breast cancer: a review

Breast cancer is the most common cancer in women worldwide. Traditional therapies are expensive and cause severe side effects. Targeted therapy is a powerful method to circumvent the problems of other therapies. It also allows drugs to localize at predefined targets in a selective manner. Currently, there are several monoclonal antibodies which target breast cancer cell surface markers. However, using antibodies has some limitations. In the last two decades, many investigators have discovered peptides that may be useful to target breast cancer cells. In this article, we provide an overview on anti-breast cancer peptides, their sources and biological activities. We further discuss the pros and cons of using anticancer peptides with further emphasis on how to improve their effectiveness in cancer therapy.

Antibody-drug conjugates in metastatic triple negative breast cancer: a spotlight on sacituzumab govitecan, ladiratuzumab vedotin, and trastuzumab deruxtecan

INTRODUCTION

Metastatic triple-negative breast cancers (mTNBC) are characterized by aggressive behavior and worse clinical outcomes than other breast cancer subtypes, as well as poor response to cytotoxic chemotherapies. The use of antibody-drug conjugates (ADCs) has been investigated as a potential treatment strategy, particularly in heavily pretreated disease.

AREAS COVERED

This article reviews the preclinical and clinical data supporting the use of the ADCs sacituzumab govitecan (SG), ladiratuzumab vedotin (LV), and trastuzumab deruxtecan (T-DXd) in mTNBC, and highlights ongoing clinical trials and future clinical applications.

EXPERT OPINION

SG, LV, and T-DXd have demonstrated their potential to meaningfully improve clinical outcomes in patients with pretreated mTNBC, as demonstrated by notable response rates in phase I/II and, for SG, phase III clinical trials. Investigation of their use in combination with other agents, including PARP inhibitors and checkpoint inhibitors, is ongoing in the metastatic setting, and their application in early-stage TNBCs are under investigation. ADCs are therefore expected to redefine treatment paradigms in TNBC.

Rational Design of Azastatin as a Potential ADC Payload with Reduced Bystander Killing

Auristatins are a class of ultrapotent microtubule inhibitors, whose growing clinical popularity in oncology is based upon their use as payloads in antibody-drug conjugates (ADCs). The most widely utilized auristatin, MMAE, has however been shown to cause apoptosis in non-pathological cells proximal to the tumour ("bystander killing"). Herein, we introduce azastatins, a new class of auristatin derivatives encompassing a side chain amine for antibody conjugation. The synthesis of Cbz-azastatin methyl ester, which included the C2-elongation and diastereoselective reduction of two proteinogenic amino acids as key transformations, was accomplished in 22 steps and 0.76 % overall yield. While Cbz-protected azastatin methyl ester (0.13-3.0 nM) inhibited proliferation more potently than MMAE (0.47-6.5 nM), removal of the Cbz-group yielded dramatically increased IC50 -values (9.8-170 nM). We attribute the reduced apparent cytotoxicity of the deprotected azastatin methyl esters to a lack of membrane permeability. These results clearly establish the azastatins as a novel class of cytotoxic payloads ideally suited for use in next-generation ADC development.

Thomas O, Barraja P, Gaudêncio SP, Bertoni F. Marine Anticancer Agents: An Overview with a Particular Focus on Their Chemical Classes

The marine environment is a rich source of biologically active molecules for the treatment of human diseases, especially cancer. The adaptation to unique environmental conditions led marine organisms to evolve different pathways than their terrestrial counterparts, thus producing unique chemicals with a broad diversity and complexity. So far, more than 36,000 compounds have been isolated from marine micro- and macro-organisms including but not limited to fungi, bacteria, microalgae, macroalgae, sponges, corals, mollusks and tunicates, with hundreds of new marine natural products (MNPs) being discovered every year. Marine-based pharmaceuticals have started to impact modern pharmacology and different anti-cancer drugs derived from marine compounds have been approved for clinical use, such as: cytarabine, vidarabine, nelarabine (prodrug of ara-G), fludarabine phosphate (pro-drug of ara-A), trabectedin, eribulin mesylate, brentuximab vedotin, polatuzumab vedotin, enfortumab vedotin, belantamab mafodotin, plitidepsin, and lurbinectedin. This review focuses on the bioactive molecules derived from the marine environment with anticancer activity, discussing their families, origin, structural features and therapeutic use.

