O 6 -Alkylguanine-DNA Alkyltransferase: A Target for the Modulation of Drug Resistance

The power and the promise of synthetic lethality for clinical application in cancer treatment

Synthetic lethality is a phenomenon wherein the simultaneous deficiency of two or more genes results in cell death, while the deficiency of any individual gene does not lead to cell death. In recent years, synthetic lethality has emerged as a significant topic in the field of targeted cancer therapy, with certain drugs based on this concept exhibiting promising outcomes in clinical trials. Nevertheless, the presence of tumor heterogeneity and the intricate DNA repair mechanisms pose challenges to the effective implementation of synthetic lethality. This review aims to explore the concepts, development, and ethical quandaries surrounding synthetic lethality. Additionally, it will provide an in-depth analysis of the clinical application and underlying mechanism of synthetic lethality.

Cell Models for Birth Defects Caused by Chloroethyl Nitrosourea-Induced DNA Lesions

ABSTRACT

Birth defects have been linked to administration of alkylating agents during pregnancy. The anti-tumor efficacy of alkylating agents correlate with their ability to induce DNA lesions, especially interstrand crosslinks (ICLs). Yet the role of DNA damages in birth defects remains to be clarified, owing, in part, to a lack of cell models. Here we generate DNA lesions in NIH/3T3 cells to mimic defects in fetus triggered by 3-Bis(2-chloroethyl)-1-nitrosourea (BCNU, carmustine). CCK-8 assay suggests that BCNU-induced cell death was dose-dependent. Alkaline comet tests and γ-H2AX staining confirm DNA ICLs and other forms of DNA damages caused by BCNUs. The cell cycle analysis shows cells arrest in G2/M phase until crosslinks repair is complete. Taken together, all these experiments demonstrate we have successfully established normal cell models for birth defects caused by BCNU-mediated DNA damages. The model can not only guide the development of effective and low-toxicity anticancer drugs, but also be of great significance for the study of neonatal malformation triggered by BCNUs.

A Drug Screening Pipeline Using 2D and 3D Patient-Derived In Vitro Models for Pre-Clinical Analysis of Therapy Response in Glioblastoma

Glioblastoma is one of the most common and lethal types of primary brain tumor. Despite aggressive treatment with chemotherapy and radiotherapy, tumor recurrence within 6-9 months is common. To overcome this, more effective therapies targeting cancer cell stemness, invasion, metabolism, cell death resistance and the interactions of tumor cells with their surrounding microenvironment are required. In this study, we performed a systematic review of the molecular mechanisms that drive glioblastoma progression, which led to the identification of 65 drugs/inhibitors that we screened for their efficacy to kill patient-derived glioma stem cells in two dimensional (2D) cultures and patient-derived three dimensional (3D) glioblastoma explant organoids (GBOs). From the screening, we found a group of drugs that presented different selectivity on different patient-derived in vitro models. Moreover, we found that Costunolide, a TERT inhibitor, was effective in reducing the cell viability in vitro of both primary tumor models as well as tumor models pre-treated with chemotherapy and radiotherapy. These results present a novel workflow for screening a relatively large groups of drugs, whose results could lead to the identification of more personalized and effective treatment for recurrent glioblastoma.

Hypoxia-targeted drug delivery

Hypoxia is a state of low oxygen tension found in numerous solid tumours. It is typically associated with abnormal vasculature, which results in a reduced supply of oxygen and nutrients, as well as impaired delivery of drugs. The hypoxic nature of tumours often leads to the development of localized heterogeneous environments characterized by variable oxygen concentrations, relatively low pH, and increased levels of reactive oxygen species (ROS). The hypoxic heterogeneity promotes tumour invasiveness, metastasis, angiogenesis, and an increase in multidrug-resistant proteins. These factors decrease the therapeutic efficacy of anticancer drugs and can provide a barrier to advancing drug leads beyond the early stages of preclinical development. This review highlights various hypoxia-targeted and activated design strategies for the formulation of drugs or prodrugs and their mechanism of action for tumour diagnosis and treatment.

