Calvin C. Willhite

Derivation of a bisphenol A oral reference dose (RfD) and drinking-water equivalent concentration.

Human exposure to bisphenol A (BPA) is due to that found in the diet, and BPA and its metabolites were detected at parts per billion (or less) concentrations in human urine, milk, saliva, serum, plasma, ovarian follicular fluid, and amniotic fluid. Adverse health effects in mice and rats may be induced after parenteral injection or after massive oral doses. Controlled ingestion trials in healthy adult volunteers with 5 mg d 16-BPA were unable to detect parent BPA in plasma despite exquisitely sensitive (limit of detection = 6 nM) methods, but by 96 h 100% of the administered dose was recovered in urine as the glucuronide. The extensive BPA glucuronidation following ingestion is not seen after parenteral injection; only the parent BPA binds plasma proteins and estrogen receptors (ER). The hypothesis that BPA dose-response may be described by a J- or U-shape curve was not supported by toxicogenomic data collected in fetal rat testes and epididymes (after repeated parenteral exposure at 2–400,000 μg/kg-d), where a clear monotonic dose-response both in the numbers of genes and magnitude of individual gene expression was evident. There is no clear indication from available data that the BPA doses normally consumed by humans pose an increased risk for immunologic or neurologic disease. There is no evidence that BPA poses a genotoxic or carcinogenic risk and clinical evaluations of 205 men and women with high-performance liquid chromatography (HPLC)-verified serum or urinary BPA conjugates showed (1) no objective signs, (2) no changes in reproductive hormones or clinical chemistry parameters, and (3) no alterations in the number of children or sons:daughters ratio. Results of benchmark dose (BMD10 and BMDL10) calculations and no-observed-adverse-effect level (NOAEL) inspections of all available and reproducible rodent studies with oral BPA found BMD and NOAEL values all greater than the 5 mg/kg-d NOAELs from mouse and rat multigeneration reproduction toxicity studies. While allometric and physiologically based pharmacokinetic (PBPK) models were constructed for interspecies scaling of BPA and its interaction with ER, multigeneration feeding studies with BPA at doses spanning 5 orders of magnitude failed to identify signs of developmental toxicity or adverse changes in reproductive tract tissues; the 5-mg/kg-d NOAELs identified for systemic toxicity in rats and mice were less than the oral NOAELs for reproductive toxicity. Thus, it is the generalized systemic toxicity of ingested BPA rather than reproductive, immunologic, neurobehavioral, or genotoxic hazard that represents the point of departure. Using U.S. Environmental Protction Agency (EPA) uncertainty factor guidance and application of a threefold database uncertainty factor (to account for the fact that the carcinogenic potential of transplacental BPA exposure has yet to be fully defined and comprehensive neurobehavioral and immunotoxicologic evaluations of BPA by relevant routes and at relevant doses have yet to be completed) to the administered dose NOAEL results in an oral RfD of 0.016 mg/kg-d. Assuming the 70-kg adult consumes 2 L of water each day and adopting the default 20% U.S. EPA drinking water relative source contribution yields a 100 μg/L BPA total allowable concentration (TAC).

