Residue Behavior and Risk Assessment of Rimsulfuron and Quizalofop-P-ethyl in Potato Under Field Conditions.

A method for simultaneous quantitation of rimsulfuron, quizalofop-P-ethyl and quizalofop-P in potato plant, soil and potato tuber samples was established. The mean recoveries of rimsulfuron, quizalofop-P-ethyl and quizalofop-P in different matrices spiked with them were 81.4%-101.1%, 76.1%-99.0% and 77.4%-106.4% with relative standard deviations (RSDs) of 2.7%-13.3%, 0.9%-5.5%, 1.7%-11.3%, respectively. The open-field trials in China were conducted in potato cultivation system of Changchun and Jinan. The results indicated that the half-lives of rimsulfuron and quizalofop-P-ethyl were 0.04-13.1 days. The residues of quizalofop-P during the harvest time in Jinan soil were < 0.01-0.044 mg kg-1, while there was no residue of target herbicides detected in all other samples. The risk assessment results demonstrated that the risk quotients (RQs) of rimsulfuron and quizalofop-P-ethyl were 7.857 × 10-5 and 8.730 × 10-3, respectively, which exhibited an acceptable dietary risk to Chinese consumers.

Dietary exposure assessment of cyadox based on tissue depletion of cyadox and its major metabolites in pigs, chickens, and carp.

The tissue kinetics of cyadox, an antibacterial agent used in food animals, and its major metabolites in pigs, chickens, and carp were investigated followed by a complete dietary exposure assessment to evaluate the food safety of cyadox. Cyadox and its major metabolites, bisdeoxycyadox (Cy1), 4-desoxycyadox (Cy2), N-(quinoxaline-2-methyl)-cyanide acetyl hydrazine (Cy4), quinoxaline-2-carboxylic acid (Cy6), and 2-hydromethyl-3-hydroxy-quinoxaline (Cy12), were simultaneously quantitated with a high-performance liquid chromatography-ultraviolet (HPLC-UV) method. Pigs, chickens, and carp were fed with 150 mg/kg cyadox in feed for consecutive 60, 40, and 30 days, respectively. The residue amount of cyadox and its major metabolites in liver, kidney, muscle, and fat (skin) tissues was determined. Cy2 was below the limit of quantitation even at the withdrawal time of 6 hr, cyadox, Cy4, Cy6, and Cy12 could be detected at 6-24 hr with low level less than 50 µg/kg. By contrast, Cy1 persisted for 3 days in the kidney of pigs and chickens, and in the liver of carp. Based on these residue depletion data and previous toxicology results, the global estimated chronic dietary exposure assessment of cyadox for general population was conducted, indicating a zero withdrawal time (WDT) may be appropriate for cyadox in food animals when used in feed for prolonged administration. These results provide analytical techniques and safety standards suitable for residue monitoring of cyadox in food animals.

Physiologically based pharmacokinetic model for quinocetone in pigs and extrapolation to mequindox.

Physiologically based pharmacokinetic (PBPK) models are scientific methods used to predict veterinary drug residues that may occur in food-producing animals, and which have powerful extrapolation ability. Quinocetone (QCT) and mequindox (MEQ) are widely used in China for the prevention of bacterial infections and promoting animal growth, but their abuse causes a potential threat to human health. In this study, a flow-limited PBPK model was developed to simulate simultaneously residue depletion of QCT and its marker residue dideoxyquinocetone (DQCT) in pigs. The model included compartments for blood, liver, kidney, muscle and fat and an extra compartment representing the other tissues. Physiological parameters were obtained from the literature. Plasma protein binding rates, renal clearances and tissue/plasma partition coefficients were determined by in vitro and in vivo experiments. The model was calibrated and validated with several pharmacokinetic and residue-depletion datasets from the literature. Sensitivity analysis and Monte Carlo simulations were incorporated into the PBPK model to estimate individual variation of residual concentrations. The PBPK model for MEQ, the congener compound of QCT, was built through cross-compound extrapolation based on the model for QCT. The QCT model accurately predicted the concentrations of QCT and DQCT in various tissues at most time points, especially the later time points. Correlation coefficients between predicted and measured values for all tissues were greater than 0.9. Monte Carlo simulations showed excellent consistency between estimated concentration distributions and measured data points. The extrapolation model also showed good predictive power. The present models contribute to improve the residue monitoring systems of QCT and MEQ, and provide evidence of the usefulness of PBPK model extrapolation for the same kinds of compounds.

Application of electrospray ionization hybrid ion trap/time-of-flight mass spectrometry in the rapid characterization of quinocetone metabolites formed in vitro.

The application of electrospray ionization hybrid ion trap/time-of-flight mass spectrometry coupled with high-performance liquid chromatography (LC/MS-IT-TOF) in the rapid characterization of in vitro metabolites of quinocetone was developed. Metabolites formed in rat liver microsomes were separated using a VP-ODS column with gradient elution. Multiple scans of metabolites in MS and MS(2) modes and accurate mass measurements were automatically performed simultaneously through data-dependent acquisition in only a 30-min analysis. Most measured mass errors were less than 10 ppm for both protonated molecules and fragment ions using external mass calibration. The elemental compositions of all fragment ions of quinocetone and its metabolites could be rapidly assigned based upon the known compositional elements of protonated molecules. The structure of metabolites were elucidated based on the combination of three techniques: agreement between their proposed structure, the accurate masses, and the elemental composition of ions in their mass spectra; comparison of their changes in accurate molecular masses and fragment ions with those of parent drug or metabolite; and the elemental compositions of lost mass numbers in proposed fragmentation pathways. Twenty-seven phase I metabolites were identified as 11 reduction metabolites, three direct hydroxylation metabolites, and 13 metabolites with a combination of reduction and hydroxylation. All metabolites except the N-oxide reduction metabolite M6 are new metabolites of quinocetone, which were not previously reported. The ability to conduct expected biotransformation profiling via tandem mass spectrometry coupled with accurate mass measurement, all in a single experimental run, is one of the most attractive features of this methodology. The results demonstrate the use of LC/MS-IT-TOF approach appears to be rapid, efficient, and reliable in structural characterization of drug metabolites.

