O6 Of Guanine Research Articles

MEN1 Deficiency-Driven Activation of the β-Catenin-MGMT Axis Promotes Pancreatic Neuroendocrine Tumor Growth and Confers Temozolomide Resistance.

O6-methylguanine DNA methyltransferase (MGMT) removes alkyl adducts from the guanine O6 position (O6-MG) and repairs DNA damage. High MGMT expression results in poor response to temozolomide (TMZ). However, the biological importance of MGMT and the mechanism underlying its high expression in pancreatic neuroendocrine tumors (PanNETs) remain elusive. Here, it is found that MGMT expression is highly elevated in PanNET tissues compared with paired normal tissues and negatively associated with progression-free survival (PFS) time in patients with PanNETs. Knocking out MGMT inhibits cancer cell growth in vitro and in vivo. Ectopic MEN1 expression suppresses MGMT transcription in a manner that depends on β-Catenin nuclear export and degradation. The Leucine 267 residue of MEN1 is crucial for regulating β-Catenin-MGMT axis activation and chemosensitivity to TMZ. Interference with β-Catenin re-sensitizes tumor cells to TMZ and significantly reduces the cytotoxic effects of high-dose TMZ treatment, and MGMT overexpression counteracts the effects of β-Catenin deficiency. This study reveals the biological importance of MGMT and a new mechanism by which MEN1 deficiency regulates its expression, thus providing a potential combinational strategy for treating patients with TMZ-resistant PanNETs.

Implications of Advances in Studies of O6-Methylguanine-DNA- Methyltransferase for Tumor Prognosis and Treatment.

O6-methylguanine-DNA-methyltransferase (MGMT) is a DNA repair enzyme, which reverses the alkylation of guanine O6 through directtransfer of the methyl group, maintains the gene stability and avoids tumor occurrence. Studies have shown that MGMT gene methylation, polymorphism and protein expression are involved in the process of various tumor development, such as colon cancer, gastric carcinoma, etc. MGMT gene promotes methylation, protein expression and enzyme activity from various tissues, which resultsin different effects on the prognosis of patients. MGMT promoter methylation is a positive factor for the prognosis of Glioblastoma (GBM), which can prolong overall survival and progression-free survival, reduce the resistance of tumor cells to temozolomide treatment, and improve the prognosis. The treatment of tumors based on MGMT focuses on three aspects: targeting MGMT to increase the sensitivity of alkylated drug therapy in tumors, immunotherapy combined with alkylated agents on tumor treatment, and treatment for patients with MGMT promoter non-methylation. Similarly, a number of studies have targeted MGMT to reduce alkylated agent resistance in other systems. Although numerous studies on MGMT in tumors have been reported, there are problems that need to be solved, such as selection and consensus of MGMT promoter methylation detection methods (CpG detection sites, cut-off value) and the treatment of MGMT non-methylated GBM patients, especially elderly patients. In this review, we describe the regulation of MGMT expression and its role inchemotherapy, especially in gliomas. Further studies exploring new methods targeting MGMT with better curative effect and less toxicity are advocated. We anticipate that these developments will be progressive and sufficiently used for clinical application.

Regioselective 2'-Silylation of Purine Ribonucleosides for Phosphoramidite RNA Synthesis.

This article describes high-yield procedures for protection of purine ribonucleosides based on a reaction that allows highly regioselective 2'-silylation. Each protocol makes use of two transient protection steps. In the case of tritylation of the 5' hydroxyl, the 2',3'-diol is protected by reaction with N,N-dimethylformamide dimethylacetal (Zemlicka, 1963) to prevent the small, but potentially troublesome, tritylation of the 2'-hydroxyl that otherwise accompanies tritylation of the 5'-hydroxyl (Zhang etal., 1997). The phenoxyacetylation of the amino group is carried out after transient hydroxyl and guanine O6 protection with trimethylchlorosilane using the hydroxybenzotriazole active ester of phenoxyacetic acid. These protocols give overall yields that are three times the best yields available by conventional procedures for adenosine and guanosine, but offer no advantage for cytidine or uridine. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Synthesis of 5'-O-(4,4'-dimethoxytrityl)-2'-O-tert-butyldimethylsilyl-6-N-acyladenosine Basic Protocol 2: Synthesis of 5'-O-(4,4'-dimethoxytrityl)-2'-O-tert-butyldimethysilyl-2-N-acylguanosine.

