Fast-atom bombardment and thermospray mass spectrometry for the characterization of two glucuronide metabolites of methapyrilene.

Two conjugated metabolites of methapyrilene hydrochloride isolated from mouse-hepatocytes were examined by mass spectrometry using fast-atom bombardment (FAB) and thermospray ionization. The major metabolite, methapyrilene glucuronide, was identified based on a prominent peak due to the protonated molecule as well as the loss of the dimethylamine and sugar moieties. Identification of the second metabolite was complicated by large signals associated with the biological sample matrix. The complementary nature of the fragmentation observed in the mass spectra using FAB and thermospray ionization allowed this metabolite to be identified as the desmethylmethapyrilene glucuronide. The fragmentation observed using FAB ionization was not greatly affected by the presence of the glucuronide moiety. While loss of the sugar moiety indicated a glucuronide, additional fragmentation confirmed the presence of the underlying ethylenediamine substructure which is characteristic of this class of antihistamines.

Characterization of the antihistamines tripelennamine, methapyrilene, and thenyldiamine and their N-oxides by thermospray mass spectrometry.

This study describes an investigation of the thermospray (TS) mass spectrometric analysis of tripelennamine, methapyrilene, thenyldiamine, and their N-oxide derivatives. These compounds were analyzed by direct injection TS mass spectrometry in the column bypass mode and with 0.1M ammonium acetate/methanol (80:20, v/v) as the mobile phase. Typically, the parent antihistamines produced only [MH]s ions under these conditions. The N-oxides provided strong [MH]s ions and multiple fragment ions. A scheme to explain the fragmentation patterns is proposed.

Glutathione and lipid peroxide levels in rat liver following administration of methapyrilene and analogs.

The possibility was examined that the induction of tumors in rat liver by feeding methapyrilene, which is not mutagenic, is related to effects on glutathione levels and lipid peroxidation. Fischer 344 rats were given single-dose and multiple-dose treatments with the anti-histamine methapyrilene (MP), which is carcinogenic in rats, and with two non-carcinogenic analogs, methafurylene (MF) and thenyldiamine (TD) and the effects on malonaldehyde (MDA) formation and glutathione (GSH) levels in the liver were investigated. After a single dose, MDA levels were increased at 6 h by MF and TD and at 24 h by MP. MDA levels returned to normal after 30 h with MP and MF, but not with TD. Levels of MDA (and other TBA-reactive products) after four daily treatments were most elevated by TD, less elevated by MP, and were lowered by MF. Forty-two hours following treatment with both MP and MF, MDA levels had returned to normal, but in TD-treated animals MDA remained high. GSH levels were highest after MF and MP, and remained high at 42 h, but TD induced only a small increase. There appears to be increased lipid peroxidation in the liver as a result of treatment of rats with MP, MF and TD. The greater response induced by TD, as well as the increased liver GSH levels after repeated administration of all three drugs indicate that lipid peroxidation in rat liver is not a particular effect related to the liver carcinogen methapyrilene.

Lack of binding of methapyrilene and similar antihistamines to rat liver DNA examined by 32P postlabeling.

The nonmutagenic carcinogen methapyrilene, together with several noncarcinogenic analogues, was administered to rats p.o. for as long as 4 wk at concentrations of 0.1%. DNA was isolated from the liver and other organs and hydrolyzed, and the identification of covalent adducts was made using the 32P postlabeling method of Randerath. Some modified procedures were also used to deal with the possibility of very mobile adducts being formed from these hydrophilic amines. Although the rats had received as much as 2 g of amine per kg of body weight, no evidence of formation of DNA adducts in liver or other organs was seen; the level of detection was between 1 in 10(8) and 1 in 10(9) nucleotides. Adduct formation from much lower doses of the mutagenic food pyrolysis product 2-amino-3-methylimidaz(4, 5f)quinoline was detectable at a level of 1 in 10(6) nucleotides in parallel analyses. These results add to the evidence that carcinogenesis by methapyrilene is through an indirect or nonmutagenic mechanism.

A comparative in vitro metabolic study of methaphenilene and pyribenzamine.

1. In vitro metabolism of methaphenilene (MFN) and pyribenzamine (PBZ) was compared to that of methapyriline (MPH) in rat, because chronic treatment with MPH causes cancer in rats, whereas MFN and PBZ cause no cancer. 2. G.l.c. and mass spectrometry were used to identify 7 metabolites of MFN and 6 of PBZ in extracts of rat liver microsome incubations. 3. Quantification of the metabolic pathways revealed that N-oxide formation is considerably more important for both MFN and PBZ than for MPH, and only MPH forms an amide as a metabolic product. 4. Quantitative balance studies show that a lower recovery is apparent for metabolic experiments with MPH than for either MFN or PBZ under all conditions examined, indicating that significant metabolic pathways for MPH exist which are not being measured under these conditions.

Mutagenicity studies of selected antihistamines, their metabolites and products of nitrosation.

Methapyrilene, four structurally related antihistamines, three metabolites of methapyrilene and two products of the reaction of methapyrilene with nitrite were all tested for mutagenicity to Salmonella typhimurium. The two products of the methapyrilene-nitrite reaction have also been identified as metabolites of methapyrilene. None was mutagenic alone, either with or without rat liver S-9 activation. After reaction with sodium nitrite in acetic acid solution (nitrosation), the products of five of the ten compounds were mutagenic. These compounds were methaphenilene, 2-thiophenemethanol, 2-thiophenecarboxylic acid, N-(2-pyridyl)-N'N'-dimethylethylenediamine and N-(2-thenylmethyl)-2-aminopyridine.

Carcinogenicity studies of some analogs of the carcinogen methapyrilene in F344 rats.

Four antihistaminic drugs similar in structure to the rat liver carcinogen methapyrilene were administered to comparable groups of male and female F344 rats in their drinking water for most of their lifetime (80-108 weeks). The concentrations were 0.1% or 0.05% and the total doses received by the animals were comparable with that of methapyrilene which induced 100% incidence of liver neoplasms. No increase in incidence of liver neoplasms was observed after treatment with any of the four compounds, thenyldiamine, chlorothen, methafurylene, or methaphenilene, although each differed structurally from methapyrilene only in one atom or one position of substitution. There were a few animals with neoplasms not usually found in untreated F344 rats, but none of these was found in statistically significant numbers. These results suggest that none of the four analogs of methapyrilene was carcinogenic under the conditions of this study, and that the property of inducing liver neoplasms in rats was confined to the intact methapyrilene molecule.