Absorption and disposition kinetics of 3,3',4,4'-tetrachloroazoxybenzene in the male Fischer 344 rat

3,3',4,4'-Tetrachloroazoxybenzene (TCAOB) is a structural analog of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). It is formed as a byproduct during the synthesis of industrial products such as herbicides. TCAOB is a ligand for the Ah receptor and, at much higher doses, exhibits toxicities similar to TCDD. Although the reduced in vivo toxicity of TCAOB probably reflects differences in disposition, this study characterized its absorption and disposition kinetics. Male Fischer 344 rats were administered [14C]TCAOB (3.4 or 34 mg/kg p.o., 3.4 mg/kg i.v.), and the excretion of the radiolabel was monitored over 96 hr. After the low and high dose, 35% and 30% of the [14C]TCAOB were eliminated in the urine, with 55% and 54% eliminated in the feces. At 96 hr, the adipose tissue:blood ratios of [14C]TCAOB equivalents were 8 and 26 for the low and high doses, respectively. After the intravenous dose of TCAOB, the adipose tissue:blood ratio was 21 at 96 hr. Other tissue:blood ratios were of little significance (0.06-3.2). Pharmacokinetic parameters indicate that the parent molecule is cleared from blood with an average half-life of 7 hr and an average clearance of 11 ml/min.kg. Absolute bioavailability was calculated to be approximately 9%. Urine contained a variety of dichlorolaniline conjugates, which support the importance of azo reduction in the disposition of TCAOB. When compared with TCDD, the absorption of TCAOB is greatly reduced and the elimination of metabolites greatly enhanced. Therefore, at equal molar oral doses, TCAOB would express lower levels of Ah receptor-mediated toxicity than those defined for TCDD.

3,3',4,4'-tetrachloroazobenzene absorption, disposition, and metabolism in male Fischer 344 rats

3,3',4,4'-Tetrachloroazobenzene (TCAB) is a contaminant generated during the synthesis of 3,4-dichloroaniline and 3,4-dichloroaniline-derived pesticides. TCAB is isosteric to 2,3,7,8-tetrachlorodibenzo-p-dioxin and has been shown to bind to the Ah receptor. Following oral administration of [14C]TCAB (3.2 and 32 mg/kg), 39-45% of the dosed radioactivity was excreted into the urine and 53-56% was recovered in the feces within 48 hr. Less than 6% of the dosed radioactivity remained in the tissues examined at 96 hr. After intravenous administration (3.2 mg/kg), 33% of the dose was excreted in the bile during 6 hr. TCAB metabolites in urine were identified using LC/MS. The major metabolites were sulfate ester conjugates of hydroxylated mono- or dichloroaniline derivatives. Some of these metabolites were also acetylated. After intravenous administration, the disappearance of [14C]TCAB from blood was monitored, and the pharmacokinetic profile was consistent with a two-compartment model. Pharmacokinetic parameters reveal that the compound is readily cleared from the blood with a t1/2 of 4.0 hr, clearance of 12.3 ml/min.kg, and an apparent volume of distribution of 4.3 liters/kg. The absolute oral bioavailability was determined to be 30%. The extensive azo reduction of TCAB decreases its systemic absorption after oral administration and thereby limits the amount of parent compound available to interact with the Ah receptor and decreases the Ah receptor-mediated toxicity.

Effect of bleomycin-induced fibrosis on pulmonary metabolism of selected xenobiotics

Few studies are available characterizing the effects of fibrosis on pulmonary disposition of drugs or environmental pollutants. Two model substrates, p-nitroanisole and carbofuran, were selected to evaluate the effect of bleomycin-induced fibrosis on pulmonary disposition and metabolism using the isolated perfused lung and in vitro enzyme preparations. The rate of p-nitroanisole oxidation in the isolated perfused lung was significantly lower in fibrotic (k = 0.0334 min-1) than in control (k = 0.0493 min-1) lungs. However, there was no difference in the amount of p-nitrophenol formed between control (38 +/- 4 micrograms) and fibrotic (47 +/- 7 micrograms) lungs. Carbofuran clearance was similar in control (t1/2 = 91 min, ke = 0.008 min-1) and fibrotic (t1/2 = 75 min, ke = 0.009 min-1) lungs and was consistent with a one-compartment model. The Km value for p-nitrophenol formation in microsomes (0.185 +/- 0.095 mM) from control lungs was similar to fibrotic lungs (0.054 +/- 0.014 mM); however, Vmax was significantly higher in healthy (34.6 +/- 6.3 pmoles/min per mg microsomal protein) than in fibrotic (11.16 +/- 3.19 pmoles/min per mg microsomal protein) lungs. In vitro carbofuran studies indicated limited metabolism of carbofuran in both healthy and fibrotic microsomal enzyme preparations (< 5% of the administered dose). Lower p-nitroanisole metabolism in fibrotic lungs was consistent with lower levels of cytochrome P-450 2B1/B2 measured in bleomycin-treated lungs. Results suggest that individuals with bleomycin-induced pulmonary fibrosis may be at greater risk when exposed to certain toxic environmental chemicals or drugs that require detoxification by pulmonary microsomal enzymes.

Effect of tolbutamide and glyburide on pyruvate kinase flux in isolated rat hepatocytes

The effects of two representative sulfonylureas, tolbutamide and glyburide, on pyruvate kinase (PK) flux were examined in fasted rat hepatocytes. PK flux was estimated by trapping 14C from NaH14CO3 in a 2 mM lactate pool, accounting for any incomplete trapping by parallel incubations with L-[1-14C]alanine. Glyburide (20 microM) and tolbutamide (1 mM) decreased glucose formation by 34.9% and 54.8%, respectively, from 2 mM lactate. This decrease in glucose formation was associated with a proportional decrease in pyruvate carboxylase (PCOX) flux (32.7% and 50.5%, respectively). Under these conditions, no net change in PK flux was observed. When hepatocytes were preincubated with lactate and/or sulfonylurea addition for 30 min prior to radiolabeling with NaH14CO3, the metabolic state of the cells changed markedly. Glyburide produced a 34.6% decrease in glucose formation and a 31.3% decrease in PCOX flux, but no change in PK flux. In contrast, tolbutamide decreased glucose formation by 12.5% and increased PK flux by 53.2%, but no change in PCOX flux was observed. Such an increase in PK flux may be linked to tolbutamide-mediated increases in fructose-1,6-bisphosphate (F16P) via fructose-2,6-bisphosphate (F26P). These findings demonstrate that tolbutamide and glyburide decrease hepatic glucose production through various alterations in carbohydrate metabolism, depending upon the metabolic state of the cell. In addition, F26P may play a larger role in the hypoglycemic mechanism of action of tolbutamide than glyburide, since pyruvate carboxylase accounted for most of the decrease in glucose formation observed with glyburide and because preincubation with tolbutamide resulted in an activation of PK.