The Experts below are selected from a list of 522 Experts worldwide ranked by ideXlab platform
Taro Tokui - One of the best experts on this subject based on the ideXlab platform.
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Intracellular activation and cytotoxic action of RS-1541 against cultured human tumor cells
Cancer Chemotherapy and Pharmacology, 1995Co-Authors: Toshiro Takatori, Taro Tokui, Akio Shiraishi, Takeshi Koizumi, Yoshihiro Mitsuhashi, Takashi TsuruoAbstract:RS-1541, an acyl-derivative of Rhizoxin (Fig. 1), is a potent antitumor compound. This agent showed cytotoxicity in vitro on some cultured human tumor cells, although it was less potent than Rhizoxin. Rhizoxin exhibited antitumor effects by inhibiting the polymerization of tubulin, whereas RS-1541 did not inhibit tubulin polymerization in vitro. However, cell cycle analysis in vivo showed that the two agents had the same mode of action. The cytotoxicity of RS-1541 was enhanced when the initial cell density of the cells was increased. The cytotoxicity was also enhanced when the membrane fraction of St-4 cells, which were the most sensitive to RS-1541 among the cell lines tested, was added to the target cells. When St-4 cells were incubated with [^14C]-RS-1541, significant amounts of [^14C]-Rhizoxin were produced within the cells. Further fractionation of the crude membrane showed that the activity that enhanced the cytotoxicity of RS-1541 (RS-1541-enhancing activity) belonged to the mitochondrial-lysosomal fraction, not to the microsomal fraction. Both the enhancing activity and the activity that converting [^14C]-RS-1541 to [^14C]-Rhizoxin (RS-1541-converting activity) were inhibited by treatment with chloroquine, an inhibitor of lysosomal function. Cholesterol esterase derived from Candida cylindracea had RS-1541-enhancing and -converting activities. These data suggest that RS-1541 exerts its cytotoxic action after being converted to Rhizoxin within the cells by a lysosomal enzyme such as cholesterol esterase.
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Tumor Selective Effect of RS-1541 (Palmitoyl-Rhizoxin) in M5076 Sarcoma and Host Tissues in Vivo
Pharmaceutical Research, 1995Co-Authors: Taro Tokui, Kazuhiko Sasagawa, Toshihiko Ikeda, Naoyuki Maeda, Chitose Kuroiwa, Takashi Inoue, Kenji Kawai, Toru KomaiAbstract:RS-1541 is a 13-O-palmitoyl derivative of Rhizoxin, an inhibitor of tubulin polymerization. After intravenous administration of RS-1541 to mice bearing M5076 sarcoma, the maximal inhibitory effect of RS-1541 on DNA synthesis in the tumor was observed 24 h after administration, in agreement with the Cmax of Rhizoxin produced from RS-1541, but not with the Cmax of RS-1541. The inhibitory effect after RS-1541 was much higher than that after Rhizoxin itself. In the spleen, thymus and bone marrow, DNA synthesis was strongly inhibited by Rhizoxin but not by RS-1541. After administration of RS-1541, no significant amounts of Rhizoxin were detected in the tissues, except for the tumor. In acute toxicity tests, RS-1541 appeared to be less toxic than Rhizoxin. These results indicate that RS-1541 possesses a high tumor-selective effect compared with Rhizoxin, because of the selective production of Rhizoxin in the tumor after administration of RS-1541.
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Delivery and cytotoxicity of RS-1541 in St-4 human gastric cancer cells in vitro by the low-density-lipoprotein pathway
Cancer Chemotherapy and Pharmacology, 1995Co-Authors: Taro Tokui, Toshiro Takatori, Nobue Shinozaki, Michi Ishigami, Akio Shiraishi, Toshihiko Ikeda, Takashi TsuruoAbstract:RS-1541 is a 13- O -palmitoyl derivative of Rhizoxin, an inhibitor of tubulin polymerization. RS-1541 has been shown to bind preferentially to plasma lipoproteins and to exhibit selective and sustained uptake by tumors in mice. To elucidate a mechanism of RS-1541 cytotoxicity, the cellular uptake and the cytotoxicity of a complex of RS-1541 with human low-density lipoprotein (RS-1541/LDL complex) were investigated in cultured St-4 human gastric cancer cells. Both the cellular uptake and the cytotoxicity of the RS-1541/LDL complex were greater in cells with higher LDL-receptor activities than in control cells. Excess amounts of LDL or 1 μ M of monensin, a proton ionophore, significantly inhibited both the uptake and the cytotoxicity of the complex. Chloroquine, an inhibitor of lysosomal enzymes, decreased the intracellular level of Rhizoxin liberated from RS-1541 and suppressed the cytotoxicity of the RS-1541/LDL complex. However, a detergent-aided solution of RS-1541 showed very low cellular uptake and cytotoxicity, irrespective of the LDL-receptor activities of these cells. These results demonstrate that the RS-1541/LDL complex is incorporated into the cells via the LDL receptor and that it manifests its cytotoxic activity after forming Rhizoxin, the original antitumor agent, in the lysosomes.
