Evaluation and modification of a physiologically based model of lead kinetics using data from a sequential isotope...

Endogenous (predominantly bone) and exogenous lead were differentially labeled in two 11-year-old female cynomolgus monkeys (Macaca fascicularis) to establish the contributions of the two sources to blood lead. The monkeys had been administered a common lead isotope "mix" at the rate of about 1300 micrograms Pb/kg body wt/day from age 10 months until the start of the study. On day 0, common lead was replaced in sequence by mixes artificially enriched in 204Pb, 206Pb, and 207Pb, given for periods of from 50 to 281 days. Total lead ingestion rate was held constant except during administration of the 207Pb-enriched mix to one of the monkeys, when it was reduced to 650 micrograms/kg/day. Blood and bone were sampled at intervals and analyzed for their content of each of the isotope mixes. A physiologically based model of human lead kinetics was scaled to the cynomolgus monkey and fit to the data to test the correctness of the model structure and to assist with interpretation of study results. Fractional absorption was varied to achieve the best visual fits of the scaled model to blood and bone concentration data for each monkey. The model failed to reproduce the sharp drop in isotope concentrations in blood observed after each exchange of isotope mix. Consequently, it was revised to include a rapid-turnover trabecular bone compartment and a slow-turnover cortical bone compartment, using estimates of trabecular and cortical bone turnover rates from histomorphometric studies in adult cynomolgus monkeys. The revised model fit most of the sets of bone and blood concentrations well. About 17% of the blood lead originated from bone after 11 years of exposure, at blood lead concentrations in excess of 50 micrograms/dl. The rate of return of common lead from bone, as estimated from the model, was 28 micrograms/day just before termination of controlled common lead exposure on day 0. Based on the success of the scaled human model in fitting these data and on the absolute and relative values of bone and blood lead concentrations, the metabolism of lead in the cynomolgus monkey appears to be similar to human lead metabolism.