Deoxypodophyllotoxin 6-hydroxylase, a cytochrome P450 monooxygenase from cell cultures of Linum flavum involved in the biosynthesis of cytotoxic lignans.

Cell-suspension cultures of Linum flavum L. (Linaceae) synthesize and accumulate aryltetrahydronaphthalene lignans with 6-methoxypodophyllotoxin as the main component. The experimental data indicate that the biosynthesis of 6-methoxypodophyllotoxin occurs via deoxypodophyllotoxin, beta-peltatin, and beta-peltatin-A methyl ether. The enzyme catalyzing the introduction of the hydroxyl group in position 6 is deoxypodophyllotoxin 6-hydroxylase (DOP6H). The enzyme was shown to be a cytochrome P450-dependent monooxygenase by blue-light reversion of carbon monoxide inhibition and inhibition by cytochrome c. DOP6H is a membrane-bound microsomal enzyme with a pH optimum of 7.6 and a temperature optimum of 26 degrees C. Deoxypodophyllotoxin is specifically accepted with an apparent Km of 20 microM and a saturation concentration of 200 microM; 4'-demethyldeoxypodophyllotoxin is the only other tested substrate accepted for hydroxylation. DOP6H predominantly accepts NADPH as electron donor; NADH can only sustain low hydroxylation activities. A synergistic effect of NADPH and NADH is not observed. The enzyme is saturated around 250 microM NADPH; the apparent Km for this substrate is 36 microM.

Isolation and identification of dihydroartemisinic acid from artemisia annua and its possible role in the biosynthesis of artemisinin

Dihydroartemisinic acid (2) was isolated as a natural product from Artemisia annua in a 66% yield, and its structure was confirmed by 1H and 13C NMR spectroscopy. Compound 2 could be chemically converted to artemisinin (4) under conditions that may also be present in the living plant. The results suggest that the conversion of 2 into 4 in the living plant might be a nonenzymatic conversion.

Artemisinin-derived sesquiterpene lactones as potential antitumour compounds: cytotoxic action against bone marrow and tumour cells.

We determined the in vitro cytotoxic activity of the sesquiterpene lactone endoperoxide artemisinin (1) and some chemically prepared derivatives, which have been found to display cytotoxicity to cloned murine Ehrlich ascites tumour (EAT) cells and human HeLa cells and against murine bone marrow using a clonogenic assay for committed progenitor cells of the granulocyte-monocyte lineage (CFU-GM assay). Comparing artemisinin (1) to deoxyartemisinin (2), the endoperoxide group appeared to play a role in cytotoxicity to CFU-GM cells. Dimers of dihydroartemisinin and dihydrodeoxyartemisinin revealed that the stereochemistry of the ether linkage of the dimers was a more important determinant for this cytotoxic activity. The nonsymmetrical dimer of dihydroartemisinin (3) and the corresponding endoperoxide-lacking dimer of dihydrodeoxyartemisinin (5) were equally cytotoxic to CFU-GM cells. Despite the differences between both systems, it may be stated that most compounds displayed higher cytotoxicity to CFU-GM cells than to EAT cells. Dimers of dihydroartemisinin (3, 4) were selected as potential antitumour compounds and subjected to the National Cancer Institute drug-screening programme consisting of about sixty human cancer cell lines derived from nine different tissues. Both compounds displayed the same specific cytotoxicity pattern. Throughout the screen dimer 3 was more active than 4.

Stereochemistry-dependent cytotoxicity of some artemisinin derivatives.

We determined the cytotoxicity of some artemisinin derivatives against EN2 tumor cells using the MTT assay. Artemisinin (1) was clearly more cytotoxic than deoxyartemisinin (2), which lacks the endoperoxide bridge. Ether-linked dimers of dihydroartemisinin with defined stereochemistry were found to differ in the extent of cytotoxic effect on EN2 cells. The nonsymmetrical dimer (3) was more cytotoxic than the symmetrical dimer (4). The nonsymmetrical dimer of dihydrodeoxyartemisinin (5) lacking the endoperoxide bridges was also effective in the MTT assay, although less cytotoxic than 3 and 4. Similarly, the symmetrical dimer (6) was less effective than 5. Epoxides of artemisitene also showed that stereochemistry was an important factor for cytotoxicity. The results suggested that the endoperoxide bridge was not crucial for cytotoxicity to the tumor cells, but contributed to the cytotoxic effect apparently exerted by the ether linkage of the dimers. Flow cytometry data indicated that the dimers 3 and 4 caused an accumulation of the cells in the G1-phase of the cell cycle. In contrast, artemisinin (1) caused a slight increase of S-phase cells.

