Modulation of O6-methylguanine-DNA methyltransferase-mediated nimustine resistance in recurrent malignant gliomas by streptozotocin--a preliminary report

This trial is based on the strategy of reversing O6-methylguanine-DNA methyltransferase (MGMT)-mediated nimustine resistance by depleting MGMT activity with streptozotocin (STZ) pretreatment. Eight patients with recurrent malignant gliomas refractory to previous nimustine chemotherapy were entered in this study. Patients received STZ (2g/m2) followed one hour with nimustine (2-3 mg/Kg) via the ipsilateral carotid artery. After 1-2 cycles of therapy, 3 patients responded, 4 stabilized, and 1 failed. Toxic effects were generally tolerated. The preliminary results indicated that nimustine-resistant tumor cells in vivo could also be sensitized by modulation of MGMT activity.

Intracellular localization and function of DNA repair methyltransferase in human cells

An antibody preparation specific for human O6-methylguanine-DNA methyltransferase (EC 2.1.1.63) was obtained by immunoaffinity purification on two types of affinity columns with the purified human and mouse methyltransferase proteins as ligands. The antibodies were used in Western blotting analysis of fractionated cell extracts. More than 90% of the methyltransferase protein was recovered in the cytoplasmic fractions with both human HeLa S3 cells and MR-M cells, the latter overproducing the enzyme 36 times as much as the former. Cytoplasmic localization of the methyltransferase in HeLa S3 cells was further confirmed by in situ immunostaining. By Western blotting analysis of fractionated cell extracts from HeLa S3 cells treated with alkylating agents, we found that amounts of the enzyme decreased more rapidly in the nuclear fraction than in the cytoplasmic fraction, and recovery of the enzyme level in the cytoplasmic fraction was slower than that in the other. These results suggest that the methyltransferase protein is degraded in the nucleus after it commits the repair reaction and that the cytoplasmic enzyme is transported into the nucleus as the nuclear methyltransferase is used up in this manner.

In vitro chemosensitivity test of malignant gliomas: clinical relevance of test results independent of adjuvant chemotherapy

Tumor specimens of malignant human gliomas were processed in a colony forming assay (CFA, n = 70) and a metabolic test system (MTT-test, n = 49) for in-vitro chemosensitivity testing. The clinical data as well as the complete follow-up of these patients were obtained until their death or at least 2 years of survival. By means of multivariate statistical analysis we demonstrate that both test systems are not influenced by biometrical data of the patient of histopathological parameters of the tumor. According to the COX regression model the results of both assays are per se no prognostic factor when regarded independently of additional treatment. However, in 33 patients treated with either ACNU or BCNU, a prospective correlative trial clearly demonstrates a predictive value of the CFA in adjuvant chemotherapy of gliomas.

[Cytotoxic action of alkylating agents in human tumor cells and its relationship to apoptosis]

Various anticancer agents have been known to induce apoptosis in certain types of human tumor cells. The fact that a variety of agents, which attack different cellular targets, induce common apoptotic cell death suggests that the nature of initial damage is not directly involved in apoptosis. The mechanism by which a damage leads to apoptosis is not known. However, modulation of this process may affect the outcome of anticancer drug treatment. This article briefly reviewed the studies of endogenous as well as exogenous factors which modulate apoptosis, and then described the characteristics of cell death induced by alkylating agents. O6-Alkylguanine, a major cytotoxic DNA damage produced by simple alkylating agents, can be repaired by the cellular enzyme O6-methylguanine-DNA methyltransferase (MGMT). About one-fifth of human tumor cell strains lack the MGMT activity and termed as Mer- cells. Mer- cells are hypersensitive to alkylating agents like chloroethyl nitrosoureas (CNUs), compared with repair-proficient Mer+ cells. It is suggested that identification of a factor which suppresses the MGMT gene expression in CNU-resistant Mer+ cells, may enable us to convert these Mer+ cells to Mer- phenotype, thus resulting in much higher sensitivity of Mer+ cells to CNUs.

