Immunotoxicologic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rabbits.

Male white New Zealand rabbits were exposed orally to 0, 0.01, 0.1, 1 and 10 micrograms/kg/wk of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for a period of 8 weeks. After 4 and 6 weeks of first TCDD administration, the rabbits were inoculated with a mixture of tetanus toxoid and Freund's adjuvant. TCDD exposure reduced the serum antitoxin titers, skin sensitivity to tuberculin, and the number of antibody producing cells in popliteal lymph nodes. At the end of the treatment period serum IgG levels were increased at the lowest dose of TCDD treatment while a marked depression was noticed at the highest dose level. An increase in the thymidine uptake by splenic lymphocytes in culture was noted at all levels of TCDD treatment whereas the response of these cells to phytomitogens was decreased at high levels of TCDD exposure. All different immunologic effects were not altered at the lowest TCDD treatment but both humoral and cell-mediated immune responses were depressed at the highest level of TCDD exposure.

Immunotoxicologic studies with vinyl chloride in rabbits and mice.

Our previous studies in mice indicated that the exposure to vinyl chloride (VC) produced a state of immunostimulation. The metabolism of VC was an important factor in this phenomenon. The present paper describes the effects of VC exposure on induced immunologic responses in rabbits. No consistent effect of VC exposure was noticed on skin reactivity to tuberculin or serum anti-tetanus titers in sensitized rabbits. Vinyl chloride produced no change in the number of antibody secreting cells in the lymph nodes of immunized rabbits. An increase in the spontaneous splenic lymphocyte transformation in immunized rabbits was observed when the animals were exposed to VC. Two known metabolites of VC, namely thiodiglycolic acid and N-acetyl-S-(hydroxyethyl)-cysteine produced little or no effect when added to mouse splenic lymphocyte cultures in vitro but in vivo administration of thiodiglycolic acid produced apparent immune stimulation in mice. The study indicated that although VC may cause an apparent enhancement of immune reactivity, it does not alter the immunologic response to simultaneously administered antigens.

Non-linear pharmacokinetic parameters need to be considered in high dose/low dose extrapolation.

It is impossible to prove that any chemical, natural or man-made, cannot cause cancer in man. However, it is possible to estimate the relative degrees of risk associated with various agents. The precision of these estimations increases as experimental procedures elucidate the basic type of mechanism associated with carcinogenesis, the role of absorption, metabolism and distribution, and excretion in increasing or decreasing activity, and the dose dependency of metabolic pathways. We must constantly strive to make the most accurate risk estimations possible so that the complex issues of risk/benefit may be properly considered.

Mechanisms of carcinogenesis: dose response.

There is great controversy whether the carcinogenicity of chemicals is dose-dependent and whether a "threshold" dose exists below which cancer will not be induced by exposure. Evidence for dose-dependency exists and is believed to be accepted generally if extricated as it should be from the "threshold" concept. The "threshold" concept conflict is not likely to be resolved in the foreseeable future; proponents and opponents argue their case in a manner similar to those arguing religion. In this paper the various arguments are reviewed. Subsequently, a chemical process model for carcinogenesis is developed based on the generally accepted evidence that the carcinogenic activity of many chemicals can be related to electrophilic alkylation of DNA. Using this model, some incidence of cancer, albeit negligible, will be predicted regardless how low the dose. However, the model reveals that the incidence of cancer induced by real-life exposures is likely to be greatly overestimated by currently used stochastic statistical extrapolations. Even more important, modeling of the chemical processes involved in the fate of a carcinogenic chemical in the body reveals experimental approaches to elucidating the mechanism(s) of carcinogenesis and ultimately a more scientifically sound basis for assessing the hazard of low-level exposure to a chemical carcinogen.

Pharmacokinetics of vinylidene chloride in the rat.

