Depletion of O6-alkylguanine-DNA alkyltransferase activity in mammalian tissues and human tumor xenografts in nude mice by treatment with O6-methylguanine

Targeting O⁶-methylguanine-DNA methyltransferase with specific inhibitors as a strategy in cancer therapy

O (6)-methylguanine-DNA methyltransferase (MGMT) repairs the cancer chemotherapy-relevant DNA adducts, O (6)-methylguanine and O (6)-chloroethylguanine, induced by methylating and chloroethylating anticancer drugs, respectively. These adducts are cytotoxic, and given the overwhelming evidence that MGMT is a key factor in resistance, strategies for inactivating MGMT have been pursued. A number of drugs have been shown to inactivate MGMT in cells, human tumour models and cancer patients, and O (6)-benzylguanine and O (6)-[4-bromothenyl]guanine have been used in clinical trials. While these agents show no side effects per se, they also inactivate MGMT in normal tissues and hence exacerbate the toxic side effects of the alkylating drugs, requiring dose reduction. This might explain why, in any of the reported trials, the outcome has not been improved by their inclusion. It is, however, anticipated that, with the availability of tumour targeting strategies and hematopoetic stem cell protection, MGMT inactivators hold promise for enhancing the effectiveness of alkylating agent chemotherapy.

In vitro comparison of O4-benzylfolate modulated, BCNU-induced toxicity in human bone marrow using CFU-GM and tumor cell lines

2-Amino-O4-benzylpteridine derivatives inactivate the human DNA repair protein O6-alkylguanine-DNA alkyltransferase and have been tested as modulators of alkylating agent chemotherapy. Recently, the therapeutic potential of O4-benzylfolate (O4BF) in modulating bis-chloroethylnitrosourea (BCNU) toxicity was demonstrated in vitro using human HT29 and KB tumor lines. The current studies replicated the previous findings in HT29 and KB cells using ATP as an endpoint. However, the effective treatment conditions were severely toxic to human neutrophil progenitors called CFU-granulocyte/macrophage (CFU-GM), which could not tolerate > or =40 microM BCNU at any O4BF concentration. A lower BCNU concentration (10 microM) in combination with O4BF (2-100 microM) was only moderately tumoricidal. To screen for conditions tolerated by CFU-GM, bone marrow (BM) cells were pre-incubated (5 h) with O4BF, co-treated with O4BF and BCNU (42 h), washed, and plated to quantify CFU-GM survival. O4BF at 2 or 5 microM progressively lowered the inhibitory concentrations (ICs) for BCNU, but further increases in O4BF concentrations did not. Increasing O4BF concentrations with constant BCNU (10 microM) under the same prolonged exposure as in the human marrow study achieved only modest tumoricidal effects. In summary, the unexpected finding that normal BM cells are impacted by an agent developed to target malignant tissue refutes speculation that normal beta-folate receptor expressing hematopoietic cells will be spared. Further, the validated IC90 endpoint from the huCFU-GM assay has provided a reference point for judging the potential therapeutic effectiveness of this investigational combination in man using in vitro assays.

Correlation of O6-methylguanine methyltransferase (MGMT) promoter methylation with clinical outcomes in glioblastoma and clinical strategies to modulate MGMT activity

Resistance to alkylating agents via direct DNA repair by O(6)-methylguanine methyltransferase (MGMT) remains a significant barrier to the successful treatment of patients with malignant glioma. The relative expression of MGMT in the tumor may determine response to alkylating agents, and epigenetic silencing of the MGMT gene by promoter methylation plays an important role in regulating MGMT expression in gliomas. MGMT promoter methylation is correlated with improved progression-free and overall survival in patients treated with alkylating agents. Strategies to overcome MGMT-mediated chemoresistance are being actively investigated. These include treatment with nontoxic pseudosubstrate inhibitors of MGMT, such as O(6)-benzylguanine, or RNA interference-mediated gene silencing of MGMT. However, systemic application of MGMT inhibitors is limited by an increase in hematologic toxicity. Another strategy is to deplete MGMT activity in tumor tissue using a dose-dense temozolomide schedule. These alternative schedules are well tolerated; however, it remains unclear whether they are more effective than the standard dosing regimen or whether they effectively deplete MGMT activity in tumor tissue. Of note, not all patients with glioblastoma having MGMT promoter methylation respond to alkylating agents, and even those who respond will inevitably experience relapse. Herein we review the data supporting MGMT as a major mechanism of chemotherapy resistance in malignant gliomas and describe ongoing studies that are testing resistance-modulating strategies.

