Combination therapy of CPT-11, a camptothecin derivative, with various antitumor drugs against L 1210 leukemia

Antitumor effect of CPT-11 in combination with cyclophosphamide (CY), nimustin hydrochloride (AC-NU), thio-TEPA (TESPA), methotrexate (MTX), 5-fluorouracil (5-FU), cytosine arabinoside (ara-C), thioinosine (6-MPR), adriamycin (ADM), bleomycin (BLM), mitomycin C (MMC), actinomycin D (ACT-D), vincristine sulfate (VCR), etoposide (VP-16) or cisplatin (CDDP) against L 1210 murine leukemia was investigated. The combination treatment of CPT-11 with CY, ACNU, ADM, CDDP, TESPA and ACT-D showed synergistic effects and significantly prolonged the survival time of L 1210-inoculated mice compared with CPT-11 alone or antitumor drug alone. Although the combination with 5-FU, 6-MPR, VP-16, MMC or VCR had synergistic effect for some schedules exceptionally with ara-C, MTX or BLM had slight synergistic effect against L 1210.     

Trifluoperazine modulation of resistance to the topoisomerase II inhibitor etoposide in doxorubicin resistant L1210 murine leukemia cells

Murine leukemia L1210 cells selected for progressive resistance to doxorubicin (DOX) display both the multidrug resistant (MDR) phenotype and reductions in drug induced topoisomerase II-mediated DNA cleavage in nuclear extracts (Ganapathi, R.; Grabowski, D.; Ford, J.; Heiss, C.; Kerrigan, D.; Pommier, Y., Cancer Commun. 1:217-224; 1989). The present study was performed to characterize the results of exposure of the sensitive (S) and progressively DOX-resistant (10-fold, R1, and 40-fold, R2) L1210 cells to the topoisomerase II inhibitor, etoposide, and to investigate the modulating effects of the calmodulin inhibitor, trifluoperazine (TFP). Immunoblotting experiments indicated no apparent decrease in the p170 or p180 isoforms of topoisomerase II in the resistant sublines versus parental sensitive cells. Cross-resistance to etoposide (VP-16) was similar to that of DOX (10- and 40-fold). A non-cytotoxic concentration of 5 microM TFP enhanced cell kill 1.5- fold in the sensitive and 3- to 5-fold in the progressively DOX-resistant cells. Accumulation of VP-16 was 30% to 50% lower in the resistant sublines versus similarly treated sensitive cells, and a marked enhancement of drug uptake in the presence of TFP was observed in the sensitive but not in the resistant cells exposed to equivalent extracellular levels of VP-16. Although equimolar concentrations of VP-16 produced fewer DNA single strand breaks (SSB) and DNA protein crosslinks (DPC) in the resistant versus sensitive cells, similar DNA damage was apparent when S and R1, but not R2, cells were treated at VP-16 concentrations that produced equivalent cell death.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of the synergistic potentiation of etoposide, doxorubicin, and vinblastine cytotoxicity by dipyridamole

Dipyridamole (DPM) enhanced sensitivity to etoposide (VP-16), doxorubicin (DOX), and vinblastine (VBL) in a human ovarian carcinoma cell line that was already relatively sensitive to all three agents. This interaction was shown to be truly synergistic by median effect analysis over a 2 log cell kill. The combination index at 50% cell kill (CI50) was used to quantitate the extent of synergy. The CI50s were 0.42, 0.66, and 0.30 for VP-16, DOX, and VBL, respectively. We compared the effect of DPM on the cellular pharmacology of each chemotherapeutic drug. DPM increased the steady state cellular content of VP-16 by a maximum of 3.2-fold, and that of DOX and VBL by 1.7- and 3.7-fold, respectively. There was a good correlation between the CI50 and the DPM-induced increase in cellular drug content (r = 0.94). DPM had no effect on the initial influx VP-16 or DOX but did increase the initial influx of VBL by 3.5-fold. DPM inhibited the initial efflux of all three compounds. However, there was no relation between the extent of efflux inhibition and the magnitude of the DPM-induced increase in cellular drug content, indicating that DPM must have other effects as well. DPM has chemical characteristics similar to other known modulators of VP-16, DOX, and VBL sensitivity. When compared to verapamil, DPM was as efficacious but twice as potent in its synergistic enhancement of VP-16 sensitivity. These results demonstrate that DPM can markedly increase the cytotoxicity of VP-16, DOX, and VBL and suggest possible clinical applications.     

