Influence of the Expression Level of O6-Alkylguanine-DNA Alkyltransferase on the Formation of DNA Interstrand Crosslinks Induced by Chloroethylnitrosoureas in Cells: A Quantitation Using High-Performance Liquid Chromatography-Mass Spectrometry

Mechanism of Action of KL-50, a Candidate Imidazotetrazine for the Treatment of Drug-Resistant Brain Cancers

Aberrant DNA repair is a hallmark of cancer, and many tumors display reduced DNA repair capacities that sensitize them to genotoxins. Here, we demonstrate that the differential DNA repair capacities of healthy and transformed tissue may be exploited to obtain highly selective chemotherapies. We show that the novel N3-(2-fluoroethyl)imidazotetrazine "KL-50" is a selective toxin toward tumors that lack the DNA repair protein O6-methylguanine-DNA-methyltransferase (MGMT), which reverses the formation of O6-alkylguanine lesions. We establish that KL-50 generates DNA interstrand cross-links (ICLs) by a multistep process comprising DNA alkylation to generate an O6-(2-fluoroethyl)guanine (O6FEtG) lesion, slow unimolecular displacement of fluoride to form an N1,O6-ethanoguanine (N1,O6EtG) intermediate, and ring-opening by the adjacent cytidine. The slow rate of N1,O6EtG formation allows healthy cells expressing MGMT to reverse the initial O6FEtG lesion before it evolves to N1,O6EtG, thereby suppressing the formation of toxic DNA-MGMT cross-links and reducing the amount of DNA ICLs generated in healthy cells. In contrast, O6-(2-chloroethyl)guanine lesions produced by agents such as lomustine and the N3-(2-chloroethyl)imidazotetrazine mitozolomide rapidly evolve to N1,O6EtG, resulting in the formation of DNA-MGMT cross-links and DNA ICLs in healthy tissue. These studies suggest that careful consideration of the rates of chemical DNA modification and biochemical DNA repair may lead to the identification of other tumor-specific genotoxic agents.

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.

Nimustine Treatment of 11 Cases of Canine Histiocytic Sarcoma

The objective of this retrospective study was to report treatment outcomes in dogs with histiocytic sarcoma (HS) that were treated with nimustine (ACNU). This study evaluated data from 11 dogs including 5 with macroscopic tumors that were treated in the primary setting and 6 that underwent aggressive local therapy while being treated in the adjuvant setting. The median ACNU starting dose was 25 mg/m² (range, 20-30 mg/m²; 3- to 5-wk intervals, 1-8 administrations). The median overall survival in the primary and adjuvant settings was 120 days (median progression-free survival [PFS], 63 days) and 400 days (median PFS, 212 days), respectively. Neutropenia was observed in eight cases (grade 1, n = 1; grade 2, n = 2; grade 3, n = 2; grade 4, n = 3) with nadir neutrophil count at 1 wk after ACNU administration. Mild gastrointestinal toxicity (grade 1-2) was observed in three cases. ACNU was well tolerated and showed a similar outcome to that seen for lomustine, which is a drug commonly used to treat canine HS, in terms of overall survival and PFS in the current study population. Further investigations will need to be undertaken to definitively determine if ACNU is an appropriate alternative to lomustine for the treatment of HS.

Chloroethylating anticancer drug-induced mutagenesis and its repair in Escherichia coli

Background

Chloroethylnitrosourea (CENU) derivatives, such as nimustine (ACNU) and carmustine (BCNU), are employed in brain tumor chemotherapy due to their ability to cross the blood-brain barrier. They are thought to suppress tumor development through DNA chloroethylation, followed by the formation of interstrand cross-links (ICLs) that efficiently block replication and transcription. However, the alkylation of DNA and ICLs may trigger genotoxicity, leading to tumor formation as a side effect of the chemotherapeutic treatment. Although the involvement of O⁶-alkylguanine-DNA alkyltransferase (AGT) in repairing chloroethylated guanine (O⁶-chloroethylguanine) has been reported, the exact lesion responsible for the genotoxicity and the pathway responsible for repairing it remains unclear.