The role of cyanide liberation in the acute toxicity of aliphatic-nitriles

Abstract The acute ip LD50 values for a series of six aliphatic nitriles were determined in mice and compared with acetone cyanohydrin and sodium cyanide. When sodium thiosulfate was given in multiple injections, it protected mice against death by acetonitrile, propionitrile, n-butyronitrile, malononitrile, or succinonitrile. In contrast, multiple injections of sodium nitrite protected mice against death by acrylonitrile, n-butyronitrile, and malononitrile, but not against acetonitrile, propionitrile, or succinonitrile. Single prophylactic doses of either thiosulfate or nitrite protected mice against death by either acetone cyanohydrin or sodium cyanide. Only sodium cyanide and acetone cyanohydrin predictably produced death within 5 min. All other nitriles produced death at widely varying intervals from a few minutes to many hours. Only acetone cyanohydrin and cyanide inhibited the activity of purified preparations of cytochrome c oxidase in vitro and in equimolar concentrations. Pretreatment of mice with carbon tetrachloride protected them against death from all nitriles except acetone cyanohydrin. Elevated concentrations of cyanide were found in the livers and brains of mice given lethal doses of all of the nitriles, acetone cyanohydrin, or sodium cyanide. The tissue concentrations of cyanide were substantially reduced in all cases when thiosulfate was also given or when nitriles were given to carbon tetrachloride-pretreated mice. Cyanide was liberated when n-butyronitrile or succinonitrile were incubated with mouse liver slices or NADPH-fortified mouse hepatic microsomal preparations. This reaction was inhibited when the livers were taken from mice pretreated with carbon tetrachloride or when SKF-525A was added in vitro to normal liver slices. Acetone cyanohydrin in all systems tested behaved qualitatively and quantitatively like its molar equivalent in cyanide. The results suggested that the other nitriles examined here possess little, if any, acute toxicity in the absence of normal hepatic function and that these nitriles were activated by hepatic mechanisms to release cyanide which can account for their major acute toxic effects.

Systematic review of potential health risks posed by pharmaceutical, occupational and consumer exposures to metallic and nanoscale aluminum, aluminum oxides, aluminum hydroxide and its soluble salts

Abstract Aluminum (Al) is a ubiquitous substance encountered both naturally (as the third most abundant element) and intentionally (used in water, foods, pharmaceuticals, and vaccines); it is also present in ambient and occupational airborne particulates. Existing data underscore the importance of Al physical and chemical forms in relation to its uptake, accumulation, and systemic bioavailability. The present review represents a systematic examination of the peer-reviewed literature on the adverse health effects of Al materials published since a previous critical evaluation compiled by 356. Challenges encountered in carrying out the present review reflected the experimental use of different physical and chemical Al forms, different routes of administration, and different target organs in relation to the magnitude, frequency, and duration of exposure. Wide variations in diet can result in Al intakes that are often higher than the World Health Organization provisional tolerable weekly intake (PTWI), which is based on studies with Al citrate. Comparing daily dietary Al exposures on the basis of “total Al”assumes that gastrointestinal bioavailability for all dietary Al forms is equivalent to that for Al citrate, an approach that requires validation. Current occupational exposure limits (OELs) for identical Al substances vary as much as 15-fold. The toxicity of different Al forms depends in large measure on their physical behavior and relative solubility in water. The toxicity of soluble Al forms depends upon the delivered dose of Al+ 3 to target tissues. Trivalent Al reacts with water to produce bidentate superoxide coordination spheres [Al(O2)(H2O4)+ 2 and Al(H2O)6 + 3] that after complexation with O2•−, generate Al superoxides [Al(O2•)](H2O5)]+ 2. Semireduced AlO2• radicals deplete mitochondrial Fe and promote generation of H2O2, O2 • − and OH•. Thus, it is the Al+ 3-induced formation of oxygen radicals that accounts for the oxidative damage that leads to intrinsic apoptosis. In contrast, the toxicity of the insoluble Al oxides depends primarily on their behavior as particulates. Aluminum has been held responsible for human morbidity and mortality, but there is no consistent and convincing evidence to associate the Al found in food and drinking water at the doses and chemical forms presently consumed by people living in North America and Western Europe with increased risk for Alzheimers disease (AD). Neither is there clear evidence to show use of Al-containing underarm antiperspirants or cosmetics increases the risk of AD or breast cancer. Metallic Al, its oxides, and common Al salts have not been shown to be either genotoxic or carcinogenic. Aluminum exposures during neonatal and pediatric parenteral nutrition (PN) can impair bone mineralization and delay neurological development. Adverse effects to vaccines with Al adjuvants have occurred; however, recent controlled trials found that the immunologic response to certain vaccines with Al adjuvants was no greater, and in some cases less than, that after identical vaccination without Al adjuvants. The scientific literature on the adverse health effects of Al is extensive. Health risk assessments for Al must take into account individual co-factors (e.g., age, renal function, diet, gastric pH). Conclusions from the current review point to the need for refinement of the PTWI, reduction of Al contamination in PN solutions, justification for routine addition of Al to vaccines, and harmonization of OELs for Al substances.

