Mutagenicity of beta-oxidized N,N,-di-n-propylnitrosamine derivatives in S. typhimurium mediated by rat and hamster tissues

Ames test study designs for nitrosamine mutagenicity testing: qualitative and quantitative analysis of key assay parameters

The robust control of genotoxic N-nitrosamine (NA) impurities is an important safety consideration for the pharmaceutical industry, especially considering recent drug product withdrawals. NAs belong to the 'cohort of concern' list of genotoxic impurities (ICH M7) because of the mutagenic and carcinogenic potency of this chemical class. In addition, regulatory concerns exist regarding the capacity of the Ames test to predict the carcinogenic potential of NAs because of historically discordant results. The reasons postulated to explain these discordant data generally point to aspects of Ames test study design. These include vehicle solvent choice, liver S9 species, bacterial strain, compound concentration, and use of pre-incubation versus plate incorporation methods. Many of these concerns have their roots in historical data generated prior to the harmonization of Ames test guidelines. Therefore, we investigated various Ames test assay parameters and used qualitative analysis and quantitative benchmark dose modelling to identify which combinations provided the most sensitive conditions in terms of mutagenic potency. Two alkyl-nitrosamines, N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) were studied. NDMA and NDEA mutagenicity was readily detected in the Ames test and key assay parameters were identified that contributed to assay sensitivity rankings. The pre-incubation method (30-min incubation), appropriate vehicle (water or methanol), and hamster-induced liver S9, alongside Salmonella typhimurium strains TA100 and TA1535 and Escherichia coli strain WP2uvrA(pKM101) provide the most sensitive combination of assay parameters in terms of NDMA and NDEA mutagenic potency in the Ames test. Using these parameters and further quantitative benchmark dose modelling, we show that N-nitrosomethylethylamine (NMEA) is positive in Ames test and therefore should no longer be considered a historically discordant NA. The results presented herein define a sensitive Ames test design that can be deployed for the assessment of NAs to support robust impurity qualifications.

N-nitrosamines: bacterial mutagenesis and in vitro metabolism

Many nitrosamines are potent mutagens. The rate-limiting step in their in vitro metabolism to mutagens is usually a single enzymatic reaction catalyzed by one or more of the many cytochrome P-450-dependent mixed-function oxidases present in the microsomal cell fraction. Current evidence indicates that this reaction activates nitrosamines to alpha-hydroxynitrosamines, which have half-lives on the order of seconds. This product decomposes to an aldehyde and a much shorter-lived ultimate metabolite which is probably an alkyl diazonium ion or an alkyl carbocation. This may react with DNA leading to premutagenic adducts. Such adducts represent a very small fraction of the ultimate mutagen, with the rest reacting with water to yield the corresponding alcohol. Evidence for this pathway includes (1) the observation of deuterium isotope effects in metabolism and mutagenesis, (2) products (aldehydes, alcohols, and N2) consistent with this pathway, (3) studies on metabolism of nitrosamines using purified cytochrome P-450, (4) formation of DNA adducts such as O6-alkylguanines which are consistent with those expected from the ultimate mutagen, (5) expected products and genotoxic effects of other sources of activated nitrosamines, e.g., alpha-acetoxynitrosamines, alkanediazotates and related compounds. Hydroxylation of nitrosamines at other positions also occurs in vitro (usually to a lesser extent), but these products are generally stable and must be further metabolized to exert mutagenic effects (with the exception of N-nitrosoalkyl(formylmethyl)amines, which are direct-acting mutagens). Because only low percentages of nitrosamines are metabolized in vitro, the contribution to mutagenesis by secondary metabolism is small. In this respect, in vitro metabolism can differ significantly from in vivo metabolism. Bacterial mutagenesis by nitrosamines has most often been studied in Salmonella typhimurium and to a lesser extent E. coli. Mutagenesis by nitrosamines generally requires a source of microsomes (a 9000 X g supernatant fraction is often used), and NADPH. Liver fractions from Aroclor-1254- or PB-induced rodents have been most frequently employed but liver fractions from untreated animals, and homogenates of other organs (lung, kidney, nasal mucosa, and pancreas) have also been utilized. Liver homogenates from humans are generally similar to those from untreated rats in metabolizing nitrosamines to mutagens but large interindividual variations are observed. Mutagenesis is often most effective using a liquid preincubation, a slightly acidic incubation mixture and hamster liver fractions.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutagenic activity and structure-activity relationships of short-chain dialkyl N-nitrosamines in a hamster hepatocyte V79 cell-mediated system