Artificial control of nuclear translocation of DNA repair methyltransferase

We constructed a recombinant plasmid carrying a chimeric cDNA that encodes a fusion protein, ER:MGMT, composed of the ligand-binding domain of the human estrogen receptor and the human O6-methylguanine-DNA methyltransferase. By introducing this plasmid into the methyltransferase-deficient human cell line HeLa MR, a system was established in which nuclear translocation of the ER:MGMT fusion protein can be controlled by estrogen. On in situ immunostaining using anti-MGMT, the cytoplasm of ER:MGMT-carrying cells was preferentially stained and nuclear staining occurred only when the cells were exposed to estrogen. The estrogen-dependent nuclear translocation of ER:MGMT was confirmed by Western blotting analysis of fractionated cell extracts. The fusion protein was translocated into the nucleus within 1 h after estrogen treatment and remained there unless estrogen was removed. The methyltransferase activity of the fusion protein was as active as the authentic methyltransferase enzyme, regardless of the presence or absence of estrogen. The ER:MGMT-producing cells were sensitive to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU) in the absence of estrogen, and estrogen treatment rendered the cells as resistant to ACNU as the ordinary Mer+ cell line, HeLa S3, thereby indicating that translocation of the methyltransferase into the nucleus is a prerequisite for repair of the chromosomal DNA damaged by alkylating agents. Taking advantage of the artificial control of cellular localization of the fusion protein, we examined the timing of the nuclear translocation required to execute efficient DNA repair. We obtained evidence that the methyltransferase must repair the DNA damage as soon as the DNA is exposed to ACNU, in order to avoid cell cycle arrest at the G2 phase.

Potential of O6-methylguanine or O6-benzylguanine in the enhancement of chloroethylnitrosourea cytotoxicity on brain tumours

The purine analogues O6-methylguanine and O6-benzylguanine are well-known as a chemical modulator of the DNA repair enzyme O6-methylguanine-DNA methyltransferase. Inactivation of the enzyme by O6-methylguanine or O6-benzylguanine is expected to enhance sensitivity of tumours to chloroethylnitrosoureas. We studied the effect of O6-methylguanine or O6-benzylguanine pretreatment on cytotoxicity of 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3- (2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) in brain tumour cells and transplanted brain tumours. Two-hour exposure of O6-methylguanine at higher concentrations (500 microM, 1,000 microM) increased ACNU cytotoxicity by only 2 times in ACNU-resistant C6-1 brain tumour cells. O6-Benzylguanine at concentrations between 10 and 100 microM markedly enhanced the cytotoxic effect. The ACNU sensitivity of the tumour cels pretreated with O6-benzylguanine was 5-40 times that of the cells without O6-benzylguanine. Neither O6-methylguanine nor O6-benzylguanine appreciably enhanced ACNU cytotoxicity of 9 L cells, which were originally sensitive to ACNU. Intracarotid ACNU with O6-methylguanine or O6-benzylguanine decreased proliferating activity of transplanted C6-1 brain tumours significantly during 48 hours. O6-Benzylguanine pretreatment resulted in a greater degree of suppression for a long time. The C6-1 tumours treated only with intracarotid ACNU showed a transient inhibition and a rapid regrowth during 24 hours after the treatment. These results indicate that O6-methylguanine or O6-benzylguanine increases ACNU cytotoxicity and may be feasible for effective combination therapy with chloroethylnitrosourea in the chemotherapy of malignant brain tumours.