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

Chloroethylnitrosoureas (CENUs), which are bifunctional alkylating agents widely used in the clinical treatment of cancer, exert anticancer activity by inducing crosslink within a guanine-cytosine DNA base pair. However, the formation of dG-dC crosslinks can be prevented by O⁶-alkylguanine-DNA alkyltransferase (AGT), ultimately leading to drug resistance. Therefore, the level of AGT expression is related to the formation of dG-dC crosslinks and the sensitivity of cells to CENUs. In this work, we determined the CENU-induced dG-dC crosslink in mouse L1210 leukemia cells and in human glioblastoma cells (SF-763, SF-767 and SF-126) containing different levels of AGT using high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry. The results indicate that nimustine (ACNU) induced more dG-dC crosslinks in L1210 leukemia cells than those induced by carmustine (BCNU), lomustine (CCNU) and fotemustine (FTMS). This result was consistent with a previously reported cohort study, which demonstrated that ACNU had a better survival gain than BCNU, CCNU and FTMS for patients with high-grade glioma. Moreover, we compared the crosslinking levels and the cytotoxicity in SF-763, SF-767 and SF-126 cells with different AGT expression levels after exposure to ACNU. The levels of dG-dC crosslink in SF-126 cells (low AGT expression) were significantly higher than those in SF-767 (medium AGT expression) and SF-763 (high AGT expression) cells at each time point. Correspondingly, the cytotoxicity of SF-126 was the highest followed by SF-767 and SF-763. The results obtained in this work provided unequivocal evidence for drug resistance to CENUs induced by AGT-mediated repair of DNA ICLs. We postulate that the level of dG-dC crosslink has the potential to be employed as a biomarker for estimating drug resistance and anticancer efficiencies of novel CENU chemotherapies.

Targeting microtubules by natural agents for cancer therapy

Natural compounds that target microtubules and disrupt the normal function of the mitotic spindle have proven to be one of the best classes of cancer chemotherapeutic drugs available in clinics to date. There is increasing evidence showing that even minor alteration of microtubule dynamics can engage the spindle checkpoint, arresting cell-cycle progression at mitosis and subsequently leading to cell death. Our improved understanding of tumor biology and our continued appreciation for what the microtubule targeting agents (MTAs) can do have helped pave the way for a new era in the treatment of cancer. The effectiveness of these agents for cancer therapy has been impaired, however, by various side effects and drug resistance. Several new MTAs have shown potent activity against the proliferation of various cancer cells, including resistance to the existing MTAs. Sustained investigation of the mechanisms of action of MTAs, development and discovery of new drugs, and exploring new treatment strategies that reduce side effects and circumvent drug resistance could provide more effective therapeutic options for patients with cancer. This review focuses on the successful cancer chemotherapy from natural compounds in clinical settings and the challenges that may abort their usefulness.

Molecular status of pituitary carcinoma and atypical adenoma that contributes the effectiveness of temozolomide

There have been several reports of temozolomide (TMZ) treatment of pituitary carcinomas and atypical adenomas. O(6)-methyl-guanine-DNA methyltransferase is not the sole molecule determining the sensitivity to TMZ in pituitary carcinomas and atypical adenomas. The Japan Society of Hypothalamic and Pituitary Tumors study suggests that MSH6, one of mismatch repair pathway enzyme, fulfills a contributory role to the efficacy of TMZ treatment for pituitary carcinomas and atypical adenomas. The preserved MSH6 function might be essential for the responsiveness to TMZ treatment in pituitary carcinomas and atypical adenomas.

4-nitrobenzyloxycarbonyl derivatives of O(6)-benzylguanine as hypoxia-activated prodrug inhibitors of O(6)-alkylguanine-DNA alkyltransferase (AGT), which produces resistance to agents targeting the O-6 position of DNA guanine