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Delivery and cytotoxicity of RS-1541 in St-4 human gastric cancer cells in vitro by the low-density-lipoprotein pathway
Cancer Chemotherapy and Pharmacology, 1995Co-Authors: Taro Tokui, Toshiro Takatori, Nobue Shinozaki, Michi Ishigami, Akio Shiraishi, Toshihiko Ikeda, Takashi TsuruoAbstract:RS-1541 is a 13- O -palmitoyl derivative of Rhizoxin, an inhibitor of tubulin polymerization. RS-1541 has been shown to bind preferentially to plasma lipoproteins and to exhibit selective and sustained uptake by tumors in mice. To elucidate a mechanism of RS-1541 cytotoxicity, the cellular uptake and the cytotoxicity of a complex of RS-1541 with human low-density lipoprotein (RS-1541/LDL complex) were investigated in cultured St-4 human gastric cancer cells. Both the cellular uptake and the cytotoxicity of the RS-1541/LDL complex were greater in cells with higher LDL-receptor activities than in control cells. Excess amounts of LDL or 1 μ M of monensin, a proton ionophore, significantly inhibited both the uptake and the cytotoxicity of the complex. Chloroquine, an inhibitor of lysosomal enzymes, decreased the intracellular level of Rhizoxin liberated from RS-1541 and suppressed the cytotoxicity of the RS-1541/LDL complex. However, a detergent-aided solution of RS-1541 showed very low cellular uptake and cytotoxicity, irrespective of the LDL-receptor activities of these cells. These results demonstrate that the RS-1541/LDL complex is incorporated into the cells via the LDL receptor and that it manifests its cytotoxic activity after forming Rhizoxin, the original antitumor agent, in the lysosomes.
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contribution of serum lipoproteins as carriers of antitumour agent rs 1541 palmitoyl Rhizoxin in mice
Biopharmaceutics & Drug Disposition, 1994Co-Authors: Taro Tokui, Kazuhiko Sasagawa, Toshihiko Ikeda, Chitose Kuroiwa, Y. Tokui, K. Kawai, Tomowo Kobayashi, Toru KomaiAbstract:The tumour uptake as well as the anti-tumour activity of RS-1541 (palmitoyl Rhizoxin), a potent antineoplastic agent, were investigated in mice bearing M5076 sarcoma. After intravenous administration, 14C-RS-1541 preferentially bound to the lipoproteins, to which 14C-Rhizoxin did not bind. 14C-RS-1541 showed persisting high concentrations of radioactivity in the plasma (T 1/2 alpha, 4.9 h). The uptake of radioactivity by the tumour was second to those by the liver and spleen, and several times greater than those by the other tissues. Selective and sustained uptake by the tumour was also demonstrated by whole-body autoradiography. A considerable amount of Rhizoxin was detected only in the tumour after administration of 14C-RS-1541, and the area under the tissue-concentration-time curve (AUCt) and the mean residence time (MRT) of Rhizoxin in the tumour were much higher than those after administration of 14C-Rhizoxin itself. The Rhizoxin formation in the tumour was significantly reduced by chloroquine, a lysosomal enzyme inhibitor. RS-1541 showed a higher therapeutic activity than Rhizoxin. At a 4 mg kg-1 dose, the maximum growth inhibition was 92% for RS-1541 and 41% for Rhizoxin. These results indicate that RS-1541, but not Rhizoxin, is taken up by the tumour via endocytosis, most likely via the low-density-lipoprotein receptor, after binding to lipoproteins. Thus, RS-1541 was considered to exhibit sustained high concentration in tumours and potent anti-tumour activity.
Takashi Tsuruo - One of the best experts on this subject based on the ideXlab platform.
