Structural insight into a quinolone-topoisomerase II-DNA complex. Further evidence for a 2:2 quinobenzoxazine-mg2+ self-assembly model formed in the presence of topoisomerase ii.
Quinobenzoxazine A-62176, developed from the antibacterial fluoroquinolones, is active in vitro and in vivo against murine and human tumors. It has been previously claimed that A-62176 is a catalytic inhibitor of mammalian topoisomerase II that does not stabilize the cleaved complex. However, at low drug concentrations and pH 6-7, we have found that A-62176 can enhance the formation of the cleaved complex at certain sites. Using a photocleavage assay, mismatched sequences, and competition experiments between psorospermin and A-62176, we pinpointed the drug binding site on the DNA base pairs between positions +1 and +2 relative to the cleaved phosphodiester bonds. A 2:2 quinobenzoxazine-Mg2+ self-assembly model was previously proposed, in which one drug molecule intercalates into the DNA helix and the second drug molecule is externally bound, held to the first molecule and DNA by two Mg2+ bridges. The results of competition experiments between psorospermin and A-62176, as well as between psorospermin and A-62176 and norfloxacin, are consistent with this model and provide the first evidence that this 2:2 quinobenzoxazine-Mg2+ complex is assembled in the presence of topoisomerase II. These results also have parallel implications for the mode of binding of the quinolone antibiotics to the bacterial gyrase-DNA complex. (+info)
Thalidomide increases both intra-tumoural tumour necrosis factor-alpha production and anti-tumour activity in response to 5,6-dimethylxanthenone-4-acetic acid.
5,6-Dimethylxanthenone-4-acetic acid (DMXAA), synthesized in this laboratory and currently in phase I clinical trial, is a low molecular weight inducer of tumour necrosis factor-alpha (TNF-alpha). Administration of DMXAA to mice with established transplantable tumours elicits rapid vascular collapse selectively in the tumour, followed by extensive haemorrhagic necrosis mediated primarily through the production of TNF-alpha. In this report we have investigated the synthesis of TNF-alpha mRNA in hepatic, splenic and tumour tissue. Co-administration of thalidomide with DMXAA increased anti-tumour activity and increased intra-tumoural TNF-alpha production approximately tenfold over that obtained with DMXAA alone. Thalidomide increased splenic TNF-alpha production slightly but significantly decreased serum and hepatic levels of TNF-alpha induced with DMXAA. Lipopolysaccharide (LPS) induced 300-fold higher serum TNF-alpha than did DMXAA at the maximum tolerated dose, but induced similar amounts of TNF-alpha in spleen, liver and tumour. Splenic TNF-alpha activity induced with LPS was slightly increased with thalidomide, but serum and liver TNF-alpha levels were suppressed. Thalidomide did not increase intra-tumoural TNF-alpha production induced with LPS, in sharp contrast to that obtained with DMXAA. While thalidomide improved the anti-tumour response to DMXAA, it had no effect on the anti-tumour action of LPS that did not induce a significant growth delay or cures against the Colon 38 tumour. The increase in the anti-tumour action by thalidomide in combination with DMXAA corresponded to an increase in intra-tumoural TNF-alpha production. Co-administration of thalidomide may represent a novel approach to improving selective intra-tumoural TNF-alpha production and anti-tumour efficacy of DMXAA. (+info)
Prostaglandin DP receptors positively coupled to adenylyl cyclase in embryonic bovine tracheal (EBTr) cells: pharmacological characterization using agonists and antagonists.
Various prostaglandin agonists representing various classes of receptor subtypes were evaluated for their ability to stimulate adenylyl cyclase via the endogenous DP receptor in embryonic bovine tracheal (EBTr) cells. Two antagonists were used to block the agonist-induced cyclic AMP production. ZK118182 (EC50 = 16+/-4 nM), RS-93520 (EC50 = 23+/- 4 nM), SQ27986 (EC50 = 33+/-9 nM), ZK110841 (EC50 = 33+/-5 nM), BW245C (EC50 = 59+/-19 nM) and PGD2 (EC50=101+/-10 nM) (n = 4-70) were the most potent agonists. Whilst most compounds were full agonists (Emax = 100% relative to PGD2), BW245C was significantly more efficacious than PGD2 (Emax = 121+/-3%; P<0.001) and RS-93520 appeared to be a partial agonist (Emax = 64+/-9%; P<0.001). Agonists from the EP (e.g. enprostil; misoprostol; butaprost), FP (e.g. cloprostenol; fluprostenol; PHXA85), IP (iloprost; PGI2) and TP (U46619) prostanoid receptor classes were weak agonists or inactive in the EBTr cell assay system. The DP-receptor antagonist, BWA868C, showed a competitive antagonist profile with pA2 values of 8.00+/-0.02 and 8.14+/-0.13 in Schild analyses with two structurally different agonists, BW245C and ZK118182, respectively (n = 3). AH6809, another purported DP-receptor antagonist, weakly inhibited PGD2- and ZK 18182-induced cyclic AMP production (K(i)s = 808+/-193 nM and 782+/-178 nM, respectively). The current studies have characterized the DP receptor positively coupled to adenylyl cyclase in EBTr cells using a wide range of agonist and antagonist prostaglandins. These data support the utility of the EBTr cell line as a useful tool for the evaluation of DP receptor agonists and antagonists and for profiling other classes of prostaglandins. (+info)
Induction of intratumoral tumor necrosis factor (TNF) synthesis and hemorrhagic necrosis by 5,6-dimethylxanthenone-4-acetic acid (DMXAA) in TNF knockout mice.
