In vivo formation and repair of O6-methylguanine in human leukocyte DNA after intravenous exposure to dacarbazine

O6-methylguanine DNA methyltransferase deficiency and response to temozolomide-based therapy in patients with neuroendocrine tumors

PURPOSE

Recent studies suggest that temozolomide has activity in neuroendocrine tumors. Low levels of the DNA repair enzyme, O(6)-methylguanine DNA methyltransferase (MGMT), are associated with sensitivity to temozolomide in other tumor types. We evaluated the prevalence of MGMT deficiency in neuroendocrine tumors and correlated MGMT deficiency with treatment response to temozolomide-based regimens.

EXPERIMENTAL DESIGN

The prevalence of MGMT deficiency, measured by immunohistochemistry, was assessed in 97 archival neuroendocrine tumor specimens. Rates of treatment response and survival were next evaluated in a cohort of 101 consecutive neuroendocrine tumor patients who had received treatment with a temozolomide-based regimen at one of three institutions. MGMT expression was directly correlated with treatment response in 21 patients who had available tumor tissue and response data.

RESULTS

In archival specimens, MGMT deficiency was observed in 19 of 37 (51%) pancreatic neuroendocrine tumors and 0 of 60 (0%) carcinoid tumors (P < 0.0001). In the clinical cohort, 18 of 53 (34%) patients with pancreatic neuroendocrine tumors but only 1 of 44 (2%) patients with carcinoid tumors (P < 0.001) experienced a partial or complete response to temozolomide-based therapy. Among 21 patients with evaluable tumor tissue who had also received treatment with temozolomide, 4 of 5 patients with MGMT-deficient tumors (all pancreatic neuroendocrine tumors) and 0 of 16 patients with tumors showing intact MGMT expression responded to treatment (P = 0.001).

CONCLUSIONS

MGMT deficiency, measured by immunohistochemistry, is more common in pancreatic neuroendocrine tumors than in carcinoid tumors as is treatment response to temozolomide-based therapy. Absence of MGMT may explain the sensitivity of some pancreatic neuroendocrine tumors to treatment.

Validation of biomarkers for the study of environmental carcinogens: a review

There is a need for validation of biomarkers. Our aim is to review published work on the validation of selected biomarkers: bulky DNA adducts, N-nitroso compounds, 1-hydroxypyrene, and oxidative damage to DNA. A systematic literature search in PubMed was performed. Information on the variability and reliability of the laboratory tests used for biomarkers measurements was collected. For the evaluation of the evidence on validation we referred to the ACCE criteria. Little is known about intraindividual variation of DNA adduct measurements, but measurements have a good repeatability irrespective of the technique used for their identification; reproducibility improved after the correction for a laboratory factor. A high-sensitivity method is available for the measurement of 1-hydroxypyrene in urine. There is consensus on validation of biomarkers of oxidative damage DNA based on the comet assay and chromatographic measurement in blood while urinary measurements by chromatographic assays are well validated, and ELISA-based assays appear to lack specificity. Immunoassays for the quantification of adducts of N-nitroso compounds are useful for large epidemiological studies, given their sensitivity, the small amount of DNA required and their potential for rapid and high-throughput analysis.

A phase II trial of lomeguatrib and temozolomide in metastatic colorectal cancer

To evaluate the tumour response to lomeguatrib and temozolomide (TMZ) administered for 5 consecutive days every 4 weeks in patients with metastatic colorectal carcinoma. Patients with stage IV metastatic colorectal carcinoma received lomeguatrib (40 mg) and TMZ (50–200 mg m⁻²) orally for 5 consecutive days every 4 weeks. Response was determined every two cycles. Pharmacokinetics of lomeguatrib and TMZ as well as their pharmacodynamic effects in peripheral blood mononuclear cells (PBMC) were determined. Nineteen patients received 49 cycles of treatments. Despite consistent depletion of O⁶-methylguanine-DNA methyltransferase in PBMC, none of the patients responded to treatment. Three patients had stable disease, one for the duration of the study, and no fall in carcinoembryonic antigen was observed in any patient. Median time to progression was 50 days. The commonest adverse effects were gastrointestinal and haematological and these were comparable to those of TMZ when given alone. This combination of lomeguatrib and TMZ is not efficacious in metastatic colorectal cancer. If further studies are to be performed, emerging data suggest that higher daily doses of lomeguatrib and a dosing period beyond that of TMZ should be evaluated.

