Synthesis and structure-activity relationship study of antimicrotubule agents phenylahistin derivatives with a didehydropiperazine-2,5-dione structure

Design, synthesis, and structure-activity relationship study of novel plinabulin derivatives as anti-tumor agents based on the co-crystal structure

Plinabulin, a 2, 5-diketopiperazine-type tubulin inhibitor derived from marine natural products, is currently undergoing Phase III clinical trials for the treatment of non-small cell lung cancer (NSCLC) and chemotherapy-induced neutropenia (CIN). To obtain novel 2, 5-diketopiperazine derivatives with higher biological activity, we designed and synthesized two series of 37 plinabulin derivatives at the C-ring, based on the co-crystal structure of compound 1 and tubulin. Their structures were characterized using NMR and HRMS. All compounds were screened in vitro using the lung cancer cell line NCI-H460 using the MTT method, and the compounds with better activity were further screened in BxPC-3 and HT-29 cells. The compounds 16c (IC50 = 2.0, NCI-H460; IC50 = 1.2 nM, BxPC-3; IC50 = 1.97 nM, HT-29) and 26r (IC50 = 0.96, NCI-H460; IC50 = 0.66 nM, BxPC-3; IC50 = 0.61 nM, HT-29) had the best activity. The cytotoxic activity of compound 26r against various tumor cell lines occurred at less than 1 nM.

Design and synthesis of novel 3-amino-5-phenylpyrazole derivatives as tubulin polymerization inhibitors targeting the colchicine-binding site

As the basic unit of microtubules, tubulin is one of the most important targets in the study of anticarcinogens. A novel series of 3-amino-5-phenylpyrazole derivatives were designed and synthesized, and evaluates for their biological activities. Among them, a majority of compounds exerted excellent inhibitory activities against five cancer cell lines in vitro. Especially, compound 5b showed a strong antiproliferative activity against MCF-7 cells, with IC50 value of 38.37 nM. Further research indicated that compound 5b can inhibit the polymerization of tubulin targeting the tubulin colchicine-binding sites. Furthermore, 5b could arrest MCF-7 cells at the G2/M phase and induce MCF-7 cells apoptotic in a dose-dependent and time-dependent manners, and regulate the level of related proteins expression. Besides, compound 5b could inhibit the cancer cell migration and angiogenesis. In addition, 5b could inhibit tumor growth in MCF-7 xenograft model without obvious toxicity. All these results indicating that 5b could be a promising antitumor agent targeting tubulin colchicine-binding site and it was worth further study.

Dehydroamino acid residues in bioactive natural products

Covering: 2000 to up to 2023α,β-Dehydroamino acids (dhAAs) are unsaturated nonproteinogenic amino acids found in a wide array of naturally occurring peptidyl metabolites, predominantly those from bacteria. Other organisms, such as fungi, higher plants and marine invertebrates, have also been found to produce dhAA-containing peptides. The α,β-unsaturation in dhAAs has profound effects on the properties of these molecules. They display significant synthetic flexibility, readily undergoing reactions such as Michael additions, transition-metal-catalysed cross-couplings, and cycloadditions. These residues in peptides/proteins also exhibit great potential in bioorthogonal applications using click chemistry. Peptides containing contiguous dhAA residues have been extensively investigated in the field of foldamers, self-assembling supermolecules that mimic biomacromolecules such as proteins to fold into well-defined conformations. dhAA residues in these peptidyl materials tend to form a 2.05-helix. As a result, stretches of dhAA residues arrange in an extended conformation. In particular, peptidyl foldamers containing β-enamino acid units display interesting conformational, electronic, and supramolecular aggregation properties that can be modulated by light-dependent E-Z isomerization. Among approximately 40 dhAAs found in the natural product inventory, dehydroalanine (Dha) and dehydrobutyrine (Dhb) are the most abundant. Dha is the simplest dehydro-α-amino acid, or α-dhAA, without any geometrical isomers, while its re-arranged isomer, 3-aminoacrylic acid (Aaa or ΔβAla), is the simplest dehydro-β-amino acid, or β-enamino acid, and displays E/Z isomerism. Dhb is the simplest α-dhAA that exhibits E/Z isomerism. The Z-isomer of Dhb (Z-Dhb) is sterically favourable and is present in the majority of naturally occurring peptides containing Dhb residues. Dha and Z-Dhb motifs are commonly found in ribosomally synthesized and post-translationally modified peptides (RiPPs). In the last decade, the formation of Dha and Dhb motifs in RiPPs has been extensively investigated, which will be briefly discussed in this review. The formation of other dhAA residues in natural products (NPs) is, however, less understood. In this review, we will discuss recent advances in the biosynthesis of peptidyl NPs containing unusual dhAA residues and cryptic dhAA residues. The proposed biosynthetic pathways of these natural products will also be discussed.

