Aaron Beekman - Academia.edu (original) (raw)
Papers by Aaron Beekman
Journal of Natural Products, 1997
This study deals with the cytotoxicity of helenanolide-type (10 alpha-methylpseudoguaianolide) se... more 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.
Planta Medica, 1998
We determined the in vitro cytotoxic activity of the sesquiterpene lactone endoperoxide artemisin... more 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.
Phytotherapy Research, 1996
Artemisinin and its derivatives possess an endoperoxide bridge, which is thought to lead to the p... more Artemisinin and its derivatives possess an endoperoxide bridge, which is thought to lead to the production of free-radical species. The cytotoxicity of some of these agents to a murine Ehrlich ascites (EN19) and a human HeLa S3 cancer cell line was determined using the MTT and the clonogenic assay. The MTT assay cannot distinguish between growth inhibition and cell killing, while the clonogenic assay detects actual cell death. The use of both assays to test a certain drug may give information on the mode of its cytotoxicity (i.e. growth inhibition versus cell killing). The endoperoxides artemisinin and the dimer of dihydroartemisinin showed much higher cytotoxicity in the MTT assay compared with the clonogenic assay. Thus these drugs mainly induced growth inhibition. For artemisitene and eupatoriopicrin, which possess an exocyclic methylene with alkylating properties, both tests yielded comparable results. For these compounds the MTT assay merely determined cell killing. For the reference drugs cisplatin and doxorubicin the MTT assay showed lower cytotoxicity than the clonogenic assay. This may be explained by the metabolic activity of cells that were clonogenically dead. Moreover, our experiments have shown that the MTT assay may lead to misinterpretations concerning the mode of action of certain drugs, when it is used as a substitute for the clonogenic assay.
Pharmacy World & Science, 1994
Artemisinin, a sesquiterpene lactone endoperoxide isolated fromArtemisia annua L., and a number o... more Artemisinin, a sesquiterpene lactone endoperoxide isolated fromArtemisia annua L., and a number of its semisynthetic derivatives have shown to possess antimalarial properties. They are all eflective againstPlasmodium parasites that are resistant to the newest and commonly used antimalarial drugs. This article gives a survey of the literature dealing with artemisinin-relaled 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, sideeffects and toxicology, and biological activities other than antimalarial activity.
Journal of Natural Products, 1997
We determined the cytotoxicity of some artemisinin derivatives against EN2 tumor cells using the ... more 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 G 1 -phase of the cell cycle. In contrast, artemisinin (1) caused a slight increase of S-phase cells.
Journal of Natural Products, 1996
Aeroplysinin-1 (1) and the structurally related dienone 2 were cytotoxic to Ehrlich ascites tumor... more 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.
Phytotherapy Research, 1996
Artemisinin and its derivatives possess an endoperoxide bridge, which is thought to lead to the p... more Artemisinin and its derivatives possess an endoperoxide bridge, which is thought to lead to the production of free-radical species. The cytotoxicity of some of these agents to a murine Ehrlich ascites (EN19) and a human HeLa S3 cancer cell line was determined using the MTT and the clonogenic assay. The MTT assay cannot distinguish between growth inhibition and cell killing, while the clonogenic assay detects actual cell death. The use of both assays to test a certain drug may give information on the mode of its cytotoxicity (i.e. growth inhibition versus cell killing). The endoperoxides artemisinin and the dimer of dihydroartemisinin showed much higher cytotoxicity in the MTT assay compared with the clonogenic assay. Thus these drugs mainly induced growth inhibition. For artemisitene and eupatoriopicrin, which possess an exocyclic methylene with alkylating properties, both tests yielded comparable results. For these compounds the MTT assay merely determined cell killing. For the reference drugs cisplatin and doxorubicin the MTT assay showed lower cytotoxicity than the clonogenic assay. This may be explained by the metabolic activity of cells that were clonogenically dead. Moreover, our experiments have shown that the MTT assay may lead to misinterpretations concerning the mode of action of certain drugs, when it is used as a substitute for the clonogenic assay.
