Mechanism of AGT-Mediated Repair of dG-dC Cross-Links in the Drug Resistance to Chloroethylnitrosoureas: Molecular Docking, MD Simulation, and ONIOM (QM/MM) Investigation

Chloroethylnitrosoureas (CENUs) are important chemotherapies applied in the treatment of cancer. They exert anticancer activity by inducing DNA interstrand cross-links (ICLs) via the formation of two O6-alkylguanine intermediates, O6-chloroethylguanine (O6-ClEtG) and N1,O6-ethanoguanine (N1,O6-EtG). However, O6-alkylguanine-DNA alkyltransferase (AGT), a DNA-repair enzyme, can restore the O6-alkylguanine damages and thereby obstruct the formation of ICLs (dG-dC cross-link). In this study, the inhibitory mechanism of ICL formation was investigated to elucidate the drug resistance of CENUs mediated by AGT in detail. Based on the structures of the substrate-enzyme complexes obtained from docking and MD simulations, two ONIOM (QM/MM) models with different sizes of the QM region were constructed. The model with a larger QM region, which included the substrate (O6-ClEtG or N1,O6-EtG), a water molecule, and five residues (Tyr114, Cys145, His146, Lys165, and Glu172) in the active pocket of AGT, accurately described the repairing reaction and generated the results coinciding with the experimental outcomes. The repair process consists of two sequential steps: hydrogen transfer to form a thiolate anion on Cys145 and alkyl transfer from the O6 site of guanine (the rate-limiting step). The repair of N1,O6-EtG was more favorable than that of O6-ClEtG from both kinetics and thermodynamics aspects. Moreover, the comparison of the repairing process with the formation of dG-dC cross-link and the inhibition of AGT by O6-benzylguanine (O6-BG) showed that the presence of AGT could effectively interrupt the formation of ICLs leading to drug resistance, and the inhibition of AGT by O6-BG that was energetically more favorable than the repair of O6-ClEtG could not prevent the repair of N1,O6-EtG. Therefore, it is necessary to completely eliminate AGT activity before CENUs medication to enhance the chemotherapeutic effectiveness. This work provides reasonable explanations for the supposed mechanism of AGT-mediated drug resistance of CENUs and will assist in the development of novel CENU chemotherapies and their medication strategies.

The O6-methyguanine-DNA methyltransferase inhibitor O6-benzylguanine enhanced activity of temozolomide + irinotecan against models of high-risk neuroblastoma

DNA-damaging chemotherapy is a major component of therapy for high-risk neuroblastoma, and patients often relapse with treatment-refractory disease. We hypothesized that DNA repair genes with increased expression in alkylating agent resistant models would provide therapeutic targets for enhancing chemotherapy. In-vitro cytotoxicity of alkylating agents for 12 patient-derived neuroblastoma cell lines was assayed using DIMSCAN, and mRNA expression of 57 DNA repair, three transporter, and two glutathione synthesis genes was assessed by TaqMan low-density array (TLDA) with further validation by qRT-PCR in 26 cell lines. O6-methylguanine-DNA methyltransferase (MGMT) mRNA was upregulated in cell lines with greater melphalan and temozolomide (TMZ) resistance. MGMT expression also correlated significantly with resistance to TMZ+irinotecan (IRN) (in-vitro as the SN38 active metabolite). Forced overexpression of MGMT (lentiviral transduction) in MGMT non-expressing cell lines significantly increased TMZ+SN38 resistance. The MGMT inhibitor O6-benzylguanine (O6BG) enhanced TMZ+SN38 in-vitro cytotoxicity, H2AX phosphorylation, caspase-3 cleavage, and apoptosis by terminal deoxynucleotidyl transferase dUTP nick end labeling. TMZ+IRN+O6BG delayed tumor growth and increased survival relative to TMZ+IRN in two of seven patient-derived xenografts established at time of death from progressive neuroblastoma. We demonstrated that high MGMT expression was associated with resistance to alkylating agents and TMZ+IRN in preclinical neuroblastoma models. The MGMT inhibitor O6BG enhanced the anticancer effect of TMZ+IRN in vitro and in vivo. These results support further preclinical studies exploring MGMT as a therapeutic target and biomarker of TMZ+IRN resistance in high-risk neuroblastoma.

Computational Study of Novel Natural Inhibitors Targeting O6-Methylguanine-DNA Methyltransferase

OBJECTIVE

To screen ideal lead compounds from a drug library (ZINC15 database) with potential inhibition effect against O⁶-methylguanine-DNA methyltransferase (MGMT) to contribute to medication design and refinement.

METHODS

A series of computer-aided virtual screening techniques were used to identify potential inhibitors of MGMT. Structure-based virtual screening by LibDock was carried out to calculate LibDock scores, followed by absorption, distribution, metabolism, and excretion and toxicity predictions. Molecule docking was employed to demonstrate binding affinity and mechanism between the selected ligands and MGMT protein. Molecular dynamics simulation was performed to evaluate stability of the ligand-MGMT complex under natural circumstances.

RESULTS

Two novel natural compounds, ZINC000008220033 and ZINC000001529323, from the ZINC15 database were found to bind with MGMT with a higher binding affinity together with more favorable interaction energy. Also, they were predicted to have less rodent carcinogenicity, Ames mutagenicity, and developmental toxicity potential as well as noninhibition with cytochrome P-450 2D6. Molecular dynamics simulation analysis demonstrated that the 2 complexes ZINC000008220033-MGMT and ZINC000001529323-MGMT had more favorable potential energy compared with reference ligand O⁶-benzylguanine, and they could exist stably in the natural environment.

CONCLUSIONS

This study elucidated that ZINC000008220033 and ZINC000001529323 were ideal lead compounds with potential inhibition targeting to MGMT protein. These compounds were selected as safe drug candidates and may contribute a solid basis for MGMT target medication design and improvement.

miR-29c contribute to glioma cells temozolomide sensitivity by targeting O6-methylguanine-DNA methyltransferases indirectely

Temozolomide (TMZ) is the most commonly used alkylating agent in glioma chemotherapy. However growing resistance to TMZ remains a major challenge to clinicians. The DNA repair protein O6-methylguanine-DNA methytransferase (MGMT) plays critical roles in TMZ resistance. Promoter methylation can inhibit MGMT expression and increase chemosensitivity. Here, we described a novel mechanism regulating MGMT expression. We showed that miR-29c suppressed MGMT expression indirectly via targeting specificity protein 1 (Sp1). MiR-29c overexpression increased TMZ efficacy in cultured glioma cells and in mouse xenograft models. The miR-29c levels were positively correlated with patient outcomes. Our data suggest miR-29c may be potential therapeutic targets for glioma treatment.

The role of O6-methylguanine-DNA methyltransferase in a long-surviving metastatic melanoma

BACKGROUND

Brain metastases commonly occur in patients with metastatic melanoma and are associated with a poor prognosis. Only a few chemotherapeutic agents have been shown to be potentially active. Resistance to chemotherapy is one of the main limitations to treatment. A key mechanism of resistance is O6-methylguanine-DNA methyltransferase (MGMT). The methylation of its promotor could inhibit the activity of this enzyme; consequently, it is very important to evaluate the methylation status of all available specimens.

CASE REPORT

We report the case of a long-surviving patient in whom combination treatment with an alkylating agent inhibiting MGMT, such as temozolomide, was useful in clinical control of the disease.

In vivo roles of conjugation with glutathione and O6-alkylguanine DNA-alkyltransferase in the mutagenicity of the bis-electrophiles 1,2-dibromoethane and 1,2,3,4-diepoxybutane in mice

Several studies with bacteria and in vitro mammalian systems have provided evidence of the roles of two thiol-based conjugation systems, glutathione (GSH) transferase and O(6)-alkylguanine DNA-alkyltransferase (AGT), in the bioactivation of the bis-electrophiles 1,2-dibromoethane and 1,2,3,4-diepoxybutane (DEB), the latter an oxidation product of 1,3-butadiene. The in vivo relevance of these conjugation reactions to biological activity in mammals has not been addressed, particularly with DEB. In this work, we used transgenic Big Blue mice, utilizing the cII gene, to examine the effects of manipulation of conjugation pathways on liver mutations arising from dibromoethane and DEB in vivo. Treatment of the mice with butathionine sulfoxime (BSO) prior to dibromoethane lowered hepatic GSH levels, dibromoethane-GSH DNA adduct levels (N(7)-guanyl), and the cII mutation frequency. Administration of O(6)-benzylguanine (O(6)-BzGua), an inhibitor of AGT, did not change the mutation frequency. Depletion of GSH (BSO) and AGT (O(6)-BzGua) lowered the mutation frequency induced by DEB, and BSO lowered the levels of GSH-DEB N(7)-guanyl and N(6)-adenyl DNA adducts. Our results provide evidence that the GSH conjugation pathway is a major in vivo factor in dibromoethane genotoxicity; both GSH conjugation and AGT conjugation are major factors in the genotoxicity of DEB. The latter findings are considered to be relevant to the carcinogenicity of 1,3-butadiene.

