Poly(ADP-ribose) polymerase-independent potentiation of nitrosourea cytotoxicity by 3-aminobenzamide in human malignant glioma cells

Poly(ADP-ribose) polymerase is a zinc-finger DNA-binding protein that detects specifically DNA strand breaks generated by genotoxic agents and is thought to be involved in DNA repair. Here, we examined the effects of 3-aminobenzamide, a poly(ADP-ribose) polymerase inhibitor, on the chemosensitivity of human malignant glioma cells. 3-Aminobenzamide selectively potentiated the cytotoxicity of the nitrosoureas, nimustine, carmustine and lomustine in 10 of 12 human malignant glioma cell lines. In contrast, 3-aminobenzamide did not modulate the cytotoxic effects of doxorubicine, teniposide, vincristine, camptothecin or cytarabine. The nitrosoureas did not induce poly(ADP-ribose) polymerase activity in the glioma cells. Ectopic expression of truncated poly(ADP-ribose) polymerase containing the poly(ADP-ribose) polymerase DNA-binding domain, which acts as a dominant-negative mutant, in LN-18 or LN-229 cells did not alter the 3-aminobenzamide effect on nitrosourea-mediated cytotoxicity. Thus, 3-aminobenzamide may target another nicotinamide adenine dinucleotide (NAD)-requiring enzyme, but not poly(ADP-ribose) polymerase, when enhancing nitrosourea cytotoxicity in human malignant glioma cells. Carmustine cytotoxicity was associated with a G2/M arrest. Coexposure to carmustine and 3-aminobenzamide overcame this G2/M arrest in T98G cells, which are sensitized to carmustine by 3-aminobenzamide, but not in U251MG cells, which are refractory to 3-aminobenzamide-mediated sensitization to carmustine. Thus, 3-aminobenzamide-mediated sensitization to carmustine cytotoxicity may result from interference with the stable G2/M arrest response to carmustine in human glioma cells.

Enhancing effect of electric stimulation on cytotoxicity of anticancer agents against rat and human glioma cells

Electropermeabilization, or electroporation, has been used to deliver genes or drugs into the cytoplasm through micropores in the cell membrane caused by electric stimulation. The cytotoxic effect of a combination of anticancer agents with electric stimulation on rat C6 and human T98G glioma cells was examined in vitro. Electric pulses of 100 microsec square waves (eight cycles at 1 Hz) at various electric fields were delivered to C6 or T98G glioma cell suspensions in combination with several anticancer agents. Cell growth was evaluated 48-72 h after treatment. Measurement of cell lysis by electric stimulation was used to assess the optimum field strength for electroporation. Electric stimulation enhanced significantly the cytotoxicity of bleomycin to both C6 and T98G cells by more than 1000-fold using an electric field of 1750 V/cm for C6 cells and 1000 V/cm for T98G cells. The enhancement disappeared when bleomycin concentration was reduced to 100 pg/ml. The cytotoxicity of carboplatin was weakly but significantly enhanced by electric stimulation when a high dose of carboplatin was used. However, there was no enhancement of the cytotoxicity of nimustine hydrochloride (ACNU), etoposide, and vincristine. These results indicate that the combination of bleomycin and electroporation is the most potent candidate for electrochemotherapy in vivo.

Actinomycin D and staurosporine, potent apoptosis inducers in vitro, are potentially effective chemotherapeutic agents against glioblastoma multiforme

PURPOSE

Although chemotherapeutic protocols that include chloroethylnitrosoureas (CENUs), such as 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) and 1, 3-bis(2-chloroethyl)-1-nitrosourea (BCNU), have been a mainstay of treatment for glioblastomas, the clinical outcomes have been unsatisfactory. More effective chemotherapeutic protocols for these tumors will require clear delineation of more cytocidal and cytostatic chemotherapeutic drugs.

