Chemotherapy-induced DNA Damage Research Articles

Pre-clinical activity of the PARP inhibitor AZD2281 in human breast cancer cell lines and in combination with DNA damaging agents.

Abstract Abstract #1038 Background: Deficiencies in DNA repair mechanisms have been associated with breast cancer. AZD2281, a potent, oral, PARP inhibitor has been shown to have clinical activity in patients with BRCA mutant breast cancers. Laboratory studies have suggested that non-BRCA mutant breast cancers may also be sensitive to PARP inhibition in tumors as a consequence of deficiencies in other homologous recombination (HR) repair components. Using a large panel of human breast cancer cell lines we tested the hypotheses that (1) there may be a subset of non-BRCA mutant breast cancers that are sensitive to single-agent AZD2281 and (2) AZD2281 would potentiate the cytotoxic effects of the DNA damaging agent cisplatin. Methods: 43 human cell lines representing known molecular subgroups of breast cancer (i.e. ER+, HER2 amplified, “triple-negative”), and 3 immortalized breast lines were treated in duplicate in adherent plates with AZD2281 using two-fold dilutions over 6 concentrations for 6 days. Dose response curves were generated using a cell count assay to calculate the IC50 of AZD2281. In addition, a subset of cell lines that grow under anchorage independent conditions were grown in triplicate in the presence and absence of 1 µM AZD2281 in soft agar for at least 3 weeks and growth inhibition was calculated as per cent of untreated control. Cell lines (both sensitive and resistant to single agent AZD2281) were also evaluated in combination with cisplatin in a cell count assay to assess the interaction between the two agents.
 Results: The majority of breast cancer cell lines evaluated in the short term 2-D growth assay did not show significant growth inhibition (IC50 < 1 µM) following AZD2281 treatment, including a known BRCA mutant cell line, suggesting this assay may not be ideal for determining sensitivity to AZD2281. However, in the longer term anchorage independent clonogenic assay, approximately half of the cell lines evaluated demonstrated an IC50 < 1 µM. Of note, the majority of the cell lines representing a “triple-negative” phenotype appeared sensitive to AZD2281 in this assay. In addition, pre-treatment with AZD2281 prior to cisplatin, potentiated the growth inhibition seen with cisplatin in both AZD2281 sensitive and resistant cell lines. Additional studies evaluating predictive markers other than BRCA status are ongoing. Conclusion: The PARP inhibitor AZD2281 has significant pre-clinical activity in human breast cancer cell lines. In a clonogenic assay, cell lines representing the “triple negative” subtype were especially sensitive to AZD2281 supporting clinical development in this population, regardless of BRCA status. In addition, these pre-clinical data support the hypothesis that PARP inhibition may potentiate the effects of chemotherapy induced DNA damage and provide further rationale for clinical development. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 1038.

The selective poly(ADP-ribose) polymerase-1(2) inhibitor, CEP-8983, increases the sensitivity of chemoresistant tumor cells to temozolomide and irinotecan but does not potentiate myelotoxicity

The effect of the potent and selective poly(ADP-ribose) (PAR) polymerase-1 [and PAR polymerase-2] inhibitor CEP-8983 on the ability to sensitize chemoresistant glioblastoma (RG2), rhabdomyosarcoma (RH18), neuroblastoma (NB1691), and colon carcinoma (HT29) tumor cells to temozolomide- and camptothecin-induced cytotoxicity, DNA damage, and G(2)-M arrest and on the potentiation of chemotherapy-induced myelotoxicity was evaluated using in vitro assays. In addition, the effect of the prodrug CEP-9722 in combination with temozolomide and/or irinotecan on PAR accumulation and tumor growth was also determined using glioblastoma and/or colon carcinoma xenografts relative to chemotherapy alone. CEP-8983 sensitized carcinoma cells to the growth-inhibitory effects of temozolomide and/or SN38 increased the fraction of and/or lengthened duration of time tumor cells accumulated in chemotherapy-induced G(2)-M arrest and sensitized tumor cells to chemotherapy-induced DNA damage and apoptosis. A granulocyte-macrophage colony-forming unit colony formation assay showed that coincubation of CEP-8983 with temozolomide or topotecan did not potentiate chemotherapy-associated myelotoxicity. CEP-9722 (136 mg/kg) administered with temozolomide (68 mg/kg for 5 days) or irinotecan (10 mg/kg for 5 days) inhibited significantly the growth of RG2 tumors (60%) or HT29 tumors (80%) compared with temozolomide or irinotecan monotherapy, respectively. In addition, CEP-9722 showed "stand alone" antitumor efficacy in these preclinical xenografts. In vivo biochemical efficacy studies showed that CEP-9722 attenuated PAR accumulation in glioma xenografts in a dose- and time-related manner. These data indicate that CEP-8983 and its prodrug are effective chemosensitizing agents when administered in combination with select chemotherapeutic agents against chemoresistant tumors.

