Ronald E. Baynes

Pharmacokinetics of melamine in pigs following intravenous administration

Melamine-contaminated pet food was recently added as a supplement to livestock feed. There is little or no information concerning the pharmacokinetics of melamine in livestock, and the aim of this study was to obtain pharmacokinetic parameters for this contaminant in pigs. Melamine was administered intravenously to five weanling pigs at a dose of 6.13 mg/kg and plasma samples were collected over 24 h, extracted for melamine, and then analyzed by HPLC-UV. The data was shown to best fit a one-compartment model with melamines half-life of 4.04 (+/- 0.37) h, clearance of 0.11 (+/- 0.01) L/h/kg, and volume of distribution of 0.61 (+/- 0.04) L/kg. These data are comparable to the only mammalian study in rats and suggests that melamine is readily cleared by the kidney and there is unlikely to be significant tissue binding. Further tissue residue studies are required to assess the depletion kinetics of this contaminant in the pig which will determine whether residue levels in the kidney should be of public health concern if pigs were exposed to a similar dose.

Use of Probabilistic Modeling within a Physiologically Based Pharmacokinetic Model To Predict Sulfamethazine Residue Withdrawal Times in Edible Tissues in Swine

ABSTRACT The presence of antimicrobial agents in edible tissues of food-producing animals remains a major public health concern. Probabilistic modeling techniques incorporated into a physiologically based pharmacokinetic (PBPK) model were used to predict the amounts of sulfamethazine residues in edible tissues in swine. A PBPK model for sulfamethazine in swine was adapted to include an oral dosing route. The distributions for sensitive parameters were determined and were used in a Monte Carlo analysis to predict tissue residue times. Validation of the distributions was done by comparison of the results of a Monte Carlo analysis to those obtained with an external data set from the literature and an in vivo pilot study. The model was used to predict the upper limit of the 95% confidence interval of the 99th percentile of the population, as recommended by the U.S. Food and Drug Administration (FDA). The external data set was used to calculate the withdrawal time by using the tolerance limit algorithm designed by FDA. The withdrawal times obtained by both methods were compared to the labeled withdrawal time for the same dose. The Monte Carlo method predicted a withdrawal time of 21 days, based on the amounts of residues in the kidneys. The tolerance limit method applied to the time-limited data set predicted a withdrawal time of 12 days. The existing FDA label withdrawal time is 15 days. PBPK models can incorporate probabilistic modeling techniques that make them useful for prediction of tissue residue times. These models can be used to calculate the parameters required by FDA and explore those conditions where the established withdrawal time may not be sufficient.

Estimating meat withdrawal times in pigs exposed to melamine contaminated feed using a physiologically based pharmacokinetic model

Recently melamine was found to have contaminated the feed of multiple food production species leading to concern over the ability to establish an appropriate withdrawal interval and protect the safety of the food supply. To establish an appropriate withdrawal interval, a physiologically based pharmacokinetic (PBPK) model for melamine was developed for rats and extrapolated to pigs. The rat model underpredicted plasma concentrations, but better predicted tissue residues. Correlation values for plasma, kidney, and liver were 0.59, 0.76, and 0.73, respectively. The pig model underpredicted early plasma time points but had greater accuracy at later time points which is relevant to withdrawal times. Correlation (R(2)) between predicted and observed plasma values was 0.89 with a negative intercept of -0.76. The pig model estimated a withdrawal interval (based on kidney tissue residues) of 19.2 and 20.9h for single oral exposures of 3.0 and 5.12 mg/kg of melamine, respectively. Chronic oral dosing (3.0 and 5.12 mg/kg twice daily for 7 days) yielded withdrawal intervals of 20 and 21.3h, respectively. PBPK models, such as this one, provide evidence of the usefulness in species extrapolation over a range of dosing scenarios and can be used to protect the food supply after accidental exposure in the face of little in the target species.

Health concerns and management of select veterinary drug residues

The aim of this manuscript is to review the potential adverse health effects in humans if exposed to residues of selected veterinary drugs used in food-producing animals. Our other objectives are to briefly inform the reader of why many of these drugs are or were approved for use in livestock production and how drug residues can be mitigated for these drugs. The selected drugs include several antimicrobials, beta agonists, and phenylbutazone. The antimicrobials continue to be of regulatory concern not only because of their acute adverse effects but also because their use as growth promoters have been linked to antimicrobial resistance. Furthermore, nitroimidazoles and arsenicals are no longer approved for use in food animals in most jurisdictions. In recent years, the risk assessment and risk management of beta agonists, have been the focus of national and international agencies and this manuscript attempts to review the pharmacology of these drugs and regulatory challenges. Several of the drugs selected for this review can cause noncancer effects (e.g., penicillins) and others are potential carcinogens (e.g., nitroimidazoles). This review also focuses on how regulatory and independent organizations manage the risk of these veterinary drugs based on data from human health risk assessments.

