The resistance of tumor cells to the cytostatic activity of methylating and chloroethylating anticancer drugs is determined by the level of expression of the DNA repair protein O6-methylguanine-DNA-methyl-transferase (MGMT). The synthesis of labelled 6-benzyloxy-9H-purin-2-ylamine derivatives should hence allow a quantification of the MGMT status of tumor and non-target tissue in vivo. 6-benzyloxy-9-(2-fluoroethyl)-9H-purin-2-yl-amine and 6-benzyloxy-7-(2-fluoroethyl)-7H-purin-2-yl-amine were synthesized and evaluated in vitro, both showing an affinity of 1.8 microM. 6-benzyloxy-9-(2-[18F]fluoroethyl)-9H-purin-2-yl-amine and 6-benzyloxy-7-(2-[18F]fluoroethyl)-7H-purin-2-yl-amine were synthesized by alkylation of 6-benzyloxy-9H-purin-2-ylamine with 1-[18F]fluoro-2-tosylethane in optimized yields of 41% and 20%, respectively. Biodistribution studies were performed in nude mice, carrying mex+ (MGMT expressing) and mex- tumors.

To study molecular aspects of cytotoxicity of the anticancer drug β-D-glucose-ifosfamide mustard we investigated the potential of the agent to induce apoptosis and DNA breakage. Since β-D-glucose-ifosfamide mustard generates DNA interstrand crosslinks, we used as an in vitro model system a pair of isogenic Chinese hamster V79 cells differing in their sensitivity to crosslinking agents. CL-V5B cells are dramatically more sensitive (30-fold based on D10 values) to the cytotoxic effects of β-D-glucose-ifosfamide mustard as compared to parental V79B cells. After 48 h of pulse-treatment with the agent, sensitive cells but not the resistant parental line undergo apoptosis and necrosis, with apoptosis being the predominant form of cell death (70 and 20% of apoptosis and necrosis, respectively). Apoptosis increased as a function of dose and was accompanied by induction of DNA double-strand breaks in the hypersensitive cells. Furthermore, a strong decline in the level of Bcl-2 protein and activation of caspases-3, -8 and -9 were observed. The resistant parental cells were refractory to all these parameters. Bcl-2 decline in the sensitive cells preceded apoptosis, and transfection-mediated overexpression of Bcl-2 protected at least in part from apoptosis. From the data we hypothesize that non-repaired crosslinks induced by β-D-glucose-ifosfamide mustard are transformed into double-strand breaks which trigger apoptosis via a Bcl-2 dependent pathway.

c-Fos and p53 are DNA damage-inducible proteins that are involved in gene regulation, cell cycle checkpoint control and cell proliferation following exposure to genotoxic agents. To investigate comparatively the role of c-Fos and p53 in the maintenance of genomic stability and the induction of apoptosis, we generated mouse fibroblast cell lines from knockout mice deficient for either c-fos (fos -/-) or p53 (p53-/-) or for both gene products (fosp53-/-). The sensitivity of these established cell lines was compared with the corresponding wild-type cells as to the cytotoxic, clastogenic and apoptosis-inducing effects of ultraviolet (UV-C) light and methyl methanesulfonate (MMS). Additionally, we analysed the frequency of apoptosis of the cell lines after treatment with ionizing radiation (IR). We observed c-fos-/-, p53-/- and fosp53-/- cells to be more sensitive than wild-type cells with respect to cell death, as measured in a cytotoxicity (MTT) assay. Regarding apoptosis, all deficient cell lines displayed hypersensitivity to UV-C light, MMS and IR. With chromosomal aberrations as the endpoint, the sensitivity of the double-knockout cells was between wild-type and single-knockouts. The results indicate that both c-Fos and p53 play an important role in protecting fibroblasts against a broad range of genotoxic agents. The results also show that, in fibroblasts, apoptosis induced by UV-C light, MMS and IR does not require p53 and that, in this cell type, p53 rather protects against DNA damage-induced apoptotic cell death.