METHODS AND RESULTS

In this study, we measured the cytocidal effects of ACNU, cisplatin, actinomycin D, and staurosporine, administered within their therapeutic dose ranges, in the treatment of glioblastoma cells. As assessed by WST-1 colorimetric assay, the number of viable cells decreased markedly in T98G cultures treated with actinomycin D or staurosporine, to less than 20% of the level in control cultures at 72 h, but did not decrease or even increased after 6 days of treatment with ACNU. After treatment with cisplatin for 5 days, cell viability decreased to 30% of control. As assessed by fluorescence microscopic examination of nuclear staining by Hoechst 33258 and by electron microscopy, the majority of dead cells treated with actinomycin D, staurosporine, or cisplatin had morphologic features of apoptosis. Caspase-3 activity increased more than 20-fold in cells treated with actinomycin D, staurosporine, or cisplatin but increased less than fivefold in ACNU-treated cells. In addition to caspase-3 activation, western blot analysis demonstrated cleavage of caspase-2 during the apoptotic process. These findings indicate that actinomycin D and staurosporine potently induce apoptosis, whereas ACNU exerts mainly a cytostatic rather than a cytocidal effect.

CONCLUSION

Actinomycin D and staurosporine and their derivatives are potentially effective chemotherapeutic agents against glioblastoma cells at least in vitro.

Expression pattern of chemoresistance-related genes in human malignant brain tumors: a working knowledge for proper selection of anticancer drugs

BACKGROUND

In addition to traditional modalities such as surgical intervention and radiotherapy, chemotherapy is a common therapeutic method for human malignant brain tumors. However, the effectiveness of chemotherapy is frequently hampered by cancer cell chemoresistance, resulting in an unsatisfactory outcome. To overcome this disadvantage, the proper selection of efficacious anticancer agents is required.

METHODS

The expression levels of chemoresistance-related genes, MGMT, mdr1, MRP, MTIIA and GST-pi, in 28 surgical specimens of human brain tumors and in 10 human glioma cell lines were examined by Northern blot analysis. In addition, the SD10 values of human glioma cell lines against ACNU, CDDP, ADM and VP16 were estimated by a cell survival assay.

RESULTS

The expression levels of each of the chemoresistance-related genes, except MRP, were generally higher in brain tumors than those in non-neoplastic brain tissues. MGMT expression correlated exclusively with ACNU resistance in all glioma cell lines examined (p = 0.0002). The transcriptional level of mdr1 in the tumor cells correlated with the SD10 values of VCR (p = 0.04) and ADM (p = 0.034). In contrast, the expression levels of MTIIA and GST-pi did not correlate with resistance to any of the drugs tested. A correlation of MRP mRNA expression with multidrug resistance was not apparent in the 10 cell lines tested.

CONCLUSIONS

The data indicate that knowledge of the expression levels of MGMT and mdr1 may be particularly useful for a more rational selection of drugs which are not influenced by these resistance genes and which have improved efficacy against human brain tumors.

Differential sensitivity to mitomycin C between human RSa cell line and its derivative UVr-1

To study cellular signaling factors responsible for the susceptibility of human cells to cell proliferation inhibition by anticancer drugs, human RSa cell line and its ultraviolet-resistant derivative UVr-1 were compared with respect to their sensitivity to the anti-proliferative effects of mitomycin C (MMC), 5-fluorouracil, nimustine (ACNU), cisplatin, pirarubicin (THP), bleomycin, methotrexate and ifosfamide. RSa cells were found to be highly sensitive to MMC by MTT assay compared to UVr-1 cells. The half maximum inhibition concentration of MMC against proliferation of RSa cells was approximately 100 ng/ml while that of UVr-1 cells was greater than 1 microgram/ml. There was no significant difference observed between RSa and UVr-1 cells in the sensitivity to other seven drugs examined. Analysis by flow cytometry revealed that the cell cycle of RSa was completely blocked at the G2/M phase 40 h after treatment with MMC at a concentration of 100 ng/ml whereas a substantial proportion of UVr-1 cells was not arrested at that phase even in the presence of MMC. Further immunoblot analysis on MMC-induced signal transduction showed that the amounts of phosphorylated ERK MAP kinases were increased in UVr-1 cells to a greater extent than those in RSa cells after treatment with MMC for longer than 2 h. However, the increase in p21Cip1 was observed in RSa cells 1 h after addition of MMC but was not observed in UVr-1 cells. These distinct signaling pathways might account for the differences in sensitivity to MMC between RSa and UVr-1 cells.