O6-alkylguanine-DNA alkyltransferase activity of human malignant glioma and its clinical implications

Activity of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) is an important determinant of responsiveness of tumor cells to chloroethylnitrosoureas (CENUs), representative chemotherapeutic agents for primary malignant gliomas. In order to assess the real states of this repair protein in human malignant gliomas, we assayed AGT activity in surgically extirpated 42 malignant glioma samples and studied the distribution of the activity under certain clinical conditions. There were wide variations in AGT activity between individuals. No significant difference in AGT activity on average was seen either between glioblastoma and anaplastic astrocytoma, nor between primary and recurrent tumors. Among 42 malignant gliomas, 7 samples (16.7%) had low AGT activity less than 0.1 pmoles/mg protein. In the case of glioblastoma, tumors possessing higher AGT activity tended to be less responsive to post-operation remission-induction therapy including CENUs. The result of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) chemosensitivity assay by using the corresponding surgical specimens suggested a close relationship between cellular resistance to CENUs and AGT activity. It was found to be unlikely that a short term administration of CENUs had a significant effect on AGT activity of brain tumors in human body. We could detect a bit of definite evidences of the relevance of AGT to resistance to CENUs and need to conduct further investigations for other resistance factors.

Identification of placental form of glutathione S-transferase in ACNU-resistant murine glioma cell lines

Expression of the placental form of glutathione S-transferase (GST-P) in 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloro-ethyl)-3-nitrosourea hydrochloride (ACNU)-sensitive 9L and C6 glioma cells, and ACNU-resistant 9L (9LR) and C6 (C6R) glioma cells was investigated by Northern blot analysis for GST-P mRNA and Western blot analysis for GST-P protein. The sensitivity of 9L, 9LR, C6 and C6R cell lines to ACNU was evaluated by microculture tetrazolium assay. Localization of GSTP-P protein in these cell lines was investigated by immunocytochemical method. Expression level of GST-P mRNA in 9LR cells was 3 times that of 9L cells and the level of GST-P protein in 9LR cells was 1.7 times that of 9L cells. On the contrary, the amount of GST-P mRNA of C6R cells was 1.3-fold larger than C6 cells and that of GST-P protein of C6R cells was 1.3-fold larger than C6 cells. Immunocytochemical investigation revealed that 9LR cells had stronger expression of GST-P in their cytoplasm than 9L cells. Expression of GST-P in both C6R and C6 cells was less than 9L and 9LR cells, and the amount was similar to each other. The present study suggests that GST-P may play an important role in detoxification of anti-cancer drugs in some glioma cells.

Characterization and chemosensitivity of two cell lines derived from human glioblastomas

We have characterized two human glioblastoma cell lines, which were designated as YH cells and AM cells. The two cell lines maintained morphological appearance observed in the primary culture and immunohistochemically expressed glial fibrillary acidic protein (GFAP) and S-100 protein. Population doubling time for YH cells and AM cells indicated 30 hours and 25 hours, respectively, in an exponential phase of culture. Inoculation of AM cells into athymic nude mice formed large tumors at a high incidence. As with chemosensitivity to chloroethylnitrosourea, O6-methylguanine-DNA methyltransferase (MGMT) activity was measured in in vitro cultured cells as well as tumor specimens obtained at surgery. YH cells showed a high MGMT activity of 1196 fmol/mg and drug resistance to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3- nitrosourea hydrochloride (ACNU) using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. YH tumor specimens indicated an MGMT activity of 301 fmol/mg, which reflected poor effectiveness of ACNU chemotherapy in the clinical evaluation. AM cells had an extremely low MGMT activity of 16 fmol/mg and were vulnerable to ACNU. Original tumor specimens of AM cells however expressed a high value of 628 fmol/mg. Considering that ACNU chemotherapy was not effective in the both patients, an MGMT activity of original tumors related with responsiveness to ACNU. Discrepancy in an MGMT activity between the in vitro cell lines and the respective tumor specimens comes from selection of ACNU-sensitive cells or alteration in biological characteristics during long term culture. These results suggest that cell lines derived human brain tumors are useful targets for understanding the chemosensitivity of human malignant gliomas and for establishing a pertinent chemosensitivity test.

