The hepatic metabolism of 15N labelled dimethylnitrosamine in the rat

Strategies for Assessing Acceptable Intakes for Novel N-Nitrosamines Derived from Active Pharmaceutical Ingredients

The detection of N-nitrosamines, derived from solvents and reagents and, on occasion, the active pharmaceutical ingredient (API) at higher than acceptable levels in drug products, has led regulators to request a detailed review for their presence in all medicinal products. In the absence of rodent carcinogenicity data for novel N-nitrosamines derived from amine-containing APIs, a conservative class limit of 18 ng/day (based on the most carcinogenic N-nitrosamines) or the derivation of acceptable intakes (AIs) using structurally related surrogates with robust rodent carcinogenicity data is recommended. The guidance has implications for the pharmaceutical industry given the vast number of marketed amine-containing drugs. In this perspective, the rate-limiting step in N-nitrosamine carcinogenicity, involving cytochrome P450-mediated α-carbon hydroxylation to yield DNA-reactive diazonium or carbonium ion intermediates, is discussed with reference to the selection of read-across analogs to derive AIs. Risk-mitigation strategies for managing putative N-nitrosamines in the preclinical discovery setting are also presented.

Metabolic Activation and DNA Interactions of Carcinogenic N-Nitrosamines to Which Humans Are Commonly Exposed

Carcinogenic N-nitrosamine contamination in certain drugs has recently caused great concern and the attention of regulatory agencies. These carcinogens-widely detectable in relatively low levels in food, water, cosmetics, and drugs-are well-established and powerful animal carcinogens. The electrophiles resulting from the cytochrome P450-mediated metabolism of N-nitrosamines can readily react with DNA and form covalent addition products (DNA adducts) that play a central role in carcinogenesis if not repaired. In this review, we aim to provide a comprehensive and updated review of progress on the metabolic activation and DNA interactions of 10 carcinogenic N-nitrosamines to which humans are commonly exposed. Certain DNA adducts such as O⁶-methylguanine with established miscoding properties play central roles in the cancer induction process, whereas others have been linked to the high incidence of certain types of cancers. We hope the data summarized here will help researchers gain a better understanding of the bioactivation and DNA interactions of these 10 carcinogenic N-nitrosamines and facilitate further research on their toxicologic and carcinogenic properties.

In vitro metabolism of bladder carcinogenic nitrosamines by rat liver and urothelial cells

In order to establish the importance of the target organ in the activation of bladder carcinogens, we compared rat liver and urothelial cell alpha-hydroxylation activities using as substrates N-nitrosobutyl(4-hydroxybutyl)amine and its metabolite N-nitrosobutyl(3-carboxypropyl)amine, two potent urinary bladder carcinogens in animals. Previous studies have shown that the production of molecular nitrogen can serve as an indicator of nitrosamine alpha-hydroxylation. The use of doubly 15N-labelled nitrosamines and the gas chromatography-mass spectrometric detection of 15N2 formed gives a measurement of the extent of this metabolic step. Various amounts of 15N-labelled substrates were incubated for 60 min at 37 degrees C with rat liver S9 preparations or urothelial cell homogenates in the presence of a NADPH generating system. Both enzyme sources metabolized 15N-labelled N-nitrosobutyl(4-hydroxybutyl)amine and N-nitrosobutyl(3-carboxypropyl)amine through the alpha-hydroxylation pathway. Using hepatic S9 fractions, 15N2 production from 15N-labelled N-nitrosobutyl(4-hydroxybutyl)amine increased from 1.69 +/- 0.02 nmol/h per mg protein (mean +/- S.E.) to 5.78 +/- 0.5 with substrate concentrations ranging between 0.55 and 5.55 mM. 15N2 produced by urothelial cell homogenates was about 40-50% that of the liver S9. 15N-labelled N-nitrosobutyl(3-carboxypropyl)amine was also metabolized through the alpha-hydroxylation pathway both by hepatic S9 and urothelial cell homogenates, though to a lesser extent. 15N2 production was about 10-times less than from 15N-labelled N-nitrosobutyl(4-hydroxybutyl)amine, but again urothelial cell 15N2 production was about 40-50% that of the liver. Treatment with phenobarbital resulted in a 2.7-fold increase in the 15N2 produced from 15N-labelled N-nitrosobutyl(4-hydroxybutyl)amine by hepatic S9. No effect was observed with urothelial cell homogenates. Acetone treatment had no effect on 15N2 production from 15N-labelled N-nitrosobutyl(4-hydroxybutyl)amine by hepatic S9, but raised 15N2 production by urothelial cell homogenates 1.8 times. Although the liver has a greater capacity than the bladder for activating the 15N-labelled nitrosamines studied, the target organ can metabolize bladder carcinogens, thus increasing the possibility of a local toxic effect. Moreover, the distribution of P-450 isozymes might be different in the bladder and this could affect the metabolism of nitrosamines reportedly formed in the human bladder in some pathological conditions.