A series of 4-nitrobenzyloxycarbonyl prodrug derivatives of O(6)-benzylguanine (O(6)-BG), conceived as prodrugs of O(6)-BG, an inhibitor of the resistance protein O(6)-alkylguanine-DNA alkyltransferase (AGT), were synthesized and evaluated for their ability to undergo bioreductive activation by reductase enzymes under oxygen deficiency. Three agents of this class, 4-nitrobenzyl (6-(benzyloxy)-9H-purin-2-yl)carbamate (1) and its monomethyl (2) and gem-dimethyl analogues (3), were tested for activation by reductase enzyme systems under oxygen deficient conditions. Compound 3, the most water-soluble of these agents, gave the highest yield of O(6)-BG following reduction of the nitro group trigger. Compound 3 was also evaluated for its ability to sensitize 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine (laromustine)-resistant DU145 human prostate carcinoma cells, which express high levels of AGT, to the cytotoxic effects of this agent under normoxic and oxygen deficient conditions. While 3 had little or no effect on laromustine cytotoxicity under aerobic conditions, significant enhancement occurred under oxygen deficiency, providing evidence for the preferential release of the AGT inhibitor O(6)-BG under hypoxia.

Levetiracetam enhances p53-mediated MGMT inhibition and sensitizes glioblastoma cells to temozolomide

Antiepileptic drugs (AEDs) are frequently used to treat seizures in glioma patients. AEDs may have an unrecognized impact in modulating O(6)-methylguanine-DNA methyltransferase (MGMT), a DNA repair protein that has an important role in tumor cell resistance to alkylating agents. We report that levetiracetam (LEV) is the most potent MGMT inhibitor among several AEDs with diverse MGMT regulatory actions. In vitro, when used at concentrations within the human therapeutic range for seizure prophylaxis, LEV decreases MGMT protein and mRNA expression levels. Chromatin immunoprecipitation analysis reveals that LEV enhances p53 binding on the MGMT promoter by recruiting the mSin3A/histone deacetylase 1 (HDAC1) corepressor complex. However, LEV does not exert any MGMT inhibitory activity when the expression of either p53, mSin3A, or HDAC1 is abrogated. LEV inhibits malignant glioma cell proliferation and increases glioma cell sensitivity to the monofunctional alkylating agent temozolomide. In 4 newly diagnosed patients who had 2 craniotomies 7-14 days apart, prior to the initiation of any tumor-specific treatment, samples obtained before and after LEV treatment showed the inhibition of MGMT expression. Our results suggest that the choice of AED in patients with malignant gliomas may have an unrecognized impact in clinical practice and research trial design.

The Fanconi anemia (FA) pathway confers glioma resistance to DNA alkylating agents

DNA alkylating agents including temozolomide (TMZ) and 1,3-bis[2-chloroethyl]-1-nitroso-urea (BCNU) are the most common form of chemotherapy in the treatment of gliomas. Despite their frequent use, the therapeutic efficacy of these agents is limited by the development of resistance. Previous studies suggest that the mechanism of this resistance is complex and involves multiple DNA repair pathways. To better define the pathways contributing to the mechanisms underlying glioma resistance, we tested the contribution of the Fanconi anemia (FA) DNA repair pathway. TMZ and BCNU treatment of FA-proficient cell lines led to a dose- and time-dependent increase in FANCD2 mono-ubiquitination and FANCD2 nuclear foci formation, both hallmarks of FA pathway activation. The FA-deficient cells were more sensitive to TMZ/BCNU relative to their corrected, isogenic counterparts. To test whether these observations were pertinent to glioma biology, we screened a panel of glioma cell lines and identified one (HT16) that was deficient in the FA repair pathway. This cell line exhibited increased sensitivity to TMZ and BCNU relative to the FA-proficient glioma cell lines. Moreover, inhibition of FA pathway activation by a small molecule inhibitor (curcumin) or by small interference RNA suppression caused increased sensitivity to TMZ/BCNU in the U87 glioma cell line. The BCNU sensitizing effect of FA inhibition appeared additive to that of methyl-guanine methyl transferase inhibition. The results presented in this paper underscore the complexity of cellular resistance to DNA alkylating agents and implicate the FA repair pathway as a determinant of this resistance.