Gallic acid provokes DNA damage and suppresses DNA repair gene expression in human prostate cancer PC-3 cells

Our earlier studies have demonstrated that gallic acid (GA) induced cytotoxic effects including induction of apoptosis and DNA damage and inhibited the cell migration and invasion in human cancer cells. However, GA-affected DNA damage and repair gene expressions in human prostate cancer cells are still unclear. In this study, we investigated whether or not GA induces DNA damage and inhibits DNA repair gene expression in a human prostate cancer cell line (PC-3). The results from flow cytometric assay indicated that GA decreased the percentage of viable PC-3 cells in a dose- and time-dependent manner. PC-3 cells after exposure to different doses (50, 100, and 200 μM) of GA and various periods of time (12, 24, and 48 h) led to a longer DNA migration smear (comet tail) occurred based on the single cell gel electrophoresis (comet assay). These observations indicated that GA-induced DNA damage in PC-3 cells, which also confirmed by 4,6-diamidino-2-phenylindole dihydrochloride staining and DNA agarose gel electrophoresis. Alternatively, results from real-time polymerase chain reaction assay also indicated that GA inhibited ataxia telangiectasia mutated, ataxia-telangiectasia and Rad3-related, O⁶-methylguanine-DNA methyltransferase, DNA-dependent serine/threonine protein kinase, and p53 mRNA expressions in PC-3 cells. Taken together, the present study showed that GA caused DNA damage and inhibited DNA repair genes as well as both effects may be the critical factors for GA-inhibited growth of PC-3 cells in vitro.

Hypoxia-selective O6-alkylguanine-DNA alkyltransferase inhibitors: design, synthesis, and evaluation of 6-(benzyloxy)-2-(aryldiazenyl)-9H-purines as prodrugs of O6-benzylguanine

O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a DNA repair protein which removes alkyl groups from the O-6 position of guanine, thereby providing strong resistance to anticancer agents which alkylate this position. The clinical usefulness of these anticancer agents would be substantially augmented if AGT could be selectively inhibited in tumor tissue, without a corresponding depletion in normal tissue. We report the synthesis of a new AGT inhibitor (5c) which selectively depletes AGT in hypoxic tumor cells.

6-Carboxyfluorescein and structurally similar molecules inhibit DNA binding and repair by O⁶-alkylguanine DNA alkyltransferase

Human O⁶-alkylguanine-DNA alkyltransferase (AGT) repairs mutagenic O⁶-alkylguanine and O⁴-alkylthymine adducts in single-stranded and duplex DNAs. These activities protect normal cells and tumor cells against drugs that alkylate DNA; drugs that inactivate AGT are under test as chemotherapeutic enhancers. In studies using 6-carboxyfluorescein (FAM)-labeled DNAs, AGT reduced the fluorescence intensity by ∼40% at binding saturation, whether the FAM was located at the 5' or the 3' end of the DNA. AGT protected residual fluorescence from quenching, indicating a solute-inaccessible binding site for FAM. Sedimentation equilibrium analyses showed that saturating AGT-stoichiometries were higher with FAM-labeled DNAs than with unlabeled DNAs, suggesting that the FAM provides a protein binding site that is not present in unlabeled DNAs. Additional fluorescence and sedimentation measurements showed that AGT forms a 1:1 complex with free FAM. Active site benzylation experiments and docking calculations support models in which the primary binding site is located in or near the active site of the enzyme. Electrophoretic analyses show that FAM inhibits DNA binding (IC₅₀∼76μM) and repair of DNA containing an O⁶-methylguanine residue (IC₅₀∼63μM). Similar results were obtained with other polycyclic aromatic compounds. These observations demonstrate the existence of a new class of non-covalent AGT-inhibitors. After optimization for binding-affinity, members of this class might be useful in cancer chemotherapy.

Intracellular delivery of bioactive molecules using light-addressable nanocapsules

This paper describes a method by which molecules that are impermeable to cells are encapsulated in dye-sensitized lipid nanocapsules for delivery into cells via endocytosis. Once inside the cells, the molecules are released from the lipid nanocapsules into the cytoplasm with a single nanosecond pulse from a laser in the far red (645 nm). We demonstrate this method with the intracellular release of the second messenger IP(3) in CHO-M1 cells and report that calcium responses from the cells changed from a sustained increase to a transient spike when the average number of IP(3) released is decreased below 50 molecules per nanocapsule. We also demonstrate the delivery of a 23 kDa O(6)-alkylguanine-DNA alkyltransferase (AGT) fusion protein into Ba/F3 cells to inhibit a key player BCR-ABL in the apoptotic pathway. We show that an average of ∼8 molecules of the inhibitor is sufficient to induce apoptosis in the majority of Ba/F3 cells.

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.

Plasma and CNS pharmacokinetics of O4-benzylfolic acid (O4BF) and metabolite in a non-human primate model

PURPOSE

O(6)-alkylguanine-DNA alkyltransferase (AGT) repairs DNA damage from alkylating agents by transferring the alkyl adducts from the O(6)-position of guanine in DNA to AGT. The folate analog O(4)-benzylfolic acid (O(4)BF) is an inhibitor of AGT with reported selectivity of the alpha-folate receptor in tumors. We studied plasma and cerebrospinal fluid (CSF) pharmacokinetics and CSF penetration of O(4)BF in a non-human primate model.

METHODS

Rhesus monkeys (Macaca mulatta) received O(4)BF (10-50 mg/kg) intravenously, and serial blood and CSF samples were obtained. Analyte concentrations in plasma were measured by HPLC/photo diode array, and an HPLC/MS/MS assay was used for CSF samples.

RESULTS

A putative metabolite of O(4)BF was detected in plasma and CSF. O(4)BF and the metabolite inactivated purified AGT with ED(50) of 0.04 mcM. The median clearance of O(4)BF was 8 ml/min/kg and half-life was 1.1 h. The metabolite had a substantially longer half-life (>20 h) and greater AUC than O(4)BF. The AUC of the metabolite increased disproportionately to the dose of O(4)BF, suggesting saturable elimination. CSF penetration of O(4)BF and its metabolite was < 1%. At the 50 mg/kg dose level, the C(max) in CSF for O(4)BF was less than 0.09 mcM and for the metabolite the C(max) ranged from 0.02 to 0.04 mcM (O(4)BF equivalents).

CONCLUSIONS

Concentrations of O(4)BF and the metabolite in CSF exceeded the ED(50) of AGT; however, recently reported lack of receptor specificity and pharmacokinetic data suggesting saturable elimination of both O(4)BF and its metabolite may limit dose-escalation and future clinical development of this agent.

Effect of O6-chloroethylguanine DNA lesions on the kinetics and mechanism of micronucleus induction in vivo

The aim of this work was to determine the kinetics of micronucleus production because of an increase in O(6)-chloroethyl guanine (O6-ChlEt-G) DNA lesions in murine bone marrow cells in vivo. We increased the frequency of O6-ChlEt-G lesions by pretreatment with an inhibitor of O(6)-methylguanine-DNA methyltransferase (MGMT), O(6)-benzylguanine (O6BG), and subsequent treatment with bis-chloroethylnitrosourea (BCNU). The kinetics of micronucleated-polychromatic erythrocyte (MN-PCE) induction was established by scoring the frequency of MN-PCEs per 2000 PCEs in peripheral blood at 8-hr intervals from immediately prior to treatment to 72-hr post-treatment. We examined groups of five mice treated with (i) dimethylsulfoxide (DMSO), (ii) O6BG in DMSO, (iii) BCNU, or (iv) O6BG in DMSO plus BCNU. The data indicate that O6BG pretreatment causes: (i) ían increase in MN-PCEs induced by BCNU, (ii) a delay in the time of maximal MN-PCE induction produced by the different BCNU doses, and (iii) an increase in cytotoxicity. These data confirm that O6-ChlEt-G is a lesion involved in DNA break induction and in the subsequent production of micronuclei, and also that these lesions seem to be stoichiometrically reduced by MGMT. These data also show that induction of MN-PCEs by BCNU is delayed by pretreatment with O6BG for more than 6 hr, perhaps due to the time required for repair of crosslinks derived from O6-ChlEt-G and/or for DNA duplication, which is required for adduct transformation into crosslinks.

p53-Mediated down-regulation of the human DNA repair gene O6-methylguanine-DNA methyltransferase (MGMT) via interaction with Sp1 transcription factor

O(6)-Methylguanine-DNA methyltransferase (MGMT), a ubiquitous DNA repair protein, reverses mutagenic and cytotoxic effects of O(6)-alkylguanine in DNA induced by chemotherapeutic N-alkyl N-nitrosourea and procarbazine type drugs by dealkylating the adduct. MGMT expression is down-regulated by wild-type p53 (WTp53) in human tumor cells. Here we report that p53 sequesters the Sp1 transcription factor to prevent its binding to the cognate cis elements in the MGMT promoter and thus inhibits MGMT expression. Sp1 overexpression abrogated the inhibitory effect of p53 on the MGMT promoter activity in a dose-dependent manner. Stable interaction of Sp1 with WTp53 was observed in HCT116 cells. Moreover, WTp53 overexpression reduced the binding of the nuclear extract to the Sp1 consensus sequence, even though recombinant p53 alone did not bind to the same sequence. Taken together, these results suggest that sequestration of Sp1 could be one of the mechanisms by which p53 negatively regulates MGMT expression, thus enhancing sensitivity of tumor cells to O(6)-alkylguanine generating drugs.