[Determination of carbadox metabolites, quinoxaline-2-carboxylic acid and desoxycarbadox, in swine muscle and liver by liquid chromatography/mass spectrometry].

A sensitive and selective method using liquid chromatography-electrospray mass spectrometry (LC-ESI-MS) for the determination of carbadox metabolites, quinoxaline-2-carboxylic acid (QCA) and desoxycarbadox (Desoxy-CDX), in swine muscle and liver has been developed. The LC separation was performed on a Cadenza CD-C18 column (10 cm x 2 mm i.d.) with a gradient system of 0.01% acetic acid-acetonitrile as the mobile phase at a flow rate of 0.2 mL/min. Negative ionization produced the [M-H]- molecular ion of QCA. On the other hand, the positive mode produced the [M+H]+ ion of Desoxy-CDX. The calibration graphs for QCA and Desoxy-CDX were rectilinear from 0.01 to 0.5 ng with selected ion monitoring (SIM). The drugs were extracted with 0.3% metaphosphoric acid-methanol (7:3), and the extracts were cleaned up on an Oasis HLB cartridge (60 mg) and by liquid-liquid extraction. The recoveries of QCA and Desoxy-CDX from swine muscle and liver fortified at 2.5 and 5 ng/g were 70.2-86.3%, and the detection limits were 1 ng/g for both drugs.

Assessment of the human exposure to heterocyclic amines.

Heterocyclic amines are possible human carcinogens and fried meat is an important source of exposure in the Western diet. To study the effect of heterocyclic amines in humans, accurate assessment of individual food consumption is essential. Parameters influencing the intake include the amount and type of meat ingested, frequency of consumption, cooking method, cooking temperature and the duration of cooking. The aim of the present study was to develop a practical method for assessing individual intakes of specific heterocyclic amines in a large sample of people. This has been done by combining information on food consumption and laboratory findings of heterocyclic amines in food products. Diet was assessed using a semi-quantitative food frequency questionnaire including photos of fried meat and, in all, 22 dishes were cooked and chemically analyzed. The method was employed in an elderly population in Stockholm to estimate the daily mean intake of the five heterocyclic amines 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (DiMeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). The total daily intake ranged from none to 1816 ng, with a mean intake of 160 ng, which is well below estimates reported previously. Highest amounts ingested were of PhIP (mean 72, range 0-865 ng/day) and MeIQx (mean 72, range 0-1388 ng/day), followed by DiMeIQx (mean 16, range 0-171 ng/day), while MeIQ and IQ were ingested only in very small amounts (mean <1 ng/day).

Assessment of the DNA adduction and pharmacokinetics of PhIP and MeIOx in rodents at doses approximating human exposure using the technique of accelerator mass spectrometry (AMS) and 32P-postlabeling.

Estimating the cancer risk posed by heterocyclic amines depends on measuring how chemical dose influences measurable indicators of cancer progression. This data ideally should encompass the range of actual human exposure, at the low dose end, and laboratory animal studies, at the high dose end. Accelerator mass spectrometry (AMS) has been used to measure the absorption, fate, and DNA adduct dosimetry of the heterocyclic amines PhIP and MeIQx at doses equivalent to human consumption following single-dose administration and chronic daily dosing. AMS is a nuclear physics technique which specifically counts nuclei of cosmogenic isotopes, rather than relying on decay. For tracing 14C, sensitivity is increased 10(6)-fold relative to decay counting. We have found that tissue clearance rates for [2-(14)C]-PhIP are rapid (t1/2 = 1 h) at low dose (41 ng/kg), with most of the radiocarbon distributed to the liver and G.I. tract. MeIQx-DNA adduct levels decrease linearly with dose (5 mg/kg-500 ng/kg) in single dose exposures. Likewise, the biologically available dose of [2-(14)C]-MeIQx decreases linearly with decreasing dose (5 mg/kg-1 ng/kg). On chronic daily dosing, it takes 40 days for adducts to reach steady-state in tissues and adduct levels appear to decrease linearly with decreasing dose, except possibly at very low doses. DNA binding of PhIP involves both sulfation or acetylation of the N-hydroxylated PhIP. Quantitatively, sulfation appears to be an important pathway for PhIp activation in rodent tissue cytosols while acetylation appears quantitatively more important in human tissue cytosols. The greatest activity is in liver and intestinal tissues for both pathways. The specific DNA adducts formed in vivo and in vitro from exposure to PhIP and MeIQx are likely guanine adducts. These data suggest that DNA adduct dosimetry responds linearly with dose but may become sub-linear at very low doses for chronic exposure and that factors other than DNA adduction may be critical to explain these heterocyclic amines' tumorigenicity.