Abstract 1843: Dianhydrogalactitol (VAL-083) for the treatment of glioblastoma multiforme (GBM): Impact of glucose transporters for crossing the blood brain barrier (BBB)

Abstract Glucose transporters at the blood brain barrier (BBB) maintain the continuous high glucose and energy demands of the brain. These transporters have been targets for the creation of therapeutic molecules as a means to cross the BBB. The BBB has been a major limitation for the successful development of treatments for brain tumors. To date, only a limited number of lipophilic small chemical entities, which can cross the BBB, have been approved for the treatment of GBM. These include the nitrosoureas such as carmustine (BCNU) and lomustine (CCNU) and temozolomide (TMZ). VAL-083 is a di-epoxide substituted hexitol sugar (galactitol). It has a molecular weight of 146 and is highly water soluble. VAL-083 is active against several GBM cell lines and animal models and acts as a DNA targeting agent rapidly forming cross-links at the N7 position of guanine, resulting in lethal DNA double strand breaks and cancer cell death. Its is active in GBM patients, both newly diagnosed and recurrent. Furthermore, its activity is independent of the DNA repair enzyme MGMT which limits the effectiveness of O6 guanine alkylating agents such as the nitrosoureas and TMZ.In clinical pharmacology studies, VAL-083 rapidly accumulates in the cerebrospinal fluid (CSF) and brain tumor tissue. Studies with radiolabeled VAL-083 have shown a half-life of ~ 20 h in CSF vs 2h for plasma post-IV infusion. Recent clinical studies in newly diagnosed GBM patients demonstrated levels of VAL-083 in CSF at least as high as in plasma two hours post infusion (ratio 1.24 ± 0.35). In comparison, peak TMZ concentrations in the CSF are ~30% of that of the plasma.Due to its structural similarity with glucose, we have initiated in vitro glucose transporter studies to identify if SGLTs (sodium dependent glucose cotransporters) or GLUTs (glucose transporters) are contributing to brain penetration. Our initial studies evaluated VAL-083 as a substrate of the human SGLT1 and SGLT2 SLC (solute carrier) transporters. HEK293 cells expressing either the SGLT1 or SGLT2 transporter were exposed to VAL-083 at 3 and 15 µM for 2 and 20 min. The probe substrate methyl-α-D- glucopyranoside (AMG) was used as a positive control, as well as comparison against the reference inhibitors phloridzin dihydrate (100 µM, 20 min) for SGLT1 and dapagliflozin (0.3 µM, 10 min) for SGLT2. Surprisingly, no transporter-specific accumulation of VAL-083 by SGLT1 or SGLT2 was observed. Ongoing studies in CaCo-2 cells are investigating whether VAL-083 is transported via other glucose transporters including GLUT1. GLUT1 is expressed on erythrocytes and the endothelial cells of the BBB and may be highly relevant to the uptake of VAL-083 into brain tissue. The results of these studies will be reported at the meeting. Citation Format: John M. Langlands, Sarath Kanekal, Anne Steino, Armando Mendez, Zoltan Karman, Beata Martonne Toth, Krisztina Heredi-Szabo, Dennis Brown. Dianhydrogalactitol (VAL-083) for the treatment of glioblastoma multiforme (GBM): Impact of glucose transporters for crossing the blood brain barrier (BBB) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1843.

World Cancer Day 2021 - Perspectives in Pediatric and Adult Neuro-Oncology.

Significant advances in our understanding of the molecular genetics of pediatric and adult brain tumors and the resulting rapid expansion of clinical molecular neuropathology have led to improvements in diagnostic accuracy and identified new targets for therapy. Moreover, there have been major improvements in all facets of clinical care, including imaging, surgery, radiation and supportive care. In selected cohorts of patients, targeted and immunotherapies have resulted in improved patient outcomes. Furthermore, adaptations to clinical trial design have facilitated our study of new agents and other therapeutic innovations. However, considerable work remains to be done towards extending survival for all patients with primary brain tumors, especially children and adults with diffuse midline gliomas harboring Histone H3 K27 mutations and adults with isocitrate dehydrogenase (IDH) wild-type, O6 guanine DNA-methyltransferase gene (MGMT) promoter unmethylated high grade gliomas. In addition to improvements in therapy and care, access to the advances in technology, such as particle radiation or biologic therapy, neuroimaging and molecular diagnostics in both developing and developed countries is needed to improve the outcome of patients with brain tumors.