Degradation of BRCA2 in alkyltransferase-mediated DNA repair and its clinical implications

Germ-line mutations in BRCA2 have been linked to early-onset familial breast cancer. BRCA2 is known to play a key role in repairing double-strand breaks. Here, we describe the involvement of BRCA2 in O6-alkylguanine DNA alkyltransferase (AGT)-mediated repair of O6-methylguanine adducts. We show that BRCA2 physically associates and undergoes repair-mediated degradation with AGT. In contrast, BRCA2 with a 29-amino-acid deletion in an evolutionarily conserved domain does not bind to alkylated AGT; the two proteins are not degraded; and mouse embryonic fibroblasts are specifically sensitive to alkylating agents that result in O6-methylguanine adducts. We show that O6-benzylguanine (O6BG), a nontoxic inhibitor of AGT, can also induce BRCA2 degradation. BRCA2 is a viable target for cancer therapy because BRCA2-deficient cells are hypersensitive to chemotherapeutic DNA-damaging agents. We show a marked effect of O6BG pretreatment on cell sensitivity to cisplatin. We also show the efficacy of this approach on a wide range of human tumor cell lines, which suggests that chemosensitization of tumors by targeted degradation of BRCA2 may be an important consideration when devising cancer therapeutics.

Methylation and intratumoural heterogeneity of 14-3-3 sigma in oral cancer

14-3-3 sigma has been a major G2/M checkpoint control gene and has demonstrated that its inactivation in various cancers occurs mostly by epigenetic hypermethylation, not by genetic change. This study investigated the methylation status and expression of the 14-3-3 sigma gene in 46 oral squamous cell carcinomas by methylation-specific polymerase chain reaction, reverse transcriptase-polymerase chain reaction, Western blotting and immunohistochemistry. Exons of the p53 gene were examined for mutations by sequencing analysis and CyclinD1 by immunohistochemistry. Methylation of the 14-3-3 sigma gene was detected in 13% (6/46) of the oral tumours, but not in corresponding adjacent non-malignant and normal gingival tissues. Intratumoural heterogeneity was found in the tumour tissues including three 14-3-3 sigma-methylated samples. Methylation of 14-3-3 sigma was detected in 3 SCC with p53 mutations and 3 with wild-type p53. Our major findings are: (a) methylation of 14-3-3 gene promoter is a rare event in oral cancer; (b) it is not always associated with 14-3-3 protein levels and there is no clear relationship between its methylation and p53 mutation; (c) loss of 14-3-3 sigma expression is associated with reduced CyclinD1 gene expression.

Inactivation of O(6)-alkylguanine-DNA alkyltransferase by folate esters of O(6)-benzyl-2'-deoxyguanosine and of O(6)-[4-(hydroxymethyl)benzyl]guanine

O6-Alkylguanine-DNA alkyltransferase (alkyltransferase) provides an important source of resistance to some cancer chemotherapeutic alkylating agents. Folate ester derivatives of O6-benzyl-2'-deoxyguanosine and of O6-[4-(hydroxymethyl)benzyl]guanine were synthesized and tested for their ability to inactivate human alkyltransferase. Inactivation of alkyltransferase by the gamma-folate ester of O6-[4-(hydroxymethyl)benzyl]guanine was similar to that of the parent base. The gamma-folate esters of O6-benzyl-2'-deoxyguanosine were more potent alkyltransferase inactivators than the parent nucleoside. The 3'-ester was considerably more potent than the 5'-ester and was more than an order of magnitude more active than O6-benzylguanine, which is currently in clinical trials to enhance therapy with alkylating agents. They were also able to sensitize human tumor cells to killing by 1,3-bis(2-chloroethyl)-1-nitrosourea, with O6-benzyl-3'-O-(gamma-folyl)-2'-deoxyguanosine being most active. These compounds provide a new class of highly water-soluble alkyltransferase inactivators and form the basis to construct more tumor-specific and potent compounds targeting this DNA repair protein.

Targeting DNA repair as a promising approach in cancer therapy

An increased DNA-repair activity in tumour cells has been associated with resistance to treatment to DNA-directed drugs, while defects in DNA repair pathways result in hypersensitivity to these agents. In the past years the unravelling of the molecular basis of these DNA pathways, with a better understanding of the DNA damage caused by different anticancer agents, has provided the rationale for the use of some DNA repair inhibitors to optimise the therapeutic use of DNA-damaging agents currently used in the treatment of tumours. In addition, the possibility to specifically target the differences in DNA repair capacity between normal and tumour cells has recently emerged as an exciting possibility. The present review will mainly cover those approaches that are currently under clinical investigation.

Randomized trial of the combination of lomeguatrib and temozolomide compared with temozolomide alone in chemotherapy naive patients with metastatic cutaneous melanoma

PURPOSE

To evaluate tumor response, pharmacodynamic effects, and safety of a combination of lomeguatrib (LM), an O6-methylguanine DNA-methyltransferase (MGMT) inactivator, and temozolomide (TMZ), TMZ alone, and LM/TMZ after disease progression on TMZ alone in patients with advanced melanoma.

PATIENTS AND METHODS

Patients with unresectable stage III or IV cutaneous melanoma who had no prior systemic chemotherapy were randomly assigned to receive either 40 to 80 mg LM and 125 mg/m2 TMZ or 200 mg/m2 TMZ on days 1 through 5 of each 28-day treatment cycle. Drugs were administered orally for up to six cycles of treatment. Patients on TMZ alone were offered LM/TMZ at progression, if fit enough to receive treatment.

RESULTS

One hundred four patients were enrolled, with 52 in each trial arm. Twenty-seven TMZ-treated patients received LM/TMZ after progression on TMZ. Unexpectedly, analysis of tumor biopsies showed rapid recovery of MGMT after LM/TMZ with 40 mg/d LM. Therefore, doses of LM were escalated to 60 then 80 mg/d. Tumor response rates were 13.5% with LM/TMZ and 17.3% with TMZ alone. No patient responded to LM/TMZ having progressed through TMZ. Median time to disease progression was 65.5 days for LM/TMZ and 68 days for TMZ. All treatments were well tolerated, although hematologic and gastrointestinal adverse events were common. A higher incidence of hematological adverse events was observed in the LM/TMZ combination arm.

CONCLUSION

The efficacy of LM and TMZ in the current dosing schedule is similar to that of TMZ alone. To maintain MGMT depletion in tumor dosing of LM needs to be continued beyond that of TMZ.

A combination of IFN-beta and temozolomide in human glioma xenograft models: implication of p53-mediated MGMT downregulation

PURPOSE

Methylation of the O(6)-methyguanine-DNA methyltransferase (MGMT) gene promoter in gliomas has been reported to be a useful predictor of the responsiveness to temozolomide (TMZ). In our previous experiments, we observed that IFN-beta sensitized TMZ-resistant glioma cells with the unmethylated MGMT promoter and that the mechanism of action was possibly due to attenuation of MGMT expression via induction of TP53. In this study, (1) we explored the synergistic effect of IFN-beta and TMZ in the animal model, and (2) clarified the role of IFN-beta induced TP53 in the human MGMT promoter.

METHODS

(1) Nude mice with either subcutaneous T98 (TMZ-resistant) or U251SP (TMZ-sensitive) tumor were treated with IFN-beta/TMZ for 5 consecutive days. (2) The MGMT promoter activity was assayed by a luciferase reporter system in Saos2 (p53-null) cells transduced with a p53-adenoviral vector, and T98 glioma cells treated with IFN-beta.

RESULTS

(1) A combination of IFN-beta/TMZ had significant synergistic antitumor activity on the growth of both T98 and U251SP tumors. (2) MGMT promoter activity was suppressed by either adenovirally transduced p53 or IFN-beta.

CONCLUSIONS

It would be appealing to consider a prospective clinical trial in which genetic markers are used for personalized drug selection, eliciting other forms of treatment or inhibition of MGMT for those with MGMT expression. In this context, IFN-beta inactivates MGMT via p53 gene induction and enhances the therapeutic efficacy to TMZ.

