Bisindolyl Maleimides and Indolylmaleimide Derivatives-A Review of Their Synthesis and Bioactivity

The evolution of bisindolyl maleimides and indolyl maleimide derivatives and their unique biological activities have stimulated great interest in medicinal chemistry programs. Bisindolylmaleimide (BIM)-type compounds arise from natural sources such as arcyriarubin and are biosynthetically related to indolocarbazoles. BIMs are commonly the immediate synthetic precursors of indolocarbazoles, lacking a central bond between the two aromatic units and making them more flexible and drug-like. Synthetic endeavours within this class of compounds are broad and have led to the development of both remarkably potent and selective protein kinase inhibitors. Clinical BIM examples include ruboxistaurin and enzastaurin, which are highly active inhibitors of protein kinase C-β. While BIMs are widely recognised as protein kinase inhibitors, other modes of activity have been reported, including the inhibition of calcium signalling and antimicrobial activity. Critically, structural differences can be used to exploit new bioactivity and therefore it is imperative to discover new chemical entities to address new targets. BIMs can be highly functionalised or chemically manipulated, which provides the opportunity to generate new derivatives with unique biological profiles. This review will collate new synthetic approaches to BIM-type compounds and their associated bioactivities with a focus on clinical applications.

29th Annual GP2A Medicinal Chemistry Conference

The 29th Annual GP₂A (Group for the Promotion of Pharmaceutical chemistry in Academia) Conference was a virtual event this year due to the COVID-19 pandemic and spanned three days from Wednesday 25 to Friday 27 August 2021. The meeting brought together an international delegation of researchers with interests in medicinal chemistry and interfacing disciplines. Abstracts of keynote lectures given by the 10 invited speakers, along with those of the 8 young researcher talks and the 50 flash presentation posters, are included in this report. Like previous editions, the conference was a real success, with high-level scientific discussions on cutting-edge advances in the fields of pharmaceutical chemistry.

Synthesis and Evaluation of Novel Ellipticines and Derivatives as Inhibitors of Phytophthora infestans

The pathogen Phytophthora infestans is responsible for worldwide catastrophic crop damage and discovery of new inhibitors of this organism is of paramount agricultural and industrial importance. Current strategies for crop treatment are inadequate with limitations of efficacy and market alternatives. Ellipticines have recently been reported to have fungicidal properties and have been assessed against P. infestans growth with promising results. We hereby report a probe of the ellipticine framework to investigate the alkyl subunit and screen a set ellipticines and derivatives to identify new lead compounds to act against P. infestans. A series of ellipticinium salt derivatives have been identified with exceptional growth inhibitory activity and apparent lack of toxicity towards a human cell-line surpassing the effect of known and marketed fungicides. This report identifies the potential of this natural product derivative as a novel fungicide.

Isoquinolinequinone N-oxides as anticancer agents effective against drug resistant cell lines

The isoquinolinequinone (IQQ) pharmacophore is a privileged framework in known cytotoxic natural product families, caulibugulones and mansouramycins. Exploiting both families as a chemical starting point, we report on the structured development of an IQQ N-oxide anticancer framework which exhibits growth inhibition in the nM range across melanoma, ovarian and leukaemia cancer cell lines. A new lead compound (16, R6 = benzyl, R7 = H) exhibits nM GI50 values against 31/57 human tumour cell lines screened as part of the NCI60 panel and shows activity against doxorubicin resistant tumour cell lines. An electrochemical study highlights a correlation between electropositivity of the IQQ N-oxide framework and cytotoxicity. Adduct binding to sulfur based biological nucleophiles glutathione and cysteine was observed in vitro. This new framework possesses significant anticancer potential.

