AGT Activity Research Articles

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.

Ligand‐Directed Caging Enables the Control of Endogenous DNA Alkyltransferase Activity with Light inside Live Cells

AbstractThe control of endogenous protein activity with light inside live cells is helpful for the high spatiotemporal probing of their dynamic roles. Herein, we report the first small‐molecule‐ligand‐directed caging approach to control the endogenous human O6‐alkylguanine‐DNA alkyltransferase (AGT) activity with light, and the caged AGT is constructed from the native intracellular AGT. The photo‐responsive O6‐benzylguanine derivative O6‐NBG3 is developed to site‐specifically cage the AGT′s catalytic cysteine residue, and the light irradiation on‐demand restores AGT′s activity in vitro, in bacteria, and in mammalian cells. With O6‐NBG3, the alkylated AGT is dealkylated for the first time to recover the DNA repair activity in breast cancer MCF‐7 cells by the dose‐dependent light irradiation. This decaging strategy enables the localized modulation of endogenous AGT activity with high temporal precision without genetic engineering, which holds great potential for therapeutic applications.

ABO blood group, glycosyltransferase activity and risk of venous thromboembolism

IntroductionABO blood group influence the risk of venous thromboembolism (VTE) by modifying A and B glycosyltransferases (AGT and BGT) activities that further modulates Factor VIII (FVIII) and von Willebrand Factor (VWF) plasma levels. The aim of this work was to evaluate the association of plasma GTs activities with VWF/FVIII plasma levels and VTE risk in a case-control study. Materials and methods420 cases were matched with 420 controls for age and ABO blood group. GT activities in plasma were measured using the quantitative transfer of tritiated N-acetylgalactosamine or galactose to the 2′-fucosyl-lactose and expressed in disintegration per minute/30 μL of plasma and 2 h of reaction (dpm/30 μL/2H). FVIII and VWF plasma levels were respectively measured using human FVIII-deficient plasma in a 1-stage factor assay and STA LIATEST VWF (Diagnostica Stago). ResultsA and B GT activities were significantly lower in cases than in controls (8119 ± 4027 vs 9682 ± 4177 dpm/30 μL/2H, p = 2.03 × 10−5, and 4931 ± 2305 vs 5524 ± 2096 dpm/30 μL/2H, p=0.043 respectively). This association was observed whatever the ABO blood groups. The ABO A1 blood group was found to explain~80% of AGT activity. After adjusting for ABO blood groups, AGT activity was not correlated to VWF/FVIII plasma levels. Conversely, there was a moderate correlation (ρ ~ 0.30) between BGT activity and VWF/ FVIII plasma levels in B blood group carriers. ConclusionWork showed, for the first time, that GT activities were decreased in VTE patients in comparison to controls with the same ABO blood group. The biological mechanisms responsible for this association remained to be determined.

Synthesis and Antitumor Activity Evaluation of a Novel Combi-nitrosourea Prodrug: BGCNU.

Chloroethylnitrosoureas (CENUs) are an important type of alkylating agent employed in the clinical treatment of cancer. However, the anticancer efficacy of CENUs is greatly decreased by a DNA repairing enzyme, O6-alkylguanine-DNA alkyltransferase (AGT), by preventing the formation of interstrand cross-links (ICLs). In this study, a combi-nitrosourea prodrug, namely, N-(2-chloroethyl)-N'-2-(O6-benzyl-9-guanine)ethyl-N-nitrosourea (BGCNU), which possesses an O6-benzylguanine (O6-BG) derivative and CENU pharmacophores simultaneously, was synthesized and evaluated for its ability to induce ICLs. The target compound is markedly more cytotoxic in human glioma cells than the clinically used CENU chemotherapies ACNU, BCNU, and their respective combinations with O6-BG. In the AGT-proficient cells, significantly higher levels of DNA ICLs were observed in the groups treated by BGCNU than those by ACNU and BCNU, which indicated that the activity of AGT was effectively inhibited by the O6-BG derivatives released from BGCNU.