The DNA-repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) is a decisive determinant of resistance of tumor cells to methylating and chloroethylating anti-cancer drugs. Therefore, selective inhibition of MGMT in tumors is expected to cause tumor sensitization. Several inhibitors of MGMT have been developed which function in both tumors and normal tissue. To deplete MGMT preferentially in tumors, strategies to target the inhibitor to the tumor tissue need to be developed. Here, we report on the properties of glucose-conjugated MGMT inhibitors that might be useful for tumor targeting since tumor cells frequently over-express glucose transporter. O(6)-Benzylguanine (O6BG), 8-aza-O(6)-benzylguanine, O(6)-(4-bromothenyl)-guanine (O6BTG) and the corresponding spacer-linked beta-D-glucose conjugates were analyzed comparatively for MGMT-inhibitory activity. Substitution at the N9 position of the purine moiety resulted generally in a reduction in the efficiency with which the inhibitors blocked MGMT. However, the inhibitory activity of the O6BTG conjugates increased with increasing spacer length, and O6BTG conjugated with a C8 spacer with beta-D-glucose was nearly as effective as O6BTG on its own. MGMT was inhibited by the conjugates both in crude cell extracts and upon treatment of intact HeLa cells, indicating efficient uptake of the glucose conjugates into cells. Since the O6BTG-C8-D-glucose conjugate 8-[O(6)-(4-bromothenyl)-guan-9-yl]-octyl-beta-D-glucoside was highly efficient at MGMT inhibition in a non-toxic concentration range, the drug might be a useful tool for specific tumor sensitization.

A series of potential inhibitors of the human DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) were synthesized, characterized in detail by NMR, and tested for their ability to deplete MGMT activity in vitro. The new compounds, omega-[O(6)-R-guan-9-yl]-(CH(2))(n)-beta-d-glucosides with R = benzyl or 4-bromothenyl and omega = n = 2, 4,. 12, were compared with the established inhibitors O(6)-benzylguanine (O(6)-BG), 8-aza-O(6)-benzylguanine (8-aza-BG), and O(6)-(4-bromothenyl)guanine (4-BTG), which exhibit in an in vitro assay IC(50) values of 0.62, 0.038, and 0.009 microM, respectively. Potential advantages of the glucosides are improved water solubility and selective uptake in tumor cells. The 4-BTG glucosides with n = 2, 4, 6 show moderate inhibition with an IC(50) of ca. 0.5 microM, while glucosides derived from BG and 8-aza-BG showed significantly poorer inhibition compared to the parent compounds. The 4-BTG glucosides with n = 8, 10, 12 were effective inhibitors with IC(50) values of ca. 0.03 microM. To understand this behavior, extensive molecular modeling studies were performed using the published crystal structure of MGMT (PDB entry: ). The inhibitor molecules were docked into the BG binding pocket, and molecular dynamics simulations with explicit water molecules were carried out. Stabilization energies for the interactions of specific regions of the inhibitor and individual amino acid residues were calculated. The alkyl spacer is located in a cleft along helix 6 of MGMT. With increasing spacer length there is increasing interaction with several amino acid residues which play an important role in the proposed nucleotide flipping mechanism required for DNA repair.

Hyperbaric oxygen (HBO) treatment (i.e. exposure to 100% oxygen at a pressure of 2.5 ATA for a total of three 20 min periods) of human subjects caused clear and reproducible DNA effects in the comet assay with leukocytes. Interestingly, DNA damage was detected only after the first treatment and not after further treatments under the same conditions, indicating an increase in antioxidant defences. We now demonstrate that blood taken 24 h after HBO treatment is well protected against the in vitro induction of DNA damage by hydrogen peroxide (H2O2). H2O2 treatment caused a significant induction of DNA effects in the comet assay and chromosome breakage in the micronucleus test in the blood of volunteers before HBO. The same treatment did not cause genotoxic effects 24 h after HBO. This protective effect lasted for at least 1 week. Experiments with isolated lymphocytes gave similar results, indicating that the adaptive response is a cellular effect. The cells were not comparably protected against the genotoxic effects of gamma-irradiation, suggesting increased scavenging of reactive oxygen species distant from nuclear DNA.

Nickel compounds are widespread carcinogens, and although only weakly mutagenic, interfere with nucleotide excision repair and with the repair of oxidative DNA base modifications. In the present study we investigated the effect of nickel(II) on the induction and repair of O6-methylguanine and N7-methylguanine after treatment with N-methyl-N-nitrosourea (MNU). We applied Chinese hamster ovary cells stably transfected with human O6-methylguanine-DNA methyltransferase (MGMT) cDNA (CHO-AT), and compared the results with the MGMT-deficient parental cell line. As determined by high-performance liquid chromatography/electrochemical detection (HPLC/ECD), there was a slight but mostly not significant reduction in the formation of both types of DNA lesions by MNU in the presence of nickel(II). Although nickel(II) did not markedly affect the repair of N7-methylguanine, it decreased the repair of O6-methylguanine in a dose-dependent manner, starting at concentrations as low as 50 μM. While the MGMT protein level was not altered in the presence of nickel(II), the MGMT activity was diminished as demonstrated in cell extracts form nickel-treated cells. This repair inhibition was accompanied by an increase in MNU-induced cytotoxicity in nickel-treated CHO-AT cells but not in MGMT-deficient control cells. There is strong evidence that O6-methylguanine is involved in tumour formation after exposure to alkylating agents. Thus, the finding that nickel(II) inhibits the repair of this lesion could be of major importance for risk assessment in case of combined exposures at work places and in the general environment.

