Sex differences in the single-dose toxicokinetics of N-nitrosomethyl(2-hydroxyethyl)amine in the rat.

The single-dose toxicokinetics of N-nitrosomethyl(2-hydroxyethyl)amine (NMHA) has been characterized in 8-week-old Fischer 344 rats by analysis using high-performance liquid chromatography of serial blood samples. An i.v. bolus dose of 0.6 mumol/kg to male rats revealed biphasic first-order elimination with a terminal half-life of 37.4 +/- 1.7 min for unchanged NMHA and 101 +/- 6 min for total radioactivity, and extensive conversion to polar metabolites was seen in the high-performance liquid chromatographic assays. The systemic blood clearance and apparent steady-state volume of distribution for unchanged NMHA were 13.1 +/- 0.9 ml/min/kg, and 685 +/- 31 ml/kg, respectively. Renal blood clearance and intrinsic hepatic clearance were estimated to be 0.805 +/- 0.024 and 16.7 +/- 2.1 ml/min/kg, respectively. A similar dose given to female rats yielded a terminal half-life for NMHA of 27.2 +/- 1.2 min, a steady-state volume of distribution of 652 +/- 23 ml/kg, and systemic blood, renal blood, and intrinsic hepatic clearances of 16.9 +/- 1.3, 1.45 +/- 0.14, and 22.5 +/- 0.3 ml/min/kg, respectively. The sex differences in terminal half-life and systemic blood, renal blood, and intrinsic hepatic clearances were significant at the P less than 0.05 level. Larger doses given by gavage, which appeared to be completely absorbed from the gut, indicated systemic bioavailabilities for unchanged NMHA of 78 +/- 10% and 69 +/- 1% for male and female rats, respectively. Binding of NMHA to plasma proteins was found to be negligible. Taken together the data allow for the conclusion that the observed sex differences in toxicokinetic parameters are due to differences in the intrinsic hepatic clearance of the compound. This difference in the ability of the liver to metabolize NMHA in vivo correlates with and may contribute to the greater susceptibility of female rats to hepatocarcinogenesis and of male rats to development of tumors in the nasal epithelium following oral exposure to NMHA.

Metabolic denitrosation of N-nitrosodimethylamine in vivo in the rat.

Enzymatic denitrosation is a potentially inactivating metabolic route that has been shown to convert carcinogenic N-nitrosodimethylamine (NDMA) to methylamine (MA) in vitro. To investigate its quantitative course in vivo, groups of 8-week-old male Fischer rats have been given small (8-15 mumol/kg) p.o. or i.v. bolus doses of 14C-labeled NDMA and the subsequent formation of radioactive MA has been monitored by high performance liquid chromatographic analysis of serially collected blood samples from each individual. Adjusting the [14C]MA fluxes observed for the previously measured rates at which MA is itself eliminated from the system after intragastric administration, denitrosation was calculated to represent a rather uniform 21.3 +/- 1.3% (SE) of total NDMA elimination in the four animals studied. By contrast, repetition of the experiment with fully deuterated NDMA (NDMA-d6) revealed a significantly wider variance in the results (39.8 +/- 8.9%). An alternative calculation using values for elimination of i.v. doses of MA and its trideuteromethyl analogue gave an even larger difference for MA formation between NDMA and NDMA-d6, the estimated extents of in vivo denitrosation in this case being 14.5 +/- 0.9% and 48.3 +/- 10.8%, respectively. The results indicate that denitrosation is a major metabolic pathway for NDMA elimination and suggest that deuteration of the carcinogen induces a shift in its metabolism toward increasing denitrosation at the expense of the competing activation pathway. Consequently, denitrosation may be the previously undefined in vivo metabolic route, the existence of which was suggested by the findings that deuteration of NDMA lowered its hepatocarcinogenicity and liver DNA alkylating ability in rats.

Carcinogenicity study of fecapentaene-12 diacetate on skin painting in SENCAR mice.