Resistance to chemotherapy in cancer: a complex and integrated cellular response

Inherent and acquired resistance pathways account for the high rate of failure in cancer chemotherapy. The mechanisms or pathways mediating resistance may be classified as pharmacokinetic (i.e. alter intratumour drug exposue) or pharmacodynamic (i.e. failure to elicit cytotoxicity). More often than not, the resistant phenotype is characterised by alterations in multiple pathways. Consequently, the pathways may act synergistically or generate a broad spectrum of resistance to anticancer drugs. There has been a great deal of systematic characterisation of drug resistance in vitro. However, translating this greater understanding into clinical efficacy has rarely been achieved. This review explores the phenomenon of drug resistance in cancer and highlights the gap between in vitro and in vivo observations. This gap presents a major obstacle in overcoming drug resistance and restoring sensitivity to chemotherapy.

Therapeutic potential of drugs to modulate DNA repair in cancer

Most established cancer therapy regimes involve DNA-damaging chemotherapy or radiotherapy. The DNA repair capacity of the tumour, therefore, represents a mechanism of therapeutic resistance. Drugs to inhibit DNA repair pathways have been developed and they demonstrate good chemosensitisation and radiosensitisation activity in preclinical models. Two classes of DNA repair inhibitors have entered clinical trial and show promising activity. Genetic instability in tumours may be at least partially due to defects in DNA repair pathways; such defects may underlie the inherent sensitivity of some tumours to certain classes of anticancer agent. DNA repair defects may also make the tumour dependent on complimentary or back-up pathways; laboratory evidence shows that targeting these complimentary pathways results in tumour-selective therapy.

Inactivation of O6-alkylguanine DNA alkyltransferase as a means to enhance chemotherapy

DNA adducts at the O6-position of guanine are a result of the carcinogenic, mutagenic and cytotoxic actions of methylating and chloroethylating agents. The presence of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) renders cells resistant to the biological effects induced by agents that attack at this position. O6-Benzylguanine (O6-BG) is a low molecular weight substrate of AGT and therefore, results in sensitizing cells and tumors to alkylating agent-induced cytotoxicity and antitumor activity. Presently, chemotherapy regimens of O6-BG in combination with BCNU, temozolomide and Gliadel are in clinical development. Other ongoing clinical trials include expression of mutant AGT proteins that confer resistance to O6-BG in bone marrow stem cells, in an effort to reduce the potential enhanced toxicity and mutagenicity of alkylating agents in the bone marrow. O6-BG has also been found to enhance the cytotoxicity of agents that do not form adducts at the O6-position of DNA, including platinating agents. O6-BG's mechanism of action with these agents is not fully understood; however, it is independent of AGT activity or AGT inactivation. A better understanding of the effects of this agent will contribute to its clinical usefulness and the design of better analogs to further improve cancer chemotherapy.