Novel post DNA synthesis chemistry for preparing oligonucleotides containing O6-modified purines

O6-alkylguanine DNA alkyltransferase (AGT) is a key target for inhibition during cancer chemotherapy. A large number of O6-modified-guanine analogues have been developed as AGT inhibitors, of which benzyl and (4-bromothenyl) have been used clinically. Since the normal AGT substrate is the alkylated guanine in DNA, the inhibition of AGT by oligonucleotides containing these compounds offers a promising therapeutic approach in terms of efficacy. In order to prepare such oligonucleotides, we have synthesised the novel phosphoramidite analogue of the 2'-deoxyriboside of 2-amino-6-methylsulfonylpurine. Following the incorporation of the analogue into DNA, the subsequent nucleophilic displacement of the methylsulfanyl group by alkoxide provides a convenient route to a variety of oligonucleotides containing O6-modified guanines.

Depletion of O6-methylguanine-DNA methyltransferase by O6-benzylguanine enhances 5-FU cytotoxicity in colon and oral cancer cell lines

O6-methylguanine-DNA methyltransferase (MGMT) is a DNA repair enzyme whose expression is controlled by its promoter methylation. A cell that expresses a low amount of MGMT is known to be more sensitive to the antiproliferative effects of alkylating agents. We have previously shown that the colorectal cancer patients treated with 5-fluorouracil (5-FU) as adjuvant chemotherapy had a better prognosis when the tumor revealed hypermethylation in its MGMT promoter. Therefore, we sought to investigate the relationship between the expression levels of MGMT and the anti-tumor effect of 5-FU in vitro by using two colon adenocarcinoma and four oral cancer cell lines with a variety of MGMT expression. We also investigated the effects of MGMT depletion by O6-benzylguanine (O6-BG), a potent inhibitor of MGMT. The 5-FU treatment uniformly depleted protein and mRNA expression of MGMT in all cell lines examined. Cell lines expressing low levels of MGMT were sensitive to 5-FU. On the other hand, cells expressing high levels of MGMT were less sensitive to 5-FU. The 5-FU treatment exhibited a better antiproliferative effect on the cells expressing high levels of MGMT by the pretreatment of O6-BG. Depletion of MGMT by O6-BG enhanced the anti-tumor effect of 5-FU. Assessment of the levels of MGMT expression in cancer cells and the control of its expression could contribute to the effective chemotherapy by 5-FU especially in patients who previously were considered as low-responsive individuals whose tumors have high levels of MGMT.

Novel role of triazenes in haematological malignancies: pilot study of Temozolomide, Lomeguatrib and IL-2 in the chemo-immunotherapy of acute leukaemia

Previous studies indicated that dacarbazine and Temozolomide could be highly effective against refractory acute leukaemia. Their activity relies mainly on the generation of methyl adducts at the O(6)-position of guanine in DNA. High levels of O(6)-methylguanine-DNA methyltransferase (MGMT) or a defective mismatch repair (MMR) system, are associated with cellular resistance to triazenes. The MGMT inhibitor, O(6)-(4-bromothenyl)guanine (Lomeguatrib), can restore in vitro sensitivity to Temozolomide in MMR-proficient blasts. In the early 1970s we discovered that, in vivo, triazene compounds induce the appearance of novel transplantation antigens in murine leukaemia ("Chemical Xenogenization", CX). Non-self peptides presented by class I MHC molecules are generated by triazene-induced somatic mutations, affecting retroviral sequences that are detectable in the mouse genome. Moreover, preliminary experiments suggested that human cancer cells can also undergo CX. Therefore, we designed a chemo-immunotherapy strategy in leukaemic patients as follows: (a) cytoreduction and a hypothetical CX phase, i.e. treatment with Lomeguatrib (to suppress MGMT activity) and Temozolomide (to kill sensitive blasts and to presumably induce CX in resistant leukaemic cells); (b) immune response recovery phase using interleukin-2 (to possibly restore an immune response and take advantage of the hypothetical, triazene-induced CX). Here we present the results of pilot study which is in progress in patients with refractory/relapsed acute leukaemia. In all tested cases, Lomeguatrib suppressed MGMT activity in vivo. Six out of eight patients showed partial or complete disappearance of blast cells in peripheral blood or in bone marrow. We observed severe and long-lasting myelosuppression, accompanied by limited non-haematological toxicity. Up to now, two patients are alive (after 9 and 10 months, respectively), four died of opportunistic infections and two of progressive disease. This investigation confirms the potential role of triazenes in leukaemia and highlights the contribution of Lomeguatrib in overcoming drug resistance. Further studies are required to establish whether Temozolomide can induce CX in human leukaemia, and thus offer a new approach to control minimal residual disease.

MGMT hypermethylation: a prognostic foe, a predictive friend

Alkylation of DNA at the O(6)-position of guanine is one of the most critical events leading to mutation, cancer, and cell death. O(6)-alkylguanine-DNA alkyltransferase (AGT), also known as O(6)-methylguanine-DNA methyltransferase (MGMT), is the DNA repair protein responsible for removing alkylation adducts from the O(6)-position of guanine in DNA. The promoter CpG island hypermethylation-associated gene silencing of MGMT is associated with a wide spectrum of human tumors. This epigenetic inactivation of MGMT has two main consequences in human cancer. First, it uncovers a new mutator pathway that causes the accumulation of G-to-A transition mutations that can affect genes required for genomic stability. Second, there is a strong and significant positive correlation between MGMT promoter hypermethylation and increased tumor sensitivity to alkylating drugs. These findings underline the importance of MGMT promoter hypermethylation in basic and translational cancer research.

MGMT inhibitors--The Trinity College-Paterson Institute experience, a chemist's perception

The DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase (MGMT) can confer resistance to the cancer chemotherapeutic effects of the class of DNA damaging drugs generally referred to as the O(6)-alkylating agents. Inactivation of MGMT is thus a practical approach to improving the efficacy of such agents. An account is given of the collaboration between groups at Trinity College, Dublin and the Paterson Institute, Manchester which led to the development of the MGMT inactivating drug, Patrin (PaTrin-2, Lomeguatrib). The development of a simpler method of synthesis of O(6)-arylmethylguanines opened up the way to make a series of O(6)-heteroalkylmethyl analogues of the archetypal MGMT pseudosubstrate, O(6)-methylguanine. Of these, the furfuryl and thenyl compounds were the most active against recombinant Human MGMT in an in vitro assay. The 4-bromothenyl derivative was chosen for clinical trial as the most active compound. The MGMT active site tolerates O(6)-substituted guanines where the side chain can be quite large, but does not tolerate those with an aromatic or heteroaromatic ring with an 'ortho' substituent.

S-alkylthiolation of O6-methylguanine-DNA-methyltransferase (MGMT) to sensitize cancer cells to anticancer therapy

O6-methylguanine DNA methyltransferase/O6-alkylguanine DNA alkyltransferase (MGMT/AGT) removes alkyl adducts from the O6-position of guanine in DNA. Expression of MGMT in human cancers has been associated with resistance to therapies using alkylating agents. MGMT promoter methylation regulates its expression and response to alkylating agents. A combination of O6-benzylguanine-based inhibitors of MGMT with alkylating agents improved the efficacy. However, this is associated with enhanced cytotoxicity and the induction of GC to AT transition mutations presumably also in progenitor/stem cells. A few recent studies have described analogs of O6-benzylguanine targeting defined pathways of cancer cells that can be used to improve the selectivity of O6-benzylguanine-based inhibitors for cancer cells. Therefore, MGMT inhibitor targeting represents a reliable strategy for improving cancer therapy with alkylating agents.

Phase I trial of polifeprosan 20 with carmustine implant plus continuous infusion of intravenous O6-benzylguanine in adults with recurrent malignant glioma: new approaches to brain tumor therapy CNS consortium trial

PURPOSE

This phase I trial was designed to (1) establish the dose of O6-benzylguanine (O6-BG) administered intravenously as a continuous infusion that suppresses O6-alkylguanine-DNA alkyltransferase (AGT) levels in brain tumors, (2) evaluate the safety of extending continuous-infusion O6-BG at the optimal dose with intracranially implanted carmustine wafers, and (3) measure the pharmacokinetics of O6-BG and its metabolite.

PATIENTS AND METHODS

The first patient cohort (group A) received 120 mg/m2 of O6-BG over 1 hour followed by a continuous infusion for 2 days at escalating doses presurgery. Tumor samples were evaluated for AGT levels. The continuous-infusion dose that resulted in undetectable AGT levels in 11 or more of 14 patients was used in the second patient cohort. Group B received the optimal dose of O6-BG for 2, 4, 7, or 14 days after surgical implantation of the carmustine wafers. The study end point was dose-limiting toxicity (DLT).