To investigate the effects of both diol esterification and coadministration with antioxidant on the tumorigenicity of fecapentaene-12 (FP-12) preparations, diacetylfecapentaene-12 (DAFP-12) in dimethylsulfoxide (DMSO) was applied to SENCAR mouse skin with or without the stabilizer, vitamin E, twice/week for 5 weeks, following which all animals were promoted for up to 25 weeks by weekly applications of 12-O-tetradecanoylphorbol-13-acetate (TPA). While positive controls receiving 7,12-dimethylbenz[a]anthracene (DMBA) instead of DAFP-12 in a similar protocol all developed papillomas (average of 23/animal), papilloma incidence in mice given DAFP-12 did not differ significantly from that of the vehicle control. We conclude that DAFP-12 shows little or no tumor initiating activity for mouse skin even when coadministered with vitamin E.

Inactivity of fecapentaene-12 as a rodent carcinogen or tumor initiator.

The possible carcinogenic activity of synthetic fecapentaene-12 (FP-12) was studied in several mammalian test systems: (a) for carcinogenicity by intrarectal instillation in male F344/NCr rats as well as by intrarectal and subcutaneous application in male B6C3F1 mice; (b) for initiation by skin painting in female SENCAR mice followed by repeated applications of 12-O-tetradecanoylphorbol-13-acetate (TPA), with 7,12-dimethylbenz[a]anthracene (DMBA) followed by TPA as positive control; (c) in a rat subcutaneous granuloma pouch assay in which mutagenicity was measured by induction of 6-thioguanine (6-TG) resistance and carcinogenicity was determined by induction of subcutaneous tumors in the pouch. There was no significant increase in tumor incidence after 72-78 weeks in test (a), although 2 rats receiving FP-12 intrarectally developed colon polyps. FP-12 did not initiate any skin tumors in test (b), nor did it significantly convert DMBA-initiated papillomas into carcinomas when 8 of the positive control mice were given FP-12 weekly for 10 weeks after 10 weeks on the DMBA-TPA regimen. Although FP-12 and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) were comparably mutagenic in test (c), FP-12 induced no tumors after more than a year in 133 rats at risk while MNNG induced 7 tumors in 107 rats. These rodent assays provide no evidence that FP-12 is a strong carcinogen, although the possibility remains that it may possess weak carcinogenic activity not revealed by these experiments.

2-Acetamido-p-benzoquinone: a reactive arylating metabolite of 3'-hydroxyacetanilide.

The covalent binding to protein of 3'-hydroxyacetanilide (3HAA), its primary metabolite 2',5'-dihydroxyacetanilide (2,5DHAA), and a putative secondary metabolite thereof, 2-acetamido-p-benzoquinone (APBQ), was studied in hepatic microsomal preparations from phenobarbital-pretreated mice. All compounds were found to bind irreversibly to microsomal protein, APBQ being by far the most effective member of the group. In the case of 3HAA, binding was dependent upon the presence in incubation media of the co-factor NADPH, indicating that metabolism of 3HAA was necessary for the generation of a reactive intermediate. In contrast, NADPH decreased by more than 2-fold the binding of both 2,5DHAA and APBQ. The free radical spin-trapping agent alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) did not reduce the binding of 3HAA to protein. These results support the contention that metabolic activation of 3HAA is a two-step process which involves initial aromatic hydroxylation to give the substituted hydroquinone, 2,5DHAA, followed by a second oxidation reaction (which may not be enzyme-mediated) to produce the benzoquinone derivative, APBQ. This quinone is a reactive, electrophilic intermediate which may either undergo reduction back to 2,5DHAA or bind covalently to cellular macromolecules.

Optimizing Caco-2 cell monolayers to increase throughput in drug intestinal absorption analysis.