Hammerhead ribozyme-mediated sensitization of human tumor cells after treatment with 1,3-bis(2-chloroethyl)-1-nitrosourea

O(6)-Methylguanine DNA methyltransferase (MGMT) protects tumor cells from the cytotoxic effects of DNA-alkylating agents such as 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU). To improve the therapeutic index of BCNU, biochemical strategies to inhibit MGMT temporarily by systemic administration of small molecules, such as O(6)-benzylguanine, have been developed and are showing promise in clinical trials. In this study, an alternative molecular strategy for modulating BCNU resistance was explored using hammerhead ribozymes (Rz) designed to degrade the long-lived MGMT mRNA. We had previously identified several ribozymes capable of decreasing MGMT levels in HeLa cells. Using colony formation assays, the BCNU-induced cell kill was shown to be increased by 1 to 3 logs in the HeLa/Rz clones compared with wild-type HeLa cells at a BCNU dose of 100 microM. In the current study, 10 randomly selected clones of Rz161, 212, and a reconstructed Rz178/212 were assayed for MGMT activity, MGMT mRNA, and sensitivity to BCNU. The 30 clones exhibited almost identical results in the three assays, i.e., nearly undetectable MGMT activity, greatly diminished MGMT mRNA, and comparable sensitivity to BCNU using the 4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate (WST-1) viability assay. The effects of catalytically inactive ribozymes carrying a single point mutation were compared with their active counterparts in vitro and in stably transfected clones to determine whether antisense inhibition was a contributor to the inhibition of MGMT activity we observed. Collectively, these results suggest that the hammerhead ribozymes characterized in this study will be excellent candidates for future gene therapy approaches targeting MGMT.

Pharmacokinetics of O6-benzylguanine (NSC637037) and its metabolite, 8-oxo-O6-benzylguanine

O6-Benzylguanine and its metabolite, 8-oxo-O6-benzylguanine, are equally potent inhibitors of the DNA repair enzyme, O6-alkylguanine-DNA alkyltransferase. Pharmacokinetic values are derived from cancer patients participating in a phase I trial (10 or 20 mg/m2 of O6-benzylguanine in a single bolus dose or 10 to 120 mg/m2 as a 60-min constant infusion). A two-compartment model fits the plasma concentration versus time profile of O6-benzylguanine. O6-Benzylguanine is eliminated rapidly from the plasma compartment in humans (t1/2 alpha and t1/2 beta are 2 +/- 2 min and 26 +/- 15 min [mean +/- SD, n = 7], respectively), and its plasma clearance (513 +/- 148 mL/min/m2) is not dose dependent. Metabolite kinetics are evaluated using both a novel approach describing the relationship between O6-benzylguanine and 8-oxo-O6-benzylguanine and classical metabolite kinetics methods. With increasing doses of O6-benzylguanine, the plasma clearance of 8-oxo-O6-benzylguanine, decreases, prolonging elimination of the metabolite. This effect is not altered by coadministration of BCNU. The urinary excretion of drug and metabolites is minimal.

Clinical relevance of MGMT in the treatment of cancer

Anumber of DNA-damaging chemotherapeutic agents attack the O(6) position on guanine, forming the most potent cytotoxic DNA adducts known. The DNA repair enzyme O(6)-alkylguanine DNA alkyltransferase (AGT), encoded by the gene MGMT, repairs alkylation at this site and is responsible for protecting both tumor and normal cells from these agents. Cells and tissues vary greatly in AGT expression, not only between tissues but also between individuals. AGT activity correlates inversely with sensitivity to agents that form O(6)-alkylguanine DNA adducts, such as carmustine (BCNU), temozolomide, streptozotocin, and dacarbazine. The one exception is those tumors lacking mismatch repair, which renders them resistant to methylating agents. A recent study in patients with gliomas confirmed the correlation between low-level expression of the MGMT gene and response and survival after BCNU. An inhibitor to AGT, O(6)-benzylguanine (BG), depletes AGT in human tumors without associated toxicity and is now in phase II clinical trials. Finally, mutations within the active site region of the MGMT gene render the AGT protein resistant to BG inactivation. As a result, mutant MGMT gene transfer into hematopoietic stem cells has been shown to selectively protect the marrow from the combination of an alkylating agent and BG, while at the same time sensitizing tumor cells. MGMT remains a paradigm for development of new agents that modulate known mechanisms of drug resistance in cancer cells and raise the spectra of combinatorial therapies that encompass known drug resistance mechanisms.