Results

We examined the mutations induced by ACNU and BCNU using a series of Escherichia coli strains, CC101 to CC111, in which reverse mutations due to each episome from F’101 to F’106 and frameshift mutations due to each episome from F’107 to F’111 could be detected. The mutant frequency increased in E. coli CC102, which can detect a GC to AT mutation. To determine the pathway responsible for repairing the CENU-induced lesions, we compared the frequency of mutations induced by CENU in the wild-type strain to those in the ada, ogt (AGT-deficient) strain, uvrA (nucleotide excision repair (NER)-deficient) strain, mismatch repair (MMR)-deficient strains, and recA (recombination deficient) strain of E. coli CC102. The frequencies of mutations induced by ACNU and BCNU increased in the ada, ogt strain, demonstrating that O⁶-chloroethylguanines were formed, and that a portion was repaired by AGT.

Mutation induced by ACNU in NER-deficient strain showed a similar profile to that in AGT-deficient strain, suggesting that an NER and AGT play at the similar efficacy to protect E. coli from mutation induced by ACNU. O⁶-Chloroethylguanine is reported to form ICLs if it is not repaired. We examined the survival rates and the frequencies of mutations induced by ACNU and BCNU in the uvrA strain, the recA strain, as well as a double-deficient strain of CC102. The mutation profile of the double-deficient strain was similar to that of the NER-deficient strain, suggesting that an NER protects E. coli from mutations but not recombination. In addition, cell death was more pronounced in the uvrA, recA double-deficient strain than in the single-deficient strains.

Conclusion

These results suggest that the toxic lesions induced by CENU were repaired additively or synergistically by NER and recombination. In other words, lesions, such as ICLs, appear to be repaired by NER and recombination independently.

High-dose fotemustine in temozolomide-pretreated glioblastoma multiforme patients: A phase I/II trial

BACKGROUND

Glioblastoma multiforme (GBM) is a rare and deadly disease, with a reported average incidence rate of 3.19 cases per 100,000 inhabitants. Fotemustine, a third-generation nitrosourea with an alanine phosphor carrier that facilitates cellular penetration, has been extensively investigated in the setting of recurrent/progressive disease after initial treatment. Fotemustine is usually administered following a schedule consisting of 3 doses every week, followed by maintenance doses administered every 3 weeks.

METHODS

In this phase I/II trial, we aimed to assess whether the use of a biweekly regimen allowed administration of higher dose than the standard 100 mg/m dose approved per label indication in a population of patients with recurrent GBM. In this phase I/II trial, fotemustine was administered intravenously over 1 hour every 2 weeks at either 120 or 140 mg/m doses for up to 1 year, until disease progression, unacceptable toxicity, or patient's request to withdraw from the study. The phase I part of the trial was conducted following the classic 3+3 study design. The phase II part of the trial was a single-arm study. The primary efficacy endpoint was the percentage of patients who had not progressed after 24 weeks (PFS-24).

RESULTS

Thirty-seven patients were enrolled in this phase I/II trial from August 2006 to November 2011. Treatment was well tolerated in the overall population. Main severe toxicity was grades 3 and 4 thrombocytopenia, which occurred in 4 of 6 patients treated at the 140 mg/m dose level and in 3 of 31 patients treated at 120 mg/m. Median PFS and overall survival were 12.1 (1-40.2) weeks and 19.7 (1-102) weeks, respectively.

CONCLUSION

We conclude that fotemustine can be safely administered at 120 mg/m biweekly. The efficacy of such modified schedule and doses should be compared to the biweekly schedule at 80 mg and the standard weekly schedule at 80 to 100 mg/m.

Single-Cell-Arrayed Agarose Chip for in Situ Analysis of Cytotoxicity and Genotoxicity of DNA Cross-Linking Agents

Development of approach or device to allow continuous multiple measurements, such as integrating cytotoxic and genotoxic analysis, is quite appealing for study of the drug's activity and mechanism of action or resistance. In this study, a single-cell-arrayed agarose chip system was developed to combine cell cultivation with subsequent in situ analysis of cytotoxicity and genotoxicity of the chemotherapeutic agent. The modified alkaline comet assay coupled with the Live/Dead assay was used to monitor the interstrand cross-links (ICLs) formation and the cytotoxic effects in different glioma cell lines. In addition, the ICL-induced double strand breaks (DSBs) was measured on the chip to reflect the level of ICLs indirectly. Compared with the traditional methods, the microarray agarose device offers higher throughput, reproducibility, and robustness, exhibiting good potential for high-content drug screening.