Mitotic Poisons in Research and Medicine

Cancer is one of the greatest challenges of the modern medicine. Although much effort has been made in the development of novel cancer therapeutics, it still remains one of the most common causes of human death in the world, mainly in low and middle-income countries. According to the World Health Organization (WHO), cancer treatment services are not available in more then 70% of low-income countries (90% of high-income countries have them available), and also approximately 70% of cancer deaths are reported in low-income countries. Various approaches on how to combat cancer diseases have since been described, targeting cell division being among them. The so-called mitotic poisons are one of the cornerstones in cancer therapies. The idea that cancer cells usually divide almost uncontrolled and far more rapidly than normal cells have led us to think about such compounds that would take advantage of this difference and target the division of such cells. Many groups of such compounds with different modes of action have been reported so far. In this review article, the main approaches on how to target cancer cell mitosis are described, involving microtubule inhibition, targeting aurora and polo-like kinases and kinesins inhibition. The main representatives of all groups of compounds are discussed and attention has also been paid to the presence and future of the clinical use of these compounds as well as their novel derivatives, reviewing the finished and ongoing clinical trials.

Natural Bioactive Thiazole-Based Peptides from Marine Resources: Structural and Pharmacological Aspects

Peptides are distinctive biomacromolecules that demonstrate potential cytotoxicity and diversified bioactivities against a variety of microorganisms including bacteria, mycobacteria, and fungi via their unique mechanisms of action. Among broad-ranging pharmacologically active peptides, natural marine-originated thiazole-based oligopeptides possess peculiar structural features along with a wide spectrum of exceptional and potent bioproperties. Because of their complex nature and size divergence, thiazole-based peptides (TBPs) bestow a pivotal chemical platform in drug discovery processes to generate competent scaffolds for regulating allosteric binding sites and peptide–peptide interactions. The present study dissertates on the natural reservoirs and exclusive structural components of marine-originated TBPs, with a special focus on their most pertinent pharmacological profiles, which may impart vital resources for the development of novel peptide-based therapeutic agents.

Antibody-Drug Conjugate Payloads; Study of Auristatin Derivatives

Auristatins are important payloads used in antibody drug conjugates (ADCs), and the most well-known compound family member, monomethyl auristatin (MMAE), is used in two Food and Drug Administration (FDA)-approved ADCs, Adcetris^® and Polivy^®. Multiple other auristatin-based ADCs are currently being evaluated in human clinical trials and further studies on this class of molecule are underway by several academic and industrial research groups. Our group's main focus is to investigate the structure-activity relationships (SAR) of novel auristatins with the goal of applying these to next generation ADCs. Modifications of the auristatin backbone scaffold have been widely reported in the chemical literature focusing on the terminal subunits: P1 (N-terminus) and P5 (C-terminus). Our approach was to modulate the activity and hydrophilic character through modifications of the central subunits P2-P3-P4 and thorough SAR study on the P5 subunit. Novel hydrophilic auristatins were observed to have greater potency in vitro and displayed enhanced in vivo antitumor activity when conjugated via protease-cleavable linkers and delivered intracellularly. Analysis of ADC aggregation also indicated that novel hydrophilic payloads enabled the synthesis of high-drug-to-antibody ratio (DAR) ADCs that were resistant to aggregation. Modification of the central peptide subunits also resulted in auristatins with potent cytotoxic activity in vitro and these azide-modified auristatins contain a handle for linker attachment from the central portion of the auristatin backbone.

From Seabed to Bedside: A Review on Promising Marine Anticancer Compounds

The marine environment represents an outstanding source of antitumoral compounds and, at the same time, remains highly unexplored. Organisms living in the sea synthesize a wide variety of chemicals used as defense mechanisms. Interestingly, a large number of these compounds exert excellent antitumoral properties and have been developed as promising anticancer drugs that have later been approved or are currently under validation in clinical trials. However, due to the high need for these compounds, new methodologies ensuring its sustainable supply are required. Also, optimization of marine bioactives is an important step for their success in the clinical setting. Such optimization involves chemical modifications to improve their half-life in circulation, potency and tumor selectivity. In this review, we outline the most promising marine bioactives that have been investigated in cancer models and/or tested in patients as anticancer agents. Moreover, we describe the current state of development of anticancer marine compounds and discuss their therapeutic limitations as well as different strategies used to overcome these limitations. The search for new marine antitumoral agents together with novel identification and chemical engineering approaches open the door for novel, more specific and efficient therapeutic agents for cancer treatment.

