Characterization of a subline of P388 leukemia resistant to amsacrine: evidence of altered topoisomerase II function

Sensitive (P388/S) and amsacrine-resistant (P388/amsacrine) sublines of P388 leukemia were cloned in vitro and tested for differential chemosensitivity against a panel of drugs. P388/amsacrine, resistant both in vivo and in vitro to amsacrine, was cross-resistant to other putative topoisomerase II inhibitors including teniposide, etoposide, bisantrene, and doxorubicin. P388/amsacrine, was however, as sensitive as cloned P388/S to camptothecin, an inhibitor of topoisomerase I. The pattern of cross-resistance suggested that an alteration in topoisomerase II may be involved in the resistance of P388/amsacrine to these drugs. No differences in the uptake of amsacrine were detected between the two sublines. Cross-resistance to vinblastine was evident in P388/amsacrine; however resistance to vinblastine was associated with alterations in uptake or efflux of the drug. The number of protein-concealed single-strand breaks induced in whole cells by amsacrine, teniposide, bisantrene, and camptothecin was measured. Diminished numbers of strand breaks in the resistant subline were consistent with decreases in DNA-protein crosslinks. In the absence of drug treatment, resistant cells sustained approximately one-half as many single-strand breaks and DNA-protein crosslinks as the sensitive cells during preparation of nuclei. As measured by the P4 phage DNA unknotting assay, 0.35 M NaCl nuclear extracts from P388/S contained approximately 2.3-fold more topoisomerase II catalytic activity than did extracts from P388/amsacrine. The amount of protein that immunoreacted with a specific antibody to calf thymus topoisomerase II was also decreased in the resistant cells. These data suggest that alterations in topoisomerase II which lead to differential drug sensitivities are partially responsible for the resistance of P388/amsacrine to a specific group of drugs.

Effect of amsacrine on ara-CTP cellular pharmacology in human leukemia cells during high-dose cytarabine therapy

The combination of high-dose cytarabine (ara-C) and amsacrine (m-AMSA) is effective treatment for relapsed adult acute leukemia. Studies were performed to determine if m-AMSA affected the pharmacokinetics of the active triphosphate ara-CTP in HL-60 and K562 cells in culture. No significant differences were observed in accumulation, rate of elimination, or total intracellular exposure to ara-CTP in cultures treated with 100 microM ara-C alone or in combination with 1 microM m-AMSA. In clinical investigations, the accumulation and retention of ara-CTP in circulating leukemic cells were studied in five patients after two serial doses of ara-C (3 g/m2 infused over 2 hours) and in six additional patients in whom the second dose of ara-C was accompanied by an infusion of m-AMSA (30 mg/m2 infused over 1 hour). While substantial differences were observed in the cellular pharmacokinetics of ara-CTP among patients, the rate of ara-CTP elimination and the total intracellular exposure to ara-CTP in individuals were remarkably similar after each ara-C infusion. Infusion of m-AMSA with the second dose of ara-C did not significantly affect the cellular pharmacokinetics of ara-CTP. These studies demonstrate the feasibility and utility of conducting investigations of the cellular pharmacology of drug-drug interactions in human leukemic cells during therapy.

The interaction between nuclear topoisomerase II activity from human leukemia cells, exogenous DNA, and 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) or 4-(4,6-O-ethylidene-beta-D-glucopyranoside) (VP-16) indicates the sensitivity of the cells to the drugs

The presumptive intracellular target of the anti-leukemia agents 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) and 4-(4,6-O-ethylidene-beta-D-glucopyranoside) (VP-16) is the enzyme topoisomerase II. We found that 350 mM NaCl extracts of nuclei from HL-60 and HL-60/AMSA, an m-AMSA resistant HL-60 subline, contained equivalent topoisomerase II activity. However, the ability of m-AMSA to stimulate cleavage of exogenous DNA and to stimulate crosslinking of exogenous DNA with protein, processes which are topoisomerase II-mediated, was greatly reduced in the HL-60/AMSA extracts compared to the HL-60 extracts. HL-60 and HL-60/AMSA were almost equally sensitive to the cytotoxic effects of VP-16 and differences in VP-16-stimulated, topoisomerase II-mediated exogenous DNA cleavage and protein crosslinking between HL-60 and HL-60/AMSA extracts were much less than the differences in m-AMSA-stimulated exogenous DNA cleavage and protein crosslinking. Thus, the interaction between topoisomerase II activity, exogenous DNA, and m-AMSA or VP-16 indicated the susceptibility HL-60 and HL-60/ AMSA to the cytotoxic effects of the drugs. A similar correlation may exist in explanted leukemia cells from patients with acute myelogenous leukemia.