Effect of axial ligands on the mechanisms of action of Ru(III) complexes structurally similar to NAMI-A: a DFT study

Although the mechanisms of action of Ru(III) anticancer drugs including NAMI-A and KP1019 are not properly understood, it is reported that they are readily aquated in vivo before reacting to the biological targets and their cytotoxic activities are mainly attributed to their binding with DNA. In the present contribution in order to understand how changing the axial ligands affects the mechanisms of action of Ru(III)-based drugs which are structurally similar to NAMI-A, we study here the aquation reactions (substitution of one chloride anion (Cl−) ligand by a water molecule) of six structurally similar Ru(III) complexes (1–6) including NAMI-A (complex 1) and KP1019 (complex 2) as well as the reactions of their mono-aquated products with the N3, N7 and O6 sites of guanine at the M06-2X/(LanL2DZ+6-311+G**)//M06-2X/(LanL2DZ+6-31G**) level of density functional theory in gas phase and aqueous media. It is found that the aquation reaction of every Ru(III) complex occurs through a single transition state where a chloride ligand of the complex is replaced by a H2O molecule. The activation free energies (ΔGb) and rate constants of aquation reactions show that the complex 6 would get aquated in biological media more readily than other complexes. The rate constant of aquation of complex 6 is 7.98 × 10−3 s−1. It is evident from our calculations that axial ligands play important role in the aquation of Ru(III) complexes structurally similar to NAMI-A, but they do not obey any specific trends. Both the calculated reaction enthalpies (ΔHf) and reaction free energies (ΔGf) show that reactions of Ru(III) complexes (1–6) at the N3, N7 and O6 sites of guanine are exothermic and would occur spontaneously. It is found that KP1019 would react more favourably than NAMI-A at the N3, N7 and O6 of guanine, in agreement with experimental observations. Further, among all the complexes studied here, reactions of complex 6 at the different sites of guanine are most exothermic in aqueous media.

BCL-2 Inhibitor Venetoclax Enhances Temozolomide Sensitivity in AML

Introduction: Many older patients with acute myeloid leukemia (AML) are ineligible for intensive chemotherapy due to frailty and co-morbidities; for such patients, existing treatments are often ineffective and new treatments are needed. Temozolomide (TMZ) is an alkylating agent that causes DNA methylation at O6 guanine, generating single strand break leading to apoptosis. However, the efficacy of TMZ depends on the DNA repair protein O6-methylguanine methyltransferase (MGMT), that maintains the genomic integrity by removing the O6-methyl group and restoring guanine nucleobase, thereby enhancing resistance to TMZ. Previous clinical trials in AML found that responses to TMZ correlated with low MGMT expression; however, even in those with low MGMT expression complete response rates were only in the 25% range. BCL-2, an anti-apoptotic protein, is overexpressed in AML cells. Direct inhibition by the selective BCL-2 inhibitor venetoclax (Venet) promotes apoptosis. This study evaluated the ability of Venet to enhance TMZ sensitivity in AML cells, including those with MGMT overexpression. Methods: KG1, MV4-11 and MOLM13 AML cell lines were studied, as well as bone marrow blast cells collected from AML patients. Western blot was used to measure MGMT and BCL-2 expression. The cells were incubated with TMZ at varying concentrations in combination with a fixed concentration of Venet. After 48 hours, cell viability and apoptosis assays were performed using spectrophotometry and flow cytometry, respectively. Synergy was evaluated by the Chou-Talalay method. Cleaved-PARP was measured by Western blot in selected combination doses after 3 hours in MV4-11 and MOLM13 and after 6 hours in KG1. Results: KG1 cells expressing high MGMT demonstrated strong resistance to TMZ; however, co-incubation with 1 uM Venet resulted in a marked enhancement of sensitivity to TMZ. Similarly, in MV4-11 and MOLM13 cell lines, which demonstrated very low or absent MGMT expression. Venet 2.5 nM in combination with TMZ markedly increased the cytotoxicity to TMZ. A synergistic effect was demonstrated in all cell lines with combination index (CI) < 1. Cells overexpressed annexin V and propidium iodide (PI) apoptotic marker after drug combination in all cell lines. Apoptotic effect with the drug combination was verified by cleaved-PARP expression. Most (6/8) AML patient samples which were resistant to TMZ in vitro became sensitized to TMZ in combination with 1 uM Venet, including those with moderate to high MGMT expression. Conclusion: Venetoclax synergizes with TMZ and induces cytotoxicity in all AML cell lines and in most AML patient samples, including those in whom MGMT was highly expressed, by activating apoptotic pathways to trigger cell death. This combination represents a potentially promising new treatment. Further studies evaluating this combination in animal models are in progress. Disclosures Brandwein: Roche: Research Funding; Novartis: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding; Pfizer: Consultancy, Honoraria, Research Funding; Otsuka: Honoraria; Jazz Pharma: Consultancy, Honoraria. OffLabel Disclosure: Temozolomide for AML