2,5-Diketopiperazines (DKPs): Promising Scaffolds for Anticancer Agents

2,5-Diketopiperazine (2,5-DKP) derivatives represent a family of secondary metabolites widely produced by bacteria, fungi, plants, animals, and marine organisms. Many natural products with DKP scaffolds exhibited various pharmacological activities such as antiviral, antifungal, antibacterial, and antitumor. 2,5-DKPs are recognized as privileged structures in medicinal chemistry, and compounds that incorporate the 2,5-DKP scaffold have been extensively investigated for their anticancer properties. This review is a thorough update on the anti-cancer activity of natural and synthesized 2,5-DKPs from 1997 to 2022. We have explored various aspects of 2,5-DKPs modifications and summarized their structure-activity relationships (SARs) to gain insight into their anticancer activities. We have also highlighted the novel approaches to enhance the specificity and pharmacokinetics of 2,5-DKP-based anticancer agents.

Design, synthesis and anti-tumor evaluation of plinabulin derivatives as potential agents targeting β-tubulin

Plinabulin is a promising microtubule destabilizing agent in phase 3 clinical stage for treating non-small cell lung cancer. However, the high toxicity and the poor water solubility of plinabulin limited its use and more plinabulin derivatives need to be explored. Here, two series of 29 plinabulin derivatives were designed, synthesized and evaluated for their anti-tumor effect against three types of cancer cell lines. Most of derivatives exerted obvious inhibition to the proliferation of the cell lines tested. Among them, compound 11c exerted stronger efficiency than plinabulin, and the reason might be the additional hydrogen bond between the nitrogen atom of the indole ring in compound 11c and Gln134 of β-tubulin. Immunofluorescence assay showed that compound 11c at 10 nM significantly disrupted tubulin structure. Compound 11c also significantly induced G2/M cell cycle arrest and apoptosis in dose dependent manner. These results suggest that compound 11c might be a potential candidate for cancer treatment as antimicrotubule agent.

Natural epidithiodiketopiperazine alkaloids as potential anticancer agents: Recent mechanisms of action, structural modification, and synthetic strategies

Cancer has become a grave health crisis that threatens the lives of millions of people worldwide. Because of the drawbacks of the available anticancer drugs, the development of novel and efficient anticancer agents should be encouraged. Epidithiodiketopiperazine (ETP) alkaloids with a 2,5-diketopiperazine (DKP) ring equipped with transannular disulfide or polysulfide bridges or S-methyl moieties constitute a special subclass of fungal natural products. Owing to their privileged sulfur units and intriguing architectural structures, ETP alkaloids exhibit excellent anticancer activities by regulating multiple cancer proteins/signaling pathways, including HIF-1, NF-κB, NOTCH, Wnt, and PI3K/AKT/mTOR, or by inducing cell-cycle arrest, apoptosis, and autophagy. Furthermore, a series of ETP alkaloid derivatives obtained via structural modification showed more potent anticancer activity than natural ETP alkaloids. To solve supply difficulties from natural resources, the total synthetic routes for several ETP alkaloids have been designed. In this review, we summarized several ETP alkaloids with anticancer properties with particular emphasis on their underlying mechanisms of action, structural modifications, and synthetic strategies, which will offer guidance to design and innovate potential anticancer drugs.

Design, Synthesis, and Anticancer Activity of Novel 3,6-Diunsaturated 2,5-Diketopiperazines

Based on the marine natural products piperafizine B, XR334, and our previously reported compound 4m, fourteen novel 3,6-diunsaturated 2,5-diketopiperazine (2,5-DKP) derivatives (1, 2, 4-6, 8-16), together with two known ones (3 and 7), were designed and synthesized as anticancer agents against the A549 and Hela cell lines. The MTT assay results showed that the derivatives 6, 8-12, and 14 had moderate to good anticancer capacities, with IC₅₀ values ranging from 0.7 to 8.9 μM. Among them, compound 11, with naphthalen-1-ylmethylene and 2-methoxybenzylidene functions at the 3 and 6 positions of 2,5-DKP ring, respectively, displayed good inhibitory activities toward both A549 (IC₅₀ = 1.2 μM) and Hela (IC₅₀ = 0.7 μM) cancer cells. It could also induce apoptosis and obviously block cell cycle progression in the G2/M phases in both cells at 1.0 μM. The electron-withdrawing functions might not be favorable for the derivatives with high anticancer activities. Additionally, compared to piperafizine B and XR334, these semi-N-alkylated derivatives have high liposolubilities (>1.0 mg mL^-1). Compound 11 can be further developed, aiming at the discovery of a novel anticancer candidate.