Interrelationship between O6-alkylguanine-DNA alkyltransferase activity and susceptibility to chloroethylnitrosoureas in several glioma cell lines

In order to study the dynamic relationship in glioma cells between O6-alkylguanine-DNA alkyltransferase (AGT) activity and resistance to the cytotoxic effect of chloroethylnitrosoureas (CENUs), we investigated the changes in sensitivity to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU) after modulation of AGT activity. In ACNU-resistant rat glioma cell lines (9LR1, 9LR3, and 9LR12) and a human glioma cell (HNG-1), O6-methylguanine enhanced cytotoxicity to ACNU following a depletion of AGT activity. But no enhancement of cytotoxicity was seen in an ACNU-sensitive rat glioma cell line (9L). In the 9L and 9LR12 cells, equivalently sublethal doses of ACNU similarly depleted AGT activity but the regeneration rates of this repair protein were different. In the case of a 7-day pretreatment with human recombinant interferon-beta (HuIFN-beta), although it could modulate AGT activity in HNG-1 cells, no definite influence on cellular sensitivity to CENUs was observed. However, a 50-day pretreatment with HuIFN-beta conferred resistance to CENUs on them despite its effect to reduce AGT activity. Thus, diversity was seen in the relation between AGT activity and resistance to CENUs when AGT activity was modulated by HuIFN-beta. The results of this study suggest that AGT activity is one of factors affecting cellular sensitivity to CENUs but that alternative mechanisms of tolerance may be induced depending upon some environmental effects.

[Treatment results of intra-arterial ACNU infusion chemotherapy for primary cerebral malignant lymphoma]

From May, 1987 to January, 1991, eight patients harboring primary cerebral malignant lymphoma (primary, six cases; recurrent, two cases) were treated by intra-arterial ACNU infusion chemotherapy. All but two cases also had hyperosmotic blood brain barrier (BBB) modification. Six primary cases had conventional teletherapy of 46-58.7 Gy (average 54.1 Gy) and one recurrent case had been irradiated again by 18 Gy. The follow-up period was 3 to 67.5 months (average 21.4 months). In six cases demonstrating measurable tumors on diagnostic images, all showed a complete response. The 1- and 2-year survival time was 50.0% and 37.5%, respectively, with a median survival time of 13.0 months. These were somewhat better than our previous results, but not better than those in the literature. Although intra-arterial ACNU infusion chemotherapy with BBB modification was not considered superior to other chemotherapy regimens, it yielded some long-surviving cases. Hence this chemotherapy is still worth using for treatment of primary cerebral malignant lymphoma.

Effect of interferon-gamma on ACNU-induced DNA damage and cytotoxicity in human glioblastoma cells

Interferons (IFNs) are a natural body defense with powerful effects on tumor growth, including gliomas. The direct effects of IFN-gamma on (1-4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride (ACNU)-induced deoxyribonucleic acid (DNA) damage and cytotoxicity were investigated in two human glioblastoma cell lines, A-172 and T98G, using a single cell microgel electrophoresis technique and a microculture tetrazolium assay. The results demonstrated a synergistic effect of IFN-gamma with ACNU on intracellular damage in both cell lines. 10 micrograms/ml ACNU induced a cell inhibition rate of 23.9% in A-172 cells, and almost no effect on T98G cells. 1000 U/ml IFN-gamma and 10 micrograms/ml ACNU caused a significant increase in cell inhibition, 51.2% for A-172 and 72.3% for T98G cells. DNA damage in individual A-172 and T98G cells exposed to ACNU was enhanced significantly by IFN-gamma (p < 0.001). The findings suggest a direct effect of IFN-gamma on ACNU-induced cell damage in human glioma, in addition to its effect on immunomodulation.

Modulation of O6-methylguanine-DNA methyltransferase-mediated 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea resistance by O6-benzylguanine in vitro and in vivo

Our previous studies have indicated that O6-methyl-guanine-DNA methyltransferase (MGMT) is a key factor determining tumor cellular resistance to 1-(4-amino-2-methyl-5-pyrimidinyl) methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU). This study describes the modulation of MGMT-mediated ACNU resistance by O6-benzylguanine pretreatment. The ACNU sensitivity of MGMT proficient human tumor HeLa S3, SMMC-7721, and Cc801 cells in tissue culture was markly enhanced by 10 mm O6-benzylguanine, and a correlation between the extent of enhancement and the level of MGMT activities was observed. A single i.p. injection of 100 mg/kg of O6-benzylguanine caused a complete inhibition of MGMT activities in HeLa S3 tumor xenografts and combination of O6-benzylguanine with ACNU (7.5 mg/kg) significantly inhibited HeLa S3 tumor growth. The results demonstrated that O6-benzylguanine could be used as a potential adjuvant in combination chemotherapy with ACNU to treat MGMT proficient tumors.