DRES-09. REGULATORY EFFECTS OF THE CILIARY GTPASE ARL13B ON PURINE METABOLISM IN GBM

Abstract Glioblastoma (GBM) is the most common form of adult primary brain cancer. Despite an aggressive treatment regimen – surgical resection, irradiation, and temozolomide (TMZ) chemotherapy – patients’ prognosis is still grim. TMZ acts by methylating purines, specifically at the O6 and N7 positions of guanine, to induce cytotoxic DNA double-strand breaks. We thus wanted to explore how purine metabolism may contribute to TMZ-resistance. In mammalian cells, purine nucleotides can be recycled by the salvage pathway or generated via de novo synthesis. The salvage pathway is energetically inexpensive relative to de novo thus, highly proliferative GBM cells preferentially utilize the salvage pathway. We have shown that salvage synthesis is reduced in response to TMZ (p-value=0.0021), hinting that the cells may utilize de novo to evade therapy induced alkylation of purines. Using immunoprecipitation-mass spectroscopy analysis, we found a novel interaction between the ciliary GTPase ARL13B and IMPDH2, the rate-limiting enzyme in de novo synthesis. We have shown that this interaction, occurring at the C-terminal domain of ARL13B, plays a significant role in the regulation of purine biosynthesis as abolishing it through ARL13B knockdown reduced flux through de novo (p-value< 0.0001) synthesis as measured by the specific activity of IMPDH2. Further, the lentiviral-mediated rescue of ARL13B brings IMPDH2 activity back to basal levels (p< 0.0001). Given its canonical function as a GTPase, we hypothesize that ARL13B acts as a novel regulator of de novo synthesis by sequestering GDP, allowing IMPDH2 to sense and respond to the cytosolic levels of guanine nucleotides. Without ARL13B the de novo pathway is halted, forcing the cells to rely on salvage to replenish nucleotide pools. Reliance on this pathway in the presence of TMZ causes cells to incorporate damaged nucleotides as a result of the drug’s alkylating action leading to the increased therapeutic efficacy of TMZ.

ACTR-06. CLINICAL TRIAL OF VAL-083 IN NEWLY DIAGNOSED MGMT-UNMETHYLATED GBM: HALF-WAY REPORT

Abstract Approximately 60% of glioblastoma multiforme (GBM) patients possess an unmethylated methylguanine DNA-methyltransferase (MGMT gene, which confers a limited response to standard of care treatment with temozolomide (TMZ) resulting in a lower survival. VAL-083 is a novel bi-functional DNA targeting agent that induces interstrand cross-links at N7-guanine, leading to DNA double-strand breaks and ultimately cell death. VAL-083 circumvents MGMT-mediated repair of the O6 guanine alkylator TMZ. A Phase 2 study has been initiated for VAL-083 in newly diagnosed MGMT unmethylated GBM. The study has 2 stages: Stage 1 is a dose-escalation and induction format to confirm the recommended dose of VAL-083 when administered concurrently with radiation therapy (RT) based on safety and tolerability. The subjects received VAL-083 at 20, 30, or 40 mg/m2/day x 3 days every 21 days along with standard radiation treatment. Stage 2 comprises an expansion phase to enroll up to 30 patients. The dose escalation stage is complete and 30 mg/m2/day of VAL-083 in combination with RT was generally safe and well-tolerated. As of 17 May, 2019, 18 patients have been enrolled. Fifteen patients have completed their prospectively planned MRI scans and had their initial assessment for tumor progression. Of these 15 patients, seven were assessed as a complete response (CR), and eight patients as having stable disease (SD). Of the remaining three patients, one died prior to their post-cycle 3 MRI and two have not been on study long enough to reach their planned post-cycle 3 MRI. As of the data cutoff, 14 of the 18 patients were still alive. Consistent with our prior experience, myelosuppression was the most common adverse event. Three dose-limiting toxicities have been reported - one at the 40 mg/m2/day and two at the 30 mg/m2/day dose. Further enrollment, safety & study updates will be presented at the meeting. Clinicaltrials.gov identifier: NCT03050736.

