Synthetic and biological studies of tubulin targeting c2-substituted 7-deazahypoxanthines derived from marine alkaloid rigidins

Marine alkaloid rigidin analogues as potential selective inhibitors of SHP1, a new strategy for cancer immunotherapeutics

SHP1 is a protein tyrosine phosphatase playing a central role in immunity, cell growth, development, and survival. The inhibition of SHP1 can help in better prognosis in various disorders like breast and ovarian cancer, melanoma, atherosclerosis, hypoxia, hypoactive immune response, and familial dysautonomia. The currently available inhibitors of SHP1 have the side effect of inhibiting the activity of SHP2, which shares >60% sequence similarity with SHP1 but has distinct biological functions. Thus, there is a need to search for novel specific inhibitors of SHP1. The current study uses a combination of virtual screening and molecular dynamic simulations, followed by PCA and MM-GBSA analysis, to screen about 35000 compounds; to predict that two rigidin analogues can potentially selectively inhibit SHP1 but not SHP2. Our studies demonstrate that these rigidin analogues are more potent at inhibiting SHP1 than the commercially available inhibitor NSC-87877. Further, cross-binding studies with SHP2 exhibited poor binding efficiency and lower stability of the complex, thus indicating a specificity of the rigidin analogues for SHP1, which is crucial in preventing side effects due to the diverse physiological functions of SHP2 in cellular signaling, proliferation, and hematopoiesis. Additionally, SHP1 is essential in mediating the inhibitory signaling in antitumor immune cells like NK and T cells. Hence, the rigidin analogues that inhibit SHP1 will potentiate the anti-tumor immune response by the release of inhibitory function of NK cells, thus driving NK activating response, in addition to their intrinsic anti-tumor function. Thus, SHP1 inhibition is a novel double-blade approach towards anti-cancer immunotherapeutics.Communicated by Ramaswamy H. Sarma.

Improved Rigidin-Inspired Antiproliferative Agents with Modifications on the 7-Deazahypoxanthine C7/C8 Ring Systems

To improve their aqueous solubility characteristics, water-solubilizing groups were added to some antiproliferative, rigidin-inspired 7-deazahypoxanthine frameworks after molecular modeling seemed to indicate that structural modifications on the C7 and/or C8 phenyl groups would be beneficial. To this end, two sets of 7-deazahypoxanthines were synthesized by way of a multicomponent reaction approach. It was subsequently determined that their antiproliferative activity against HeLa cells was retained for those derivatives with a glycol ether at the 4'-position of the C8 aryl ring system, while also significantly improving their solubility behavior. The best of these compounds were the equipotent 6-[4-(2-ethoxyethoxy)benzoyl]-2-(pent-4-yn-1-yl)-5-phenyl-1,7-dihydro-4H-pyrrolo[2,3-d]pyrimidin-4-one 33 and 6-[4-(2-ethoxyethoxy)benzoyl]-5-(3-fluorophenyl)-2-(pent-4-yn-1-yl)-1,7-dihydro-4H-pyrrolo[2,3-d]pyrimidin-4-one 59. Similarly to the parent 1, the new derivatives were also potent inhibitors of tubulin assembly. In treated HeLa cells, live cell confocal microscopy demonstrated their impact on microtubulin dynamics and spindle morphology, which is the upstream trigger of mitotic delay and cell death.

Novel 4-Aryl-4H-chromene derivative displayed excellent in vivo anti-glioblastoma efficacy as the microtubule-targeting agent