[Expression and functions of adaptive response genes in Escherichia coli treated with mono- and bifunctional alkylating agents. Interference with SOS response]

The expression of genes belonging to the Ada regulon of Escherichia coli under the action of mono- and bifunctional alkylating agents--high-efficiency antitumor HMM, ACNU, and BCNU preparations--was studied. The functional specificity of the alkA, alkB, and aidB1 genes concerning both the structure and volume of DNA alkylation and the specificity of cell preadaptation was revealed. Additional experimental evidence for the role of the aidB1 gene as a unique "hazard gene", a component of the E. coli ada operon, was obtained. A phenomenon of positive interference between alternative SOS and Ada responses was observed for the first time upon gene expression.

CD95-mediated apoptosis: no variation in cellular sensitivity during cell cycle progression

Sensitivity of CD95-mediated apoptosis has been reported to vary during cell cycle progression (FEBS Lett. (1997) 412, 91-93). Here, we report that three human glioma cell lines with different p53 status (i) undergo growth arrest and synchronous cell cycle re-entry after prolonged serum deprivation, (ii) do not exhibit cell cycle-related changes in CD95 expression at the cell surface, and (iii) do not exhibit cell cycle-related changes in susceptibility to DC95 ligand-induced apoptosis. In contrast, cell cycle-specific activity was demonstrated for various cancer chemotherapy drugs. Further, CD95 expression and susceptibility to CD95 ligand-induced apoptosis does not vary during cell cycle progression of Jurkat T cells, HeLa cervical carcinoma and HepG2 hepatocellular carcinoma cells. These results do not support a role for the cell cycle phase as an important predictor of vulnerability to CD95-mediated apoptosis.

Adenovirus-mediated p16 gene transfer prevents drug-induced cell death through G1 arrest in human glioma cells

This study examined the effects of full-length p16 gene transfer by recombinant adenovirus on cell growth and on sensitivity to CDDP or ACNU chemotherapies. We developed a recombinant adenovirus expressing the full-length human p16 gene (AxCA-hp16) by the COS-TPC method. AxCA-hp16 was infected into the p16-null human glioma cell line, U251MG. AxCA-hp16 infection inhibited proliferation of U251MG cells. A proliferation assay employing MTT showed that AxCA-hp16 infection induced chemoresistance, preventing CDDP-induced cell death (11- to 15-fold) and ACNU-induced cell death (80- to 92-fold). In the absence of AxCA-hp16, cell death was induced with CDDP or ACNU at 3 to 5 days after treatment, as demonstrated by Trypan-blue exclusion. Flow-cytometric analysis showed that CDDP or ACNU arrested cells in the G2 phase on day 1 and that cells re-entered the cycle on day 3. However, the cells infected with AxCA-hp16 after CDDP or ACNU treatment showed G1 arrest on day 5 after re-entering the cycle from G2 arrest on day 3. The cells infected with AxCA-hp16 before CDDP or ACNU treatment showed G1 arrest over the 5 days after the infection. This study demonstrated that G1 arrest induced with p16-gene expression prevents ACNU- or CDDP-induced cell death. The cell death induced by ACNU and CDDP therefore appears to occur in the phase after the G1/S check point.

[Sensitivity of human melanoma xenografts to nitrosoalkylurea antineoplastic agents]

We studied the sensitivity of human melanoma (Bro strain) xenografts to drugs of the nitrosoalkylurea (NAU) class: nitrosomethylurea (NMM), karmustin (BCNU), nimustin (ACNU), nitrulin, and ADEKO. High antitumor activity of NAM was shown when the drugs were applied not only at the early, but also at the late stages of tumor progression (tumor mass 400 and 1200 mg, respectively). The therapeutic effect of the drugs was estimated with the use of criteria characterizing the kinetics of tumor regression, increased life span, and survival of treated animals. After early administration of the drugs (Day 4 after tumor transplantation), 67% and 50% of animals survive under the influence of nitrulin and ACNU, respectively, while the rate of tumor regression increased in the sequence nitrulin < karmustin < NMM < ACNU. After late administration (11 days after tumor transplantation), NMM was most effective at increasing survival (35% of survived animals by 35 days of observation), while the rate of tumor regression increased in the sequence ADEKO < NMM < karmustin < nitrulin < ACNU.

