Quest for Efficacious Next-Generation Taxoid Anticancer Agents and Their Tumor-Targeted Delivery

Effect of substituents at the C3´, C3´N, C10 and C2-meta-benzoate positions of taxane derivatives on their activity against resistant cancer cells

We tested the effect of substituents at the (1) C3´, C3´N, (2) C10, and (3) C2-meta-benzoate positions of taxane derivatives on their activity against sensitive versus counterpart paclitaxel-resistant breast (MCF-7) and ovarian (SK-OV-3) cancer cells. We found that (1) non-aromatic groups at both C3´ and C3´N positions, when compared with phenyl groups at the same positions of a taxane derivative, significantly reduced the resistance of ABCB1 expressing MCF-7/PacR and SK-OV-3/PacR cancer cells. This is, at least in the case of the SB-T-1216 series, accompanied by an ineffective decrease of intracellular levels in MCF-7/PacR cells. The low binding affinity of SB-T-1216 in the ABCB1 binding cavity can elucidate these effects. (2) Cyclopropanecarbonyl group at the C10 position, when compared with the H atom, seems to increase the potency and capability of the derivative in overcoming paclitaxel resistance in both models. (3) Derivatives with fluorine and methyl substituents at the C2-meta-benzoate position were variously potent against sensitive and resistant cancer cells. All C2 derivatives were less capable of overcoming acquired resistance to paclitaxel in vitro than non-substituted analogs. Notably, fluorine derivatives SB-T-121205 and 121,206 were more potent against sensitive and resistant SK-OV-3 cells, and derivatives SB-T-121405 and 121,406 were more potent against sensitive and resistant MCF-7 cells. (4) The various structure-activity relationships of SB-T derivatives observed in two cell line models known to express ABCB1 favor their complex interaction not based solely on ABCB1.

A general strategy for the synthesis of taxane diterpenes

The carbon skeleton of any organic molecule serves as the foundation for its three-dimensional structure, playing a pivotal role in determining its physical and biological properties1. As such, taxane diterpenes are one of the most well-known natural product families, primarily owing to the success of their most prominent compound, paclitaxel, an effective anticancer therapeutic for more than 25 years2-6. In contrast to classical taxanes, the bioactivity of cyclotaxanes (also referred to as complex taxanes) remains significantly underexplored. The carbon skeletons of these two groups of taxanes differ significantly, and so would typically their own distinct synthetic approaches. Here we report a versatile synthetic strategy based on the interconversion of complex molecular frameworks, providing general access to the wider taxane diterpene family. A range of classical and cyclotaxane frameworks was prepared including, among others, the total syntheses of taxinine K (2), canataxapropellane (5) and dipropellane C from a single advanced intermediate. The synthetic approach deliberately eschews biomimicry, emphasizing instead the power of stereoelectronic control in orchestrating the interconversion of polycyclic frameworks.

Growing Utilization of Radical Chemistry in the Synthesis of Pharmaceuticals

Our current unhealthy lifestyle and the exponential surge in the population getting affected by a variety of diseases have made pharmaceuticals or drugs an imperative part of life, making the development of innovative strategies for drug discovery or the introduction of refined, cost-effective and modern technologies for the synthesis of clinically used drugs, a need of the hour. Ever since their discovery, free radicals and radical cations or anions as reactive intermediates have captivated the chemists, resulting in an exceptional utilization of these moieties throughout the field of chemical synthesis, owing to their unprecedented and widespread reactivity. Sticking with the idea of not judging the book by its cover, despite the conventional thought process of radicals being unstable and difficult to control entities, scientists and academicians around the globe have done an appreciable amount of work utilizing both persistent as well as transient radicals for a variety of organic transformations, exemplifying them with the synthesis of significant biologically active pharmaceutical ingredients. This review truly accounts for the organic radical transformations including radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling with metal-complexes and radical cations coupling with nucleophiles, that offers fascinating and unconventional approaches towards the construction of intricate structural frameworks of marketed APIs with high atom- and step-economy; complementing the otherwise employed traditional methods. This tutorial review presents a comprehensive package of diverse methods utilized for radical generation, featuring their reactivity to form critical bonds in pharmaceutical total synthesis or in building key starting materials or intermediates of their synthetic journey, acknowledging their excellence, downsides and underlying mechanisms, which are otherwise poorly highlighted in the literature. Despite great achievements over the past few decades in this area, many challenges and obstacles are yet to be unraveled to shorten the distance between the academics and the industry, which are all discussed in summary and outlook.

Doxorubicin and other anthracyclines in cancers: Activity, chemoresistance and its overcoming

Anthracyclines have been important and effective treatments against a number of cancers since their discovery. However, their use in therapy has been complicated by severe side effects and toxicity that occur during or after treatment, including cardiotoxicity. The mode of action of anthracyclines is complex, with several mechanisms proposed. It is possible that their high toxicity is due to the large set of processes involved in anthracycline action. The development of resistance is a major barrier to successful treatment when using anthracyclines. This resistance is based on a series of mechanisms that have been studied and addressed in recent years. This work provides an overview of the anthracyclines used in cancer therapy. It discusses their mechanisms of activity, toxicity, and chemoresistance, as well as the approaches used to improve their activity, decrease their toxicity, and overcome resistance.

