Route-dependent pharmacokinetics, distribution, and placental permeability of organic and inorganic selenium in hamsters

Antioxidant Supplements Improve Profiles of Hepatic Oxysterols and Plasma Lipids in Butter-fed Hamsters

Hypercholesterolemic diets are associated with oxidative stress that may contribute to hypercholesterolemia by adversely affecting enzymatically-generated oxysterols involved in cholesterol homeostasis. An experiment was conducted to examine whether the cholesterol-lowering effects of the antioxidants selenium and α-tocopherol were related to hepatic oxysterol concentrations. Four groups of male Syrian hamsters (n = 7-8) were fed high cholesterol and saturated fat (0.46% cholesterol, 14.3% fat) hypercholesterolemic semi-purified diets: 1) Control; 2) Control + α-tocopherol (67 IU all-racemic-α-tocopheryl-acetate/kg diet); 3) Control + selenium (3.4 mg selenate/kg diet); and 4) Control + α-tocopherol + selenium. Antioxidant supplementation was associated with lowered plasma cholesterol concentrations, decreased tissue lipid peroxidation and higher hepatic oxysterol concentrations. A second experiment examined the effect of graded selenium doses (0.15, 0.85, 1.7 and 3.4 mg selenate/kg diet) on mRNA expression of the oxysterol-generating enzyme, hepatic 27-hydroxylase (CYP27A1, EC 1.14.13.15), in hamsters (n = 8-9) fed the hypercholesterolemic diets. Supplementation of selenium at 3.4 mg selenate/kg diet was not associated with increased hepatic 27-hydroxylase mRNA. In conclusion, the cholesterol lowering effects of selenium and α-tocopherol were associated with increased hepatic enzymatically generated oxysterol concentrations, which appears to be mediated via improved antioxidant status rather than increased enzymatic production.

Kinetics of selenium incorporation into tissues of female mallard ducks

Selenium is essential for both mammalian and avian species, although its metabolism in birds has been less thoroughly studied. Little information has been available on the kinetics of selenium in birds, especially as it relates to the teratogenicity seen in waterfowl consuming excessive amounts. This study describes the pharmacokinetics of small amounts of 75Se as selenious acid injected into female mallard ducks. Labeled selenium was injected into a wing vein of restrained animals and tissues taken at five different time points up to 24 h post-injection. Selenium levels as percent of injected dose were determined in liver, kidney, heart, lung, adrenals, thyroid, spleen, pancreas, ovaries, intestine, muscle and plasma. Estimates of kinetic parameters (uptake and elimination rates, time of maximum concentration and maximum concentration) were obtained with a non-linear kinetics computer program (PCNONLIN, SCI Software, Lexington, KY). Results showed four basic patterns of distribution, uptake and elimination. Visceral tissues exhibited a triphasic pattern with a rapid rise, a decline followed by a distinctive increase in levels between the last two time points. Brain uptake was continuous over the 24 h. Plasma rose rapidly and then declined to a constant level. The ovaries as a tissue of interest relating to the teratogenic effects of selenium showed the greatest relative increase at 24 h, suggesting kinetic mechanisms consistent with a pathway that might lead to accumulation of toxic levels and teratogenic effects during embryo development.

Selenium kinetics, placental transfer, and neonatal exposure in cynomolgus macaques (Macaca fascicularis)

Forty pregnant cynomolgus macaques were treated daily from gestational day 20 to 50 by nasogastric intubation of 0, 25, 150, or 300 micrograms selenium as L-selenomethionine/kg body weight. In each group, 7-8 pregnancies were terminated by hysterotomy at gestational day 100 +/- 2 and the fetuses were examined, while 2-3 pregnancies in each group were allowed to proceed to term. Selenium and soluble glutathione peroxidase were measured in: maternal, neonatal, and fetal plasma and erythrocytes; fetal kidney, liver, muscle, and placenta; and maternal breast milk. The area under the multidose maternal plasma selenium concentration:time curve, the maximum maternal plasma selenium concentration, and the maternal urinary selenium excretion rates were proportional to the L-selenomethionine dose. Selenium concentrations in all fetal and neonatal, tissues were also proportional to maternal L-selenomethionine dose. Glutathione peroxidase was affected only in maternal erythrocytes, fetal kidney, and neonatal plasma. The selenium concentration in fetal plasma was an average 33% of that in maternal plasma. Although selenium concentrations in macaque milk were doubled by the highest dose, intrauterine selenium accumulation accounted for the majority of the neonatal selenium body burden. Despite the elevated selenium concentrations in fetal tissues, neonatal blood, and milk, no deleterious effects on neonates were observed. These results suggest that primate fetuses are well protected against selenium toxicity arising from high maternal L-selenomethionine intakes.