Enriching cancer pharmacology with drugs of marine origin

Marine natural products have proven, over the last half-century, to be effective biological modulators. These molecules have revealed new targets for cancer therapy as well as dissimilar modes of action within typical classes of drugs. In this scenario, innovation from marine-based pharmaceuticals has helped advance cancer chemotherapy in many aspects, as most of these are designated as first-in-class drugs. Here, by examining the path from discovery to development of clinically approved drugs of marine origin for cancer treatment-cytarabine (Cytosar-U®), trabectedin (Yondelis®), eribulin (Halaven®), brentuximab vedotin (Adcetris®), and plitidepsin (Aplidin®)- together with those in late clinical trial phases-lurbinectedin, plinabulin, marizomib, and plocabulin-the present review offers a critical analysis of the contributions given by these new compounds to cancer pharmacotherapy.

The "Utility" of Highly Toxic Marine-Sourced Compounds

Currently a few compounds isolated from marine sources have become drugs, mainly directed towards cancer and pain. Compounds from marine sources have exquisite potencies against eukaryotic cells, as they act as protective agents against attack by predators in the marine environment. Their toxicities act as a “double-edged sword” as they are often too toxic for direct use in humans and thus have to be chemically modified. By linking suitably modified compounds to monoclonal antibodies directed against specific epitopes in mammalian cancer cells, they can be delivered to a specific cell type in humans. This review updates and extends an article published in early 2017, demonstrating how by careful chemical modifications, highly toxic compounds, frequently peptidic in nature, can be utilized as antitumor drug candidates. The antibody-drug- conjugates (ADCs) discussed are those that are currently in clinical trials listed in the NIH Clinical Trials Registry as, “currently active, recruiting or in some cases, recently completed”. There are also some ADCs discussed that are at the advanced preclinical stage, that in some cases, are repurposing current drug entities, and the review finishes with a short discussion of the aplyronines as potential candidate warheads as a result of scalable synthetic processes.

Anti-tubulin agents of natural origin: Targeting taxol, vinca, and colchicine binding domains

Microtubules are a protein which is made of α- and β-heterodimer. It is one of the main components of the cell which play a vital role in cell division especially in G2/M-phase. It exists in equilibrium dynamic of polymerization and depolymerization of α- and β-heterodimer. It is one of the best targets for developing anti-cancer drugs. Various natural occurring molecules are well known for their anti-tubulin effect such as vinca, paclitaxel, combretastatin, colchicine etc. These microtubule-targeted drugs are acted through two processes (i) inhibiting depolymerization of tubulin (tubulin stabilizing agents) and (ii) inhibiting polymerization of tubulin (tubulin destabilizing agents). Now days, various binding domains have been explore through which these molecules are binding to tubulin but the three major binding domain of tubulin are taxol, vinca and colchicine binding domain. The present article mainly focus on the classification of various naturally occurring compounds on the basis of their inhibition processes (depolymerization and polymerization) and the site of interaction (targets taxol, vinca and colchicine binding domain) which has been hitherto reported. By placing all the naturally occurring taxol, vinca and colchicine binding site analogues at one place makes a better understanding of the tubulin interactions with known natural tubulin binders that would helps in the discovery of new and potent natural, semi-synthetic and synthetic analogues for treating cancer.

New naphthopyran analogues of LY290181 as potential tumor vascular-disrupting agents

A series of 19 analogues of the antiproliferative naphthopyran LY290181 were prepared for structure-activity relationship studies. We found the best activities for test compounds bearing small substituents at the meta position of the phenyl ring. The mode of action of LY290181 and eight new analogues was studied in detail. The compounds were highly anti-proliferative with IC₅₀ values in the sub-nanomolar to triple-digit nanomolar range. The new analogues led to G2/M arrest due to interruption of the microtubule dynamics. In 518A2 melanoma cells they caused a mitotic catastrophe which eventually led to apoptosis. The naphthopyrans also induced a disruption of the vasculature in the chorioallantoic membrane (CAM) of fertilized chicken eggs as well as in xenograft tumors in mice. In a preliminary therapy trial, the difluoro derivative 2b retarded the growth of resistant xenograft tumors in mice.