Identification of cellular defect in UVS1, a UV-sensitive Chinese hamster ovary mutant cell line

UVS1 is an intermediately UV-sensitive Chinese hamster ovary mutant originally isolated by its hypersensitivity to an anticancer drug, 1-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-3-(2-chloroethyl)-3-nitrosour ea hydrochloride. By cell fusion analysis, UVS1 complemented the UV sensitivity of the mouse lymphoma cell line US31 from the eighth complementation group of UV-sensitive rodent cell lines. By enzyme-linked immunosorbent assay we found that within 3 h after UV irradiation both pyrimidine dimers and (6-4)photoproducts in UVS1 were not removed from chromosomal DNA in UVS1 at all. Twenty-four h after UV irradiation the removal rate of (6-4)photoproducts was intermediate between CHO9, the parental cell line, and 43-3B, a UV-hypersensitive Chinese hamster ovary mutant of the complementation group 1, whereas the pyrimidine dimers in UVS1 were removed less efficiently as 43-3B. Alkaline elution assay showed that the incising activity to damaged DNA after UV irradiation of UVS1 was as low as that of 43-3B. The number of 1-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-3-(2-chloroethyl)-3-nitrosour ea hydrochloride-induced DNA interstrand cross-links of UVS1 was almost equal to that of 43-3B and about 1.5 times more than that of CHO9, suggesting that the gene products defective in UVS1 and 43-3B are essential for the excision repair of DNA damages produced by 1-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-3-(2-chloroethyl)-3-nitrosour ea hydrochloride.

Effects of ACNU and cranial irradiation on the mouse immune system

The effects of ACNU and cranial irradiation on the immune system were studied in three groups of 90 mice: Group A, intraperitoneal injection of ACNU (30 mg/kg); Group B, single exposure of 10 Gy to the head; and Group C, intraperitoneal injection of ACNU (30 mg/kg) and single exposure of 10 Gy to the head. Peripheral white blood cell counts, spleen cell subsets, natural killer (NK) cell activity, lymphocyte blastogenesis, and production of interferon (IFN)-gamma were analyzed once a week for 6 weeks after treatment. In Group A, NK cell activity decreased between weeks 4-5, concanavalin A blastogenesis decreased during weeks 1-5, and the levels of L3T4 (CD4) and Lyt2 cells (CD8) and IFN-gamma production decreased during weeks 2-5. However, all tested parameters returned to the normal range at 6 weeks. In Group B, all parameters except for the L3T4 cell level and the IFN-gamma production decreased during week 1, and returned to the normal range thereafter. The concentration of L3T4 cells decreased during week 2 and between weeks 5-6. The IFN-gamma production increased during week 1, decreased during week 2, and returned to the normal range thereafter. In Group C, the suppressive effects were severe and continued for a longer period than in either Group A or B. Concanavalin A blastogenesis, L3T4 cell concentration, and IFN-gamma production were still suppressed after 6 weeks. Therefore, intensive radiochemotherapy for brain tumor may suppress the immunological function.

In vitro measurement of chemosensitivity of human small cell lung and gastric cancer cell lines toward cell cycle phase-nonspecific agents under the clinically equivalent area under the curve

BACKGROUND

Based on the previous finding that cell killing effects of cell cycle phase-nonspecific agents depend on the concentration-time product (C x T) or the area under the curve (AUC), the authors investigated in vitro cytotoxic effects of nimustine hydrochloride (ACNU) and mitomycin C (MMC) under an experimental condition in which the assay AUC was equivalent to their clinically achievable AUC.