Role of Mono- and Divalent Ions in Peptide Glu-Asp-Arg-DNA Interaction.

The interaction of the regulatory biologically active peptide Glu-Asp-Arg (EDR) with DNA is considered by spectral, NMR, viscosimetry, and molecular dynamics methods. It was shown that EDR can partly penetrate into the major groove of DNA and affect the base atoms, mainly the N7 and O6 of guanine. It was observed that Mg2+ ions can promote DNA-EDR interaction due to their effective screening of the negatively charged phosphate groups of DNA. This action of Mg2+ remains in salted solution as well.

Phase 2 clinical trial of VAL-083 as first-line treatment in newly-diagnosed MGMT-unmethylated glioblastoma multiforme (GBM): Halfway report

Background and Aim: Approximately 60% of glioblastoma multiforme (GBM) patients possess an unmethylated O-6-methylguanine-DNA methyltransferase (MGMT) gene, which confers a limited response to standard-of-care treatment with temozolomide (TMZ), resulting in a lower survival. Dianhydrogalactitol (VAL-083) is a novel bi-functional DNA-targeting agent that induces interstrand cross-links at N7-guanine, leading to DNA double-strand breaks and ultimately cell death. VAL-083 circumvents MGMT-mediated repair of the O6 guanine alkylator TMZ. A Phase 2 study has been initiated for VAL-083 in newly diagnosed MGMT unmethylated GBM. Subjects and Methods: The study has two parts: part 1 is a dose–escalation and induction format to enroll up to ten patients in which they received VAL-083 at 20, 30, or 40 mg/m2 per day for 3 days every 21 days concurrently with standard radiation treatment and VAL-083 for up to eight additional cycles. Part 2 comprises an expansion phase to enroll up to twenty additional patients. This study was performed with approval by the Institutional Review Board of Sun Yat-sen University Cancer Center (B2016-058-01) on January 13, 2017, and registered with the ClinicalTrials.gov (NCT03050736) on February 13, 2017. Results: After completion of dose escalation, VAL-083, 30 mg/m2 per day, in combination with radiation therapy, was generally safe and well tolerated. At the cutoff date, 23 patients had been enrolled, 14 of whom had been treated in the expansion phase. Consistent with prior studies, myelosuppression was the most common adverse event. Pharmacokinetic assessment indicated that the levels of VAL-083 were as high in the cerebrospinal fluid as in plasma, 2 h postinfusion. Of the 22 patients who had reached their four precycle magnetic resonance imaging assessments, 12 were assessed with disease progression, with a median progression-free survival of 9.9 (95% confidence interval 7.3–12.0) months for all the patients studied. Conclusion: These preliminary data support VAL-083 as a potentially valuable treatment option for newly diagnosed GBM.

A DFT study of both the hydrolytic degradation and protonation of semustine in variation conditions of pH and interaction of drug with DNA nucleobases