Marine drugs for cancer: surfacing biotechnological innovations from the oceans

This review will discuss the contributions of marine natural molecules, a source only recently found to have pharmaceutical prospects, to the development of anticancer drugs. Of the seven clinically utilized compounds with a marine origin, four are used for the treatment of cancer. The development of these drugs has afforded valuable knowledge and crucial insights to meet the most common challenges in this endeavor, such as toxicity and supply. In this context, the development of these compounds will be discussed herein to illustrate, with successful examples provided by cytarabine, trabectedin, eribulin and brentuximab vedotin, the steps involved in this process as well as the scientific advances and technological innovation potential associated with developing a new drug from marine resources.

Synthesis and Evaluation of Linear and Macrocyclic Dolastatin 10 Analogues Containing Pyrrolidine Ring Modifications

Because of their potent cytotoxic activity, members of the auristatin family (synthetic analogues of the naturally occurring dolastatin 10) have remained a target of significant research, most notably in the context of antibody drug conjugate payloads. Typically, modifications of the backbone scaffold of dolastatin 10 have focused on variations of the N-terminal (P1) and C-terminal (P5) subunits. Scant attention has been paid thus far to the P4 subunit in the scientific literature. In this paper, we introduce an azide functional group at the P4 subunit, resulting in potent cytotoxic activity seen in vitro. Another highly active compound in this study contained azide functional groups in both the P2 and P4 subunits and required dolavaline as the P1 subunit and a phenylalanine as the P5 subunit. Furthermore, these two azide groups served not only as modifiers of cytotoxicity but also as handles for linker attachment or as a tether for use in the synthesis of a macrocyclic analogue.

Antitumour bioactive peptides isolated from marine organisms

Marine organisms are an important source of antitumour active substances. Thus, pharmaceutical research in recent years has focused on exploring new antitumour drugs derived from marine organisms, and, many peptide drugs with strong antitumour activities have been successfully extracted. Based on different mechanisms, this paper reviews the research on several typical antitumour bioactive peptides in marine drugs and the latest progress therein. Additionally, the development prospects for these antitumour bioactive peptide-based drugs are discussed so as to provide a reference for future research in this field.

A New Protein Extract Inhibitor from Hypobranchial Purple Gland of Hexaplex trunculus, a Mediterranean Mollusk, Impairs the Motility of Human Glioblastoma U87 and the HeLa Cell Line of Cervical Carcinoma Cells

The aim of this study is to evaluate the effect of hypobranchial gland protein extracts (HGPEs) of Hexaplex trunculus on the viability, cell adhesion, and migration of human U87 glioblastoma cells and the HeLa cell line obtained from epithelial cervical carcinoma cells. Analysis of the HGPE on polyacrylamide gel (12%) shows a variety of proteins whose molecular weights vary between 12 and 1OO kDa. Chromatographic analysis shows 16 peaks obtained at various retention times. Cytotoxic effect was observed after 24 hours of incubation at the concentrations 20, 40, and 60 μg/ml in a dose-dependent manner. Concentrations giving 50% inhibition (IC₅₀) are 22 μg/ml for U87 and 15 μg/ml for HeLa cells. Our results show inhibition of U87 and HeLa cancer cell adhesion at concentrations of 10 and 20 µg/ml, respectively. High-pressure liquid chromatography fractions did not show antiadhesive effect on both cancer cell lines. The presence of HGPEs completely blocked the migration of the two cancer cell lines at 10 µg/ml. This inhibition is dose-dependent. IC₅₀ is about 2.5 μg/ml for both cancer cells. The HGPE of Hexaplex trunculus may have the potential to serve as a model for future anticancer drug development with probably a synergistic activity of the proteins of this extract.

Synthesis and Evaluation of Dolastatin 10 Analogues Containing Heteroatoms on the Amino Acid Side Chains

Synthetic analogues of the natural occurring dolastatin 10 are of great interest in cancer due to their potent in vitro activity and their uses as payloads in antibody drug conjugates (ADCs). Modification of the dolastatin 10 core scaffold has mainly focused on modifications of the P1, N-terminus, and P5, C-terminus, with minimal attention to the P2 subunit. In this paper we discuss the introduction of heteroatoms to the P2 side chain, which results in potent activity in vitro. The most active compounds contained azides in the P2 unit and required a phenylalanine-derived P5 subunit.