METHODS

The cytotoxic effects of these agents on human cancer cell lines, consisting of 9 small cell lung carcinomas (SCLC) and 10 gastric cancers, were measured by a tetrazolium-based colorimetric assay (MTT assay).

RESULTS

These cell lines individually responded to ACNU and MMC in this assay condition. When the authors considered 60% or greater cell kill to be effective, the in vitro response rates of SCLC to ACNU and MMC were 22% (two of nine carcinomas) and 67% (six of nine carcinomas), respectively. The response rates of gastric cancer to ACNU and MMC were 10% (1 of 10 carcinomas) and 40% (4 of 10 carcinomas), respectively. Except for the response of SCLC to ACNU, these in vitro response rates corresponded well to the clinical rates (SCLC to ACNU and MMC, 47% [14 of 30 carcinomas] and 50% [17 of 34 carcinomas], respectively; gastric cancer to ACNU and MMC, 11% [4 of 37 carcinomas] and 30% [63 of 211 carcinomas], respectively).

CONCLUSIONS

These results suggest that the introduction of the clinically equivalent AUC to the in vitro chemosensitivity test for cell cycle phase-nonspecific agents may improve its clinical predictability.

[Quantitative analysis of glutathione and glutathione S-transferase in human brain tumors, C6 rat glioma cells and drug resistant C6 cells]

Glutathione (GSH) and Glutathione S-transferase (GST) plays an important role in the protection of cells against damage from free radicals and also influences cytotoxicity to some kinds of chemotherapeutic agents. GST comprises a group of abundant and widely distributed catalytic and binding proteins that facilitate the conjugation of GSH with the electrophilic center of a large spectrum of hydrophilic molecules. Multiple GST isozymes in mammalian tissues arise from dimeric combination of a number of distinct subunits grouped into three major classes: alpha (alpha), mu (mu), and pi (p). We report the total GST, GST-p activity and GSH content of human brain tumors, C6 rat glioma cells and drug resistant C6 cells. The values of total GST activity in 42 normal brain and brain tumors were quantitatively analyzed. Total GST activity was 92.6 +/- 25.1 units (mean +/- standard deviation) in 8 samples of normal brain tissues, 126 +/- 58.8 units in five grade II or III astrocytomas (154 +/- 63.3 units in grade II astrocytomas, 84.4 +/- 2.7 units in 2 grade III astrocytoma), 66.2 +/- 29.3 in 5 glioblastoma cases, 94.7 +/- 47.7 units in 3 metastatic tumors, 302 +/- 114 unit in 8 meningiomas and 213 +/- 90.4 units in 3 neurinomas. Differences of GST activity between glioblastomas and meningiomas, grade II or III astrocytomas and meningioma, in normal brain tissues and meningioma were statistically significant (p < 0.01). The difference between normal brain tissues and benign tumors (meningiomas and neurinomas), gliomas and benign tumors were also statistically significant (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

[Chemo-sensitivity test for malignant brain tumors by DNA histogram, in vitro]

Application of flow cytometric DNA analysis was tried to determine the sensitivity of ACNU, one of nitrosourea derivatives which had been used very commonly for malignant brain tumors, to tumor cells. To evaluate the degree of chemo-sensitivity of ACNU, factor B was introduced, indicating the accumulated cells in SG2M phases after ACNU treatment as the percentage of cells that was previously in the SG2M phases and it revealed that ACNU sensitive cell clones gave much larger values of Factor B than ACNU resistant ones at the concentration of 10 micrograms/ml ACNU treatments. Twenty clinical materials obtained by operation were examined by measuring the degree of values of this Factor B, and the findings indicated that this method would be applicable as a clinical test for chemo-sensitivity of malignant brain tumors, after some improvement of the method.