In this study, a series of novel 4-Aryl-4H-chromene derivatives (D1-D31) were designed and synthesized by integrating quinoline heterocycle to crolibulin template molecule based on the strategy of molecular hybridization. One of these compounds D19 displayed positive antiproliferative activity against U87 cancer cell line (IC50 = 0.90 ± 0.03 μM). Compound D19 was verified as the microtubule-targeting agent through downregulating tubulin related genes of U87 cells, destroying the cytoskeleton of tubulins and interacting with the colchicine-binding site to inhibit the polymerization of tubulins by transcriptome analysis, immune-fluorescence staining, microtubule dynamics and EBI competition assays as well as molecular docking simulations. Moreover, compound D19 induced G2/M phase arrest, resulted in cell apoptosis and inhibited the migration of U87 cells by flow cytometry analysis and wound healing assays. Significantly, compound D19 dose-dependently inhibited the tumor growth of orthotopic glioma xenografts model (GL261-Luc) and effectively prolonged the survival time of mice, which were extremely better than those of positive drug temozolomide (TMZ). Compound D19 exhibited potent in vivo antivascular activity as well as no observable toxicity. Furthermore, the results of in silico simulation studies and P-gp transwell assays verified the positive correlation between compound D19's Blood-Brain Barrier (BBB) permeability and its in vivo anti-GBM activity. Overall, compound D19 can be used as a promising anti-GBM lead compound for the treatment of glioblastoma.

Red-Light Activation of a Microtubule Polymerization Inhibitor via Amide Functionalization of the Ruthenium Photocage

Photoactivated chemotherapy (PACT) is a promising cancer treatment modality that kills cancer cells via photochemical uncaging of a cytotoxic drug. Most ruthenium-based photocages used for PACT are activated with blue or green light, which penetrates sub-optimally into tumor tissues. Here, we report amide functionalization as a tool to fine-tune the toxicity and excited states of a terpyridine-based ruthenium photocage. Due to conjugation of the amide group with the terpyridine π system in the excited state, the absorption of red light (630 nm) increased 8-fold, and the photosubstitution rate rose 5-fold. In vitro, red light activation triggered inhibition of tubulin polymerization, which led to apoptotic cell death both in normoxic (21 % O2 ) and hypoxic (1 % O2 ) cancer cells. In vivo, red light irradiation of tumor-bearing mice demonstrated significant tumor volume reduction (45 %) with improved biosafety, thereby demonstrating the clinical potential of this compound.

A new route for the synthesis of 1-deazaguanine and 1-deazahypoxanthine

Imidazopyridines and pyrrolopyrimidines are an important class of compounds in medicinal chemistry. They can also be considered as deaza-modified purine nucleobases, and as such have attracted a lot of interest recently in the context of RNA atomic mutagenesis. In particular, for 1-deazaguanine (c1G base), a significant increase in demand is apparent. Synthetic access is challenging and the few reports found in the literature suffer from the requirement of hazardous intermediates and harsh reaction conditions. Here, we report a new six-step synthesis for c1G base, starting from 6-iodo-1-deazapurine. The key transformations are copper catalyzed C–O-bond formation followed by site-specific nitration. A further strength of our route is divergency, additionally enabling the synthesis of 1-deazahypoxanthine (c1I base).

Glioblastoma: Current Status, Emerging Targets, and Recent Advances

Glioblastoma (GBM) is a highly malignant brain tumor characterized by a heterogeneous population of genetically unstable and highly infiltrative cells that are resistant to chemotherapy. Although substantial efforts have been invested in the field of anti-GBM drug discovery in the past decade, success has primarily been confined to the preclinical level, and clinical studies have often been hampered due to efficacy-, selectivity-, or physicochemical property-related issues. Thus, expansion of the list of molecular targets coupled with a pragmatic design of new small-molecule inhibitors with central nervous system (CNS)-penetrating ability is required to steer the wheels of anti-GBM drug discovery endeavors. This Perspective presents various aspects of drug discovery (challenges in GBM drug discovery and delivery, therapeutic targets, and agents under clinical investigation). The comprehensively covered sections include the recent medicinal chemistry campaigns embarked upon to validate the potential of numerous enzymes/proteins/receptors as therapeutic targets in GBM.

