Plutonium Systemic Biokinetic Model for Rats

A baseline compartmental model (relative to modeling decorporation) of the distribution and retention of plutonium (Pu) in the rat for a systemic intake is derived. The model is derived from data obtained from a study designed to evaluate the behavior of plutonium in the first 28 days after incorporation. The model is based on a recently published model of americium (Am) in rats, which incorporated a pharmacokinetic (PK)-front-end modeling approach, which was used to specify transfer to and from the extracellular fluids (ECF) in the various tissues in terms of vascular flow and volumes of ECF. In the americium model, the approach was "cell-membrane limited," meaning that rapid diffusion of americium occurred throughout all the extracellular fluids (i.e., the blood plasma and interstitial fluids), while back-end rates representing transport into and out of the cells were determined empirically. However, this approach was inconsistent with the plutonium dataset. A good fit to the data is obtained by incorporating aspects of the Durbin et al. model structure, with plutonium in plasma separated into "free" and "bound" components. Free plutonium uses a cell-membrane-limited front end as for americium. Bound plutonium uses a capillary-wall-limited front end, where transfer rates from blood plasma into the interstitial fluids are relatively slow, and must be determined either empirically or from a priori knowledge. As in the Durbin et al. model, both free and bound plutonium are available for deposition in bone. In addition, our model has some bound plutonium associated with uptake to the gastrointestinal (GI) tract. Uncertainties in transfer rates were investigated using Markov Chain Monte Carlo (MCMC). It is anticipated that this model structure of plutonium will also be useful in interpreting comparable data from decorporation studies done in experimental animals.