Enhancement of radiosensitivity by tamoxifen in C6 glioma cells

The antiestrogen drug tamoxifen, which is used extensively in the treatment of breast cancer, has also been reported to inhibit the proliferation of some estrogen receptor-negative cell lines, including malignant glioma in vitro. To explore the possible role of tamoxifen in the treatment of malignant glioma, we have investigated its effects on cell growth and radiosensitivity in C6 glioma cells using a colony-forming assay and a tetrazolium-formazan growth rate assay. Pretreatment of C6 cells with tamoxifen resulted in dose-dependent inhibition of cell growth and enhancement of the antitumor effects of ACNU and irradiation. The radiosensitivity of the treated cells was enhanced by the administration of 5 mumol/L of tamoxifen either before and during irradiation or continuously before, during, and after irradiation [37% survival dose (Do) = 2.68 +/- 0.19 and 2.64 +/- 0.04 Gy, respectively, P < 0.01)], as compared with controls (Do = 3.79 +/- 0.25 Gy). In addition, protein kinase C activity was also inhibited by tamoxifen at the concentration in which the radiosensitivity was augmented in C6 cells. Taken together, our results demonstrate a synergistic effect of tamoxifen with radiation on intracellular damage in C6 glioma cells, which may in part be due to the inhibition of protein kinase C, suggesting that tamoxifen serves as a useful agent in combination therapy of glioma.

DNA repair pathways in mammalian cells analyzed by isolation of ACNU-sensitive Chinese hamster ovary cells

1-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-3-(2-chloroethyl)-3- nitrosourea hydrochloride (ACNU) causes chloroethylation of DNA strand followed by cross linking through an ethylene bridge. We recently isolated two ACNU sensitive mutants from mutagenized Chinese hamster ovary cells, and found them to be new drug sensitive recessive mutants (Hata et al. 1991). The O6-methyl guanine DNA methyl transferase (MT) activities of these cells were undetectable as the parental cell line, indicating that the sensitivity of the mutant cell lines to ACNU was not due to the decreased cellular level of this enzyme. By complementation analysis with the 7 established UV-sensitive CHO cell lines, one of the mutants, UVS1, turned out to complement their UV-sensitivity and, therefore, build a new complementation group among all the CHO cell lines ever reported. The other mutant, CNU1 showed hypersensitivity only to chlorethylating agents (ACNU, CCNU) and exhibited a slightly reduced unscheduled DNA synthesis (UDS) induced by UV. It is, therefore, suggestive that this mutant is defective in a specific step of DNA repair systems, which is important for the processing of DNA damages produced by ACNU. Only cell lines from the complementation group 1 and 4 out of 7 established complementation groups of UV-sensitive CHO mutants were more sensitive to ACNU than UVS1 and CNU1, indicating some steps of excision repair pathways as well as specific repair system play important roles in repairing ACNU-induced DNA damages.

Isolation and preliminary characterization of ACNU-resistant sublines of rat brain tumors in vivo

Two variant cells lines resistant to the nitrosourea derivative ACNU ((1-4-amino-2-methyl-5-pyrimidinyl)-methyl-3-(2-chloroethyl)-3-nitrosourea hydrochloride), namely C6/ACNU and 9L/ACNU, were selected in vivo from rat brain tumors. Stable resistance to ACNU proved to be a characteristic of these cell lines, whether they were grown in vivo or in vitro. These cell lines exhibited a different pattern of cross-resistance to a wide range of chemotherapeutic agents with dissimilar chemical structures and mechanisms of action as compared with that of other ACNU-resistant cell lines established in vitro. Distinct cross-resistance was observed in both the C6/ACNU and 9L/ACNU cell lines to chloroethyl-nitrosoureas such as BCNU (carmustine), CCNU (lomustine), and methyl CCNU and, additionally, to vincristine, vinblastine, Adriamycin (doxorubicin), and arabinosylcytosine, but not to bleomycin, methotrexate, cis-platinum, and 5-fluorouracil. This might point to a multifactorial mechanism of drug resistance in ACNU-resistant cell lines derived from rat C6 and 9L brain tumor cells.