Multidrug resistance in glioblastoma. Chemosensitivity testing and immunohistochemical demonstration of P-glycoprotein

Chemosensitivity of previously untreated glioblastomas to mitoxantrone, methotrexate, ACNU and BCNU was tested on cultured tissue. Sixteen of 62 tumors were partially chemosensitive in vitro. The monoclonal antibody C 219 was used to demonstrate the presence of p-glycoprotein in the 16 sensitive and five highly resistant glioblastomas. All 21 tumors identically expressed p-glycoprotein. These results show that untreated glioblastomas primarily express p-glycoprotein even if they are at least partially chemosensitive in vitro. Therefore, immunohistochemical demonstration of p-glycoprotein with the monoclonal antibody C 219 can not provide reliable information on short term resistance of the individual tumors to antineoplastic drugs. P-glycoprotein expression could, however, help to explain the disappointing overall long-term efficacy of chemotherapy by showing the existence of cell populations with early drug resistance in these tumors.

Effects of treatment with nimustine (ACNU), a bifunctional alkylating anticancer agent, on cultured resting L1210 cells

Resting L1210 cells were treated with nimustine (ACNU), a bifunctional alkylating anticancer agent, for 2 h in a nutrient-depleted medium. The cells were then transferred to a fresh medium and incubated for a further 48 h. Functions of the cells thus prepared were examined in terms of the dye-exclusion of the membrane, 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl] -2H-tetrazolium hydroxide, inner salt, sodium salt (XTT)-reducing ability of the mitochondria, and heat generation due to vital metabolism as the measure of cell viability. The cells treated with ACNU were functioning normally in all the cell functions examined but were completely devoid of proliferating capacity. These results suggest the possibility that ACNU might impair the proliferative capacity of the resting cell population inside a solid tumor without causing such impairment to the cells of normal organs and tissues composed of intrinsically non-proliferative cells.

Sensitization of acnu killing effects on HeLa S3 cells by MGMT antisense RNA transfection

O6-methylguanine-DNA-methyltransferase (MGMT) plays a very important role in the cellular resistance to nitrosoureas drugs. Inhibition of MGMT might be a useful approach in tumor chemotherapy. In this study, the depletion of MGMT activity by retroviral-mediated antisense RNA transfection were reported. Three retroviral vectors expressing MGMT antisense RNA were constructed and transfected into HeLa S3 cells. The difference of MGMT mRNA, MGMT activity as well as cellular resistance to ACNU before and after transfection were observed. It was found that antisense RNA targeting 5' region and whole length of MGMT mRNA could partially deplete MGMT activity and enhance killing effects of ACNU. However, 3' region antisense RNA had no effect on MGMT modulation.

p53-independent WAF1 induction by ACNU in human glioblastoma cells

The induction of WAF1 gene expression after the treatment with the anticancer agent 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU; nimustine hydrochloride) was studied in two human glioblastoma cell lines: U-87MG, which bears the wild-type p53 gene, and T98G, which bears the mutant p53 gene. A marked accumulation of WAF1 was observed 3 h after ACNU treatment in both cell lines. The induction of WAF1 mRNA by ACNU was detected by northern blot analysis in these cells. Binding activity of p53 to a p53 consensus sequence increased after treatment in U-87MG cells but not in T98G cells. The existence of a p53-independent WAF1 induction pathway was supported by the apparent accumulation of WAF1 after ACNU treatment in the p53-null human osteosarcoma cell line Saos-2. These findings suggest that there are two possible pathways for WAF1 induction: the p53-dependent pathway through the p53-responsive element and the p53-independent pathway through other elements.