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Intracellular activation and cytotoxic action of RS-1541 against cultured human tumor cells
Cancer Chemotherapy and Pharmacology, 1995Co-Authors: Toshiro Takatori, Taro Tokui, Akio Shiraishi, Takeshi Koizumi, Yoshihiro Mitsuhashi, Takashi TsuruoAbstract:RS-1541, an acyl-derivative of Rhizoxin (Fig. 1), is a potent antitumor compound. This agent showed cytotoxicity in vitro on some cultured human tumor cells, although it was less potent than Rhizoxin. Rhizoxin exhibited antitumor effects by inhibiting the polymerization of tubulin, whereas RS-1541 did not inhibit tubulin polymerization in vitro. However, cell cycle analysis in vivo showed that the two agents had the same mode of action. The cytotoxicity of RS-1541 was enhanced when the initial cell density of the cells was increased. The cytotoxicity was also enhanced when the membrane fraction of St-4 cells, which were the most sensitive to RS-1541 among the cell lines tested, was added to the target cells. When St-4 cells were incubated with [^14C]-RS-1541, significant amounts of [^14C]-Rhizoxin were produced within the cells. Further fractionation of the crude membrane showed that the activity that enhanced the cytotoxicity of RS-1541 (RS-1541-enhancing activity) belonged to the mitochondrial-lysosomal fraction, not to the microsomal fraction. Both the enhancing activity and the activity that converting [^14C]-RS-1541 to [^14C]-Rhizoxin (RS-1541-converting activity) were inhibited by treatment with chloroquine, an inhibitor of lysosomal function. Cholesterol esterase derived from Candida cylindracea had RS-1541-enhancing and -converting activities. These data suggest that RS-1541 exerts its cytotoxic action after being converted to Rhizoxin within the cells by a lysosomal enzyme such as cholesterol esterase.
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Delivery and cytotoxicity of RS-1541 in St-4 human gastric cancer cells in vitro by the low-density-lipoprotein pathway
Cancer Chemotherapy and Pharmacology, 1995Co-Authors: Taro Tokui, Toshiro Takatori, Nobue Shinozaki, Michi Ishigami, Akio Shiraishi, Toshihiko Ikeda, Takashi TsuruoAbstract:RS-1541 is a 13- O -palmitoyl derivative of Rhizoxin, an inhibitor of tubulin polymerization. RS-1541 has been shown to bind preferentially to plasma lipoproteins and to exhibit selective and sustained uptake by tumors in mice. To elucidate a mechanism of RS-1541 cytotoxicity, the cellular uptake and the cytotoxicity of a complex of RS-1541 with human low-density lipoprotein (RS-1541/LDL complex) were investigated in cultured St-4 human gastric cancer cells. Both the cellular uptake and the cytotoxicity of the RS-1541/LDL complex were greater in cells with higher LDL-receptor activities than in control cells. Excess amounts of LDL or 1 μ M of monensin, a proton ionophore, significantly inhibited both the uptake and the cytotoxicity of the complex. Chloroquine, an inhibitor of lysosomal enzymes, decreased the intracellular level of Rhizoxin liberated from RS-1541 and suppressed the cytotoxicity of the RS-1541/LDL complex. However, a detergent-aided solution of RS-1541 showed very low cellular uptake and cytotoxicity, irrespective of the LDL-receptor activities of these cells. These results demonstrate that the RS-1541/LDL complex is incorporated into the cells via the LDL receptor and that it manifests its cytotoxic activity after forming Rhizoxin, the original antitumor agent, in the lysosomes.
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Delivery and cytotoxicity of RS-1541 in St-4 human gastric cancer cells in vitro by the low-density-lipoprotein pathway
Cancer Chemotherapy and Pharmacology, 1995Co-Authors: Taro Tokui, Toshiro Takatori, Nobue Shinozaki, Michi Ishigami, Akio Shiraishi, Toshihiko Ikeda, Takashi TsuruoAbstract:RS-1541 is a 13- O -palmitoyl derivative of Rhizoxin, an inhibitor of tubulin polymerization. RS-1541 has been shown to bind preferentially to plasma lipoproteins and to exhibit selective and sustained uptake by tumors in mice. To elucidate a mechanism of RS-1541 cytotoxicity, the cellular uptake and the cytotoxicity of a complex of RS-1541 with human low-density lipoprotein (RS-1541/LDL complex) were investigated in cultured St-4 human gastric cancer cells. Both the cellular uptake and the cytotoxicity of the RS-1541/LDL complex were greater in cells with higher LDL-receptor activities than in control cells. Excess amounts of LDL or 1 μ M of monensin, a proton ionophore, significantly inhibited both the uptake and the cytotoxicity of the complex. Chloroquine, an inhibitor of lysosomal enzymes, decreased the intracellular level of Rhizoxin liberated from RS-1541 and suppressed the cytotoxicity of the RS-1541/LDL complex. However, a detergent-aided solution of RS-1541 showed very low cellular uptake and cytotoxicity, irrespective of the LDL-receptor activities of these cells. These results demonstrate that the RS-1541/LDL complex is incorporated into the cells via the LDL receptor and that it manifests its cytotoxic activity after forming Rhizoxin, the original antitumor agent, in the lysosomes.