Design and Synthesis of Novel Phenylahistin Derivatives Based on Co-Crystal Structures as Potent Microtubule Inhibitors for Anti-Cancer Therapy

Phenylahistin is a naturally occurring marine product with a diketopiperazine structure that can bind to the colchicine site of microtubulin as a possible anticancer agent. To develop more potent microtubule inhibitors, novel phenylahistin derivatives were designed and synthesized based on the co-crystal complexes of phenylahistin derivatives and microtubulin. We established a focused library of imidazole-type molecules for the introduction of different groups to the C-ring and A-ring of phenylahistin. Structure-activity relationship studies indicated that appropriate hydrocarbon substituents and unsaturated alkenyl substituents at the 1-position of the imidazole group are important for improving the activity of such compounds. In addition, this study found that propylamine groups could maintain the activity of these compounds, as exemplified by compound 16d (IC₅₀ = 5.38 nM, NCI-H460). Compound 15p (IC₅₀ = 1.03 nM, NCI-H460) with an allyl group exhibited potent cytotoxic activity at the nanomolar level against human lung cancer cell lines. Immunofluorescence assay indicated that compound 15p could efficiently inhibited microtubule polymerization and induced a high expression of caspase-3. 15p also displayed good pharmacokinetic characteristics in vitro. Additionally, the growth of H22 transplanted tumors was significantly inhibited in BALB/c mice when 15p alone was administered at 4 mg/kg, and the tumor inhibition rate was as much as 65%. Importantly, the continuous administration of 15p resulted in a lower toxicity than that of docetaxel (10 mg/kg) and cyclophosphamide (20 mg/kg). Overall, the novel allyl-imidazole-diketopiperazine-type derivatives could be considered safe and effective potential agents for cancer treatment.

An intramolecular hydrogen bond-promoted "green" Ugi cascade reaction for the synthesis of 2,5-diketopiperazines with anticancer activity

We report a "green chemistry"-based Ugi cascade reaction to furnish a series of 2,5-diketopiperazines (through nucleophilic attack of amides upon ketones in Ugi adducts) at moderate-to-good yields. Investigation with the MTT assay revealed compound (±) 5c to exhibit potent anticancer activities against acute myeloid leukaemia (MV411; IC₅₀ = 1.7 μM) and acute T lymphocyte leukaemia (Jurkat; IC₅₀ = 5.7 μM) cell lines.

In Vitro Identification of Potential Metabolites of Plinabulin (NPI 2358) in Hepatic Preparations Using Liquid Chromatography-Ion Trap Mass Spectrometry

Plinabulin (1, NPI2358), a vascular disrupting agent (VDA) molecule, is a synthetic analogue of the natural product phenylahistin (2, NPI 2350), which is isolated from Aspergillus ustus. Evaluation of the in vitro metabolic profile of VDA plinabulin using human liver microsomes (HLMs) and HepaRG Cells Cryopreserved is described. HLMs and HepaRG Cells Cryopreserved were prepared in-house and incubated with plinabulin according to published methodologies. The incubated mixtures were analyzed by liquid chromatography-ion trap mass spectrometry to identify possible metabolic products. The incubated plinabulin (1) revealed the presence of several peaks representing 19 tentative metabolites in HLMs and HepaRG Cells Cryopreserved in the presence of NADPH (nicotinamide adenine dinucleotide phosphate) and in the absence of NADPH-generating system, respectively. However, in NADPH absence, no metabolites and microsomes were generated for 1 in incubated HLMs, indicating a likely involvement of CYP450 enzymes in the metabolism. The metabolite structures, obtained from HLMs and HepaRG Cells Cryopreserved incubations, were elucidated by LC-MS/MS fragmentation study. Seventeen phase-I metabolites were proposed to be the results of isomerization, hydroxylation, hydration, and oxygenation of 1 in HLMs and two isomeric phase-II sulfate conjugate metabolites of 1 in HepaRG Cells Cryopreserved incubation.

