Recent advances in the chemistry and biology of new generation taxoids

Third-generation taxanes SB-T-121605 and SB-T-121606 are effective in pancreatic ductal adenocarcinoma

Pancreatic cancer is a severe malignancy with increasing incidence and high mortality due to late diagnosis and low sensitivity to treatments. Search for the most appropriate drugs and therapeutic regimens is the most promising way to improve the treatment outcomes of the patients. This study aimed to compare (1) in vitro efficacy and (2) in vivo antitumor effects of conventional paclitaxel and the newly synthesized second (SB-T-1216) and third (SB-T-121605 and SB-T-121606) generation taxanes in KRAS wild type BxPC-3 and more aggressive KRAS G12V mutated Paca-44 pancreatic cancer cell line models. In vitro, paclitaxel efficacy was 27.6 ± 1.7 nM, while SB-Ts showed 1.7-7.4 times higher efficacy. Incorporation of SB-T-121605 and SB-T-121606 into in vivo therapeutic regimens containing paclitaxel was effective in suppressing tumor growth in Paca-44 tumor-bearing mice at small doses (≤3 mg/kg). SB-T-121605 and SB-T-121606 in combination with paclitaxel are promising candidates for the next phase of preclinical testing.

Anticancer regimens containing third generation taxanes SB-T-121605 and SB-T-121606 are highly effective in resistant ovarian carcinoma model

Taxanes are widely used in the treatment of ovarian carcinomas. One of the main problems with conventional taxanes is the risk of development of multidrug resistance. New-generation synthetic experimental taxoids (Stony Brook Taxanes; SB-T) have shown promising effects against various resistant tumor models. The aim of our study was to compare the in vitro efficacy, intracellular content, and in vivo antitumor effect of clinically used paclitaxel (PTX) and SB-Ts from the previously tested second (SB-T-1214, SB-T-1216) and the newly synthesized third (SB-T-121402, SB-T-121605, and SB-T-121606) generation in PTX resistant ovarian carcinoma cells NCI/ADR-RES. The efficacy of the new SB-Ts was up to 50-times higher compared to PTX in NCI/ADR-RES cells in vitro. SB-T-121605 and SB-T-121606 induced cell cycle arrest in the G2/M phase much more effectively and their intracellular content was 10-15-times higher, when compared to PTX. Incorporation of SB-T-121605 and SB-T-121606 into therapeutic regimens containing PTX were effective in suppressing tumor growth in vivo in NCI/ADR-RES based mice xenografts at small doses (≤3 mg/kg), where their adverse effects were eliminated. In conclusion, new SB-T-121605 and SB-T-121606 analogs are promising candidates for the next phase of preclinical testing of their combination therapy with conventional taxanes in resistant ovarian carcinomas.

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.

Age-dependent modulation of oleoresin production in the stem of Sindora glabra

Plants produce specialized metabolites in various organs which serve important functions in defense and development. However, the molecular regulatory mechanisms of oleoresin production in stems from broadleaved tree species are not fully understood. To determine whether endogenous developmental cues play a role in the regulation of oleoresin biosynthesis in tree stems, anatomy, multi-omics and molecular experiments were utilized to investigate the change of secretory structures, chemical profiles and gene expression in different ontogenetic stages of Sindora glabra tree, which accumulates copious amount of sesquiterpene-rich oleoresin in stems. The size of secretory canals and the concentration of five sesquiterpenes in Sindora stems exhibited obvious increase with plant age, from 0.5- to 20-year-old plants. Moreover, α-copaene and β-copaene were found to be stem-specific sesquiterpenes. Metabolomic analysis revealed that salicylic acid highly accumulated in mature stems, but the content of triterpenes was greatly decreased. The expression of three repressors AUX/IAA, DELLA and JAZ involved in hormone signaling transduction pathways was significantly downregulated in stems of 10- and 20-year-old plants. Two key genes SgTPS3 and SgTPS5 were identified, whose expression was highly correlated with the accumulation patterns of specific sesquiterpenes and their enzymatic products were consistent with the chemical profiles in the stem. The promoters of three SgTPSs exhibiting high activity were isolated. Furthermore, we demonstrated that SgSPL15 directly interacts with SgTPS3 and SgTPS5 promoters and activates SgTPS5 expression but SgSPL15 inhibits SgTPS3 expression. In addition, SgSPL15 enhanced sesquiterpene levels by upregulating AtTPSs expression in Arabidopsis. These results suggested that sesquiterpene biosynthesis in S. glabra stem was dependent on the regulation of endogenous hormones as well as plant age, and SgSPL15 might act as a buffering factor to regulate sesquiterpene biosynthesis by targeting SgTPS genes.

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.

