Drug Distribution into Peripheral Nerve

Little is known about the impact of the blood-nerve barrier (BNB) on drug distribution into peripheral nerves. In this study, we examined the peripheral nerve penetration in rats of 11 small-molecule drugs possessing diverse physicochemical and transport properties and ProTx-II, a tarantula venom peptide with molecular mass of 3826 Daltons. Each drug was administered as constant rate intravenous infusion for 6 hours (small molecules) or 24 hours (ProTx-II). Blood and tissues including brain, spinal cord, sciatic nerve, and dorsal root ganglion (DRG) were collected for drug concentration measurements. Unbound fractions of a set of compounds were determined by equilibrium dialysis method in rat blood, brains, spinal cords, sciatic nerves, and DRG. We also investigated the influence of N-[4-[2-(6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-yl)ethyl]phenyl]-5-methoxy-9-oxo-10H-acridine-4-carboxamide (GF120918), a P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) inhibitor, on the peripheral nerve and central nervous system (CNS) tissue penetration of imatinib. We found that: 1) the unbound fraction in brain tissue homogenate highly correlates with that in the spinal cord, sciatic nerve, and DRG for a set of compounds and thus provides a good surrogate for spinal cord and peripheral nerve tissues, 2) small-molecule drugs investigated can penetrate the DRG and sciatic nerve, 3) P-gp and BCRP have a limited impact on the distribution of small-molecule drugs into peripheral nerves, and 4) DRG is permeable to ProTx-II, but its distribution into sciatic nerve and CNS tissues is restricted. These results demonstrate that small-molecule drugs investigated can penetrate peripheral nerve tissues, and P-gp/BCRP may not be a limiting factor at the BNB. Biologics as large as ProTx-II can access the DRG but not sciatic nerve and CNS tissues.