Interdomain interactions rearrangements control the reaction steps of a thermostable DNA alkyltransferase

BackgroundAlkylated DNA-protein alkyltransferases (AGTs) are conserved proteins that repair alkylation damage in DNA by using a single-step mechanism leading to irreversible alkylation of the catalytic cysteine in the active site. Trans-alkylation induces inactivation and destabilization of the protein, both in vitro and in vivo, likely triggering conformational changes. A complete picture of structural rearrangements occurring during the reaction cycle is missing, despite considerable interest raised by the peculiarity of AGT reaction, and the contribution of a functional AGT in limiting the efficacy of chemotherapy with alkylating drugs. MethodsAs a model for AGTs we have used a thermostable ortholog from the archaeon Sulfolobus solfataricus (SsOGT), performing biochemical, structural, molecular dynamics and in silico analysis of ligand-free, DNA-bound and mutated versions of the protein. ResultsConformational changes occurring during lesion recognition and after the reaction, allowed us to identify a novel interaction network contributing to SsOGT stability, which is perturbed when a bulky adduct between the catalytic cysteine and the alkyl group is formed, a mandatory step toward the permanent protein alkylation. ConclusionsOur data highlighted conformational changes and perturbation of intramolecular interaction occurring during lesion recognition and catalysis, confirming our previous hypothesis that coordination between the N- and C-terminal domains of SsOGT is important for protein activity and stability. General significanceA general model of structural rearrangements occurring during the reaction cycle of AGTs is proposed. If confirmed, this model might be a starting point to design strategies to modulate AGT activity in therapeutic settings.

Synthesis and antitumor activity evaluation of a novel combi-nitrosourea prodrug: Designed to release a DNA cross-linking agent and an inhibitor of O6-alkylguanine-DNA alkyltransferase

The drug resistance of CENUs induced by O6-alkylguanine-DNA alkyltransferase (AGT), which repairs the O6-alkylated guanine and subsequently inhibits the formation of dG–dC cross-links, hinders the application of CENU chemotherapies. Therefore, the discovery of CENU analogs with AGT inhibiting activity is a promising approach leading to novel CENU chemotherapies with high therapeutic index. In this study, a new combi-nitrosourea prodrug 3-(3-(((2-amino-9H-purin-6-yl)oxy)methyl)benzyl)-1-(2-chloroethyl)-1-nitrosourea (6), designed to release a DNA cross-linking agent and an inhibitor of AGT, was synthesized and evaluated for its antitumor activity and ability to induce DNA interstrand cross-links (ICLs). The results indicated that 6 exhibited higher cytotoxicity against mer+ glioma cells compared with ACNU, BCNU, and their respective combinations with O6-benzylguanine (O6-BG). Quantifications of dG–dC cross-links induced by 6 were performed using HPLC–ESI-MS/MS. Higher levels of dG–dC cross-link were observed in 6-treated human glioma SF763 cells (mer+), whereas lower levels of dG–dC cross-link were observed in 6-treated calf thymus DNA, when compared with the groups treated with BCNU and ACNU. The results suggested that the superiority of 6 might result from the AGT inhibitory moiety, which specifically functions in cells with AGT activity. Molecular docking studies indicated that five hydrogen bonds were formed between the O6-BG analogs released from 6 and the five residues in the active pocket of AGT, which provided a reasonable explanation for the higher AGT-inhibitory activity of 6 than O6-BG.

Abstract 5498: Changes in expression of DNA repair genes in human leukemia cells treated with laromustine

Abstract The anticancer prodrug laromustine induces cytotoxic DNA damage and has had clinical success against acute myelogenous leukemia and glioblastoma multiforme. The causative DNA damage is principally a G-C interstrand crosslink preceded by 2-chloroethylation of guanine O6 by a subspecies of laromustine generated in situ upon base-catalyzed activation. Another cogenerated electrophile, methylisocyanate, contributes synergistically with the DNA alkylating activity towards cytotoxicity. Given this synergism, DNA repair enzymes are considered likely targets of methylisocyanate, which can carbamoylate amines and sulfhydryls. The inhibition of O6-alkylguanine-DNA alkyltransferase (AGT) and DNA polymerase beta by laroumustine's carbamoylating activity, observed in vitro, may contribute to cytotoxicity. To further investigate the relationship between laromustine and DNA repair, drug-induced changes in the transcription of 88 DNA repair genes were measured in cultured human promyelocytic (HL-60) cells using quantitative real-time reverse transcriptase PCR. Cells were treated with laromustine or vehicle for six hours before harvesting mRNA for analysis. The expression of a subset of the tested genes emerged as significantly different in cells treated with laromustine as compared to control cells. Included among those genes with significantly increased expression were: NTHL1 (17 fold), PARP1 (8 fold), and AGT (5 fold). Included among those genes with significantly decreased expression were: PARP2 (-5 fold) and MSH6 (-74 fold). Increased expression of NTHL1, which encodes for an N-glycosylase that can remove damaged guanine bases, may reduce the effectiveness of laromustine's alkylation of guanine O6. A strong negative correlation between the level of AGT activity in patient samples and the efficacy of laromustine has already been established. Some of the gene products identified in this study, such as NTHL1, may emerge as possible cotherapeutic targets or markers for clinical screening. Citation Format: Kristen N. Robinson, Kathryn A. Coe, Justin E. Owumi, Kevin P. Rice. Changes in expression of DNA repair genes in human leukemia cells treated with laromustine. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5498. doi:10.1158/1538-7445.AM2014-5498