The Role of TRIP6, ABCC3 and CPS1 Expression in Resistance of Ovarian Cancer to Taxanes

The main problem precluding successful therapy with conventional taxanes is de novo or acquired resistance to taxanes. Therefore, novel experimental taxane derivatives (Stony Brook taxanes; SB-Ts) are synthesized and tested as potential drugs against resistant solid tumors. Recently, we reported alterations in ABCC3, CPS1, and TRIP6 gene expression in a breast cancer cell line resistant to paclitaxel. The present study aimed to investigate gene expression changes of these three candidate molecules in the highly resistant ovarian carcinoma cells in vitro and corresponding in vivo models treated with paclitaxel and new experimental Stony Brook taxanes of the third generation (SB-T-121605 and SB-T-121606). We also addressed their prognostic meaning in ovarian carcinoma patients treated with taxanes. We estimated and observed changes in mRNA and protein profiles of ABCC3, CPS1, and TRIP6 in resistant and sensitive ovarian cancer cells and after the treatment of resistant ovarian cancer models with paclitaxel and Stony Brook taxanes in vitro and in vivo. Combining Stony Brook taxanes with paclitaxel caused downregulation of CPS1 in the paclitaxel-resistant mouse xenograft tumor model in vivo. Moreover, CPS1 overexpression seems to play a role of a prognostic biomarker of epithelial ovarian carcinoma patients’ poor survival. ABCC3 was overexpressed in EOC tumors, but after the treatment with taxanes, its up-regulation disappeared. Based on our results, we can suggest ABCC3 and CPS1 for further investigations as potential therapeutic targets in human cancers.

Regulatory role of the transforming growth factor-β signaling pathway in the drug resistance of gastrointestinal cancers

Gastrointestinal (GI) cancer, including esophageal, gastric, and colorectal cancer, is one of the most prevalent types of malignant carcinoma and the leading cause of cancer-related deaths. Despite significant advances in therapeutic strategies for GI cancers in recent decades, drug resistance with various mechanisms remains the prevailing cause of therapy failure in GI cancers. Accumulating evidence has demonstrated that the transforming growth factor (TGF)-β signaling pathway has crucial, complex roles in many cellular functions related to drug resistance. This review summarizes current knowledge regarding the role of the TGF-β signaling pathway in the resistance of GI cancers to conventional chemotherapy, targeted therapy, immunotherapy, and traditional medicine. Various processes, including epithelial-mesenchymal transition, cancer stem cell development, tumor microenvironment alteration, and microRNA biogenesis, are proposed as the main mechanisms of TGF-β-mediated drug resistance in GI cancers. Several studies have already indicated the benefit of combining antitumor drugs with agents that suppress the TGF-β signaling pathway, but this approach needs to be verified in additional clinical studies. Moreover, the identification of potential biological markers that can be used to predict the response to TGF-β signaling pathway inhibitors during anticancer treatments will have important clinical implications in the future.

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.

Synthesis and biological evaluation of novel cabazitaxel analogues

Cabazitaxel is one of the most recently FDA-approved taxane anticancer agent. In view of the advantages in preclinical and clinical data of cabazitaxel over former toxoids, the synthesis and biological evaluation of novel cabazitaxel analogues were conducted. First, a novel semi-synthesis of cabazitaxel was described. This strategy is concise and efficient, which needs five steps from the 10-deacetylbaccatin III (10-DAB) moiety and a commercially available C13 side chain precursor with a 32% overall yield. Besides, this strategy avoids using many hazardous reagents that involved in the previously reported processes. Then, a panel of cabazitaxel analogues were prepared basing on this strategy. The cytotoxicity evaluations showed that the majority of these cabazitaxel analogues are potent against both A549 and KB cells and their corresponding drug-resistant cell lines KB/VCR, and A549/T, respectively. Further in vivo antitumor efficacies assessment of 7,10-di-O-methylthiomethyl (MTM) modified cabazitaxel (compounds 16 and 19) on SCID mice A549 xenograft model showed they both had similar antitumor activity to the cabazitaxel. Since compound 19 was observed causing more body wight loss on the mice than 16, these preliminary studies suggest 16 might be a potent drug candidate for further preclinical evaluation.

Peptidomimetics - An infinite reservoir of metal binding motifs in metabolically stable and biologically active molecules

The involvement of metal ions in interactions with therapeutic peptides is inevitable. They are one of the factors able to fine-tune the biological properties of antimicrobial peptides, a promising group of drugs with one large drawback - a problematic metabolic stability. Appropriately chosen, proteolytically stable peptidomimetics seem to be a reasonable solution of the problem, and the use of D-, β-, γ-amino acids, unnatural amino acids, azapeptides, peptoids, cyclopeptides and dehydropeptides is an infinite reservoir of metal binding motifs in metabolically stable, well-designed, biologically active molecules. Below, their specific structural features, metal-chelating abilities and antimicrobial potential are discussed.

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.

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.