Effects of microtubule inhibitors on etoposide accumulation and DNA damage in human K562 cells in vitro

The effects of microtubule inhibitors on cellular accumulation of 4'-demethylepipodophyllotoxin-9-(4,6-O-ethylidine-beta-D-glu copyranoside) (VP-16) and subsequent epipodophyllotoxin-induced DNA single-strand breaks were investigated in human leukemia K562 cells. At concentrations of 0.05-20 microM, vinblastine, vincristine, and maytansine similarly increased the steady-state cell concentration of VP-16 (2.5 microM) up to 2-fold. Following removal of extracellular vinblastine, the elevation of cell VP-16 was maintained through an additional 55-min incubation period. Washing cells free of extracellular VP-16 resulted in a nonexchangeable (or bound) component comprising 15-17% of the VP-16 concentration found before removal of extracellular drug. In cells incubated with VP-16 alone, removal of extracellular drug resulted in less than 5% cell retention of drug. At vinblastine concentrations of 0.05-0.2 microM, the increase in cell VP-16 was due to a progressive increase in nonexchangeable VP-16. At greater vinblastine concentrations, up to 10 microM, there was no further increase in nonexchangeable VP-16 but there was a 1.6-fold increase in the exchangeable (or free) concentration of VP-16. Similar elevation of both nonexchangeable and exchangeable VP-16 by 10 microM vincristine and maytansine was observed; however, 50-100 microM podophyllotoxin or taxol was required for comparable elevation of exchangeable drug with no increase of nonexchangeable VP-16. Elevation of exchangeable VP-16 in the presence of vinblastine was due to inhibition of the unidirectional efflux of this epipodophyllotoxin with a 69% decline in the rate constant for efflux. There were no effects of vinblastine on VP-16 influx. There was no enhancement of DNA single-strand break frequency when cells were incubated with 2.5 microM VP-16 and 0.2 microM vinblastine, a concentration of the Vinca alkaloid that increased only nonexchangeable VP-16. VP-16-induced DNA damage was enhanced by vinblastine concentrations above 0.5 microM, concentrations that elevated exchangeable VP-16, with a maximum doubling of radiation equivalent single-strand break frequency observed with 20 microM vinblastine, consistent with the maximum elevation of cell VP-16 with 20 microM Vinca alkaloid. These results indicate that vinblastine and other microtubule inhibitors elevate cell VP-16 by inhibition of the efflux of exchangeable drug and by increasing the level of nonexchangeable drug. Potentiation of VP-16-induced DNA damage is observed only at microtubule inhibitor concentrations which elevate exchangeable VP-16.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dipyridamole modulates multidrug resistance and intracellular as well as nuclear levels of doxorubicin in B16 melanoma cells

Simultaneous occurrence of resistance to many chemothera-peutic agents, termed multidrug resistance (MDR), is a complex phenotype. MDR occurs due to several reasons, including over-expression of a 170-kDa membrane-bound protein, called P-glycoprotein (P-gp), which apparently participates in active drug efflux. Multidrug-resistant cells also frequently exhibit an altered pattern of intracellular drug distribution, resulting in a reduction in the nuclear level of drugs such as doxorubicin (DOX). In this study, the effect of dipyridamole (DP) on drug resistance and on intracellular as well as nuclear levels of DOX in multidrug-resistant melanoma cells has been examined. For this purpose, drug-sensitive murine melanoma cells (B16V) and their multidrug-resistant variant cells, (B16VDXR; selected for resistance to DOX) which over-produce P-gp, were employed. B16VDXR cells were cross-resistant to several anti-cancer agents including etoposide (VP-16) and mitoxantrone (Mitox). DP (10 m?M) significantly potentiated the cytotoxicity of DOX, VP-16 and Mitox towards multidrug-resistant B16VDXR cells but not in parental drug-sensitive B16V cells. The presence of DP resulted in a 3.7-fold increase in the total cellular level and a 4.2-fold increase in the nuclear content of DOX in the resistant cells. Isobologram analysis indicates that DP at several pharmacologically relevant concentrations synergistically potentiates the activity of DOX in B16VDXR cells.     