Molecular interactions at the colchicine binding site in tubulin: An X-ray crystallography perspective

Tubulin is an important cancer drug target. Compounds that bind at the colchicine site in tubulin have attracted significant interest as they are generally less affected by multidrug resistance than other potential drugs. Modeling is useful in understanding the interactions between tubulin and colchicine binding site inhibitors (CBSIs), but because the colchicine binding site contains two flexible loops whose conformations are highly ligand-dependent, modeling has its limitations. X-ray crystallography provides experimental pictures of tubulin-ligand interactions at this challenging colchicine site. Since 2004, when the first X-ray structure of tubulin in complex with N-deacetyl-N-(2-mercaptoacetyl)-colchicine (DAMA-colchicine) was published, many X-ray crystal structures have been reported for tubulin complexes involving the colchicine binding site. In this review, we summarize the crystal structures of tubulin in complexes with various CBSIs, aiming to facilitate the discovery of new generations of tubulin inhibitors.

Histamine H3 receptor antagonists with peptidomimetic (keto)piperazine structures to inhibit Aβ oligomerisation

Alzheimeŕs disease (AD) is the most prominent neurodegenerative disorder with high medical need. Protein-protein-interactions (PPI) interactions have a critical role in AD where β-amyloid structures (Aβ) build toxic oligomers. Design of disease modifying multi target directed ligand (MTDL) has been performed, which disable PPI on the one hand and on the other hand, act as procognitive antagonists at the histamine H₃ receptor (H₃R). The synthetized compounds are structurally based on peptidomimetic amino acid-like structures mainly as keto, diketo-, or acyl variations of a piperazine moiety connected to an H₃R pharmacophore. Most of them showed low nanomolar affinities at H₃R and some with promising affinity to Aβ-monomers. The structure-activity relationships (SAR) described offer new possibilities for MTDL with an optimized profile combining symptomatic and potential causal therapeutic approaches in AD.

Cyclic Dipeptide: A Privileged Molecular Scaffold to Derive Structural Diversity and Functional Utility

Cyclic dipeptides (CDPs) are the simplest form of cyclic peptides with a wide range of applications from therapeutics to biomaterials. CDP is a versatile molecular platform endowed with unique properties such as conformational rigidity, intermolecular interactions, structural diversification through chemical synthesis, bioavailability and biocompatibility. A variety of natural products with the CDP core exhibit anticancer, antifungal, antibacterial, and antiviral activities. The inherent bioactivities have inspired the development of synthetic analogues as drug candidates and drug delivery systems. CDP plays a crucial role as conformation and molecular assembly directing core in the design of molecular receptors, peptidomimetics and fabrication of functional material architectures. In recent years, CDP has rapidly become a privileged scaffold for the design of advanced drug candidates, drug delivery agents, bioimaging, and biomaterials to mitigate numerous disease conditions. This review describes the structural diversification and multifarious biomedical applications of the CDP scaffold, discusses challenges, and provides future directions for the emerging field.

Modifications of diketopiperazines assembled by cyclodipeptide synthases with cytochrome P450 enzymes

2,5-Diketopiperazines are the smallest cyclic peptides comprising two amino acids connected via two peptide bonds. They can be biosynthesized in nature by two different enzyme families, either by nonribosomal peptide synthetases or by cyclodipeptide synthases. Due to the stable scaffold of the diketopiperazine ring, they can serve as precursors for further modifications by different tailoring enzymes, such as methyltransferases, prenyltransferases, oxidoreductases like cyclodipeptide oxidases, 2-oxoglutarate-dependent monooxygenases and cytochrome P450 enzymes, leading to the formation of intriguing secondary metabolites. Among them, cyclodipeptide synthase-associated P450s attracted recently significant attention, since they are able to catalyse a broader variety of astonishing reactions than just oxidation by insertion of an oxygen. The P450-catalysed reactions include hydroxylation at a tertiary carbon, aromatisation of the diketopiperazine ring, intramolecular and intermolecular carbon-carbon and carbon-nitrogen bond formation of cyclodipeptides and nucleobase transfer reactions. Elucidation of the crystal structures of three P450s as cyclodipeptide dimerases provides a structural basis for understanding the reaction mechanism and generating new enzymes by protein engineering. This review summarises recent publications on cyclodipeptide modifications by P450s.Key Points• Intriguing reactions catalysed by cyclodipeptide synthase-associated cytochrome P450s• Homo- and heterodimerisation of diketopiperazines• Coupling of guanine and hypoxanthine with diketopiperazines.