Targeted Modulation of MGMT: Clinical Implications: Fig. 1

O(6)-Methylguanine DNA methyltransferase (MGMT) has been studied for >20 years as a gene that is associated with the mutagenicity and cytotoxicity induced by either methylating carcinogens or alkylating (methylating and chloroethylating) therapeutic agents. Pioneering studies of alkylating agents identified alkylated guanine at the O(6) position, the substrate of MGMT, as a potentially promutagenic and lethal toxic DNA lesion. MGMT plays a prominent role in DNA adduct repair that limits the mutagenic and cytotoxic effect of alkylating agents. Because of its role in cancer etiology and chemotherapy resistance, MGMT is of particular interest. In this article, the clinical effect of MGMT expression and targeted modulation of MGMT will be summarized.

The emerging role of DNA repair proteins as predictive, prognostic and therapeutic targets in cancer

Advanced cancer is the second leading cause of death in the western world. Chemotherapy and radiation are the two main treatment modalities currently available to improve patient outcomes, but treatment related toxicity and the emergence of resistance limit their effectiveness. Hence there is an urgent need to develop novel treatment strategies. Rapid advances in cancer biology have identified key pathways involved in the repair of DNA damage induced by chemotherapeutic agents and irradiation. Efficient DNA repair in the cancer cell is an important mechanism for therapeutic resistance. Up to 130 genes have been identified that are associated with human DNA repair. Several of these proteins are emerging as important predictive and prognostic factors in solid tumours. Inhibition of DNA repair has the potential to enhance the efficacy of currently available DNA damaging agents. In recent years, several promising drug targets have been identified and novel drugs synthesised that target specific DNA repair proteins. These agents have shown impressive anti-cancer effects in preclinical studies in combination with chemotherapy or irradiation. Their role in human cancer is now being investigated in early phase clinical trials in combination with chemotherapy. MGMT inhibitors, PARP inhibitors and methoxyamine are currently in early stages of clinical development. Innovative clinical trial designs are essential to evaluate the potential of DNA repair inhibitor in cancer therapy.

Isolated limb infusion with fotemustine after dacarbazine chemosensitisation for inoperable loco-regional melanoma recurrence

BACKGROUND

Isolated limb infusion (ILI) is a simple yet effective alternative to conventional isolated limb perfusion for the treatment of advanced melanoma of the extremities.

PATIENTS AND METHODS

The study group comprised 13 patients with very advanced limb disease who had failed to achieve a satisfactory response to one or more ILIs with melphalan, and in whom amputation was the only other realistic treatment option. The aim of this study was to evaluate the efficacy and toxicity of ILI with fotemustine after systemic chemosensitisation with dacarbazine (DTIC).

RESULTS

Complete remission was achieved in four patients and partial remission in eight patients, with a median response duration of 3 months. Limb salvage was achieved in five of 12 assessable patients (42%). Limb toxicity peaked 9 days after ILI; two patients experienced Wieberdink grade IV (severe) toxicity and four patients had grade V toxicity (requiring early amputation).

CONCLUSIONS

ILI with fotemustine after DTIC chemosensitisation can be successful when gross limb disease has not been controlled by one or more ILIs with melphalan. However, it cannot be recommended as a routine method of treatment for advanced melanoma of the extremities because of the high incidence of severe limb toxicity.

Methylation assay by nucleotide incorporation: a quantitative assay for regional CpG methylation density

Although the aberrant methylation in CpG islands is of great interest as a causative role in human malignancies, it has been very difficult to accurately determine methylation density. Here we report a novel microplate-based quantitative methylation assay, designated MANIC, for a region containing a number of CpG sites based on incorporation of hapten-labeled dCTP at cytosine sites where the methylated cytosines have not been converted to uracil by the bisulfite treatment. Validation using control DNAs revealed that the method was sensitive enough to detect < 1.25% methylated DNA and that calibration curve was linear. With this approach, we determined relative methylation density of O6-methylguanine-DNA methyltransferase gene promoter containing 12 CpG sites among the 12 colorectal cancers and corresponding normal mucosal tissues. Consequently, MANIC showed a high concordance with results by a quantitative method, bisulfite PCR single-stranded conformational polymorphism (BiPS). MANIC is a technique that avoids cumbersome procedures such as electrophoresis or the use of radiolabeling and is applicable to any sequence regardless of the total number of CpG sites or heterogeneity in methylation status.

