Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:5.99.1.2 (topoisomerase)
 9,166 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The development of tumor drug resistance is the major obstacle to successful systemic chemotherapy. Therefore, devising methods for reversing drug resistance is a high priority and could lead to significant improvements in cancer treatment. The mechanisms of tumor drug resistance are manifold and are not well understood. The phenomenon of multidrug resistance (MDR) represents the development of resistance to most drugs, regardless of their chemical structure. Several types of MDR are known, for example, the overexpression of a cell membrane glycoprotein (P-170), increased activity of glutathione S-transferase, elevated levels of glutathione (GSH), and alterations in topoisomerase action. A partial reversal of tumor drug resistance has been achieved by the use of competitive inhibitors for the function of glycoprotein P-170, or by the inhibition of GSH synthesis; however, this strategy has not been substantially successful for improving the response of human tumors to clinical therapy. We have recently used electroporation, in conjunction with the cytotoxic drug, cisplatin (cDDP), in an attempt to circumvent drug resistance in cDDP-resistant mouse tumor cells (RIF/Ptr1). Electroporation is the application of a high-voltage electric shock which is known to create transient pores in plasma membranes of cultured cells. Electroporation plus cDDP treatment increased intracellular cDDP concentration and reversed cellular resistance to cDDP-induced cell killing.
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PMID:New approaches to the study of tumor drug resistance. 136 47

pH-mediated conversions in the structure of the topoisomerase (topo) I inhibitors camptothecin (CPT) and its analogues have strong implications for the pharmacokinetics and pharmacodynamics of these novel anticancer agents. Because the cell-penetrating and biologically active lactone isomers predominate at acidic conditions, we have tested if low pH potentiates the cytotoxic and antitumor effects of CPT and its water-soluble derivative topotecan (TPT). In L1210 leukemia cells, rapid initial uptake of radiolabeled CPT and TPT was followed by a gradual release from cells at physiological pH 7.4, whereas high drug levels were maintained in cells at pH 6.2. Steady-state uptake levels of CPT increased proportionally, up to 5-fold, with decreasing pH of the incubating medium (from 7.4 to 6.0). With TPT, a maximum 3-fold increase was observed at pH 6.8 to 6.4. By contrast, the cellular pharmacokinetics of the topoisomerase II inhibitor etoposide (ETP) were independent of the ambient pH. The large increases in intracellular CPT and TPT levels caused only moderate potentiation of cytotoxicity in short-term incubations. Conditions of very low pH < or =6.2 even antagonized the cytotoxicity of the topo I and topo II inhibitors, due to inhibition of DNA synthesis by intracellular acidification. However, in clinically relevant schedules of prolonged exposures at low drug concentration, low pH potentiated the cytotoxicity of CPT and TPT by 2-3-fold. To investigate the effect of local pH in vivo, the basal interstitial pH of 6.8 of RIF-1 tumors was selectively lowered by i.p. injection of the host animals with the mitochondrial inhibitor meta-iodobenzylguanidine (32 mg/kg) and glucose (1.5 g/kg). In accordance with the pH optimum for TPT uptake at pH 6.8 to 6.4, tumor acidification had no effect on the antitumor effect of this analogue. By contrast, the intervention significantly potentiated the response of tumors to CPT. The results indicate that local pH is an important determinant of the cellular pharmacokinetics and the antitumor activity of CPT and analogues.
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PMID:Cellular pharmacokinetics and cytotoxicity of camptothecin and topotecan at normal and acidic pH. 935 43

AQ4 (1,4-Bis-[[2-(dimethylamino-N-oxide)ethyl]amino]5,8-dihydroxyanthrace ne-9, 10-dione) is a prodrug designed to be excluded from cell nuclei until bioreduced in hypoxic cells to AQ4, a DNA intercalator and topoisomerase II poison. Thus, AQ4N is a highly selective bioreductive drug that is activated in, and is preferentially toxic to, hypoxic cells in tumours. Five murine tumours (MAC16, MAC26, NT, SCCVII and RIF-1) have been used to investigate the anti-tumour effects of AQ4N. In only one tumour (MAC16) was AQ4N shown to be active as a single agent. However, when combined with methods to increase the hypoxic tumour fraction in RIF-1 (by physical clamping) and MAC26 tumours (using hydralazine) there was a substantial enhancement in anti-tumour effect. Notably, RIF-1 tumours treated with AQ4N (250 mg kg(-1)) followed 15 min later by physically occluding the blood supply to the tumour for 90 min, resulted in a 13-fold increase in growth delay. When combined with radiation or chemotherapy, AQ4N substantially increased the effectiveness of these modalities in a range of in vivo model systems. AQ4N potentiates the action of radiation in both a drug and radiation dose-dependent manner. Further the enhancement observed is schedule-independent with AQ4N giving similar effects when given at any time within 16 h before or after the radiation treatment. In combination with chemotherapy it is shown that AQ4N potentiates the activity of cyclophosphamide, cisplatin and thiotepa. Both the chemotherapeutic drugs and AQ4N are given at doses which individually are close to their estimated maximum tolerated dose (data not included) which provides indirect evidence that in the combination chemotherapy experiments there is some tumour selectivity in the enhanced action of the drugs.
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PMID:Enhancement of chemotherapy and radiotherapy of murine tumours by AQ4N, a bioreductively activated anti-tumour agent. 1086 7