INTRODUCTION: The aim of this investigation was to evaluate methods for increasing Caco-2 cell throughput for assessing drug intestinal absorption. The use of 6-, 12-, and 24-well membranes and the effect of membrane size on permeability and the integrity of the Caco-2 cell monolayer were assessed. In an effort to optimize the assessment of drug permeability, increased throughput was investigated by testing compounds singly or as mixtures of analytes. METHOD: The transepithelial electrical resistance (TEER) of cell monolayers was measured on 0.33, 1.0, and 4.7 cm2 polycarbonate membranes using EVOM, over a 25-day period. Absorptive transport was determined on all compounds tested using LC-MS/MS assays, or liquid scintillation spectrometry. RESULTS: The effect of multiple compounds in one well compared to single compounds was assessed with atenolol, nadolol, metoprolol, and propranolol for mixtures of four compounds and with RWJ-53308, atenolol, terbutaline, propranolol, naproxen, piroxicam, topiramate, and furosemide for mixtures of eight compounds. The apparent permeability (Papp) values correlated well between single analytes and mixtures of four and eight analytes in each well. Drug permeability decreased slightly with an increase in well size. The TEER value increased with the number of days in culture for each of the 6-, 12-, and 24-well sizes. DISCUSSION: It was demonstrated that the 24-well format system is ideal for high-throughput assessment. Furthermore, the approach of mixing four or eight analytes in each well to further increase throughput was also demonstrated to be valid.

Structural characterization of the major covalent adduct formed in vitro between acetaminophen and bovine serum albumin.

The structure of the covalent adduct formed in vitro between [14C]-acetaminophen ([14C]APAP) and bovine serum albumin (BSA) has been investigated with the aid of new analytical methodology. The APAP-BSA adduct, isolated from mouse liver microsomal incubations to which the radiolabeled drug and BSA had been added, was cleaved using a combination of specific (cyanogen bromide) and non-specific (acid hydrolysis) procedures, following which the mixture of amino acids obtained was derivatized, in aqueous solution, with ethyl chloroformate. The resulting ethoxycarbonyl derivatives were recovered by extraction into ethylacetate, methylated and subjected to profile analysis using both reverse-phase and normal-phase HPLC techniques. In each HPLC step, one major radioactive amino acid adduct was detected and was identified by mass spectrometry as the derivative of 3-cystein-S-yl-4-hydroxyaniline. Based on this finding, and with a knowledge of the behavior under acidic hydrolysis conditions of the 3-cysteinyl conjugate of APAP, it could be concluded that the major APAP-BSA adduct is one in which the drug is bound, via a thioether linkage at the C-3 position, to a sulfhydryl group on the protein. Furthermore, it could be established that this -SH function almost certainly is that associated with the cys-34 residue of BSA.

The covalent binding of acetaminophen to protein. Evidence for cysteine residues as major sites of arylation in vitro.

Covalent binding of the reactive metabolite of acetaminophen has been investigated in hepatic microsomal preparations from phenobarbital-pretreated mice. Low molecular weight thiols (cysteine and glutathione) were found to inhibit this binding, whereas several other amino acids which were tested did not. Bovine serum albumin (BSA), which contains a single free sulfhydryl group per molecule and which thus represents a macromolecular thiol compound, inhibited covalent binding of the reactive acetaminophen metabolite to microsomal protein in a concentration-dependent manner. The acetaminophen metabolite also became irreversibly bound to BSA in these experiments, although this binding was reduced by approx. 47% when the thiol function of BSA was selectively blocked prior to incubation. Covalent binding of the acetaminophen metabolite to bovine alpha s1-casein, a soluble protein which does not contain any cysteine residues, was found to occur to an extent of 37% of that which became bound to native BSA. These results were taken to indicate that protein thiol groups are major sites of covalent binding of the reactive metabolite of acetaminophen in vitro. The covalent binding characteristics of synthetic N-acetyl-p-benzoquinoneimine (NAPQI), the putative electrophilic intermediate produced during oxidative metabolism of acetaminophen, paralleled closely those of the reactive species generated metabolically. These findings support the contention that NAPQI is indeed the reactive arylating metabolite of acetaminophen which binds irreversibly to protein.