A Randomized Phase I and Pharmacological Trial of Sequences of 1,3-bis(2-Chloroethyl)-1-Nitrosourea and Temozolomide in Patients with Advanced Solid Neoplasms

PURPOSE

O(6)-alkylguanine-DNA alkyltransferase (AGAT) is modulated by methylating agents, which, in turn, abrogates nitrosourea resistance in preclinical studies. The feasibility of administering various sequences of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and temozolomide (TEM) in patients with advanced solid neoplasms was evaluated in this Phase I and pharmacological study to assess this premise in the clinical setting. The study also sought to determine the maximum tolerated dose (MTD) levels of BCNU and TEM as a function of Seq, to characterize the pharmacokinetic (PK) behavior of TEM administered both before and after BCNU, assess AGAT fluctuations in peripheral blood mononuclear cells (PBMCs), and seek preliminary evidence of anticancer activity.

EXPERIMENTAL DESIGN

Sixty-three patients were randomized to receive treatment with oral TEM daily on days 1-5 and BCNU administered i.v., either on day 1 before TEM [Sequence (Seq) B-->T] or day 5 after TEM (Seq T-->B). Treatment was repeated every 6 weeks. Blood sampling for PK studies was performed on both days 1 and 5 of course one. PBMCs were sampled to evaluate major sequence-dependent effects on AGAT levels.

RESULTS

Neutropenia and thrombocytopenia were the principal dose-limiting toxicities of the BCNU/TEM regimen. These effects were more prominent in patients receiving Seq T-->B, resulting in a much lower MTD of 80/100 mg/m(2)/day compared with 150/110 mg/m(2)/day for Seq B-->T. Notable antitumor activity was observed in patients with glioblastoma multiforme, sarcoma, and ovarian carcinoma. No sequence-dependent PK effects were noted to account for sequence-dependent toxicological effects. At the MTD level, AGAT activity in PBMCs decreased 3-fold, on average, and AGAT fluctuations did not appear to be sequence-dependent.

CONCLUSIONS

The principal toxicities of the BCNU/TEM regimen were neutropenia and thrombocytopenia, which were consistent and predictable, albeit sequence-dependent. Seq T-->B was substantially more myelosuppressive, resulting in disparate MTDs and dose levels recommended for subsequent disease-directed evaluations (150/110 and 80/100 mg/m(2)/day for Seq B-->T and T-->B, respectively). Sequence-dependent differences in TEM PK do not account for this clinically relevant magnitude of sequence-dependent toxicity. The characteristics of the myelosuppressive effects of BCNU/TEM, the paucity of severe nonhematological toxicities, and antitumor activity at tolerable doses warrant disease-directed evaluations on this schedule.

Phase I-II study on isolation antiblastic fotemustine perfusion after dacarbazine chemosensitization for advanced melanoma of the extremities

Isolation limb perfusion (ILP) is the treatment of choice for locally advanced limb melanoma. With melphalan, the referral drug, complete response (CR) is achieved in about 50% of patients, but significant local toxicity occurs in up to 30%. The aim of the present phase I-II study was to challenge fotemustine (F) in ILP after systemic chemosensitization with dacarbazine (DTIC), given its lower toxicity and greater efficacy, as reported in a previous pilot study. Eleven patients with locally advanced limb melanoma were subdivided into triplets, and given F ILP at escalating doses (starting from 25 mg/l) after intravenous administration of 500 mg/m2 DTIC. Acute and chronic locoregional and systemic toxicity, tumour response and clinical outcome were evaluated. Two patients in the first triplet had G3-G4 local toxicity, so that the scheduled F dosage was halved. At drug levels of 12.5, 15.6 and 18.2 mg/l, local toxicity decreased, but only one of eight patients showed CR. The trial was then interrupted due to the low tolerability and poor efficacy of this perfusion regimen. At present, F ILP after DTIC chemosensitization should not be recommended for the treatment of locally advanced limb melanoma.