The Preparation, Determination of a Flexible Complex Liposome Co-Loaded with Cabazitaxel and β-Elemene, and Animal Pharmacodynamics on Paclitaxel-Resistant Lung Adenocarcinoma

Paclitaxel is highly effective at killing many malignant tumors; however, the development of drug resistance is common in clinical applications. The issue of overcoming paclitaxel resistance is a difficult challenge at present. In this study, we developed nano drugs to treat paclitaxel-resistant lung adenocarcinoma. We selected cabazitaxel and β-elemene, which have fewer issues with drug resistance, and successfully prepared cabazitaxel liposome, β-elemene liposome and cabazitaxel-β-elemene complex liposome with good flexibility. The encapsulation efficiencies of cabazitaxel and β-elemene in these liposomes were detected by precipitation microfiltration and microfiltration centrifugation methods, respectively. Their encapsulation efficiencies were all above 95%. The release rates were detected by a dialysis method. The release profiles of cabazitaxel and β-elemene in these liposomes conformed to the Weibull equation. The release of cabazitaxel and β-elemene in the complex liposome were almost synchronous. The pharmacodynamics study showed that cabazitaxel flexible liposome and β-elemene flexible liposome were relatively good at overcoming paclitaxel resistance on paclitaxel-resistant lung adenocarcinoma. As the flexible complex liposome, the dosage of cabazitaxel could be reduced to 25% that of the cabazitaxel injection while retaining a similar therapeutic effect. It showed that β-elemene can replace some of the cabazitaxel, allowing the dosage of cabazitaxel to be reduced, thereby reducing the drug toxicity.

Recent progress in the strategic incorporation of fluorine into medicinally active compounds

This account exemplifies our recent progress on the strategic incorporation of fluorine and organofluorine groups to (i) taxoid anticancer agents, (ii) acylhydrazone-based antifungal agents and (iii) inhibitors of matrix metalloproteinase 9 (MMP9) for medicinal chemistry and chemical biology studies. In the case study (i), a series of next-generation fluorotaxoids, bearing m-OCF3 or m-OCF2H group in the C2-benzoate moiety was designed, synthesized and examined for their potencies. A number of these fluorotaxoids possess two orders of magnitude greater potency in different drug-resistant cancer cell lines as compared to paclitaxel. One of these next-generation fluorotaxoids, SB--121205wasselected for detailed mechanistic study against highly paclitaxel-resistant human breast cancer cell line, MCF-7/PTX, which disclosed a unique mechanism of action. Recently, glucosylceramide (GlcCer) synthesis emerged as a promising target for next-generation antifungal agents, especially against cryptococcosis, candidiasis and pulmonary aspergillosis. The HTP screening of compound libraries identified several acylhydrazones as hit compounds. In the case study (ii), fluoro-acylhydrazones containing F, OCF3, OCHF2, o-F/p-OCF3, as well as o-F/p-CF3 functional groups in the ring A and ring B were designed based on these hit compounds, synthesized and examined for their potencies against C. neoformans. A number of those novel fluoro-acylhydrazones exhibited high potency and excellent killing properties. The hemopexin-like domain of matrix metalloproteinases (MMPs) is a highly promising target to circumvent the critical issue in the development of MMP inhibitors for the treatment of various cancers. In the case study (iii), a small optimization library of compounds, based on the OCHF2-containing hit compound, SB-M-001, was generated and evaluated, which identified a fluorine-containing new lead compound, SB-M-103. SB-M-103 was found to inhibit tumor cell growth, migration, and invasion by effectively disrupting the MMP-9 homodimerization.

Aliphatic C-H Oxidations for Late-Stage Functionalization

The atomistic change of C( sp3)-H to C( sp3)-O can have a profound impact on the physical and biological properties of small molecules. Traditionally, chemical synthesis has relied on pre-existing functionality to install new functionality, and directed approaches to C-H oxidation are an extension of this logic. The impact of developing undirected C-H oxidation reactions with controlled site-selectivity is that scientists gain the ability to diversify complex structures at sites remote from existing functionality, without having to carry out individual de novo syntheses. This Perspective offers a historical view of why, as recently as 2007, it was thought that the differences between aliphatic C-H bonds of the same bond type (for example, 2° aliphatic) were not large enough to distinguish them preparatively with small-molecule catalysis in the absence of directing groups or molecular recognition elements. We give an account of the discovery of Fe(PDP)-catalyzed non-directed aliphatic C-H hydroxylations and how the electronic, steric, and stereoelectronic rules for predicting site-selectivity that emerged have affected a shift in how the chemical community views the reactivity among these bonds. The discovery that site-selectivity could be altered by tuning the catalyst [i.e., Fe(CF3-PDP)] with no changes to the substrate or reaction now gives scientists the ability to exert control on the site of oxidation on a range of functionally and topologically diverse compounds. Collectively, these findings have made possible the emerging area of late-stage C-H functionalizations for streamlining synthesis and derivatizing complex molecules.