A physiologically based pharmacokinetic computer model for human pregnancy

A physiologically based pharmacokinetic (PBPK) model for human pregnancy must incorporate many factors that are not usually encountered in PBPK models of mature animals. Models for pregnancy must include the large changes that take place in the mother, the placenta and the embryo/fetus over the period of pregnancy. The embryo/fetal weight change was modeled using the Gompertz equation for growth which gave a good fit to extensive pooled weight data of the human embryo/fetus from 25 to 300 days of gestation. This equation is based on a growth rate that is proportional to the total weight of the organism with the proportionality factor decreasing exponentially with time. Allometric equations, which are widely used to relate organ weights, blood flow rates and other attributes of mature animals to total weight, were adapted to correlate fetal organ weights with total fetal weight. Allometric relationships were also developed for plasma flow rates and other organ-related parameters. The computer model, written in FORTRAN 77, included 27 compartments for the mother and 16 for the fetus; it also accommodates two substances allowing representation of a parent compound and a metabolite (or a second drug or environmental substance). Although this model is large, the inherent sparsity in the equations allow it to be solved numerically in a reasonable time on currently available, reasonably priced desktop computers. A nonlinear regression routine is included to fit key model parameters to experimental data. Concentrations of chemicals administered and measured in the mother may be simulated in both maternal and fetal organs at any day(s) between 25 days and 300 days of gestation. Allometric relationships are also utilized to adopt this human model for use with data obtained from animal experiments.

Effects of excess selenomethionine on selenium status indicators in pregnant long-tailed macaques (Macaca fascicularis)

Forty pregnant long-tailed macaques were treated daily for 30 d with 0, 25, 150, or 300 micrograms selenium as L-selenomethionine/kg body weight. Erythrocyte and plasma selenium and glutathione peroxidase specific activities, hair and fecal selenium, and urinary selenium excretion were increased by and were linearly related to L-selenomethionine dose. Hair selenium was most sensitive to L-selenomethionine dose, with an 84-fold increase in the 300 micrograms selenium/(kg-d) group relative to controls (r = 0.917). Daily urinary selenium excretion (80-fold, r = 0.958), plasma selenium (22-fold, r = 0.885), erythrocyte selenium (24-fold, r = 0.920), and fecal selenium (18-fold, r = 0.911) also responded strongly to L-selenomethionine. Erythrocyte and plasma glutathione peroxidase specific activities increased 154% and 69% over controls, respectively. Toxicity was associated with erythrocyte selenium > 2.3 micrograms/mL, plasma selenium > 2.8 micrograms/mL, and hair selenium > 27 micrograms/g. Plasma, erythrocyte, and hair selenium concentrations may be useful for monitoring and preventing the toxicity of L-selenomethionine administered to humans in cancer chemoprevention trials.

Selecting exposure parameters in developmental neurotoxicity assessments

Numerous factors must be considered in selecting exposure parameters for developmental neurotoxicity investigations. Whether employing a single dose during pregnancy, or continuous exposure from prepregnancy through early postnatal developmental periods, the following primary factors should be addressed: 1) Purpose of the study; 2) pharmacokinetics/pharmacodynamics; 3) biotransformation; 4) genotypic variables; 5) limiting factors, including the availability of test compounds for evaluation; and, 6) several general, miscellaneous factors. Whether a single, large dose of an exogenous agent is more toxic to the developing nervous system than a series of smaller doses depends upon the interaction of the physicochemical, pharmacokinetic, and pharmacodynamic properties of the agent with the genotypic features of the test organism.