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)

Effects of pyrazole on nitrosodimethylamine demethylase and other microsomal xenobiotic metabolising activities

Pyrazole administered to immature rats at one day or on four successive days prior to sacrifice increased a microsomal NDMAD with apparent Km 0.04 mM. Aniline hydroxylase activity was also increased by these treatments. Ethoxycoumarin deethylase and amino pyrine demethylase activities were not altered when animals were treated with pyrazole one day prior to sacrifice but were reduced to below control activity when animals were treated for four successive days. All microsomal mono-oxygenases were decreased when animals received a single administration of pyrazole four days prior to sacrifice and the cytochrome P-450 content of these microsomes was reduced by up to 50%. When microsomes from untreated animals or animals treated for four successive days were incubated with pyrazole in the presence of NADPH, cytochrome P-450 content decreased in a time dependent process to a limiting value. The effect was dependent on pyrazole concentration and saturable. These results suggest that pyrazole induces a cytochrome P-450 isoenzyme with high affinity for NDMA but also acts as a suicide inhibitor of the cytochrome.

Formation and metabolism of nitrosamines in vivo, monitored by 14N-stable isotope labelling

Microsomal metabolism of N-nitrosodimethylamine entails release of molecular nitrogen; the extent is determined by 15N stable isotope labelling and mass-spectrometric isotope ratio measurements. Exhalation of labelled nitrogen by rats treated with 15N-dimethylamine and nitrite or 15N-nitrite alone indicates that nitrogen may arise from nitrite via two pathways: either directly from nitrosation of primary amines or from secondary and tertiary amines with subsequent enzymic N-demethylation. The overall yield of nitrosamine formation, N-demethylation and nitrogen-release represent about 0.3-6% or the administered dose of dimethylamine (1.1 mmol/kg), depending upon the dose of nitrite (0.55-2.2 mmol/kg). 15N-stable isotope labelling and mass-spectrometric isotope ratio measurements are powerful tools for assessment of endogenous nitrosamine formation from nitrite. One hundred nmol of labelled nitrogen are easily detectable in vivo; with further methodological refinement the limit of detection may be lowered by two orders of magnitude.

Metabolism of 15N-labelled N-nitrosodimethylamine and N-nitroso-N-methylaniline by isolated rat hepatocytes

The N-demethylation of 15N-labeled N-nitrosodimethylamine (DMN) and N-nitroso-N-methylaniline (NMA) by isolated rat hepatic cells has been investigated. The values obtained in this system for molecular nitrogen formed during metabolism, compared with substrate consumed, were DMN 47%, NMA 23%, and N-nitroso-N-methylurea (NMU) 105%. The results for DMN are roughly halfway between those previously determined with rat liver S-9 fraction in vitro (33%) and in vivo (67%). For NMA, the hepatocyte data are closer to those obtained from S-9 in vitro (19%), rather than the in vivo (52%). No mixed nitrogen ( 15N14N ) or labeled nitrogen oxides were found.

Studies on the relationship between dimethylnitrosamine-demethylase activity and dimethylnitrosamine-dependent mutagenesis in Drosophila melanogaster

The relationship between dimethylnitrosamine (DMN) demethylase activity and DMN-induced mutagenesis was investigated in Drosophila melanogaster. The activity of DMN-demethylase was at least 10-fold greater in the Hikone-R strain than in three other Drosophila strains. However, the sex-linked recessive lethal (SLRL) mutations induced by DMN in the four strains differed by less than 2-fold. Several possibilities to explain the lack of correlation between DMN-demethylase activity and DMN-induced mutations were tested and eliminated. They include: (i) the presence of inhibitors of DMN-demethylase in extracts of low-activity strains, (ii) a sex bias in the Hikone-R strain in which the enzyme activity is confined to the females, (iii) the possibility that DMN treatment induces DMN-demethylase activity in the low-activity strains and (iv) the possibility that Hikone-R has a much more efficient DNA repair system than the other strains. The results are discussed in terms of what is known about the role of DMN-demethylase in the metabolic activation of DMN in other systems.