Topoisomerase II-mediated DNA breaks and cytotoxicity in relation to cell proliferation and the cell cycle in NIH 3T3 fibroblasts and L1210 leukemia cells

The DNA intercalator, 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) and the nonintercalator, etoposide (VP-16) produce topoisomerase II-mediated protein-linked DNA strand breaks. This function of topoisomerase II was investigated in relation to cell proliferation and cell cycle. Mouse fibroblasts NIH 3T3 and mouse leukemia L1210 cells stop proliferation when they reach a certain density. Nuclei were isolated from proliferative or quiescent cells and then treated with drug for 30 min. DNA modifications were assayed by alkaline elution. We found that the frequencies of m-AMSA- or VP-16-induced DNA-protein links were higher in nuclei from exponentially growing than in those from quiescent cells in both the 3T3 and the L1210 lines. Drug-induced protein-associated DNA breaks were also studied as a function of the cell cycle in 3T3 cells that had been arrested by contact inhibition in medium containing 1% calf serum and then stimulated to proliferate by raplating at a lower cell density in medium containing 10% serum. In these synchronized cells, a large peak of [3H]thymidine incorporation occurred 18-30 h after replating. The yield of DNA-protein cross-links produced by 30-min drug treatments of nuclei isolated at various times after growth initiation increased concomitantly with the peak of the DNA synthesis. The topoisomerase II activity of nuclear extracts, as measured by kinetoplast DNA decatenation followed a similar pattern. Using colony-forming assays, we also observed that m-AMSA and VP-16 were most cytotoxic in proliferative cells and during DNA synthesis. These results suggest that alkaline elution measurement of m-AMSA- or VP-16-induced protein-linked DNA breaks reflects the association of topoisomerase II with DNA. This association is increased during DNA replication, making the cells more vulnerable to m-AMSA and VP-16 at this time.

Amsacrine evaluation

Amsacrine, an antineoplastic agent currently undergoing clinical trials in the U.S., has been shown to be active against adult and pediatric leukemias, Hodgkin's disease, and non-Hodgkin's lymphomas. Amsacrine is highly bound to plasma proteins and is eliminated primarily via hepatic metabolism. Severe hepatic dysfunction will result in a decreased excretion rate of the drug. The primary side effect is a dose-related suppression of bone marrow function. Other reported toxic effects include mucositis, nausea, vomiting, cardiotoxicity, liver dysfunction, and alopecia. Despite these negative effects, amsacrine appears to have a role in the combination therapy of acute leukemias.

Sensitivity and resistance to chemotherapy in acute leukemia: correlation with in vitro drug uptake and lack of potentiation by verapamil

The calcium channel blocker verapamil has been reported to circumvent acquired resistance to different antitumor agents in tumor cell lines in vitro. We studied its effect on in vitro uptake of m-AMSA and adriamycin in fresh leukemic cells from 11 leukemia patients. Six previously untreated patients were sensitive to m-AMSA (obtained remission). Four were clinically resistant to m-AMSA, and two of these also to adriamycin. Leukemic cells were incubated in pharmacological doses of 14C-adriamycin and 14C-m-AMSA for up to 2 h. Samples were supplemented with verapamil (750 ng ml-1) 30 min prior to the addition of m-AMSA or adriamycin. Drug uptake was measured at 15 min intervals up to 2 h and drug retention was measured during 30 min after the end of incubation, following washing and resuspension in fresh medium without cytotoxic drugs. Adriamycin uptake was the same irrespective of verapamil in all four cell samples, two of which were derived from patients resistant to adriamycin. The cellular m-AMSA uptake was higher in cells from clinically sensitive than from resistant patients (510 +/- 155 fg cell-1 vs 275 +/- 125 fg cell-1; P less than 0.01). Retention of m-AMSA 30 min after incubation was higher in cells from sensitive compared to resistant patients (187 +/- 78 vs 25 +/- 7; P less than 0.05). Our data suggest: (1) in vitro uptake greater than or equal to 350 fg cell-1 and subsequent retention greater than 75 fg cell-1 correlate to clinical sensitivity to the drug; and (2) neither m-AMSA nor adriamycin uptake could be significantly increased by verapamil.