Covalent binding of acetaminophen to mouse hemoglobin. Identification of major and minor adducts formed in vivo and implications for the nature of the arylating metabolites.

When hepatotoxic doses of [ring-U-14C]acetaminophen ([ring-U-14C]APAP) were administered to mice, radioactivity became bound irreversibly to hemoglobin as well as to proteins in the liver and kidney. The covalent binding to hemoglobin was dose-dependent, and in phenobarbital-pretreated mice occurred to the extent of approximately 8% of the corresponding binding to liver proteins. Degradation of the modified globin by acid hydrolysis yielded 3-cystein-S-yl-4-hydroxyacetanilide as the major radioactive product, accounting for approximately 70% of protein-bound drug residues. This finding is consistent with the view that the majority of covalent binding of APAP to proteins is mediated by N-acetyl-p-benzoquinone imine (NAPQI), a reactive metabolite which preferentially arylates cysteinyl thiol residues. However, after administration of [acetyl-3H]APAP to mice, it was found that approximately 20% of the drug bound to hemoglobin had lost the N-acetyl side-chain, indicating the existence of a second type of APAP-protein adduct. One minor component of the globin hydrolysate was identified as S-(2,5-dihydroxyphenyl)-cysteine, which most likely arises from binding to hemoglobin of p-benzoquinone, a hydrolysis product of NAPQI. The two adducts reported represent the first identified examples of arylating drugs binding to hemoglobin. Experiments on the influence of different cytochrome P-450 inducing agents on the ratio of drug bound to hemoglobin versus hepatic proteins suggested that the reactive metabolites of APAP are formed in the liver and migrate to the erythrocyte, rather than being produced by hemoglobin-catalyzed oxidation of APAP. These findings imply that the reactive metabolites of APAP escape from hepatocytes in some latent forms, which then participate in the arylation of protein thiols in red blood cells and, possibly, at other remote sites.

Body fat in children does not adversely influence bone development: a 7-year longitudinal study (EarlyBird 18).

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT: Both negative and positive associations have been reported between body fat and bone density. Extra mechanical loading from excess fat may lead to greater bone mass. Excess ectopic fat may lead to bone demineralisation through inflammatory pathways. WHAT THIS STUDY ADDS: Longitudinally collected data from narrow-angle beam densitometry gives a novel insight into bone growth through adolescence. There is no evidence of a deleterious effect of body fat on children's growing bones after adjustment for height and age. Body fat, mediated by puberty, is associated with larger bones in boys and bones that are both denser and larger in girls. OBJECTIVE: Bone growth is an important determinant of peak bone mass and fracture risk, but there is limited data on the impact of fat-on-bone development at a time when childhood obesity is reaching epidemic proportions. Accordingly, we explored the effect of body fat (BF) on bone growth over time in the context of age, pubertal tempo and gender. METHOD: A cohort of 307 children was measured biannually from 9-16 years for height and weight, and every 12 months for percent BF, bone area (BA), bone mineral content and areal bone mineral density (aBMD) by dual-energy X-ray absorptiometry. Pubertal tempo was determined quantitatively by age at peak height velocity. RESULTS: Percent BF increased and then fell in the boys, but increased throughout in the girls. aBMD and BA increased in both genders (P < 0.001). Greater BF was associated with higher aBMD and BA in girls (P < 0.001), but only BA in boys (P < 0.001). The extra aBMD associated with increased BF was greater in older girls. The rise in aBMD and BA was associated with earlier puberty in both genders (P < 0.001). The impact of BF on aBMD was greater in later puberty in girls (0.0025 g cm(-2) per 10% BF at 10 years versus 0.016 g cm(-2) per 10% BF at 14 years, P < 0.001). CONCLUSION: Greater BF is associated with larger bones, but also denser bones in girls. The effects of fat and puberty are complex and gender specific, but BF of contemporary UK children does not appear to be deleterious to bone quality.

The novel, orally active, delta opioid RWJ-394674 is biotransformed to the potent mu opioid RWJ-413216.