RESULTS

Thirty-eight patients were accrued. In group A, 12 of 13 patients had AGT activity levels of less than 10 fmol/mg protein with a continuous-infusion O6-BG dose of 30 mg/m2/d. Group B patients were enrolled onto 2-, 4-, 7-, and 14-day continuous-infusion cohorts. One DLT of grade 3 elevation in ALT was seen. Other non-DLTs included ataxia and headache. For up to 14 days, steady-state levels of O6-BG were 0.1 to 0.4 micromol/L, and levels for O6-benzyl-8-oxoguanine were 0.7 to 1.3 micromol/L.

CONCLUSION

Systemically administered O6-BG can be coadministered with intracranially implanted carmustine wafers, without added toxicity. Future trials are required to determine if the inhibition of tumor AGT levels results in increased efficacy.

Role of O6-methylguanine-DNA methyltransferase in protecting from alkylating agent-induced toxicity and mutations in mice

The DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) protects from toxicity and mutations incurred following alkylating agents by removing O(6)-alkylguanine lesions. Using Mgmt-/- mice, we examined MGMT's role in protecting from in vivo mutations induced by three different alkylating agents, temozolomide (TMZ), 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) and cyclophosphamide. Mutant frequencies were determined in the hypoxanthine-guanine phosphoribosyltransferase gene of splenic T-lymphocytes from C57BL/6 mice (Mgmt+/+ and Mgmt-/-) following TMZ, BCNU or cyclophosphamide. Following TMZ, the mutation frequency was significantly greater in Mgmt-/- mice (5.5 and 9.8 x 10(-6) for 7 and 10 mg/kg TMZ, respectively) compared with vehicle-treated mice (1.0 x 10(-6), P <or= 0.05). In contrast, TMZ-induced mutations were not increased over vehicle in Mgmt+/+ mice. The mutation frequency of mice treated with BCNU (7.5 mg/kg) was the same regardless of Mgmt status. Similarly, pretreatment of Mgmt+/+ mice with 30 mg/kg O(6)-benzylguanine, a potent inactivator of MGMT, prior to BCNU (15 mg/kg) did not result in significantly more mutations than mice treated with BCNU alone. Following cyclophosphamide, mutation frequencies significantly increased from 1.8 x 10(-6) in control-treated mice to 12.9 x 10(-6) in Mgmt+/+ and 18.1 x 10(-6) in Mgmt-/- mice, although the difference in Mgmt-/- compared with Mgmt+/+ was not significant. Acrolein and chloroacetaldehyde, metabolites of cyclophosphamide, were not mutagenic in Mgmt+/+ and Mgmt-/- mice. These results demonstrate that MGMT significantly protects against in vivo TMZ-induced mutations and that MGMT deficiency does not result in greater mutation frequency following cyclophosphamide or BCNU compared with wild-type mice.

A single cycle of treatment with temozolomide, alone or combined with O(6)-benzylguanine, induces strong chemoresistance in melanoma cell clones in vitro: role of O(6)-methylguanine-DNA methyltransferase and the mismatch repair system

Clinically achievable concentrations of temozolomide (TMZ) produce cytotoxic effects only in mismatch repair (MMR)-proficient cells endowed with low O6-methylguanine-DNA methyltransferase (MGMT) activity. Aim of the present study was to investigate the molecular mechanisms underlying acquired resistance of melanoma cells to TMZ and the effect of O6-benzylguanine (BG), a specific MGMT inhibitor, on the development of a TMZ-resistant phenotype. Three MMR-proficient melanoma cell clones with low or no MGMT activity were treated daily for 5 days with 50 micromol/l TMZ, alone or in combination with 5 micromol/l BG. Parental clones and sublines established after one or four cycles of treatment were analyzed for sensitivity to TMZ or TMZ+BG and for other parameters. The sublines established after one cycle of TMZ or TMZ+BG exhibited a marked increase in MGMT activity and resistance to TMZ alone. BG only partially reversed acquired resistance to the drug. In some cases, alterations in the MMR system accounted for MGMT-independent resistance to TMZ. Up-regulation of MGMT activity was associated with either demethylation of the MGMT promoter or hypermethylation of the body of the gene, and partially reversed by 5-aza-2'-deoxycytidine. The sublines established after four cycles of TMZ or TMZ+BG did not show a further increase in resistance to TMZ alone. However, two out of three sublines established after TMZ+BG treatment exhibited increased resistance to TMZ+BG. In conclusion, our data demonstrate that a single cycle of TMZ is sufficient to induce high levels of drug resistance in melanoma clones, principally, but not exclusively, via up-regulation of MGMT expression. Exposure to TMZ+BG favors the development of MGMT-independent mechanisms of TMZ resistance.

Exploiting the role of O6-methylguanine-DNA-methyltransferase (MGMT) in cancer therapy

Improving the efficacy of standard chemotherapy by targeting DNA repair mechanisms remains an important area of research. O6-methylguanine-DNA-methyltransferase (MGMT), which repairs alkylating agent damage, is one such target. Downregulation of the gene through epigenetic silencing has been shown to predict response to alkylating agent therapy in selected malignancies. Platinums have also been found to downregulate MGMT expression and this approach is currently under exploration. Another way to deplete O6-alkylguanine DNA alkyltransferase (AGT) levels is to modify methylating agent scheduling. Extended dosing has met with early favourable results. However, pseudosubstrates used to inhibit AGT activity have had limited success because of dose-limiting myelotoxicity. Topoisomerase I is 'trapped' on DNA by alteration of ligation kinetics following alkylating agent damage, leading to interest in combining AGT inhibitors or O6-alkylating agents with topoisomerase I inhibitors. DNA repair by AGT is an interesting target for cancer therapy that remains to be fully evaluated. The best results are likely to be achieved where its inhibition is part of treatment targeting multiple DNA damage processing pathways.

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.

Lomeguatrib, a potent inhibitor of O6-alkylguanine-DNA-alkyltransferase: phase I safety, pharmacodynamic, and pharmacokinetic trial and evaluation in combination with temozolomide in patients with advanced solid tumors

PURPOSE

A major mechanism of resistance to temozolomide involves the DNA repair protein O6-alkylguanine-DNA-alkyltransferase (ATase). The main aims of this phase I trial were to determine an ATase-depleting dose (ADD) of lomeguatrib, a potent pseudosubstrate inhibitor, and to define a suitable dose of temozolomide to be used in combination with lomeguatrib in patients with advanced cancer.

EXPERIMENTAL DESIGN

Lomeguatrib was administered at dose levels of 10 to 40 mg/m2 days 1 to 5, as a single agent, and also in combination with temozolomide. Once the ADD of lomeguatrib was identified, the dose of temozolomide in combination was increased, in successive patient cohorts, from 50 to 175 mg/m2 on days 1 to 5 of a 28-day cycle to define the maximal tolerated dose and dose-limiting toxicity of the combination.

RESULTS

Thirty-eight patients with advanced solid tumors were enrolled. More than 95% ATase depletion within 4 hours of the first dose was achieved in peripheral blood mononuclear cells at lomeguatrib doses of > or =10 mg/m2/d i.v. or > or =20 mg/m2/d orally, and tumor biopsies showed > or =92% ATase depletion. At the ADD of lomeguatrib i.v., the maximal tolerated dose of temozolomide in combination was 150 mg/m2 days 1 to 5. The dose limiting toxicity of the combination of lomeguatrib and temozolomide was myelosuppression. The toxicity of lomeguatrib alone was minimal. In 23 patients with measurable disease, one complete response was seen and 12 patients had stable disease for at least 3 months.

CONCLUSION

This first administration of lomeguatrib to man successfully established an oral ADD of lomeguatrib and identified a combination regimen with temozolomide suitable for future clinical evaluation.

Phase II trial of the O6-alkylguanine DNA alkyltransferase inhibitor O6-benzylguanine and 1,3-bis(2-chloroethyl)-1-nitrosourea in advanced melanoma

PURPOSE

1,3-Bis(2-chloroethyl)-1-nitrosourea (BCNU) induces DNA damage via a chloroethyl adduct at the O(6) position of guanine, which can be repaired by O(6)-alkylguanine DNA alkyltransferase (AGT) expressed in melanoma. We postulated that the addition of O(6) benzylguanine (O(6)BG), a potent inactivator of AGT, would improve the clinical response to BCNU in melanoma.

EXPERIMENTAL DESIGN

Patients had measurable disease, adequate organ function, and a corrected Diffusing capacity of the lung for carbon monoxide (DLCO) of > or =70% predicted. They were accrued into two cohorts based on prior chemotherapy. O(6)BG (120 mg/m(2)) was administered i.v. followed by BCNU (40 mg/m(2)) on an outpatient basis. Peripheral blood mononuclear cells (PBMC) were collected pre- and 18 hours post-O(6)BG to analyze AGT depletion. Treatment was every 6 weeks, and clinical response was assessed after every two cycles.