Verapamil-induced augmentation of etoposide accumulation in L1210 cells in vitro

The effects of the calcium antagonist verapamil on the intracellular disposition of 4'-demethylepipodophyllotoxin-9-(4,6-O-ethylidene-beta-D-glucopyra noside) (etoposide) (VP-16) as well as on subsequent DNA damage and cytotoxicity were studied in L1210 cells in vitro. At extracellular VP-16 concentrations of 1 to 5 microM, verapamil (10 microM) addition resulted in an increase of DNA single-strand break frequency comparable to that found when VP-16 was present alone at a 3-fold higher concentration. In addition, the elevation of cellular VP-16 levels in the presence of verapamil was linearly correlated with the enhancement of DNA damage and increased cell kill. Verapamil-mediated increase in net VP-16 transport was rapid (1 to 2 min), and allowed for the same elevation of steady-state VP-16 concentration, whether verapamil was added simultaneously with VP-16 or was added after a steady-state level of VP-16 was achieved. Verapamil-mediated elevation of VP-16 levels was not seen at reduced temperature (0 degrees C). Studies of bidirectional VP-16 transport revealed that verapamil (40 microM) did not alter influx of VP-16 (15 microM), but lowered the unidirectional rate constant for efflux by 93%, resulting in the observed increase of steady-state level of the epipodophyllotoxin. Removal of verapamil resulted in a rapid return of VP-16 to levels comparable to that seen with VP-16 alone. When VP-16 was allowed to flow out of the cell in the presence of verapamil, less than 5% of cellular epipodophyllotoxin was retained, suggesting that the calcium antagonist is not acting by enhancing intracellular binding of VP-16. These results indicate that verapamil potentiates VP-16 activity by elevation of intracellular exchangeable epipodophyllotoxin; an activity which seems to be due to inhibition of the efflux mechanism for VP-16. The low intracellular retention of this epipodophyllotoxin and the good correlation between intracellular VP-16 and subsequent DNA damage and cytotoxicity suggest that the epipodophyllotoxin class of anticancer agents may be more useful for probing calcium antagonist effects on drug transport in sensitive cells and in cells exhibiting pleiotropic drug resistance than the vinca alkaloids and anthracyclines which have large tight binding intracellular components.     

Comparison of cellular basis of drug sensitivity of human colon,pancreatic, and renal carcinoma cell lines with that of leukemia cell lines

Summary In an attempt to find how much the low therapeutic effectiveness of antitumor drugs against so-called chemotherapy-refractory tumors such as colon carcinoma depends on drug sensitivity at the cellular level, sensitivity of five carcinoma cell lines (three colorectal, one pancreatic, and one renal) to nine typical anticancer agents was compared in vitro with that of four generally chemotherapysusceptible leukemia cell lines. Sensitivity was assesed in terms of the percentage cell growth in control cultures, which was determined by exposing exponentially growing cells for 48 h to the following antitumor drugs: 1-(4-amino-2-methylpyridine-5-yl)-methyl-3-(2-chloroethyl)3-nitrosourea hydrochloride (ACNU), adriamycin (ADM), bleomycin (BLM), cisplatin (DDP), etoposide (VP-16), 5-fluorouracil (5FU), mitomycin C (MMC), methotrexate (MTX), and vinblastine (VLB). As expected, 10-fold or greater differences in sensitivity were scarcely ever observed between the two kinds of cell lines. Thus, we recorded a result of more (or less) sensitivity when there was a difference of 3-fold or more; and compared the drug sensitivity in every pair of carcinoma and leukemia cell lines (20 pairs for each drug). We found that carcinoma cell lines were less sensitive to VP-16, ADM, DDP, and MTX than leukemia cell lines in 18, 15, 12, and 10 of 20 pairs, respectively; only one opposite case was observed, with DDP. On the other hand, no such tendency between the two groups was observed with BLM, 5FU, or MMC. Overall, significantly different sensitivities were observed between them in 91 out of 180 pairs (i.e., 9 antitumor drugsx5 carcinomasx4 leukemias), and carcinoma cell lines were less sensitive than leukemia cell lines in 79 of these 91 pairs. These results suggest that the refractoriness of colon carcinoma, etc. to chemotherapy is, at least in part, due to low drug sensitivity of the tumor cell itself.This study was supported in part by Grants-in-Aid for Cancer Research from the Ministry of Education, Science and Culture, Japan     

Comparative effectiveness of mitoxantrone and doxorubicin in overcoming experimentally induced drug resistance in murine and human tumour cell lines in vitro