Effects of a new amsacrine derivative, N-5-dimethyl-9-(2-methoxy-4-methylsulfonylamino)phenylamino-4- acridinecarboxamide, on cultured mammalian cells

The effects of N-5-dimethyl-9-(2-methoxy-4-methylsulfonylamino)-phenylamino-4- acridinecarboxamide (CI-921; NSC 343499), a lipophilic and water-soluble derivative of amsacrine (NSC 249992), on cell viability, growth, clonogenicity, and progression through the cell cycle were investigated in suspension cultures of Friend erythroleukemic cells and in in suspension cultures of Friend erythroleukemic cells and in adherent cultures of Chinese hamster ovary cells. CI-921 was less toxic toward stationary than toward exponentially growing Chinese hamster ovary cells; colony formation was inhibited by 50% following a 1-h pulse of 190 versus 80 nM CI-921, respectively. Cell viability was unaffected in Friend erythroleukemic cell cultures at concentrations up to 50 nM, although growth was inhibited by 50% following 24 h of continuous exposure to 9.5 nM or a 1 h pulse of 67.5 nM CI-921. Constant exposure of Friend erythroleukemic cells to 10 nM CI-921 slowed proliferation and resulted in prolongation of cell transit through late S and G2 phases. Higher drug concentrations (50 nM) caused a complete cessation of growth marked by greatly suppressed cell transit through S phase and an irreversible block in G2 phase, about 30 min prior to division. In such cases, unbalanced growth was observed with total RNA and protein content of drug-treated cells increasing by 74 and 34%, respectively. Pulse exposure of cells to CI-921 resulted in transient accumulations of cells in S and/or G2 phase depending upon dose. The cell cycle distribution of stationary cultures treated for 1 h with drug and replated at a low cell density were identical to that of controls. Binding of the drug affected the sensitivity of DNA in situ to acid denaturing conditions which provides additional evidence that CI-921 binds to DNA by intercalation.

Cell cycle dependence of sister chromatid exchange induction by DNA topoisomerase II inhibitors in Chinese hamster V79 cells

The cell cycle dependence of sister chromatid exchange (SCE) induced by topoisomerase II inhibitors was studied in Chinese hamster V79 cells. 4'-(9-Acridinylamino)methansulfon-m-anisidide, which increases the concentration of covalently linked DNA-topoisomerase II complexes (cleavable complexes), induces SCE strongly in only a short period of the cell cycle. The sensitive period was identified an occurring in early to mid-S phase through the use of labeled thymidine incorporation and flow cytometry. Novobiocin, an inhibitor which prevents formation of the cleavable complex, did not induce SCEs in any part of the cell cycle. However, novobiocin did decrease the level of 4'-(9-acridinylamino)methansulfon-m-anisidide-induced SCEs. These results indicate that the cleavable complex may be important in 4'-(9-acridinylamino)methansulfon-m-anisidide-induced SCE.

Charge transfer-oxy radical mechanism for anticancer agents: mAMSA derivatives, rhodamine 123, and nickel salicylaldoximate

The proposal is advanced that many anticancer agents may function via redox reactions resulting in generation of toxic oxy radicals which destroy neoplastic cells. Cyclic voltammetry was performed with some of the main types: iminium ions (protonated mAMSA derivatives), quinone derivatives (rhodamine 123) and metal complexes (nickel(II) salicylaldoximate). In addition, relevant literature data are provided. A rationale is offered that relates electrochemical data to physiological activity.

