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Biomedical subjects

L R Rhomberg

Publications and source records attributed to L R Rhomberg.

7 recordsLinked to original sources
PubMed2001

Reproductive and developmental risks from ethylene oxide: a probabilistic characterization of possible regulatory thresholds.

Ethylene oxide is a gas produced in large quantities in the United States that is used primarily as a chemical intermediate in the production of ethylene glycol, propylene glycol, non-ionic surfactants, ethanolamines, glycol ethers, and other chemicals. It has been well established that ethylene oxide can induce cancer, genetic, reproductive and developmental, and acute health effects in animals. The U.S. Environmental Protection Agency is currently developing both a cancer potency factor and a reference concentration (RfC) for ethylene oxide. This study used the rich database on the reproductive and developmental effects of ethylene oxide to develop a probabilistic characterization of possible regulatory thresholds for ethylene oxide. This analysis was based on the standard regulatory approach for noncancer risk assessment, but involved several innovative elements, such as: (1) the use of advanced statistical methods to account for correlations in developmental outcomes among littermates and allow for simultaneous control of covariates (such as litter size); (2) the application of a probabilistic approach for characterizing the uncertainty in extrapolating the animal results to humans; and (3) the use of a quantitative approach to account for the variation in heterogeneity among the human population. This article presents several classes of results, including: (1) probabilistic characterizations of ED10s for two quantal reproductive outcomes-resorption and fetal death, (2) probabilistic characterizations of one developmental outcome-the dose expected to yield a 5% reduction in fetal (or pup) weight, (3) estimates of the RfCs that would result from using these values in the standard regulatory approach for noncancer risk assessment, and (4) a probabilistic characterization of the level of ethylene oxide exposure that would be expected to yield a 1/1,000 increase in the risk of reproductive or developmental outcomes in exposed human populations.

Animals↗
PubMed2000

Dose-response analyses of the carcinogenic effects of trichloroethylene in experimental animals.

In lifetime bioassays, trichloroethylene (TCE, CAS No. 79-01-6) causes liver tumors in mice following gavage, liver and lung tumors in mice following inhalation, and kidney tumors in rats following gavage or inhalation. Recently developed pharmacokinetic models provide estimates of internal, target-organ doses of the TCE metabolites thought responsible for these tumor responses. Dose-response analyses following recently proposed methods for carcinogen risk assessment from the U.S. Environmental Protection Agency (U.S. EPA) are conducted on the animal tumor data using the pharmacokinetic dosimeters to derive a series of alternative projections of the potential carcinogenic potency of TCE in humans exposed to low environmental concentrations. Although mechanistic considerations suggest action of possibly nonlinear processes, dose-response shapes in the observable range of tumor incidence evince little sign of such patterns. Results depend on which of several alternative pharmacokinetic analyses are used to define target-organ doses. Human potency projections under the U.S. EPA linear method based on mouse liver tumors and internal dosimetry equal or somewhat exceed calculations based on administered dose, and projections based on mouse liver tumors exceed those from mouse lung or rat kidney tumors. Estimates of the carcinogenic potency of the two primary oxidative metabolites of TCE--trichloroacetic acid and dichloroacetic acid, which are mouse liver carcinogens in their own right--are also made, but it is not clear whether the carcinogenic potency of TCE can be quantitatively ascribed to metabolic generation of these metabolites.

Animals↗
PubMed1998

Empirical scaling of single oral lethal doses across mammalian species based on a large database.

The scaling of administered doses to achieve equal degrees of toxic effect in different species has been relatively poorly examined for noncancer toxicity, either empirically or theoretically. We investigate empirical patterns in the correspondence of single oral dose LD50 values across several mammalian species for a large number of chemicals based on data reported in the RTECS database maintained by the National Institute for Occupational Safety and Health. We find a good correspondence of LD50 values across species when the dose levels are expressed in terms of mg administered per kg of body mass. Our findings contrast with earlier analyses that support scaling doses by the 3/4-power of body mass to achieve equal subacute toxicity of antineoplastic agents. We suggest that, especially for severe toxicity, single- and repeated-dosing regimes may have different cross-species scaling properties, as they may depend on standing levels of defenses and rate of regeneration of defenses, respectively.

Administration, Oral↗
PubMedDate not supplied

Evidence for a link between local and seasonal cycles in gene frequencies and latitudinal gene clines in a cyclic parthenogen.

In an earlier study (Rhombergh et al., Can. J. Genet. Cytol. 27: 224-232, 1985) of natural populations of the cyclic parthenogenetic Rose aphids, Macrosiphum rosae, 6 out of 31 loci were found to be polymorphic and one locus (Esterase-4) showed cyclic seasonal changes in gene and genotypic frequencies. Assuming that the Est-4 polymorphism was balanced and due to some climatic factor that varies seasonally, and realizing that most environmental factors that vary seasonally also vary latitudinally, we predicted existence of a latitudinal gene cline at this locus. In the present study we surveyed four polymorphic loci (chosen to be used as markers) in six geographic populations spanning over 1200 km between the United States and Canada and found all four loci to have latitudinal clines. We think that the gene clines are due to a latitudinal cline in the degree of advancement of local populations through the seasonal cycle, and have called such a pattern a 'seasonal phase cline'. The results are discussed in relation to the temporal instability of local patterns and persistence of genetic variability on the large scale in aphids. It is argued that population structure of aphids makes retention of selectively neutral or weakly selected polymorphisms difficult.

Animals↗