Studies on the metabolism of dimethylnitrosamine in vitro by rat-liver preparations. II. Inhibition by substrates and inhibitors of monoamine oxidase

1. The metabolism of dimethylnitrosamine (DMN) to formaldehyde by rat-hepatic postmitochondrial supernatant fractions has been compared with the activities of several cytochrome P-450-dependent mixed-function oxidase enzymes and the Ziegler mixed-function amine oxidase enzyme (EC 1.14.13.8). 2. A variety of monoamine oxidase (MAO, EC 1.4.3.4) inhibitors of diverse chemical structure inhibited the metabolism of DMN. In parallel studies a number of MAO substrates, but not their deaminated products, also inhibited DMN metabolism, whereas substrates of diamine oxidase were ineffective. 3. At concentrations which inhibited DMN metabolism several MAO substrates and inhibitors did not inhibit the N-oxidation of N, N-dimethylaniline and an inhibitor and an activator of the Ziegler enzyme had no corresponding effect on DMN metabolism. 4. The metabolism of DMN and a number of MAO enzyme activities were stable to storage under conditions where mixed-function oxidase enzymes were not. 5. These results are consistent with the suggestion that DMN may, at least in part, be metabolized by hepatic enzyme(s) not dependent on cytochrome P-450 and that a microsomal amine oxidase enzyme, unrelated to the Ziegler enzyme, may be involved in the hepatic degradation of this nitrosamine. The present data does, however, suggest a role for microsomal NADPH-cytochrome c reductase in hepatic DMN metabolism.

alpha-Hydroxylation pathway in the in vitro metabolism of carcinogenic nitrosamines: N-nitrosodimethylamine and N-nitroso-N-methylaniline

Evolution of 15N2-labeled molecular nitrogen was used to gauge the extent of alpha-hydroxylation during rat liver homogenate metabolism of doubly 15N-labeled N-nitrosodimethylamine (DMN) and N-nitrosomethylaniline (NMA). These measurements were correlated with the extent of total metabolism as measured by the disappearance of the nitrosamines and by the formation of formaldehyde. The results indicate that approximately 34% of DMN and 19% of NMA were metabolized by the alpha-hydroxylation pathway. Positive controls utilizing doubly 15N-labeled N-nitroso-N-methylurea yielded 96% of labeled nitrogen. These results are in variance with previously published data which claimed that either less than 5% or about 100% of DMN is metabolized by that route in vitro. Formaldehyde formation was shown to be a poor measure of the extent of metabolism. Semicarbazide gave rise to both formaldehyde and nitrogen, which makes it an undesirable component of the in vitro metabolism mixtures, particularly when those two substances are being measured.

Substrates and inhibitors of hepatic amine oxidase inhibit dimethylnitrosamine-induced mutagenesis in Salmonella typhimurium

The mutagenic effect of dimethylnitrosamine in Salmonella typhimurium TA100, in the presence of a rat-liver homogenate derived from animals treated with Aroclor 1254, was inhibited by substrates and inhibitors of monoamine oxidase. Substrates of diamine oxidase did not inhibit dimethylnitrosamine mutagenesis and, furthermore, monoamine oxidase inhibitors had no effect on mutagenesis by benzo[a]pyrene or aflatoxin B1. The results suggest that monoamine oxidase participates in the activation of dimethylnitrosamine to a mutagen.

Interaction of vitamin E and selenium with the hepatotoxic agent dimethylnitrosamine

Pretreatment of rats with vitamin E, 0.02% w/w of the diet and a sc dose, 200 mg/kg, given 48 hrs. before dimethylnitrosamine, DMNA, was found to ameliorate the acute hepatotoxicity of DMNA (30 mg/kg) as reflected in reduced plasma asparagine-amino-transferase (AspAT) activity. This effect was confirmed by histological evaluation. No significant effect of DMNA on plasma levels of vitamin E was observed, however, DMNA significantly increased the hepatic level of vitamin E supplemented rats. Pretreatment with selenium, 0.5 mg/kg given intraperitoneally 48 hrs. before DMNA, was found to enhance the acute hepatotoxicity of DMNA as reflected in increased elevation of plasma AspAT activity. This effect was not confirmed morphologically. DMNA did not have any effect on the hepatic selenium state in selenium pretreated rats; however, selenium pretreatment tended to decrease hepatic and plasma tocopherol levels. To explain the effects observed in the present investigation, various mechanisms were discussed. If the compounds were acting as antioxidants, then the difference in intracellular localization had to be important. More likely a specific biochemical function involving drug metabolizing enzymes could be involved. Finally vitamin E could protect membranes from damage during the necrotizing action of DMNA.

[On the carcinogenetic action of N-nitroso compounds. 7th communication: methyl-, trideuteromethyl-, ethyl-, n-propyl-, n-butyl-, acetoxymethyl-nitrosamine, and methyl-butyroxymethyl-nitrosamine (author's transl)]

The homologons alkyl-acetoxymethyl-nitrosamines were tested for carcinogenicity in SD rats. All compounds were found to be carcinogenic and induced within the same time carcinomas of the forestomach. The total doses necessary for induction of tumors are related to the length of the alkyl chain and hence to the watersolubility. These results are discussed.