Advances of antitumor drug discovery in traditional Chinese medicine and natural active products by using multi-active components combination

The antitumor efficacy of Chinese herbal medicines has been widely recognized. Leading compounds such as sterols, glycosides, flavonoids, alkaloids, terpenoids, phenylpropanoids, and polyketides constitute their complex active components. The antitumor monomers derived from Chinese medicine possess an attractive anticancer activity. However, their use was limited by low bioavailability, significant toxicity, and side effects, hindering their clinical applications. Recently, new chemical entities have been designed and synthesized by combining natural drugs with other small drug molecules or active moieties to improve the antitumor activity and selectivity, and reduce side effects. Such a novel conjugated drug that can interact with several vital biological targets in cells may have a more significant or synergistic anticancer activity than a single-molecule drug. In addition, antitumor conjugates could be obtained by combining pharmacophores containing two or more known drugs or leading compounds. Based on these studies, the new drug research and development could be greatly shortened. This study reviews the research progress of conjugates with antitumor activity based on Chinese herbal medicine. It is expected to serve as a valuable reference to antitumor drug research and clinical application of traditional Chinese medicine.

Discovery and development of botanical natural products and their analogues as therapeutics for ovarian cancer

Covering: 2015 through the end of July 2022Ovarian cancer is one of the most common cancers affecting the female reproductive organs and has the highest mortality rate among gynecological cancers. Although botanical drugs and their derivatives, namely members of the taxane and camptothecin families, represent significant therapeutics currently available for the treatment of ovarian cancer, new drugs that have alternative mechanisms of action are still needed to combat the disease. For this reason, many efforts to identify additional novel compounds from botanical sources, along with the further development of existing therapeutics, have continued to appear in the literature. This review is designed to serve as a comprehensive look at both the currently available small-molecule therapeutic options and the recently reported botanically-derived natural products currently being studied and developed as potential future therapeutics that could one day be used against ovarian cancer. Specifically, key properties, structural features, and biological data are highlighted that are important for the successful development of potential agents. Recently reported examples are specifically discussed in the context of "drug discovery attributes," including the presence of structure-activity relationship, mechanism of action, toxicity, and pharmacokinetic studies, to help indicate the potential for future development and to highlight where these compounds currently exist in the development process. The lessons learned from both the successful development of the taxanes and camptothecins, as well as the strategies currently being employed for new drug development, are expected to ultimately help guide the future development of botanical natural products for ovarian cancer.

Multifunctional small-molecule theranostic agents for tumor-specific imaging and targeted chemotherapy

Cancer is one of the leading causes of death worldwide. Although great progress has been achieved in cancer diagnosis and treatment, novel therapies are still urgently needed to increase the efficacy and reduce the side effects of conventional therapies. Personalized medicine involves administering patients drugs that are specific to the characteristics of their tumors, and has significantly reduced side effects and increased overall survival rates. Multifunctional theranostic drugs are designed to combine diagnostic and therapeutic functions into a single molecule, which reduces the number of drugs administered to patients and increases patient compliance, and have shown great potential in propelling personalized medicine. This review focuses on multifunctional small-molecule theranostic agents for tumor-specific imaging and targeted chemotherapy, with a particular emphasis placed on highlighting design strategies and application in vitro or in vivo. The challenges and future perspectives of multifunctional small molecules are also discussed.

Biotinylated selenocyanates: Potent and selective cytostatic agents

Most of the currently available cytotoxic agents for tackling cancer are devoid of selectivity, thus causing severe side-effects. This situation stimulated us to develop new antiproliferative agents with enhanced affinity towards tumour cells. We focused our attention on novel chalcogen-containing compounds (thiosemicarbazones, disulfides, selenoureas, thio- and selenocyanates), and particularly on selenium derivatives, as it has been documented that this kind of compounds might act as prodrugs releasing selenium-based reactive species on tumour cells. Particularly interesting in terms of potency and selectivity was a pharmacophore comprised by a selenocyanato-alkyl fragment connected to a p-phenylenediamine residue, where the nature of the second amino moiety (free, Boc-protected, enamine-protected) provided a wide variety of antiproliferative activities, ranging from the low micromolar to the nanomolar values. The optimized structure was in turn conjugated through a peptide linkage with biotin (vitamin B₇), a cellular growth promoter, whose receptor is overexpressed in numerous cancer cells; the purpose was to develop a selective vector towards malignant cells. Such biotinylated derivative behaved as a very strong antiproliferative agent, achieving GI₅₀ values in the low nM range for most of the tested cancer cells; moreover, it was featured with an outstanding selectivity, with GI₅₀ > 100 µM against human fibroblasts. Mechanistic studies on the mode of inhibition of the biotinylated selenocyanate revealed (Annexin-V assay) a remarkable increase in the number of apoptotic cells compared to the control experiment; moreover, depolarization of the mitochondrial membrane was detected by flow cytometry analysis, and with fluorescent microscopy, what supports the apoptotic cell death. Prior to the apoptotic events, cytostatic effects were observed against SW1573 cells using label-free cell-living imaging; therefore, tumour cell division was prevented. Multidrug resistant cell lines exhibited a reduced sensitivity towards the biotinylated selenocyanate, probably due to its P-gp-mediated efflux. Remarkably, antiproliferative levels could be restored by co-administration with tariquidar, a P-gp inhibitor; this approach can, therefore, overcome multidrug resistance mediated by the P-gp efflux system.

Synergistic effect of docetaxel and gambogic acid on bone metastasis of lung cancer

INTRODUCTION

Gambogic acid (GA) as an active compound isolated from Gamboge, have been investigated for many years and proved to be a promising natural anticancer agent for clinical treatment. This study aimed to investigate the inhibitory effect of docetaxel (DTX) combined with gambogic acid on bone metastasis of lung cancer.

