O6-Benzylguanine potentiates BCNU but not busulfan toxicity in hematopoietic stem cells

DNA Adducts in Cancer Chemotherapy

DNA adducting drugs, including alkylating agents and platinum-containing drugs, are prominent in cancer chemotherapy. Their mechanisms of action involve direct interaction with DNA, resulting in the formation of DNA addition products known as DNA adducts. While these adducts are well-accepted to induce cancer cell death, understanding of their specific chemotypes and their role in drug therapy response remain limited. This perspective aims to address this gap by investigating the metabolic activation and chemical characterization of DNA adducts formed by the U.S. FDA-approved drugs. Moreover, clinical studies on DNA adducts as potential biomarkers for predicting patient responses to drug efficacy are examined. The overarching goal is to engage the interest of medicinal chemists and stimulate further research into the use of DNA adducts as biomarkers for guiding personalized cancer treatment.

Personalized prediction of overall survival in patients with AML in non-complete remission undergoing allo-HCT

Allogenic hematopoietic stem cell transplantation (allo‐HCT) is the standard treatment for acute myeloid leukemia (AML) in non‐complete remission (non‐CR); however, the prognosis is inconsistent. This study aimed to develop and validate nomograms and a web application to predict the overall survival (OS) of patients with non‐CR AML undergoing allo‐HCT (cord blood transplantation [CBT], bone marrow transplantation [BMT], and peripheral blood stem cell transplantation [PBSCT]). Data from 3052 patients were analyzed to construct and validate the prognostic models. The common significant prognostic factors among patients undergoing allo‐HCT were age, performance status, percentage of peripheral blasts, cytogenetic risk, chemotherapy response, and number of transplantations. The conditioning regimen was a significant prognostic factor only in patients undergoing CBT. Compared with cyclophosphamide/total body irradiation, a conditioning regimen of ≥3 drugs, including fludarabine, with CBT exhibited the lowest hazard ratio for mortality (0.384; 95% CI, 0.266–0.554; p < 0.0001). A conditioning regimen of ≥3 drugs with CBT also showed the best leukemia‐free survival among all conditioning regimens. Based on the results of the multivariable analysis, we developed prognostic models showing adequate calibration and discrimination (the c‐indices for CBT, BMT, and PBSCT were 0.648, 0.600, and 0.658, respectively). Our prognostic models can help in assessing individual risks and designing future clinical studies. Furthermore, our study indicates the effectiveness of multi‐drug conditioning regimens in patients undergoing CBT.

MGMT inhibition suppresses survivin expression in pancreatic cancer

OBJECTIVES

Survivin, an antiapoptotic gene inhibited by p53, is overexpressed in human cancers and correlates with chemotherapy resistance. Here, we investigated the mutual regulatory mechanism between MGMT (O-methylguanine DNA methyltransferase) and survivin.

METHODS

This study used standard techniques for protein and messenger RNA levels, promoter activity, protein-DNA interaction, cell viability, and correlative animal model.

RESULTS

O-benzylguanine (BG), a potent inhibitor of MGMT (a DNA repair protein), curtails the expression of survivin in pancreatic cancer. Silencing MGMT by small interfering RNA down-regulates survivin transcription. p53 inhibition enhances MGMT and survivin expressions. When p53 was silenced, BG-induced MGMT inhibition was not associated with the down-regulation of survivin, underscoring the regulatory role of p53 in the MGMT-survivin axis. O-benzylguanine inhibits survivin and PCNA (proliferating cell nuclear antigen) at messenger RNA and protein levels in PANC-1 and L3.6pl cells and decreases survivin promoter activity via increased p53 recruitment to the survivin promoter. In orthotopic pancreatic xenografts established in nude mice, BG ± gemcitabine (GEM) decrease survivin expression in tumor tissue; protein levels and immunohistochemistry show significant decrease in survivin and PCNA levels, which correlate with increased sensitivity to GEM.