Summary Using a range of cell lines of murine and human tumour origin in which relatively modest levels (2- to 17-fold) of drug resistance have been selected in vitro by exposure to a range of standard antitumour drugs, we compared the cytotoxic effects of doxorubicin (DOX) and mitoxantrone (MITO). In general, significantly lower concentrations of MITO than of DOX were required to achieve comparable cytotoxicity, confirming previously published data. MITO appears more generally effective against the murine L5178Y drug-resistant sublines than DOX, although there was no expression of collateral sensitivity to this newer agent. In the various human tumour lines there was a lack of cross-resistance to both DOX and MITO in two 5-fluorouracil (FU)-resistant lines and one of two cisplatin (CDDP)-resistant cells, but cross-resistance was expressed in one subline resistant to vincristine (VCR) and two etoposide (VP-16)-resistant sublines. One murine and two human DOX-resistant sublines were effectively killed by MITO, whilst DOX proved effective against the human MITO-resistant subline. This apparent lack of cross-resistance between DOX and MITO in these resistant sublines expressing low levels of resistance in vitro therefore appears to contrast with previous reports involving highly multidrug-resistant DOX-selected sublines. However, since the latter lines generally exhibited profound cross-resistance to VCR and definite cross-resistance to VP-16, this may at least in part dictate their responses to MITO. Therefore, attempts to use experimentally derived drug-resistant sublines for preclinical drug screening should be approached with caution, since patterns of drug response appear to be influenced by the level of drug resistance expressed. The need remains to determine which type of model system provides the most relevant clinical information.     

Topoisomerase II-dependent and -independent mechanisms of etoposide resistance in Chinese hamster cell lines

Resistance to etoposide (VP-16), amsacrine (mAMSA), and doxorubicin (Adriamycin) was studied in two Chinese hamster cell lines primarily selected for resistance to the epipodophyllotoxin. Both lines demonstrated profound resistance to VP-16, and mAMSA stimulated DNA breakage. However, the resistance to mAMSA cytotoxicity in both lines was less than expected from the level of resistance to the effects of topoisomerase II inhibition. Similarly, resistance to the cytotoxicity of high VP-16 concentrations in one of the lines was less than expected from the resistance to inhibition of topoisomerase II. An analysis of the relation of DNA breaks to drug cytotoxicity suggests that cross-resistance to mAMSA was mainly conferred through loss of mAMSA-stimulated, topoisomerase II-mediated DNA breaks. This mechanism also contributed towards reduced VP-16 cytotoxicity. However, our studies suggest that additional mechanisms, independent of resistance to VP-16-mediated topoisomerase II effects, greatly increased the resistance to this agent. Resistance to VP-16 cytotoxicity, not dependent on resistance to drug-mediated DNA cleavage, could be overcome at high drug concentrations in one of the resistant lines and might be responsible for the greater relative resistance to VP-16 than to mAMSA. These findings suggest the presence of two distinct mechanisms of resistance to VP-16 cytotoxicity, one presumably mediated by topoisomerase II and dependent on resistance to drug-mediated DNA scission, and a second mechanism independent of the effects of the drug on topoisomerase II.     

DNA strand breaks produced by etoposide (VP-16,213) in sensitive and resistant human breast tumor cells: implications for the mechanism of action

Pleotropic resistant human breast cancer cells (MCF-7), selected for resistance to Adriamycin, were used to study the production of DNA strand breaks by etoposide (VP-16) and its relationship to drug cytotoxicity. It was shown that the resistant MCF-7 cell line was cross-resistant to VP-16, and the degree of resistance was found to be 125-200-fold. Alkaline elution studies indicated that the parental cell line was very sensitive to VP-16 which caused extensive DNA strand breakage. In contrast, little DNA strand breakage was detected in the resistant MCF-7 cells, even at very high drug concentrations, indicating a good agreement between strand breaks and cytotoxicity. Further studies indicated that the nuclei isolated from the parental cell line were more resistant to VP-16-induced DNA strand breaks than the intact cells, while the opposite was found in the resistant cell line. In addition, the alkaline elution studies in isolated nuclei showed only a 2-fold reduction of VP-16-induced DNA breaks in nuclei from the resistant cells. In agreement with this result, it was found that nuclear extract from the resistant cells produced 2-3-fold less VP-16-induced DNA breaks than that from the sensitive cells in 32P-end-labeled SV40 DNA. VP-16 uptake and efflux studies indicated that there was a 2-3-fold decrease in net cellular accumulation of VP-16 in the resistant cells. Although the reduced uptake of VP-16 and decreased drug sensitivity of topoisomerase II appear to contribute to the mechanism of action and the development of resistance to VP-16, they do not completely explain the degree of resistance to VP-16 in this multidrug-resistant MCF-7 cell line indicating that other biochemical factors, such as activation of VP-16, are also involved in drug resistance and suggesting that the resistance is multifactorial.     