RESULTS

Forty-two patients were enrolled, 22 of these patients were chemotherapy-naïve. In the chemotherapy-naïve cohort, there was a patient with a complete response (CR), 4 with stable disease (SD), 13 with progressive disease (PD), and 4 nonevaluable patients; the median time to progression was 80 days and the median survival was 211 days. In the prior-chemotherapy cohort, there were no responses, 3 SD, 15 PD, and 2 nonevaluable patients; median time to progression was 54 days and median survival was 120 days. AGT was depleted from PBMC in the 15 patients tested. Grades 3 to 4 myelosuppression was seen in 57% of patients; toxicities were similar between the two cohorts.

CONCLUSIONS

O(6)BG/BCNU was successfully administered on an outpatient basis and depleted AGT from PBMC. However, significant myelosuppression was observed and the clinical outcome was not improved. Alternative mechanisms of resistance to melanoma cell death need to be investigated.

Modulation of chemotherapy resistance in regional therapy: a novel therapeutic approach to advanced extremity melanoma using intra-arterial temozolomide in combination with systemic O6-benzylguanine

This study investigated whether the therapeutic index of regional melanoma therapy using parenteral temozolomide could be improved by chemomodulation with O6-benzylguanine (O6BG), an inhibitor of the DNA repair enzyme O6-alkylguanine-DNA alkyltransferase (AGT). Using a nude rat s.c. human melanoma xenograft model of the extremity, tumors were analyzed for AGT level 2 to 3 hours after the i.p. injection of 3.5 to 70.0 mg/kg O6BG to inhibit AGT activity. Survival studies were conducted using animals that were treated with a 15-minute isolated limb infusion with 10% DMSO in PBS (control), temozolomide alone, or temozolomide in conjunction with single or multiple doses of i.p. O6BG. Tumor volume and toxicity level were monitored every other day. Administration of 3.5 mg/kg O6BG depleted tumor AGT activity by 93.5% (P < 0.01). Groups treated with regional temozolomide alone (350 mg/kg), systemic temozolomide with O6BG, or vehicle combined with O6BG showed no significant tumor responses compared with controls. Whereas use of regional temozolomide alone at a higher dose (750 mg/kg) showed some degree of tumor response, regional temozolomide given in conjunction with multiple dosages of O6BG showed a marked (P < 0.01) reduction in tumor growth with minimal toxicity. Our findings suggest that AGT modulation by the administration of O6BG in combination with temozolomide regional chemotherapy leads to a significant improvement in melanoma antitumor responses. Clinical trials using chemotherapy modulation may improve response rates in future regional infusion and perfusion drug trials.

A phase II trial of O6-benzylguanine and carmustine in patients with advanced soft tissue sarcoma

PURPOSE

Tumor resistance to alkylating agents such as carmustine (BCNU) has been found to be associated with intracellular expression of O6-methylguanine-DNA methyltransferase (MGMT). Administration of O6-benzylguanine (O6-BG), a substrate that inactivates MGMT, may help overcome chemotherapy resistance. We performed a phase II study to explore the activity of O6-BG in combination with BCNU in patients with advanced soft tissue sarcoma.

EXPERIMENTAL DESIGN

Informed consent was obtained from patients with metastatic soft tissue sarcoma naïve to systemic chemotherapy (adjuvant chemotherapy allowed). Patients received O6-BG 120 mg/m2 I.V. followed by BCNU 40 mg/m2 I.V. Treatment was repeated every 6 weeks until disease progression or development of unacceptable toxicity.

RESULTS

No objective responses were observed in 12 enrolled patients. Four patients exhibited stable disease lasting 11-25+ weeks. The median overall survival was 16.9 months (95% CI, 2.9-NR). The most common grade 3-4 toxicities were neutropenia, thrombocytopenia, and anemia. Depletion of MGMT activity was demonstrated in peripheral blood mononuclear cells. Immunohistochemical estimation of MGMT expression from archival tissue ranged from 20 to 99% positive staining cells.

CONCLUSIONS

Observed toxicities were consistent with previous studies of O6-BG plus BCNU. The degree of MGMT expression was variable in this small sample of heterogeneous sarcomas. Further development of this regimen and dose for the treatment of soft tissue sarcoma is not warranted due to the lack of objective responses.

O6-(4-bromothenyl)guanine (PaTrin-2), a novel inhibitor of O6-alkylguanine DNA alkyl-transferase, increases the inhibitory activity of temozolomide against human acute leukaemia cells in vitro

Anti-tumour activity of triazene compounds of clinical interest [i.e. dacarbazine and temozolomide (TMZ)] relies mainly on the generation of methyl adducts to purine bases of DNA. Two DNA repair enzyme systems, i.e. the O6-guanine-alkyl-transferase (MGMT) and mismatch repair (MMR), play a predominant role in conditioning the cytotoxic effects of triazenes. In particular, high levels of MGMT associated with target cells are responsible of resistance to triazenes. On the contrary, the presence of MMR is required for the cytotoxic effects of these compounds. Previous studies performed by our group and a more recent clinical investigation reported by Karen Seiter, pointed out that triazene compounds could play an important role in the treatment of refractory acute leukaemia. Leukaemia blasts, especially of lymphoblastic leukaemia, show frequently high levels of MGMT activity. Therefore, it reasonable to hypothesize that combined treatment of leukaemia patients with triazene compounds along with MGMT inhibitors could lead to a better control of the disease. PaTrin-2 (O6-(4-bromothenyl)guanine, PAT) is a potent and scarcely toxic MGMT inhibitor recently introduced in clinical trials. This drug is used in combination with triazene compounds in order to augment their anti-tumour efficacy against neoplastic cells endowed with high MGMT activity. The present report describes, for the first time, pre-clinical in vitro studies on the cytotoxic activity of combined treatment with PAT+TMZ against long-term cultured leukaemia cells and primary leukaemia blasts obtained from patients with acute lymphoblastic leukaemia or acute myeloblastic leukaemia. The results point out that, both in long-term cultured leukaemia cell lines and in primary blast samples, PAT could improve dramatically the sensitivity of malignant cells to the cytotoxic effects of TMZ. This sensitizing effect is detectable when leukaemia cells show resistance mechanisms based on a MGMT-proficient phenotype. On the contrary, when resistance to TMZ is dependent on MMR deficiency, no influence of PAT can be detected in various experimental conditions. In conclusion, these results appear to provide disease-oriented rational basis to design novel clinical protocols for the treatment of acute leukaemia with combined administration of PAT and triazene compounds.

Synthesis of 131I-Labeled Glucose-Conjugated Inhibitors of O6-Methylguanine-DNA Methyltransferase (MGMT) and Comparison with Nonconjugated Inhibitors as Potential Tools for in Vivo MGMT Imaging

O(6)-Substituted guanine derivatives are powerful agents used for tumor cell sensitization by inhibition of the DNA repair enzyme O(6)-methylguanine-DNA methyltransferase (MGMT). To provide targeted accumulation of MGMT inhibitors in tumor tissue as well as tools for in vivo imaging, we synthesized iodinated C(8)-alkyl-linked glucose conjugates of 2-amino-6-(5-iodothenyl)-9H-purine (O(6)-(5-iodothenyl) guanine, ITG) and 2-amino-6-(3-iodobenzyloxy)-9H-purine (O(6)-(5-iodobenzyl) guanine, IBG). These compounds have MGMT inhibitor constants (IC(50) values) of 0.8 and 0.45 microM for ITGG and IBGG, respectively, as determined in HeLa S3 cells after 2-h incubation with inhibitor. To substantiate that the (131)I-(hetero)arylmethylene group at the O(6)-position of guanine is transferred to MGMT, both the glucose conjugated inhibitors ITGG and IBGG and the corresponding nonglucose conjugated compounds ITG and IBG were labeled with iodine-131. The radioiodinations of all compounds with [(131)I]I(-) were performed with radiochemical yields of >70% for the destannylation of the corresponding tri-n-butylstannylated precursors. The binding ability of [(131)I]ITGG, [(131)]IBGG, [(131)I]ITG, and [(131)I]IBG to purified MGMT was tested. All radioactive compounds were substrates for MGMT, as demonstrated using a competitive repair assay. The newly synthesized radioactive inhibitors were utilized to study ex vivo biodistribution in mice, and the tumor-to-blood ratio of tissue uptake of [(131)I]IBG and [(131)I]IBGG was determined to be 0.24 and 0.76 after 0.5 h, respectively.