Semustine as an anti-cancer medicine is decomposed by hydrolysis under pH-dependent conditions, forming chlorocarbonium ion and interaction with nucleic acid molecules. This medicine can prevent the growth of cancer cells. In this study, the hydrolysis of medicine under various conditions is investigated, to find out the energy differences and the effects of an acidic condition on its activation energy. The mechanisms of protonation, degradation, and interaction with DNA bases were treated using density functional theory (B3LYP) and by utilizing the complete basis set 6–31 + G(d) method in gas phase. Also, in order to clarify the role of the acidic conditions on the increased stability and mechanism of the drug, the protonation of all possible sites was investigated. The obtained results depicted that the protonation of the oxygen of the carbonyl group (O1) is the preferable site for protonation that would stabilize the system by about 20.27 kcal mol−1 more than the next favorable protonation site (N1). The comparison of the hydrolysis rates indicates that the tendency to hydrolysis of medicine under acidic conditions increases by protonation of O1 site. While the rate of degradation enhances by decreasing of pH, it never comes to a complete halt under basic conditions. In order to investigate the medicine interaction with the bases of DNA profile, the mechanism of chlorocarbonium ion interaction was examined with the bases of DNA. The outcome indicates that the values of energies for O6 and N7 of guanine are − 70.26 kcal mol−1 and − 55.19 kcal mol−1, respectively, and probably the drug exerts its anti-cancer effect through interaction with these sites.

TLR8 activation and inhibition by guanosine analogs in RNA: Importance of functional groups and chain length

Toll-like receptor 8 (TLR8) is an important component of the human innate immune system that recognizes single stranded RNA (ssRNA). Recent X-ray crystal structures of TLR8 bound to ssRNA revealed a previously unrecognized binding site for a 5′-UpG-3′ dinucleotide. Here we use an atomic mutagenesis strategy coupled with a cellular TLR8 activation assay to probe the importance of specific functional groups present on the guanine base in RNA-mediated receptor agonism and antagonism. Results from RNA analogs containing 7-deazaguanosine, 2-aminopurine and inosine confirm the importance of guanine N7, O6 and N2, respectively, in TLR8 activation. Nevertheless, these RNAs each retained TLR8 antagonism activity. RNA containing 7-deaza-8-azainosine (7d8aI) was prepared from a novel phosphoramidite and found to be a weaker TLR8 activator than guanosine-containing RNA. However, 7d8aI-containing RNA also retained TLR8 antagonism activity indicating that removal of multiple TLR8 H-bonding sites on guanine is insufficient for blocking TLR8 antagonism by guanine-containing RNA. We also identified an oligoribonucleotide length dependence on both TLR8 activation and antagonism. These studies extend our understanding of the effects of nucleobase modification on immune stimulation and will inform the design of novel RNA-based therapeutics.

The effect of S-substitution at the O6-guanine site on the structure and dynamics of a DNA oligomer containing a G:T mismatch.

The effect of S-substitution on the O6 guanine site of a 13-mer DNA duplex containing a G:T mismatch is studied using molecular dynamics. The structure, dynamic evolution and hydration of the S-substituted duplex are compared with those of a normal duplex, a duplex with S-substitution on guanine, but no mismatch and a duplex with just a G:T mismatch. The S-substituted mismatch leads to cell death rather than repair. One suggestion is that the G:T mismatch recognition protein recognises the S-substituted mismatch (GS:T) as G:T. This leads to a cycle of futile repair ending in DNA breakage and cell death. We find that some structural features of the helix are similar for the duplex with the G:T mismatch and that with the S-substituted mismatch, but differ from the normal duplex, notably the helical twist. These differences arise from the change in the hydrogen-bonding pattern of the base pair. However a marked feature of the S-substituted G:T mismatch duplex is a very large opening. This showed considerable variability. It is suggested that this enlarged opening would lend support to an alternative model of cell death in which the mismatch protein attaches to thioguanine and activates downstream damage-response pathways. Attack on the sulphur by reactive oxygen species, also leading to cell death, would also be aided by the large, variable opening.

Abstract 2109: Transcriptional analysis of human leukemia cells treated with the experimental anticancer drug laromustine