Alkaline comet assay study with breast cancer patients: evaluation of baseline and chemotherapy-induced DNA damage in non-target cells

The sensitivity of the alkaline comet assay for the evaluation of baseline and treatment-induced DNA damage in white blood cells of breast cancer patients receiving adjuvant chemotherapy according to three conventional anthracycline- and cyclophosphamide-containing protocols was investigated. Additionally, baseline DNA damage in cancer patients was compared with the levels of DNA damage recorded in healthy women. Altogether 30 patients with diagnosed breast cancer and 30 female blood donors with no known familial history of breast cancer participated in the study. Alkaline comet assay was performed according to standard protocol and DNA migration in peripheral blood leukocytes was measured by a computer-based image analysis system. For each subject the frequency of "damaged" cells, i.e., long-tailed nuclei (LTN) with tail length exceeding 95th percentile for the considered parameter among controls, is also reported. Breast cancer patients had significantly increased background levels of DNA damage in their peripheral blood leukocytes as compared to healthy women. Prior to the chemotherapy, a majority of patients showed a statistically significant increase in the number of LTN compared to healthy blood donors. Marked interindividual variations in baseline DNA damage among patients were recorded, some of them related to the disease stage and status. The present study confirmed the alkaline comet assay as a sensitive technique able to detect significantly elevated DNA migration in blood cells of patients already one hour after completion of the first cycle of chemotherapy. Administration of antineoplastic drugs in three chemotherapy protocols studied induced a similar increase of primary DNA damage in nontarget cells. The evaluation of the LTN frequencies indicates the best response to the protocol containing cyclophosphamide, methotrexate and 5-fluorouracil (CMF). Our results point to the significance of simultaneous evaluation of DNA migration and frequency of LTN in the same subject and approved the use of alkaline comet assay as a suitable method for the routine detection of critical DNA lesions produced after administration of antineoplastic drugs in the clinical settings.

Temozolomide: a milestone in neuro-oncology and beyond?

Temozolomide (Temodal®, Temodar ), an imidazol derivative, is a second-generation alkylating agent. The orally available prodrug with the capacity of crossing the blood–brain barrier received accelerated US FDA approval in 1999. Three pivotal Phase II trials showed modest activity in the treatment of recurrent anaplastic astrocytoma glioblastoma. In 2005, the FDA and the European Agency for the Evaluation of Medicinal Products approved temozolomide for use in newly diagnosed glioblastoma, in conjunction with radiotherapy, based on an European Organisation for Research and Treatment of Cancer/National Cancer Institute of Canada Phase III trial. The adverse events associated with temozolomide are mild-to-moderate and generally predictable; the most serious are noncumulative and reversible myelosuppression and, in particular, thrombocytopenia, which occurs in less than 5% of patients. Continuous temozolomide administration is associated with profound CD4-selective lymphocytopenia. Molecular studies have suggested that the benefit of temozolomide chemotherapy is restricted to patients whose tumors have a methylated methylguanine methyltransferase gene promotor and are thus unable to repair some of the chemotherapy-induced DNA damage. Temozolomide is under investigation for other disease entities, in particular lower-grade glioma, brain metastases and melanoma.

Effect of Chemotherapy-Induced DNA Repair on Oncolytic Herpes Simplex Viral Replication