Progressive resistance to doxorubicin in mouse leukemia L1210 cells with multidrug resistance phenotype: reductions in drug-induced topoisomerase II-mediated DNA cleavage

Cells selected for resistance to doxorubicin (DOX) express the multidrug resistance (MDR) phenotype, and resistance has been suggested to be due primarily to enhanced cellular efflux of drug. A progressively DOX-resistant (10- and 40-fold) L1210 mouse leukemia model system, which does not exhibit enhanced DOX efflux as a primary mechanism of resistance, was found to display the MDR phenotype, based on overexpression of P-glycoprotein in western blots and cross-resistance to vinca alkaloids. Cross-resistance to another topoisomerase II inhibitor, etoposide (VP-16), was similar to that of DOX (10- and 40-fold), whereas resistance to N-[4-(9-acridinylamino)-3-methoxyphenyl]methanesulfonamide (m-AMSA) was 5-fold lower. In contrast, no cross-resistance to camptothecin, an inhibitor of topoisomerase I, was observed. Topoisomerase II decatenation activity in nuclear extracts from 10- and 40-fold DOX-resistant cells was 2- and 4-fold lower, respectively, when compared to sensitive cells. In these cells, however, marked reductions in m-AMSA- and VP-16-induced topoisomerase II mediated DNA cleavage were found to exceed decreases in the catalytic activity of the enzyme. Results from this study demonstrated that, in progressively DOX-resistant L1210 mouse leukemia cells with the MDR phenotype, a better relation existed between the degree of resistance and reduced VP-16- and m-AMSA-induced topoisomerase II mediated DNA cleavage, than between increases in P-glycoprotein and concomitant reduction in DOX accumulation.     

Correlation between decreased apoptosis and multidrug resistance (MDR) in murine leukemic T cell lines.

Cancer cells may frequently develop cross-resistance to structurally dissimilar chemotherapeutic agents. However, the molecular mechanisms for sensitivity and resistance of tumor cells towards chemotherapy are still partially understood. Antineoplasic drugs have been shown to induce apoptosis in chemosensitive leukemias and solid tumors. In this work, cross-resistance among vincristine (VCR), doxorubicin (DOX) and other antineoplasic agents commonly used in the treatment of leukemia such as etoposide (VP-16), methotrexate (MTX), cyclophosphamide (CTX), dexamethasone (DEX), cytarabine (Ara-C) and L-asparaginase on vincristine resistant (LBR-V160), doxorubicin resistant (LBR-D160) and sensitive (LBR-) murine leukemic T cell lines, was determined. The effect of antineoplasic agents was assayed by tritiated thymidine incorporation. Our results showed that VCR exhibited cross-resistance with DOX, VP-16, DEX and MTX, while DOX demonstrated cross-resistance with VCR, VP-16 and MTX. Ara-C failed to present cross-resistance with any cell line. Apoptosis induced by the above drugs on the same cell lines was analyzed by acridine orange and ethidium bromide staining, DNA hypoploidy (flow cytometry) and oligonucleosomal fragmentation of nuclear DNA showing that therapeutic concentrations of these chemotherapeutic agents induced apoptosis in the LBR- cell line. Our results demonstrated that, except for DEX, none of the drugs presenting cross-resistance were able to induce cell death on LBR-V 160 or LBR-D 160 cell lines.     

Calmodulin inhibitor trifluoperazine in combination with doxorubicin induces the selection of tumour cells with the multidrug resistant phenotype.