Discovery of novel N-benzylbenzamide derivatives as tubulin polymerization inhibitors with potent antitumor activities

A series of novel N-benzylbenzamide derivatives were designed and synthesized as tubulin polymerization inhibitors. Among fifty-one target compounds, compound 20b exhibited significant antiproliferative activities with IC₅₀ values ranging from 12 to 27 nM against several cancer cell lines, and possessed good plasma stability and satisfactory physicochemical properties. Mechanism studies demonstrated that 20b bound to the colchicine binding site and displayed potent anti-vascular activity. Notably, the corresponding disodium phosphate 20b-P exhibited an excellent safety profile with the LD₅₀ value of 599.7 mg/kg (i.v. injection), meanwhile, it significantly inhibited tumor growth and decreased microvessel density in liver cancer cell H22 allograft mouse model without obvious toxicity. Collectively, 20b and 20b-P are novel promising anti-tubulin agents with more druggable properties and deserve to be further investigated for cancer therapy.

Selective release of a potent anticancer agent from a supramolecular hydrogel using green light

Selective green-light triggered release of an anticancer agent under physiological conditions from a supramolecular hydrogel.

Structure-based design and synthesis of novel furan-diketopiperazine-type derivatives as potent microtubule inhibitors for treating cancer

Plinabulin, a synthetic analog of the marine natural product "diketopiperazine phenylahistin," displayed depolymerization effects on microtubules and targeted the colchicine site, which has been moved into phase III clinical trials for the treatment of non-small cell lung cancer (NSCLC) and the prevention of chemotherapy-induced neutropenia (CIN). To develop more potent anti-microtubule and cytotoxic derivatives, the co-crystal complexes of plinabulin derivatives were summarized and analyzed. We performed further modifications of the tert-butyl moiety or C-ring of imidazole-type derivatives to build a library of molecules through the introduction of different groups for novel skeletons. Our structure-activity relationship study indicated that compounds 17o (IC₅₀ = 14.0 nM, NCI-H460) and 17p (IC₅₀ = 2.9 nM, NCI-H460) with furan groups exhibited potent cytotoxic activities at the nanomolar level against various human cancer cell lines. In particular, the 5-methyl or methoxymethyl substituent of furan group could replace the alkyl group of imidazole at the 5-position to maintain cytotoxic activity, contradicting previous reports that the tert-butyl moiety at the 5-position of imidazole was essential for the activity of such compounds. Immunofluorescence assay indicated that compounds 17o and 17p could efficiently inhibit microtubule polymerization. Overall, the novel furan-diketopiperazine-type derivatives could be considered as a potential scaffold for the development of anti-cancer drugs.

Comparative studies on similarities and differences of cyclodipeptide oxidases for installation of C-C double bonds at the diketopiperazine ring

Cyclodipeptide oxidases (CDOs) perform dehydrogenations on diketopiperazines and play an important role in the cyclodipeptide diversification. In this study, we investigated the two known CDOs AlbA/B and Ndas_1146/7 and one new member, CDO-Np. LC-MS monitoring of 32 cyclodipeptide biotransformations in E. coli revealed good consumption of cyclodipeptides containing aromatic amino acids. Cyclodipeptides consisting solely of aliphatic amino acids were poor substrates. In vitro assays of 34 substrates with crude enzyme extracts and product identification proved that the CDO-Np-containing extract catalyzes the formation of two C-C double bonds in many cases. The extracts containing the two other enzymes had lower activities and catalyzed mainly didehydrogenations. For didehydrogenation, the phenylalanyl or tyrosyl site was usually preferred. No or very low acceptance of benzodiazepinediones and a 2,6-diketopiperazine proved the importance of the 2,5-diketopiperazine ring. N-Methylation at the diketopiperazine ring or prenylation of the tryptophan-containing cyclodipeptides influences the enzyme activity and product spectrum. KEY POINTS: • Comparison of catalytic activities of three enzymes; Diverse cyclodipeptides and derivatives as substrates; Determination of double bond formation using²H-labeled substrates; Product identification also by interpretation of MS²fragmentation pattern.

Development of novel phenoxy-diketopiperazine-type plinabulin derivatives as potent antimicrotubule agents based on the co-crystal structure

The co-crystal structure of Compound 6b with tubulin was prepared and solved for indicating the binding mode and for further optimization. Based on the co-crystal structures of tubulin with plinabulin and Compound 6b, a total of 27 novel A/B/C-rings plinabulin derivatives were designed and synthesized. Their biological activities were evaluated against human lung cancer NCI-H460 cell line. The optimum phenoxy-diketopiperazine-type Compound 6o exhibited high potent cytotoxicity (IC₅₀ = 4.0 nM) through SAR study of three series of derivatives, which was more potent than plinabulin (IC₅₀ = 26.2 nM) and similar to Compound 6b (IC₅₀ = 3.8 nM) against human lung cancer NCI-H460 cell line. Subsequently, the Compound 6o was evaluated against other four human cancer cell lines. Both tubulin polymerization assay and immunofluorescence assay showed that Compound 6o could inhibit microtubule polymerization efficiently. Furthermore, theoretical calculation of the physical properties and molecular docking were elucidated for these plinabulin derivatives. The binding mode of Compound 6o was similar to Compound 6b based on the result of molecular docking. The theoretical calculated LogPo/w and PCaco of Compound 6o were better than Compound 6b, which could enhance its cytostatic activity. Therefore, Compound 6o might be developed as a novel potent anti-microtubule agent.