Gliomas treated with the alkylating agent temozolomide have incomplete responses in part because of tumoral repair of chemotherapy-induced DNA damage. Data from phase I trials suggest that G207, an oncolytic herpes simplex virus (HSV) with mutated ribonucleotide reductase (RR) and gamma34.5 genes, is safe but needs greater viral oncolysis to be effective. We hypothesized that temozolomide and G207 treatment limitations could be jointly addressed using temozolomide-induced tumor-protective DNA repair pathways to enhance viral replication. Human glioblastoma cells (U87, T98, and U373) and U87 cells transfected with the gene for the DNA repair enzyme O6-methylguanine DNA methyltransferase (MGMT) were treated with G207 and/or temozolomide. Drug interactions, expression of the growth arrest DNA damage 34 (GADD34) and RR transcripts before and after their knockdown with short interfering RNAs, DNA strand breaks, and apoptosis were measured using Chou-Talalay analysis, real-time reverse transcription-polymerase chain reaction, the comet assay, and flow cytometry, respectively. Survival of mice (groups of ten) with intracranial U87 xenograft tumors treated with temozolomide and/or G207 was analyzed using Kaplan-Meier analysis. Temozolomide exhibited strong synergy with G207 in both MGMT-negative and the MGMT inhibitor O6-benzylguanine-treated MGMT-expressing gliomas (Chou-Talalay combination indices = 0.005 to 0.39) and induced GADD34 expression primarily in nonapoptotic MGMT-negative U87 glioma cells (fold difference = 16, 95% confidence interval [CI] = 12.6 to 20.4, compared with untreated cells). MGMT-expressing T98 and U87/MGMT cells treated with temozolomide plus O6-benzylguanine had higher RR expression than untreated cells (fold difference =14.9, 95% CI = 10.1 to 22.0 [T98]; 9.9, 95% CI = 7.0 to 13.8 [U87/MGMT]). GADD34 and RR knockdown increased temozolomide-induced DNA damage and inhibited the synergy of G207 and temozolomide in U87 and O6-benzylguanine-treated U87/MGMT cells. Mice bearing intracranial U87 tumors survived longer after combination therapy (100% survival at 90 days) than after single-agent therapy (median survival = 46 and 48 days with G207 and temozolomide treatment, respectively). Temozolomide-induced DNA repair pathways vary with MGMT expression and enhance HSV-mediated oncolysis in glioma cells. These findings unveil the potential of HSV to target cells surviving temozolomide treatment.

Phase I trial combining gemcitabine and treosulfan in advanced cutaneous and uveal melanoma patients

Gemcitabine and treosulfan are DNA-damaging agents. Preclinical studies suggest that synergism exists when melanoma cells are exposed to both drugs concurrently. We conducted a phase I trial in advanced melanoma patients to determine the optimal dose of gemcitabine to be combined with treosulfan. Cohorts of three patients received increasing doses of gemcitabine, commencing at 0.5 g m−2, followed by a fixed dose of 5.0 g m−2 treosulfan on day one of a 21-day cycle. Patients alternately received a first cycle of single-agent gemcitabine or treosulfan before subsequent cycles of both drugs. Peripheral blood lymphocytes were collected in cycles 1 and 2 at various time points until 48 h post-treatment. The single-cell gel electrophoresis (Comet) assay was used to measure chemotherapy-induced DNA damage. A total of 27 patients were enrolled, no objective responses were observed, but two uveal melanoma patients had minor responses. Dose-limiting myelosuppression was reached at 3.0 g m−2 gemcitabine. DNA single-strand breaks were detected 4 h post-gemcitabine, repaired by 24 h. DNA interstrand crosslinks were detected 4 h post-treosulfan, fully removed by 48 h. Following combination chemotherapy, treosulfan-induced DNA crosslinks persisted, still being detectable 48 h post-treatment, supporting the hypothesis that gemcitabine potentiates treosulfan-induced cytotoxicity. The recommended regimen for further study is 2.5 g m−2 gemcitabine combined with 5.0 g m−2 treosulfan.

Genetic variation in XPD predicts treatment outcome and risk of acute myeloid leukemia following chemotherapy

AbstractThe xeroderma pigmentosum group D (XPD) gene encodes a DNA helicase that functions in nucleotide excision repair of chemotherapy-induced DNA damage, the efficiency of which is predicted to be affected by a lysine to glutamine variant at codon 751. We hypothesized that this constitutive genetic variant may modify clinical response to chemotherapy, and we have examined its association with outcome following chemotherapy for acute myeloid leukemia (AML) in 341 elderly patients entered into the United Kingdom Medical Research Council AML 11 trial, and with the risk of developing chemotherapy-related AML. Among subjects treated for AML, disease-free survival at one year was 44% for lysine homozygotes, compared with 36% for heterozygotes and 16% for glutamine homozygotes (hazard ratio [HR], 1.30; 95% confidence interval [CI], 1.01-1.70; P = .04). Similarly, overall survival at one year was 38% for lysine homozygotes, 35% for heterozygotes, and 23% for glutamine homozygotes (HR, 1.18; 95% CI, 0.99-1.41; P = .07). Furthermore, homozygosity for the XPD codon 751 glutamine variant was associated with a significantly increased risk of developing AML after chemotherapy (odds ratio, 2.22 for Gln/Gln vs Lys/Lys; 95% CI, 1.04-4.74). These data suggest that the XPD codon 751 glutamine variant protects against myeloid cell death after chemotherapy. (Blood. 2004;104:3872-3877)