Phase I Study of Temozolomide and Laromustine (VNP40101M) in Patients With Relapsed or Refractory Leukemia

Although alkylators are known to be effective against some myeloid leukemias, resistance is often mediated via O6-alkylguanine-DNA alkyltransferase (AGT). Temozolomide's inhibition of AGT may sensitize leukemia cells to the novel alkylator laromustine. We conducted a phase I translational study to evaluate the toxicities and estimate the maximum tolerated dose (MTD) of laromustine when administered with temozolomide (TMZ) in patients with hematologic malignancies. TMZ was delivered twice daily for 5 doses followed by a single infusion of laromustine. The target TMZ dose was the dose that would reliably result in > 90% AGT depletion. Once the target TMZ dose was identified, the laromustine dose was escalated. A total of 35 patients with relapsed/refractory leukemia were treated. Treatment with TMZ 300 mg for 5 doses resulted in > 90% depletion of AGT levels in 5 of 6 patients. The MTD of the combination was established at TMZ 1500 mg and laromustine 300 mg/m2. Three of the 7 patients assayed from cohort 1 achieved > 90% depletion of AGT activity (range, 77%-100% depletion; median, 88%). Five of 6 patients enrolled in cohort 2 achieved > 90% depletion of AGT activity (range, 92%-100% depletion; median, 93.5%). This established that the 300-mg dose of TMZ (1500 mg total) would be maintained in subsequent cohorts. The majority of adverse events were primarily hematologic, with infectious and pulmonary complications also noted. Three (9%) of the patients with previous refractory disease achieved a complete remission, and 5 (14%) of the patients achieved a morphologic, leukemia-free, but persistent hypocellular bone marrow status. Laromustine in combination with TMZ is tolerable and manageable at doses that predictably suppress AGT. Reliable TMZ-induced inhibition of AGT was observed in doses that are clinically tolerable. Evidence of antitumor effect was observed with this combination, suggesting that further efficacy studies should be performed.

Development of an O6-alkylguanine–DNA alkyltransferase assay based on covalent transfer of the benzyl moiety from [benzene- 3H] O6-benzylguanine to the protein

Although it is known that (i) O 6-alkylguanine–DNA alkyltransferase (AGT) confers tumor cell resistance to guanine O 6-targeting drugs such as cloretazine, carmustine, and temozolomide and that (ii) AGT levels in tumors are highly variable, measurement of AGT activity in tumors before treatment is not a routine clinical practice. This derives in part from the lack of a reliable clinical AGT assay; therefore, a simple AGT assay was devised based on transfer of radioactive benzyl residues from [benzene- 3H] O 6-benzylguanine ([ 3H]BG) to AGT. The assay involves incubation of intact cells or cell homogenates with [ 3H]BG and measurement of radioactivity in a 70% methanol precipitable fraction. Approximately 85% of AGT in intact cells was recovered in cell homogenates. Accuracy of the AGT assay was confirmed by examination of AGT levels by Western blot analysis with the exception of false-positive results in melanin-containing cells due to [ 3H]BG binding to melanin. Second-order kinetic constants for human and murine AGT were 1100 and 380 M −1 s −1, respectively. AGT levels in various human cell lines ranged from less than 500 molecules/cell (detection limit) to 45,000 molecules/cell. Rodent cell lines frequently lacked AGT expression, and AGT levels in rodent cells were much lower than in human cells.

