Using structural information to create physiologically based pharmacokinetic models for all polychlorinated biphenyls. II. Rates of metabolism.

Physiologically based pharmacokinetic (PBPK) models are useful in describing the distribution, metabolism, and fate of xenobiotics across multiple species. The eventual goal of the present research is to create PBPK models for all 209 polychlorinated biphenyls (PCBs). Key parameters in any PBPK model are the metabolic rates. Data on metabolic rates of PCBs were derived from in vitro experiments and from fitting of PBPK models to in vivo data. The rate of metabolism was assumed to be a linear function of PCB concentration. Structural descriptors suggested by the literature were used in a stepwise regression to find an expression for the metabolic rate of PCBs as a function of five structural descriptors related to the degree and pattern of chlorine substitution. R2 for the fit of the model to the data is 0.9606.

Using structural information to create physiologically based pharmacokinetic models for all polychlorinated biphenyls.

Physiologically based pharmacokinetic (PBPK) models are useful in describing the distribution, metabolism, and fate of xenobiotics across multiple species. The eventual goal of the present research is to create PBPK models for all 209 polychlorinated biphenyls (PCBs). Key parameters in any PBPK model are the tissue-to-blood partition coefficients. Tissue:blood partition coefficients relate the compound's concentration in a target tissue to its concentration in blood under equilibrium conditions. Data on the adipose:plasma partition coefficients of 24 PCBs were used in a regression analysis to find an expression for the adipose:plasma partition coefficient as a function of molecular structure. Using stepwise regression, it was found that three simple structural descriptors were sufficient to predict adipose:plasma partition coefficients for all 209 PCB congeners. Data on the distribution of PCBs among blood components were used to derive the adipose:blood partition coefficient from the adipose:plasma partition coefficient. The lipid contents of liver, muscle, and skin were used to derive the tissue:blood partition coefficient for those tissues from the adipose:blood partition coefficient. These results allow for the calculation of tissue:blood partition coefficients for liver, skin, muscles, and fat for all 209 PCB congeners.

Comparative absorption of lead from contaminated soil and lead salts by weanling Fischer 344 rats.

A 44-day dosed feed study was performed to compare the bioavailability of lead from contaminated soil versus two lead salts and the effect of soil on gastrointestinal absorption of ingested lead. Male Fischer rats (approximately 4 weeks of age) received lead, 17, 42, or 127 ppm, in the form of lead acetate, lead sulfide, lead-contaminated soil, or combinations thereof in the diet for 7, 15, or 44 days. Control soil was added to the diets of some animals to determine how it might alter lead bioavailability. Blood Delta-aminolevulinic acid dehydratase (Delta-ALAD) and blood, bone, kidney, and liver lead were determined in groups of animals at each time-point. Blood Delta-ALAD was inhibited in a dose-dependent manner and to the greatest degree in the lead acetate and lead acetate/control soil groups, followed by the lead sulfide and lead-contaminated soil groups. Bone and tissue lead levels increased in a dose-dependent manner and were greatest in animals receiving lead acetate and significantly less in animals receiving lead sulfide and lead-contaminated soil. Blood lead levels were generally greatest by 7 days and stabilized at lower levels thereafter. Bone lead concentration-time patterns did not demonstrate the biphasic change seen with tissues and continued to increase in most treatment groups through the course of the study. The presence of soil in the diet clearly attenuated the absorption of lead acetate, but had little effect on the absorption of lead sulfide. Results of these studies confirm previous observations that lead absorption is highly dependent on the form of lead ingested and the matrix in which it is ingested. More important, these studies demonstrate that lead in soil may be significantly less available than estimated by current default assumptions and that the presence of soil may decrease the availability of lead from lead salts on which the default assumptions are based. Results presented here also demonstrate that the weanling rat may represent an appropriate model that could be used to obtain relatively rapid and economical estimates of the availability of lead in complex matrices such as soil.