The Role of BRCA1 in the Cellular Response to Chemotherapy

Germline mutations of the BRCA1 gene account for approximately 5% of breast and ovarian cancer cases, and lower than normal BRCA1 expression or function may be an important contributing factor in sporadic cancers. The major role of BRCA1 is to respond to DNA damage by participating in cellular pathways for DNA repair, mRNA transcription, cell cycle regulation, and protein ubiquitination. Because most chemotherapeutic agents function by directly or indirectly damaging DNA, the role of BRCA1 as a regulator of chemotherapy-induced DNA damage has been the subject of an increasing number of investigations. We review published preclinical and clinical evidence that the level of BRCA1 function in an individual patient's tumor can guide the choice of chemotherapeutic agents for breast and ovarian cancer. We conclude that a loss of BRCA1 function is associated with sensitivity to DNA-damaging chemotherapy and may also be associated with resistance to spindle poisons. We recommend that prospective clinical studies investigating the role of BRCA1 in the response to chemotherapy be conducted.

High specificity of quantitative excision repair cross-complementing 1 messenger RNA expression for prediction of minor histopathological response to neoadjuvant radiochemotherapy in esophageal cancer.

The excision repair cross-complementing 1 (ERCC1) gene is coding for a nucleotide excision repair protein involved in the repair of radiation- and chemotherapy-induced DNA damage. We examined the potential of quantitative ERCC1 mRNA expression to predict minor or major histopathological response to neoadjuvant radiochemotherapy (cisplatin, 5-fluorouracil, and 36 Gy of radiation) followed by transthoracic en bloc esophagectomy in patients with locally advanced esophageal cancer (cT(2-4), N(x), M(0)). Tissue samples were collected by endoscopic biopsy before treatment. RNA was isolated from biopsies, and quantitative real-time reverse transcriptase PCR assays were performed to determine ERCC1 mRNA expression. Relative mRNA levels (tumor/normal ratios) were calculated as (ERCC1/beta-actin in tumor)/(ERCC1/beta-actin in paired normal tissue). ERCC1 expression levels were correlated with the objective histopathological response in resected specimens. Histomorphological regression was defined as major response when resected specimens contained <10% of residual vital tumor cells or in case a pathologically complete response was achieved. Twelve of 36 tumors showed a major histopathological response, and 24 of 36 showed a minor histopathological response. Relative expression levels of ERCC1 of >1.09 were not associated with a major histopathological response (sensitivity, 62.5%; specificity, 100%) and 15 of 24 patients with minor histopathological response to the delivered neoadjuvant radiochemotherapy could be unequivocally identified. This association of dichotomized relative ERCC1 mRNA expression and histopathological response was statistically significant (P < 0.001). Relative expression levels of ERCC1 mRNA determined by quantitative real-time reverse transcriptase-PCR appear highly specific to predict minor response to our neoadjuvant radiochemotherapy protocol in patients with locally advanced esophageal cancer and could be applied to prevent expensive, noneffective, and potentially harmful therapies in a substantial number (42%) of patients.

Assessment of in vivo chemotherapy-induced DNA damage in a p53-mutated rat tumor by micronuclei assay.

Production of DNA damage is the basis of cancer treatments such as chemo- and radiotherapy. Such treatments induce mitotic catastrophe, a form of cell death resulting from abnormal mitosis and leading to the formation of interphase cells with multiple micronuclei. In this study, we compared apoptosis induction and micronuclei formation to assess the DNA damage provoked in vivo by cytotoxic agents in established 9L rat gliosarcoma tumors expressing a mutated p53 gene. Results from TUNEL assays revealed the efficiency of local gamma-irradiation at the tumor site to induce apoptosis within 9L tumor mass. However, little or no apoptosis was detected after systemic (ip) injection of cisplatin (1 mg/kg). Interestingly, the micronuclei assays showed that not only gamma-irradiation but also cisplatin treatment led to an increase in the emergence of binucleated cells with micronuclei. Apoptosis induction and micronuclei emergence are thus not absolutely correlated. However, micronuclei assays, rarely performed on solid tumors, appear more sensitive than apoptosis assays in evaluating DNA damage linked to chemotherapy.

