Detection by alkaline elution of rat liver DNA damage induced by simultaneous subacute administration of nitrite and aminopyrine

Alkaline DNA Fragmentation in Vivo: Borderline or Negative Results Obtained Respectively with 7,12-Dimethylbenz[A]Anthracene and Benzo[A]Pyrene

Using the in vivo DNA damage alkaline elution assay, a satisfactory correlation with carcinogenicity in the same target organ has been previously shown for a variety of chemical agents. This work was intended to enlarge the exploration of the predictivity of this test. Benzo[a]pyrene (BP) was found negative for damage to liver DNA of mice and rats, and 7,12-dimethylbenz[a]anthracene (DMBA) negative for damage to liver and bone marrow DNA of mice and slightly positive for damage to mammary gland DNA of young female rats. The results were found to be correlated with the extension of DNA arylation in target organs in similar experimental conditions. From carcinogenicity data reported in the Survey of Compounds Which Have Been Tested for Carcinogenic Activity (vols. 1961-1973) BP and DMBA were both found to be essentially negative as liver carcinogens; however, DMBA was a potent carcinogen in inducing mammary tumors.

Genotoxic and carcinogenic risk to humans of drug-nitrite interaction products

The large majority of N-nitroso compounds (NOC) have been found to produce genotoxic effects and to cause tumor development in laboratory animals; four NOC have been classified by the International Agency for Research on Cancer (IARC) as probably and another 15 as possibly carcinogenic to humans. A considerable fraction of drugs are theoretically nitrosatable due to the presence of amine, amide or other groups which by reacting with nitrite in the gastric environment, or even in other sites, can give rise to the formation of NOC, and in some cases other reactive species. This review provides a synthesis of information on the chemistry of NOC formation, the carcinogenic activity of NOC in animals and humans and the inhibitors of nitrosation reactions. It contains information on the drugs which have been tested for the formation of NOC by reaction with nitrite and the genotoxic-carcinogenic effects of their nitrosation products. In an extensive search we have found that 182 drugs, representing a wide variety of chemical structures and therapeutic activities, were examined in various experimental conditions for their ability to react with nitrite, and 173 (95%) of them were found to form NOC or other reactive species. Moreover, 136 drugs were examined in short-term genotoxicity tests and/or in long-term carcinogenesis assays, either in combination with nitrite or using their nitrosation product, in order to establish whether they produce genotoxic and carcinogenic effects; 112 (82.4%) of them have been found to give at least one positive response. The problem of endogenous drug nitrosation is largely unrecognized. Only a small fraction of theoretically nitrosatable drugs have been examined for the possible formation of genotoxic-carcinogenic NOC, guidelines for genotoxicity testing of pharmaceuticals do not indicate the need of performing the appropriate tests, and patients are not informed that the drug-nitrite interaction and the consequent risk can be reduced to a large extent by consuming the nitrosatable drug with ascorbic acid.

Application of alkaline unwinding to analysis of strand breaks induced by bleomycin in hamster lung DNA in vivo

Strand breaks in hamster lung DNA were analyzed following in vivo exposure to bleomycin. An alkaline unwinding procedure involving separation of single-strand from double-strand DNA by hydroxylapatite chromatography followed by fluorescence detection of DNA with bisbenzimide was adapted for these studies. Procedures were developed that allowed preparation of DNA from lungs of control animals with a relatively low amount of single-strand DNA relative to double-strand DNA. Time dependence of unwinding was demonstrated using samples that were damaged deliberately by brief probe sonication. To verify that residual bleomycin remaining in lungs at the time of sacrifice did not cause strand breaks during sample preparation, bleomycin was added to minced lung in vitro. Under these conditions, the ratio of single-strand to double-strand DNA was not increased significantly. Substantial strand breaks were produced in vivo at 15 min and 1 h following intratracheal instillation of bleomycin into hamsters, as evidenced by a 5-6-fold increase in the ratio of single-strand to double-strand DNA relative to controls. The DNA damage appeared to be repaired within 1 day following exposure.