CONCLUSIONS

MGMT inhibition is associated with decrease in survivin expression and increase in sensitivity to GEM in pancreatic cancer.

In vivo selection of autologous MGMT gene-modified cells following reduced-intensity conditioning with BCNU and temozolomide in the dog model

Chemotherapy with 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU) and temozolomide (TMZ) is commonly used for the treatment of glioblastoma multiforme (GBM) and other cancers. In preparation for a clinical gene therapy study in patients with glioblastoma, we wished to study whether these reagents could be used as a reduced-intensity conditioning regimen for autologous transplantation of gene-modified cells. We used an MGMT(P140K)-expressing lentivirus vector to modify dog CD34(+) cells and tested in four dogs whether these autologous cells engraft and provide chemoprotection after transplantation. Treatment with O(6)-benzylguanine (O6BG)/TMZ after transplantation resulted in gene marking levels up to 75%, without significant hematopoietic cytopenia, which is consistent with hematopoietic chemoprotection. Retrovirus integration analysis showed that multiple clones contribute to hematopoiesis. These studies demonstrate the ability to achieve stable engraftment of MGMT(P140K)-modified autologous hematopoietic stem cells (HSCs) after a novel reduced-intensity conditioning protocol using a combination of BCNU and TMZ. Furthermore, we show that MGMT(P140K)-HSC engraftment provides chemoprotection during TMZ dose escalation. Clinically, chemoconditioning with BCNU and TMZ should facilitate engraftment of MGMT(P140K)-modified cells while providing antitumor activity for patients with poor prognosis glioblastoma or alkylating agent-sensitive tumors, thereby supporting dose-intensified chemotherapy regimens.

Bone marrow transplantation in mice as a tool for studying the role of hematopoietic cells in metabolic and cardiovascular diseases

Hematopoietic cells have been established as major players in cardiovascular disease, with an important role in the etiology of atherosclerotic plaque. In addition, hematopoietic cells, and in particular the cells of monocyte and macrophage lineages, have recently been unmasked as one of the main causes of metabolic abnormalities leading to insulin resistance and type 2 diabetes. With the availability of transgenic mouse models that reproduce many aspects of these diseases, research in these areas has been able to make exceptional progress. Much of the work exploring the role of hematopoietic cells has been carried out on chimeric mice made by the recipient disease model mice being given donor bone marrow cells from transgenic mice harboring a genetic alteration in a relevant pathway. Here, we will describe the potential of the bone marrow transplantation approach and discuss recent developments, including the use of virally transduced cells. We will explain some of the caveats, their effect on the experimental outcomes, and some possible solutions. Taken as a whole, this technology offers great advantages in efficiency and cost-effectiveness, and it is expected to continue to be a crucial technique in cardiovascular research work.

Repair of O6-G-alkyl-O6-G interstrand cross-links by human O6-alkylguanine-DNA alkyltransferase

O (6)-Alkylguanine-DNA alkyltransferase (AGT) plays an important role by protecting cells from alkylating agents. This reduces the frequency of carcinogenesis and mutagenesis initiated by such agents, but AGT also provides a major resistance mechanism to some chemotherapeutic drugs. To improve our understanding of the AGT-mediated repair reaction and our understanding of the spectrum of repairable damage, we have studied the ability of AGT to repair interstrand cross-link DNA damage where the two DNA strands are joined via the guanine- O (6) in each strand. An oligodeoxyribonucleotide containing a heptane cross-link was repaired with initial formation of an AGT-oligo complex and further reaction of a second AGT molecule yielding a hAGT dimer and free oligo. However, an oligodeoxyribonucleotide with a butane cross-link was a very poor substrate for AGT-mediated repair, and only the first reaction that forms an AGT-oligo complex could be detected. Models of the reaction of these substrates in the AGT active site show that the DNA duplex is forced apart locally to repair the first guanine. This reaction is greatly hindered with the butane cross-link, which is mostly buried in the active site pocket and limited in conformational flexibility. This limitation also prevents the adoption of a conformation for the second reaction to repair the AGT-oligo complex. These results are consistent with the postulated mechanism of AGT repair that involves DNA binding and flipping of the substrate nucleotide and indicate that hAGT can repair some types of interstrand cross-link damage.