Trifluoperazine (TFP) is effective in modulating DNA damage/repair in doxorubicin (DOX) treated cells. In the present study we have characterised the resistance phenotype of parental sensitive L1210 mouse leukaemia cells (L1210/S) adapted to grow in the presence of 0.017 microns DOX+5 microM TFP (L1210/DT). Although with prolonged exposure, 0.017 microM DOX alone produced < 35% cell kill in L1210/S cells, similar cytotoxicity was achieved at 0.43 microM DOX in L1210/S cells selected in the presence of 0.017 microM DOX+5 microM TFP. L1210/DT cells were > 30-fold resistant to DOX following a 3 h drug exposure in a soft agar colony assay. In contrast, DOX sensitivity in cells adapted to grow in 5 microM TFP alone was comparable to L1210/S cells. Resistance to other inhibitors of topoisomerase II in L1210/DT cells was > 30-fold to etoposide and > 6-fold to amsacrine. The levels of the 170 kDa and 180 kDa isoforms of topoisomerase II in an immunoblot were comparable between the L1210/S and L1210/DT cells. Cross resistance to vincristine in the L1210/DT cells was accompanied by the overexpression of plasma membrane P-glycoprotein. Although a 1.5-2-fold decrease in accumulation of etoposide and DOX was observed in the L1210/DT cells, drug levels for equivalent DNA damage in the alkaline elution assay were > 5-fold higher in the L1210/DT versus L1210/S cells. No abrogation in the modulating effects of TFP on DOX, VP-16 or amsacrine induced cytotoxicity was apparent in the L1210/DT cells. Results suggest that: (a) TFP in combination with low concentrations DOX can induce the selection of cells with the multidrug resistant phenotype; and (b) characteristics of cells selected for resistance to DOX or DOX plus TFP are comparable.     

Potentiation by novobiocin of the cytotoxic activity of etoposide (VP-16) and teniposide (VM-26).

The coumermycin antibiotic novobiocin, which interacts with the nuclear enzyme topoisomerase II, produced supra-additive toxicity to WEHI-3B D+ leukemia cells at clinically achievable concentrations, when combined with teniposide (VM-26) or etoposide (VP-16). Simultaneous exposure of cells to both agents was required for maximum efficacy of the combination. Novobiocin also produced supra-additive toxicity to A549 human lung carcinoma cells when combined with VM-26 or VP-16. At concentrations above the peak plasma levels achievable in patients, novobiocin lost its potentiating activity. Exposure of WEHI-3B D+ cells to novobiocin did not modify the cytotoxicity produced by the topoisomerase II inhibitor m-AMSA, whereas, in contrast, novobiocin antagonized the cytotoxicity of m-AMSA in A549 cells. Although it has been suggested that inhibitors of the syntheses of DNA and RNA interfere with the cytotoxic activity of the epipodophyllotoxins, maximum potentiation of the cytotoxicities of VP-16 and VM-26 occurred at novobiocin concentrations that decreased the rates of synthesis of both DNA and RNA in WEHI-3B D+ cells by about 50%. The number of DNA-topoisomerase-II covalent complexes stabilized by VM-26 in WEHI-3B D+ cells was greatly increased when cells were exposed simultaneously to VM-26 and novobiocin for 1 hr, but not when cells were treated with m-AMSA and novobiocin for the same period of time. Novobiocin did not affect the amount of covalent complexes produced by VM-26 in isolated nuclei, suggesting that the potentiating activity of novobiocin was not due to its direct interaction with the nuclear topoisomerase II enzyme. Our findings suggest that therapeutic levels of novobiocin may be capable of enhancing the clinical activities of VP-16 and VM-26.     

Combined modalities of resistance in etoposide-resistant human KB cell lines

The alkaloid derivative 4'-demethylepipodophyllotoxin 9-(4,6-O-ethylidene)-beta-D-glucopyranoside (etoposide, VP-16) is believed to exert cytotoxicity by causing double-stranded DNA breaks through interruption of the breaking-resealing reaction of topoisomerase II (topo II). Thus it was conceivable that cells could become resistant to VP-16 by a decrease in topo II enzyme level, since this would lead to fewer DNA breaks. As well, given the structure of VP-16, it was also possible that a pleiotropic mechanism of resistance could decrease sensitivity to this drug. To study these possibilities, a series of VP-16-resistant human KB cell lines was established by stepwise selection. The concentrations of VP-16 required to inhibit cell proliferation by 50% in the parent line and KB/1c, KB/7d, KB/20a, and KB/40a lines were, respectively, 0.16, 4.7, 24, 31, and 47 microM. These cell lines expressed cross-resistance to 4'-(9-acridinylamino)methanesulfon-m-anisidide, doxorubicin, vincristine, and methotrexate, although the pattern of relative drug sensitivity was quite different from that of pleiotropic resistant cell lines reported elsewhere. The resistance to vincristine and methotrexate did not increase above the level of the KB/1c cells, and resistance to VP-16, doxorubicin, and especially vincristine was unstable in VP-16-resistant cells cultured in the absence of drug. Although the drug resistance marker Mr 180,000 glycoprotein could not be detected in any of our cell lines, cellular accumulation of [3H]VP-16 was reduced 50-75% in the resistant lines compared with parent KB. With increasing VP-16 resistance, the level of topo II protein, detected by antibody staining, decreased at each step of selection, concomitant with a general decrease in topo II unknotting activity. Sensitivity of the topo II unknotting assay to inhibition by VP-16 was the same for the parent and all resistant cell lines. The level of topo I activity and enzyme increased slightly in the resistant cells. Thus, these cell lines are resistant to VP-16 by virtue of at least two mechanisms: (a) reduced levels of topo II, which confers cross-resistance to other compounds which are topo II-dependent cytotoxic agents; and (b) reduced accumulation of drug, which is likely also responsible for vincristine and methotrexate resistance. However, the possible existence of other mechanisms of resistance cannot be ruled out.     