Electron donor properties of the antitumour drug amsacrine as studied by fluorescence quenching of DNA-bound ethidium

The effect of the antitumour acridine derivative amsacrine [4'-(9-acridinylamino)methanesulphon-m-anisidide] on the fluorescence lifetime of DNA-bound ethidium has been investigated using a synchronously pumped cavity dumped dye laser producing picosecond pulses for sample excitation and a time-correlated single photon counting detection system. As the proportion of DNA-bound amsacrine on the synthetic DNA polymer poly[deoxyadenylic-thymidylic acid] is increased, the fluorescence decay curve of ethidium can be accurately resolved into two exponential components. The short lifetime component, whose proportion increases with increasing proportions of DNA-bound amsacrine, has a lifetime of between 3 and 4 ns, significantly longer than that of ethidium in aqueous solution (1.63 ns). The magnitude of the long lifetime component decreases from 25.4 to 14 ns with increasing proportions of bound amsacrine. It is concluded that a new fluorescence state of ethidium (lifetime 3-4 ns) is present, probably resulting from reversible electron transfer between ethidium and amsacrine. The ability of various 9-anilinoacridine derivatives to quench the fluorescence of DNA-bound ethidium appears to be related to the electron donor properties of the substituents on the anilino ring, as well as to experimental antitumour activity. The electron donor properties of DNA-bound amsacrine may therefore be relevant to its antitumour action.

Effect of cell proliferation and chromatin conformation on intercalator-induced, protein-associated DNA cleavage in human brain tumor cells and human fibroblasts

Antineoplastic intercalating agents such as 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) stabilize a cleavable complex between topoisomerase II and DNA. The production of protein-associated DNA cleavage in whole cells exposed to m-AMSA is thought to represent the cellular correlate of this topoisomerase II-mediated reaction. Protein-associated DNA cleavage can be quantified in mammalian cells by using alkaline elution technology. In an attempt to understand the impact of phenotypic and biochemical cellular characteristics on protein-associated DNA cleavage, we quantified m-AMSA-induced DNA cleavage in quiescent or proliferative normal human fibroblasts (cell strain 1508) and human glioblastoma cells (line T98G) as well as in asynchronously proliferating HeLa cells. The magnitude of DNA cleavage in quiescent fibroblasts and quiescent glioblastoma cells was identical and low relative to that observed in the HeLa cells. The magnitude of DNA cleavage was enhanced in both cell types following proliferation. This enhancement was greater in the glioblastoma cells than in the fibroblasts. These results were not due to alterations in cellular m-AMSA uptake. Chromatin was more elongated (open) in the quiescent glioblastoma cells than in the quiescent fibroblasts (as visualized by using the premature chromosome condensation assay), suggesting chromatin accessibility to drug per se may not be a critical determinant of the magnitude of m-AMSA-induced DNA cleavage. The onset of the enhanced m-AMSA-induced DNA cleavability that accompanied proliferation closely followed the formation of regions of localized chromatin decondensation, a late G1 event, and coincided with the onset of enhanced thymidine uptake, a marker for the onset of S phase. m-AMSA-induced cytotoxicity was also enhanced in proliferating compared with quiescent cells. The major finding of this study is that the cellular target for m-AMSA, putatively topoisomerase II, is more susceptible to drug action in proliferating cells than in quiescent cells. Effects of chromatin conformation or cellular phenotype upon topoisomerase II-mediated events such as m-AMSA-induced DNA cleavage are less certain.

Characterization of the topoisomerase II-induced cleavage sites in the c-myc proto-oncogene. In vitro stimulation by the antitumoral intercalating drug mAMSA

In an attempt to get an insight into the activity of mAMSA (a DNA topoisomerase II-mediated drug) on the human proto-oncogene c-myc, an in vitro system consisting of purified calf thymus DNA topoisomerase II and a c-myc DNA inserted in lambda phage was utilized. The occurrence of discrete bands, detected by hybridization of Southern blots with appropriate c-myc probes, indicated the presence of cleavage sites in the sole presence of DNA topoisomerase II. The band intensity increased in the presence of mAMSA, while no significant difference occurred in the cleavage pattern. The location of the cleavage sites along the c-myc locus revealed a striking correspondence with that of some DNase hypersensitive sites. These results indicate that DNA topoisomerase II is most certainly implicated in the mAMSA activity and that the drug stimulates the topoisomerase II cleaving activity at specific sites, which may be involved in the biological activity of the drug.