Transient induction of the MRP/GS-X pump and gamma-glutamylcysteine synthetase by 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3- nitrosourea in human glioma cells

Treatment of human glioma A172 cells with 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU), an alkylating antitumor agent the primary target of which has been thought to be DNA, resulted in elevated expression of mRNA for multidrug resistance-associated protein (MRP) within the first 2 h and then a decrease in expression 24 h after the treatment. Western blot analyses revealed that levels of MRP in these ACNU-treated cells paralleled mRNA levels. Membrane vesicles prepared from ACNU-treated cells also displayed elevated transport activities for leukotriene C4, a known substrate for MRP. Gamma-glutamylcysteine synthetase (gamma-GCS) mRNA expression was coinduced with MRP by ACNU. Because gamma-GCS is the rate-limiting enzyme involved in the de novo biosynthesis of glutathione, increases in glutathione were also transiently induced by ACNU. These results demonstrate for the first time that the expression of functional MRP and gamma-GCS can be transiently coinduced by ACNU. Multiple short exposures (1 h) of ACNU following a long duration (1 week) of drug-free conditions resulted in the development of an ACNU-resistant population (designated A172R) that overexpressed MRP/gamma-GCS mRNA and had elevated transport activities for leukotriene C4. A172R exhibited cross-resistance to the antitumor drug doxorubicin and heavy metal sodium arsenate but not to cisplatin. Our results also demonstrate that intermittent treatments of human glioma cells with ACNU can lead to the development of MRP-related multidrug resistance. These results, taken together, reveal a possible new mechanism of the development of drug resistance for the antitumor nitrosoureas.

Posttranscriptional regulation of MRP/GS-X pump and gamma-glutamylcysteine synthetase expression by 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea and by cycloheximide in human glioma cells

Treatment of human glioma A172 cells with 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethy-3-nitrosourea (ACNU) for 2 to 4 hr resulted in a 2- to 3-fold increase in steady-state levels of multidrug resistance-associated protein (MRP) and gamma-glutamylcysteine synthetase (gamma-GCS) mRNA. Nuclear run-on assays revealed a less than 0.5-fold increase in transcription rates of these genes under the same treatment conditions, suggesting that posttranscriptional regulation plays an important role for the increased mRNA levels. In the absence of ACNU, rates of MRP and gamma-GCS mRNA degradation were similar in A172 cells as determined by incubating cells with the RNase inhibitor, Actinomycin D. ACNU treatments resulted in increased MRP mRNA stability. Induction of MRP and gamma-GCS mRNA by ACNU apparently did not require de novo protein synthesis as determined by the use of protein synthesis inhibitor cycloheximide (CHX). However, CHX alone could induce accumulation of gamma-GCS mRNA, also by posttranscriptional mechanism. Taken together, these results demonstrate that (i) posttranscriptional regulation is primarily involved in the induction of MRP and gamma-GCS expression by ACNU and CHX in human glioma cells; and (ii) despite the fact that these two genes have been reported to be frequently co-expressed, their responses to the treatments of RNA and protein synthesis inhibitors are not the same.

Chemotherapy of human malignant glioma: prevention of efficacy by dexamethasone?

Steroids are commonly administered for the control of edema, mass effect, and side effects from therapy to patients with malignant glioma who are receiving radiotherapy and chemotherapy. Here, we report that therapeutic concentrations of dexamethasone (DEX) attenuate cytotoxicity and growth inhibition of human malignant glioma cells induced by exposure to several chemotherapeutics, including ACNU, VM-26, vincristine, cytarabine, methotrexate, and adriamycin. DEX-mediated cytoprotection is not linked to DEX effects on glioma cell proliferation. However, the cytoprotective effects of DEX appeared to be more prominent in cell lines with wild-type p53 status (n = 2) than in p53 mutant cell lines (n = 3). Further, DEX-mediated rescue from chemotherapy does not directly involve Bcl-2 family proteins since DEX failed to change the expression of Bcl-2 or Bax proteins and since bcl-2 gene transfer-mediated cytoprotection was not redundant with the effects of DEX. DEX thus appears to control a common, bcl-2-independent death pathway in glioma cells that is not limited to specific drug actions. Chemotherapy is usually given as an elective, adjuvant treatment to glioma patients in stable condition who can tolerate steroid withdrawal. To maximize therapeutic efficacy, steroids should be withdrawn from glioma patients prior to chemotherapy.