Membrane transport of naphthalene and dodecane in jet fuel mixtures

Jet fuels are formulated with numerous aliphatic and aromatic components that are thought to cause dermal irritation in air force personnel. However, diffusion of these components in such a complex mixture is not well understood. The purpose of this study is to evaluate the physicochemical properties of these mixtures in the context of how they influence partitioning, diffusion, and absorption of aromatic (14C-naphthalene) and aliphatic (14C-dodecane) markers in porcine skin and silastic membranes in vitro. In these 5-h flowthrough diffusion studies, Jet-A, JP-8, and JP-8(100) fuels, and weathered JP-8 (JP-8 (Puddle)) were tested. In both membrane systems and across all jet fuels tested, naphthalene absorption (1.29-1.84% dose) was significantly greater than dodecane absorption (0.14-0.28% dose). However, significantly more dodecane than naphthalene was observed in the stratum corneum (SC; 4.23-7.23% dose vs. 1.88-4.08% dose) and silastic membranes (59.2-81.7% dose vs. 30.5-36.7% dose). Naphthalene was least likely to be retained on the skin surface compared to dodecane, while this trend was reversed in silastic membranes. In porcine skin. weathered JP-8 significantly increased dodecane absorption, permeability (0.19×10−4 cm/h), and diffusivity, and also naphthalene deposition in the SC compared to other jet fuels. In contrast, weathered JP-8 appears to decrease naphthalene flux (1.56 μg/cm2/h) and permeability (1.14×10−4 cm/h) in skin. There were no differences among the three jet fuels in terms of their ability to influence naphthalene or dodecane disposition in skin and, generally, no significant differences among the four jet fuel mixtures were observed in silastic membranes. In conclusion, these transport studies suggest that absorption and deposition of naphthalene and dodecane are different when dosed in various jet fuel mixtures, and disposition in weathered jet fuel is significantly different from that in commercial and military fuels. These interactions may not only be related to the unique chemistry of these components, but also specific membrane interactions in the SC and viable epidermis.

Pyridostigmine bromide modulates topical irritant-induced cytokine release from human epidermal keratinocytes and isolated perfused porcine skin.

Gulf War personnel were given pyridostigmine bromide (PB) as a prophylactic treatment against organophosphate nerve agent exposure, and were exposed to the insecticide permethrin and the insect repellent N,N-diethyl-m-toluamide (DEET). The purpose of this study was to assess the effects of PB to modulate release of inflammatory biomarkers after topical chemical exposure to chemical mixtures containing permethrin and DEET applied in ethanol or water vehicles. Treatments were topically applied to isolated perfused porcine skin flaps (IPPSFs). Concentrations of interleukin-8 (IL-8), tumor necrosis factor-alpha (TNF-alpha) and prostaglandin E(2) (PGE(2)) were assayed in perfusate to probe for potential inflammatory effects after complex mixture application. IPPSFs (n=4/treatment) were topically dosed with mixtures of permethrin, DEET, and permethrin/DEET, in ethanol. Each treatment was repeated with perfusate spiked with 50 ng/ml of PB. Perfusate was also spiked with 30 ng/ml diisopropylfluorophosphate to simulate low level organophosphate nerve agent exposure. Timed IPPSF venous effluent samples (0.5,1,2,4, and 8 h) were assayed by ELISA for IL-8 and TNF-alpha and by EIA for PGE(2). Overall, PB infusion caused a decrease or IL-8 and PGE(2) release. Effects on TNF-alpha were vehicle dependent. To probe the potential mechanism of this PB effect, human epidermal keratinocyte HEK cell cultures were exposed to permethrin DEET permethrin/DEET, with and without PB in DMSO. IL-8 was assayed at 1, 2, 4, 8, 12 and 24 h. PB suppressed IL-8 in permethrin and ethanol treatment from 4 to 24 h confirming the IPPSF results. In conclusion, these studies suggest that systemic exposure to PB suppressed IL-8 release at multiple time points in two skin model systems. This interaction merits further study.

Disposition of melamine residues in blood and milk from dairy goats exposed to an oral bolus of melamine

There have been numerous reports of melamine-related illnesses following oral exposure to this contaminant. These studies have been in monogastrics, but there are few reports of adverse effects and pharmacokinetics of melamine in ruminants. The purpose of this project was to determine how melamine is systemically cleared from the blood and milk in lactating animals. Five lactating goats were given a single oral dose of 40 mg/kg body weight. Milk and blood samples were collected for 144 h and analyzed to determine key pharmacokinetic parameters. The apparent plasma half-life (11.12h) was 3 times longer in these ruminants than that reported in monogastrics and the apparent volume of distribution was more than 6 times greater than that reported in monogastrics. The milk had an apparent half-life of 9.44h and less than 0.4% of the melamine dose was eliminated in milk. All milk samples were below the LOQ at 4 days (96 h) after exposure. In summary, the pharmacokinetics of melamine in ruminants is not predictive from monogastrics and milk from similarly exposed animals should be condemned for at least 4days after the last exposure to avoid violation of proposed MRLs or safe levels for milk.