METHODS

The anti-proliferation effect of the combination of DTX and GA on Lewis lung cancer (LLC) cells was determined by MTT assays. The anticancer effect of the combination of DTX and GA on bone metastasis of lung cancer in vivo was explored. Evaluation of the efficacy of drug therapy was performed by comparing the degree of bone destruction and the pathological section of bone tissue of the treated mice with that of the control mice.

RESULTS

In vitro cytotoxicity, cell migration, and osteoclast-induced formation assay showed that GA enhanced the therapeutic effect of DTX in Lewis lung cancer cell with a synergistic effect. In an orthotopic mouse model of bone metastasis, the average survival of the DTX+GA combination group (32.61d±1.06 d) was significantly increased compared with that of the DTX group (25.75 d±0.67 d) or GA group (23.99 d±0.58 d), *P<0.01.

CONCLUSION

The combination of DTX and GA has synergistic effect and resulted in more effective inhibition of tumor metastasis, providing a strong preclinical rationale for the clinical development of the DTX+GA combination for treating bone metastasis of lung cancer.

Total Synthesis of Taxol Enabled by Inter- and Intramolecular Radical Coupling Reactions

Taxol is a clinically used drug for the treatment of various types of cancers. Its 6/8/6/4-membered ring (ABCD-ring) system is substituted by eight oxygen functional groups and flanked by four acyl groups, including a β-amino acid side chain. Here we report a 34-step total synthesis of this unusually oxygenated and intricately fused structure. Inter- and intramolecular radical coupling reactions connected the A- and C-ring fragments and cyclized the B-ring, respectively. Functional groups of the A- and C-rings were then efficiently decorated by employing newly developed chemo-, regio-, and stereoselective reactions. Finally, construction of the D-ring and conjugation with the β-amino acid delivered taxol. The powerful coupling reactions and functional group manipulations implemented in the present synthesis provide new valuable information for designing multistep target-oriented syntheses of diverse bioactive natural products.

General Synthetic Approach to Diverse Taxane Cores

Chemical synthesis of natural products is typically inspired by the structure and function of a target molecule. When both factors are of interest, such as in the case of taxane diterpenoids, a synthesis can both serve as a platform for synthetic strategy development and enable new biological exploration. Guided by this paradigm, we present here a unified enantiospecific approach to diverse taxane cores from the feedstock monoterpenoid (S)-carvone. Key to the success of our approach was the use of a skeletal remodeling strategy which began with the divergent reorganization and convergent coupling of two carvone-derived fragments, facilitated by Pd-catalyzed C-C bond cleavage tactics. This coupling was followed by additional restructuring using a Sm(II)-mediated rearrangement and a bioinspired, visible-light induced, transannular [2 + 2] photocycloaddition. Overall, this divergent monoterpenoid remodeling/convergent fragment coupling approach to complex diterpenoid synthesis provides access to structurally disparate taxane cores which have set the stage for the preparation of a wide range of taxanes.

Strategies for the drug discovery and development of taxane anticancer therapeutics

INTRODUCTION

Paclitaxel and docetaxel have been extensively used in the clinic over the past three decades. Although the patents of these first-generation taxanes have expired, their clinical applications, particularly new formulations and combination therapies, are under active investigations. Inspired by the notable success of Abraxane and Lipusu, new formulations have been extensively developed. In parallel, to overcome multidrug resistance (MDR) and to eradicate cancer stem cells, immense efforts have been made on the discovery and development of new-generation taxanes with improved potency and superior pharmacological properties.

AREAS COVERED

This review covers (a) natural sources of advanced intermediates used for semi-synthesis of taxane API, (b) new formulations, (c) the major issues of FDA approved taxanes, (d) the design and development of next-generation taxanes, (e) new mechanisms of action, and (f) a variety of taxane-based drug delivery systems.

EXPERT OPINION

As the highly potent next-generation taxanes can eradicate cancer stem cells and overcome MDR, the priority is to develop these compounds as an integral part of cancer therapy, especially for pancreatic, colon and prostate cancers which hardly respond to checkpoint inhibitors. In order to mitigate undesirable side effects, the exploration of effective nanoformulations and tumor-targeted drug delivery systems are essential.

Narrow-Gap Rheometry: A Novel Method for Measuring Cell Mechanics

The viscoelastic properties of a cell cytoskeleton contain abundant information about the state of a cell. Cells show a response to a specific environment or an administered drug through changes in their viscoelastic properties. Studies of single cells have shown that chemical agents that interact with the cytoskeleton can alter mechanical cell properties and suppress mitosis. This envisions using rheological measurements as a non-specific tool for drug development, the pharmacological screening of new drug agents, and to optimize dosage. Although there exists a number of sophisticated methods for studying mechanical properties of single cells, studying concentration dependencies is difficult and cumbersome with these methods: large cell-to-cell variations demand high repetition rates to obtain statistically significant data. Furthermore, method-induced changes in the cell mechanics cannot be excluded when working in a nonlinear viscoelastic range. To address these issues, we not only compared narrow-gap rheometry with commonly used single cell techniques, such as atomic force microscopy and microfluidic-based approaches, but we also compared existing cell monolayer studies used to estimate cell mechanical properties. This review provides insight for whether and how narrow-gap rheometer could be used as an efficient drug screening tool, which could further improve our current understanding of the mechanical issues present in the treatment of human diseases.

Potent antitumor activity of novel taxoids in anaplastic thyroid cancer

PURPOSE

Anaplastic thyroid cancer (ATC) is the most aggressive form of thyroid cancers and it is rapidly fatal without any effective therapeutic regimens. There are some clinical trials showing that paclitaxel-based chemotherapy for ATC can achieve a relatively high response rate and low incidence of adverse reaction. The aim of this study was to evaluate potential therapeutic activity of novel taxoids in ATC cells.