5,6-Dimethylxanthenon-4-acetic acid (DMXAA) is a new antitumor drug currently undergoing clinical trial. Administration of DMXAA to mice with tumors leads to cessation of tumor blood flow and the onset of tumor hemorrhagic necrosis, accompanied by the production of the cytokine tumor necrosis factor (TNF). Previous studies have shown that DMXAA induces both tumor and host cells to synthesize TNF and that induced intratumoral TNF production correlates with the antitumor activity of DMXAA. To explore the hypothesis that TNF production by tumor cells contributed to the induction of hemorrhagic necrosis by DMXAA, TNF-/- (C57Bl/6 background) mice were used as recipients for the s.c. implantation of (TNF positive) colon 38 adenocarcinoma. Tumors removed 24 h after treatment with DMXAA (66 or 100 micromol/kg) were found to be hemorrhagic and necrotic. Cells expressing TNF mRNA in tumors removed 2 h after treatment with DMXAA (160 micromol/kg) were found by in situ hybridization to be comparable in frequency and distribution with those in tumors from C57Bl/6 TNF-positive mice. However, the amount of TNF protein extracted from tumors from TNF knockout mice was lower than that from TNF-positive mice. Spleen and liver tissue from TNF knockout mice, in contrast to that from TNF-positive mice, produced no TNF mRNA. TNF protein was undetectable in liver and spleen tissue from TNF knockout mice, but was evident in tissue from TNF-positive mice. These results confirm that DMXAA has the novel ability of inducing tumors to synthesize TNF in situ. (+info)
Enhancement of antibody-directed enzyme prodrug therapy in colorectal xenografts by an antivascular agent.
The irregular nature of solid tumor vasculature produces a heterogeneous distribution of antibody-targeted therapies within the tumor mass, which frequently results in reduced therapeutic efficacy. We have, therefore, combined two complementary therapies: Antibody-directed Enzyme Prodrug Therapy (ADEPT), which targets tumor cells, and an agent that selectively destroys tumor vasculature. A single i.p. dose (27.5 mg/kg) of the drug 5,6-dimethylxanthenone-4-acetic acid (DMXAA), given to nude mice bearing the LS174T colorectal xenograft, destroyed all but a peripheral rim of tumor cells, without enhancing survival. The ADEPT system, in which a pretargeted enzyme activates a prodrug, consisted of the F(ab')2 fragment of anti-carcinoembryonic antigen antibody A5B7 conjugated to the bacterial enzyme carboxypeptidase G2 and the prodrug 4-[(2-chloroethyl)(2-mesyloxyethyl)amino]benzoyl-L-glutamic acid, which was given i.p. in three doses of 500 mg/kg at 72, 84, and 96 h post-conjugate administration (25 units of carboxypeptidase G2). The antibody-enzyme conjugate could be selectively retained at approximately twice the control levels by administration of the antivascular agent at the time of optimal conjugate localization within the tumor (20 h post-conjugate administration), as demonstrated by gamma counting, phosphor plate image analysis, and active enzyme measurement. This resulted in significantly enhanced tumor growth inhibition in groups of six mice, compared to conventional ADEPT therapy, with no concomitant increase in systemic toxicity. In a separate experiment, aimed at trapping the prodrug within the tumor, a 16-fold increase over control values was produced (means, 44.8 versus 2.8 microg/g tumor) when DMXAA was given 4 h prior to 4-[(2-chloroethyl)(2-mesyloxyethyl)amino]benzoyl-L-glutamic acid. The therapeutic window was small, with no significant enhancement of prodrug retention when DMXAA was given at either earlier or later time points. This correlated with the time of vascular shut-down induced by the antivascular agent. We are currently investigating whether it is more advantageous to trap increased levels of conjugate or prodrug within the tumor for maximal enhancement of conventional ADEPT. These studies demonstrate that combined use of antibody-directed and antivascular therapies can significantly benefit the therapeutic outcome of either strategy alone. (+info)
Chinonin, a novel drug against cardiomyocyte apoptosis induced by hypoxia and reoxygenation.