Photopiperazines A-D, Photosensitive Interconverting Diketopiperazines with Significant and Selective Activity against U87 Glioblastoma Cells, from a Rare, Marine-Derived Actinomycete of the Family Streptomycetaceae

Photopiperazines A-D (1-4), unsaturated diketopiperazine derivatives, were isolated from the culture broth of a rare, marine-derived actinomycete bacterium, strain AJS-327. This strain shows very poor 16S rRNA sequence similarity to other members of the actinomycete family Streptomycetaceae, indicating it is likely a new lineage within this group. The structures of the photopiperazines were defined by analysis of HR-ESI-TOF-MS spectra in conjunction with the interpretation of 1D and 2D NMR data. The photopiperazines are sensitive to light, causing interconversion among the four olefin geometrical isomers, which made purification of each isomer challenging. The photopiperazines are highly cytotoxic metabolites that show selective toxicity toward U87 glioblastoma and SKOV3 ovarian cancer cell lines.

Polymorphs, co-crystal structure and pharmacodynamics study of MBRI-001, a deuterium-substituted plinabulin derivative as a tubulin polymerization inhibitor

MBRI-001, a deuterium-substituted plinabulin derivative, has been reported to have better pharmacokinetic and similar antitumor effects in comparison with plinabulin. In this approach, we further carried out its polymorphs, co-crystal structure of MBRI-001-tubulin and tubulin inhibition study. Among the different polymorphs, Form F (MBRI-001/H₂O) was prepared and evaluated, which had better physical stability and suitable process for scale-up production. Co-crystal structure of MBRI-001-tubulin (PDB:5XI5) was prepared and analyzed. The result of tubulin polymerization assay demonstrated that MBRI-001 could inhibit tubulin polymerization which was similar as plinabulin. Subsequently, the anti-proliferative activities of plinabulin and MBRI-001 were evaluated against two different human lung cancer cell lines. In vivo study, MBRI-001 revealed similar antitumor inhibition in comparison with plinabulin in A549 xenograft tumor model. Therefore, we suggested that MBRI-001 could be developed as a promising anti-cancer agent in near future.

Benzophenone: a ubiquitous scaffold in medicinal chemistry

The benzophenone scaffold represents a ubiquitous structure in medicinal chemistry because it is found in several naturally occurring molecules which exhibit a variety of biological activities, such as anticancer, anti-inflammatory, antimicrobial, and antiviral. In addition, various synthetic benzophenone motifs are present in marketed drugs. They also represent important ingredients in perfumes and can act as photoinitiators. This review will provide an overview of benzophenone moieties with medicinal aspects synthesized in the last 15 years and will cover the most potent molecule in each report. In this review, only benzophenones with substitutions on their aryl rings, i.e. diphenyl ketone analogues, have been covered.

In vitro and in vivo pharmacokinetic and pharmacodynamic study of MBRI-001, a deuterium-substituted plinabulin derivative as a potent anti-cancer agent

MBRI-001 was demonstrated preliminary better pharmacokinetics and antitumor effects than that of plinabulin in vivo. In this approach, we further carried out systematic pharmacokinetic and pharmacodynamic study of MBRI-001 in vitro and in vivo. MBRI-001 was tested stable in rat plasma and more stable in liver microsomes than plinabulin in vitro. In vivo, MBRI-001 could be distributed rapidly and widely in various tissues, especially the concentration of MBRI-001 in lung was remarkably higher than other tissues. Excretion study indicated that MBRI-001 might been decomposed and excreted as metabolites. Additionally, the combination treatment of MBRI-001 and gefitinib revealed better antitumor inhibition rate than monotherapy in vivo. Therefore, we suggest that MBRI-001 could be developed as a promising anti-cancer agent in near future.