Toru Komai - One of the best experts on this subject based on the ideXlab platform.
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Tumor Selective Effect of RS-1541 (Palmitoyl-Rhizoxin) in M5076 Sarcoma and Host Tissues in Vivo
Pharmaceutical Research, 1995Co-Authors: Taro Tokui, Kazuhiko Sasagawa, Toshihiko Ikeda, Naoyuki Maeda, Chitose Kuroiwa, Takashi Inoue, Kenji Kawai, Toru KomaiAbstract:RS-1541 is a 13-O-palmitoyl derivative of Rhizoxin, an inhibitor of tubulin polymerization. After intravenous administration of RS-1541 to mice bearing M5076 sarcoma, the maximal inhibitory effect of RS-1541 on DNA synthesis in the tumor was observed 24 h after administration, in agreement with the Cmax of Rhizoxin produced from RS-1541, but not with the Cmax of RS-1541. The inhibitory effect after RS-1541 was much higher than that after Rhizoxin itself. In the spleen, thymus and bone marrow, DNA synthesis was strongly inhibited by Rhizoxin but not by RS-1541. After administration of RS-1541, no significant amounts of Rhizoxin were detected in the tissues, except for the tumor. In acute toxicity tests, RS-1541 appeared to be less toxic than Rhizoxin. These results indicate that RS-1541 possesses a high tumor-selective effect compared with Rhizoxin, because of the selective production of Rhizoxin in the tumor after administration of RS-1541.
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contribution of serum lipoproteins as carriers of antitumour agent rs 1541 palmitoyl Rhizoxin in mice
Biopharmaceutics & Drug Disposition, 1994Co-Authors: Taro Tokui, Kazuhiko Sasagawa, Toshihiko Ikeda, Chitose Kuroiwa, Y. Tokui, K. Kawai, Tomowo Kobayashi, Toru KomaiAbstract:The tumour uptake as well as the anti-tumour activity of RS-1541 (palmitoyl Rhizoxin), a potent antineoplastic agent, were investigated in mice bearing M5076 sarcoma. After intravenous administration, 14C-RS-1541 preferentially bound to the lipoproteins, to which 14C-Rhizoxin did not bind. 14C-RS-1541 showed persisting high concentrations of radioactivity in the plasma (T 1/2 alpha, 4.9 h). The uptake of radioactivity by the tumour was second to those by the liver and spleen, and several times greater than those by the other tissues. Selective and sustained uptake by the tumour was also demonstrated by whole-body autoradiography. A considerable amount of Rhizoxin was detected only in the tumour after administration of 14C-RS-1541, and the area under the tissue-concentration-time curve (AUCt) and the mean residence time (MRT) of Rhizoxin in the tumour were much higher than those after administration of 14C-Rhizoxin itself. The Rhizoxin formation in the tumour was significantly reduced by chloroquine, a lysosomal enzyme inhibitor. RS-1541 showed a higher therapeutic activity than Rhizoxin. At a 4 mg kg-1 dose, the maximum growth inhibition was 92% for RS-1541 and 41% for Rhizoxin. These results indicate that RS-1541, but not Rhizoxin, is taken up by the tumour via endocytosis, most likely via the low-density-lipoprotein receptor, after binding to lipoproteins. Thus, RS-1541 was considered to exhibit sustained high concentration in tumours and potent anti-tumour activity.
