Prediction of in vivo tissue distribution from in vitro data. 2. Influence of albumin diffusion from tissue pieces during an in vitro incubation on estimated tissue-to-unbound plasma partition coefficients (Kpu).

Purpose: To determine the extent of albumin diffusion from tissue pieces into medium during in vitro incubations, to develop and assess the utility of mathematical models describing this effect on the estimation of tissue-to-unbound plasma partition coefficients (Kpu) of drug substances and to derive factors to correct for associated errors.

Methods: Twelve separate tissues were obtained from rats sacrificed by cervical dislocation, 48 h after an intravenous dose of 125I-human albumin, and tissue pieces incubated to determine the efflux of albumin into media over 2 to 4 h. A mathematical model was developed to predict and correct for the effect of albumin diffusion on the measured Kpu values of drugs.

Results: The model predicted that the effect of albumin diffusion from tissue pieces during in vitro incubation (ranging from 14 to 59% remaining in tissue) on Kpu values was generally minimal, except for compounds that are highly plasma bound and have a low measured Kpu. Under these circumstances, the measured Kpu substantially underestimates the true value. An equation was derived from readily available or measurable parameters to correct for this underestimation.

Conclusions: Albumin diffuses from tissue pieces into protein free media during in vitro incubations until equilibrium is reached, defined by the albumin Kpu. Model predictions indicated that for the majority of compounds albumin diffusion would have a minimal effect on the measured Kpu value and that a correction factor could be calculated to account for any deviation.