Pluronic block copolymers for overcoming drug resistance in cancer

Pluronic block copolymers have been used extensively in a variety of pharmaceutical formulations including delivery of low molecular mass drugs and polypeptides. This review describes novel applications of Pluronic block copolymers in the treatment of drug-resistant tumors. It has been discovered that Pluronic block copolymers interact with multidrug-resistant cancer (MDR) tumors resulting in drastic sensitization of these tumors with respect to various anticancer agents, particularly, anthracycline antibiotics. Furthermore, Pluronic affects several distinct drug resistance mechanisms including inhibition of drug efflux transporters, abolishing drug sequestration in acidic vesicles as well as inhibiting the glutathione/glutathione S-transferase detoxification system. All these mechanisms of drug resistance are energy-dependent and therefore ATP depletion induced by Pluronic block copolymers in MDR cells is considered as one potential reason for chemosensitization of these cells. Following validation using in vitro and in vivo models, a formulation containing doxorubicin and Pluronic mixture (L61 and F127), SP1049C, has been evaluated in phase I clinical trials. Further mechanistic studies and clinical evaluations of these systems are in progress.

The DNA repair protein, O(6)-methylguanine-DNA methyltransferase is a proteolytic target for the E6 human papillomavirus oncoprotein

We have previously shown that O(6)-methylguanine-DNA methyltransferase (MGMT), a DNA repair protein that protects tissues against toxic and carcinogenic effects of alkylating agents, is degraded through ubiquitination-dependent proteolysis. Here, we investigated the role of the human papillomavirus (HPV) E6 protein in MGMT degradation. In three pairs of isogenic human tumor cell lines in which a member of each pair expressed the E6 protein through stable transfection (HCT116/HCT116-E6, MCF7/MCF7-E6, and RKO/RKO-E6), we found a consistent 40-55% reduction in the MGMT protein level and its activity in all E6-expressing cells compared with the parent cells (P=<0.05). E6 expression did not, however, alter the levels of MGMT mRNA. Addition of the recombinant MGMT (rMGMT) protein to extracts of HCT116/E6 cells resulted in the binding of E6 to MGMT. Further, the purified E6 protein promoted the degradation of rMGMT in rabbit reticulocyte lysates. Immunoprecipitation assays showed the presence of a ternary protein complex between MGMT, E6, and the cellular ubiquitin-ligase E6-associated protein (E6-AP). Transient transfection of the p53-null H1299 lung tumor cells with an E6 construct also down-regulated the MGMT. The MGMT protein also showed structural features that are compatible for interaction with the E6, and E6-AP components. Collectively, these data suggest that the oncogenic E6 proteins enhance the ubiquitin-dependent proteolysis of MGMT.

Phosphorylation of O6-alkylguanine-DNA alkyltransferase: experience with a GST-fusion protein and a new pull-down assay

We showed recently that human O6-alkylguanine-DNA alkyltransferase (AGT), a key target for enhancing the efficacy of anticancer alkylating agents, is regulated by phosphorylation in brain tumor cells. This report describes the problems we encountered in using a glutathione S-transferase (GST)-tagged AGT as the substrate in our search for cellular AGT kinases, validation of a new pull-down assay for AGT phosphorylation, and its wide applicability for quantitating protein kinases in crude extracts and purified fractions. The GST-tag present in the fusion protein, by itself, was found to undergo significant phosphorylation by tumor cell extracts and contribute to spurious results. Instead, we used a histidine-tagged AGT protein, and its micro-scale purification with Talon resin as the basis for a quantitative pull-down assay, and applied it for measuring AGT phosphorylation by protein kinase C (PKC) and other cellular kinases. The pull-down procedure can be easily adopted for quantitating protein kinases in a variety of settings, as it overcomes the need for substrate immunoprecipitation when whole cell extracts are used, and eliminates the autophosphorylated kinase proteins, when purified kinases are used. Our observations call for caution in interpreting the results with GST-fusion proteins in phosphorylation studies.