Potentiation of chemotherapeutic agents following antagonism of nuclear factor kappa B in human gliomas.

Future success using chemotherapy against human gliomas may result from exploiting unique molecular vulnerabilities of these tumors. Chemotherapy frequently results in DNA damage. When such damage is sensed by the cell, programmed cell death, or apoptosis, may be initiated. However, chemotherapy-induced DNA damage may activate nuclear factor kappa B (NF-kappaB) and block apoptosis. We inhibited NF-kappaB using a gene therapy approach to determine whether this would render human glioma cells more susceptible to chemotherapy. U87 and U251 glioma cell lines were infected with either treatment adenovirus containing the gene for a mutant non-degradable form of IkappaBalpha, which is an inhibitor of NF-kappaB nuclear translocation, or empty control virus. Following viral infection, cells were treated either with BCNU, carboplatin, tumor necrosis factor alpha (TNF-alpha), or SN-38. Chemotherapy resulted in a marked increase in active intranuclear NF-kappaB. This response was greatly decreased by insertion of the mutant repressor gene. Similarly, a significant increase in cell killing by all chemotherapy age was demonstrated following infection with treatment virus. Expression of the mutant repressor gene also resulted in increased apoptosis by TUNEL assay following chemotherapy. Numerous genes are responsible for glioma chemoresistance. DNA damage by chemotherapy may induce the antiapoptotic factor NF-kappaB and prevent programmed cell death. Insertion of a mutant inhibitor of NF-kappaB strips cells of this antiapoptotic defense and renders them more susceptible to killing by chemotherapy via increased apoptosis.

Upper GI and Oncology 02

Traditional models of studying chemotherapy efficacy do not take into account microenvironment nor do they behave like naturally occurring human tumours. An isolated human liver model with tumour was developed to provide information for overcoming barriers to drug delivery and for developing novel therapeutic strategies. Liver specimens obtained from eight patients undergoing hemihepatectomy for hepatocellular carcinoma (HCC) or colorectal cancer (CRC) metastases were perfused via portal vein and hepatic artery branches (one control; three cisplatin: 23.3, 50 and 100 µm), two etoposide and two nitrogen mustard (NM; 50 and 100 µm). Chemotherapy-induced DNA damage was measured in individual tumour cells from fine needle aspiration biopsies using the Comet Assay. Thirteen tumours (median size 5 cm, range 2–17) were perfused (1 HCC, 12 CRC). Median duration of perfusion was 300 min (range 240–360), and median total perfusion flow was 300 mL min−1 (range 110–240). Laser Doppler imaging confirmed adequate perfusion, and livers remained viable on gross appearance, stable portal vein and hepatic artery pressures, bile production and maintenance of DNA integrity of the control. DNA damage increased linearly with cisplatin concentration (23.3, 50 and 100 µm caused 35, 45 and 60 per cent decrease in tail moment, respectively), but not with etoposide and NM. Seven of the 12 tumours demonstrated increased DNA damage superficially compared with deeper levels (three cisplatin; two etoposide and two NM). Perfusion of human livers obtained during hepatic resection is a simple procedure that provides new insights into the distribution patterns of chemotherapy within tumours in an ex vivo model.

Expression of Bcl-xL Can Confer a Multidrug Resistance Phenotype

It has been suggested that genes that regulate apoptotic cell death may play an important role in determining the sensitivity of tumor cells to chemotherapy. We have recently cloned a member of the bcl-2 family, bcl-x. To test whether bcl-XL expression affects the sensitivity of tumor cells to chemotherapy, we have created stable cell lines overexpressing bcl-XL and have tested these cells for resistance to cell death induced by metabolic inhibitors and chemotherapeutic agents. Bcl-XL expression dramatically reduces the cytotoxicity of bleomycin, cisplatin, etoposide, vincristine, hygromycin B, and mycophenolic acid for up to 4 days in culture. Bcl-XL does not prevent cells from undergoing cell cycle arrest in response to these drugs, but rather prevents treated cells from undergoing apoptosis. Cell-cycle analysis on cells treated with the chemotherapeutic agents bleomycin, cisplatin, etoposide, and vincristine, show that the drugs cause growth arrest in different positions within the cell cycle. Bcl-XL expressing cells treated with chemotherapeutic drugs retain their proliferative ability after the drugs are removed. Interestingly, vincristine-treated cells expressing bcl-XL become polyploid after drug removal. These data show that bcl-XL protects cells from a wide variety of apoptotic stimuli, acts in multiple positions within the cell cycle, and confers a multidrug resistance phenotype. The ability of bcl-XL to prevent apoptotic cell death in response to chemotherapy-induced DNA damage and cell-cycle arrest may contribute to the accumulation of chromosomal aberrations within tumors. The expression of bcl-XL in tumor cells is likely to be an important indicator of chemotherapeutic efficacy.

