Effects of pethoxamid treatment on the disposition of thyroxine in rats.

Pethoxamid (PXA) is a chloroacetamide herbicide that works by inhibiting the germination of target weeds in crops. PXA is not a genotoxic agent, however, in a two-year chronic toxicity study, incidence of thyroid follicular cell hyperplasia was observed in male rats treated at a high dose. Many non-mutagenic chemicals, including agrochemicals are known to produce thyroid hyperplasia in rodents through a hepatic metabolizing enzyme induction mode of action (MoA). In this study, the effects of oral gavage PXA treatment at 300 mg/kg for 7 days on the disposition of intravenously (iv) administered radio-labeled thyroxine ([125I]-T4) was assessed in bile-duct cannulated (BDC) rats. Another group of animals were treated with phenobarbital (PB, 100 mg/kg), a known enzyme inducer, serving as a positive control. The results showed significant increase (p < 0.01) in the mean liver weights in the PB and PXA-treated groups relative to the control group. The serum total T4 radioactivity Cmax and AUC0-4 values for PB and PXA-treated groups were lower than for the control group, suggesting increased clearance from serum. The mean percentages of administered radioactivity excreted in bile were 7.96 ± 0.38%, 16.13 ± 5.46%, and 11.99 ± 2.80% for the control, PB and PXA groups, respectively, indicating increased clearance via the bile in the treated animals. These data indicate that PXA can perturb the thyroid hormone homeostasis in rats by increasing T4 elimination in bile, possibly through enzyme induction mechanism similar to PB. In contrast to humans, the lack of high affinity thyroid binding globulin (TBG) in rats perhaps results in enhanced metabolism of T4 by uridine diphosphate glucuronosyl transferase (UGT). Since this liver enzyme induction MoA for thyroid hyperplasia by PB is known to be rodent specific, PXA effects on thyroid can also be considered not relevant to humans. The data from this study also suggest that incorporating a BDC rat model to determine thyroid hormone disposition using [125I]-T4 is valuable in a thyroid mode of action analysis.

Nondetectable or minimal detectable residue levels of N-(n-butyl) thiophosphoric triamide in bovine tissues and milk from a 28-d NBPT dosing study.

N-(n-butyl) thiophosphoric triamide (NBPT) (Figure 1) is an active ingredient in nitrogen stabilizer (urease inhibitor), which temporarily inhibits the action of the urease enzyme to improve the efficiency of urea-containing fertilizers. Given the potential for NBPT residues to be present in milk and tissues of dairy cattle, due diligence is needed to demonstrate the safety of NBPT in urea-based fertilizers used on forages and crops intended for consumption by Holstein dairy cows. This study used controlled dosing of NBPT in capsule form to dairy cattle for 28 d, followed by a 14-d depuration phase to assess the potential for residues to exist in milk and tissues of dairy cattle at exaggerated use levels. Fourteen lactating cows were selected for the dosing and depuration phases of the study, based on health, body weight (BW), and milk production. There were four treatment groups: 0 mg NBPT/kg BW (Control) (n = 2 cows), 1 mg NBPT/kg BW (1×) (n = 3 cows), 3 mg NBPT/kg BW (3×) (n = 3 cows), and 10 mg NBPT/kg/BW (10×) (n = 6 cows); levels were based on maximum tolerable amount of urea that a cow can ingest on a daily basis (1×) and the maximum concentration of NBPT commercially used when treating urea (0.1 wt% NBPT in urea). At the end of the 28-d dosing phase, cows were randomly selected for the 14-d depuration phase of the study (one control and three 10× cows). The results showed no NBPT residue is detectable at all dose levels, except that a residue level was above the lower limit of quantitation in a single milk and subcutaneous fat sample in the highest (10×) treatment group, which represents the level of NBPT that would be theoretically present in 10× the lethal dosing of daily consumable urea to a cow. Overall, the study demonstrated that it is unlikely for NBPT residues to be present in cattle milk or edible tissues or to cause negative effects on animal health under good agricultural practice.

Synthesis and in vivo antitumor evaluation of 2-methoxyestradiol 3-phosphate, 17-phosphate, and 3,17-diphosphate.

A prodrug strategy was investigated to address the problem of limited aqueous solubility and the resulting limited bioavailability of the antitumor agent 2-methoxyestradiol. The 3-phosphate, 17-phosphate, and 3,17-diphosphate of 2-methoxyestradiol were synthesized. 2-methoxyestradiol 3-phosphate was metabolized more efficiently to the parent compound in vivo than 2-methoxyestradiol 17-phosphate, and it was also more cytotoxic in cancer cell cultures than either the 17-phosphate or the 3,17-diphosphate. These results agree with the in vivo anticancer activity of 2-methoxyestradiol 3-phosphate in a mouse Lewis lung carcinoma experimental metastasis model as opposed to the 17-phosphate and 3,17-diphosphate, both of which were inactive. The in vivo antitumor activity of 2-methoxyestradiol 3-phosphate at a dose of 200 mg/kg per day was comparable to that of a maximally tolerated dose of cyclophosphamide.

Quantitative determination of tilmicosin in canine serum by high performance liquid chromatography-tandem mass spectrometry.

A highly sensitive and quantitative LC/MS/MS assay for the determination of tilmicosin in serum has been developed and validated. For sample preparation, 0.2 mL of canine serum was extracted with 3 mL of methyl tert-butyl ether. The organic layer was transferred to a new vessel and dried under nitrogen. The sample was then reconstituted for analysis by high performance liquid chromatography-tandem mass spectrometry. A Phenomenex Luna C8(2) analytical column was used for the chromatographic separation. The eluent was subsequently introduced to the mass spectrometer by electrospray ionization. A single range was validated for 50-5000 ng/mL for support of toxicokinetic studies. The inter-day relative error (inaccuracy) for the LLOQ samples ranged from -5.5% to 0.3%. The inter-day relative standard deviations (imprecision) at the respective LLOQ levels were < or =10.1%.