Mismatch Repair Deficiency Does Not Mediate Clinical Resistance to Temozolomide in Malignant Glioma

A major mechanism of resistance to methylating agents, including temozolomide, is the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT). Preclinical data indicates that defective DNA mismatch repair (MMR) results in tolerance to temozolomide regardless of AGT activity. The purpose of this study was to determine the role of MMR deficiency in mediating resistance in samples from patients with both newly diagnosed malignant gliomas and those who have failed temozolomide therapy. The roles of AGT and MMR deficiency in mediating resistance in glioblastoma multiforme were assessed by immunohistochemistry and microsatellite instability (MSI), respectively. The mutation status of the MSH6 gene, a proposed correlate of temozolomide resistance, was determined by direct sequencing and compared with data from immunofluorescent detection of MSH6 protein and reverse transcription-PCR amplification of MSH6 RNA. Seventy percent of newly diagnosed and 78% of failed-therapy glioblastoma multiforme samples expressed nuclear AGT protein in > or = 20% of cells analyzed, suggesting alternate means of resistance in 20% to 30% of cases. Single loci MSI was observed in 3% of patient samples; no sample showed the presence of high MSI. MSI was not shown to correlate with MSH6 mutation or loss of MSH6 protein expression. Although high AGT levels may mediate resistance in a portion of these samples, MMR deficiency does not seem to be responsible for mediating temozolomide resistance in adult malignant glioma. Accordingly, the presence of a fraction of samples exhibiting both low AGT expression and MMR proficiency suggests that additional mechanisms of temozolomide resistance are operational in the clinic.

Phase II trial of Gliadel plus O6-benzylguanic (O6-BG) for patients with recurrent glioblastoma multiforme

2036 Background: The major mechanism of resistance to alkylnitrosourea therapy involves the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) which removes chloroethylation or methylation damage from the O6- position of guanine. O6-BG is an AGT substrate that inhibits AGT by suicide inactivation. A previous phase III randomized, placebo- controlled trial has shown that Gliadel wafer (G) significantly prolongs 6-month survival (55.5% for G vs. 35.6% for placebo) and median survival (28 weeks for G vs. 20 weeks for placebo) in patients with recurrent glioblastoma multiforme (GBM) (Brem et al 1995). Despite the success of G in prolonging survival we may be able to improve on this success by depleting AGT. Methods: Thus, we have designed a phase 2 trial where we define the activity and the toxicity of G in combination with a 5-day infusion of O6-BG in patients with recurrent GBM. In a prior study the O6-BG dose found to be effective in depleting tumor AGT activity at 48 hours was an IV bolus of 120 mg/m2 over 1 hour followed by a continuous infusion of 30 mg/m2/d for 48 hours. In order to guarantee depletion of tumor AGT activity for at least 5 days after G placement, this O6-BG bolus was repeated on days 3 and 5 while continuing the infusion. Results: To date, 47 patients have been enrolled out of a planned accrual of 50 patients. The 6-month survival is 80% and the median survival is 47 weeks. The adverse events include the following: 3 episodes of grade 3 CSF leak (6%), 7 episodes of grade 3 wound infection at craniotomy site (15%), 6 episodes of hyponatremia (13%), 3 episodes of hydrocephalus (6%), 1 episode of hygroma (2%), 1 episode of infectious meningitis (2%), 1 episode of arachnoiditis (2%), 1 episode of grade 3 fever (2%). Conclusions: Thus far, this data demonstrates an increase in the efficacy of G when combined with O6-BG. Three additional patients will be enrolled for a total accrual of 50 patients. [Table: see text]

Defining regional infusion treatment strategies for extremity melanoma: comparative analysis of melphalan and temozolomide as regional chemotherapeutic agents