Abstract Laromustine is an experimental anticancer prodrug that demonstrated significant activity against acute myeloid leukemia in human clinical trials. Upon base-catalyzed activation, laromustine yields two reactive, electrophilic species in situ: 90CE, which can chloroethylate the O6 position of guanine in DNA causing lethal interstrand crosslinks, and methyl isocyanate, which can carbamoylate biochemically relevant nucleophiles, such as cysteine sulfhydryl groups. Previous in vitro and ex vivo investigations into laromustine’s molecular mechanism of action reveal several biochemical consequences of methyl isocyanate, many of which may explain either the acute cytotoxicity of the drug’s carbamoylating activity or the observed synergism with the cytotoxicity of guanine O6 chloroethylation. The combined activities of the electrophilic subspecies of laromustine are believed to induce apoptosis in leukemia cells. Presented herein are extensive flow cytometry experiments using FITC-AnnexinV and propidium iodide suggesting a dose-dependent cytotoxicity that includes apoptotosis. So as to gain a more complete understanding of the drug’s toxicity, an investigation into how laromustine modulates gene transcription in HL60 cells is also carried out and reported here. Total mRNA was harvested from cultured HL60 cells treated with sub-lethal doses of laromustine or analogs lacking either carbamoylating or chloroethylating activity. These samples were subjected to GeneChip analyses against control mRNA. More than 4,000 genes were significantly dysregulated upon the cells’ exposure to laromustine. The analog of laromustine possessing only carbamoylating activity caused the dysregulation of nearly 3,000 genes, the vast majority of which were also dysregulated in response to laromustine. 90CE, which lacks carbamoylating activity but retains chloroethylating activity, understood to be principally responsible for therapeutic cytotoxicity, demonstrated very little effect on gene transcription in HL60 cells. Preliminary analysis of gene pathways affected by laromustine suggests that among the altered pathways, those associated with cancer progression, the G1/S checkpoint, and hematological disorders are particularly stressed. The carbamoylating activity of laromustine is responsible for most of these effects. Identifying genetic pathways disrupted by this potentially useful drug may inform clinical strategies or identify potential targets for co-therapeutic agents. Citation Format: Amanda J. Loya, Dylan J. Cincotta, Abdel Elkahloun, David Bodine, Kevin P. Rice. Transcriptional analysis of human leukemia cells treated with the experimental anticancer drug laromustine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2109. doi:10.1158/1538-7445.AM2017-2109

Gas-Phase Conformations and N-Glycosidic Bond Stabilities of Sodium Cationized 2'-Deoxyguanosine and Guanosine: Sodium Cations Preferentially Bind to the Guanine Residue.

2'-Deoxyguanosine (dGuo) and guanosine (Guo) are fundamental building blocks of DNA and RNA nucleic acids. In order to understand the effects of sodium cationization on the gas-phase conformations and stabilities of dGuo and Guo, infrared multiple photon dissociation (IRMPD) action spectroscopy experiments and complementary electronic structure calculations are performed. The measured IRMPD spectra of [dGuo+Na]+ and [Guo+Na]+ are compared to calculated IR spectra predicted for the stable low-energy structures computed for these species to determine the most favorable sodium cation binding sites, identify the structures populated in the experiments, and elucidate the influence of the 2'-hydroxyl substituent on the structures and IRMPD spectral features. These results are compared with those from a previous IRMPD study of the protonated guanine nucleosides to elucidate the differences between sodium cationization and protonation on structure. Energy-resolved collision-induced dissociation (ER-CID) experiments and survival yield analyses of protonated and sodium cationized dGuo and Guo are performed to compare the effects of these cations toward activating the N-glycosidic bonds of these nucleosides. For both [dGuo+Na]+ and [Guo+Na]+, the gas-phase structures populated in the experiments are found to involve bidentate binding of the sodium cation to the O6 and N7 atoms of guanine, forming a 5-membered chelation ring, with guanine found in both anti and syn orientations and C2'-endo (2T3 or 3T2) puckering of the sugar. The ER-CID results, IRMPD yields and the computed C1'-N9 bond lengths indicate that sodium cationization activates the N-glycosidic bond less effectively than protonation for both dGuo and Guo. The 2'-hydroxyl substituent of Guo is found to impact the preferred structures very little except that it enables a 2'OH···3'OH hydrogen bond to be formed, and stabilizes the N-glycosidic bond relative to that of dGuo in both the sodium cationized and protonated complexes.

Do Solvated Electrons (e(aq)⁻) Reduce DNA Bases? A Gaussian 4 and Density Functional Theory-Molecular Dynamics Study.