[Study on potentiation of nitrosourea-cytotoxicity by DNA repair enzyme inhibitors in human brain tumor cells]

Nitrosoureas are antitumor alkylating agents widely used in the chemotherapy of malignant brain tumors. However, the effectiveness of adjuvant nitrosourea chemotherapy has proved inadequate, failing to provide any significant prolongation of survival time. One of the reasons for the poor results is a drug resistance system in the form of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT). O6-alkylguanine derivatives are well known to be inhibitors of MGMT, and inactivation of MGMT by these derivatives leads to increased tumor cell sensitivity to nitrosoureas. In this study, the authors tested the ability of O6-benzylguanine, O6-(4-, 3- and 2-fluorobenzyl) guanines, O6-(4-, 3- and 2-trifluoromethylbenzyl) guanines, O6-(4-, 3- and 2-pyridylmethyl) guanines and O6-(2- and 1-naphthylmethyl) guanines to reduce MGMT activity in SF-188 cell-free extract by using [3H] methylated substrate DNA and analyzed their enhancing effect on the cytotoxicity of 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl) -3-nitrosourea hydrochloride (ACNU) by using a calorimetric cytotoxicity assay. The MGMT activity in the SF-188 cell-free extract was 944 +/- 43 fmol/mg protein (Mean +/- SD, n = 5). O6-(4- and 3-fluorobenzyl) guanines were found to be more effective in inactivating MGMT than O6-benzylguanine. O6-(4-trifluoromethylbenzyl) guanine considerably reduced MGMT activity as did O6-benzylguanine. O6-(3-trifluoromethylbenzyl) guanine, O6-(4- and 3-pyridylmethyl) guanines, and O6-(2-naphthylmethyl) guanine were intermediately effective, but O6-(2-fluorobenzyl) guanine, O6-(2-trifluoromethylbenzyl) guanine and O6-(1-naphthylmethyl) guanine were less effective. ACNU cytotoxicity in SF-188 cells was strongly enhanced by pretreatment with O6-(4- and 3-fluorobenzyl) guanines and O6-(4-trifluoromethylbenzyl) guanine and moderately enhanced by O6-(3- trifluoromethylbenzyl) guanine and O6-(4- and 3-pyridylmethyl) guanines, but not enhanced by O6-(2-fluorobenzyl) guanine, O6-(2-trifluoromethylbenzyl) guanine and O6-(1-naphthylmethyl) guanine. The test compounds were not cytotoxic at concentrations between 0.5 and 5.0 microM. The enhancing effects on ACNU cytotoxicity were consistent with the inhibition of MGMT activity after two-hour pretreatment with O6-arylmethylguanine derivatives. These results indicate that the 2-position of the O6-benzyl group plays an important role in the inactivation of the MGMT activity and the potentiation of ACNU cytotoxicity.

Transgenerational effects of maternal exposure to chemicals on the functional development of the brain in the offspring

In order to prevent health risk from environmental chemicals, particularly for progeny, we have been performing a risk assessment for various chemicals including therapeutic agents. This paper reports the functional effects of maternal exposure to psychoactive drugs, anticancer drugs, or herbicides on the offspring of rats. Maternal exposure to imipramine in a dose equivalent to the therapeutic dose per unit body weight induced hyperthermic response to chlorpromazine in the male offspring, while normal control rats showed a marked hypothermia. Exposure to ethosuximide resulted in an increase in play fighting behavior in young offspring that was fostered by lactating normal mothers. Single exposures to nimustine or cisplatin, anticancer drugs, at a different gestational stage resulted in an acceleration of growth when exposed at the earlier stage of gestation. Moreover, cisplatin-exposed rats were emotionally unstable, showing a short latent time to the first line-crossing in an open-field during infantile period. The rats exposed to glufosinate ammonium, an herbicide, during the time of neurogenesis in the hippocampus showed a decrease in the wet-dog shakes response to kainic acid at six weeks of age. These results suggest that maternal exposure to chemicals during pregnancy induces a variety of functional abnormalities in the brain of the offspring dependent on the pharmacologic action of chemicals and the stage of gestation even with a single exposure.