In vitro drug response and molecular markers associated with drug resistance in malignant gliomas

PURPOSE

Drug resistance in malignant gliomas contributes to poor clinical outcomes. We determined the in vitro drug response profiles for 478 biopsy specimens from patients with the following malignant glial histologies: astrocytoma (n = 71), anaplastic astrocytoma (n = 39), glioblastoma multiforme (n = 259), oligodendroglioma (n = 40), and glioma (n = 69).

EXPERIMENTAL DESIGN

Samples were tested for drug resistance to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), cisplatin, dacarbazine, paclitaxel, vincristine, and irinotecan. Biomarkers associated with drug resistance were detected by immunohistochemistry, including multidrug resistance gene-1, glutathione S-transferase pi (GSTP1), O(6)-methylguanine-DNA methyltransferase (MGMT), and mutant p53.

RESULTS

In vitro drug resistance in malignant gliomas was independent of prior therapy. High-grade glioblastomas showed a lower level of extreme drug resistance than low-grade astrocytomas to cisplatin (11% versus 27%), temozolomide (14% versus 27%), irinotecan (33% versus 53%), and BCNU (29% versus 38%). A substantial percentage of brain tumors overexpressed biomarkers associated with drug resistance, including MGMT (67%), GSTP1 (49%), and mutant p53 (41%). MGMT and GSTP1 overexpression was independently associated with in vitro resistance to BCNU, whereas coexpression of these two markers was associated with the greatest degree of BCNU resistance.

CONCLUSIONS

Assessment of in vitro drug response and profiles of relevant tumor-associated biomarkers may assist the clinician in stratifying patient treatment regimens.

Extent of damage and repair in the p53 tumor-suppressor gene after treatment of myeloma patients with high-dose melphalan and autologous blood stem-cell transplantation is individualized and may predict clinical outcome

PURPOSE

To quantitate the individual levels of melphalan-induced DNA damage formation and repair in vivo and to search for possible correlations with clinical outcome in patients with multiple myeloma (MM).

PATIENTS AND METHODS

The formation and subsequent repair of DNA damage (monoadducts and interstrand cross-links) in the p53 tumor-suppressor gene, the proto-oncogene N-ras, and the housekeeping gene beta-actin during the first 24 hours after treatment with high-dose melphalan (HDM; 200 mg/m2) supported by autologous blood stem-cell transplantation (ABSCT) was measured in blood leukocytes of 26 patients with MM. The peak DNA adduct levels, the total amount of adducts over time, and the rate of adducts repair in each gene were correlated with response and time to progression after HDM.

RESULTS

The levels of gene-specific DNA damage formation and the individual repairing capacity varied up to 16-fold among patients, indicating that the melphalan-induced biologic effect in vivo is highly individualized. A significantly greater DNA damage and a slower rate of repair in p53 for all end points under study were found in patients who achieved tumor reduction compared with nonresponding patients. Furthermore, longer progression-free survival correlated with increased peak monoadduct levels in the p53 gene (P = .032).

CONCLUSION

Increased DNA damage and slower repairing capacity in the p53 gene from blood leukocytes after HDM correlate with improved outcome of patients with MM who undergo ABSCT. These results suggest that quantitation of such biologic end points may identify patients who are more likely to benefit from this procedure.