Alkaline elution of rat testicular DNA: detection of DNA cross-links after in vivo treatment with chemical mutagens

The alkaline elution technique was used to measure DNA damage in the rat testis after intraperitoneal injection of 3 chemicals known to cause heritable mutations in rodents. These 3 chemicals are triethylenemelamine (TEM), mitomycin C, and cyclophosphamide. All three of these chemicals produced DNA damage which was readily detectable by alkaline elution. Both TEM and mitomycin C produced DNA interstrand cross-links, although TEM was a more potent cross-linker on an equimolar basis than mitomycin C. Cyclophosphamide produced both DNA cross-links and DNA strand breaks. Alkaline elution in the absence of proteinase K indicated that some of the strand breaks appeared to be closely associated with protein. These studied indicate that the alkaline elution technique is capable of detecting DNA damage in mammalian germ cells produced by chemical mutagens. This technique may prove useful as a screening tool for identifying chemicals which cause heritable mutations in mammals.

Quantitative correlation with carcinogenic potency of different short term tests

The following short term parameters evaluating essentially genotoxic effects were considered: liver DNA alkaline fragmentation (DFI), morphological transformation in hamster embryo cells (TPI), and mutagenicity in the Ames' test (MPI). The internal consistency of the carcinogenicity data (OPI) was rather high (r approximately or equal to 0.8). The correlation with OPI of DFI and MPI was statistically significant but rather modest, about 0.4-0.5. The best correlation between OPI and TPI was 0.65. This level of correlation was observed only when some kind of dose-response relationship for transformation could be established. When comparisons were attempted for exactly the same 27 compounds for all three tests, a general decrease in predictability was observed. This could be mainly due to problems of representation of the ideal population of chemicals using small samples. The more general problem of the quantitative approach to the predictability of short term tests was also discussed.

The unique role of rodents in the detection of possible human carcinogens and mutagens

Some of the probable reasons underlying the observation that not all chemicals shown to be genotoxic in vitro are capable of eliciting tumours in rodents or humans are discussed using appropriate examples. It is suggested that a substantial proportion of the resources currently available for conducting rodent carcinogenicity bioassays should be employed in the short-term evaluation in vivo of some of the many hundreds of chemicals recently defined as genotoxic in vitro, rather than in the protracted evaluation of a few chemicals, often of unknown activity in vitro, for carcinogenicity. A decision tree approach to the evaluation of chemicals for human mutagenic/carcinogenic potential is presented which is at variance with the construction and philosophy of many of the current legislative guidelines. The immediate need for the adoption of one of the available short-term in vivo liver assays, and/or the development of a short-term in vivo rodent assay capable of concomitantly monitoring different genetic end-points in a range of organs or tissues is emphasized.

Quantitative correlation between carcinogenicity and sister chromatid exchange induction in vivo for a group of 11 N-nitroso derivatives

The quantitative correlation between induction of sister chromatid exchanges (SCEs) in vivo and carcinogenic potency was examined for 11 nitroso derivatives and was compared with the correlation of alkaline DNA fragmentation in liver DNA in vivo and with the Ames test. The correlation between DNA adducts and SCEs was also evaluated. DNA damage was slightly more predictive and the Ames test less predictive than SCE evaluation. The predictivity of these tests for this class of compounds was compared with the predictivity shown for different classes of chemical compounds.

Quantitative correlations amongst alkaline DNA fragmentation, DNA covalent binding, mutagenicity in the Ames test and carcinogenicity, for 21 compounds

21 compounds from different chemical classes were quantitatively compared for their carcinogenic potency according to 4 parameters: (1) potency in inducing covalent binding with DNA in vivo; (2) potency in inducing alkaline DNA fragmentation after treatment in vivo; (3) acute toxicity; (4) mutagenic potency in the Ames test. Establishing well-defined conditions for normalization of the different types of data and determination of the set that had to be submitted to statistical analysis appeared to be a difficult task, for which only compromise solutions were possible. A statistical analysis of the data suggested that all parameters considered were correlated with carcinogenic potency. However, we found that there are about 3 chances to 1 that carcinogenicity is better correlated with DNA covalent binding in vivo than it is to mutagenicity in the Ames test. With due precautions, even acute toxicity could be of predictive value. DNA adducts and DNA fragmentation, both in vivo, appeared to be 2 parameters strongly correlated between them. From a multivariate statistical analysis it appeared that: (1) a significant improvement of quantitative predictability is in principle obtainable with a battery of short-term test; and (2) the improvement is obtainable only if the short-term tests considered, while all correlated with carcinogenicity, are relatively independent amongst themselves.