METHODS

We evaluated antitumor activity of five novel 3'-difluorovinyltaxoids (DFV-taxoids) in anaplastic thyroid cancer cells by a series of in vitro and in vivo experiments. Besides, we also explored the potential mechanism underlying the difference among the taxoids and paclitaxel by molecular docking and tubulin polymerization assays.

RESULTS

Our data showed that these novel DFV-taxoids were more effective than paclitaxel in ATC cell lines and xenografts, as reflected by the inhibition of cell proliferation, colony formation and tumorigenic potential in nude mice, and the induction of G₂/M phase arrest and cell apoptosis. Using tubulin polymerization assays and molecular docking analysis, we found that these DFV-taxoids promoted more rapid polymerization of β-tubulin than paclitaxel.

CONCLUSIONS

Our data demonstrate that these novel taxoids exhibit stronger antitumor activity in ATC cells than paclitaxel, thereby providing a promising therapeutic strategy for the patients with ATC.

Design, synthesis and SAR study of Fluorine-containing 3rd-generation taxoids

It has been shown that the incorporation of fluorine or organofluorine groups into pharmaceutical and agricultural drugs often induces desirable pharmacological properties through unique protein-drug interactions involving fluorine. We have reported separately remarkable effects of the 2,2-difluorovinyl (DFV) group at the C3' position, as well as those of the CF₃O and CHF₂O groups at the 3-position of the C2-benzoyl moiety of the 2nd- and 3rd-generation taxoids on their potency and pharmacological properties. Thus, it was very natural for us to investigate the combination of these two modifications in the 3rd-generation taxoids and to find out whether these two modifications are cooperative at the binding site in the β-tubulin or not, as well as to see how these effects are reflected in the biological activities of the new 3rd-generation DFV-taxoids. Accordingly, we designed, synthesized and fully characterized 14 new 3rd-generation DFV-taxoids. These new DFV-taxoids exhibited remarkable cytotoxicity against human breast, lung, colon, pancreatic and prostate cancer cell lines. All of these new DFV-taxoids exhibited subnanomolar IC₅₀ values against drug-sensitive cell lines, A549, HT29, Vcap and PC3, as well as CFPAC-1. All of the novel DFV-taxoids exhibited 2-4 orders of magnitude greater potency against extremely drug-resistant cancer cell lines, LCC6-MDR and DLD-1, as compared to paclitaxel, indicating that these new DFV-taxoids can overcome MDR caused by the overexpression of Pgp and other ABC cassette transporters. Dose-response (kill) curve analysis of the new DFV-taxoids in LCC6-MDR and DLD-1 cell lines revealed highly impressive profiles of several new DFV-taxoids. The cooperative effects of the combination of the 3'-DFV group and 3-CF₃O/CHF₂O-benzoyl moiety at the C2 position were investigated in detail by molecular docking analysis. We found that both the 3'-DFV moiety and the 3-CF₃O/3-CHF₂O group of the C2-benzoate moiety are nicely accommodated to the deep hydrophobic pocket of the paclitaxel/taxoid binding site in the β-tubulin, enabling an enhanced binding mode through unique attractive interactions between fluorine/CF₃O/CHF₂O and the protein beyond those of paclitaxel and new-generation taxoids without bearing organofluorine groups, which are reflected in the remarkable potency of the new 3rd-generation DFV-taxoids.

Docetaxel loaded mPEG-PLA nanoparticles for sarcoma therapy: preparation, characterization, pharmacokinetics, and anti-tumor efficacy

Sarcoma represents one of the most common malignant tumors with poor treatment outcomes and prognosis. Docetaxel (DTX) is acknowledged as one of the most important chemotherapy agents. The aim of this study was to improve the efficacy of docetaxel by incorporation into the mPEG-PLA nanoparticle (DTX NP) for the treatment of sarcoma. The DTX NP was prepared by emulsion solvent diffusion method and the prescription and preparation process were optimized through a single factor experiment. The optimized DTX NP was characterized by drug loading, encapsulation efficiency, drug release, etc. Then, the pharmacokinetics was conducted on rats and tumor-bearing ICR mice. Finally, the anti-tumor efficacy of DTX NP with different dosages was evaluated on tumor-bearing ICR mice. The optimized DTX NP was characterized by around 100 nm sphere nanoparticles, sustained in vitro drug release with no obvious burst drug release. Compared with DTX injection, the AUC of DTX NP increased by 94.7- and 35.1-fold on the rats and tumor-bearing ICR mice models, respectively. Moreover, the intra-tumoral drug concentration increased by 5.40-fold. The tumor inhibition rate of DTX NP reached 94.66%, which was 1.24 times that of DTX injection (76.11%) at the same dosage, and the bodyweight increase rate was also higher than the DTX injection. The study provided a DTX NP, which could significantly improve the bioavailability and therapeutic efficacy of DTX as well as reduced its toxicity. It possessed a certain prospect of application for sarcoma treatment.