Oxidation of sitosterol and transport of its 7-oxygenated products from different tissues in humans and ApoE knockout mice

The most common phytosterols in the human diet are sitosterol and campesterol, which originate exclusively from plant derived food. These phytosterols are taken up by NPC1L1 transport from the intestine into the enterocytes together with cholesterol and other xenosterols. Phytosterols are selectively pumped back from the enterocytes into the intestinal lumen and on the liver site from hepatocytes into bile by heterodimeric ABCG5/G8 transporters. Like cholesterol, both phytosterols are prone to ring and side chain oxidation. It could be shown that oxyphytosterols, found in atherosclerotic tissue, are most likely of in situ oxidation (Schött et al.; Biochem. Biophys. Res. Commun. 2014 Apr 11;446(3):805-10). However, up to now, the entire mechanism of phytosterol oxidation is not clearly understood. Here, we provide further information about the oxidation of sitosterol and the transport of its oxidation products out of tissue. Our survey includes data of 104 severe aortic stenosis patients that underwent an elective aortic valve cusp replacement. We studied their phytosterol concentrations, as well as absolute and substrate corrected oxyphytosterol levels in plasma and valve cusp tissue. In addition, we also examined phytosterol and oxyphytosterol concentrations in plasma and tissues (from brain and liver) of 10 male ApoE knockout mice. The ratio of 7-oxygenated-sitosterol-to-sitosterol exceeds the ratio for 7-oxygenated-campesterol-to-campesterol in plasma and tissue of both humans and mice. This finding indicates that sitosterol is oxidized to a higher amount than campesterol and that a selective oxidative mechanism might exist which can differentiate between certain phytosterols. Secondly, the concentrations of oxyphytosterols found in plasma and tissue support the idea that oxysitosterols are preferably transported out of individual tissues. Selective oxidation of sitosterol and preferred transport of sitosterol oxidation products out of tissue seem to be a metabolic pathway of forced sitosterol clearance from tissue compartments.

The impact of storage conditions upon gentamicin coated antimicrobial implants

A systematic approach was developed to investigate the stability of gentamicin sulfate (GS) and GS/poly (lactic-co-glycolic acid) (PLGA) coatings on hydroxyapatite surfaces. The influence of environmental factors (light, humidity, oxidation and heat) upon degradation of the drug in the coatings was investigated using liquid chromatography with evaporative light scattering detection and mass spectrometry. GS coated rods were found to be stable across the range of environments assessed, with only an oxidizing atmosphere resulting in significant changes to the gentamicin composition. In contrast, rods coated with GS/PLGA were more sensitive to storage conditions with compositional changes being detected after storage at 60 °C, 75% relative humidity or exposure to light. The effect of γ-irradiation on the coated rods was also investigated and found to have no significant effect. Finally, liquid chromatography-mass spectrometry analysis revealed that known gentamines C1, C1a and C2 were the major degradants formed. Forced degradation of gentamicin coatings did not produce any unexpected degradants or impurities.

Activation of multiple chemotherapeutic prodrugs by the natural enzymolome of tumour-localised probiotic bacteria

Some chemotherapeutic drugs (prodrugs) require activation by an enzyme for efficacy. We and others have demonstrated the ability of probiotic bacteria to grow specifically within solid tumours following systemic administration, and we hypothesised that the natural enzymatic activity of these tumour-localised bacteria may be suitable for activation of certain such chemotherapeutic drugs. Several wild-type probiotic bacteria; Escherichia coli Nissle, Bifidobacterium breve, Lactococcus lactis and Lactobacillus species, were screened against a panel of popular prodrugs. All strains were capable of activating at least one prodrug. E. coli Nissle 1917 was selected for further studies because of its ability to activate numerous prodrugs and its resistance to prodrug toxicity. HPLC data confirmed biochemical transformation of prodrugs to their toxic counterparts. Further analysis demonstrated that different enzymes can complement prodrug activation, while simultaneous activation of multiple prodrugs (CB1954, 5-FC, AQ4N and Fludarabine phosphate) by E. coli was confirmed, resulting in significant efficacy improvement. Experiments in mice harbouring murine tumours validated in vitro findings, with significant reduction in tumour growth and increase in survival of mice treated with probiotic bacteria and a combination of prodrugs. These findings demonstrate the ability of probiotic bacteria, without the requirement for genetic modification, to enable high-level activation of multiple prodrugs specifically at the site of action.