Prenatal and Developmental Toxicology of Arsenicals

A variety of species, including the human, have been shown to be susceptible to the embryotoxic effects of inorganic arsenic. Malformations of the axial skeleton, neurocranium, viscerocranium, eyes, and genitourinary systems as well as prenatal death followed a bolus dose of trivalent or pentavalent inorganic arsenic. Trivalent arsenic was more teratogenic than pentavalent arsenic; in contrast, the methylated metabolites of arsenic possessed only limited teratogenic activity. Administration of inorganic arsenic to mammals results in concentration of arsenic within the placenta and small amounts are deposited within the embryo. Studies concerning the pathogenesis of arsenic-induced axial skeletal lesions revealed early failure of neural fold elevation and a subsequent, persistent failure of closure of the neural tube. Physical factors, drugs and heavy metals may modify the response to a teratogenic dose of inorganic arsenic. Medical problems associated with industrial or agricultural arsenicalism are most often typified by chronic exposure; future studies should emphasize those routes of administration and types of exposure that are characteristic of arsenic intoxication.

Toxicity of Retinoids in Humans and Animals

AbstractThe retinoids comprise a diverse group of chemical compounds similar in structure to vitamin A (retinol). Several retinoid analogs are employed or are undergoing clinical trials to determine their therapeutic effects on dermatological disorders, preneoplastic and neoplastic lesions. Due to their structural similarities with retinol, the retinoids also possess similar toxic effects, manifested as the so-called hypervitaminosis A syndrome. The toxic effects, which are very similar in humans and laboratory animals, include liver and lipid alterations, skin, bone, and connective tissue toxicity. Severe congenital malformations induced by retinoids are similar in humans and animals; humans appear to be more susceptible to retinoid teratogenic effects than hamsters. The toxic effects of retinoids are dose-related and abate upon discontinuance of the treatment, with the exception of permanent teratogenic effects. The retinoids are not mutagenic or carcinogenic. The most active retinoids are those with o...

Dose Response and Structure-Activity Considerations in Retinoid-Induced Dysmorphogenesis

Synthetic retinoids induce terata by the same molecular mechanism as that associated with elevated concentrations of their naturally occurring congeners. The evidence for definite retinoid structure-activity relationships and for specific cellular receptors in their biochemical mechanism of action illustrates the threshold basis of retinoid teratogenesis

Embryotoxicity and dose-response relationships of selenium in hamsters

Pregnant hamsters were treated with selenite, selenate, and selenomethionine during the critical stages of embryogenesis. The dosing regimens were oral, intravenous, and osmotic minipump infusion. Malformations, mainly encephaloceles, were noted with oral and intravenous selenite and selenate but were associated with maternal toxicity manifested by inanition and weight loss. Fetal body weights and lengths were reduced in a dose-dependent manner with the inorganic forms. Single oral doses of selenomethionine above 77 mumol/kg induced similar malformations but not when the dose was delivered orally over four days nor by minipump over several days. Fetal body weights and lengths were decreased by selenomethionine in a dose-dependent manner. Maternal toxicity was pronounced with the higher doses of selenomethionine. Assigning a specific teratogenic effect to selenium is confounded by maternal toxicity.

Weight-of-evidence versus strength-of-evidence in toxicologic hazard identification: Di(2-ethylhexyl)phthalate (DEHP).