Antibody-Drug Conjugates (ADCs) for Personalized Treatment of Solid Tumors: A Review

Attaching a cytotoxic "payload" to an antibody to form an antibody-drug conjugate (ADC) provides a mechanism for selective delivery of the cytotoxic agent to cancer cells via the specific binding of the antibody to cancer-selective cell surface molecules. The first ADC to receive marketing authorization was gemtuzumab ozogamicin, which comprises an anti-CD33 antibody conjugated to a highly potent DNA-targeting antibiotic, calicheamicin, approved in 2000 for treating acute myeloid leukemia. It was withdrawn from the US market in 2010 following an unsuccessful confirmatory trial. The development of two classes of highly potent microtubule-disrupting agents, maytansinoids and auristatins, as payloads for ADCs resulted in approval of brentuximab vedotin in 2011 for treating Hodgkin lymphoma and anaplastic large cell lymphoma, and approval of ado-trastuzumab emtansine in 2013 for treating HER2-positive breast cancer. Their success stimulated much research into the ADC approach, with >60 ADCs currently in clinical evaluation, mostly targeting solid tumors. Five ADCs have advanced into pivotal clinical trials for treating various solid tumors-platinum-resistant ovarian cancer, mesothelioma, triple-negative breast cancer, glioblastoma, and small cell lung cancer. The level of target expression is a key parameter in predicting the likelihood of patient benefit for all these ADCs, as well as for the approved compound, ado-trastuzumab emtansine. The development of a patient selection strategy linked to target expression on the tumor is thus critically important for identifying the population appropriate for receiving treatment.

Merocyclophanes C and D from the Cultured Freshwater Cyanobacterium Nostoc sp. (UIC 10110)

Merocyclophanes C and D (1 and 2) were isolated from the cell extract of the cultured cyanobacterium UIC 10110. The structures were determined by one-dimensional nuclear magnetic resonance (NMR) and high-resolution electrospray ionization mass spectrometry and confirmed by 2D NMR techniques. The absolute configurations were determined using electronic circular dichroism spectroscopy. Merocyclophanes C and D represent the first known analogues of the merocyclophane core structure, a recently discovered scaffold of [7,7] paracyclophanes characterized by an α-branched methyl at C-1/C-14; 1 and 2 showed antiproliferative activity against the MDA-MB-435 cell line with IC50 values of 1.6 and 0.9 μM, respectively. Partial 16S analysis determined UIC 10110 to be a Nostoc sp., and it was found to clade with UIC 10062 Nostoc sp., the only other strain known to produce merocyclophanes. The genome of UIC 10110 was sequenced, and a biosynthetic gene cluster was identified that is proposed to encode type I and type III polyketide synthases that are potentially responsible for production of the merocyclophanes; however, further experiments will be required to verify the true function of the gene cluster. The gene cluster provides a genetic basis for the observed structural differences of the [7,7] paracyclophane core structures.

Peptides, Peptidomimetics, and Polypeptides from Marine Sources: A Wealth of Natural Sources for Pharmaceutical Applications

Nature provides a variety of peptides that are expressed in most living species. Evolutionary pressure and natural selection have created and optimized these peptides to bind to receptors with high affinity. Hence, natural resources provide an abundant chemical space to be explored in peptide-based drug discovery. Marine peptides can be extracted by simple solvent extraction techniques. The advancement of analytical techniques has made it possible to obtain pure peptides from natural resources. Extracted peptides have been evaluated as possible therapeutic agents for a wide range of diseases, including antibacterial, antifungal, antidiabetic and anticancer activity as well as cardiovascular and neurotoxin activity. Although marine resources provide thousands of possible peptides, only a few peptides derived from marine sources have reached the pharmaceutical market. This review focuses on some of the peptides derived from marine sources in the past ten years and gives a brief review of those that are currently in clinical trials or on the market.