Phase II trial of carmustine plus O(6)-benzylguanine for patients with nitrosourea-resistant recurrent or progressive malignant glioma

PURPOSE

We conducted a phase II trial of carmustine (BCNU) plus the O(6)-alkylguanine-DNA alkyltransferase inhibitor O(6)-benzylguanine (O(6)-BG) to define the activity and toxicity of this regimen in the treatment of adults with progressive or recurrent malignant glioma resistant to nitrosoureas.

PATIENTS AND METHODS

Patients were treated with O(6)-BG at an intravenous dose of 120 mg/m(2) followed 1 hour later by 40 mg/m(2) of BCNU, with cycles repeated at 6-week intervals.

RESULTS

Eighteen patients were treated (15 with glioblastoma multiforme, two with anaplastic astrocytoma, and one with malignant glioma). None of the 18 patients demonstrated a partial or complete response. Two patients exhibited stable disease for 12 weeks before their tumors progressed. Three patients demonstrated stable disease for 6, 12, and 18 weeks before discontinuing therapy because of hematopoietic toxicity. Twelve patients experienced reversible > or = grade 3 hematopoietic toxicity. There was no difference in half-lives (0.56 +/- 0.21 hour v 0.54 +/- 0.20 hour) or area under the curve values (4.8 +/- 1.7 microg/mL/h v 5.0 +/- 1.3 microg/mL/h) of O(6)-BG for patients receiving phenytoin and those not treated with this drug.

CONCLUSION

These results indicate that O(6)-BG plus BCNU at the dose schedule used in this trial is unsuccessful in producing tumor regression in patients with nitrosourea-resistant malignant glioma, although stable disease was seen in five patients for 6, 12, 12, 12, and 18 weeks. Future use of this approach will require strategies to minimize dose-limiting toxicity of BCNU such as regional delivery or hematopoietic stem-cell protection.

Resistance-modifying agents. 8. Inhibition of O(6)-alkylguanine-DNA alkyltransferase by O(6)-alkenyl-, O(6)-cycloalkenyl-, and O(6)-(2-oxoalkyl)guanines and potentiation of temozolomide cytotoxicity in vitro by O(6)-(1-cyclopentenylmethyl)guanine

A series of O(6)-allyl- and O(6)-(2-oxoalkyl)guanines were synthesized and evaluated, in comparison with the corresponding O(6)-alkylguanines, as potential inhibitors of the DNA-repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT). Simple O(6)-alkyl- and O(6)-cycloalkylguanines were weak AGT inactivators compared with O(6)-allylguanine (IC(50) = 8.5 +/- 0.6 microM) with IC(50) values ranging from 100 to 1000 microM. The introduction of substituents at C-2 of the allyl group of O(6)-allylguanine reduced activity compared with the parent compound, while analogous compounds in the O(6)-(2-oxoalkyl)guanine series exhibited very poor activity (150-1000 microM). O(6)-Cycloalkenylguanines proved to be excellent AGT inactivators, with 1-cyclobutenylmethylguanine (IC(50) = 0.55 +/- 0.02 microM) and 1-cyclopentenylmethylguanine (IC(50) = 0.39 +/- 0.04 microM) exhibiting potency approaching that of the benchmark AGT inhibitor O(6)-benzylguanine (IC(50) = 0.18 +/- 0.02 microM). 1-Cyclopentenylmethylguanine also inactivated AGT in intact HT29 human colorectal carcinoma cells (IC(50) = 0.20 +/- 0.07 microM) and potentiated the cytotoxicity of the monomethylating antitumor agent Temozolomide by approximately 3- and 10-fold, respectively, in the HT29 and Colo205 tumor cell lines. The observation that four mutant AGT enzymes resistant to O(6)-benzylguanine also proved strongly cross-resistant to 1-cyclopentenylmethylguanine indicates that the O(6)-substituent of each compound makes similar binding interactions within the active site of AGT.