Although the mu opioid receptor is the primary target of marketed opioid analgesics, several studies suggest the advantageous effect of combinations of mu and delta opioids. The novel compound RWJ-394674 [N,N-diethyl-4-[(8-phenethyl-8-azabicyclo]3.2.1]oct-3-ylidene)-phenylmethyl]-benzamide]; bound with high affinity to the delta opioid receptor (0.2 nM) and with weaker affinity to the mu opioid receptor (72 nM). 5'-O-(3-[(35)S]-thio)triphosphate binding assay demonstrated its delta agonist function. Surprisingly given this pharmacologic profile, RWJ-394674 exhibited potent oral antinociception (ED(50) = 10.5 micromol/kg or 5 mg/kg) in the mouse hot-plate (48 degrees C) test and produced a moderate Straub tail. Antagonist studies in the more stringent 55 degrees C hot-plate test demonstrated the antinociception produced by RWJ-394674 to be sensitive to the nonselective opioid antagonist naloxone as well as to the delta- and mu-selective antagonists, naltrindole and beta-funaltrexamine, respectively. In vitro studies demonstrated that RWJ-394674 was metabolized by hepatic microsomes to its N-desethyl analog, RWJ-413216 [N-ethyl-4-[(8-phenethyl-8-azabicyclo[3.2.1]oct-3-ylidene)-phenylmethyl]-benzamide], which, in contrast to RWJ-394674, had a high affinity for the mu rather than the delta opioid receptor and was an agonist at both. Pharmacokinetic studies in the rat revealed that oral administration of RWJ-394674 rapidly gave rise to detectable plasma levels of RWJ-413216, which reached levels equivalent to those of RWJ-394674 by 1 h. RWJ-413216 itself demonstrated a potent oral antinociceptive effect. Thus, RWJ-394674 is a delta opioid receptor agonist that appears to augment its antinociceptive effect through biotransformation to a novel mu opioid receptor-selective agonist.

The in vitro metabolism of hydralazine.

Hydralazine was metabolized in vitro to phthalazine and phthalazine-1-one by microsomal enzymes requiring NADPH. Hydrazine was not detected as a metabolite under these in vitro conditions. Addition of 1-hydrazinophthalazin-4-one, a possible intermediate metabolite, decreased covalent binding to protein but also decreased metabolism. Phthalazine and phthalazin-1-one also decreased covalent binding to protein to a lesser extent. N-Acetylcysteine significantly decreased the level of phthalazine and phthalazin-1-one detected in microsomal incubations. The results are consistent with a reactive intermediate, but not 1-hydrazinophthalazin-4-one, being responsible for both covalent binding and production of phthalazine.

Enzyme-mediated covalent binding of hydralazine to rat liver microsomes.

Hydralazine was metabolized in vitro to a reactive metabolite(s) which bound to microsomal protein. The binding was dependent on mixed function oxidase activity, was proportional to time and microsomal protein concentration, and was not removed by acid washing. The apparent Km was 3.55 X 10(-5) M and the Vmax was 0.342 nmol/mg protein/min. Binding was inhibited by carbon monoxide and required oxygen and enzyme activity. Microsomes from animals pretreated with 3-methylcholanthrene showed a slight but significant increase in covalently bound hydralazine metabolite(s) whereas piperonyl butoxide treatment significantly reduced this binding. Glutathione, cysteine, and N-acetylcysteine all reduced the binding when added to incubations. Therefore, hydralazine is metabolized by the microsomal enzymes to a metabolite(s) capable of reacting covalently with cellular macromolecules.

Studies on the in vivo metabolism of hydralazine in the rat.