DNA adducts from chemotherapeutic agents

The guiding principle of early work was the hypothesis that the anti-cancer alkylating drugs acted through their ability to cross-link macromolecules essential for cell division. Not long afterwards, DNA was specified as the essential target, and support for the hypothesis came from evidence that the archetypal agent, mustard gas, could link guanine bases in DNA through their N-7 atoms. Quantitative correlations between alkylation of DNA and its inactivation as a template followed, with bacteriophage as a simple test object, showing that the mean lethal dose was close to a single cross-link in the genome. This conclusion applied to either mustard gas or the more recently introduced platinum drugs. Although both inter- and intra-strand cross-links were effective, it was thought that in cells the inter-strand cross-link would, by preventing the separation of the strands necessary for cell division, and by being more difficult to repair, constitute the more effectively lethal lesion. With repair-deficient bacteria, it also emerged that a single cross-link in the genome was lethal, but proficient bacteria could remove about 20 cross-links through excision repair. Mono-7-alkylguanines were not removed and were evidently inert. Thus, only a few percent of the total alkylation products were the most effective lesions. Parallel studies with cultured mammalian cells gave a rather different picture, in that the mean lethal doses of even hypersensitive cell lines were around 20 or more cross-links per genome, about the same as for resistant strains of bacteria. Most cells could withstand several hundreds of cross-links per genome, and although adducts were removed, there was incomplete removal of cross-links. Some, but not all, sensitive cell lines were deficient in excision repair. Methods were devised for measuring the extents of alkylation of DNA in cells of patients treated with chemotherapeutic drugs; these are mainly immunoassays, and were applied generally to peripheral blood leukocytes, although some tumours were studied. Extents of alkylation of leukocyte DNA were generally of the same order as, or rather less than the mean lethal doses of cultured cells of the 'normal' type, but in some reports for cisplatin-treated patients, very wide variability between individuals was found. A positive correlation between adduct levels, and particularly a very minor adduct recognised specifically by one antibody, and favourable therapeutic outcome was discerned, and suggested to have a pharmacogenetic basis. In several instances, extents of alkylation of tumours were significantly higher than the average for leukocytes; for ovarian and a testicular tumour for cisplatin, and for a plasma cell tumour for melphalan. Nevertheless, these favourable examples would not constitute more than three or four mean lethal doses in the tumour cells, assuming that they had the same sensitivity as 'normal' cell lines: the therapeutic effect would of course be much more favourable if the tumour cells resembled 'sensitive' cell lines. This lack of a favourable difference between extents of alkylation in DNA of patients and the mean lethal dose for normal cells was particularly obvious with the methylating drugs dacarbazine and procarbazine. These considerations stress the need for higher extents of alkylation to be achieved in target tumour DNA for successful chemotherapy. One approach is to give a higher overall dose, and to 'rescue' the bone marrow (known from the earliest report on mustard gas to be the most susceptible tissue) by autologous transplantation. The second, which has yet to reach the clinic, is to convert unreactive prodrugs through enzymic activation into alkylating agents specifically in tumours (see Bagshawe, 1994).

Mutations induced by dacarbazine activated with cytochrome P-450

The mutagenicity of the antitumor drug dacarbazine (DTIC) is due to alkylation of cellular DNA by metabolites resulting from the metabolism of this drug by the mixed function oxidase system. In the present study, we used an in vitro shuttle vector assay to study the base and sequence specificity of mutagenesis by DTIC. The shuttle vector plasmid pSP189 was treated with DTIC (1-2.5 mM) in vitro in a reconstituted cytochrome P-450 system at 37 degrees C for either 30 or 60 min. SupF tRNA gene insert contained in the plasmid was sequenced after replication of the drug-treated plasmid in human Ad 293 cells followed by amplification in indicator bacteria. Mutagenesis of DTIC in this system was dependent upon the presence of the cytochrome P-450 reconstituted system and NADPH. Mutations induced by DTIC included single base substitutions (35%), single base deletions (30.5%), single base insertions (19.4%) and large deletions (13.8%). Among the substitutions, transversions and transitions were in the ratio of 1:0.7. Base pairs 108 and 127 in the SupF tRNA of the pSP189 were identified as mutational hot spots.

Repair of O6-alkylguanines in the nuclear DNA of human lymphocytes and leukaemic cells: analysis at the single-cell level

Inter-individual and cell-cell variability of repair of O6-alkylguanines (O6-AlkGua) in nuclear DNA was studied at the single-cell level in peripheral lymphocytes from healthy donors and in leukaemic cells isolated from patients with chronic lymphatic leukaemia (CLL) or acute myeloid leukaemia (AML). Cells were pulse exposed to N-ethyl- or N-(n-)butyl-N-nitrosourea in vitro, and O6-AlkGua residues in DNA were quantified using an anti-(O6-AlkGua) monoclonal antibody and electronically intensified fluorescence. The kinetics of O6-AlkGua elimination revealed considerable inter-individual differences in O6-ethylguanine (O6-EtGua) half-life (t1/2) values in DNA, ranging from 1.5 to 4.5 h (five AML patients), from 0.8 to 2.8 h (five CLL patients) and from 1.2 to 7.3 h (five healthy donors). The elimination from DNA of equimolar amounts of O6-butylguanine was generally 3-5 times slower in comparison with O6-EtGua. The t1/2 values of individual samples varied in parallel for both DNA alkylation products. Upon preincubation with O6-benzylguanine, the activity of the DNA repair protein O6-alkylguanine-DNA alkyltransferase (AT) in both lymphocytes and leukaemic blasts was reduced to < or = 1%. However, while the rate of O6-EtGua elimination from DNA was decelerated it was not abolished, suggesting the possible involvement of additional repair systems that might be co-regulated with AT. Within individual samples, no major cell subpopulations were observed whose repair kinetics would differ significantly from the remaining cells.