Antibacterial, antifungal and antimycobacterial compounds from cyanobacteria

Infections from multidrug resistant (MDR) pathogenic bacteria, fungi and Mycobacterium tuberculosis remain progressively intractable. The search of effective antimicrobials from other possible non-conventional sources against MDR pathogenic bacteria, fungi and mycobacteria is call of the day. This review considers 121 cyanobacterial compounds or cyano-compounds with antimicrobial activities. Chemical structures of cyano-compounds were retrieved from ChemSpider and PubChem databases and were visualized by the software ChemDraw Ultra. Chemical information on cyano-compounds pertaining to Lipinski rules of five was assessed. The reviewed cyano-compounds belong to the following chemical classes (with examples): alkaloids (ambiguine isonitriles and 12-epi-hapalindole E isonitrile), aromatic compounds (benzoic acid and cyanobacterin), cyclic depsipeptides (cryptophycin 52 and lyngbyabellin A), cyclic peptides (calophycin and tenuecyclamides), cyclic undecapeptides (kawaguchipeptins and lyngbyazothrin A), cyclophane (carbamidocyclophane), extracellular pigment (nostocine A), fatty acids (alpha-dimorphecolic acid and majusculonic acid), linear peptides (muscoride A), lipopeptides (fischerellins and scytonemin A), nucleosides (tolytoxin and tubercidin), phenols (ambigols and 4-4'-hydroxybiphenyl), macrolides (scytophycin A and tolytoxin), polyketides (malyngolide and nostocyclyne), polyphenyl ethers (crossbyanol A), porphinoids (tolyporphin J) and terpenoids (noscomin and scytoscalarol). Cyanobacteria appear to be a diverse source of compounds with antimicrobial activity. Further attention is required to elucidate whether those could be applied as pharmaceuticals.

Current Status of Marine-Derived Compounds as Warheads in Anti-Tumor Drug Candidates

In this review, we have attempted to describe all of the antibody–drug conjugates using a marine-derived compound as the “warhead”, that are currently in clinical trials as listed in the current version of the NIH clinical trials database (clinicaltrials.gov). In searching this database, we used the beta-test version currently available, as it permitted more specific search parameters, since the regular version did not always find trials that had been completed in the past with some agents. We also added small discussion sections on candidates that are still at the preclinical stage, including a derivative of diazonamide that has an unusual interaction with tubulin (DZ-23840), which may also be a potential warhead in the future.

Synthesis and Structure Revision of Symplocin A

Symplocin A, a linear peptide possessing N-terminal N,N-dimethylisoleucine, statine, and valic acid residues, has been synthesized for the first time employing our previously established 'one-pot intramolecular tandem protocol'. Moreover, the stereochemistry of natural symplocin A was unambiguously revised through the confirmation by 1D NMR, 2D NMR, and HPLC comparisons with authentic natural product.

Microtubule destabilising agents: far more than just antimitotic anticancer drugs

Vinca alkaloids have been approved as anticancer drugs for more than 50 years. They have been classified as cytotoxic chemotherapy drugs that act during cellular mitosis, enabling them to target fast growing cancer cells. With the evolution of cancer drug development there has been a shift towards new "targeted" therapies to avoid the side effects and general toxicities of "cytotoxic chemotherapies" such as the vinca alkaloids. Due to their original classification, many have overlooked the fact that vinca alkaloids, taxanes and related drugs do have a specific molecular target: tubulin. They continue to be some of the most effective anticancer drugs, perhaps because their actions upon the microtubule network extend far beyond the ability to halt cells in mitosis, and include the induction of apoptosis at all phases of the cell cycle. In this review, we highlight the numerous cellular consequences of disrupting microtubule dynamics, expanding the textbook knowledge of microtubule destabilising agents and providing novel opportunities for their use in cancer therapy.

Cyanobacterial Metabolite Calothrixins: Recent Advances in Synthesis and Biological Evaluation

The marine environment is host to unparalleled biological and chemical diversity, making it an attractive resource for the discovery of new therapeutics for a plethora of diseases. Compounds that are extracted from cyanobacteria are of special interest due to their unique structural scaffolds and capacity to produce potent pharmaceutical and biotechnological traits. Calothrixins A and B are two cyanobacterial metabolites with a structural assembly of quinoline, quinone, and indole pharmacophores. This review surveys recent advances in the synthesis and evaluation of the biological activities of calothrixins. Due to the low isolation yields from the marine source and the promise this scaffold holds for anticancer and antimicrobial drugs, organic and medicinal chemists around the world have embarked on developing efficient synthetic routes to produce calothrixins. Since the first review appeared in 2009, 11 novel syntheses of calothrixins have been published in the efforts to develop methods that contain fewer steps and higher-yielding reactions. Calothrixins have shown their potential as topoisomerase I poisons for their cytotoxicity in cancer. They have also been observed to target various aspects of RNA synthesis in bacteria. Further investigation into the exact mechanism for their bioactivity is still required for many of its analogs.