Nab-paclitaxel promotes the cancer-immunity cycle as a potential immunomodulator

Paclitaxel is a widely used anti-tumor chemotherapeutic drug. Solvent-based paclitaxel causes bone marrow suppression, allergic reactions, neurotoxicity and systemic toxicity, which are associated with non-specific cytotoxicity and side effects of fat-soluble solvents. Studies have explored various new nano-drug strategies of paclitaxel, including nanoparticle albumin-bound paclitaxel (nab-paclitaxel) to improve the water solubility and safety of paclitaxel. Nab-paclitaxel is a targeted solvent-free formulation that inhibits microtubule depolymerization to anticancer. It is easily taken up by tumor and immune cells owing to the nano-scaled size and superior biocompatibility. The internalized nab-paclitaxel exhibits significant immunostimulatory activities to promote cancer-immunity cycle. The aim of this study was to explore the synergistic effect of nab-paclitaxel in tumor antigen presentation, T cell activation, reversing the immunosuppressive pattern of tumor microenvironment (TME), and the synergistic effect with cytotoxic lymphocytes (CTLs) in clearance of tumor cells. The effects of nab-paclitaxel on modulation of cancer-immunity cycle, provides potential avenues for combined therapeutic rationale to improve efficacy of immunotherapy.

A comprehensive survey upon diverse and prolific applications of chitosan-based catalytic systems in one-pot multi-component synthesis of heterocyclic rings

Heterocyclic compounds are among the most prestigious and valuable chemical molecules with diverse and magnificent applications in various sciences. Due to the remarkable and numerous properties of the heterocyclic frameworks, the development of efficient and convenient synthetic methods for the preparation of such outstanding compounds is of great importance. Undoubtedly, catalysis has a conspicuous role in modern chemical synthesis and green chemistry. Therefore, when designing a chemical reaction, choosing and or preparing powerful and environmentally benign simple catalysts or complicated catalytic systems for an acceleration of the chemical reaction is a pivotal part of work for synthetic chemists. Chitosan, as a biocompatible and biodegradable pseudo-natural polysaccharide is one of the excellent choices for the preparation of suitable catalytic systems due to its unique properties. In this review paper, every effort has been made to cover all research articles in the field of one-pot synthesis of heterocyclic frameworks in the presence of chitosan-based catalytic systems, which were published roughly by the first quarter of 2020. It is hoped that this review paper can be a little help to synthetic scientists, methodologists, and catalyst designers, both on the laboratory and industrial scales.

Current Perspectives on Taxanes: Focus on Their Bioactivity, Delivery and Combination Therapy

Taxanes, mainly paclitaxel and docetaxel, the microtubule stabilizers, have been well known for being the first-line therapy for breast cancer for more than the last thirty years. Moreover, they have been also used for the treatment of ovarian, hormone-refractory prostate, head and neck, and non-small cell lung carcinomas. Even though paclitaxel and docetaxel significantly enhance the overall survival rate of cancer patients, there are some limitations of their use, such as very poor water solubility and the occurrence of severe side effects. However, this is what pushes the research on these microtubule-stabilizing agents further and yields novel taxane derivatives with significantly improved properties. Therefore, this review article brings recent advances reported in taxane research mainly in the last two years. We focused especially on recent methods of taxane isolation, their mechanism of action, development of their novel derivatives, formulations, and improved tumor-targeted drug delivery. Since cancer cell chemoresistance can be an unsurpassable hurdle in taxane administration, a significant part of this review article has been also devoted to combination therapy of taxanes in cancer treatment. Last but not least, we summarize ongoing clinical trials on these compounds and bring a perspective of advancements in this field.

Taxanes in cancer treatment: Activity, chemoresistance and its overcoming

Since 1984, when paclitaxel was approved by the FDA for the treatment of advanced ovarian carcinoma, taxanes have been widely used as microtubule-targeting antitumor agents. However, their historic classification as antimitotics does not describe all their functions. Indeed, taxanes act in a complex manner, altering multiple cellular oncogenic processes including mitosis, angiogenesis, apoptosis, inflammatory response, and ROS production. On the one hand, identification of the diverse effects of taxanes on oncogenic signaling pathways provides opportunities to apply these cytotoxic drugs in a more rational manner. On the other hand, this may facilitate the development of novel treatment modalities to surmount anticancer drug resistance. In the latter respect, chemoresistance remains a major impediment which limits the efficacy of antitumor chemotherapy. Taxanes have shown impact on key molecular mechanisms including disruption of mitotic spindle, mitosis slippage and inhibition of angiogenesis. Furthermore, there is an emerging contribution of cellular processes including autophagy, oxidative stress, epigenetic alterations and microRNAs deregulation to the acquisition of taxane resistance. Hence, these two lines of findings are currently promoting a more rational and efficacious taxane application as well as development of novel molecular strategies to enhance the efficacy of taxane-based cancer treatment while overcoming drug resistance. This review provides a general and comprehensive picture on the use of taxanes in cancer treatment. In particular, we describe the history of application of taxanes in anticancer therapeutics, the synthesis of the different drugs belonging to this class of cytotoxic compounds, their features and the differences between them. We further dissect the molecular mechanisms of action of taxanes and the molecular basis underlying the onset of taxane resistance. We further delineate the possible modalities to overcome chemoresistance to taxanes, such as increasing drug solubility, delivery and pharmacokinetics, overcoming microtubule alterations or mitotic slippage, inhibiting drug efflux pumps or drug metabolism, targeting redox metabolism, immune response, and other cellular functions.

Design, synthesis of novel C-3'-N-sulfonyl modified taxane analogues from 1-deoxybaccatin VI and their impact on anti-HCC activity

A new series of C-3'-N-sulfonyl paclitaxel analogs were designed and synthesized from 1-deoxybaccatin VI and their structures were confirmed by ¹H NMR, ¹³C NMR and high resolution MS. The synthesized compounds were evaluated for their in vitro anti-Hepatocellular carcinoma (HCC) activity against human hepatoma (HepG2) cell line. Bioassay results showed that compounds 17c, 17d and 17f exhibited more potent inhibitory activity against HepG2 cell line in comparison with paclitaxel. It is suggested that paclitaxel analogs containing the C-3'-N-sulfonyl could be considered as a precursor structure for further synthesis of more potent analogues.