Modulation of Nitrosourea Resistance in Myeloid Leukemias

Drug resistance in myeloid leukemias may be mediated by an increased capacity to repair chemotherapy-induced DNA damage. Some tumor cell lines that are resistant to nitrosoureas contain the DNA repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase). This protects cells by removing cytotoxic, nitrosourea-induced O6-alkylguanine adducts. We measured the level of alkyltransferase activity in myeloid leukemic cells freshly obtained from patients to determine whether the alkyltransferase was an important factor in nitrosourea resistance in these cells and whether inactivation of this protein could sensitize leukemic cells to nitrosoureas. Myeloid leukemic cells from patients with acute nonlymphocytic leukemia and chronic myelogenous leukemia had higher levels of alkyltransferase than did myeloid precursors from normal donors (P less than .01). This difference did not appear to be due to the state of differentiation of the leukemic or normal cells. To show that this repair protein mediated nitrosourea resistance in leukemic cells, cells were treated with the modified base O6-methylguanine to selectively and irreversibly inactivate the alkyltransferase and then exposed to 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU). An 18-hour incubation in 0.5 mmol/L O6-methylguanine caused an 87% +/- 3.6% decrease in alkyltransferase activity in leukemic cells and a 73% +/- 8.6% decrease in normal myeloid precursors. After treatment with O6-methylguanine, clonogenic leukemic cells from ten different donors became much more sensitive to BCNU, with a decrease in the dose needed to reduce colony survival by 50% (LD50) of 6.3 +/- 1.4-fold. A lesser effect was seen on CFU-GM, BFU-E, and CFU-GEM where the LD50 decreased two- to threefold. These studies show that nitrosourea resistance in myeloid leukemic cells can be abrogated by inactivation of the DNA repair protein O6-alkylguanine-DNA alkyltransferase. This method of biochemical modulation of DNA repair will sensitize leukemic cells to nitrosoureas in vitro and has the potential of increasing the therapeutic index of nitrosoureas in this disease.

Modulation of nitrosourea resistance in myeloid leukemias

Drug resistance in myeloid leukemias may be mediated by an increased capacity to repair chemotherapy-induced DNA damage. Some tumor cell lines that are resistant to nitrosoureas contain the DNA repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase). This protects cells by removing cytotoxic, nitrosourea-induced O6-alkylguanine adducts. We measured the level of alkyltransferase activity in myeloid leukemic cells freshly obtained from patients to determine whether the alkyltransferase was an important factor in nitrosourea resistance in these cells and whether inactivation of this protein could sensitize leukemic cells to nitrosoureas. Myeloid leukemic cells from patients with acute nonlymphocytic leukemia and chronic myelogenous leukemia had higher levels of alkyltransferase than did myeloid precursors from normal donors (P less than .01). This difference did not appear to be due to the state of differentiation of the leukemic or normal cells. To show that this repair protein mediated nitrosourea resistance in leukemic cells, cells were treated with the modified base O6- methylguanine to selectively and irreversibly inactivate the alkyltransferase and then exposed to 1,3-bis (2-chloroethyl)-1- nitrosourea (BCNU). An 18-hour incubation in 0.5 mmol/L O6- methylguanine caused an 87% +/- 3.6% decrease in alkyltransferase activity in leukemic cells and a 73% +/- 8.6% decrease in normal myeloid precursors. After treatment with O6-methylguanine, clonogenic leukemic cells from ten different donors became much more sensitive to BCNU, with a decrease in the dose needed to reduce colony survival by 50% (LD50) of 6.3 +/- 1.4-fold. A lesser effect was seen on CFU-GM, BFU- E, and CFU-GEM where the LD50 decreased two- to threefold. These studies show that nitrosourea resistance in myeloid leukemic cells can be abrogated by inactivation of the DNA repair protein O6-alkylguanine- DNA alkyltransferase. This method of biochemical modulation of DNA repair will sensitize leukemic cells to nitrosoureas in vitro and has the potential of increasing the therapeutic index of nitrosoureas in this disease.