A Humanized Anti-CD22-Onconase Antibody-Drug Conjugate Mediates Highly Potent Destruction of Targeted Tumor Cells

Antibody-drug conjugates (ADCs) have evolved as a new class of potent cancer therapeutics. We here report on the development of ADCs with specificity for the B-cell lineage specific (surface) antigen CD22 being expressed in the majority of hematological malignancies. As targeting moiety a previously generated humanized anti-CD22 single-chain variable fragment (scFv) derivative from the monoclonal antibody RFB4 was reengineered into a humanized IgG1 antibody format (huRFB4). Onconase (ranpirnase), a clinically active pancreatic-type ribonuclease, was employed as cytotoxic payload moiety. Chemical conjugation via thiol-cleavable disulfide linkage retained full enzymatic activity and full binding affinity of the ADC. Development of sophisticated purification procedures using size exclusion and ion exchange chromatography allowed the separation of immunoconjugate species with stoichiometrically defined number of Onconase cargos. A minimum of two Onconase molecules per IgG was required for achieving significant in vitro cytotoxicity towards lymphoma and leukemia cell lines. Antibody-drug conjugates with an Onconase to antibody ratio of 3 : 1 exhibited an IC₅₀ of 0.08 nM, corresponding to more than 18,400-fold increased cytotoxicity of the ADC when compared with unconjugated Onconase. These results justify further development of this ADC as a promising first-in-class compound for the treatment of CD22-positive malignancies.

Stabilizing versus destabilizing the microtubules: a double-edge sword for an effective cancer treatment option?

Microtubules are dynamic and structural cellular components involved in several cell functions, including cell shape, motility, and intracellular trafficking. In proliferating cells, they are essential components in the division process through the formation of the mitotic spindle. As a result of these functions, tubulin and microtubules are targets for anticancer agents. Microtubule-targeting agents can be divided into two groups: microtubule-stabilizing, and microtubule-destabilizing agents. The former bind to the tubulin polymer and stabilize microtubules, while the latter bind to the tubulin dimers and destabilize microtubules. Alteration of tubulin-microtubule equilibrium determines the disruption of the mitotic spindle, halting the cell cycle at the metaphase-anaphase transition and, eventually, resulting in cell death. Clinical application of earlier microtubule inhibitors, however, unfortunately showed several limits, such as neurological and bone marrow toxicity and the emergence of drug-resistant tumor cells. Here we review several natural and synthetic microtubule-targeting agents, which showed antitumor activity and increased efficacy in comparison to traditional drugs in various preclinical and clinical studies. Cryptophycins, combretastatins, ombrabulin, soblidotin, D-24851, epothilones and discodermolide were used in clinical trials. Some of them showed antiangiogenic and antivascular activity and others showed the ability to overcome multidrug resistance, supporting their possible use in chemotherapy.

Management of relapsed or refractory hodgkin lymphoma with second-generation antibody-drug conjugates: focus on brentuximab vedotin

Brentuximab vedotin (Adcetris, Seattle Genetics) is an antibody-drug conjugate (ADC) that joins an anti-CD 30 monoclonal antibody with the anti-tubulin agent monomethyl auristatin E via a dipeptide linker. It has demonstrated significant activity in CD 30-positive lymphomas and is currently approved by the US FDA for treatment of Hodgkin lymphoma that has relapsed following autologous stem-cell transplantation, or after two lines of chemotherapy in non-transplant candidates. Brentuximab vedotin has also been approved for the treatment of relapsed anaplastic large-cell lymphoma after front-line chemotherapy. We briefly review the biology of Hodgkin lymphoma, with a focus on the pathogenic role of CD 30 as well as the development of CD 30-targeted therapy. We also discuss both the current role of brentuximab vedotin in the management of relapsed and refractory Hodgkin lymphoma and the likely future developments for this agent.