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Contribution of serum lipoproteins as carriers of antitumour agent RS‐1541 (palmitoyl Rhizoxin) in mice
Biopharmaceutics & drug disposition, 1994Co-Authors: Taro Tokui, Kazuhiko Sasagawa, Toshihiko Ikeda, Chitose Kuroiwa, Y. Tokui, K. Kawai, Tomowo Kobayashi, Toru KomaiAbstract:The tumour uptake as well as the anti-tumour activity of RS-1541 (palmitoyl Rhizoxin), a potent antineoplastic agent, were investigated in mice bearing M5076 sarcoma. After intravenous administration, 14C-RS-1541 preferentially bound to the lipoproteins, to which 14C-Rhizoxin did not bind. 14C-RS-1541 showed persisting high concentrations of radioactivity in the plasma (T 1/2 alpha, 4.9 h). The uptake of radioactivity by the tumour was second to those by the liver and spleen, and several times greater than those by the other tissues. Selective and sustained uptake by the tumour was also demonstrated by whole-body autoradiography. A considerable amount of Rhizoxin was detected only in the tumour after administration of 14C-RS-1541, and the area under the tissue-concentration-time curve (AUCt) and the mean residence time (MRT) of Rhizoxin in the tumour were much higher than those after administration of 14C-Rhizoxin itself. The Rhizoxin formation in the tumour was significantly reduced by chloroquine, a lysosomal enzyme inhibitor. RS-1541 showed a higher therapeutic activity than Rhizoxin. At a 4 mg kg-1 dose, the maximum growth inhibition was 92% for RS-1541 and 41% for Rhizoxin. These results indicate that RS-1541, but not Rhizoxin, is taken up by the tumour via endocytosis, most likely via the low-density-lipoprotein receptor, after binding to lipoproteins. Thus, RS-1541 was considered to exhibit sustained high concentration in tumours and potent anti-tumour activity.
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studies on macrocyclic lactone antibiotics xiii anti tubulin activity and cytotoxicity of Rhizoxin derivatives synthesis of a photoaffinity derivative
The Journal of Antibiotics, 1991Co-Authors: Yuzo Kato, Shigeo Iwasaki, Tomowo Kobayashi, Yuji Ogawa, Takashi Imada, Naomi Shimazaki, Toru KomaiAbstract:Chemical modification of the side chain in Rhizoxin, a potent antimitotic agent, was attempted in order to study structure-activity relationships and also to devise a probe for photoaffinity labeling of tubulin. An OsO 4 /NaIO 4 oxidation gave a nor-Rhizoxin 20-al (5) which was converted to 20-ol (6) by a NaBH 3 CN reduction. Starting from these two compounds as key intermediates, a series of Wittig reaction products 7∼12. and of 20-o-acylates 13∼21 were prepared and their anti-tubulin activity and cytotoxicity were determined. An aryl azide derivative 23 was synthesized as a photoaffinity analogue
Toshihiko Ikeda - One of the best experts on this subject based on the ideXlab platform.
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Tumor Selective Effect of RS-1541 (Palmitoyl-Rhizoxin) in M5076 Sarcoma and Host Tissues in Vivo
Pharmaceutical Research, 1995Co-Authors: Taro Tokui, Kazuhiko Sasagawa, Toshihiko Ikeda, Naoyuki Maeda, Chitose Kuroiwa, Takashi Inoue, Kenji Kawai, Toru KomaiAbstract:RS-1541 is a 13-O-palmitoyl derivative of Rhizoxin, an inhibitor of tubulin polymerization. After intravenous administration of RS-1541 to mice bearing M5076 sarcoma, the maximal inhibitory effect of RS-1541 on DNA synthesis in the tumor was observed 24 h after administration, in agreement with the Cmax of Rhizoxin produced from RS-1541, but not with the Cmax of RS-1541. The inhibitory effect after RS-1541 was much higher than that after Rhizoxin itself. In the spleen, thymus and bone marrow, DNA synthesis was strongly inhibited by Rhizoxin but not by RS-1541. After administration of RS-1541, no significant amounts of Rhizoxin were detected in the tissues, except for the tumor. In acute toxicity tests, RS-1541 appeared to be less toxic than Rhizoxin. These results indicate that RS-1541 possesses a high tumor-selective effect compared with Rhizoxin, because of the selective production of Rhizoxin in the tumor after administration of RS-1541.