Structure-cytotoxicity relationships of a series of natural and semi-synthetic simple coumarins as assessed in two human tumour cell lines.

The cytotoxicity of 22 natural and semi-synthetic simple coumarins was evaluated in GLC4, a human small cell lung carcinoma cell line, and in COLO 320, a human colorectal cancer cell line, using the microculture tetrazolium (MTT) assay. With IC50 values > 100 microM, following a continuous (96 h) incubation, most coumarins exhibited only low cytotoxicity. Several compounds, however, displayed significant potencies. As far as the structure--cytotoxicity relationship is concerned, it is conspicuous that all the potentially active natural compounds possess at least two phenolic groups in either the 6,7- or 6,8-positions. In addition, the 5-formyl-6-hydroxy substituted semi-synthetic analogue was found to be potent, reflecting the importance of at least two polar functions for high cytotoxicity.

Structure-cytotoxicity relationships of some helenanolide-type sesquiterpene lactones.

This study deals with the cytotoxicity of helenanolide-type (10 alpha-methylpseudoguaianolide) sesquiterpene lactones. We determined the influence of substitution patterns on the toxicity of 21 helenanolides to a cloned Ehrlich ascites tumor cell line, EN2. Within a series of helenalin esters, the acetate (2) and isobutyrate (3) were more toxic than helenalin itself (1). Esters with larger acyl groups (tiglate 4 and isovalerate 5) exhibited a decreased toxicity compared with the parent alcohol (1). Similar relationships were observed between the 6,8-diastereomer of helenalin, mexicanin I (6) and its acetate (7) and isovalerate (8). In contrast, cytotoxicity within a series of 11 alpha, 13-dihydrohelenalin esters (9-12) was shown to be directly related to the size and lipophilicity of the ester side chain, dihydrohelenalin (9) being the least toxic compound in this group. Investigation of several 2,3-dihydrohelenalin derivatives (13-21) with 2 alpha-hydroxy-4-oxo- and 2 alpha,4 alpha-dihydroxy- or -O-acyl-substituted cyclopentane rings (arnifolins and chamissonolides, respectively), for which no pharmacological data have been reported so far, revealed further interesting influences of the substitution pattern on cytotoxicity. The results may be interpreted in terms of lipophilicity and steric effects on the accessibility of the reactive sites considered responsible for biological activity.

Stability of artemisinin in aqueous environments: impact on its cytotoxic action to Ehrlich ascites tumour cells.

We have recently shown artemisinin to be cytotoxic against Ehrlich ascites tumour cells. The aim of this study was to investigate the stability of this compound in the aqueous environment of the in-vitro Ehrlich ascites tumour cell system (RPMI 1640 cell culture medium supplemented with 10% foetal bovine serum (RPMI/FBS) with reference to its cytotoxic action. Literature data show that artemisinin can react with Fe2+ yielding reactive intermediates leaving artemisinin G as a major end-product. The current study showed that only excess addition of Fe2+ to artemisinin in distilled water, phosphate-buffered saline (PBS) and RPMI/FBS and incubation for 24 h led to degradation of artemisinin and yielded artemisinin G. If Fe2+ was not added results from HPLC analysis were indicative of complete recovery of artemisinin from distilled water and RPMI/FBS, with or without cells, at 37 degrees C for at least 24 h. In addition, incubation of artemisinin in RPMI/FBS with or without cells at 37 degrees C for 24 h before cytotoxicity assay did not change its cytotoxic action. On the basis of these results, we suggest that cytotoxicity to tumour cells was caused by unchanged artemisinin. This is not so for the antimalarial activity of artemisinin and derivatives, for which the presence of a pool of (haem) Fe2+ is a prerequisite resulting in free radicals or electrophilic intermediates or both.