Nitrogen-doped carbon nanodots for bioimaging and delivery of paclitaxel

Carbon nanodots (CNDs) hold great potential in imaging and drug delivery applications. In this study, nitrogen-doped CNDs (NCNDs) were coupled to the anticancer agent paclitaxel (PTX) through a labile ester bond. NCNDs showed excellent cell viability and endowed the NCND-PTX conjugate with good water solubility. The hybrid integrates the optical properties of the nanodots with the anticancer function of the drug into a single unit. Cytotoxicity was evaluated in breast, cervix, lung, and prostate cancer cell lines by the MTT assay while the cellular uptake was monitored using confocal microscopy. NCND-PTX induced apoptosis in cancer cells exhibiting slightly better anticancer activity compared to the drug alone. Moreover, the course of the NCND-PTX interaction with cancer cells was monitored using an xCELLigence system. The NCND-based conjugate represents a promising platform for bioimaging and drug delivery.

Synthesis and biological evaluation of novel larotaxel analogues

Taxoids are a class of successful drugs and have been successfully used in chemotherapy for a variety of cancer types. However, despite the hope and promises that these taxoids have engendered, their utility is hampered by some clinic limitations. Extensive structure-activity relationship (SAR) studies of toxoids have been performed in many different laboratories. Whereas, SAR studies that based on the new-generation toxoid, larotaxel, have not been reported yet. In view of the advantages in preclinical and clinical data of larotaxel over former toxoids, new taxoids that strategicly modified at the C3'/C3'-N and C2 positions of larotaxel were designed, semi-synthesized, and examined for their potency and efficacy in vitro. As a result, it has been shown that the majority of these larotaxel analogues are exceptionally potent against both drug-sensitive tumor cells and tumor cells with drug resistance arising from P-glycoprotein over expression. Further in vivo antitumor efficacies investigations revealed A2 might be a potent antitumor drug candidate for further preclinical evaluation.

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

Paclitaxel and docetaxel are among the most widely used chemotherapeutic drugs against various types of cancer. However, these drugs cause undesirable side effects as well as drug resistance. Therefore, it is essential to develop next-generation taxoid anticancer agents with better pharmacological properties and improved activity especially against drug-resistant and metastatic cancers. The SAR studies by the authors have led to the development of numerous highly potent novel second- and third-generation taxoids with systematic modifications at the C-2, C-10, and C-3' positions. The third-generation taxoids showed virtually no difference in potency against drug-resistant and drug-sensitive cell lines. Some of the next-generation taxoids also exhibited excellent potency against cancer stem cells. This account summarizes concisely investigations into taxoids over 25 years based on a strong quest for the discovery and development of efficacious next-generation taxoids. Discussed herein are SAR studies on different types of taxoids, a common pharmacophore proposal for microtubule-stabilizing anticancer agents and its interesting history, the identification of the paclitaxel binding site and its bioactive conformation, characteristics of the next-generation taxoids in cancer cell biology, including new aspects of their mechanism of action, and the highly efficacious tumor-targeted drug delivery of potent next-generation taxoids.

Strategic Incorporation of Fluorine into Taxoid Anticancer Agents for Medicinal Chemistry and Chemical Biology Studies

This account exemplifies our recent progress on the strategic incorporation of fluorine and organofluorine groups to taxoid anticancer agents and their tumor-targeted drug delivery systems (TTDDSs) for medicinal chemistry and chemical biology studies. Novel 3'-difluorovinyltaxoids were strategically designed to block the metabolism by cytochrome P-450, synthesized, and evaluated for their cytotoxicity against drug-sensitive and multidrug-resistant (MDR) human cancer cell lines. 3'-Difluorovinyltaxoids exhibited impressive activities against these cancer cell lines. More significantly, a representative 3'-difluorovinyltaxoid exhibited 230-33,000 times higher potency than conventional anticancer drugs against cancer stem cell-enriched HCT-116 cell line. Studies on the mechanism of action (MOA) of these fluorotaxoids were performed by tubulin polymerization assay, morphology analysis by electron microscopy (EM) and protein binding assays. Novel ¹⁹F NMR probes, BLT-F₂ and BLT-S-F₆, were designed by strategically incorporating fluorine, CF₃ and CF₃O groups into tumor-targeting drug conjugates. These ¹⁹F-probes were designed and synthesized to investigate the mechanism of linker cleavage and factors that influence their plasma and metabolic stability by real-time ¹⁹F NMR analysis. Time-resolved ¹⁹F NMR study on probe BLT-F₂ revealed a stepwise mechanism for the release of a fluorotaxoid, which might not be detected by other analytical methods. Probe BLT-S-F₆ were very useful to study the stability and reactivity of the drug delivery system in human blood plasma by ¹⁹F NMR. The clean analysis of the linker stability and reactivity of drug conjugates in blood plasma by HPLC and ¹H NMR is very challenging, but the use of ¹⁹F NMR and suitable ¹⁹F probes can provide a practical solution to this problem.