The metabolism of inhaled vinylidene chloride in rats represents a balance of biotransformation pathways leading to the formation of a reactive alkylating species which is normally detoxified by conjugation with glutathione. Detoxification of the reactive intermediate formed from inhaled VDC is dependent upon the availability of hepatic glutathione (GSH); as VDC exposure concentrations are increased, the fraction of the dose detoxified by conjugation with GSH decreases markedly, commensurate with depletion of hepatic GSH. This reactive intermediate in the absence of GSH alkylates hepatic macromolecules and causes cell death. Similarly, hepatic GSH plays a vital role in the detoxification of the reactive metabolite formed from inhaled vinyl chloride (VC). However, the dose--response relationships for the utilization of GSH and the accumulation of alkylating metabolites following inhalation exposure to either VDC or VC point to distinct differences which may explain the differing biological activities of the two materials. Finally, preliminary pharmacokinetic data for inhaled VDC in mice indicate an enhanced susceptibility to VDC by virtue of an increased ability for production of alkylating VDC metabolites over that observed in the rat. The importance of these findings in light of recent evidence for a carcinogenic effect of VDC in mice is discussed.

Dose-dependent fate of vinyl chloride and its possible relationship to oncogenicity in rats.

Studies on the fate of 14C-labeled vinyl chloride (VC) following oral administration and inhalation exposure in rats demonstrated that the disposition of VC in the body is a function of the dose. More importantly, from the data available, it appears that a correlation exists between doses of VC which cause tumors and those that saturate metabolic or detoxifying pathways. Additional studies characterized the depression of liver non-protein sulfhydryl content (primarily GSH) with the duration and concentration of exposure to VC. The results of these investigations indicate that statistical projections utilizing data collected from rats exposed to high doses of VC are invalid for predicting the hazard of low level exposure, because such projections violate a priori assumption that the dynamics governing the fate of VC in the body are unaltered.

Preliminary studies on the fate of inhaled vinyl chloride monomer (VCM) in rats.

Rats were exposed to vinyl chloride monomer gas (VCM) in a closed recirculating system. The rate at which VCM was removed from the system via metabolism was determined for rats exposed to initial concentrations of VCM ranging from 50 to 1167 ppm. Upon exposure to initial concentrations of 50 to 105 ppm, the rate of metabolism was 8.04 plus or minus 3.40 x 10(-3) min-1. Upon exposure to initial concentrations ranging from 202 to 1167 ppm, the rate constants were less; the mean value being 2.65 plus or minus 1.35 x 10(-3) min-1. Regardless of concentration, the disappearance followed apparent first order kinetics. Pretreatment of rats with pyrazole prior to exposure to initial concentrations of 65 and 1234 ppm VCM caused 71 and 87% reductions in the rate of metabolism. Ethanol caused 96% and 83% reductions in the rate of VCM metabolism by rats exposed to 56 and 97 ppm VCM, respectively. Ethanol was less effective in blocking the rate of metabolism by rats exposed to high concentrations of VCM; 46 and 36% in rats exposed to 1025 and 1034 ppm VCM. In rats exposed to an initial concentration of 65 ppm VCM, SKF-525-A administration caused no inhibition of the rate of VCM metabolism; however, a 19% inhibition was seen in rats exposed to 1038 ppm. The nonprotein sulfhydryl content of the liver (glutathione and cysteine) of rats exposed to VCM concentrations ranging from 50 to 15,000 ppm VCM is reduced without a relationship to dose. With repeated daily exposure the degree of reduction is reduced. Preliminary results indicate that the primary metabolites of VCM react with the nonprotein sulfhydryl. Final metabolic products excreted in the urine appear to be S-(2-hydroxyethyl) cysteine and S-(2-carboxymethyl)cysteine and the respective N-acetyl derivatives. Monochloroacetic acid was identified as another potential metabolite. Considering the results in toto, it is hypothesized that VCM is readily and extensively metabolized. Metabolism via the primary pathway, postulated to involve alcohol dehydrogenase, is swamped by exposures to concentrations exceeding 220 ppm. In rats exposed to concentrations at and exceeding this level, metabolism occurs via a secondary pathway(s), postulated to be epoxidation and/or peroxidation. These results are considered pertinent is assessing the potential hazard at low level exposures to VCM.