A single dose of [1-14C]hydralazine is extensively metabolized in the rat as no unchanged drug is excreted in the urine, the major route of elimination of the drug. However, only a small proportion of the dose could be accounted for as known metabolites. The lack of expired 14CO2 suggests that the phthalazine ring is metabolically stable. The metabolites were qualitatively but not quantitatively similar to those excreted by human subjects. The three major urinary metabolites were found to be 3-methyl-s-triazolo[3,4a] phthalazine and acid-labile conjugates of hydralazine and 1-hydrazinophthalazin-4-one. There were also small amounts of s-triazolo[3,4-a]-phthalazine, 3-hydroxymethyl-s-triazolo[3,4-a]-phthalazine, hydrazine, and phthalazin-1-one. Induction of the microsomal enzymes by pretreatment with 3-methylcholanthrene reduced the excretion of the acetylated metabolite 3-methyl-s-triazolo[3,4-a]phthalazine, of conjugates of hydralazine and 1-hydrazinophthalazin-4-one. Pretreatment with phenobarbital reduced excretion of 3-methyl-s-triazolo[3,4-a]phthalazine. Conversely, inhibition of the microsomal enzymes by pretreatment with piperonyl butoxide increased the excretion of 3-methyl-s-triazolo[3,4-a]phthalazine and decreased the excretion of 1-hydrazinophthalazin-4-one conjugates. [14C]Hydralazine or a metabolite was covalently bound to tissue protein, particularly in the aorta, lungs, and spleen. Induction of the microsomal enzymes with 3-methylcholanthrene reduced and inhibition of the microsomal enzymes increased the binding to the aorta. In conclusion, the covalent binding of [14C]hydralazine or a metabolite to protein does not appear to be mediated by the microsomal enzymes in the rat and the metabolism of hydralazine in the rat shows considerable quantitative differences from that in man.

Evidence for the involvement of a nitrenium ion in the covalent binding of nitrofurazone to DNA.

We have shown that the xanthine oxidase-catalyzed anaerobic reduction of nitrofurazone in the presence of added DNA leads to the formation of covalently bound adducts. Further, by systematically decreasing the pH of the reaction mixture, we have demonstrated that generation of the reactive species is facilitated under mildly acidic conditions. From these observations, we conclude that it is the nitrenium ion formed from nitrofurazone which binds to DNA.

Studies on hydrazine hepatotoxicity. 2. Biochemical findings.

Hydrazine causes a dose-related increase in liver triglycerides and in liver weight and causes a decrease in hepatic glutathione. The threshold dose for the toxic effect is around 10 mg/kg, and the optimal effect is seen after a dose of 40 mg/kg. The effect of hydrazine on liver weight and glutathione was detectable within 30 min of dosing, but the elevation of hepatic triglycerides was not detectable until 4 h after dosing. At 24 h after a dose of 60 mg hydrazine/kg, hepatic reduced glutathione was approximately 50% of the control value and triglycerides were about 7 times the normal level. In vitro studies indicated that hydrazine is metabolized by rat liver microsomal enzymes, this being dependent on NADPH and oxygen. Pretreatment of animals with phenobarbital or piperonyl butoxide respectively decreases and increases the hepatotoxicity. Prior depletion of hepatic glutathione by administration of diethyl maleate had no effect on the toxicity. Pyruvate azine, a probable metabolite of hydrazine, is much less toxic than hydrazine itself on a molar basis. These and other results suggest that although hydrazine is metabolized via several routes, the hepatotoxicity may well be due to the parent compound rather than a metabolite.

Investigations of the N-hydroxylation of 3'-hydroxyacetanilide, a non-hepatotoxic positional isomer of acetaminophen.

The hydroxamic acid of 3'-hydroxyacetanilide (AMAP) was synthesized to test the hypothesis that different reactive metabolites of AMAP and acetaminophen account for similarities in covalent binding of the two positional isomers to hepatic proteins, but for differences in their ability to cause hepatotoxicity. N-OH-AMAP was found to be a relatively stable hydroxamic acid, but it was not detected as a metabolite of AMAP formed in vitro by mouse liver microsomes or in urine of mice administered AMAP. Therefore, metabolites other than N-OH-AMAP must be responsible for covalent binding observed with AMAP to mouse liver proteins.