Alkylation and oxidative-DNA damage repair activity in blood leukocytes of smokers and non-smokers

The levels of 3 DNA repair enzymes involved in alkylation and oxidative DNA damage repair in human peripheral blood leukocytes were measured in 20 smokers and 17 non-smokers. No differences in O6-alkylguanine-DNA-alkyltransferase (AGT) activity were found between the 2 groups and the AGT distribution within the population appeared to be unimodal. In contrast, the mean activities of both the methylpurine (MeP)- and the 2-6-diamino-4-hydroxy-5N formamidopyrimidine (FaPy)-DNA glycosylases were higher in the smokers, although only the difference between the MeP-DNA glycosylase means was statistically significant. The standard deviations of these 2 enzymes were also higher in the smokers. The MeP-DNA glycosylase activity showed a bimodal distribution when all subjects were considered. This may in part be due to the smoking habit; 83% of the subjects with enzyme activities higher than 500 fmoles/mg protein were current smokers, whilst 85% of the non-smokers had lower enzyme activities. However, if the smokers were considered separately, a bimodal distribution of this enzyme activity could still be observed. No strong correlation was observed between enzyme activity and age, although the slopes of the regression lines of enzyme activity on age were all negative. The relationship between enzyme activities was studied by bivariate distribution and a strong correlation was only found between the MeP-DNA glycosylase and the FaPy-glycosylase, with the highest values of both enzyme activities being observed in the smokers and the lowest in the non-smokers. Our results suggest that the activity of certain DNA repair enzymes can be modulated by environmental exposure.

Molecular epidemiology in cancer risk assessment and prevention: recent progress and avenues for future research

Molecular epidemiology is increasingly being applied in studies of cancer risks derived from exposure to environmental carcinogens of both endogenous and exogenous origins. Analytical methods have been developed that are capable of detecting and quantifying levels of covalent adducts of several important classes of carcinogens with cellular DNA and blood proteins. Methods of sufficient sensitivity and specificity to detect ambient levels of exposure are in current use. These are being used in studies related to tobacco use (polycyclic aromatic hydrocarbons, aromatic amines, tobacco-specific nitrosamines); dietary exposures (aflatoxins, N-nitrosamines, heterocyclic amines); medicinal exposures (cisplatin, alkylating agents, 8-methoxypsoralen, ultraviolet photoproducts); occupational exposures (aromatic amines, polycyclic aromatic hydrocarbons, oxides of ethylene and styrene, and vinyl chloride); and oxidative damage (8-hydroxyguanine, thymine glycol). Methodologic improvements together with their expanded use in feasibility studies continue to produce results that support the validity of this approach for detecting and quantifying exposure to carcinogens. Genetic markers are also being used to detect early biological responses in efforts to link carcinogen exposure to initiating events in the carcinogenesis process. These include, in addition to traditional cytogenetic markers (e.g., chromosomal aberrations, sister chromatid exchange, micronuclei), other alterations in chromosomal structure such as restriction fragment length polymorphisms, loss of heterozygosity, and translocation markers. Specific genetic changes have recently been identified as critical molecular events in the initiation and development of many cancers. Important among these are activation of oncogenes, especially those of the ras family, and inactivation of tumor-suppressor genes (e.g., p53 and Rb) by point mutations and/or chromosomal deletions and other structural changes. Although some of these changes are known to occur in chemically induced tumors of experimental animals, the possible role of chemical carcinogens in the induction of genetic abnormalities in human cancers has yet to be determined. Continuing investigations employing the methods of molecular epidemiology promise to provide further evidence concerning these relationships. Future investigations employing newly developed molecular biological methods, in particular those based on polymerase chain reaction amplification of DNA, to identify alterations in DNA and chromosomal structure, combined with methods for characterizing exposure to carcinogens and early effects, have great potential for further elucidating the role of genotoxic agents in the etiology of human cancers and also for the development of strategies for their prevention.