Five different human melanoma xenografts were used in a xenograft model of extremity melanoma to evaluate the variability of tumor response to regionally administered melphalan or temozolomide and to determine if various components of pertinent drug resistance pathways for melphalan [glutathione S-transferase (GST)/glutathione] and temozolomide [O(6)-alkylguanine DNA alkyltranferase (AGT)/mismatch repair (MMR)] could be predictive of tumor response. Xenograft-bearing rats underwent regional isolated limb infusion with either melphalan (90 mg/kg) or temozolomide (2,000 mg/kg). The levels of AGT activity, GST activity, glutathione level, and GST/AGT expression were examined in this group of xenografts and found to be quite heterogeneous. No correlation was identified between melphalan sensitivity and the GST/glutathione cellular detoxification pathway. In contrast, a strong correlation between the levels of AGT activity and percentage increase in tumor volume on day 30 (r = 0.88) was noted for tumors treated with temozolomide. Regional therapy with temozolomide was more effective when compared with melphalan for the xenograft with the lowest AGT activity, whereas melphalan was more effective than temozolomide in another xenograft that had the highest AGT activity. In three other xenografts, there was no significant difference in response between the two chemotherapy agents. This study shows that AGT activity may be useful in predicting the utility of temozolomide-based regional therapy for advanced extremity melanoma tumors. Our observations also point out the limited ability of analysis of the GST/glutathione pathway to predict response to chemotherapies like melphalan whose resistance is primarily mediated through a complex mechanism of detoxification.

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

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. 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). 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. 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.

Quantitative analysis of O6-alkylguanine-DNA alkyltransferase in malignant glioma

Promoter hypermethylation of the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) has been associated with an enhanced response to chloroethylating and methylating agents in patients with malignant glioma. The purpose of this study was to compare three distinct yet related indices for measuring AGT to determine if these assays could be used interchangeably when AGT status is to be used to guide chemotherapeutic decisions. Real-time methylation-specific PCR (MSP), assessed as the ratio of methylated AGT copies to internal beta-actin control, was used to quantitate AGT hypermethylation in 32 glioma samples. Data were compared with AGT enzyme activity as well as immunohistochemical detection of AGT protein from the same samples. Hypermethylation of the AGT promoter was detected in 19 of 31 (61%) samples evaluable by MSP. Low-level AGT, defined as <20% nuclear AGT staining by immunohistochemistry, was found in 10 of 32 samples (31%), whereas 12 of 32 (38%) had low levels of AGT activity. Correlation of immunohistochemistry to AGT activity was statistically significant (P = 0.014) as was the correlation of immunohistochemistry to MSP (P = 0.043), whereas MSP compared with AGT activity (P = 0.246) was not significant. Cross-tabulation of immunohistochemistry and MSP data based on prognostic groups, where good prognosis was represented by an immunohistochemistry of <20% and an MSP ratio >12, showed no significant relationship (P = 0.214), suggesting that one assay cannot be used interchangeably for another. The observed discordance between respective measures of AGT based on prognosis supports further standardization of AGT assays designed to guide therapeutic practice. The data also suggest that consideration be given to the large population of AGT-expressing cells within samples when therapeutic strategies based on tumor methylation are used.

Phase II trial of Gliadel plus O6-benzylguanine (O6-BG) for patients with recurrent glioblastoma multiforme

1568 Background. The major mechanism of resistance to alkylnitrosourea therapy involves the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) which removes chloroethylation or methylation damage from the O6-position of guanine. O6-BG is an AGT substrate that inhibits AGT by suicide inactivation. A previous phase III randomized, placebo-controlled trial has shown that Gliadel wafer significantly prolongs 6-month survival (55.5% for Gliadel vs. 35.6% for placebo) and median survival (28 weeks for Gliadel vs. 20 weeks for placebo) in patients with recurrent glioblastoma multiforme (GBM) (Brem et al 1995). Despite the success of Gliadel in prolonging survival we may be able to improve on this success by depleting AGT. Methods. Thus, we have designed a phase 2 trial where we define the activity and the toxicity of Gliadel in combination with a 5-day infusion of O6-BG in patients with recurrent GBM. In a prior study the O6-BG dose found to be effective in depleting tumor AGT activity at 48 hours was an IV bolus of 120 mg/m2 over 1 hour followed by a continuous infusion of 30 mg/m2/d for 48 hours. In order to guarantee depletion of tumor AGT activity for at least 5 days after Gliadel placement, this O6-BG bolus was repeated on days 3 and 5 while continuing the infusion. Results. To date, 24 patients have been enrolled out of a planned accrual of 50 patients. Seventeen of these patients received prior nitrosourea therapy. The 6-month survival is 68% and the median survival is 36 weeks. The adverse events include the following: 2 episodes of CSF leak (8%), 4 episodes of wound infection at craniotomy site (16%), 5 episodes of grade ≥ 3 seizures (21%) and 3 episodes of hyponatremia (12%). These adverse events were similar in frequency to those seen in patients receiving Gliadel in prior placebo-controlled Gliadel trials. Conclusions. Thus far, this data demonstrates an increase in the efficacy of Gliadel when combined with O6-BG. Twenty-six additional patients will be enrolled for a total accrual of 50 patients. [Table: see text]

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.