7β-Hydroxysitosterol crosses the blood-brain barrier more favored than its substrate sitosterol in ApoE-/- mice

In this study, we compare the distribution of intraperitoneally injected sitosterol, 7β-hydroxysitosterol or vehicle only (control) for 28days in male ApoE-/- mice. Furthermore we examine its impact on surrogate markers of cholesterol biosynthesis and sterol absorption rate in plasma, brain and liver tissues from these animals. Injection of sitosterol revealed a 32.1% (P=0.013) lower plasma total cholesterol compared with control. Cholesterol corrected plasma and absolute brain and liver levels of sitosterol are 4.1-, 1.7-, and 7.2-fold (P<0.001 for all) higher, respectively. This is in accordance with a reduced plasma campesterol to cholesterol ratio (-16.2%; P=0.018) together with a 24.1% (P=0.047) lower concentration of hepatic lathosterol. After injection of 7β-hydroxysitosterol the concentrations of 7β-hydroxysitosterol in plasma, brain and liver are 21.0-, 65.8- and 42.7-fold (P<0.001 for all) higher, respectively, compared with control. Injection of 7β-hydroxysitosterol revealed significantly lower plasma cholesterol corrected cholestanol and campesterol (-44.2%; P=0.001 and -24.5; P=0.004) as well as lower absolute liver cholestanol levels (-31.9%; P<0.001) compared with control. Intraperitoneally injected sitosterol and 7β-hydroxysitosterol differently influence cholesterol metabolism in plasma and liver. We conclude that the polar 7β-hydroxysitosterol compound can pass the blood brain barrier with higher efficacy than its substrate, sitosterol. Though present in higher amounts in the brain, both, sitosterol and 7β-hydroxysitosterol do not influence cholesterol metabolism in the brain as proven by our surrogate markers.

Synthesis and identification of novel indolo[2,3-a]pyrimido[5,4-c]carbazoles as a new class of anti-cancer agents

A range of 5,6-bisindole and 5-indole-6-(7-azaindole)pyrimidinones were synthesised via a β-keto ester intermediate and a screen of cyclisation conditions undertaken. The optimised route to this new class of indolocarbazoles and azaindolocarbazoles involved photocyclisation, with typical yields of 50-60%. These derivatives were screened for anti-cancer activity via topo II inhibition and the NCI-60 cell line assay, with the screening indicating a lack of topo II inhibition, but significant in vitro growth inhibition within this new chemical class, in the low micromolar range against renal cancer, melanoma and colon cancer cell lines.

Oxidized derivatives of dihydrobrassicasterol: cytotoxic and apoptotic potential in U937 and HepG2 cells

The ability of phytosterol compounds to reduce plasma serum cholesterol levels in humans is well investigated. However, phytosterols are structurally similar to cholesterol with a double bond at the C5-6 position and are therefore susceptible to oxidation. Much research has been carried out on the biological effects of cholesterol oxidation products (COPs) in vitro. In contrast, there is less known about phytosterol oxidation products (POPs). From previous studies, it is apparent that oxidized derivatives of the phytosterols, β-sitosterol and stigmasterol, are cytotoxic in vitro but are less potent than their COP counterparts. In the present study, the cytotoxic and apoptotic potential of oxidized derivatives of dihydrobrassicasterol (DHB) including 5α,6α-epoxyergostan-3β-ol (α-epoxide), 5β,6β-epoxyergostan-3β-ol (β-epoxide), ergost-5-en-7-on-3β-ol (7-keto), ergost-5-ene-3β,7β-diol (7-β-OH), and ergostane-3β,5α,6β-triol (triol) were evaluated in the U937 and HepG2 cell lines. In general, 7-keto, 7-β-OH, and triol derivatives had a significant cytotoxic impact on U937 and HepG2 cells. The oxides appear to be more toxic toward U937 cells. In line with previous findings, the POPs investigated in this study were less potent than the equivalent COPs. The results add to the body of data on the toxicity of individual POPs.