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Delivery and cytotoxicity of RS-1541 in St-4 human gastric cancer cells in vitro by the low-density-lipoprotein pathway
Cancer Chemotherapy and Pharmacology, 1995Co-Authors: Taro Tokui, Toshiro Takatori, Nobue Shinozaki, Michi Ishigami, Akio Shiraishi, Toshihiko Ikeda, Takashi TsuruoAbstract:RS-1541 is a 13- O -palmitoyl derivative of Rhizoxin, an inhibitor of tubulin polymerization. RS-1541 has been shown to bind preferentially to plasma lipoproteins and to exhibit selective and sustained uptake by tumors in mice. To elucidate a mechanism of RS-1541 cytotoxicity, the cellular uptake and the cytotoxicity of a complex of RS-1541 with human low-density lipoprotein (RS-1541/LDL complex) were investigated in cultured St-4 human gastric cancer cells. Both the cellular uptake and the cytotoxicity of the RS-1541/LDL complex were greater in cells with higher LDL-receptor activities than in control cells. Excess amounts of LDL or 1 μ M of monensin, a proton ionophore, significantly inhibited both the uptake and the cytotoxicity of the complex. Chloroquine, an inhibitor of lysosomal enzymes, decreased the intracellular level of Rhizoxin liberated from RS-1541 and suppressed the cytotoxicity of the RS-1541/LDL complex. However, a detergent-aided solution of RS-1541 showed very low cellular uptake and cytotoxicity, irrespective of the LDL-receptor activities of these cells. These results demonstrate that the RS-1541/LDL complex is incorporated into the cells via the LDL receptor and that it manifests its cytotoxic activity after forming Rhizoxin, the original antitumor agent, in the lysosomes.
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Delivery and cytotoxicity of RS-1541 in St-4 human gastric cancer cells in vitro by the low-density-lipoprotein pathway
Cancer Chemotherapy and Pharmacology, 1995Co-Authors: Taro Tokui, Toshiro Takatori, Nobue Shinozaki, Michi Ishigami, Akio Shiraishi, Toshihiko Ikeda, Takashi TsuruoAbstract:RS-1541 is a 13- O -palmitoyl derivative of Rhizoxin, an inhibitor of tubulin polymerization. RS-1541 has been shown to bind preferentially to plasma lipoproteins and to exhibit selective and sustained uptake by tumors in mice. To elucidate a mechanism of RS-1541 cytotoxicity, the cellular uptake and the cytotoxicity of a complex of RS-1541 with human low-density lipoprotein (RS-1541/LDL complex) were investigated in cultured St-4 human gastric cancer cells. Both the cellular uptake and the cytotoxicity of the RS-1541/LDL complex were greater in cells with higher LDL-receptor activities than in control cells. Excess amounts of LDL or 1 μ M of monensin, a proton ionophore, significantly inhibited both the uptake and the cytotoxicity of the complex. Chloroquine, an inhibitor of lysosomal enzymes, decreased the intracellular level of Rhizoxin liberated from RS-1541 and suppressed the cytotoxicity of the RS-1541/LDL complex. However, a detergent-aided solution of RS-1541 showed very low cellular uptake and cytotoxicity, irrespective of the LDL-receptor activities of these cells. These results demonstrate that the RS-1541/LDL complex is incorporated into the cells via the LDL receptor and that it manifests its cytotoxic activity after forming Rhizoxin, the original antitumor agent, in the lysosomes.
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contribution of serum lipoproteins as carriers of antitumour agent rs 1541 palmitoyl Rhizoxin in mice
Biopharmaceutics & Drug Disposition, 1994Co-Authors: Taro Tokui, Kazuhiko Sasagawa, Toshihiko Ikeda, Chitose Kuroiwa, Y. Tokui, K. Kawai, Tomowo Kobayashi, Toru KomaiAbstract:The tumour uptake as well as the anti-tumour activity of RS-1541 (palmitoyl Rhizoxin), a potent antineoplastic agent, were investigated in mice bearing M5076 sarcoma. After intravenous administration, 14C-RS-1541 preferentially bound to the lipoproteins, to which 14C-Rhizoxin did not bind. 14C-RS-1541 showed persisting high concentrations of radioactivity in the plasma (T 1/2 alpha, 4.9 h). The uptake of radioactivity by the tumour was second to those by the liver and spleen, and several times greater than those by the other tissues. Selective and sustained uptake by the tumour was also demonstrated by whole-body autoradiography. A considerable amount of Rhizoxin was detected only in the tumour after administration of 14C-RS-1541, and the area under the tissue-concentration-time curve (AUCt) and the mean residence time (MRT) of Rhizoxin in the tumour were much higher than those after administration of 14C-Rhizoxin itself. The Rhizoxin formation in the tumour was significantly reduced by chloroquine, a lysosomal enzyme inhibitor. RS-1541 showed a higher therapeutic activity than Rhizoxin. At a 4 mg kg-1 dose, the maximum growth inhibition was 92% for RS-1541 and 41% for Rhizoxin. These results indicate that RS-1541, but not Rhizoxin, is taken up by the tumour via endocytosis, most likely via the low-density-lipoprotein receptor, after binding to lipoproteins. Thus, RS-1541 was considered to exhibit sustained high concentration in tumours and potent anti-tumour activity.