The solvated electron (e(aq)⁻) is a primary intermediate after an ionization event that produces reductive DNA damage. Accurate standard redox potentials (E(o)) of nucleobases and of e(aq)⁻ determine the extent of reaction of e(aq)⁻ with nucleobases. In this work, E(o) values of e(aq)⁻ and of nucleobases have been calculated employing the accurate ab initio Gaussian 4 theory including the polarizable continuum model (PCM). The Gaussian 4-calculated E(o) of e(aq)⁻ (-2.86 V) is in excellent agreement with the experimental one (-2.87 V). The Gaussian 4-calculated E(o) of nucleobases in dimethylformamide (DMF) lie in the range (-2.36 V to -2.86 V); they are in reasonable agreement with the experimental E(o) in DMF and have a mean unsigned error (MUE) = 0.22 V. However, inclusion of specific water molecules reduces this error significantly (MUE = 0.07). With the use of a model of e(aq)⁻ nucleobase complex with six water molecules, the reaction of e(aq)⁻ with the adjacent nucleobase is investigated using approximate ab initio molecular dynamics (MD) simulations including PCM. Our MD simulations show that e(aq)⁻ transfers to uracil, thymine, cytosine, and adenine, within 10 to 120 fs and e(aq)⁻ reacts with guanine only when a water molecule forms a hydrogen bond to O6 of guanine which stabilizes the anion radical.

Guanine nucleobase adducts formed by a monofunctional complex: [Pt(N-(6-methyl-2-picolyl)-N-(2-picolyl)amine)Cl]Cl

Monofunctional Pt(II) complexes bind to G residues in DNA and, if the carrier ligands are bulky, cause DNA structural distortions that lead to anticancer activity. We assessed the steric effects of the tridentate carrier ligand, N(H)6-Medpa (N-(6-methyl-2-picolyl)-N-(2-picolyl)amine), bearing a 6-methyl group and a 6′-proton projecting toward the nucleobase in Pt(N(H)6-Medpa)G adducts (G=9-ethylguanine, 3′-GMP, 5′-GMP, 5′-GTP). Pt(N(H)6-Medpa)G adducts form syn and anti rotamers with the guanine O6 and the central N–H of N(H)6-Medpa on the same or opposite side of the coordination plane, respectively. Pt(N(H)6-Medpa)G adducts have some properties (ease of rotamer interchange and extent of conversion to bis adducts, Pt(N(H)6-Medpa)G2) intermediate to properties reported for analogs having a tridentate ligand with zero or two methyl groups. However, in comparison, the syn rotamer of Pt(N(H)6-Medpa)G adducts has an unexpectedly high abundance. This result is attributable to guanine base canting, such that the 6-membered guanine ring is positioned away from the bulky 6-Me group. This canting both relieves electrostatic repulsion between the partially positive H6′ and the guanine H8 protons and creates a favorable electrostatic attraction between the H6′ proton and the partially negative guanine O6. This combined information provides insight useful for designing monofunctional anticancer agents.

DNA3'pp5'G de-capping activity of aprataxin: effect of cap nucleoside analogs and structural basis for guanosine recognition.

DNA3′pp5′G caps synthesized by the 3′-PO4/5′-OH ligase RtcB have a strong impact on enzymatic reactions at DNA 3′-OH ends. Aprataxin, an enzyme that repairs A5′pp5′DNA ends formed during abortive ligation by classic 3′-OH/5′-PO4 ligases, is also a DNA 3′ de-capping enzyme, converting DNAppG to DNA3′p and GMP. By taking advantage of RtcB's ability to utilize certain GTP analogs to synthesize DNAppN caps, we show that aprataxin hydrolyzes inosine and 6-O-methylguanosine caps, but is not adept at removing a deoxyguanosine cap. We report a 1.5 Å crystal structure of aprataxin in a complex with GMP, which reveals that: (i) GMP binds at the same position and in the same anti nucleoside conformation as AMP; and (ii) aprataxin makes more extensive nucleobase contacts with guanine than with adenine, via a hydrogen bonding network to the guanine O6, N1, N2 base edge. Alanine mutations of catalytic residues His147 and His149 abolish DNAppG de-capping activity, suggesting that the 3′ de-guanylylation and 5′ de-adenylylation reactions follow the same pathway of nucleotidyl transfer through a covalent aprataxin-(His147)–NMP intermediate. Alanine mutation of Asp63, which coordinates the guanosine ribose hydroxyls, impairs DNAppG de-capping.

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