Percutaneous Absorption of Topical N , N -Diethyl- m -Toluamide (Deet): Effects of Exposure Variables and Coadministered Toxicants

Exposure to N,N-diethyl-m-toluamide (DEET) commonly occurs in the general population and has been implicated as a contributory factor to the Gulf War Illness. The focus of the present studies was to determine the effect of coexposure factors, potentially encountered in a military environment, that could modulate transdermal flux of topically applied DEET. Factors investigated were vehicle, dose, coexposure to permethrin, low-level sulfur mustard, occlusion, and simultaneous systemic exposure to pyridostigmine bromide and the nerve agent simulant diisopropylfluorophosphate (DFP). Studies were conducted using the isolated perfused porcine skin flap (IPPSF), with a few mechanistically oriented studies conducted using in vitro porcine skin and silastic membrane diffusion cells. DEET was quantitated using high-performance liquid chromatography. The vehicle-control transdermal DEET flux in the IPPSF was approximately 2 w g/cm 2 /h for both 7.5 and 75% DEET concentrations, a value similar to that reported in humans. DEET absorption was enhanced by coinfusion of pyridostigmine bromide and DFP, by the presence of sulfur mustard, or by dosing under complete occlusion. The greatest increase in baseline flux was fivefold. In vitro diffusion cell studies indicated that silastic membranes had two orders of magnitude greater permeability than porcine skin, and showed vehicle effects on flux that were not detected in the IPPSF. These results suggest that coexposure to a number of chemicals that potentially could be encountered in a military environment may modulate the percutaneous absorption of topically applied DEET beyond that seen for normal vehicles at typically applied concentrations.

Pharmacokinetics of tulathromycin following subcutaneous administration in meat goats

Tulathromycin is a triamilide antibiotic that maintains therapeutic concentrations for an extended period of time. The drug is approved for the treatment of respiratory disease in cattle and swine and is occasionally used in goats. To investigate the pharmacokinetics of tulathromycin in meat goats, 10 healthy Boer goats were administered a single 2.5 mg/kg subcutaneous dose of tulathromycin. Plasma concentrations were measured by ultra-high pressure liquid chromatography tandem mass spectrometry (UPLC-MS/MS) detection. Plasma maximal drug concentration (Cmax) was 633 ± 300 ng/ml (0.40 ± 0.26 h post-subcutaneous injection). The half-life of tulathromycin in goats was 110 ± 19.9 h. Tulathromycin was rapidly absorbed and distributed widely after subcutaneous injection 33 ± 6 L/kg. The mean AUC of the group was 12,500 ± 2020 h ng/mL for plasma. In this study, it was determined that the pharmacokinetics of tulathromycin after a single 2.5 mg/kg SC injection in goats were very similar to what has been previously reported in cattle.

Plasma pharmacokinetics and milk residues of flunixin and 5-hydroxy flunixin following different routes of administration in dairy cattle

The objective of this study was to determine if the plasma pharmacokinetics and milk elimination of flunixin (FLU) and 5-hydroxy flunixin (5OH) differ following intramuscular and subcutaneous injection of FLU compared with intravenous injection. Twelve lactating Holstein cows were used in a randomized crossover design study. Cows were organized into 2 groups based on milk production (<20 or >30 kg of milk/d). All cattle were administered 2 doses of 1.1mg of FLU/kg at 12-h intervals by intravenous, intramuscular, and subcutaneous injections. The washout period between routes of administration was 7d. Blood samples were collected from the jugular vein before FLU administration and at various time points up to 36 h after the first dose of FLU. Composite milk samples were collected before FLU administration and twice daily for 5d after the first dose of FLU. Samples were analyzed by ultra-HPLC with mass spectrometric detection. For FLU plasma samples, a difference in terminal half-life was observed among routes of administration. Harmonic mean terminal half-lives for FLU were 3.42, 4.48, and 5.39 h for intravenous, intramuscular, and subcutaneous injection, respectively. The mean bioavailability following intramuscular and subcutaneous dosing was 84.5 and 104.2%, respectively. The decrease in 5OH milk concentration versus time after last dose was analyzed with the nonlinear mixed effects modeling approach and indicated that both the route of administration and rate of milk production were significant covariates. The number of milk samples greater than the tolerance limit for each route of administration was also compared at each time point for statistical significance. Forty-eight hours after the first dose, 5OH milk concentrations were undetectable in all intravenously injected cows; however, one intramuscularly injected and one subcutaneously injected cow had measurable concentrations. These cows had 5OH concentrations above the tolerance limit at the 36-h withdrawal time. The high number of FLU residues identified in cull dairy cows by the United States Department of Agriculture Food Safety Inspection Service is likely related to administration of the drug by an unapproved route. Cattle that received FLU by the approved (intravenous) route consistently eliminated the drug before the approved withdrawal times; however, residues can persist beyond these approved times following intramuscular or subcutaneous administration. Cows producing less than 20 kg of milk/d had altered FLU milk clearance, which may also contribute to violative FLU residues.