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Contribution of serum lipoproteins as carriers of antitumour agent RS‐1541 (palmitoyl Rhizoxin) in mice
Biopharmaceutics & drug disposition, 1994Co-Authors: Taro Tokui, Kazuhiko Sasagawa, Toshihiko Ikeda, Chitose Kuroiwa, Y. Tokui, K. Kawai, Tomowo Kobayashi, Toru KomaiAbstract:The tumour uptake as well as the anti-tumour activity of RS-1541 (palmitoyl Rhizoxin), a potent antineoplastic agent, were investigated in mice bearing M5076 sarcoma. After intravenous administration, 14C-RS-1541 preferentially bound to the lipoproteins, to which 14C-Rhizoxin did not bind. 14C-RS-1541 showed persisting high concentrations of radioactivity in the plasma (T 1/2 alpha, 4.9 h). The uptake of radioactivity by the tumour was second to those by the liver and spleen, and several times greater than those by the other tissues. Selective and sustained uptake by the tumour was also demonstrated by whole-body autoradiography. A considerable amount of Rhizoxin was detected only in the tumour after administration of 14C-RS-1541, and the area under the tissue-concentration-time curve (AUCt) and the mean residence time (MRT) of Rhizoxin in the tumour were much higher than those after administration of 14C-Rhizoxin itself. The Rhizoxin formation in the tumour was significantly reduced by chloroquine, a lysosomal enzyme inhibitor. RS-1541 showed a higher therapeutic activity than Rhizoxin. At a 4 mg kg-1 dose, the maximum growth inhibition was 92% for RS-1541 and 41% for Rhizoxin. These results indicate that RS-1541, but not Rhizoxin, is taken up by the tumour via endocytosis, most likely via the low-density-lipoprotein receptor, after binding to lipoproteins. Thus, RS-1541 was considered to exhibit sustained high concentration in tumours and potent anti-tumour activity.
Christian Hertweck - One of the best experts on this subject based on the ideXlab platform.
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Twofold polyketide branching by a stereoselective enzymatic Michael addition.
Chemical communications (Cambridge England), 2015Co-Authors: Daniel Heine, Srividhya Sundaram, Tom Bretschneider, Christian HertweckAbstract:The versatility of the branching module of the Rhizoxin polyketide synthase was tested in an in vitro enzyme assay with a polyketide mimic and branched (di)methylmalonyl-CoA extender units. Comparison of the products with synthetic reference compounds revealed that the module is able to stereoselectively introduce two branches in one step by a Michael addition–lactonisation sequence, thus expanding the scope of previously studied PKS systems.
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Chemical and biological features of endofungal bacteria.
2013Co-Authors: Gerald Lackner, Christian HertweckAbstract:(A) Chemical structures of Rhizoxin, an antimitotic macrolide, and rhizonin A, a hepatotoxic cyclopeptide. (B) Light micrograph of a sporangium of R. microsporus (ATCC 62147) stained with a viability assay system (Invitrogen). Green spots are living cells of the bacterial endosymbiont B. Rhizoxinica. (C) Life cycle of R. microsporus strains and their endosymbionts B. Rhizoxinica and B. endofungorum. Endobacteria are propagated within fungal spores (vertical transmission). Under laboratory conditions, isolated bacteria can infect compatible host strains (horizontal transmission).
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Symbiotic cooperation in the biosynthesis of a phytotoxin.