Review of Chromatographic Bioanalytical Assays for the Quantitative Determination of Marine-Derived Drugs for Cancer Treatment

The discovery of marine-derived compounds for the treatment of cancer has seen a vast increase over the last few decades. Bioanalytical assays are pivotal for the quantification of drug levels in various matrices to construct pharmacokinetic profiles and to link drug concentrations to clinical outcomes. This review outlines the different analytical methods that have been described for marine-derived drugs in cancer treatment hitherto. It focuses on the major parts of the bioanalytical technology, including sample type, sample pre-treatment, separation, detection, and quantification.

Synthesis of monoalkylidene diketopiperazines and application to the synthesis of barettin

The synthesis of barettin, a selective serotonin receptor inhibitor and potent antibiofouling natural product, is described. The synthesis starts with the diketopiperazine nucleus intact and the side chains are installed using iterative aldol condensations. The route represents a general strategy for synthesis of a wide array of mono-alkylidene diketopiperazine structures, including those derived from non-canonical amino acid residues.

Design, synthesis and biological evaluation of anti-pancreatic cancer activity of plinabulin derivatives based on the co-crystal structure

Based on the co-crystal structures of tubulin with plinabulin and Compound 1 (a derivative of plinabulin), a total of 18 novel plinabulin derivatives were designed and synthesized. Their biological activities were evaluated against human pancreatic cancer BxPC-3 cell lines. Two novel Compounds 13d and 13e exhibited potent activities with IC₅₀ at 1.56 and 1.72 nM, respectively. The tubulin polymerization assay indicated that these derivatives could inhibit microtubule polymerization. Furthermore, the interaction between tubulin and these compounds were elucidated by molecular docking. The binding modes of Compounds 13d and 13e were similar to the co-crystal structure of Compound 1. H-π interaction was observed between the aromatic hydrogen of thiophene moiety with Phe20, which could enhance their binding affinities.

Biological activity and interaction mechanism of the diketopiperazine derivatives as tubulin polymerization inhibitors

Microtubules are a favorable target for development of anticancer agents. In this study, the anti-proliferative activities of plinabulin and six diketopiperazine derivatives were evaluated against human lung cancer cell line NCI-H460 and human pancreatic cancer cell line BxPC-3. The inhibition activities on these microtubules were assessed by tubulin polymerization and immunofluorescence assays. To gain insight into the interaction mechanism of the derivatives and tubulin, a molecular dynamics simulation was performed. We discovered that the diketopiperazine derivatives could prevent tubulin assembly through conformational changes. Molecular Mechanics/Poisson–Boltzmann Surface Area (MM-PBSA) calculations showed that the trend of the binding free energies of these inhibitors was in agreement with the trend of their biological activities. Introducing hydrophobic groups into the A-ring was favorable for binding. Energy decomposition indicated that van der Waals interaction played an essential role in the binding affinity of tubulin polymerization inhibitors. In addition, the key residues responsible for inhibitor binding were identified. In summary, this study provided valuable information for development of novel tubulin polymerization inhibitors as anticancer agents.

Microtubules are a favorable target for development of anticancer agents.

Mutations of Residues in Pocket P1 of a Cyclodipeptide Synthase Strongly Increase Product Formation

Expression of a cyclodipeptide synthase gene from Nocardiopsis prasina (CDPS-Np) in Escherichia coli resulted in the formation of cyclo-(l-Tyr-l-Tyr) (1) as the minor and cyclo-(l-Tyr-l-Phe) (2) as the major products. Site-directed mutagenesis revealed a strong influence on product accumulation of the amino acid residues in pocket P1. An 8-fold increase in product formation for 1 and 10-fold for 2 were detected in the double mutant T82V_Y196F compared with the wild type.

Tubulin inhibitors targeting the colchicine binding site: a perspective of privileged structures

The vital roles of microtubule in mitosis and cell division make it an attractive target for antitumor therapy. Colchicine binding site of tubulin is one of the most important pockets that have been focused on to design tubulin-destabilizing agents. Over the past few years, a large number of colchicine binding site inhibitors (CBSIs) have been developed inspired by natural products or synthetic origins, and many moieties frequently used in these CBSIs are structurally in common. In this review, we will classify the CBSIs into classical CBSIs and nonclassical CBSIs according to their spatial conformations and binding modes with tubulin, and highlight the privileged structures from these CBSIs in the development of tubulin inhibitors targeting the colchicine binding site.