Microtubule-stabilizing agents: New drug discovery and cancer therapy

Microtubule-stabilizing agents (MSAs) have been highly successful in the treatment of cancer in the past 20years. To date, three classes of MSAs have entered the clinical trial stage or have been approved for clinical anticancer chemotherapy, and more than 10 classes of novel structural MSAs have been derived from natural resources. The microtubule typically contains two MSA-binding sites: the taxoid site and the laulimalide/peloruside site. All defined MSAs are known to bind at either of these sites, with subtle but significant differences. MSAs with different binding sites may produce a synergistic effect. Although having been extensively applied in the clinical setting, paclitaxel and other approved MSAs still pose many challenges such as multidrug resistance, low bioavailability, poor solubility, high toxicity, and low passage through the blood-brain barrier. A variety of studies focus on the structure-activity relationship in order to improve the pharmaceutical properties of these agents. Here, the mechanisms of action, advancements in pharmacological research, and clinical developments of defined MSAs during the past decade are discussed. The latest discovered MSAs are also briefly introduced in this review. The increasing number of natural MSAs indicates the potential discovery of more novel, natural MSAs with different structural bases, which will further promote the development of anticancer chemotherapy.

Taxane anticancer agents: a patent perspective

INTRODUCTION

Paclitaxel and docetaxel were two epoch-making anticancer drugs and have been successfully used in chemotherapy for a variety of cancer types. In the year 2010, a new taxane, cabazitaxel, was approved by FDA for use in combination with prednisone for the treatment of metastatic hormone-refractory prostate cancer. Albumin-bound paclitaxel (nab™-paclitaxel; abraxane) nanodroplet formulation was another notable invention (FDA approval 2005 for refractory, metastatic, or relapsed breast cancer). Abraxane in combination with gemcitabine for the treatment of pancreatic cancer was approved by FDA in 2013. Accordingly, there have been a huge number of patent applications dealing with taxane anticancer agents in the last 5 years. Thus, it is a good time to review the progress in this area and find the next wave for new developments.

AREA COVERED

This review covers the patent literature from the year 2010 to early 2015 on various aspects of taxane-based chemotherapies and drug developments.

EXPERT OPINION

Three FDA-approved taxane anticancer drugs will continue to expand their therapeutic applications, especially through drug combinations and new formulations. Inspired by the success of abraxane, new nano-formulations are emerging. Highly potent new-generation taxanes will play a key role in the development of efficacious tumor-targeted drug delivery systems.

Poly(2-oxazoline) based micelles with high capacity for 3rd generation taxoids: preparation, in vitro and in vivo evaluation

The clinically and commercially successful taxanes, paclitaxel and docetaxel suffer from two major drawbacks, namely their very low aqueous solubility and the risk of developing resistance. Here, we present a method that overcomes both drawbacks in a very simple manner. We formulated 3rd generation taxoids, able to avoid common drug resistance mechanisms with doubly amphiphilic poly(2-oxazoline)s (POx), a safe and highly efficient polymer for the formulation of extremely hydrophobic drugs. We found excellent solubilization of different 3rd generation taxoids irrespective of the drug's chemical structures with essentially quantitative drug loading and final drug to polymer ratios around unity. The small, highly loaded micelles with a hydrodynamic diameter of less than 100nm are excellently suited for parenteral administration. Moreover, a selected formulation with the taxoid SB-T-1214 is about one to two orders of magnitude more active in vitro than paclitaxel in the multidrug resistant breast cancer cell line LCC6-MDR. In contrast, in wild-type LCC6, no difference was observed. Using a q4d×4 dosing regimen, we also found that POx/SB-T-1214 significantly inhibits the growth of LCC6-MDR orthotropic tumors, outperforming commercial paclitaxel drug Taxol and Cremophor EL formulated SB-T-1214.

Design, Synthesis and Application of Fluorine-Labeled Taxoids as 19F NMR Probes for the Metabolic Stability Assessment of Tumor-Targeted Drug Delivery Systems

Novel tumor-targeting drug conjugates, BLT-F₂ (1) and BLT-S-F₆ (2), bearing a fluorotaxoid as the warhead, a mechanism-based self-immolative disulfide linker, and biotin as the tumor-targeting module, were designed and synthesized as ¹⁹F NMR probes. Fluorine atoms and CF₃ groups were strategically incorporated into the conjugates to investigate the mechanism of linker cleavage and factors that influence their plasma and metabolic stability by real-time monitoring with ¹⁹F NMR. Time-resolved ¹⁹F NMR study on probe 1 disclosed a stepwise mechanism for release of a fluorotaxoid, which might not have been detected by other analytical methods. Probe 2 was designed to bear two CF₃ groups in the taxoid moiety as "3-FAB" reporters for enhanced sensitivity and a polyethylene glycol oligomer insert to improve solubility. The clean analysis of the linker stability and reactivity of drug conjugates in blood plasma or cell culture media by HPLC and ¹H NMR is troublesome, due to the overlap of key signals/peaks with background arising from highly complex ingredients in biological systems. Accordingly, the use of ¹⁹F NMR would provide a practical solution to this problem. In fact, our "3-FAB" probe 2 was proven to be highly useful to investigate the stability and reactivity of the self-immolative disulfide linker system in human blood plasma by ¹⁹F NMR. It has also been revealed that the use of polysorbate 80 as excipient for the formulation of probe 2 dramatically increases the stability of the disulfide linker system. This finding further indicates that the tumor-targeting drug conjugates with polysorbate 80/EtOH/saline formulation for in vivo studies would have high stability in blood plasma, while the drug release in cancer cells proceeds smoothly.