Bioactive Compounds with Antiglioma Activity from Marine Species

The search for new chemical compounds with antitumor pharmacological activity is a necessary process for creating more effective drugs for each specific malignancy type. This review presents the outcomes of screening studies of natural compounds with high anti-glioma activity. Despite significant advances in cancer therapy, there are still some tumors currently considered completely incurable including brain gliomas. This review covers the main problems of the glioma chemotherapy including drug resistance, side effects of common anti-glioma drugs, and genetic diversity of brain tumors. The main emphasis is made on the characterization of natural compounds isolated from marine organisms because taxonomic diversity of organisms in seawaters significantly exceeds that of terrestrial species. Thus, we should expect greater chemical diversity of marine compounds and greater likelihood of finding effective molecules with antiglioma activity. The review covers at least 15 classes of organic compounds with their chemical formulas provided as well as semi-inhibitory concentrations, mechanisms of action, and pharmacokinetic profiles. In conclusion, the analysis of the taxonomic diversity of marine species containing bioactives with antiglioma activity is performed noting cytotoxicity indicators and to the tumor cells in comparison with similar indicators of antitumor agents approved for clinical use as antiglioblastoma chemotherapeutics.

Marine Natural Products with High Anticancer Activities

This review covers recent literature from 2012-2019 concerning 170 marine natural products and their semisynthetic analogues with strong anticancer biological activities. Reports that shed light on cellular and molecular mechanisms and biological functions of these compounds, thus advancing the understanding in cancer biology are also included. Biosynthetic studies and total syntheses, which have provided access to derivatives and have contributed to the proper structure or stereochemistry elucidation or revision are mentioned. The natural compounds isolated from marine organisms are divided into nine groups, namely: alkaloids, sterols and steroids, glycosides, terpenes and terpenoids, macrolides, polypeptides, quinones, phenols and polyphenols, and miscellaneous products. An emphasis is placed on several drugs originating from marine natural products that have already been marketed or are currently in clinical trials.

Retracted Article: The synthesis and biological activity of marine alkaloid derivatives and analogues

The ocean is the origin of life, with a unique ecological environment, which has given birth to a wealth of marine organisms. The ocean is an important source of biological resources and tens of thousands of monomeric compounds have been separated from marine organisms using modern separation technology. Most of these monomeric compounds have some kind of biological activity that has attracted extensive attention from researchers. Marine alkaloids are a kind of compound that can be separated from marine organisms. They have complex and special chemical structures, but at the same time, they can show diversity in biological activities. The biological activities of marine alkaloids mainly manifest in the form of anti-tumor, anti-fungus, anti-viral, anti-malaria, and anti-osteoporosis properties. Many marine alkaloids have good medicinal prospects and can possibly be used as anti-tumor, anti-viral, and anti-fungal clinical drugs or as lead compounds. The limited amounts of marine alkaloids that can be obtained by separation, coupled with the high cytotoxicity and low selectivity of these lead compounds, has restricted the clinical research and industrial development of marine alkaloids. Marine alkaloid derivatives and analogues have been obtained via rational drug design and chemical synthesis, to make up for the shortcomings of marine alkaloids; this has become an urgent subject for research and development. This work systematically reviews the recent developments relating to marine alkaloid derivatives and analogues in the field of medical chemistry over the last 10 years (2010–2019). We divide marine alkaloid derivatives and analogues into five types from the point-of-view of biological activity and elaborated on these activities. We also briefly discuss the optimization process, chemical synthesis, biological activity evaluation, and structure–activity relationship (SAR) of each of these compounds. The abundant SAR data provides reasonable approaches for the design and development of new biologically active marine alkaloid derivatives and analogues.

The ocean is the origin of life, with a unique ecological environment, which has given birth to a wealth of marine organisms.