Different behavior of SCE-eliciting lesions induced by low and high doses of busulfan

Our previous studies suggested a dose-dependent transition in the types of DNA lesions induced by busulfan, as measured using the comet assay and by micronuclei analyses. The aim of the present study was to investigate the dose-dependent induction of different sister-chromatid exchange-eliciting lesions; lesions were distinguished by their efficiency in producing sister-chromatid exchange (SCE), and by their reparability during G1. Synchronously dividing murine salivary gland cells were assayed in vivo. Groups of mice were intraperitoneally injected with either 30 or 80 micromol busulfan/kg body weight solution at early or late G1. The rate of SCE/micromol busulfan/kg body weight obtained by exposure at late G1 with the high dose was twice that of the low dose. SCE induction during early G1 was higher than at late G1 with both doses; only the low-dose response was statistically significant. The frequency distribution of SCEs per cell demonstrated that cells exposed at the late G1 phase showed typical profiles that closely fit a Gaussian curve. However, an irregular profile was obtained for cells treated during early G1, which showed some cells with high-SCE frequency. Cells treated in early G1 have more time to repair lesions before DNA synthesis; therefore, the results suggest that instead of repair, secondary SCE-eliciting lesions during G1 were produced, especially at the lower dose. The results obtained in this study indicate that there are dose-dependent differences in the types of SCE-eliciting lesions induced by busulfan.

Kinetic of genotoxic expression in the pharmacodynamics of busulfan

BACKGROUND

Busulfan (BUS) is a highly toxic antineoplastic bifunctional-alkylating agent and has a narrow therapeutic window. Our previous study revealed a narrow dose-range of BUS, which causes a sudden dose-dependent transition from early- to late-expressing micronucleus induction and from a non-cytotoxic to a cytotoxic effect. In the present study, the kinetics of DNA-damaged cell induction by BUS and its dose-effect relationship were established.

METHODS

This was achieved by using the kinetics of DNA-damaged cell induction, determined by the comet assay in murine peripheral blood leukocytes of mice, after the intraperitoneal exposure to 16, 30, 45, 60 or 80 micromol/kg of BUS.

RESULTS

Doses of 15 or 30 micromol/kg of BUS were able to increase DNA-damaged cell frequency, but doses of 45 micromol/kg body weight or higher caused a sudden drop in this frequency.

CONCLUSIONS

This suggests that higher doses cause lesions that inhibit the expression of damage as comets, i.e., DNA-protein or interstrand crosslinks. The latter could be explained by sudden monoadduct-to-crosslink transformation due to a DNA conformational change induced by monoadduct accumulation that facilitates crosslink formation. This narrow dose-dependent transition could contribute to the narrow therapeutic window of BUS.

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.

Phase 1 study of 28-day, low-dose temozolomide and BCNU in the treatment of malignant gliomas after radiation therapy

We conducted a study to determine the dose-limiting toxicity of an extended dosing schedule of temozolomide (TMZ) when used with a fixed dose of BCNU, or 1,3-bis(2-chloroethyl)-1-nitrosourea (carmustine), taking advantage of TMZ's ability to deplete O6-alkylguanine-DNA-alkyltransferase and the synergistic activity of these two agents. Patients with malignant gliomas who had undergone radiation therapy were eligible. Patients were treated with TMZ for 28 days, followed by a 28-day rest (1 cycle). The TMZ was started at 50 mg/m2 and increased in 10-mg/m2 increments; a fixed dose of BCNU (150 mg/m2) was given within 72 h of starting TMZ. A standard phase 1 dose-escalation scheme was used with 3 patients per cohort. Fourteen glioblastoma patients and 10 anaplastic astrocytoma patients were treated. The dose-limiting toxicity was myelosuppression at 90 mg/m2 of TMZ. The total number of cycles given was 73 (median number was 2). Six patients (25%) required a dose reduction in BCNU, and six were removed from study for hematologic toxicity after cycle 1; three patients overlapped. The median time to progression and overall survival were, respectively, 82 and 132 weeks for anaplastic astrocytomas and 14 and 69 weeks for glioblastomas. We conclude that the combination of BCNU and the extended dosing schedule of TMZ is feasible and that the maximal tolerated dose of a 28-day course of TMZ is 80 mg/m2 when combined with a fixed dose of BCNU at 150 mg/m2. This is the recommended dose for phase 2, but myelosuppression after cycle 1 suggests that long-term treatment may be difficult.