Isolation of two cytotoxic diterpenes from the fern Pteris multifida.

From aerial parts of the fern Pteris multifida Poir. (Polypodiaceae) two diterpenes, entkaurane-2 beta, 16 alpha-diol and ent-kaur-16-ene-2 beta, 15 alpha-diol, were isolated by repeated column chromatography using silica gel and silica gel impregnated with silver nitrate. The structures were confirmed by spectroscopic methods. Both compounds showed a moderate cytotoxicity to Ehrlich ascites tumour cells.

Cytotoxicity and mode of action of aeroplysinin-1 and a related dienonefrom the sponge Aplysina aerophoba.

Aeroplysinin-1 (1) and the structurally related dienone 2 were cytotoxic to Ehrlich ascites tumor (EAT) cells and HeLa tumor cells in the microculture tetrazolium (MTT) and clonogenic assays. Both compounds are bromotyrosine derivatives, isolated from the marine spong Aplysina aerophoba. As the effective concentrations in the MTT assay were lower than in the clonogenic assay, 1 and 2 are able to cause growth inhibition as well as actual cell death in these cell lines. With an IC50 value of 8.2 microM (MTT assay, 2-h incubation, EAT cells), 1 was the more toxic compound. When the cells were depleted of glutathione by pretreatment with buthionine sulfoximine, they were significantly more sensitive toward 1 and 2 in the MTT assay. A dose-enhancement factor as high as 11.8 was found in EAT cells after 2-h incubation with 2. Using electron paramagnetic resonance we were able to measure free radical formation of 1 and 2, yielding the semiquinone structures 3 and 4, respectively, in a culture medium with tumor cells. It is concluded that free radicals are, at least in part, responsible for the cytotoxicity of 1 and 2. This conclusion is in line with expectations derived from the chemical structures of both compounds.

Synthesis and cytotoxicity of novel lignans.

In this study the syntheses of 11 novel lignans are described. Their cytotoxicities are studied in GLC4, a human small cell lung carcinoma cell line, using the microculture tetrazolium (MTT) assay. Ten of these compounds were substituted with a menthyloxy group on the 5-position of the lactone. These compounds can easily be prepared in (novel) 'one-pot', three- or four-step syntheses. In addition, methods for controlling the stereogenic centers are described. Furthermore, five naturally occurring podophyllotoxin-related compounds were tested. The cytotoxicities of all lignan compounds, and of three non-lignan intermediates originating from the syntheses, were compared with the clinically applied anticancer agents etoposide, teniposide, and cisplatin. Most compounds showed moderate to high activities against GLC4, and two of the compounds containing a menthyloxy group showed activities comparable to the reference cytotoxic agents.

Decreased helenalin-induced cytotoxicity by flavonoids from Arnica as studied in a human lung carcinoma cell line.

The effect of the flavones apigenin, luteolin, hispidulin and eupafolin, and of the flavonols kaempferol, quercetin, 6-methoxykaempferol and patuletin from Arnica spp. on the cytotoxicity of the sesquiterpene lactone helenalin was studied in the human lung carcinoma cell line GLC4 using the microculture tetrazolium (MTT) assay. The tumour cells were exposed to the test compounds for 2h. Helenalin concentrations around its control IC(50) value, 0.5 µM, were combined with flavonoid concentrations ranging from 0.01 to 20 µM. At non-toxic concentrations, up to 10µM, all flavonoids except kaempferol significantly reduced the helenalin-induced cytotoxicity. Hispidulin and patuletin displayed their modulating effect on helenalin-induced cytotoxicity in the broadest concentration range. The strongest effect was found with 5 and 10µM hispidulin, 0.05 µM quercetin, and 1 µM patuletin, increasing the IC(50) value of helenalin with circa 40%. No dose-dependency was found in the concentration range tested.

Cytotoxicity of flavonoids and sesquiterpene lactones from Arnica species against the GLC4 and the COLO 320 cell lines.