DNA repair and gene therapy: implications for translational uses

Gene therapy has been proposed to have implications in the treatment of cancer. By genetically manipulating the hematopoietic stem cell compartment with genes that confer resistance to chemotherapeutic agents, the dose escalation that is necessary to effectively treat the cancers could potentially be achieved. DNA repair genes are some of the potential candidates to confer increased resistance to chemotherapeutic agents. Although initial focus in this area has been on the direct reversal protein (MGMT), its protective ability is limited to those agents that produce O(6)-methylGuanine cross-links-agents that are not extensively used clinically (e.g., nitrosoureas). Furthermore, most alkylating agents attack more sites in DNA other than O(6)-methylGuanine, such that the protections afforded by MGMT may prevent the initial cytotoxicity, but at a price of increased mutational burden and potential secondary leukemias. Therefore, some of the genes that are being tested as candidates for gene transfer are base excision repair (BER) genes. We and others have found that overexpression of selective BER genes confers resistance to chemotherapeutic agents such as thiotepa, ionizing radiation, bleomycin, and other agents. As these "proof of concept" analyses mature, many more clinically relevant chemotherapeutic agents can be tested for BER protective ability.

Cancer surveillance series [corrected]: brain and other central nervous system cancers: recent trends in incidence and mortality

BACKGROUND

During the 1980s, the incidence of primary malignant brain and other central nervous system tumors (hereafter called brain cancer) was reported to be increasing among all age groups in the United States, while mortality was declining for persons younger than 65 years. We analyzed these data to provide updates on incidence and mortality trends for brain cancer in the United States and to examine these patterns in search of their causes.

METHODS

Data on incidence, overall and according to histology and anatomic site, and on relative survival were obtained from the Surveillance, Epidemiology, and End Results (SEER) Program of the National Cancer Institute for 1975 through 1995. Mortality data were obtained from the National Center for Health Statistics. Medicare procedure claims from the National Cancer Institute's SEER-Medicare database were used for imaging trends. Statistically significant changes in incidence trends were identified, and annual percent changes were computed for log linear models.

RESULTS/CONCLUSIONS

Rates stabilized for all age groups during the most recent period for which SEER data were available, except for the group containing individuals 85 years of age or older. Mortality trends continued to decline for the younger age groups, and the steep increases in mortality seen in the past for the elderly slowed substantially. Patterns differed by age group according to the site and grade of tumors between younger and older patients. During the last decade, use of computed tomography scans was relatively stable for those 65-74 years old but increased among those 85 years old or older.

IMPLICATIONS

Improvements in diagnosis and changes in the diagnosis and treatment of elderly patients provide likely explanations for the observed patterns in brain cancer trends.

O6-Alkylguanine-DNA alkyltransferase: influence on susceptibility to the genetic effects of alkylating agents

O6-Alkylguanine-DNA alkyltransferase (AGT) repairs DNA containing O6-alkylguanine by a suicide mechanism involving transfer of the alkyl group to its active site. In this way AGT protects cells from the mutagenic and cytotoxic effects of O6-alkylguanine-type lesions such as O6-methylguanine (O6-meG), an observation which has during recent years been confirmed by studies in transgenic animals either over-expressing or completely lacking this activity. While the levels of expression of AGT have been shown to affect strongly the repair of O6-meG after high doses of methylating agents inducing complete and prolonged depletion of the cellular AGT pool, other data suggest that within smaller variations of AGT levels (such as the interindividual variations observed in man or as observed after low or moderate exposures to alkylating agents) the dependence of O6-meG repair is limited. This phenomenon may reflect the intracellular distribution of the repair protein and must be taken into account when assessing the role of AGT in determining susceptibility to alkylating agents of environmental or clinical significance.

Challenging drug resistance in cancer therapy--review of the First Nordic Conference on Chemoresistance in Cancer Treatment, October 9th and 10th, 1997

The First Nordic Conference on Chemoresistance in Cancer Treatment was held in the Danish town of Helsingør on October 9th and 10th, 1997, under the auspices of the Nordic Cancer Chemoresistance Group (NCCG). The meeting focused on biochemical chemoresistance in a multidisciplinary approach. There were 19 oral and 15 poster presentations documenting recent advances in experimental and clinical research of drug transport mechanisms, DNA repair systems, detoxifying enzymes, drug target regulation, in vitro sensitivity tests, apoptosis inhibition, and strategies to circumvent chemoresistance. In the present paper we review the main issues that were addressed and discuss the findings with reference to the current literature in the field. The meeting demonstrated the plurality and the complexity of chemoresistance, which is a major obstacle to successful chemotherapy in cancer patients. The new insights to mechanisms of drug resistance and sensitization represent a useful basis for further development of strategies to circumvent chemoresistance in clinical practice.