Significance of Nuclear Glutathione S-Transferase π in Resistance to Anti-cancer Drugs

Recent study has shown that nuclear glutathione S-transferase (GST) π accumulates in cancer cells resistant to doxorubicin hydrochloride (DOX) and may function to prevent nuclear DNA damage caused by DOX (Goto et al., FASEB J. , 15, 2702–2714 (2001)). It is not clear if the amount of nuclear GSTπ increases in response to other anti-cancer drugs and if so, what is the physiological significance of the nuclear transfer of GSTπ in the acquisition of drug-resistance in cancer cells. In the present study, we employed three cancer cell lines, HCT8 human colonic cancer cells, A549 human lung adenocarcinoma cells, and T98G human glioblastoma cells. We estimated the nuclear transfer of GSTπ induced by the anti-cancer drugs cisplatin (CDDP), irinotecan hydrochloride (CPT-11), etoposide (VP-16) and 5-fluorouracil (5-FU). It was found that: (1) Nuclear GSTjt accumulated in these cancer cells in response to CDDP, DOX, CPT-11, VP-16 and 5-FU. (2) An inhibitor of the nuclear transport of GSTπ, edible mushroom lectin ( Agaricus bisporus lectin, ABL), increased the sensitivity of the cancer cells to DOX and CDDP, and partially to CPT-11. Treatment with ABL had no apparent effect on the cytotoxicity of VP-16 and 5-FU. These results suggest that inhibitors of the nuclear transfer of GSTπ have practical value in producing an increase of sensitivity to DOX, CDDP and CPT-11.     

Mechanism of increased sensitivity to etoposide in a mitomycin C-resisitant human bladder cancer cell line

The mechanism of increased sensitivity to etoposide (VP-16) in a human bladder cancer cell line (J82/MMC-2), which is >9-fold more resistant to mitomycin C (MMC) compared with parental cells (J82/WT), was investigated. Colony formation assays, following 1 hr drug exposure, revealed that about a 2.2-fold higher concentration of VP-16 was required to kill 50% of the J82/WT cell line compared with J82/MMC-2. The MTT assays, following continuous drug exposure, also showed that the J82/MMC-2 cell line was significantly more sensitive to VP-16 compared with J82/WT. Accumulation of VP-16 was significantly higher in the J82/MMC-2 cell line compared with J82/WT at every drug concentration tested. Likewise, intracellular VP-16 retention was significantly higher in the J82/MMC-2 cell line compared with J82/WT when drug uptake was measured as a function of varying incubation time and at a fixed VP-16 concentration. The efflux of VP-16 from the J82/MMC-2 cell line was equivalent to that from J82/WT. In agreement with the results of drug uptake studies, the levels of VP-16-induced protein-DNA complexes were markedly higher in the J82/MMC-2 cell line compared with J82/WT. The catalytic activity of topoisomerase II (topo II) in 0.35 M NaCl nuclear extract of J82/WT cells was equivalent to that of J82/MMC-2. The levels of topo II mRNA were also comparable in these cells. Our results suggest that the mechanism responsible for the collateral sensitivity of the J82/MMC-2 cell line to VP-16 may be attributable to a relatively higher drug accumulation in this cell line compared with parental cells. Int. J. Cancer 70:612–618. © 1997 Wiley-Liss Inc.     