Microtubule-Targeting 7-Deazahypoxanthines Derived from Marine Alkaloid Rigidins: Exploration of the N3 and N9 Positions and Interaction with Multidrug-Resistance Proteins

Our laboratories have been investigating synthetic analogues of marine alkaloid rigidins that possess promising anticancer activities. These analogues, based on the 7-deazahypoxanthine skeleton, are available in one- or two-step synthetic sequences and exert cytotoxicity by disrupting microtubule dynamics in cancer cells. In the present work we extended the available structure-activity relationship (SAR) data to N3- and N9-substituted derivatives. Although N3 substitution results in loss of activity, the N9-substituted compounds retain nanomolar antiproliferative activities and the anti-tubulin mode of action of the original unsubstituted compounds. Furthermore, our results also demonstrate that multidrug-resistance (MDR) proteins do not confer resistance to both N9-unsubstituted and -substituted compounds. It was found that sublines overexpressing ABCG2, ABCC1, and ABCB1 proteins are as responsive to the rigidin analogues as their parental cell lines. Thus, the study reported herein provides further impetus to investigate the rigidin-inspired 7-deazahypoxanthines as promising anticancer agents.

Recent advances in the discovery of small molecules targeting glioblastoma

Glioblastoma (GBM) is one of the most common central nervous system cancers. It is characterized as a fast-growing tumor that arises from multiple cell types with neural stem-cell-like properties. Additionally, GBM tumors are highly invasive, which is attributed to the presence of glioblastoma stem cells that makes surgery ineffective in most cases. Currently, temozolomide is the unique chemotherapy option approved by the U.S. Food and Drug Administration for GBM treatment. This review analyzes the emergence and development of new synthetic small molecules discovered as promising anti-glioblastoma agents. A number of compounds were described herein and grouped according to the main chemical class used in the drug discovery process. Importantly, we focused only on synthetic compounds published in the last 10 years, thus excluding natural products. Furthermore, we included in this review only those most biologically active compounds with proven in vitro and/or in vivo efficacy.

Pyrrolopyrimidines: An update on recent advancements in their medicinal attributes

Fused heterocycles are reported to demonstrate variety of biological activities such as anticancer, antibacterial, antifungal and anti-inflammatory, and are thus exhaustively utilized in the field of medicinal chemistry. Pyrrolopyrimidines is one of the major classes of fused heterocycles which are extensively reported throughout the literature. Several reports suggest that pyrrolopyrimidine as fused scaffold possess more diverse and potent pharmacological profile than individual pyrrole and pyrimidine nucleus. Different pathological targets require different structural attributes reflected via varied substitutions, thus in recent years, researchers have employed various synthetic strategies to achieve desired substitutions on the pyrrolopyrimidine nucleus. In this review, authors highlight the recent advancement in this area, special focus was laid on the pharmacological profile and structure-activity relationship studies (SAR) of various synthesized pyrrolopyrimidine derivatives.

The Rigidins: Isolation, Bioactivity, and Total Synthesis-Novel Pyrrolo[2,3-d]Pyrimidine Analogues Using Multicomponent Reactions

Rigidins (2-6) are pyrrolopyrimidine alkaloids isolated from marine tunicates. Since their isolation, refinement of their total syntheses, and biochemical evaluation, interest toward this pyrrolo[2,3-d]pyrimidine scaffold as a medicinal candidate has been triggered. The derivatization of these natural products has led to the discovery of a novel range of 7-deazahypoxanthines, which exhibit extremely potent anticancer activity in human cancer cell lines. A major breakthrough toward the synthesis of rigidin and various rigidin analogues has been the application of multicomponent reactions (MCRs). The rapid assembly of molecular diversity and flexibility displayed by MCRs makes it an attractive strategy for the preparation of rigidin-inspired small molecules. Furthermore, a number of rigidin-like 7-deazaxanthine compounds have been reported in the literature and the popularity of implementing MCRs to construct these 7-deazaxanthines is highlighted here. It is our hope that the synthetic methods described in this chapter will result in the further generation of rigidin-inspired compounds that will move on from being "hits" into "leads" in the medicinal chemistry drug discovery pipeline and potentially into anticancer therapeutics.

