Rainer Schobert

Tetramic and tetronic acids: An update on new derivatives and biological aspects

Significant developments in the isolation of tetramic acids and tetronic acids, in the elucidation of their biosyntheses and their biological activities and in laboratory syntheses are reviewed with a focus on those derivatives with medicinal and pharmacological relevance. Important new members of the title compound families isolated since the year 2000 are covered as well as new biological aspects of some earlier congeners.

4-(3-Halo/amino-4,5-dimethoxyphenyl)-5-aryloxazoles and -N-methylimidazoles that are cytotoxic against combretastatin A resistant tumor cells and vascular disrupting in a cisplatin resistant germ cell tumor model.

New combretastatin A analogues featuring oxazole or N-methylimidazole bridged Z-alkenes and halo- or amino-substituted A-rings were tested against various cancer cell lines and in testicular germ cell tumor xenografts in mice. Imidazoles with 3-halo-4,5-dimethoxy substituted A-rings and 3-amino-4-methoxy substituted B-rings (7b and 8b) were efficacious at nanomolar concentrations against cells of combretastatin A refractory HT-29 colon carcinoma, multidrug-resistant MCF-7/Topo breast carcinoma, and cisplatin-resistant 1411HP testicular germ cell tumor. They induced apoptosis and inhibited tubulin polymerization. While well tolerated by mice at high doses, these imidazoles initiated extensive intratumoral hemorrhage and regressions of highly vascularized 1411HP xenografts.

Gold(I)-NHC complexes of antitumoral diarylimidazoles: structures, cellular uptake routes and anticancer activities.

Five new heterocyclic gold carbene complexes were prepared, four chlorido-[1,3-dimethyl-4,5-diarylimidazol-2-ylidene]gold complexes 6a-d and a chlorido-[1,3-dibenzylimidazol-2-ylidene]gold complex 11, and three of them were characterised by X-ray single crystal analyses. They were tested for cytotoxicity against a panel of four human cancer cell lines and non-malignant fibroblasts, for tubulin interaction, and for the pathways of their uptake into 518A2 melanoma cells. All complexes showed cytotoxic activity in the micromolar IC(50) range with distinct selectivities for certain cell lines. In stark contrast to related metal-free 1-methyl-4,5-diarylimidazoles, the complexes 6 and 11 did not noticeably inhibit the polymerisation of tubulin to give microtubules. The cellular uptake of complexes 6 occurred mainly via the copper transporter (Ctr1) and the organic cation transporters (OCT-1/2). Complex 11 was accumulated preferentially via the organic cation transporters and by Na(+)/K(+)-dependent endocytosis. The new gold carbene complexes seem to operate by a mechanism different from that of the parent 1-methylimidazolium ligands.

Terpene conjugates of the Nigella sativa seed-oil constituent thymoquinone with enhanced efficacy in cancer cells.

Thymoquinone (TQ; 1) is a weak anticancer constituent of black seed oil. Derivatives bearing terpene‐terminated 6‐alkyl residues were tested in cells of human HL‐60 leukemia, 518A2 melanoma, multidrug‐resistant KB‐V1/Vbl cervix, and MCF‐7/Topo breast carcinomas, as well as in non‐malignant human foreskin fibroblasts. Derivatives with a short four‐atom spacer between quinone and cyclic monoterpene moieties were more antiproliferative than analogues with longer spacers. 6‐(Menthoxybutyryl)thymoquinone (3a) exhibited single‐digit micromolar IC50 (72 h) values in all four cell lines. It was seven times more active than TQ (1) in 518A2 melanoma cells and four times in KB‐V1/Vbl cervix carcinoma cells, while only half as toxic in the fibroblasts. Compound 3a was also not a substrate for the P‐gp and BCRP drug transporters of the resistant cancer cells. The caryophyllyl and germacryl conjugates 3e and 3f specifically inhibited the growth of the resistant MCF‐7 breast carcinoma cells. Conjugation of TQ with the triterpene betulinic acid via the OH group as in 3g led to a loss in activity, while conjugation via the carboxylic acid afforded compound 4 with nanomolar IC50 (72 h) activity against HL‐60 cells. All anticancer‐active derivatives of TQ (1) induced apoptosis associated with DNA laddering, a decrease in mitochondrial membrane potential and a slight increase in reactive oxygen species.

Metabolome Analysis of Arabidopsis thaliana Roots Identifies a Key Metabolic Pathway for Iron Acquisition

Fe deficiency compromises both human health and plant productivity. Thus, it is important to understand plant Fe acquisition strategies for the development of crop plants which are more Fe-efficient under Fe-limited conditions, such as alkaline soils, and have higher Fe density in their edible tissues. Root secretion of phenolic compounds has long been hypothesized to be a component of the reduction strategy of Fe acquisition in non-graminaceous plants. We therefore subjected roots of Arabidopsis thaliana plants grown under Fe-replete and Fe-deplete conditions to comprehensive metabolome analysis by gas chromatography-mass spectrometry and ultra-pressure liquid chromatography electrospray ionization quadrupole time-of-flight mass spectrometry. Scopoletin and other coumarins were found among the metabolites showing the strongest response to two different Fe-limited conditions, the cultivation in Fe-free medium and in medium with an alkaline pH. A coumarin biosynthesis mutant defective in ortho-hydroxylation of cinnamic acids was unable to grow on alkaline soil in the absence of Fe fertilization. Co-cultivation with wild-type plants partially rescued the Fe deficiency phenotype indicating a contribution of extracellular coumarins to Fe solubilization. Indeed, coumarins were detected in root exudates of wild-type plants. Direct infusion mass spectrometry as well as UV/vis spectroscopy indicated that coumarins are acting both as reductants of Fe(III) and as ligands of Fe(II).