The DNA repair protein O6-methylguanine-DNA methyl-transferase (MGMT) is a main determinant of resistance of cells to the cytostatic effects of O6-alkylguanine-generating alkylating agents. The purpose of our study was to assay MGMT activity in cells of lung cancers and to correlate MGMT levels with chemotherapy response to cyclophosphamide (CTX) and cisplatin (DDP). MGMT levels were determined in 14 human lung tumor xenografts. There was a wide variation of MGMT expression in these tumors, ranging from 10 to 984 fmol/mg protein. There was also a wide range in the sensitivity of the xenografts to CTX and DDP, as measured by specific growth delay. When the MGMT levels of the different xenograft lines were compared with the corresponding responses to CTX and DDP, a close correlation was found between MGMT activity and CTX (lin reg., r = -0.83, p < 0.05). The higher the MGMT activity, the less pronounced was the growth-inhibiting effect of CTX. With DDP, no such correlation was found. Our results indicate that the in vivo response of tumors to CTX is related to the level of MGMT expression.

The DNA repair protein O 6 -methylguanine-DNA methyltransferase (MGMT) is inducible by genotoxic stress. MGMT induction results from transcriptional activation of the MGMT gene which is a specific response to DNA damage. A possible factor involved in triggering MGMT induction might be p53, because both p53 and MGMT are activated by DNA breaks. To study the effect of p53 on induction of the MGMT gene, we compared the presence of functional wild-type (wt) and mutant p53 with MGMT expression level in various mouse fibroblasts and rat hepatoma cell lines upon genotoxic treatment. Cells which responded to ionizing radiation (IR) by MGMT induction displayed functional p53, whereas in cells not expressing wt p53, MGMT induction was not observed. Also, the cloned MGMT promoter was inducible by IR upon transfection into p53 wt cells, but not in cells deficient for p53. Thus, expression of wt p53 appears to be required for induction of MGMT mRNA and protein by IR. On the other hand, transfection of a MGMT-promoter-CAT construct together with p53 (either wt or mutant) in cells expressing wt p53 markedly reduced the basal activity of the MGMT promoter whereas cotransfection with a p53 antisense construct slightly increased MGMT promoter activity. Furthermore, cotransfection of MGMT promoter with wt or mutant p53 in p53 wt cells reduced radiation evoked MGMT promoter induction. Thus, transfection mediated high level expression of p53 has inhibitory effect both on basal MGMT promoter activity and its activation by IR. The results give evidence for involvement of p53 in DNA damage-induced MGMT promoter activation.

The DNA repair protein O-6-methylguanine-DNA methyltransferase (MGMT) is a main determinant of resistance of cells towards the cytostatic effect of N-nitrosoureas. Here we report the activity of MGMT in different types of human tumors. Colorectal tumors showed the highest mean of MGMT activity (481 +/- 258 fmol/mg protein) followed by ovarian tumors (437 +/- 291 fmol/mg), breast (306 +/- 204 fmol/mg), testicular (299 +/- 179 fmol/mg), and brain tumors (55 +/- 44 fmol/mg). Only in breast and brain tumors were MGMT-deficient samples found, with a frequency of 3 and 5%, respectively. No significant difference in mean MGMT activity was observed between breast tumors of grading I, II, and III. For brain tumors there was a tendency of MGMT to decline with increasing grading. Breast and colorectal carcinomas were compared with the corresponding normal tissue obtained from the same patient. The data showed for 11 out of 12 pairs of breast tissue and for 13 out of 15 pairs of colorectal tissue that MGMT activity in the tumor was equal to or, more frequently, higher than the activity found in the corresponding normal tissue. Thus, it appears that in these groups of tumors, increase of MGMT activity during tumor formation and progression is the rule rather than the exception.