The cytotoxicity of 21 flavonoids and 5 sesquiterpene lactones, as present in Arnica species, was studied in GLC4, a human small cell lung carcinoma cell line, and in COLO 320, a human colorectal cancer cell line, using the microculture tetrazolium (MTT) assay. Following continuous incubation, most flavonoids showed moderate to low cytotoxicity, as compared with the reference compound cisplatin (IC50 = 1.1 microM against GLC4 and 2.9 microM against COLO 320). Their IC50 values varied from 17 to > 200 microM. The most toxic compound was the flavone jaceosidin. Of the sesquiterpene lactones tested, helenalin, possessing both the reactive alpha-methylene-gamma-lactone moiety and a reactive alpha,beta-unsubstituted cyclopentenone ring, displayed the strongest cytotoxicity. For 2 h exposure, its IC50 value was 0.44 microM against GLC4 and 1.0 microM against COLO 320. COLO 320 was more sensitive than GLC4 for many flavonoids (especially for flavones), but more resistant to the cytotoxic effect of the sesquiterpene lactones bearing an exocylic methylene group fused to the lactone function.

Progress in the research of artemisinin-related antimalarials: an update.

Artemisinin, a sesquiterpene lactone endoperoxide isolated from Artemisia annua L., and a number of its semisynthetic derivatives have shown to possess antimalarial properties. They are all effective against Plasmodium parasites that are resistant to the newest and commonly used antimalarial drugs. This article gives a survey of the literature dealing with artemisinin-related antimalarial issues that have appeared from the end of 1989 up to the beginning of 1994. A broad range of medical and pharmaceutical disciplines is covered, including phytochemical aspects like the selection of high-producing plants, analytical procedures, and plant biotechnology. Furthermore, the organic synthesis of artemisinin derivatives is discussed, as well as their mechanism of action and antimalarial activity, metabolism and pharmacokinetics, clinical studies, side-effects and toxicology, and biological activities other than antimalarial activity.

Artemisinin, Related Sesquiterpenes, and Essential Oil in Artemisia annua During a Vegetation Period in Vietnam.

The active principle of ARTEMISIA ANNUA L., artemisinin, is currently being developed to a registered antimalarial drug. For production purposes, plants with a high artemisinin content are required. We followed the development of the artemisinin content and of the biosynthetically related sesquiterpenes artemisinic acid, arteannuin B, and artemisitene in A. ANNUA plants, during a vegetation period in Vietnam, where this species is indigenous. In addition, the essential oil content and composition were studied. Samples of leaves, buds, flowers, or post-bloom flowers and fruits were taken at different stages: vegetative (5, 6, and 8 months old), at mass formation of buds (9 months), at full bloom (10 months), and post-bloom (10S months). The highest artemisinin content (0.86% dry wt) was present in the leaves of 5 months-old plants. At this stage also the highest leaf yield was found. Subsequently, the artemisinin content gradually dropped. At the age of 5 months the highest artemisinic acid and arteannuin B contents, 0.16 and 0.08% dry wt, respectively, were found as well. Artemisitene was present at all stages of development, ranging from 0.002 to 0.09% dry wt. With 1.9% v/w, the essential oil content was maximal just before flowering and was composed of 55% monoterpenes and 45% sesquiterpenes. At all other stages (0.4 - 1.0% v/w oil) this ratio was ca. 30%/70%. The main components of the oil were camphor and germacrene-D.

Mucuna pruriens: improvement of the biotechnological production of the anti-Parkinson drug L-dopa by plant cell selection.

Routinely grown cell suspension cultures of Mucuna pruriens L. (Fabaceae) were able to endogenously accumulate the anti-Parkinson drug L-dihydroxyphenylalanine (L-dopa) in the range between 0.2 and 2% on a dry weight (DW) basis. The green colour that developed in light-exposed cultures, appeared to be a suitable marker to select cells with an increased L-dopa biosynthesis and/or phenoloxidase activity. For this purpose, saccharose concentrations from 0 to 4% (w/v), and light intensities of 1,000 and 2,000 lux, were involved in the selection procedure. After 6 months, photomixotrophic callus cultures with a rapid growth and a high L-dopa content of 0.9% (DW) were obtained on 2% saccharose and under 1,000 lux. The cell suspensions, derived from these calli, accumulated up to 6% (DW) L-dopa, which was the highest stable content ever measured in cultures of M. pruriens. An L-dopa yield of approximately 1.2 g/l was calculated after 6 days of growth. In contrast, compared wtih the standard-grown parent cell line, the phenoloxidase activity, and consequently the bioconversion capacity as measured after entrapment in calcium alginate, of these high-producing cultures was approximately threefold lower.