Domino syntheses of bioactive tetronic and tetramic acids

Natural products containing tetronic acid or tetramic acid moieties continue to attract the interest of chemists, biologists, and physicians due to their challenging structures and to the wide range of biological activities they display. This review portrays the structural varieties of tetronic and tetramic acids and the spectrum of possible therapeutically relevant effects in man for exemplary derivatives. Their biosynthetic origin from α-amino and α-hydroxy acids is briefly discussed as is the relationship between their structures and their modes of interaction with biochemical effectors such as metal cations or enzymes. A short overview of laboratory syntheses of the heterocyclic core structures of tetramic and tetronic acids is provided with an emphasis on those emulating the biosynthesis. A synthesis from the α-amino or α-hydroxy esters and the cumulated phosphorus ylide Ph3PCCO based upon a domino addition–intra-Wittig alkenation sequence is presented with applications to the preparation of the antibiotics reutericyclin and tenuazonic acid, the cytotoxic melophlin B, and the enzyme inhibitor RK-682. Procedural advantages of immobilizing either starting component by attaching it to a resin and its exploitation in the parallel synthesis of libraries of potential drug candidates are described. The basic domino reaction can even be extended by further C–C bond forming steps when starting from suitable α-hydroxy or α-amino allyl esters. Depending on the chosen reaction conditions, bioactive intermediates of formally three to seven step long cascades can be obtained. Among them, herbicidal 3-alkyltetronic acids and lactone endoperoxides with antiplasmodial activity exceeding that of the natural antimalarial lead artemisinin. Hence, this domino reaction gives access to diversely functionalized derivatives of tetronic and tetramic acids. As it can also be ported to solid phase, it is ideally suited for parallel and combinatorial processing. Future developments might include running such domino sequences in continuous mode in arrays of “labs on microchips”.

Coinage metal complexes against breast cancer.

Breast cancer is still the leading cause of cancer deaths among women worldwide, and new therapies to treat this dangerous disease are desperately needed. The serendipitously found anticancer drug cisplatin and its second-generation congener carboplatin appear to be promising drug systems for the treatment of breast tumors, in particular of multidrug resistant and highly aggressive triple-negative subtypes. In the wake of these platinum drugs, complexes of the coinage metals copper, silver, and gold were developed that showed enhanced selectivity for breast cancer while causing fewer and weaker side-effects. This review takes stock of the latest developments in the field of coinage metal anticancer drugs with an emphasis on their biological and mechanistic aspects. Pertinent literature is covered up to 2012.

Anticancer Active Illudins: Recent Developments of a Potent Alkylating Compound Class

An overview of anticancer active spirocyclopropanes of the illudin class is provided. After a short introduction on the history and general chemistry of illudins M and S, new discoveries concerning their mode of action and metabolism are reported as well as new synthetic endeavors towards derivatives with improved selectivity for and efficacy against cancer cells. In addition, common and recently tapped biological sources and isolation procedures for known and new illudins are discussed. Pertinent literature is covered up to 2010.

Modulation of doxorubicin activity in cancer cells by conjugation with fatty acyl and terpenyl hydrazones.

Doxorubicin N-acylhydrazones derived from saturated, unsaturated and terpene-terminated fatty acids were tested for anticancer activity in cells of human HL-60 leukaemia, 518A2 melanoma, MCF-7/Topo breast and KB-V1/Vbl cervix carcinomas. In the latter, the N-heptadecanoyl hydrazone was more cytotoxic than its unsaturated C18-fatty acyl analogues and even three times more than doxorubicin. The (menthoxycarbonyl)undecanoyl hydrazone was twice as active as doxorubicin in these multidrug resistant KB-V1/Vbl and in the 518A2 cells and also more efficacious in KB-V1 and MCF-7 cells that had been desensitised for doxorubicin. All hydrazones induced apoptosis albeit by slightly different mechanisms. While apoptosis induction by the menthoxymalonyl hydrazone was characterized by an upfront increase in caspase-8 activity, all other hydrazones elicited a hike in caspase-9 activity. Treatment of HL-60 and 518A2 cells with doxorubicin or its heptadecanoyl, linolenoyl, (menthoxycarbonyl)undecanoyl or menthoxymalonyl hydrazones also led to diverging increases of the ratio of bax to bcl-2 mRNA expression, of reactive oxygen species and of mitochondrial membrane damage.

Adjusting the DNA Interaction and Anticancer Activity of Pt(II) N-Heterocyclic Carbene Complexes by Steric Shielding of the Trans Leaving Group.

Five platinum(II) complexes bearing a (1,3-dibenzyl)imidazol-2-ylidene ligand but different leaving groups trans to it were examined for cytotoxicity, DNA and cell cycle interference, vascular disrupting properties, and nephrotoxicity. The cytotoxicity of complexes 3a-c increased with the steric shielding of their leaving chloride ligand, and complex 3c, featuring two triphenylphosphanes, was the most efficacious, with submicromolar IC50 concentrations. Complexes 3a-c interacted with DNA in electrophoretic mobility shift and ethidium bromide binding assays. The cationic complex 3c did not bind coordinatively to DNA but led to its aggregation, damage that is not amenable to the usual repair mechanisms. Accordingly, it arrested the cell cycle of melanoma cells in G1 phase, whereas cis-dichlorido[(1,3-dibenzyl)imidazol-2-ylidene](dimethyl sulfoxide) platinum(II) 3a induced G2/M phase arrest. Complex 3c also disrupted the blood vessels in the chorioallantoic membrane of fertilized chicken eggs. Ex vivo studies using precision-cut tissue slices suggested the nephrotoxicities of 3a-c to be clinically manageable.