Metabolic changes of glioma following chemotherapy: an experimental study using four PET tracers

To shed light on the metabolic changes in glioma following therapy, uptake changes among 18F-fluoro-2'-deoxyuridine (18FUdR), 14C-thymidine (dThd), 14C-methionine (Met) and 3H-deoxyglucose (DG) in glioma model after chemotherapy were studied, as a means for interpreting clinical PET results, together with the changes in the bromodeoxyuridine (BUdR) labeling index. 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(-2chloroethyl)-3-nitro sourea hydrochloride (ACNU) was administered intraperitoneally in the tumor-bearing rats and uptake of the tracers or BUdR labeling index in tumor tissue were measured. The metabolic response following chemotherapy was a sharp fall immediately for 14C-dThd and 18FUdR and a moderate fall for 14C-Met whereas there was a fall in 3H-DG from 1 week after chemotherapy. The changes of BUdR labeling index paralleled that in the uptake for dThd and FUdR. These result indicate that PET scans using a variety of tracers in conjunction could be used for clinical diagnosis and evaluation of therapy in glioma cases. 18FUdR is a promising tracer of nucleic acid metabolism to evaluate the proliferative potential of brain gliomas.

[Experimental study on the effect of injection with anticancer agent-oil suspension]

Local injection of an anticancer agent guided by endoscopy is thought to be effective for cancerous lesion associated with lymph node metastasis, if the anticancer drugs are drained into the lymph nodes. In the experimental study, anticancer drug-oil (nimustine-Lipiodol) (N-L) suspension (5 mg/ml) was injected into the tumor (Lewis lung cancer) that had been implanted sub dorsally in mice (57 black/6 mice) for the purpose of finding out the antitumor effect on the primary lesion. Then it was injected into sarcoma-180 that had been implanted into hind feet of mice (ICR mice) for the purpose of finding out the antitumor effect on metastatic lymph nodes. The results showed that the N-L suspension was effective for the primary cancerous lesion and metastatic lesions.

Expression of O6-methylguanine-DNA methyltransferase and chloroethylnitrosourea resistance of human brain tumors

Northern blot analysis with O6-methylguanine-DNA methyltransferase (MGMT) cDNA as a probe was used to analyze the MGMT activity regulating drug resistance of human cells to chloroethylnitrosoureas (CENUs). By this method, the expression levels of MGMT mRNA in six human glioma cell lines and 12 human brain tumor tissues from surgical specimens were determined. These MGMT mRNA levels were compared with the SD10 values of the tumor cells, estimated by cell survival assay, which indicated their resistance to the anticancer drug, 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea (ACNU). Human brain tumors that were highly resistant to ACNU, such as glioblastoma Gbl1 and metastatic brain tumor Col1 with SD10 values (microM) of above 100, expressed markedly increased amounts of 0.95 kb MGMT mRNA. In contrast, tumor cells such as U-87MG, U-251MG, U-343MG, U-373MG and SF-126 with SD10 values of under 14 indicating low resistance to ACNU scarcely synthesized any MGMT mRNA. These results indicated that the level of expression of MGMT mRNA in human brain tumors determined by Northern blot analysis truly reflects their cellular resistance to ACNU. Thus the Northern method with MGMT cDNA probe reported here is a practical and reliable method for estimation of cellular resistance to CENUs such as ACNU and for screening the chemotherapeutic response to CENUs of human brain tumors.

The MTT assay for chemosensitivity testing of human tumors of the central nervous system. Part II: Evaluation of patient- and drug-specific variables

In this study we assessed the influence of patient- and drug-specific parameters in the short-term MTT-chemosensitivity assay in 150 primary cell cultures derived from human brain tumors. In 45 patients the MTT assay was directly compared with the CFA (Colony Forming Assay). Resistance was 10-20% higher in the MTT assay than in the CFA, but there was a good agreement in both assays, that more malignant gliomas had a higher in vitro chemosensitivity against ACNU and BCNU. Overall the results demonstrate, that there is no uniform correlation between the in vitro chemosensitivity and the histopathological classification of the tumors, which corresponds well to the clinical situation. On the basis of this study we suggest prospective clinical trials with the MTT assay in human brain tumors.