Isolation, purification, and cytotoxicity of 5-methoxypodophyllotoxin, a lignan from a root culture of Linum flavum.

A method has been developed for the large scale isolation of 5-methoxypodophyllotoxin [1] from a high-producing root culture derived from Linum flavum. A closely related lignan, 5'-demethoxy-5-methoxypodophyllotoxin [2], was also present in the root culture and was the cause of the main isolation difficulties. Essential steps in the isolation procedure are CH2Cl2 and XAD-4 extraction and XAD-8 cc followed by Si gel chromatography, using two different mobile phases. The isolated 5-methoxypodophyllotoxin [1] was very pure (greater than 99%) and possessed the desired stereochemical configuration, namely (-)-5-methoxypodophyllotoxin [1]. The in vitro cytotoxicity of 5-methoxypodophyllotoxin [1] against EAT and HeLa cells was determined and compared with those of podophyllotoxin [3], etoposide (VP-16-213) [4], teniposide (VM-26) [5], and 5-methoxypodophyllotoxin-4-beta-D-glucoside [6]. It appeared that 5-methoxypodophyllotoxin [1] has about the same cytotoxic potency as podophyllotoxin [3].

The accumulation and isolation of coniferin from a high-producing cell suspension of Linum flavum L.

Cell suspensions of Linum flavum L. contained large amounts (2 g·l(-1)) of the glucoside coniferin which was accumulated endogenously up to 12.4% on a dryweight basis. Callus material contained 5.6%, while in leaves of in-vitro-grown plantlets, the origin of the callus and suspension cultures, no coniferin could be detected. Leaf, callus and suspension material were compared for metabolite accumulation and associated enzyme activities. High coniferin contents corresponded with low 5-methoxypodophyllotoxin levels. A reciprocal relationship between ß-glucosidase (E.C. 3.2.1.21) activity and coniferin accumulation was found. No relationship between peroxidase (E.C. 1.11.1.7) activity and coniferin accumulation or 5-methoxypodophyllotoxin could be demonstrated. Finally, a rapid and effective isolation procedure for coniferin was developed.

Increased podophyllotoxin production in Podophyllum hexandrum cell suspension cultures after feeding coniferyl alcohol as a ß-cyclodextrin complex.

Cell suspension cultures, derived from roots of Podophyllum hexandrum Royle (Berberidaceae), accumulate podophyllotoxin. In this study the use of ß-cyclodextrin in feeding the poorly water-soluble precursor coniferyl alcohol to these cultures is described. By complexation with ß-cyclodextrin, a solution of 3 mM coniferyl alcohol could be fed, resulting in enhanced podophyllotoxin accumulation. The same concentration of non-complexed suspended coniferyl alcohol had only little effect on the podophyllotoxin accumulation. ß-Cyclodextrin itself was proven to be non-toxic for the cells. It did not influence the podophyllotoxin content and it was not metabolized or used as a carbon source by the cells. For comparison, coniferin, the water-soluble ß-D-glucoside of coniferyl alcohol, was also fed in the same concentration. The effect of coniferin on the podophyllotoxin accumulation was stronger than that of coniferyl alcohol complexed with ß-cyclodextrin, but coniferin is not commercially available.

The accumulation of podophyllotoxin-ß-D-glucoside by cell suspension cultures derived from the conifer Callitris drummondii.

Podophyllotoxin was produced by cell cultures derived from needles of Callitris drummondii. The needles of this conifer contained 1.56% podophyllotoxin on a dry weight basis, 32% being present in the ß-glucosidic form. Trace amounts of desoxypodophyllotoxin and matairesinol were also detected. In dark-grown cell cultures, ca. 0.02 % podophyllotoxin was accumulated, 85-90 % in the ß-D-glucosidic form. Moreover, traces of the lignans matairesinol, hinokinin and asarinin were detected. Illumination stimulated the endogenous production of podophyllotoxin-ß-D-glucoside; contents of up to 0.11 % could be measured.