Anticancer Properties of Halogenated Pyrrolo[3,2-d]pyrimidines with Decreased Toxicity via N5 Substitution

Halogenated pyrrolo[3,2-d]pyrimidine analogues have shown antiproliferative activity in recent studies, with cell accumulation occurring in the G2 /M stage without apoptosis. However, the mechanism of action and pharmacokinetic (PK) profile of these compounds has yet to be determined. To investigate the PK profile of these compounds, a series of halogenated pyrrolo[3,2-d]pyrimidine compounds was synthesized and first tested for activity in various cancer cell lines followed by a mouse model. EC50 values ranged from 0.014 to 14.5 μm, and maximum tolerated doses (MTD) in mice were between 5 and 10 mg kg-1 . This indicates a wide variance in activity and toxicity that necessitates further study. To decrease toxicity, a second series of compounds was synthesized with N5-alkyl substitutions in an effort to slow the rate of metabolism, which was thought to be leading to the toxicity. The N-substituted compounds demonstrated comparable cell line activity (EC50 values between 0.83-7.3 μm) with significantly decreased toxicity (MTD=40 mg kg-1 ). Finally, the PK profile of the active N5-substituted compound shows a plasma half-life of 32.7 minutes, and rapid conversion into the parent unsubstituted analogue. Together, these data indicate that halogenated pyrrolo[3,2-d]pyrimidines present a promising lead into potent antiproliferative agents with tunable activity and toxicity, and rapid metabolism.

G2/M inhibitors as pharmacotherapeutic opportunities for glioblastoma: the old, the new, and the future

Glioblastoma (GBM) is one of the deadliest tumors and has a median survival of 3 months if left untreated. Despite advances in rationally targeted pharmacological approaches, the clinical care of GBM remains palliative in intent. Since the majority of altered signaling cascades involved in cancer establishment and progression eventually affect cell cycle progression, an alternative approach for cancer therapy is to develop innovative compounds that block the activity of crucial molecules needed by tumor cells to complete cell division. In this context, we review promising ongoing and future strategies for GBM therapeutics aimed towards G2/M inhibition such as anti-microtubule agents and targeted therapy against G2/M regulators like cyclin-dependent kinases, Aurora inhibitors, PLK1, BUB, 1, and BUBR1, and survivin. Moreover, we also include investigational agents in the preclinical and early clinical settings. Although several drugs were shown to be gliotoxic, most of them have not yet entered therapeutic trials. The use of either single exposure or a combination with novel compounds may lead to treatment alternatives for GBM patients in the near future.

Synthesis and in vitro growth inhibitory activity of novel silyl- and trityl-modified nucleosides

Seventeen silyl- and trityl-modified (5'-O- and 3',5'-di-O-) nucleosides were synthesized with the aim of investigating the in vitro antiproliferative activities of these nucleoside derivatives. A subset of the compounds was evaluated at a fixed concentration of 100μM against a small panel of tumor cell lines (HL-60, K-562, Jurkat, Caco-2 and HT-29). The entire set was also tested at varying concentrations against two human glioma lines (U373 and Hs683) to obtain GI50 values, with the best results being values of ∼25μM.

Novel Microtubule-Targeting 7-Deazahypoxanthines Derived from Marine Alkaloid Rigidins with Potent in Vitro and in Vivo Anticancer Activities

Docking studies of tubulin-targeting C2-substituted 7-deazahypoxanthine analogues of marine alkaloid rigidins led to the design and synthesis of compounds containing linear C2-substituents. The C2-alkynyl analogue was found to have double- to single-digit nanomolar antiproliferative IC50 values and showed statistically significant tumor size reduction in a colon cancer mouse model at nontoxic concentrations. These results provide impetus and further guidance for the development of these rigidin analogues as anticancer agents.