Cytotoxic and apoptotic effects of the oxidized derivatives of stigmasterol in the U937 human monocytic cell line

Dietary exposure to phytosterols has increased in recent years due to the incorporation of these compounds into cholesterol-lowering products. Previous studies have investigated the cytotoxic effects of the oxidized derivatives of β-sitosterol and determined that phytosterol oxidation products (POP) have a similar but less potent toxicity compared to their cholesterol equivalents. In the present study, the cytotoxicity of the oxidized derivatives of stigmasterol were investigated in the U937 cell line. The stigmasta-5,22-diene-3β,7β-diol (7β-OH), 5,6-epoxystigmasta-22,23-diol (epoxydiol), 5,6,22,23-diepoxystigmastane (diepoxide), and (22R,23R)-stigmast-5-ene-3β,22,23-triol (22R,23R-triol) derivatives were identified as the most cytotoxic, and the mode of cell death was identified as apoptosis in cells incubated with 7β-OH, epoxydiol, and diepoxide stigmasterol. The antioxidants α-tocopherol, γ-tocopherol, and β-carotene did not protect against apoptosis induced by 7β-OH and diepoxide stigmasterol; however, α-tocopherol was found to protect against epoxydiol-induced apoptosis. The cellular antioxidant, glutathione, was depleted and the apoptotic protein, Bcl-2, was down-regulated by the stigmasterol oxides identified as apoptotic.

Synthesis and characterization of stigmasterol oxidation products

The synthesis and structural characterization of a series of oxides of stigmasterol is described providing a valuable series of reference standards for these oxides, analogous to the cholesterol oxidation products (COPs) which have been shown to have detrimental biological effects. Biological evaluation of the oxides of phytosterols is significant in the context of increased dietary use of phytosterols in the drive to reduce cholesterol absorption.

Computer simulations reveal a novel nucleotide-type binding orientation for ellipticine-based anticancer c-kit kinase inhibitors

Receptor tyrosine kinase (RTK) enzymes regulate cell signaling pathways and so are an important target for cancer chemotherapy. Current inhibitors of c-kit, a key RTK stem cell factor receptor, are inactive against the most common mutated variant Asp816Val, associated with highly malignant cancers. Recent combined experimental/simulation work has highlighted the utility of the ellipticine pharmacore in inhibiting mutant c-kit, and the present simulation study applies a combination of high-level simulation tools to probe further the binding of ellipticine-based derivatives to c-kit. We find a large preference for protonation of bound ellipticine, which stabilizes the negative protein residues that coordinated ADP.Mg (2+) in the native complex. The resulting ellipticine inhibitor binding mode resembles the native nucleotide complex and serves to explain some existing experimental data on binding specificities, indicating that functionalization at the C4/C5 sites of ellipticine derivatives may be important for the design of novel nucleotide analogues that inhibit mutant c-kit.

Exploring DNA topoisomerase I inhibition by the benzo[c]phenanthridines fagaronine and ethoxidine using steered molecular dynamics

The benzo[c]phenanthridines (BCPs) are a group of compounds that are believed to express their antitumor activity through the inhibition of topoisomerase I. The enzyme is crucial to cell cycle division and progression, and regulates the equilibrium between relaxed and supercoiled DNA that occurs during DNA replication. Over the years, we have prepared a number of BCPs and employed a number of biophysical techniques to explore their mechanism of action and improve their activity against this particular enzyme. The naturally occurring alkaloid fagaronine 1 and the synthetic compound ethoxidine 3 are two of the most active compounds, although their inhibitory mechanisms are different, being a poison and suppressor, respectively. We have modified the approach of steered molecular dynamics to create a torque on the intercalator to comprehensively sample the DNA binding site, and using topoisomerase I crystal structures, have proposed a model to explain the different mechanisms of action for these two BCP compounds.

Synthesis, isolation and characterisation of β-sitosterol and β-sitosterol oxide derivatives

Beta-sitosterol is the most prevalent plant cholesterol derivative (phytosterol) and can undergo similar oxidation to cholesterol, leading to beta-sitosterol oxides. The biological impact of phytosterol oxides has only been evaluated in a phytosterol blend (usually of beta-sitosterol, campesterol, stigmasterol and dihydrobrassicasterol). The lack of pure phytosterols, including beta-sitosterol, hinders the collection of significant toxicity data on the individual beta-sitosterol oxides. An efficient synthetic route to multi-gram quantities of pure beta-sitosterol is described here, together with the first syntheses and characterisation of pure beta-sitosterol oxides.