Design, synthesis, and biological evaluation of theranostic vitamin-linker-taxoid conjugates

Novel tumor-targeting theranostic conjugates 1 and 2, bearing either a fluorine-labeled prosthetic as a potential (18)F-PET radiotracer (1) or a fluorescence probe (2) for internalization studies in vitro, were designed and synthesized. We confirmed efficient internalization of 2 in biotin-receptor positive (BR+) cancer cells via receptor-mediated endocytosis (RME) based on flow cytometry and confocal fluorescence microscopy (CFM) analyses, which exhibited very high specificity to BR+ cancer cells. The potency and cancer-cell selectivity of 1 were evaluated against MX-1, L1210FR and ID8 cancer cells (BR+) as well as L1210 cells and WI38 normal human lung fibroblast cells (biotin-receptor negative: BR-). In particular, we designed and performed an assay in the presence of glutathione ethyl ester (GSH-OEt) wherein only 1 molecules internalized into cells via RME in the first 24 h period exert cytotoxic effect. The observed selectivity of 1 was remarkable, with 2 orders of magnitude difference in IC50 values between BR+ cancer cells and WI38 cells, demonstrating a salient feature of this tumor-targeted drug delivery system.

Characterization of in vitro metabolites of TM-2, a potential antitumor drug, in rat, dog and human liver microsomes using liquid chromatography/tandem mass spectrometry

RATIONALE

TM-2 (13-(N-Boc-3-i-butylisoserinoyl-4,10-β-diacetoxy-2-α-benzoyloxy-5-β,20-epoxy-1,13-α-dihydroxy-9-oxo-19-norcyclopropa[g]tax-11-ene) is a novel semi-synthetic taxane derivative. Our previous study demonstrated that it is a promising taxane derivative. The in vitro comparative metabolic profile of a drug between animals and humans is a key issue that should be investigated at early stages of drug development to better select drug candidates. In this study, the in vitro metabolic pathways of TM-2 in rat, dog and human liver microsomes were established and compared.

METHODS

TM-2 was incubated with liver microsomes in the presence of NADPH. Two different types of mass spectrometers - a hybrid linear trap quadrupole orbitrap (LC/LTQ-Orbitrap) mass spectrometer and a triple-quadrupole tandem mass spectrometer (LC/QqQ) were employed to acquire structural information of TM-2 metabolites. Accurate mass measurement using LC/LTQ-Orbitrap was used to determine the accurate mass data and elemental compositions of metabolites thereby confirming the proposed structures of the metabolites. For the chemical inhibition study, selective P450 inhibitors were added to incubations to initially characterize the cytochrome P450 (CYP) enzymes involved in the metabolism of TM-2.

RESULTS

A total of 12 components (M1-M12) were detected and identified as the metabolites of TM-2 in vitro. M1-M5 were formed by hydroxylation of the taxane ring or the lateral chain. Hydroxylated products can be further oxidized to the dihydroxylated metabolites M6-M10. M11 was a trihydroxylated metabolite. M12 was tentatively identified as a carboxylic acid derivative. The metabolism of TM-2 is much the same in all three species with some differences. The chemical inhibition study initially demonstrated that the formation of M2, the major metabolite of TM-2, is mainly mediated by CYP3A4.

CONCLUSIONS

Hydroxylation is the major biotransformation of the TM-2 pathway in vitro. CYP3A4 may play a dominant role in the formation of M2 in liver microsomes. The knowledge of the metabolic pathways of TM-2 is important to support further research of TM-2.

Drug discovery targeting cell division proteins, microtubules and FtsZ

Eukaryotic cell division or cytokinesis has been a major target for anticancer drug discovery. After the huge success of paclitaxel and docetaxel, microtubule-stabilizing agents (MSAs) appear to have gained a premier status in the discovery of next-generation anticancer agents. However, the drug resistance caused by MDR, point mutations, and overexpression of tubulin subtypes, etc., is a serious issue associated with these agents. Accordingly, the discovery and development of new-generation MSAs that can obviate various drug resistances has a significant meaning. In sharp contrast, prokaryotic cell division has been largely unexploited for the discovery and development of antibacterial drugs. However, recent studies on the mechanism of bacterial cytokinesis revealed that the most abundant and highly conserved cell division protein, FtsZ, would be an excellent new target for the drug discovery of next-generation antibacterial agents that can circumvent drug-resistances to the commonly used drugs for tuberculosis, MRSA and other infections. This review describes an account of our research on these two fronts in drug discovery, targeting eukaryotic as well as prokaryotic cell division.