Cytotoxicity of fluoroethylating agents is potentiated by O6-benzylguanine

O6-Benzylguanine (BG) is a potent depleter of a repair enzyme O6-alkylguanine-DNA alkyltransferase. Pretreatment of cells with BG potentiates the cytotoxicity of chloroethylating anti-cancer agents. In this study we used HeLa S3 cells to examine the cytotoxic potentiation of 39 compounds after BG pretreatment. Compounds tested included anti-cancer agents and carcinogens, and among them only the cytotoxicity of methylating, chloroethylating and fluoroethylating agents was potentiated. This is the first description of the cytotoxic potentiation of fluoroethylating agents. Potentiation ratios were found to vary even among compounds possessing the same alkylating group. By pretreatment with 10 microM of BG, the cytotoxicity of methylating agents such as N-methyl-N-nitrosourea and streptozotocine was potentiated 3.7 and 9.4 fold, respectively. For chloroethylating agents, the potentiation ratios were 3.5 for N-chloroethyl-N-nitrosourea, 8.6 for N-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-N'-(2-chloroethyl)-N'-nitroso urea (ACNU), 2.2 for N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU), 3.0 for N-(2-chloroethyl)-N'-cyclohexyl-N-nitrosourea (CCNU) and 5.2 for chloroethyl methanesulfonate. With respect to fluoroethylating agents, the potentiation ratios were 7.2 for N-fluoroethyl-N-nitrosourea, 2.0 for N-cyclohexyl-N'-fluoroethyl-N'-nitrosourea and 5.5 for fluoroethyl methanesulfonate. No effect was observed with the bromoethylating agent, N-bromoethyl-N-nitrosourea. There was no potentiation of the cytotoxicity of anti-cancer agents such as mitomycin C (MMC), cisplatin (CDDP), 5-fluorouracil (5FU), bleomycin (BLM), prednisolone, camptothecin, etoposide, methotrexate or vinblastine. A possible mechanism for the cytotoxic potentiation of the test compounds by BG pretreatment is discussed.

[Chemosensitivity of cultured meningiomas]

Meningioma is one of the popular benign brain tumors. However, the recurrence of this tumor is not infrequently encountered. In an attempt to establish the useful adjuvant therapy for the recurrent meningioma, in vitro chemosensitivity study for meningioma was conducted. Among various chemotherapeutic agents tested here, cisplatin showed highest cytotoxicity on cultured meningioma cells. In conclusion, cisplatin may be useful in adjuvant chemotherapy for the recurrent meningioma.

[Flow cytometric analysis of cell cycle for the action mechanism of antineoplastic agents]

Cell kinetics of cancers have been described in books, texts and other reports, but the correlation with action mechanism of antineoplastic agents has rarely been mentioned in the literature. The action mechanism of the antineoplastic agents such as interferon, ACNU and cisplatin was analyzed with use of propidium iodide and BrdU double staining by flow cytometer. Interferon showed S phase accumulation, ACNU and cisplatin blocked the stage of G(2)M phase. Flow cytometry was useful for the analysis of cell kinetics.

Transfer and expression of human O6-methylguanine-DNA methyltransferase cDNA confers resistance of Mer- HeLa MR cells to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosoure a

Previous studies have demonstrated that O6-methylguanine-DNA methyltransferase (MGMT) is a major contributor to tumor cellular resistance toward chloroethylnitrosoureas. To further clarify the effect of MGMT gene expression on cellular chemosensitivity to 1-(4-amino-2-methyl-5-pyrimidinyl)-3-(2-chloroethyl)-3-nitrosourea (ACNU) in human cells, a repair-deficient human tumor cell line (HeLa MR) was transfected with a human MGMT expression vector (pSV2MGMT-neo). Multiple unique transfectants were isolated which exhibited variable levels of MGMT mRNA by Northern hybridization analysis. Vector-transfected controls were generated simultaneously. Transfectants expressing high levels of MGMT activity showed an increased resistance to ACNU-induced cytotoxicity. Furthermore, the levels of the protective effect against ACNU correlated generally with the levels of introduced MGMT expression. This study further provided direct evidence of MGMT contribution to ACNU resistance in human tumor cells. Based on the results presented here, we also discussed the perspective of the clinical utility of MGMT cDNA transfer and expression.