Development of MBRI-001, a deuterium-substituted plinabulin derivative as a potent anti-cancer agent

Plinabulin, a drug targeting microtubule of cancer cells, has been currently tried in its phase III clinical study. However, low efficacy caused by poor pharmacokinetic (PK) properties has been considered to be the main obstacle to approved by the Food and Drug Administration. Herein, we introduced a deuterium atom as an isostere in its structure to become a new compound named (MBRI-001, No. 9 in a series of deuterium-substituted compounds). The structure of MBRI-001 was characterized by HRMS, NMR, IR and a single crystal analysis. MBRI-001 exhibited better pharmacokinetic characteristics than that of plinabulin. Additionally, its antitumor activity is in a low nanomolar level for a variety of cancer cell lines and high activity against human NCI-H460 xenograted in mice intravenous administration. Importantly, continuous administration of MBRI-001 exhibited lower toxicity compared to docetaxel. We thus suggest that MBRI-001 could be developed as a promising anti-cancer agent in near future.

Isomethoxyneihumicin, a new cytotoxic agent produced by marine Nocardiopsis alba KM6-1

A new cytotoxic agent designated isomethoxyneihumicin (1 and 2), a mixture of lactam-lactim tautomers, was isolated along with methoxyneihumicin (3) from the culture broth of the marine Nocardiopsis alba KM6-1. The structures of 1 and 2 were elucidated in spectroscopic analyses (1D and 2D NMR data, and ROESY correlations). Isomethoxyneihumicin (15.0 μM) and 3 (15.0 μM) arrested the cell cycle of Jurkat cells at the G2/M phase (66 and 67%) in 12 h. Isomethoxyneihumicin and 3 exhibited cytotoxicity against Jurkat cells with IC₅₀ values of 6.98 and 30.5 μM in 20 h, respectively. These results strongly suggest that isomethoxyneihumicin and 3 exhibit cytotoxicity against Jurkat cells by inhibiting the cell cycle at the G2/M phase.

Novel Hybrid Compound of a Plinabulin Prodrug with an IgG Binding Peptide for Generating a Tumor Selective Noncovalent-Type Antibody-Drug Conjugate

Although several approaches for making antibody-drug conjugates (ADC) have been developed, it has yet to be reported that an antibody binding peptide such as Z33 from protein A is utilized as the pivotal unit to generate the noncovalent-type ADC (NC-ADC). Herein we aim to establish a novel probe for NC-ADC by synthesizing the Z33-conjugated antitumor agent, plinabulin. Due to the different solubility of two components, including hydrophobic plinabulin and hydrophilic Z33, an innovative method with a solid-supported disulfide coupling reagent is required for the synthesis of the target compounds with prominent efficiency (29% isolated yield). We demonstrate that the synthesized hybrid exhibits a binding affinity against the anti-HER2 antibody (Herceptin) and the anti-CD71 antibody (6E1) (Kd = 46.6 ± 0.5 nM and 4.5 ± 0.56 μM, respectively) in the surface plasmon resonance (SPR) assay. In the cell-based assays, the hybrid provides a significant cytotoxicity in the presence of Herceptin against HER2 overexpressing SKBR-3 cells, but not against HER2 low-expressing MCF-7 cells. Further, it is noteworthy that the hybrid in combination with Herceptin induces cytotoxicity against Herceptin-resistant SKBR-3 (SKBR-3HR) cells. Similar results are obtained with the 6E1 antibody, suggesting that the synthesized hybrid can be widely applicable for NC-ADC using the antibody of interest. In summary, a series of evidence presented here strongly indicate that NC-ADCs have high potential for the next generation of antitumor agents.

Early investigational tubulin inhibitors as novel cancer therapeutics

INTRODUCTION

Microtubules represent one of the most logical and strategic molecular targets amongst the current targets for chemotherapy, alongside DNA. In the past decade, tubulin inhibitors as cancer therapeutics have been an area of focus due to the improved understanding and biological relevance of microtubules in cellular functions. Fueled by the objective of developing novel chemotherapeutics and with the aim of establishing the benefits of tubulin inhibition, several clinical trials have been conducted with others ongoing.

AREA COVERED

At present, the antitubulin development pipeline contains an armful of agents under clinical investigation. This review focuses on novel tubulin inhibitors as cancer therapeutics. The article covers the agents which have completed the phase II studies along with the agents demonstrating promising results in phase I studies.

EXPERT OPINION

Countless clinical trials evaluating the efficacy, safety and pharmacokinetics of novel tubulin inhibitors highlights the scientific efforts being paid to establish their candidature as cancer therapeutics. Colchicine binding site inhibitors as vascular disrupting agents (VDAs) and new taxanes appear to be the most likely agents for future clinical interest. Numerous agents have demonstrated clinical benefits in terms of efficacy and survival in phase I and II studies. However conclusive benefits can only be ascertained on the basis of phase III studies.