Beyond Overton’s Rule: Quantitative Modeling of Passive Permeation through Tight Cell Monolayers

TLDR

Optimizing transport across cell barriers remains a major challenge in pharmacokinetics. This study models permeation across a tight endothelial monolayer using a series of NBD‑labeled alkyl‑chain amphiphiles to derive structure–permeability rules. The model follows amphiphile movement from serum—equilibrating with albumin and lipoproteins—through sequestration, endothelial interaction, and tissue accumulation, and uses sensitivity analysis to identify rate‑limiting steps. Permeability shows a parabolic dependence on alkyl‑chain length, but beyond a hydrophobicity threshold further increases slow tissue permeation; serum sequestration and endothelial desorption dominate for some amphiphiles, others are limited by translocation, and for some multiple steps contribute, with analytical equations derived for single‑step‑controlled cases.

Abstract

One of the great challenges in pharmacokinetics is to find a means to optimize the transport across cell barriers. In this work, permeation across a cell monolayer, such as the tight endothelia in the blood–brain barrier, was modeled using a homologous series of amphipatic molecules, 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-labeled alkyl chain amphiphiles (NBD-Cn, n = 2 to 16), to obtain rules that relate permeant structure to permeability. The amphiphile enters the system from the serum, equilibrated with serum albumin and lipoproteins, and its sequestration by serum components, interaction with the endothelium, and accumulation in the tissue is followed over time. The dependence of the permeability coefficient on the number of carbons of the amphiphile's alkyl chain has a parabolic-like shape. After a threshold value, an increase in the hydrophobicity of the amphiphile, along the homologous series, results in a decrease in the characteristic rate of permeation to the tissue. A sensitivity analysis was performed, and the rate limiting steps for permeation of each amphiphile were identified. Sequestration in the serum and rate of interaction with the endothelium, particularly the rate of desorption, were found to be the determinant processes for some amphiphiles, while for others translocation was the rate limiting step. Additionally, for some amphiphiles a single rate limiting step could not be identified, with several steps contributing significantly to the overall permeation. Finally, we derived analytical equations that adequately describe the rate of amphiphile accumulation in the tissue for the cases where permeation is controlled by a single rate limiting step.