Design, synthesis, and biological evaluations of tumor-targeting dual-warhead conjugates for a taxoid-camptothecin combination chemotherapy

Novel tumor-targeting dual-warhead conjugates, 2 (DW-1) and 3 (DW-2), which consist of a next-generation taxoid, 1 (SB-T-1214), and camptothecin as two warheads, self-immolative disulfide linkers for drug release, biotin as the tumor-targeting moiety, and 1,3,5-triazine as the tripod splitter module, were designed and synthesized. The potency of 2 was evaluated against MX-1, MCF-7, ID8, L1210FR (BR+, biotin receptor overexpressed) and WI38 (BR-, normal) cell lines in the absence and presence of glutathione (GSH), which is an endogenous thiol that triggers drug release inside the cancer cells. With the GSH and resuspension protocol, 2 exhibited IC50 values of 3.22-9.80 nM against all BR+ cancer cell lines, and 705 nM against WI38. Thus, there was a two orders of magnitude higher selectivity to cancer cells. Also, a clear cooperative effect was observed for the taxoid-camptothecin combination when two drugs were delivered to the cancer cells specifically in the form of a dual-warhead conjugate.

Recent progress with microtubule stabilizers: new compounds, binding modes and cellular activities

Nature has yielded numerous classes of chemically distinct microtubule stabilizers. Several of these, including paclitaxel (Taxol) and docetaxel (Taxotere), are important drugs used in the treatment of cancer. New microtubule stabilizers and novel formulations of these agents continue to provide advances in cancer therapy. In this review we cover recent progress in the chemistry and biology of these diverse microtubule stabilizers focusing on the wide range of organisms that produce these compounds, their mechanisms of inhibiting microtubule-dependent processes, mechanisms of drug resistance, and their interactions with tubulin including their distinct binding sites and modes. A new potential role for microtubule stabilizers in neurodegenerative diseases is reviewed.

A structure-based design of new C2- and C13-substituted taxanes: tubulin binding affinities and extended quantitative structure-activity relationships using comparative binding energy (COMBINE) analysis

Ten novel taxanes bearing modifications at the C2 and C13 positions of the baccatin core have been synthesized and their binding affinities for mammalian tubulin have been experimentally measured. The design strategy was guided by (i) calculation of interaction energy maps with carbon, nitrogen and oxygen probes within the taxane-binding site of β-tubulin, and (ii) the prospective use of a structure-based QSAR (COMBINE) model derived from an earlier series comprising 47 congeneric taxanes. The tubulin-binding affinity displayed by one of the new compounds (CTX63) proved to be higher than that of docetaxel, and an updated COMBINE model provided a good correlation between the experimental binding free energies and a set of weighted residue-based ligand-receptor interaction energies for 54 out of the 57 compounds studied. The remaining three outliers from the original training series have in common a large unfavourable entropic contribution to the binding free energy that we attribute to taxane preorganization in aqueous solution in a conformation different from that compatible with tubulin binding. Support for this proposal was obtained from solution NMR experiments and molecular dynamics simulations in explicit water. Our results shed additional light on the determinants of tubulin-binding affinity for this important class of antitumour agents and pave the way for further rational structural modifications.

Fluorine-Containing Taxoid Anticancer Agents and Their Tumor-Targeted Drug Delivery

A long-standing problem of conventional chemotherapy is the lack of tumor-specific treatments. Traditional chemotherapy relies on the premise that rapidly proliferating cancer cells are more likely to be killed by a cytotoxic agent. In reality, however, cytotoxic agents have very little or no specificity, which leads to systemic toxicity, causing undesirable severe side effects. Consequently, various "molecularly targeted cancer therapies" have been developed for use in specific cancers, including tumor-targeting drug delivery systems. In general, such a drug delivery system consists of a tumor recognition moiety and a cytotoxic "warhead" connected through a "smart" linker to form a conjugate. When a multi-functionalized nanomaterial is used as the vehicle, a "Trojan Horse" approach can be used for mass delivery of cytotoxic "warheads" to maximize the efficacy. Exploitation of the special properties of fluorine has proven successful in the development of new and effective biochemical tools as well as therapeutic agents. Fluorinated congeners can also serve as excellent probes for the investigation of biochemical mechanisms. ¹⁹F-NMR can provide unique and powerful tools for mechanistic investigations in chemical biology. This account presents our recent progress, in perspective, on the molecular approaches to the design and development of novel tumor-targeted drug delivery systems for new generation chemotherapy by exploiting the unique nature of fluorine.

Asymmetric synthesis of anti-β-amino-α-hydroxy esters via dynamic kinetic resolution of β-amino-α-keto esters

A method for the asymmetric synthesis of enantioenriched anti-α-hydroxy-β-amino acid derivatives by enantioconvergent reduction of the corresponding racemic α-keto esters is presented. The requisite α-keto esters are prepared via Mannich addition of ethyl diazoacetate to imines followed by oxidation of the diazo group with Oxone. Implementation of a recently developed dynamic kinetic resolution of β-substituted-α-keto esters via Ru(II)-catalyzed asymmetric transfer hydrogenation provides the title motif in routinely high diastereo- and enantioselectivity.