The inhibitory effects of Chinonin, a natural antioxidant extracted from a Chinese medicine, on apoptotic and necrotic cell death of cardiomyocytes in hypoxia-reoxygenation process were observed in this study. The possible mechanisms of Chinonin on scavenging reactive oxygen species and regulating apoptotic related genes bcl-2 and p53 were also investigated. Neonatal rat cardiomyocytes were subjected to 24-h hypoxia and 4-h reoxygenation. Cell death was evaluated by DNA electrophoresis on agarose gel, cell death ELISA and annexin-V-FLUOS/propidium iodide (PI) double staining cytometry. Hypoxia caused the increase of apoptotic rates and the release of lactate dehydrogenase (LDH), while reoxygenation not only further increased the apoptotic rates and leakage of LDH, but also induced necrosis of cardiomyocytes. In addition, hypoxia increased the levels of NO(2)(-)/NO(3)(-) and thiobarbituric acid reacted substances (TBARS), while reoxygenation decreased NO(2)(-)/NO(3)(-), but further increased TBARS in the cultured media. Moreover, hypoxia up-regulated the expression levels of bcl-2 and p53 proteins, while reoxygenation down-regulated bcl-2 and further up-regulated p53. Chinonin significantly decreased the rates of apoptotic and necrotic cardiomyocytes, and inhibited the leakage of LDH. It also diminished NO(2)(-)/NO(3)(-) and TBARS, down-regulated the expression level of p53 protein, and up-regulated bcl-2 protein, respectively. The results suggest that Chinonin has preventive effects against apoptotic and necrotic cell death and its protective mechanisms are related to the antioxidant properties of scavenging nitric oxide and oxygen free radicals, and the modulating effects on the expression levels of bcl-2 and p53 proteins. (+info)
Scavenging effect of chinonin on free radicals studied with quantum chemistry.
AIM: To study whether the xanthonoid structure can enhance the ability of chinonin to scavenge free radicals and to understand the sequence of activity of chinonin hydroxyls. METHODS: Semiempirical quantum chemistry method AM1 was employed to calculate delta HOF values, the differences between heat of formations of mother molecule and its free radicals, and spin density distribution of different states of chinonin. delta HOF values were used as theoretical indices to measure scavenging activity of chinonin on free radicals and effects of structure on delta HOF values were investigated. RESULTS: delta HOF values of different phenolics were calculated to be 139.09 kJ.mol-1 (O5-H6), 141.46 kJ.mol-1 (O4-H5), 185.14 kJ.mol-1 (O2-H2) and 195.71 kJ.mol-1 (O1-H1). Spin density distribution of free radicals were obtained as well. CONCLUSION: Xanthonoid structure of chinonin made ring C to display high passive effect on ring B, which reduced scavenging activity of phenolics of ring B on free radicals. The sequence of activities of chinonin hydroxyls was O5-H6 > O4-H5 > O2-H2 > O1-H1. (+info)
Xanthones as antimalarial agents: stage specificity.
The erythrocytic development of Plasmodium falciparum is divided into the ring, trophozoite, and schizont stages based on morphologic assessment. Using highly synchronous ring and trophozoite cultures of P. falciparum, we observed considerable differences in their sensitivity to hydroxyxanthones: trophozoites were much more sensitive to the drugs than ring-stage parasites. Trophozoites treated with a prototypic xanthone, the 2,3,4,5,6-pentahydroxy derivative (X5), were arrested in their development and became degenerate in appearance within 24 hr of drug exposure. These morphologic changes appeared to reflect the cytotoxic nature of the action of the drug against the parasite, since daughter ring-stage forms were not observed following addition of the drug. That X5 was more active against parasites in the later stages of intraerythrocytic development is consistent with the proposed mode of action, inhibition of heme polymerization. Knowledge of the structure-activity relationships for xanthones as antimalarial agents has also been expanded. Xanthones with a hydroxyl group in the peri-position exhibited decreased antimalarial activity, possibly due to intramolecular hydrogen bonding with the carbonyl and consequent reduced affinity for heme. Paired hydroxyls attached to the lower half of the xanthone greatly enhanced drug potency. (+info)