DNA intrastrand cross-link at the 5'-GA-3' sequence formed by busulfan and its role in the cytotoxic effect

Busulfan (1,4-butanediol dimethanesulfonate) has been used widely for the treatment of patients with chronic myelogenous leukemia. Busulfan is bifunctional and thus may effectively induce DNA damage, which may play an important role in the cytotoxicity. In this study, we compared the cytotoxicity of bifunctional busulfan with that of monofunctional ethyl methanesulfonate (EMS) in human promyelocytic leukemia HL-60 cells. Busulfan showed a significant inhibitory effect on cell growth, whereas the cells grew in the presence of EMS. To clarify the mechanism of cytotoxicity of busulfan, we investigated DNA damage induced by busulfan using 32P-5'-end-labeled DNA fragments obtained from the human p16 tumor suppressor gene. Busulfan induced DNA damage dose-dependently, whereas EMS caused little DNA damage. DNA-sequencing experiments using piperidine and 3-methyladenine DNA glycosylase indicated that busulfan caused double-base lesions mainly at 5'-GA-3' and, to a lesser extent, at 5'-GG-3' sequences. Time of flight mass spectrometry confirmed that busulfan forms an intrastrand cross-link at the 5'-GA-3' sequence, in addition to mono-alkylation. The mechanism and the role of cross-linking at the 5'-GA-3' sequence are discussed in relation to the cytotoxicity induced by busulfan.

Cytotoxicity, DNA damage, and apoptosis induced by new fotemustine analogs on human melanoma cells in relation to O6-methylguanine DNA-methyltransferase expression

Fotemustine is a third generation chloroethylnitrosourea that has demonstrated significant antitumoral effects in malignant melanoma. However, its use is somewhat limited by its toxic side effects and chemoresistance caused by direct repair of O6-alkyl groups by the enzyme O6-methylguanine DNA-methyltransferase (MGMT). The aim of this work was to determine to what extent the expression of MGMT influences cytotoxicity, DNA damage, and apoptosis induced by new nitrososulfamide analogs of fotemustine (compounds 4 and 8), which have previously demonstrated interesting antiproliferative properties. We carried out complementary strategies that consisted of MGMT cDNA transfection in CAL77 Mer- melanoma cells and of MGMT inhibition with O6-benzylguanine (BG) in A375 Mer+ melanoma cells. MGMT-transfected cells were 7 to 9 times less sensitive to fotemustine than parent cells, whereas no difference between the transfected and parent cells was observed for nitrososulfamide analogs. The cytotoxicity of these analogs vis à vis a MGMT-proficient A375 melanoma cell line was approximately 3 times greater than that of fotemustine. Coincubation of these cells with O6-benzylguanine significantly increased the cytotoxicity of fotemustine and compound 8, whereas BG had little effect on the cytotoxicity of compound 4. Furthermore, DNA fragmentation determined by a comet assay was greater with nitrososulfamide analogs than with fotemustine. O6-benzylguanine increased DNA fragmentation for fotemustine and compound 8, but not for compound 4, which induced comets with a typical apoptotic appearance. The ability of this compound to induce apoptosis in the absence of BG was confirmed by a specific enzyme-linked immunosorbent assay apoptotic assay using a single-stranded DNA monoclonal antibody.