DNA repair inhibition: a selective tumour targeting strategy

Advanced cancer is a leading cause of death in the developed world. Chemotherapy and radiation are the two main treatment modalities currently available. The cytotoxicity of many of these agents is directly related to their propensity to induce DNA damage. However, the ability of cancer cells to recognize this damage and initiate DNA repair is an important mechanism for therapeutic resistance and has a negative impact upon therapeutic efficacy. Pharmacological inhibition of DNA repair, therefore, has the potential to enhance the cytotoxicity of a diverse range of anticancer agents. Moreover, the use of inhibitors of DNA repair or DNA damage signalling pathways appears to provide an exciting opportunity to target the genetic differences that exist between normal and tumour tissue.

Role ofO6-Alkylguanine-DNA Alkyltransferase in Protecting against 1,3-Bis(2-chloroethyl)-1-nitrosourea (BCNU)-Induced Long-Term Toxicities

O6-alkylguanine-DNA alkyltransferase (AGT) protects from the mutagenic and toxic lesions induced by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and in many tumors, AGT overexpression provides a means of resistance. To circumvent this, O6-benzylguanine, an inactivator of AGT, has been developed and is currently in clinical development with BCNU; however, the potential long-term toxicities associated with this treatment are unknown. With the inactivation of AGT by O6-benzylguanine, a higher number of toxic and mutagenic O6-alkylguanine lesions introduced by methylating or chloroethylating agents would be expected. In this study, cohorts of mice were treated with vehicle, O6-benzylguanine (30 mg/kg), BCNU alone (low dose of 15 mg/kg or high dose of 50 mg/kg), or O6-benzylguanine (30 mg/kg) plus BCNU (15 mg/kg) and followed for 12 months post-treatment. Mice treated with O6-benzylguanine plus BCNU or high-dose BCNU died significantly earlier (p < 0.0001) than mice in the other three cohorts with a median survival of 8.3 (O6-benzylguanine plus BCNU) and 7.9 months (high-dose BCNU). Histopathologic sections of tissues revealed that the most common morphological diagnosis in animals treated with O6-benzylguanine plus BCNU (15 mg/kg) or BCNU (50 mg/kg) was cytomegaly in the lung with greater severity observed in mice receiving the combination O6-benzylguanine plus BCNU. Four of five mice analyzed in this cohort had alveolar histiocytosis, with one also having alveolar edema. In contrast, liver and kidney toxicity was only observed in mice treated with BCNU (50 mg/kg). These results suggest that O6-benzylguanine enhances long-term pulmonary toxicity associated with BCNU in mice.

O6-methylguanine-DNA methyltransferase, O6-benzylguanine, and resistance to clinical alkylators in pediatric primary brain tumor cell lines

PURPOSE

Primary brain tumors are the leading cause of cancer death in children. Our purpose is (a) to assess the contribution of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) to the resistance of pediatric brain tumor cell lines to clinical alkylating agents and (b) to evaluate variables for maximal potentiation of cell killing by the MGMT inhibitor O6-benzylguanine, currently in clinical trials. Few such data for pediatric glioma lines, particularly those from low-grade tumors, are currently available.

EXPERIMENTAL DESIGN

We used clonogenic assays of proliferative survival to quantitate cytoxicity of the chloroethylating agent 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and the methylating agent temozolomide in 11 glioma and five medulloblastoma lines. Twelve lines are newly established and characterized here, nine of them from low-grade gliomas including pilocytic astrocytomas.

RESULTS

(a) MGMT is a major determinant of BCNU resistance and the predominant determinant of temozolomide resistance in both our glioma and medulloblastoma lines. On average, O(6)-benzylguanine reduced LD10 for BCNU and temozolomide, 2.6- and 26-fold, respectively, in 15 MGMT-expressing lines. (b) O6-Benzylguanine reduced DT (the threshold dose for killing) for BCNU and temozolomide, 3.3- and 138-fold, respectively. DT was decreased from levels higher than, to levels below, clinically achievable plasma doses for both alkylators. (c) Maximal potentiation by O6-benzylguanine required complete and prolonged suppression of MGMT.

CONCLUSIONS

Our results support the use of O6-benzylguanine to achieve full benefit of alkylating agents, particularly temozolomide, in the chemotherapy of pediatric brain tumors.

Engineering Substrate Specificity of O6-Alkylguanine-DNA Alkyltransferase for Specific Protein Labeling in Living Cells

Fusion proteins of human O(6)-alkylguanine-DNA alkyltransferase (AGT) can be specifically labeled with a wide variety of synthetic probes in mammalian cells; this makes them an attractive tool for studying protein function. However, to avoid undesired labeling of endogenous wild-type AGT (wtAGT), the specific labeling of AGT fusion proteins has been restricted to AGT-deficient mammalian cell lines. We present here the synthesis of an inhibitor of wtAGT and the generation of AGT mutants that are resistant to this inhibitor. This enabled the inactivation of wtAGT and specific labeling of fusion proteins of the AGT mutant in vitro and in living cells. The ability to specifically label AGT fusion proteins in the presence of endogenous AGT, after brief incubation of the cells with a small-molecule inhibitor, should significantly broaden the scope of application of AGT fusion proteins for studying protein function in living cells.

The Antineoplastic Efficacy of the Prodrug CloretazineTM Is Produced by the Synergistic Interaction of Carbamoylating and Alkylating Products of its Activation

Cloretazine {1,2-bis(methylsulfonyl)-1-[(2-chloroethyl)-2-(methylamino)carbonyl]hydrazine; VNP40101M; 101M} is a sulfonylhydrazine prodrug that possesses broad spectrum antitumor efficacy against transplanted murine and human tumor models and has shown activity in clinical trials against relapsed or refractory acute myeloid leukemia. Base catalyzed activation of this prodrug generates two different reactive intermediates: chloroethylating species that covalently interact with DNA at the O6-position of guanine residues that progress to a G-C interstrand cross-link, and a carbamoylating agent, methyl isocyanate. Previous findings from this laboratory have provided initial evidence that methyl isocyanate can contribute to the efficacy of Cloretazine by enhancing the cytotoxicity of the generated chloroethylating species. This action may be due in part to inhibition of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT); however, activity in cells devoid of AGT indicates that other actions are involved in the synergistic cytotoxicity. Herein we demonstrate that O6-benzylguanine can also produce synergistic cell kill with the alkylating component of Cloretazine but differs from methyl isocyanate in that the enhancement occurs in AGT-containing cells, but not in cells devoid of AGT. Methyl isocyanate generated by the decomposition of 1,2-bis(methylsulfonyl)-1-[methylaminocarbonyl]hydrazine also acts to enhance the activity of a variety of DNA cross-linking agents, while only producing additive cytotoxicity with methylating agents. Flow cytometric studies using annexin as a marker for apoptosis indicate that in Chinese hamster ovary cells and in human leukemia cells Cloretazine-induced apoptosis is primarily caused by the generated methyl isocyanate. Comet assays designed to detect DNA cross-links in intact cells indicate that the chloroethylating species generated by the activation of Cloretazine produce DNA cross-links, with the co-generated methyl isocyanate increasing the degree of cross-linking produced by the reactive chloroethylating species. These findings provide further evidence that the methyl isocyanate produced by the activation of Cloretazine can be a major contributor to the cytotoxicity produced by this antineoplastic agent.

Inactivated MGMT by O6-benzylguanine is associated with prolonged G2/M arrest in cancer cells treated with BCNU

HCT116 and HCT15 cells that highly express O(6)-methylguanine-DNA-methyltransferase (MGMT) displayed a transient cell cycle G2/M arrest in response to exposure to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) alone; however, 70-80% of cells were arrested in G2/M after treatment with O(6)-benzylguanine (BG) and BCNU. Cells accumulated in G2/M showed elevated levels of an inactive form of cyclin B1/p-Cdc2 (Tyr15) complex that was not associated with activation of Chk1/p-Cdc25C and was independent of p53/p21 status. The most prominent feature of cell death was the appearance of enlarged and multinucleated cells that was related to the inhibition of mitotic entry. In contrast, BG-resistant cell lines, HCT116 BBR and HCT15 BBR cells that contain mutations K165E and K165N of MGMT, respectively, displayed a normal cell cycle progression with a slight and transient increase in G2/M arrest at 24 h after treatments with either BCNU alone or BG combined with BCNU. The differences in the ability to progress toward G2/M after treatment with BG and BCNU between cells expressing wild-type MGMT and mutated MGMT were confirmed in CHO cells transfected with human wild type and K165E mutant MGMT cDNA, respectively. Thus, our findings suggest that BG-inactivated MGMT may be linked to cell signaling events, forcing cells into a permanent G2/M arrest in response to the DNA damages induced by BCNU.