Differential requirement of DNA replication for the cytotoxicity of DNA topoisomerase I and II inhibitors in Chinese hamster DC3F cells

The cytotoxicity of topoisomerase inhibitors is thought to result from the induction of enzyme-mediated DNA breaks. The fact that these breaks reverse rapidly in cells programmed to die, led us to investigate further the cytotoxic mechanisms of topoisomerase I (camptothecin) and topoisomerase II inhibitors (VP-16 and amsacrine) in Chinese Hamster lung fibroblasts (DC3F). Exposures (30 min) to camptothecin produced limited cell killing with approximately 20% of the cells naturally resistant. This resistance was overcome by increasing the drug exposure time. Inhibition of DNA synthesis by 5-min pretreatments with aphidicolin or hydroxyurea abolished the cytotoxicity of camptothecin without changing the level of camptothecin-induced DNA breaks. A good correlation was found between the degree of DNA synthesis inhibition by aphidicolin and the reduction of camptothecin cytotoxicity. In similar experiments performed with topoisomerase II inhibitors, aphidicolin prevented only partially against VP-16- and amsacrine-induced cytotoxicities, yet had no effect upon drug-induced DNA breaks. These results indicate that the production of topoisomerase-mediated DNA breaks by antitumor drugs is not sufficient for cell killing. Instead, an interference of moving DNA replication forks with drug-stabilized topoisomerase-DNA complexes is critical for cell death. The cytotoxicity of camptothecin seemed to be completely related to this process, while that of topoisomerase II inhibitors seemed to involve additional mechanisms in DC3F cells.     

Role of xanthine oxidase in the potentiation of doxorubicin-induced cardiotoxicity by mitomycin C

Clinical evidence has suggested that mitomycin C (MMC) potentiates doxorubicin (DOX) induced cardiotoxicity. In this study a mouse model was used to examine the effect of DOX on the ability of cardiac tissue to bioactivate MMC to generate oxygen radicals. Cardiac damage was assessed by measuring serum CPK-MB isoenzyme levels and thiobarbituric acid reactive substances (TBARS) in the cardiac tissue. The exposure of animals to DOX or DOX and MMC over a three week period led to an increase in serum CPK-MB isoenzyme levels as well as TBARS. Treatment with DOX led to an increase in MMC-dependent, NADH-dependent, cyanide insensitive oxygen consumption, compared to control animals, thereby suggesting increased MMC-dependent oxygen radical generation. Levels of xanthine oxidase (XO; EC 1.1.3.22) and NADPH:cytochrome C reductase, two enzymes known to bioactivate MMC with subsequent oxygen radical generation, were measured in cardiac tissue with a 4.5 x increase in XO activity seen in DOX treated animals vs controls and no change in NADPH:cytochrome C reductase activity. Cardiac levels of xanthine dehydrogenase (XDH; EC 1.1.1.204) activity in DOX treated animals decreased while the XO/XDH ratio increased, suggesting a conversion of XDH to XO following DOX treatment.     

Antagonistic effect of aclarubicin on the cytotoxicity of etoposide and 4'-(9-acridinylamino)methanesulfon-m-anisidide in human small cell lung cancer cell lines and on topoisomerase II-mediated DNA cleavage

The effect of combinations of the anthracycline aclarubicin and the topoisomerase II targeting drugs 4'-demethylepipodophyllotoxin-9-(4,6-O-ethylidene-beta-D-glucopyra noside) (VP-16) and 4'-(9-acridinylamino)methanesulfon-m-anisidide (m-AMSA) was investigated in a clonogenic assay. The cytotoxicity of VP-16 was almost completely antagonized by preincubating cells with nontoxic concentrations of aclarubicin. The inhibition of cytotoxicity was not seen when the cells were exposed to aclarubicin after exposure to VP-16. The inhibition was significant over a wide range of aclarubicin concentrations (3 nM to 0.4 microM), above which the toxicity of aclarubicin became apparent. A similar effect was seen on the toxicity of m-AMSA. In contrast to aclarubicin, preincubation with Adriamycin did not antagonize the effect of VP-16. With purified topoisomerase II and naked DNA, aclarubicin did not stimulate the formation of cleavable complexes between topoisomerase II and DNA. Aclarubicin concentrations above 1 microM inhibited the baseline formation of cleavable complexes elicited with the enzyme alone. Low (1 to 10 nM) aclarubicin concentrations increased the formation of cleavable complexes obtained with VP-16 and m-AMSA; however, at aclarubicin concentrations above 1 microM an antagonistic effect was obtained. In cells, the m-AMSA- and VP-16-induced, protein-concealed DNA strand breaks were completely inhibitable by aclarubicin preincubation with no synergic dose levels. Our results suggest that aclarubicin inhibits topoisomerase II-mediated DNA cleavage. This inhibition could represent the mechanism of action of the drug and explain the lack of cross-resistance to the classical anthracyclines. The observed antagonism could have consequences for scheduling of aclarubicin with topoisomerase II-active anticancer drugs.