A series of 19 short chain dialkyl N-nitrosamines was studied for mutagenic activity in an uninduced hamster hepatocyte V79 cell-mediated mutagenesis system. Ouabain was used as the selective agent to quantitatively analyze for chemically induced mutants. None of the nitrosamines was mutagenic in the absence of hamster hepatocyte activation. The relative mutagenic activities of the nitrosamines at an equimolar dose are presented. The results of the study indicated that: increasing alkyl chain length decreased mutagenic activity; oxidation of the 2-carbon position to a carbonyl group increased the mutagenic activity of symmetrical and asymmetrical nitrosamines, whereas oxidation to a hydroxyl group only increased the mutagenic activity of the asymmetrical nitrosamines tested; and the carbon position at which oxidation occurred was important in determining mutagenic activity. The relationships between structure, metabolic activation, and mechanisms of mutagenic activity are discussed.

Activation of carcinogenic N-nitrosopropylamines to mutagens by lung and pancreas S9 fractions from various animal species and man

The mutagenic potential of 7 carcinogenic N-nitrosopropylamines was examined by the Ames liquid incubation assay, using lung and pancreas 9000 X g supernatant (S9) fractions from rats, hamsters, mice, rabbits, monkeys and humans for metabolic activation. N-Nitroso(2-hydroxypropyl)(2-oxopropyl)amine (HPOP), N-nitrosobis(2-oxopropyl)amine (BOP) and N-nitrosomethyl(2-oxopropyl)amine (MOP) showed positive mutagenicity in strain TA100 in the presence of lung S9 from each of the uninduced animals and humans. Besides the 3 N-nitrosopropylamines, N-nitrosomethyl(2-hydroxypropyl)amine (MHP) was also positive in the presence of lung S9 from polychlorinated biphenyl (PCB)-induced rats, hamsters and mice. On the other hand, in the presence of pancreas S9 from uninduced or PCB-induced animals, only HPOP and BOP showed positive mutagenicity. In contrast, N-nitrosobis(2-hydroxypropyl)amine (BHP), N-nitrosobis(2-acetoxypropyl)amine (BAP) and N-nitroso-2,6-dimethylmorpholine (NDMM) showed negative mutagenicity in the presence of lung and pancreas S9 from either uninduced or PCB-induced animals and humans. HPOP was a direct-acting mutagen, and lung and pancreas S9 from 5 animal species and man did not affect the activity. BOP was mutagenic even in the presence of bovine serum albumin. The mutagenic activation of MHP by lung S9 from PCB-induced rats, hamsters and mice was completely inhibited by preincubation in an atmosphere of carbon monoxide or by addition of cytochrome c or metyrapone to the S9 mixture, whereas 7,8-benzoflavone totally lacked this effect. However, that of MOP was insensitive to these inhibitors. These results of mutagenicity assay indicate that only the methyl derivatives of N-nitrosopropylamines, MHP and MOP are activated by the lung from 5 animal species and man, whereas the pancreas from all the tested animals did not activate the 7 N-nitrosopropylamines to mutagens, and that the phenobarbital-inducible major cytochrome P-450 in the lung of rodents is involved in the mutagenic activation of MHP.

Inhibitory effect of organic solvents on the mutagenicity of N-nitrosodialkylamines in Salmonella

The influence of organic solvents on the mutagenicity of 11 N-nitrosamines was examined in Salmonella typhimurium TA100 using the Ames's liquid incubation assay in the presence of rat-liver S9. The mutagenic activities of N-nitrosodimethylamine, N-nitrosodiethylamine, 6 oxidative derivatives of N-nitrosopropylamine and N-nitroso-2,6-dimethylmorpholine were considerably decreased by addition of dimethyl sulfoxide, dimethyl formamide, acetone, 95% ethanol or acetonitrile, which are recommended for use as solvents in the assay by Ames's group, to the incubation mixture. The mutagenic activities of N-nitrosodipropylamine and N-nitrosodibutylamine, which are barely soluble in water, were also suppressed by increasing concentrations of dimethyl sulfoxide. These organic solvents did not appear to exert their influence by desmutagenic and antimutagenic actions. In contrast, the recoveries of unmetabolized carcinogens from preincubation mixtures and from agar plates were significantly higher in the presence of organic solvents than in their absence. The results indicate that the inhibitory effect is a result of interference with the process of metabolic activation by liver S9.

