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Background  The goal of physiologically based pharmacokinetics (PBPK) is to predict drug kinetics from an understanding of the organ/blood exchange. The standard approach is to assume that the organ is "flow limited" which means that the venous blood leaving the organ equilibrates with the well-stirred tissue compartment. Although this assumption is valid for most solutes, it has been shown to be incorrect for several very highly fat soluble compounds which appear to be "diffusion limited". This paper describes the physical basis of this adipose diffusion limitation and its quantitative dependence on the blood/water (K bld-wat ) and octanol/water (K ow ) partition coefficient.    Methods  Experimental measurements of the time dependent rat blood and adipose concentration following either intravenous or oral input were used to estimate the "apparent" adipose perfusion rate (F A ) assuming that the tissue is flow limited. It is shown that the ratio of F A  to the anatomic perfusion rate (F) provides a measure of the diffusion limitation. A quantitative relationship between this diffusion limitation and K bld-wat  and K ow  is derived. This analysis was applied to previously published data, including the Oberg et. al. measurements of the rat plasma and adipose tissue concentration following an oral dose of a mixture of 13 different polychlorinated biphenyls.    Results  Solutes become diffusion limited at values of log K ow  greater than about 5.6, with the adipose-blood exchange rate reduced by a factor of about 30 for a solute with a log K ow  of 7.36. Quantitatively, a plot of F A /F versus K ow  is well described assuming an adipose permeability-surface area product (PS) of 750/min. This PS corresponds to a 0.14 micron aqueous layer separating the well-stirred blood from the adipose lipid. This is approximately equal to the thickness of the rat adipose capillary endothelium.    Conclusions  These results can be used to quantitate the adipose-blood diffusion limitation as a function of K ow . This is especially important for the highly fat soluble persistent organic chemicals (e.g. polychlorinated biphenyls, dioxins) whose pharmacokinetics are primarily determined by the adipose-blood exchange kinetics.

Quantitative relationship between the octanol/water partition coefficient and the diffusion limitation of the exchange between adipose and blood