Effect of cell cycle inhibition on Cisplatin-induced cytotoxicity

Pharmacological inhibitors of cyclin-dependent kinase (CDK)2 are currently in preclinical and clinical development. The purpose of our work was to evaluate a series of guanine derivatives for their ability to inhibit CDK2, affect cell cycle progression, and enhance the cytotoxic and apoptotic effects of cisplatin. A panel of guanine derivatives, including O(6)-benzylguanine (O(6)-BG), S(6)-benzyl-6-thioguanine (S(6)-BG), S(6)-[(cyclohexyl)methyl]-6-thioguanine (S(6)-CMG), O(6)-[(cyclohexyl)methyl]guanine (O(6)-CMG), O(6)-benzyl-9-methylguanine (9-CH(3)-BG), O(6)-[(cyclohexyl)methyl]-9-methyl-guanine (9-CH(3)-CMG), and 7-benzylguanine (N7-BG), exhibited varying degrees of CDK2 inhibition with O(6)-CMG being the most potent and 9-CH(3)-BG, 9-CH(3)-CMG, and N7-BG the least potent compounds. Treatment with S(6)-CMG and O(6)-CMG significantly decreased the percentage of cells in S phase. In SQ20b and SCC61 head and neck cancer cell lines, the most potent CDK2 inhibitor, O(6)-CMG, was also the most effective at enhancing cisplatin-induced cytotoxicity and apoptosis. Cisplatin-induced DNA platination increased in SQ20b cells pretreated with S(6)-BG, S(6)-CMG, and O(6)-CMG. Treatment with both O(6)-BG and trichostatin A, an indirect cell cycle inhibitor, demonstrated additive effects on cisplatin-induced cytotoxicity. In summary, we have identified a group of guanine derivatives that were effective modulators of cisplatin-induced cytotoxicity and apoptosis.

Pharmacokinetics of O6-benzylguanine in Pediatric Patients with Central Nervous System Tumors

PURPOSE

To report the results of the first pharmacokinetic study in pediatric patients of O(6)-benzylguanine (O(6)BG), which irreversibly inactivates the DNA repair protein alkylguanine-alkyltransferase, thus enhancing the cytotoxicity of nitrosoureas.

EXPERIMENTAL DESIGN

As part of a Pediatric Oncology Group Phase I study, 120 mg/m(2) of O(6)BG was administered i.v. over 1 h, before 1,3-bis(2-chloroethyl)-1-nitrosourea administration in children with recurrent or refractory brain tumors. Serial blood samples for plasma pharmacokinetic studies were obtained. Concentrations of O(6)BG and its active metabolite O(6)-benzyl-8-oxoguanine (8-oxo-O(6)BG) were measured by high-performance liquid chromatography. A pharmacokinetic model and additional first-order elimination rate constants for each compound were developed.

RESULTS

O(6)BG concentration versus time data were evaluated for 25 patients. The peak concentration of O(6)BG (mean +/- SD) was 11 +/- 4 microm, and the peak concentration of its active metabolite, 8-oxo-O(6)BG, was 35 +/- 10 microm. O(6)BG was rapidly eliminated with a half-life of 85 +/- 140 min, area under the curve of 795 +/- 320 microm. min and clearance of 760 +/- 400 ml/min/m(2). The area under the curve of 8-oxo-O(6)BG when extrapolated to infinity was 22,700 +/- 11,800 microm. min. The clearance and terminal half-life of 8-oxo-O(6)BG were 30 +/- 15 ml/min/m(2) and 360 +/- 220 min, respectively.

CONCLUSIONS

There is rapid elimination of O(6)BG after i.v. administration over 1 h. In contrast, the terminal half-life for the active metabolite, 8-oxo-O(6)BG, is 4-fold longer. The pharmacokinetic parameters for O(6)BG and 8-oxo-O(6)BG are similar to those reported previously in adults.

Inhibition of O6-methylguanine-DNA methyltransferase by glucose-conjugated inhibitors: comparison with nonconjugated inhibitors and effect on fotemustine and temozolomide-induced cell death

The DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) is an important suicide enzyme involved in the defense against O(6)-alkylating mutagens. It also plays a role in the resistance of tumors to anticancer drugs targeting the O(6)-position of guanine, such as temozolomide and fotemustine. Several potent MGMT inhibitors have been developed sensitizing cells to O(6)-alkylating agents. Aimed at targeting MGMT inhibitors to tumor cells, we synthesized MGMT inhibitory compounds conjugated with glucose to improve uptake in tumor cells. Here, we compared O(6)-benzylguanine, O(6)-2-fluoropyridinylmethylguanine (O(6)FPG), O(6)-3-iodobenzylguanine, O(6)-4-bromothenylguanine, and O(6)-5-iodothenylguanine with the corresponding C8-linker beta-d-glucose derivatives. All glucose conjugated inhibitors were 3- to 5-fold less effective than the corresponding nonconjugated drugs as to MGMT inhibition that was measured in cell extracts (in vitro) and cultivated HeLaS3 cells (in vivo). Except for O(6)FPG, IC(50) values of the guanine derivatives applied in vitro and in vivo were correlated. A similar correlation was not obvious for the corresponding glucosides, indicating differences in cellular uptake. C8-alpha-d-glucosides were less effective than beta-glucosides. From the newly developed glucose-conjugated inhibitors tested, O(6)-4-bromothenylguanine-C8-beta-d-glucoside (O(6)BTG-C8-betaGlu) was most potent in inhibiting MGMT both in vitro and in vivo. At a concentration of 0.1 microM, it inhibited cellular MGMT to completion. It was not toxic, even when applied chronically to cells at high dose (up to 20 microM). O(6)BTG-C8-betaGlu strongly potentiated the killing effect of fotemustine and temozolomide, causing reversal from MGMT+ to MGMT- phenotype. Therefore, O(6)BTG-C8-betaGlu seems to be especially suitable for approaching MGMT inhibitor targeting in tumor therapy.

2-Amino-O4-benzylpteridine Derivatives: Potent Inactivators ofO6-Alkylguanine-DNA Alkyltransferase

2-amino-O4-benzylpteridine (1), 2-amino-O4-benzyl-6,7-dimethylpteridine (2), 2-amino-O4-benzyl-6-hydroxymethylpteridine (4), 2-amino-O4-benzylpteridine-6-carboxylic acid (5), 2-amino-O4-benzyl-6-formylpteridine (6), and O4-benzylfolic acid (7) are shown to be as potent or more potent inactivators of the human DNA repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase) in vitro than O6-benzylguanine, the prototype alkyltransferase inactivator currently in clinical trials. Additionally, the negatively charged (at physiological pH) inactivators 2-amino-O4-benzylpteridine-6-carboxylic acid (5) and O4-benzylfolate (7) are far more water soluble than O6-benzylguanine. The activity of O4-benzylfolic acid (7) is particularly noteworthy because it is roughly 30 times more active than O6-benzylguanine against the wild-type alkyltransferase and is even capable of inactivating the P140K mutant alkyltransferase that is resistant to inactivation by O6-benzylguanine. All the pteridine derivatives except 2-amino-O4-benzylpteridine-6-carboxylic acid are effective in enhancing cell killing by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). However, the effectiveness of O4-benzylfolate as an adjuvant for cell killing by BCNU appears to be a function of a cell's alpha-folate receptor expression. Thus, O4-benzylfolate is least effective as an adjuvant in A549 cells (which express little if any receptor), is moderately effective in HT29 cells (which express low levels of the receptor), but is very effective in KB cells (which are known to express high levels of the alpha-folate receptor). Therefore, O4-benzylfolic acid shows promise as an agent for possible tumor-selective alkyltransferase inactivation, which suggests it may prove to be superior to O6-benzylguanine as a chemotherapy adjuvant.

Repression of MLH1 and MGMT genes in colon mucosa adjacent to implanted cancer in athymic mouse

Hyperplastic mucosa adjacent to colon cancer, being a reactive change, accelerates cancer progression and its metastasis through expression of angiogenic factors. We investigated promoter methylation in hyperplastic mucosa adjacent to orthotopic KM12SM colon cancer in mice. In the hyperplastic mucosa adjacent to KM12SM tumors in the cecum of athymic mice, reductions in the levels of the mutL homologue 1 (MLH1) and O6-methylguanine-DNA methyltransferase (MGMT) proteins were detected by immunohistochemistry and immunoblotting. To examine the effects of growth factors and cytokines on promoter methylation and repressed expression of the MLH1 and MGMT genes, a rat intestinal epithelial cell line, IEC6, was treated with epidermal growth factor (EGF) and interleukin (IL)-15 for 35 days. Protein levels of MLH1 and MGMT were reduced in EGF- and IL-15-treated IEC6 cells. A methylation-sensitive restriction enzyme assay revealed that CpG methylation was present in the promoter regions of the MLH1 and MGMT genes in DNAs extracted from hyperplastic mucosa adjacent to KM12SM tumors. These findings suggest that promoter CpG methylation affects expression of MLH1 MGMT genes in hyperplastic mucosa adjacent to colon cancer.