Two New Taxane Diterpenoids From Taxus baccata

Two new diterpenoids named 13-oxo-wollifoliane-10,15-olide (1) and 19-acetoxy-7,9,10-deacetyl-baccatin VI (2), along with 14 known taxanes (3-16), were isolated from Taxus baccata. The structures of these compounds were elucidated by 1-dimensional and 2-dimensional nuclear magnetic resonance spectra, high-resolution electrospray ionization-mass spectrometry, and infrared spectroscopy. Structurally, 13-oxo-wollifoliane-10,15-olide (1) is the first taxane diterpenoid possessing an unusual carbonyl group at the C-13 position of the 11(15→1),11(10→9)bis- abeo-taxane structure (5/6/6/6/4 skeleton), and 19-acetoxy-7,9,10-deacetyl-baccatin VI (2) is a new compound containing an acetoxy group at the C-19 position of 6/8/6/4-taxane. Among the 14 known taxane compounds 3-16, compounds 7 and 9 were first isolated and reported from T. baccata. Several compounds (3-16) were evaluated for cytotoxicity against MCF-7 and HCT116 human cancer cell lines, but none of them showed considerable cytotoxic activity.

An Overview of Antibody Conjugated Polymeric Nanoparticles for Breast Cancer Therapy

Nanoparticles (NPs) are promising drug delivery systems (DDS) for identifying and treating cancer. Active targeting NPs can be generated by conjugation with ligands that bind overexpressed or mutant cell surface receptors on target cells that are poorly or not even expressed on normal cells. Receptor-mediated endocytosis of the NPs occurs and the drug is released inside the cell or in the surrounding tissue due to the bystander effect. Antibodies are the most frequently used ligands to actively target tumor cells. In this context, antibody-based therapies have been extensively used in HER2+ breast cancer. However, some patients inherently display resistance and in advanced stages, almost all eventually progress. Functionalized NPs through conjugation with antibodies appear to be a promising strategy to optimize targeted therapies due to properties related to biocompatibility, suitable delivery control and efficiency of functionalization. This review is focused on the different strategies to conjugate antibodies into polymeric NPs. Recent antibody conjugation approaches applied to the improvement of breast cancer therapy are highlighted in this review.

Tracking the Footprints of Paclitaxel Delivery and Mechanistic Action via SERS Trajectory in Glioblastoma Cells

The design and development of an efficacious tumor-specific drug-delivery system is a challenging task. In this study, we have synthesized target-specific small peptide substrates on an octaguanidine sorbitol scaffold, named small molecular targeted drug-delivery conjugate (SMTDDC). The SMTDDC fabrication, with dual targeting cRGD and Cathepsin B (Cath B)-specific tripeptide (Glu-Lys-Phe), altered the microtubule network of glioblastoma cells by the orchestrated release of the cytotoxic paclitaxel (PTX). Cath B assisted PTX delivery was monitored by high-performance liquid chromatography and Surface-Enhanced Raman Scattering (SERS) modalities. The time-dependent SERS fingerprinting and imaging revealed a fast and accurate PTX release profile and subsequent in vitro cytotoxicity as well as the apoptotic events and microtubule network alteration in U-87 MG glioblastoma cells. Furthermore, SMTDDC displayed adequate stability under physiological conditions and demonstrated biocompatibility toward red blood cells and lymphocytes. This study indicated a new insight on SERS-guided peptidomimetic sorbitol molecular transporter, enabling a greater promise with high potential for the further development of PTX delivery in glioblastoma treatment.

A novel taxane, difluorovinyl-ortataxel, effectively overcomes paclitaxel-resistance in breast cancer cells

Paclitaxel (PTX) is widely used to treat breast and ovarian cancers, but innate and acquired resistance often compromises its applications. The objective of this study was to screen new-generation taxanes for their efficiency against both PTX-sensitive and PTX-resistant breast cancer cells. From twelve compounds, difluorovinyl-ortataxel (DFV-OTX) displayed potent cytotoxic activities against both PTX-sensitive and PTX-resistant breast cancer cells. Moreover, DFV-OTX effectively induced tubulin/microtubule polymerization and G2/M phase arrest, leading to apoptosis in both PTX-sensitive and PTX-resistant cancer cells. Molecular docking analysis showed that DFV-OTX possesses unique hydrogen-bonding and van der Waals interactions with β-tubulin. LC-MS/MS analysis also demonstrated that the intracellular drug amount of DFV-OTX was lower than that of PTX, which would be critical to overcome PTX-resistance. Furthermore, DFV-OTX exhibited clear efficacy in the MCF-7R and MDA-MB-231R tumor xenografts in mouse models. Taken together, our results demonstrate that the novel taxane, DFV-OTX, can effectively overcome PTX-resistance in MDA-MB-231R cells, wherein the drug resistance was attributed to ABCB1/ABCG2 upregulation and a distinct mode of action in MCF-7R cells. Our results strongly indicate that DFV-OTX is a promising chemotherapeutic agent for the treatment of PTX-resistant cancers.