Mutagenicity of N-nitrosobis(2-hydroxypropyl)amine and its related compounds in the presence of rat lung and liver S9

The mutagenic activities of N-nitrosobis(2-hydroxypropyl)amine (BHP) and its related compounds were studied in Salmonella typhimurium TA100 and TA98 strains by Ames's liquid incubation assay in the presence or absence of lung and liver S9 of rats treated with polychlorinated biphenyl (PCB). BHP and its related compounds, N-nitroso-(2-hydroxypropyl)(2-oxopropyl)amine (HPOP), N-nitrosobis(2-oxopropyl)amine (BOP), N-nitrosobis(2-acetoxypropyl)amine (BAP), and N-nitroso-2,6-dimethylmorpholine (NDMM) showed negative mutagenicity in the absence of lung and liver S9 in TA100 and TA98 strains while those compounds showed positive in the presence of liver S9 in TA100 strain. HPOP and BOP showed positive mutagenic activity in the presence of lung S9 in TA100 strain. HPOP showed the strongest mutagenic activity in the presence of lung and liver S9.

Mutagenicity of n-nitrosodiethanolamine and its acetyl derivatives

The mutagenicity of N-nitrosodiethanolamine and its mono- and di-acetyl derivatives was tested in the S. typhimurium test system, in cytogenetic studies and in the micronucleus test. N-nitrosodiethanolamine had no mutagenic effects towards several strains of S. typhimurium either in the absence or in the presence of metabolic activating systems. Its diacetyl derivative exerted mutagenic effects towards the S. typhimurium strains TA1530 and TA100. Neither compound increased significantly the nubmer of chromosomal aberrations or of micronuclei in mice.

Mutagenicity of several pancreatic carcinogenic derivatives of N-nitrosodipropylamine in the Ames assay

The mutagenic activity of several N-nitrosamines related to the potent hamster pancreatic carcinogen N-nitrosobis(2-oxopropyl)amine (BOP) has been investigated using the Ames Salmonella/microsomal mutagenicity test system. S9 from the livers of phenobarbital-pretreated hamsters was the source of activating enzyme, and strain TA1530 was the indicator organism. Mutagenicity assays of BOP, N-nitroso(2-hydroxypropyl)(2-oxopropyl)amine (HPOP), N-nitrosobis(2-hydroxypropyl)amine (BHP), N-nitroso-2,6-dimethylmorpholine (NDMM) and N-nitrosomorpholine (NM) indicate that only HPOP was strongly mutagenic in the absence of the hamster-liver preparation. In the presence of this activation system, NDMM was the most mutagenic, and cis NDMM was 2-3 times more mutagenic than the trans isomer of this compound. BOP and BHP were considerably less mutagenic than HPOP.

Mutagenicity of aliphatic nitrosamines in Salmonella typhimurium

25 aliphatic nitrosamines were examined in the Ames assay for bacterial mutagens, using rat liver "S-9" for activation. Of them, 8 carcinogens were mutagenic and 5 non-carcinogens were not mutagenic. However, 2 compounds not carcinogenic in rats were mutagenic and 9 carcinogens were not mutagenic, including 6 that are liver carcinogens in rats.

The mutagenicity of dialkyl nitrosamines in the Salmonella plate assay

N-nitrosodimethylamine (DMN) is not mutagenic in the standard Salmonella plate incorporation assay (Ames test) in the presence of an in vitro metabolic activation system (S-9) derived from rat liver. When the S-9 was derived from Aroclor- or phenobarbital-induced mouse or hamster liver or from uninduced hamster liver, mutagenic activity was observed. Increasing the amount of S-9 above the usual maximum level of 50 microliter per plate increased the mutagenic response. Similarly, the mutagenicity of N-nitrosodiethylamine (DEN) and N-nitrosodi(n-butyl)amine (DBN) was greater in the presence of hamster liver S-9 than when mouse or rat liver was used. Data are also presented indicating that the ability of rat liver S-9 to mediate the mutagenic activity of DMN in the "preincubation" assay is due to the fact that the various components are present in this assay at several times the concentrations attained in the standard plate incorporation assay.

Comparison of the effect of beta-oxidized dipropylnitrosamine metabolites administered at equimolar doses to Syrian hamsters

After subcutaneous administration of dipropylnitrosamine (DPN) to Syrian hamsters, gas-liquid chromatographic analysis of the 16-h urine revealed the DPN metabolites, 2-hydroxypropyl-, 2-oxopropyl-, and methylpropylnitrosamines. In a related series of experiments, hamsters received equimolar doses of the above compounds and of N-nitrosobis(2-hydroxypropyl)-amine (BHP) and 2,2'-dimethyldipropylnitrosamine (DMDPN). The metabolites as well as BHP and DMDPN had a weaker effect than did DPN on the rate and/or latency of respiratory tumors. In the respiratory tract, the segmental tumor distribution and histological types varied according to the compounds. The metabolites of DPN induced additional tumors in the digestive tract. These experiments do not support the concept that the beta-oxidized metabolites of DPN are the proximate carcinogens of the parent compound.