Journal of Natural Products, 1997
This study deals with the cytotoxicity of helenanolide-type (10 alpha-methylpseudoguaianolide) se... more 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.
Planta Medica, 1998
We determined the in vitro cytotoxic activity of the sesquiterpene lactone endoperoxide artemisin... more 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.
Phytotherapy Research, 1996
Artemisinin and its derivatives possess an endoperoxide bridge, which is thought to lead to the p... more Artemisinin and its derivatives possess an endoperoxide bridge, which is thought to lead to the production of free-radical species. The cytotoxicity of some of these agents to a murine Ehrlich ascites (EN19) and a human HeLa S3 cancer cell line was determined using the MTT and the clonogenic assay. The MTT assay cannot distinguish between growth inhibition and cell killing, while the clonogenic assay detects actual cell death. The use of both assays to test a certain drug may give information on the mode of its cytotoxicity (i.e. growth inhibition versus cell killing). The endoperoxides artemisinin and the dimer of dihydroartemisinin showed much higher cytotoxicity in the MTT assay compared with the clonogenic assay. Thus these drugs mainly induced growth inhibition. For artemisitene and eupatoriopicrin, which possess an exocyclic methylene with alkylating properties, both tests yielded comparable results. For these compounds the MTT assay merely determined cell killing. For the reference drugs cisplatin and doxorubicin the MTT assay showed lower cytotoxicity than the clonogenic assay. This may be explained by the metabolic activity of cells that were clonogenically dead. Moreover, our experiments have shown that the MTT assay may lead to misinterpretations concerning the mode of action of certain drugs, when it is used as a substitute for the clonogenic assay.
Pharmacy World & Science, 1994
Artemisinin, a sesquiterpene lactone endoperoxide isolated fromArtemisia annua L., and a number o... more Artemisinin, a sesquiterpene lactone endoperoxide isolated fromArtemisia annua L., and a number of its semisynthetic derivatives have shown to possess antimalarial properties. They are all eflective againstPlasmodium parasites that are resistant to the newest and commonly used antimalarial drugs. This article gives a survey of the literature dealing with artemisinin-relaled 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, sideeffects and toxicology, and biological activities other than antimalarial activity.
Journal of Natural Products, 1997
We determined the cytotoxicity of some artemisinin derivatives against EN2 tumor cells using the ... more 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 G 1 -phase of the cell cycle. In contrast, artemisinin (1) caused a slight increase of S-phase cells.
Journal of Natural Products, 1996
Aeroplysinin-1 (1) and the structurally related dienone 2 were cytotoxic to Ehrlich ascites tumor... more 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.
Phytotherapy Research, 1996
Artemisinin and its derivatives possess an endoperoxide bridge, which is thought to lead to the p... more Artemisinin and its derivatives possess an endoperoxide bridge, which is thought to lead to the production of free-radical species. The cytotoxicity of some of these agents to a murine Ehrlich ascites (EN19) and a human HeLa S3 cancer cell line was determined using the MTT and the clonogenic assay. The MTT assay cannot distinguish between growth inhibition and cell killing, while the clonogenic assay detects actual cell death. The use of both assays to test a certain drug may give information on the mode of its cytotoxicity (i.e. growth inhibition versus cell killing). The endoperoxides artemisinin and the dimer of dihydroartemisinin showed much higher cytotoxicity in the MTT assay compared with the clonogenic assay. Thus these drugs mainly induced growth inhibition. For artemisitene and eupatoriopicrin, which possess an exocyclic methylene with alkylating properties, both tests yielded comparable results. For these compounds the MTT assay merely determined cell killing. For the reference drugs cisplatin and doxorubicin the MTT assay showed lower cytotoxicity than the clonogenic assay. This may be explained by the metabolic activity of cells that were clonogenically dead. Moreover, our experiments have shown that the MTT assay may lead to misinterpretations concerning the mode of action of certain drugs, when it is used as a substitute for the clonogenic assay.