Recent advances in the development of new auristatins: structural modifications and application in antibody drug conjugates

Dolastatin 10 is a powerful antineoplastic agent and microtubule inhibitor that was discovered by Pettit et al. and published in 1987. Since then, many research groups have engaged in SAR studies of synthetic analogues, termed "auristatins". It was eventually discovered that auristatins are of great value as payloads in antibody drug conjugates (ADCs), which led to the FDA-approved ADC brentuximab vedotin (Seattle Genetics). Currently, over 30 ADCs in clinical trials employ auristatins as payloads, and there is a great interest in the research community, both on academic and industrial sides, to further study these analogues. This review will provide an overview of the recent advancements in auristatin development spanning a time frame of about the past ten years. The main focus will be to describe structural changes made to the auristatin peptide and their resulting biological activities in tumor cell proliferation assays. Selected ADC examples will also be described.

Discovery of cytotoxic dolastatin 10 analogues with N-terminal modifications

Auristatins, synthetic analogues of the antineoplastic natural product Dolastatin 10, are ultrapotent cytotoxic microtubule inhibitors that are clinically used as payloads in antibody-drug conjugates (ADCs). The design and synthesis of several new auristatin analogues with N-terminal modifications that include amino acids with α,α-disubstituted carbon atoms are described, including the discovery of our lead auristatin, PF-06380101. This modification of the peptide structure is unprecedented and led to analogues with excellent potencies in tumor cell proliferation assays and differential ADME properties when compared to other synthetic auristatin analogues that are used in the preparation of ADCs. In addition, auristatin cocrystal structures with tubulin are being presented that allow for the detailed examination of their binding modes. A surprising finding is that all analyzed analogues have a cis-configuration at the Val-Dil amide bond in their functionally relevant tubulin bound state, whereas in solution this bond is exclusively in the trans-configuration. This remarkable observation shines light onto the preferred binding mode of auristatins and serves as a valuable tool for structure-based drug design.

Cyanobacteria from terrestrial and marine sources contain apoptogens able to overcome chemoresistance in acute myeloid leukemia cells

In this study, we investigated forty cyanobacterial isolates from biofilms, gastropods, brackish water and symbiotic lichen habitats. Their aqueous and organic extracts were used to screen for apoptosis-inducing activity against acute myeloid leukemia cells. A total of 28 extracts showed cytotoxicity against rat acute myeloid leukemia (IPC-81) cells. The design of the screen made it possible to eliminate known toxins, such as microcystins and nodularin, or known metabolites with anti-leukemic activity, such as adenosine and its analogs. A cytotoxicity test on human embryonic kidney (HEK293T) fibroblasts indicated that 21 of the 28 extracts containing anti-acute myeloid leukemia (AML) activity showed selectivity in favor of leukemia cells. Extracts L26-O and L30-O were able to partly overcome the chemotherapy resistance induced by the oncogenic protein Bcl-2, whereas extract L1-O overcame protection from the deletion of the tumor suppressor protein p53. In conclusion, cyanobacteria are a prolific resource for anti-leukemia compounds that have potential for pharmaceutical applications. Based on the variety of cellular responses, we also conclude that the different anti-leukemic compounds in the cyanobacterial extracts target different elements of the death machinery of mammalian cells.

Microbial natural products: molecular blueprints for antitumor drugs

Microbes from two of the three domains of life, the Prokarya, and Eukarya, continue to serve as rich sources of structurally complex chemical scaffolds that have proven to be essential for the development of anticancer therapeutics. This review describes only a handful of exemplary natural products and their derivatives as well as those that have served as elegant blueprints for the development of novel synthetic structures that are either currently in use or in clinical or preclinical trials together with some of their earlier analogs in some cases whose failure to proceed aided in the derivation of later compounds. In every case, a microbe has been either identified as the producer of secondary metabolites or speculated to be involved in the production via symbiotic associations. Finally, rapidly evolving next-generation sequencing technologies have led to the increasing availability of microbial genomes. Relevant examples of genome mining and genetic manipulation are discussed, demonstrating that we have only barely scratched the surface with regards to harnessing the potential of microbes as sources of new pharmaceutical leads/agents or biological probes.