Topoisomerase inhibitors can selectively interfere with different stages of simian virus 40 DNA replication

I have found that antineoplastic drugs which are known to be inhibitors of mammalian DNA topoisomerases have pronounced and selective effects on simian virus 40 DNA replication. Ellipticine, 4'-(9-acridinylamino)methanesulfon-m-aniside, and Adriamycin blocked decatenation of newly replicated simian virus 40 daughter chromosomes in vivo. The arrested decatenation intermediates produced by these drugs contained single-strand DNA breaks. Ellipticine in particular produced these catenated dimers rapidly and efficiently. Removal of the drug resulted in rapid reversal of the block and completion of decatenation. The demonstration that these drugs interfere with decatenation suggests that they may exert their cytotoxic and antineoplastic effects by preventing the separation of newly replicated cellular chromosomes. Camptothecin rapidly breaks replication forks in growing Cairns structures. It is likely that the target of camptothecin is the "swivel" topoisomerase required for DNA replication and that it is located at or very near the replication fork in vivo. Evidence is presented that many of the broken Cairns structures are in fact half-completed sister chromatid exchanges. One pathway for the resolution of these structures is completion of the sister chromatid exchange to produce a circular head-to-tail dimer.

Nucleic acid binding drugs--XIV. The crystal structure of 1-methyl amsacrine hydrochloride; relationships to DNA-binding ability and anti-tumour activity

The crystal structure of the 1-methyl derivative of the anticancer drug amsacrine [4'-(acridin-9-ylamino)-3'-methoxy-methanesulphonanilide+ ++] as its hydrochloride salt has been determined. The compound crystallizes in the monoclinic space group P21/n with cell dimensions a = 15.302(3), b = 8.035(2), c = 18.258(4) A and beta = 102.68(2) degrees, and has been refined to a final R of 0.055. The acridine chromophore is significantly non-planar, with a butterfly conformation about the C(9)-N(11) bond. The bonding geometry about the C(9) atom has been significantly altered compared to non-distorted amsacrine structures, as a result of this non-planarity. Energy calculations have been used to examine the flexibility of the molecule with respect to rotations about the C(9)-N(11) and N(11)-C(12) bonds, and with respect to intercalation into a dinucleoside duplex model for DNA. The latter calculations have been compared with solution DNA-binding and in vitro activity data for 1-methyl-amsacrine hydrochloride. The molecular modelling studies find that the energy of interaction between 1-methyl-amsacrine and a DNA intercalation fragment is significantly higher than for amsacrine itself, in accord with the biological data.

Effects of anthrapyrazole antineoplastic agents on lipid peroxidation

The effects of three anthrapyrazoles and an aminoacridine derivative on doxorubicin- and iron-stimulated lipid peroxidation in rabbit hepatic microsomes have been characterized. Two anthrapyrazoles, CI-937 and CI-942, were potent inhibitors of lipid peroxidation with 15 microM drug inhibiting the rate of peroxidation 70 to 90%. In contrast CI-941 was relatively ineffective in inhibiting lipid peroxidation with only 35% inhibition occurring at 100 microM drug. CI-921, an aminoacridine derivative, diminished lipid peroxidation by 65% at 15 microM. All four drugs failed to decrease the rate of doxorubicin-stimulated NADPH oxidation at concentrations less than 50 microM, suggesting that inhibition of lipid peroxidation was not the result of diminished enzyme activity. CI-937 formed a 2:1 complex with ferric ion, KD = 47 microM, which was reversible with EDTA.

In vitro and in vivo assessment of activity of new anilino-substituted analogues of amsacrine against Lewis lung carcinoma

A number of new anilino ring variants of the anti-tumour drug amsacrine have been synthesised and their anti-tumour activity evaluated. In vitro selectivity, as measured by the logarithmic ratio of IC50 growth inhibition assays against P388 leukaemia and Lewis lung carcinoma cells, was significantly correlated with the increase in life span in vivo with the P388 leukaemia and Lewis lung lines, whereas the growth inhibition IC50 values alone correlated with the dose potency in mice. It was thus possible to predict both in vivo anti-tumour activity and dose potency, identifying compounds with high therapeutic activity, using a combination of two in vitro assays. Two new compounds have been identified which provide, along with an acridine-substituted analogue of amsacrine which is at present in clinical trial (CI-921), a high proportion of cures against the Lewis lung tumour in vivo. Since amsacrine is thought to interact with the enzyme topoisomerase II, and because the anilino group of 9-anilinoacridine derivatives is thought to project from the DNA intercalation site of the drug-DNA complex, these compounds may be of particular interest in mode of action studies.