Establishing chemoresistance in hematopoietic progenitor cells

An attractive approach to circumvent chemotherapy-induced myelosuppression is the use of gene-transfer technology to introduce new genetic material into hematopoietic cells. Several pre-clinical studies have demonstrated that increasing the expression of genes encoding proteins that modulate drug resistance in hematopoietic cells provides significant protection against chemotherapy-induced myelosuppression both in vitro and in vivo. Most work in this area has focused on the use of recombinant retroviruses as vectors for the delivery of DNA sequences into hematopoietic stem cells and progenitor cells. Based on these studies, clinical trials are now under way to evaluate the potential use of two gene sequences-multidrug resistance gene 1 and O6-methylguanine DNA methyltransferase. Reducing chemotherapy-induced myelosuppression by increasing the expression of genes that modulate drug resistance via gene transfer into bone marrow cells might allow dose-intensification of chemotherapy, which might result in an improvement in the clinical outcome of patients with high-risk tumors.

Retroviral transduction of a mutant methylguanine DNA methyltransferase gene into human CD34 cells confers resistance to O6-benzylguanine plus 1,3-bis(2-chloroethyl)-1-nitrosourea

Human CD34 cells express low levels of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) and are sensitive to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). Gene transfer of the AGT gene, methylguanine DNA methyltransferase (MGMT), results in only modest BCNU resistance. Recently, an AGT inhibitor, O6-benzylguanine (BG), entered clinical trials. In preclinical studies, BG potentiated the cytotoxic effect of BCNU in tumors but increased toxicity to normal CD34 cells. We transferred a mutant MGMT containing a glycine-to-alanine mutation at position 156, resulting in marked resistance to BG, into Chinese hamster cells; the K562 cell line and human CD34 cells used the retroviral backbone MFG. In each instance, cells expressed increased AGT and were much more resistant to the combination of BG and BCNU than the parental cells or cells transduced with wild-type MGMT. Furthermore, the transduction efficiency in human CD34 cells was in excess of 70%, and the proportion of CD34 transduced cells resistant to the combination was > 30%. Thus, retroviral-mediated transduction of a mutant MGMT into CD34 cells appears to be an effective way to induce selective resistance to a drug combination designed to overcome a significant resistance mechanism to nitrosoureas in tumors.

Localization of O6-alkylguanine transferase in cancer susceptible cells of human female breast

The cell type specific distribution of O6-alkylguanine-DNA-alkyltransferase (AGT) protein was assessed using immunohistochemical localization in human female breast tissue sections and biochemical quantitation of fractionated cell extracts. The results demonstrated that the AGT protein is predominantly localized in luminal epithelial and myoepithelial cells of the intralobular mammary ducts. Western blot analysis revealed that the AGT level in epithelial cell rich organoid fraction was substantially higher than the whole tissue and fibro-collagenous stromal cell fraction of the normal breast. The quantitative activity measurements confirmed the occurrence of a statistically significant 2.7-fold (P = 0.05) and 4.0-fold (P = 0.04) enriched AGT activity in extracts prepared from the organoids compared to the whole tissue homogenate and fractionated stromal cells, respectively. The results suggest that the invariably high AGT level in malignant compared to the normal breast tissue could be due to AGT accumulation in luminal epithelial and myoepithelial cell mass, increasing as a consequence of the uncontrolled proliferation and differentiation of ductal cells in invasive carcinoma.

Role of DNA repair in resistance to drugs that alkylate O6 of guanine

The mechanism of cytotoxicity of a number of chemotherapeutic agents involves alkylation at the O6 position of guanine, a site that strongly influences cytotoxicity. Repair of these lesions by the alkyltransferase protects from cytotoxicity and is a major mechanism of resistance to these agents. O6-benzylguanine inhibition of alkyltransferase sensitizes tumor cells, and clinical trials are underway to determine its efficacy. The use of gene therapy to enhance the expression of alkyltransferase in hematopoietic cells may prevent dose-limiting myelosuppression and may enhance the utility of this class of chemotherapeutic agents.