Design, Synthesis, and Anticancer Activity of Natural Product Hybrids With Paclitaxel Side Chain Inducing Apoptosis in Human Colon Cancer Cells

Based on the strong activity dependence of paclitaxel (PTX; Taxol) or docetaxel (Taxotere) on the C-13 side chain, a small library of dehydroepiandrosterone, cholesterol, vitamin D2, and alkaloids talatisamine and songorine-PTX hybrids have been synthesized and evaluated for in vitro anticancer activity by MTT assay against human breast (MCF-7), colon (HCT116), lung carcinoma (A549), and renal adenocarcinoma (786-0) cancer cell lines. Most hybrids (11b, 12b, 13b, 15b, and 18b) reduced the growth of MCF-7 and 786-0 cells with low PTX sensitivity in vitro. Among the synthesized compounds, hybrid 11b was better in inhibiting the growth of the 4 cells than PTX. A relatively low IC50 value of compound 11b (8.16 ± 0.04 μM) was also examined after exposure for 48 hours. Hybrid 11b showed a proapoptotic effect in HCT116 cells evaluated by Annexin V/propidium iodide binding assay. The level of hybrid 11b leading to protective cell death in HCT116 cells was detected using western blot and not easily observed in our basic examinations.

Palladium-Catalyzed Synthesis and Anticancer Activity of Paclitaxel-Dehydroepiandrosterone Hybrids

According to the activity-structure relationship of the C-13 side chain in paclitaxel or docetaxel, eighteen novel paclitaxel-dehydroepiandrosterone (DHEA) hybrids were designed and synthesized by Pd(II)-catalyzed Suzuki-Miyaura cross-coupling of 17-trifluoromethanesulfonic enolate-DHEA with different aryl boronic acids. The in vitro anticancer activity of the hybrids against a human liver cancer cell line (HepG-2) was evaluated by MTT assay, showing that most of these hybrids possessed moderate antiproliferative activity against the HepG-2 cancer cell line. Among these hybrids, three ones (7b, 7g, and 7i) with ortho-substituents in the phenyl group of the D-ring of DHEA analogues exhibited moderate anticancer activity. The optimal compound 7i showed superior anticancer activity against the HepG-2 cell line with an IC₅₀ value of 26.39 μM.

Improvement of conventional anti-cancer drugs as new tools against multidrug resistant tumors

Multidrug resistance (MDR) is the dominant cause of the failure of cancer chemotherapy. The design of antitumor drugs that are able to evade MDR is rapidly evolving, showing that this area of biomedical research attracts great interest in the scientific community. The current review explores promising recent approaches that have been developed with the aim of circumventing or overcoming MDR. Encouraging results have been obtained in the investigation of the MDR-modulating properties of various classes of natural compounds and their analogues. Inhibition of P-gp or downregulation of its expression have proven to be the main mechanisms by which MDR can be surmounted. The use of hybrid molecules that are able to simultaneously interact with two or more cancer cell targets is currently being explored as a means to circumvent drug resistance. This strategy is based on the design of hybrid compounds that are obtained either by merging the structural features of separate drugs, or by conjugating two drugs or pharmacophores via cleavable/non-cleavable linkers. The approach is highly promising due to the pharmacokinetic and pharmacodynamic advantages that can be achieved over the independent administration of the two individual components. However, it should be stressed that the task of obtaining successful multivalent drugs is a very challenging one. The conjugation of anticancer agents with nitric oxide (NO) donors has recently been developed, creating a particular class of hybrid that can combat tumor drug resistance. Appropriate NO donors have been shown to reverse drug resistance via nitration of ABC transporters and by interfering with a number of metabolic enzymes and signaling pathways. In fact, hybrid compounds that are produced by covalently attaching NO-donors and antitumor drugs have been shown to elicit a synergistic cytotoxic effect in a variety of drug resistant cancer cell lines. Another strategy to circumvent MDR is based on nanocarrier-mediated transport and the controlled release of chemotherapeutic drugs and P-gp inhibitors. Their pharmacokinetics are governed by the nanoparticle or polymer carrier and make use of the enhanced permeation and retention (EPR) effect, which can increase selective delivery to cancer cells. These systems are usually internalized by cancer cells via endocytosis and accumulate in endosomes and lysosomes, thus preventing rapid efflux. Other modalities to combat MDR are described in this review, including the pharmaco-modulation of acridine, which is a well-known scaffold in the development of bioactive compounds, the use of natural compounds as means to reverse MDR, and the conjugation of anticancer drugs with carriers that target specific tumor-cell components. Finally, the outstanding potential of in silico structure-based methods as a means to evaluate the ability of antitumor drugs to interact with drug transporters is also highlighted in this review. Structure-based design methods, which utilize 3D structural data of proteins and their complexes with ligands, are the most effective of the in silico methods available, as they provide a prediction regarding the interaction between transport proteins and their substrates and inhibitors. The recently resolved X-ray structure of human P-gp can help predict the interaction sites of designed compounds, providing insight into their binding mode and directing possible rational modifications to prevent them from becoming P-gp drug substrates. In summary, although major efforts were invested in the search for new tools to combat drug resistant tumors, they all require further implementation and methodological development. Further investigation and progress in the abovementioned strategies will provide significant advances in the rational combat against cancer MDR.

Functionalizing nanoparticles with cancer-targeting antibodies: A comparison of strategies

Standard cancer therapies sometimes fail to deliver chemotherapeutic drugs to tumor cells in a safe and effective manner. Nanotechnology takes the lead in providing new therapeutic options for cancer due to major potential for selective targeting and controlled drug release. Antibodies and antibody fragments are attracting much attention as a source of targeting ligands to bind specific receptors that are overexpressed on cancer cells. Therefore, researchers are devoting time and effort to develop targeting strategies based on nanoparticles functionalized with antibodies, which hold great promise to enhance therapeutic efficacy and circumvent severe side effects. Several methods have been described to immobilize antibodies on the surface of nanoparticles. However, selecting the most appropriate for each application is challenging but also imperative to preserve antigen binding ability and yield stable antibody-conjugated nanoparticles. From this perspective, we aim to provide considerable knowledge on the most widely used methods of functionalization that can be helpful for decision-making and design of conjugation protocols as well. This review summarizes adsorption, covalent conjugation (carbodiimide, maleimide and "click" chemistries) and biotin-avidin interaction, while discussing the advantages, limitations and relevant therapeutic approaches currently under investigation.