Directed evolution of O6-alkylguanine-DNA alkyltransferase for applications in protein labeling

The specific reaction of O6-alkylguanine-DNA alkyltransferase (AGT) with O6-benzylguanine (BG) derivatives allows for a specific labeling of AGT fusion proteins with chemically diverse compounds in living cells and in vitro. The efficiency of the labeling depends on a number of factors, most importantly on the reactivity, selectivity and stability of AGT. Here, we report the use of directed evolution and two different selection systems to further increase the activity of AGT towards BG derivatives by a factor of 17 and demonstrate the advantages of this mutant for the specific labeling of AGT fusion proteins displayed on the surface of mammalian cells. The results furthermore identify two regions of the protein outside the active site that influence the activity of the protein towards BG derivatives.

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.

Phase I Study of Cloretazine (VNP40101M), a Novel Sulfonylhydrazine Alkylating Agent, Combined with Cytarabine in Patients with Refractory Leukemia

Cloretazine (VNP40101M) is a novel sulfonylhydrazine alkylating agent with significant antileukemia activity. A phase I study of cloretazine combined with cytarabine (1-beta-d-arabinofuranosylcytosine, ara-C) was conducted in patients with refractory disease. Ara-C was given i.v. at a fixed dose of 1.5 gm/m(2)/d by continuous infusion for 4 days (patients ages <65 years at time of diagnosis) or 3 days (patients ages > or =65 years). Cloretazine was given i.v. over 15 to 60 minutes on day 2 at a starting dose of 200 mg/m(2), with escalation in 100 mg/m(2) increments in cohorts of three to six patients until a maximum tolerated dose was established. The DNA repair enzyme O(6)-alkylguanine DNA alkyltransferase (AGT) was measured at baseline. Forty patients, including 32 with acute myeloid leukemia, received 47 courses of treatment. Complete responses were seen at cloretazine dose levels of > or =400 mg/m(2) in 10 of 37 (27%) evaluable patients, and in this patient subset, AGT activity was significantly lower in patients that responded to treatment than in patients who did not (P < or = 0.027). Dose-limiting toxicities (gastrointestinal and myelosuppression) were seen with 500 and 600 mg/m(2) of cloretazine combined with the 4-day ara-C schedule but not seen with the 3-day schedule. The recommended cloretazine dose schedule for future studies is 600 mg/m(2) combined with 1.5 gm/m(2)/d continuous infusion of ara-C for 3 days. The cloretazine and ara-C regimen has significant antileukemic activity. AGT activity may be a predictor of response to cloretazine.

Phase I Trial of Temozolomide Plus O6-Benzylguanine for Patients With Recurrent or Progressive Malignant Glioma

We conducted a two-phase clinical trial in patients with progressive malignant glioma (MG). The first phase of this trial was designed to determine the dose of O6-BG effective in producing complete depletion of tumor AGT activity for 48 hours. The second phase of the trial was designed to define the maximum tolerated dose (MTD) of a single dose of temozolomide when combined with O6-BG. In addition, plasma concentrations of O6-BG and O6-benzyl-8-oxoguanine were evaluated after O6-BG. For our first phase of the clinical trial, patients were scheduled to undergo craniotomy for AGT determination after receiving a 1-hour O6-BG infusion at 120 mg/m2 followed by a continuous infusion at an initial dose of 30 mg/m2/d for 48 hours. The dose of the continuous infusion of O6-BG escalated until tumor AGT was depleted. Once the O6-BG dose was established a separate group of patients was enrolled in the second phase of clinical trial, in which temozolomide, administered as a single dose at the end of the 1-hour O6-BG infusion, was escalated until the MTD was determined. The O6-BG dose found to be effective in depleting tumor AGT activity at 48 hours was an IV bolus of 120 mg/m2 over 1 hour followed by a continuous infusion of 30 mg/m2/d for 48 hours. On enrolling 38 patients in six dose levels of temozolomide, the MTD was established at 472 mg/m2 with dose-limiting toxicities limited to myelosuppression. This study provides the foundation for a phase II trial of O6-BG plus temozolomide in temozolomide-resistant MG.