[Modulation of the antitumor activity of 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosoure a by O(6)-methyl-2'-deoxyguanosine--a new inhibitor of O(6)-alkylguanine-DNA-alkyltransferase]

O6-Methyl-2'-deoxyguanosine (O6-MedG), a novel inhibitor of O6-alkylguanine-DNA alkyltransferase (O6-AGT), has been synthesized. The ability of O6-MedG to deplete the O6-AGT activity in leukemia L1210 and melanoma B16 cells in vivo has been studied. After intraperitoneal administration of O6-MedG to mice bearing leukemia L1210 or melanoma B16, the activity of O6-AGT in tumour cells decreased by 50%. Pretreatment of leukemia L1210 bearing mice with O6-MedG (200 mg/kg) 24 hours prior to ACNU (15 mg/kg) administration resulted in six out of seven 60-day survivors. Treatment of mice with ACNU (15 mg/kg) alone increased the life span by 200%. Treatment of melanoma B16 bearing mice with O6-MedG and 3 hours thereafter with ACNU resulted in a 50% inhibition of tumour growth, whereas the inhibiting effect of ACNU alone was 16%. There was no difference in leukemia growth when L1210/BCNU bearing mice were treated with O6-MedG followed by ACNU treatment. In vivo ACNU (15 mg/kg) produced a deep and prolonged inhibition of DNA, RNA and protein synthesis in leukemia L1210 cells. The DNA synthesis in leukemia L1210/BCNU cells was shown to recover more rapidly than in L1210 cells. The activities of DNA-polymerases alpha and beta and, especially, of O6-AGT were elevated in ACNU-resistant leukemia cells as compared with ACNU-sensitive cells. The activation of some repairing enzymes, such as O6-AGT, DNA-polymerases alpha and beta as well as increased levels of GSH may play a role in the development of drug resistance to ACNU.

Experimental study of intraoperative local chemotherapy with fibrin glue containing nitrosourea for malignant gliomas

BACKGROUND

Local control of the tumor bed after removal of a tumor is one of the most important points in the treatment of malignant gliomas. This study was designed to examine whether fibrin glue is useful as a vehicle for sustained release of intraoperative local chemotherapy with nitrosourea (ACNU).

METHODS

The growth-inhibiting activity of ACNU on C6 glioma cells and ACNU released into 5-mL supernatant saline from fibrin glue containing 5 mg/mL (10 mg) of ACNU was measured in vitro. C6 tumor inoculated in rat brains was covered with fibrin glue containing either 2 mg/mL or 5 mg/mL of ACNU for 5 days, and the histologic changes were examined.

RESULTS

ACNU inhibited the growth of C6 glioma cells in a dose-dependent manner, and the drug concentration required for 50% inhibition of cell growth (IC50) was about 4 micrograms/mL with 1 hour of treatment. Although about 50% of all ACNU included in the fibrin glue was released in the first hour, an effective concentration over the value of IC50 was sustained even after 12 hours. A histologic examination showed tumor cells damaged within a depth of about 2-3 mm from the tumor surface covered with fibrin glue containing ACNU.

CONCLUSIONS

Fibrin glue may be useful as a vehicle for sustained-release chemotherapy, and intraoperative local chemotherapy with fibrin glue containing anticancer agents such as nitrosourea may be helpful in the local control of malignant gliomas.

Intra-arterial ACNU and cisplatin chemotherapy for the treatment of glioblastoma multiforme

Intra-arterial (IA) chemotherapy has achieved no obvious clinical superiority as a treatment for glioblastoma multiforme despite the many theoretical advantages. The clinical courses of 38 patients who underwent surgery and radiotherapy with IA 1-(4-amino-2-methyl-5-pyrimidinyl)-methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) and cisplatin were reviewed. Tumor regrowth was evaluated by comparison of contrast-enhanced areas on computed tomographic scans. The initial response rate was 19 of 32 patients evaluated, and the median survival time (MST) for all 38 patients was 53 weeks. Local recurrence was observed in 20 patients, and distant recurrence (areas more than 3 cm from the original tumor margin) was observed in 15 patients. The MST was 59 weeks for patients without distant recurrence, and 42 weeks for patients with distant recurrence (statistically not significant). Adjuvant IA ACNU and cisplatin chemotherapy did not improve the survival time. An important clinical feature was the high incidence of distant recurrence, in contrast to experience with other conventional therapy regimens. Distant recurrence, without extended survival, may suggest insufficient control of tumor regrowth.