Synthetic and Biological Studies of Sesquiterpene Polygodial: Activity of 9-Epipolygodial against Drug-Resistant Cancer Cells

Polygodial, a terpenoid dialdehyde isolated from Polygonum hydropiper L., is a known agonist of the transient receptor potential vanilloid 1 (TRPV1). In this investigation a series of polygodial analogues were prepared and investigated for TRPV1-agonist and anticancer activities. These experiments led to the identification of 9-epipolygodial, which has antiproliferative potency significantly exceeding that of polygodial. 9-Epipolygodial was found to maintain potency against apoptosis-resistant cancer cells as well as those displaying the multidrug-resistant (MDR) phenotype. In addition, the chemical feasibility for the previously proposed mechanism of action of polygodial, involving the formation of a Paal-Knorr pyrrole with a lysine residue on the target protein, was demonstrated by the synthesis of a stable polygodial pyrrole derivative. These studies reveal rich chemical and biological properties associated with polygodial and its direct derivatives. These compounds should inspire further work in this area aimed at the development of new pharmacological agents, or the exploration of novel mechanisms of covalent modification of biological molecules with natural products.

Fungal metabolite ophiobolin A as a promising anti-glioma agent: In vivo evaluation, structure-activity relationship and unique pyrrolylation of primary amines

Glioblastoma, the most common form of malignant primary brain tumor, is characterized by resistance to apoptosis, which is largely responsible for the low effectiveness of the classical chemotherapeutic approaches based on apoptosis induction in cancer cells. Previously, a fungal secondary metabolite ophiobolin A was found to have significant activity against apoptosis-resistant glioblastoma cells through the induction of a non-apoptotic cell death, thus, offering an innovative strategy to combat this type of cancer. The current work describes the results of a preliminary evaluation of ophiobolin A in an in vivo glioblastoma model and its chemical derivatization to establish first synthetically generated structure-activity relationship. The synthetic work has also led to the discovery of a unique reaction of ophiobolin A with primary amines suggesting the possibility of pyrrolylation of lysine residues on its intracellular target protein(s).

Wittig derivatization of sesquiterpenoid polygodial leads to cytostatic agents with activity against drug resistant cancer cells and capable of pyrrolylation of primary amines

Many types of cancer, including glioma, melanoma, non-small cell lung cancer (NSCLC), among others, are resistant to proapoptotic stimuli and thus poorly responsive to current therapies based on the induction of apoptosis in cancer cells. The current investigation describes the synthesis and anticancer evaluation of unique C12-Wittig derivatives of polygodial, a sesquiterpenoid dialdehyde isolated from Persicaria hydropiper (L.) Delabre. These compounds were found to undergo an unprecedented pyrrole formation with primary amines in a chemical model system, a reaction that could be relevant in the biological environment and lead to the pyrrolation of lysine residues in the target proteins. The anticancer evaluation of these compounds revealed their promising activity against cancer cells displaying various forms of drug resistance, including resistance to proapoptotic agents. Mechanistic studies indicated that compared to the parent polygodial, which displays fixative general cytotoxic action against human cells, the C12-Wittig derivatives exerted their antiproliferative action mainly through cytostatic effects explaining their activity against apoptosis-resistant cancer cells. The possibility for an intriguing covalent modification of proteins through a novel pyrrole formation reaction, as well as useful activities against drug resistant cancer cells, make the described polygodial-derived chemical scaffold an interesting new chemotype warranting thorough investigation.

Anticancer properties of lamellarins

In 1985 the first lamellarins were isolated from a small oceanic sea snail. Today, more than 50 lamellarins have been inventoried and numerous derivatives synthesized and tested as antiviral or anticancer agents. The lead compound in the family is lamellarin D, characterized as a potent inhibitor of both nuclear and mitochondrial topoisomerase I but also capable of directly interfering with mitochondria to trigger cancer cell death. The pharmacology and chemistry of lamellarins are discussed here and the mechanistic portrait of lamellarin D is detailed. Lamellarins frequently serve as a starting point in the design of anticancer compounds. Extensive efforts have been devoted to create novel structures as well as to improve synthetic methods, leading to lamellarins and related pyrrole-derived marine alkaloids.