Phase I trial of carmustine plus O6-benzylguanine for patients with recurrent or progressive malignant glioma

PURPOSE

The major mechanism of resistance to alkylnitrosourea therapy involves the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT), which removes chloroethylation or methylation damage from the O(6) position of guanine. O(6)-benzylguanine (O(6)-BG) is an AGT substrate that inhibits AGT by suicide inactivation. We conducted a phase I trial of carmustine (BCNU) plus O(6)-BG to define the toxicity and maximum-tolerated dose (MTD) of BCNU in conjunction with the preadministration of O(6)-BG with recurrent or progressive malignant glioma.

PATIENTS AND METHODS

Patients were treated with O(6)-BG at a dose of 100 mg/m(2) followed 1 hour later by BCNU. Cohorts of three to six patients were treated with escalating doses of BCNU, and patients were observed for at least 6 weeks before being considered assessable for toxicity. Plasma samples were collected and analyzed for O(6)-BG, 8-oxo-O(6)-BG, and 8-oxoguanine concentration.

RESULTS

Twenty-three patients were treated (22 with glioblastoma multiforme and one with anaplastic astrocytoma). Four dose levels of BCNU (13.5, 27, 40, and 55 mg/m(2)) were evaluated, with the highest dose level being complicated by grade 3 or 4 thrombocytopenia and neutropenia. O(6)-BG rapidly disappeared from plasma (elimination half-life = 0. 54 +/- 0.14 hours) and was converted to a longer-lived metabolite, 8-oxo-O(6)-BG (elimination half-life = 5.6 +/- 2.7 hours) and further to 8-oxoguanine. There was no detectable O(6)-BG 5 hours after the start of the O(6)-BG infusion; however, 8-oxo-O(6)-BG and 8-oxoguanine concentrations were detected 25 hours after O(6)-BG infusion. The mean area under the concentration-time curve (AUC) of 8-oxo-O(6)-BG was 17.5 times greater than the mean AUC for O(6)-BG.

CONCLUSION

These results indicate that the MTD of BCNU when given in combination with O(6)-BG at a dose of 100 mg/m(2) is 40 mg/m(2) administered at 6-week intervals. This study provides the foundation for a phase II trial of O(6)-BG plus BCNU in nitrosourea-resistant malignant glioma.

Inhibition of DNA repair as a means of increasing the antitumor activity of DNA reactive agents

Chemotherapeutic alkylnitrosoureas (BCNU, CCNU, streptozotocin) and alkyltriazenes (DTIC, temozolomide) produce a cytotoxic lesion at the O(6)-position of guanine. The DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase removes damage from the O(6)-position in a single-step mechanism without co-factors. There is extensive evidence that this protein is one of the most important factors contributing to alkylnitrosourea and alkyltriazene treatment failure. There is an inverse correlation between the level of this protein and the sensitivity of cells to the cytotoxic effects of O(6)-alkylating agents. Attempts have been made to modulate AGT activity using anti-sense technology, methylating agents, O(6)-alkylguanines, and O(6)-benzylguanine analogs. O(6)-Benzylguanine and its analogs are clearly the most potent direct inactivators of the AGT protein. The mechanism involves O(6)-benzylguanine acting as a low-molecular weight substrate with transfer of the benzyl group to the cysteine residue within the active site of the repair protein. Pretreatment of cells with non-toxic doses of O(6)-benzylguanine results in an increase in the sensitivity to O(6)-alkylating agents. Animal studies revealed that the therapeutic index of BCNU increased when administered in combination with O(6)-benzylguanine. This drug is currently in phase I clinical trials. Evidence from animal studies indicates that myelosuppression may be the dose-limiting toxicity, thus, efforts are aimed at improving the therapeutic index by the stable expression of O(6)-benzylguanine-resistant AGT proteins into targeted normal tissue such as bone marrow. The successful modulation of alkyltransferases brings on an exciting new era for alkylnitrosoureas and alkyltriazenes.

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.

