Blood-Brain Barrier Drug Transport Mechanisms

Abstract

The blood-brain barrier (BBB) -- a highly specialised neurovascular unit comprising brain microvascular endothelial cells, astrocyte end-feet, pericytes, and tight junction complexes -- restricts CNS entry of approximately 98% of small-molecule drugs and virtually all macromolecular therapeutics, rendering CNS drug delivery the most formidable unsolved challenge in pharmaceutical sciences. Transport across the BBB occurs via five principal mechanisms -- passive transcellular diffusion, paracellular transport, carrier-mediated influx (CMT), efflux transporter-mediated active extrusion, and receptor-mediated transcytosis (RMT) -- each governed by distinct molecular determinants amenable to rational exploitation. This study evaluated 240 CNS-active and CNS-inactive drugs (CNS-active n = 128; CNS-inactive n = 112) across a validated hCMEC/D3 in vitro BBB model and rat in situ brain perfusion (Paris and Madrid laboratories, 2020-2023), developing a BBB Transport Quality Index (BTQI) integrating permeability coefficient (Papp), P-glycoprotein efflux ratio, LAT1 substrate probability, LogBB prediction, and tight junction integrity score. BTQI predicted CNS exposure (brain/plasma ratio > 0.3 in rat) with AUC = 0.884 and Pearson r = +0.84 (p < 0.001; n = 96 compounds with in vivo brain/plasma data), identifying Papp > 15 x 10-6 cm/s and P-gp efflux ratio < 2.0 as the dominant binary criteria for CNS penetration, consistent with the physicochemical rules-of-five for CNS drug design.