Toxicokinetic and mode of action data for DEHP reduce the concern for its potential carcinogenic hazard to human health. Chronic, high dose ingestion of DEHP and related peroxisome proliferators (PP) by mice and rats precipitate the following: activation of peroxisome proliferator activated receptor (PPARalpha) and its binding to peroxisome proliferator response elements (PPREs) within promoters of PP-responsive genes, peroxisome proliferation, increased microsomal fatty acid oxidation, increased hepatic hydrogen peroxide, hepatomegaly, hyperplasia and subsequent neoplasia. Neither peroxisome proliferation nor increased liver cancer occur in patients treated with pharmacologic doses of PP. Species differences in endogenous PPARalpha expression and differential activity of the peroxisome proliferator response element (PPRE) contribute to the failure of humans to respond in a manner qualitatively similar to that of rats or mice. Where it can be demonstrated that a mechanism for rodent tumor formation has no relevance for humans, then a substance which elicits a carcinogenic response in the test species via that mechanism should not be classified as anything other than an animal carcinogen. Systemic noncarcinogenic endpoints are available for definition of a DEHP reference dose. Considerable difficulty is encountered in the revision of promulgated regulations and in public risk communication when a material is no longer considered a carcinogenic hazard to humans.

Structure-activity relationships of retinoids in developmental toxicology. II. Influence of the polyene chain of the vitamin A molecule.

The comparative teratogenic potencies of 7,8-dihydroretinoic acid, 7,8-dehydroretinoic acid, 9,10-dihydroretinoic acid, 9-cis-retinal, hydroxenin, oxenin, the C15 analog of all-trans-retinoic acid, abscissic acid, beta-C14 aldehyde, beta-ionone, and psuedoionone were determined in golden Syrian hamsters and compared with that of all-trans-retinoic acid. A single oral dose of each retinoid on Day 8 of gestation failed to induce signs of retinoid intoxication in the dams and the maternal weight gain was not affected significantly, except after treatment with beta-C14 aldehyde, pseudoionone, and the lowest dose of 9-cis-retinal, where maternal weight was depressed. A significant teratogenic response was observed after intubation of 7,8-dihydroretinoic acid, 7,8-dehydroretinoic acid, or 9-cis-retinal. The highest dose of hydroxenin (equivalent to nearly seven times an equimolar teratogenic dose of all-trans-retinoic acid) showed a limited teratogenic response. None of the remaining retinoids induced terata. The results suggest that the side chain plays an important role in retinoid embryotoxicity and that even minor structural alterations of retinoids at the side chain can abolish teratogenic activity. The present results indicate that a polyene chain of greater than five carbon atoms was necessary for retinoid teratogenic activity and that the presence of the ring, the nine-carbon side chain, and the acidic polar terminus was insufficient to ensure teratogenic activity. Retinoid teratogenicity in hamsters required the hydrophobic ring, a terminal polar group with an acidic pKa, and the hydrophobic, curved plane of the tetraene chain.

Comparative distribution, pharmacokinetics and placental permeabilities of all-trans-retinoic acid, 13-cis-retinoic acid, all-trans-4-oxo-retinoic acid, retinyl acetate and 9-cis-retinal in hamsters.

Pregnant hamsters were given a single oral dose (35 μmol/kg) of all-trans-retinoic acid, 13-cis-retinoic acid, all-trans-4-oxo-retinoic acid, 9-cis-retinal or all-trans-retinyl acetate during the early primitive streak stage of development. The radioactivity associated with the acidic retinoids was distributed to all tissues sampled (including placenta and fetus), with the largest accumulation in the liver and the least accumulation in fat. Radioactivity from 9-cis-retinal or retinyl acetate concentrated in the liver and lung. The all-trans-retinoic acid was oxidized in vivo to all-trans-4-oxo-retinoic acid and isomerized to 13-cis-retinoic acid; 13-cis-retinoic acid was oxidized to 13-cis-4-oxo-retinoic acid and isomerized to all-trans-retinoic acid. No parent 9-cis-retinal or retinyl acetate could be detected in maternal plasma. Plasma concentrations of the parent acidic retinoids reached their maxima within 60 min and then followed exponential decay. Of all the retinoids examined here, 13-cis-retinoic acid showed the largest area under the plasma curve, the slowest clearance and the longest elimination t 1/2. Total plasma radioactivity, consisting of unidentified metabolites, remained elevated at 4 days after dosing. Maternal peak circulating concentrations of the parent retinoids, total radioactivity, plasma pharmacokinetic parameters or the total concentrations of residual radioactivity in fetal tissues could not be correlated with the differential teratogenic potencies of these retinoids.