Treatment of subcutaneous and intracranial brain tumor xenografts with O6-benzylguanine and 1,3-bis(2-chloroethyl)-1-nitrosourea

O6-Alkylguanine-DNA alkyltransferase (AT) is a cellular protein that protects cells from the cytotoxic effects of nitrosoureas by repairing alkyl lesions at the O6 position of guanine. We have studied the ability of O6-benzylguanine to deplete AT activity in brain tumor xenografts and thereby increase the sensitivity of these tumors to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). In toxicity studies, pretreatment of athymic mice with O6-benzylguanine increased the toxicity of BCNU significantly. After i.p. injection of O6-benzylguanine into athymic mice carrying subcutaneous (s.c.) D341MED, a human medulloblastoma xenograft with a high AT activity, the AT activity of the tumors became undetectable within 1 h and remained depleted until 36 h. In s.c. xenografts to D341MED, treatment with O6-benzylguanine followed 1 h later by BCNU produced a significantly greater growth delay (14.8 days) than was seen with BCNU alone (2.3 days). A lower pretreatment dose of O6-benzylguanine produced a significantly smaller therapeutic effect. Delaying the administration of BCNU until 36 h after O6-benzylguanine resulted in a growth delay (1.2 days) that was not significantly different from that produced by the control or BCNU alone. In athymic mice with intracranial (i.c.) xenografts of D341MED, pretreatment with O6-benzylguanine followed 1 h later by BCNU produced a significantly increased survival as compared with that of the control, BCNU alone, O6-benzylguanine alone, and O6-benzylguanine followed 36 h later by BCNU. In experiments with s.c. xenografts of D245MG, a human glioma xenograft with undetectable AT activity, pretreatment with O6-benzylguanine 1 h prior to BCNU produced a significantly greater effect than was seen with BCNU treatment alone. The combination regimen, however, was not as effective as an equitoxic dose of BCNU alone. These studies suggest that O6-benzylguanine may be a useful adjuvant to nitrosourea therapy in human malignancies that exhibit a range of AT activities and that dose and timing are important variables in achieving therapeutic success. These data also indicate that therapeutic potentiation of BCNU by O6-benzylguanine can be achieved in i.c. tumors. As a result, this approach may be useful in the treatment of neoplasms of the central nervous system.

Depletion of mammalian O6-alkylguanine-DNA alkyltransferase activity by O6-benzylguanine provides a means to evaluate the role of this protein in protection against carcinogenic and therapeutic alkylating agents

O6-Alkylguanine-DNA alkyltransferase was rapidly and irreversibly inactivated by exposure to O6-benzylguanine or the p-chlorobenzyl and p-methylbenzyl analogues. This inactivation was much more rapid than with O6-methylguanine: incubation with 2.5 microM O6-benzylguanine led to more than a 90% loss of activity within 10 min, whereas 0.2 mM O6-methylguanine for 60 min was required for the same reduction. O6-Benzylguanine was highly effective in depleting the alkyltransferase activity of cultured human colon tumor (HT29) cells. Complete loss of activity was produced within 15 min after addition of O6-benzylguanine to the culture medium and a maximal effect was obtained with 5 microM. In contrast, at least 100 microM O6-methylguanine for 4 hr was needed to get a maximal effect, and this reduced the alkyltransferase by only 80%. Pretreatment of HT29 cells with 10 microM O6-benzylguanine for 2 hr led to a dramatic increase in the cytotoxicity produced by the chemotherapeutic agents 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) or 2-chloroethyl(methysulfonyl)methanesulfonate (Clomesone). Administration of O6-benzylguanine to mice at a dose of 10 mg/kg reduced alkyltransferase levels by more than 95% in both liver and kidney. These results indicate that depletion of the alkyltransferase by O6-benzylguanine may be used to investigate the role of the DNA repair protein in carcinogenesis and mutagenesis and that this treatment may be valuable to increase the chemotherapeutic effectiveness of chloroethylating agents.