Effects of the bifunctional antitumor intercalator ditercalinium on DNA in mouse leukemia L1210 cells and DNA topoisomerase II

Ditercalinium, a 7H-pyridocarbazole dimer (bisintercalator) belongs to a new class of antineoplastic intercalating agents. To investigate its mechanism of cytotoxicity, the effects of ditercalinium on DNA were assessed using normal (L1210) and drug-resistant (L1210/PyDi1) mouse leukemia cells. Alkaline elution assays demonstrated that ditercalinium produced no DNA strand breaks, DNA-protein cross-links, or DNA-DNA cross-links, eliminating these effects as cytotoxic lesions. This result sets ditercalinium apart from other intercalating agents with respect to its interaction with DNA. Nucleoids (histone-depleted chromatin) from ditercalinium-treated L1210 cells were considerably more compact than those from untreated cells, as determined by sedimentation in neutral sucrose gradients. In contrast, nucleoids from ditercalinium-treated L1210/PyDi1 (resistant) cells were similar in compactness to those from control cells. Thus, ditercalinium altered chromatin structure in vivo. The effect of the bisintercalator on purified DNA topoisomerase II, an intracellular target of monointercalators, was measured in vitro. Ditercalinium (5 X 10(-7) M) completely inhibited both the formation of covalent complexes between this enzyme and simian virus 40 DNA and the enzyme-induced DNA cleavage. In addition, ditercalinium induced DNA catenation in the presence of topoisomerase II and adenosine triphosphate. Thus, the cytotoxicity of ditercalinium may derive from a mechanism that, although involving topoisomerase II, is manifested by condensation of DNA rather than by the induction of protein-associated DNA strand breaks.

AMSA: in vivo log cell kill for leukemic clonogenic cells versus toxicity for normal hemopoietic stem cells in a rat model for human acute myelocytic leukemia (BNML)

The efficacy of AMSA was evaluated quantitatively in a rat model (BNML) relevant for human acute myelocytic leukemia. The LD50 values observed in normal and leukemic Brown-Norway rats were 26.4 and 28.3 mg/kg respectively. In the higher dose ranges, the major cause of death was acute cardio-pulmonary toxicity. After single dose treatment, 20 mg AMSA/kg resulted in a surviving fraction of 5.5 X 10(2) for normal pluripotent hemopoietic stem cells and 4.1 X 10(-5) for in vivo clonogenic leukemic cells. With repeated administration of the drug amounting to the same total dose, even a 4 log difference in cell kill was observed between both cell populations. These studies provide quantitative information on the therapeutic index of AMSA and support the inclusion of this drug in first-line treatment regimens for acute myelocytic leukemia.

Steric constraints for DNA binding and biological activity in the amsacrine series

To determine the relation between DNA binding mode and biological activity in compounds related to the clinical anti-leukaemia drug amsacrine, a series of acridine-substituted derivatives has been synthesized and compared for lipophilicity, DNA-binding affinity, DNA-binding geometry and anti-leukaemia activity in vitro and in vivo. DNA-binding affinity, as estimated either by ethidium displacement or equilibrium dialysis, decreased progressively as the bulk of the substituent increased. Substitution at the 2 position provided the largest effects. The DNA unwinding angles, estimated by changes in viscosity of closed circular duplex DNA in the presence of drug, decreased significantly with ethyl and isopropyl substituents. However, with tertbutyl groups in the 2, 3 or 4 position of the acridine ring, unwinding was not observed even though DNA binding was measurable. Anti-leukaemia activity in vivo was also abolished when the acridine ring was substituted with a tertbutyl group. The results suggest that methyl substitution of the acridine ring inhibits DNA binding at the 2 position but not at the 3 and 4 positions, that ethyl and isopropyl substitution inhibits intercalative binding at all positions and that tertbutyl substitution abolishes intercalative binding. Biological activity in vitro is dependent on both lipophilicity and DNA binding and activity in vivo requires intercalative DNA binding.