Design, synthesis and SAR study of 3rd-generation taxoids bearing 3-CH3, 3-CF3O and 3-CHF2O groups at the C2-benzoate position

It has been shown that inclusion of CF₃O and CHF₂O groups to drug candidates often improve their pharmacological properties, especially metabolic stability, membrane permeability and PK profile. Moreover, the unique non-spherical structure of the OCHF₂ group can provide interesting and beneficial characteristics. Accordingly, new 3rd-generation taxoids, bearing 3-OCF₃ or 3-OCF₂H (and 3-CH₃ for comparison) at the C2 benzoate moiety, were synthesized and their potencies against drug-sensitive and drug-resistant cancer cell lines examined. In this study, our previous SAR studies on 3rd-generation taxoids were expanded to disclose that CH₃, CF₃O and CHF₂O groups are well tolerated at this position and enhance potency, especially against MDR-cancer cell lines so that these taxoids can virtually overcome MDR. These new taxoids exhibit up to 7 times higher cytotoxicity (IC₅₀) than paclitaxel against drug-sensitive cancer cell lines (MCF7 and LCC6-WT) and 2-3 orders of magnitude higher potency than paclitaxel against drug-resistant ovarian, breast and colon cancer cell lines with MDR-phenotype (NCI/ADR, LCC6-MDR and LDL-1), as well as pancreatic cancer cell line, CFPAC-1. Since it has been shown that a bulky group at this position reduces potency, it is noteworthy that rather bulky CF₃O and CHF₂O groups are well tolerated. Molecular modeling analysis indicated the favorable van der Waals interactions of CF₃O and CHF₂O groups in the binding site. It is also worthy of note that new taxoids, bearing a CHF₂O group at the C2 benzoate position (1-06 series), exhibited the highest potencies against MDR-cancer cell lines and cancer stem cell (CSC)-enriched cancer cell lines. These new 3rd-generation taxoids are promising candidates for highly potent chemotherapeutic agents, as well as payloads for tumor-targeting drug conjugates such as antibody-drug conjugates (ADCs).

Total Synthesis of 1-Hydroxytaxinine

1-Hydroxytaxinine (1) is a cytotoxic taxane diterpenoid. Its central eight-membered B-ring possesses four oxygen-functionalized centers (C1, C2, C9, and C10) and two quaternary carbon centers (C8 and C15), and is fused with six-membered A- and C-rings. The densely functionalized and intricately fused structure of 1 makes it a highly challenging synthetic target. Reported here is an efficient radical-based strategy for assembling 1 from A- and C-ring fragments. The A-ring bearing an α-alkoxyacyl telluride moiety underwent intermolecular coupling with the C-ring fragment by a Et₃ B/O₂ -promoted decarbonylative radical formation. After construction of the C8-quaternary stereocenter, a pinacol coupling reaction using a low-valent titanium reagent formed the B-ring with stereoselective installation of the C1,C2-diol. Subsequent manipulations at the A- and C-rings furnished 1 in 26 total steps.

Convergent Synthesis of Taxol Skeleton via Decarbonylative Radical Coupling Reaction

The highly oxygenated 6/8/6-membered ABC-ring 2 of taxol was assembled in a convergent fashion. A decarbonylative radical reaction between α-alkoxyacyl telluride 4 and cyanocyclohexenone 5 linked the A- and C-rings and stereoselectively installed the C2- and C3-tertiary carbon centers of 3. After the C8-quaternary stereocenter was constructed, the C9-methyl ketone and the C11-vinyl triflate of 30 participated in Pd(0)-promoted cyclization of the eight-membered B-ring, giving rise to the taxol skeleton 2.

Docetaxel Down-Regulates PD-1 Expression via STAT3 in T Lymphocytes

BACKGROUND

Lung tumor is a major cause of cancer incidence and patient death. Chemotherapy is the primary therapy used to treat lung cancer. In addition to direct cytotoxic effect on tumor cells, chemotherapeutic drugs activate immune responses to exert antitumor function. Here, the effects of docetaxel on the inhibitory molecules, programmed cell death 1 (PD-1), cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), and T-cell immunoglobulin and mucin domain 3 (TIM-3) in T lymphocytes were explored in patients with lung adenocarcinoma.

PATIENTS AND METHODS

Peripheral blood mononuclear cells were isolated from lung adenocarcinoma patients receiving cisplatin-docetaxel chemotherapy. By flow cytometry and PCR, the expressions of CTLA-4, PD-1 and TIM-3 in T cell subsets were analyzed. Health subjects were used as control group.

RESULTS

During chemotherapy, suppressive markers were down-regulated in peripheral CD4⁺ and CD8⁺ T cells from patients with partial remission or stable disease. Additionally, interferon-γ production was also augmented during this period. In vitro assay showed that docetaxel reduced the expression of PD-1 on T-cell subsets without altering cell death. Further tests in Jurkat T cells demonstrated that docetaxel activated signal transduction and activator of transcription 3 (STAT3) signaling to suppress PD-1 expression, whereas STAT3 inhibition reversed the down-regulation of PD-1.

CONCLUSION

Our data support the hypothesis that chemotherapeutic drugs are not only purely cytotoxic but are also immune modulators.