Small molecule, big difference: the role of water in the crystallization of paclitaxel

Paclitaxel is an important antineoplastic drug, which is used widely in the treatment of many forms of cancer. The crystal structures of the anhydrous form and the hemihydrate were determined from laboratory X-ray powder diffraction data, whereas the dihydrate was solved from single-crystal synchrotron diffraction data. Intermolecular spaces allow for the inclusion of loosely bound water molecules, which are then lost easily upon heating. All three forms were found to crystallize in the orthorhombic spacegroup P2(1)2(1)2(1), with Z' = 2. The unit cell parameters were found to be a = 9.6530(3) Å, b = 28.1196(8) Å, c = 33.5378(14) Å, and V = 9103.5(5) Å for the anhydrous form (363 K); a = 9.6890(2) Å, b = 28.0760(4) Å, c = 33.6166(8) Å, and V = 9144.7(3) Å(3) for the hemihydrate (333 K); and a = 9.512(6) Å, b = 28.064(16) Å, c = 33.08(2) Å, and V = 8829.0(9) Å(3) for the dihydrate (120 K). Water loss occurs in two steps between 120 K ≤ t ≤ 363 K. The thermal stability of the hydrates and accompanying unit cell changes were observed in situ via temperature-resolved X-ray powder diffraction and thermogravimetric analysis.

Molecular simulations of drug–receptor complexes in anticancer research

Molecular modeling and computer simulation techniques have matured significantly in recent years and proved their value in the study of drug–DNA, drug–DNA–protein, drug–protein and protein–protein interactions. Evolution in this area has gone hand-in-hand with an increased availability of structural data on biological macromolecules, major advances in molecular mechanics force fields and considerable improvements in computer technologies, most significantly processing speeds, multiprocessor programming and data-storage capacity. The information derived from molecular simulations of drug–receptor complexes can be used to extract structural and energetic information that is usually beyond current experimental possibilities, provide independent accounts of experimentally observed behavior, help in the interpretation of biochemical or pharmacological results, and open new avenues for research by posing novel relevant questions that can guide the design of new experiments. As drug-screening tools, ligand- and fragment-docking platforms stand out as powerful techniques that can provide candidate molecules for hit and lead development. This review provides an overall perspective of the main methods and focuses on some selected applications to both classical and novel anticancer targets.

Analogue-based drug discovery: Contributions to medicinal chemistry principles and drug design strategies. Microtubule stabilizers as a case in point (Special Topic Article)

The benefits of utilizing marketed drugs as starting points to discover new therapeutic agents have been well documented within the IUPAC series of books that bear the title Analogue-based Drug Discovery (ABDD). Not as clearly demonstrated, however, is that ABDD also contributes to the elaboration of new basic principles and alternative drug design strategies that are useful to the field of medicinal chemistry in general. After reviewing the ABDD programs that have evolved around the area of microtubule-stabilizing chemo-therapeutic agents, the present article delineates the associated research activities that additionally contributed to general strategies that can be useful for prodrug design, identifying pharmacophores, circumventing multidrug resistance (MDR), and achieving targeted drug distribution.

Abietane diterpenes induce cytotoxic effects in human pancreatic cancer cell line MIA PaCa-2 through different modes of action

Abietane diterpenes, especially those containing quinone moieties, are often reported to have cytotoxic effects on cancer cell lines. They deserve greater attention because several cancer chemotherapeutic agents also possess the quinone structural feature. To date, very little is known about their cytotoxic molecular modes of action. In the present study, five diterpenes, 7 alpha-acetoxyroyleanone, horminone, royleanone, 7-ketoroyleanone and sugiol which have been previously isolated from the medicinal plant Peltodon longipes were shown to possess cytotoxic activity against the human pancreatic cancer cell line MIA PaCa-2. 7 alpha-Acetoxyroyleanone, horminone and royleanone were demonstrated to possess alkylating properties using the nucleophile 4-(4-nitrobenzyl)pyridine. However, no clear correlation between the alkylating properties and cytotoxicity of these diterpenes was observed. Furthermore, the relaxation activity of human DNA topoisomerases I and II was found to be influenced by these compounds, with 7-ketoroyleanone and sugiol being the most active. These two diterpenes preferentially inhibited topoisomerase I and exhibited lower IC(50) values than the classical topoisomerase I inhibitor camptothecin. Molecular docking studies revealed possible interactions of diterpenes with topoisomerase I, indicating that these compounds do not form the drug-enzyme-DNA covalent ternary complex as observed with camptothecin. A binding pocket located at the surface of the DNA-interaction site was proposed. Moreover, the ability of the five diterpenes to generate DNA-strand breaks in single cells was confirmed using the alkaline comet assay. As expected, these diterpenes also influenced cell cycle progression and arrested cells in different phases of the cell cycle, primarily the G1/G0 and S-phases. Interestingly, the diterpenes only exhibited a slight ability to induce apoptotic cell death and failed to generate intracellular reactive oxygen species. These results provide additional understanding of the cytotoxic effects of abietane diterpenes. Depending on their functional groups, we propose that abietane diterpenes utilise different mechanisms to induce cell death.