Angewandte Chemie (International ed. in English), 2012Co-Authors: Kirstin Scherlach, Gerald Lackner, Benjamin Busch, Uta Paszkowski, Christian HertweckAbstract:Natural products play a key role in symbiotic interactions between microorganisms and higher organisms, covering all kingdoms of life. The function of these secondary metabolites may range from signaling compounds in mutualism to virulence factors and antibiotics in parasitic relationships. In many cases the interactions involve multiple partners and thus the biogenetic basis of chemical mediators can be quite complex. This complexity is well exemplified by the unparalleled tripartite relationship among the rice-seedling-blight fungus Rhizopus microsporus, its host plant Oryza sativa and endosymbiotic bacteria that reside in the fungal cytosol. The bacterial symbionts (Burkholderia species) produce a phytotoxin complex to assist the phytopathogenic fungus in colonizing rice seedlings. In turn the bacteria profit from a safe niche and access to nutrients released from the decaying plant. Initially, the macrolide Rhizoxin (1, Figure 1) and various congeners such as WF-1360F (2) were isolated from cultures of R. microsporus van Tieghem var. chinensis and identified as the causative agent of rice seedling blight. Rhizoxin efficiently inhibits eukaryotic cell proliferation by binding to b-tubulin and thus blocking the formation of the mitotic spindle. Notably, the pure compound alone evokes the typical symptoms of seedling root swelling. Only recently, through detection, isolation, and cultivation of the endosymbionts we could unequivocally prove that actually associated bacteria are the true producers of the toxin complex. The importance of this metabolic capability has been underlined by the finding that fungal reproduction depends entirely on the presence of the bacterial symbionts. Survival of the toxinogenic symbiosis is warranted by the strict sporulation control and exclusive dispersal of spores harboring endosymbionts. Moreover, the unusual mutualism has been fine-tuned through symbiosis factors such as a type-III secretion system and a novel lipopolysaccharide O-antigen that sets the symbionts into a “stealth mode” by decorating the outer membrane of the endosymbionts. The host, on the other hand, acquired resistance towards Rhizoxin by mutation of the b-tubulin. Because of its ecological and medicinal relevance as an antimitotic agent, the biosynthesis of Rhizoxin has been studied. Cloning, sequencing, and molecular analyses of the Rhizoxin (rhi) biosynthetic gene cluster in the genome of Burkholderia Rhizoxinica revealed the molecular basis for a complex polyketide assembly line required for the biosynthesis of the virulence factor. Whereas the biosynthesis of the macrolide backbone has been decoded by mutational analyses, polyketide tailoring mechanisms and the biological role of the bis(epoxidation) have remained elusive. Herein we elucidate the dual epoxidation of Rhizoxin and its impact on rice seedling blight and report an unprecedented case for symbiotic cooperation in the biosynthesis of an ecologically relevant natural product. In a broader survey on Rhizoxin-positive Rhizopus species we discovered that the unusual bacterial–fungal association is not restricted to a single isolate but has spread worldwide. We have identified eight related Burkholderia–Rhizopus associations from geographically highly different regions on five continents; these findings underlign the ecological imporFigure 1. A) Structures of Rhizoxin and congeners. B) Phylogenetic relationship of Rhizopus microsporus strains; structures of the corresponding metabolites indicate which strains can produce bisepoxides. The numbers on top of the branches indicate the clade probability values; the scale on the left site relates the length of a branch to the distance (number of changes that have taken place along a branch).
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photochemical oxazole nitrile conversion downstream of Rhizoxin biosynthesis and its impact on antimitotic activity
Organic and Biomolecular Chemistry, 2012Co-Authors: Kirstin Scherlach, Hans-martin Dahse, Christian Hertweck, Nicole Brendel, Keishi IshidaAbstract:Through metabolic profiling of mutants and wild type of the endofungal bacterium Burkholderia Rhizoxinica two novel Rhizoxin derivatives with unusual nitrile substitutions were discovered. The nitrile groups result from a photochemical oxidative cleavage of the oxazolyl moiety. In vitro studies revealed that the photooxidation by singlet oxygen also takes place in the absence of a photosensitizer, and that also a thiazolyl-substituted Rhizoxin analogue undergoes the same transformation. The resulting nitriles have antimitotic properties but are significantly less active than the parent compounds. These results highlight the impact of photoreactions onto the antiproliferative agent and encourage the introduction of bioisosteric groups that render the compound less susceptible towards photooxidation.
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Photochemical oxazole–nitrile conversion downstream of Rhizoxin biosynthesis and its impact on antimitotic activity
Organic & biomolecular chemistry, 2012Co-Authors: Kirstin Scherlach, Hans-martin Dahse, Nicole Brendel, Keishi Ishida, Christian HertweckAbstract:Through metabolic profiling of mutants and wild type of the endofungal bacterium Burkholderia Rhizoxinica two novel Rhizoxin derivatives with unusual nitrile substitutions were discovered. The nitrile groups result from a photochemical oxidative cleavage of the oxazolyl moiety. In vitro studies revealed that the photooxidation by singlet oxygen also takes place in the absence of a photosensitizer, and that also a thiazolyl-substituted Rhizoxin analogue undergoes the same transformation. The resulting nitriles have antimitotic properties but are significantly less active than the parent compounds. These results highlight the impact of photoreactions onto the antiproliferative agent and encourage the introduction of bioisosteric groups that render the compound less susceptible towards photooxidation.
