The Influx/efflux Mechanisms of D-peptide Ligand of NAChRs Across the Blood-brain Barrier and Its Therapeutic Value in Treating Glioma

A d-peptide ligand of the nicotine acetylcholine receptors (nAChRs), termed CDX, enables drug delivery to the brain when incorporated into liposomes and has shown promise as a nanocarrier for treating brain diseases. However, few reports have described the mechanisms whereby CDX-modified liposomes traverse the blood-brain barrier (BBB). Here, we studied the molecular mechanisms enabling CDX (and its associated liposomes) to cross an in vitro BBB using a simulated cerebral endothelium monolayer formed by brain capillary endothelial cells (bEnd.3 cells). We also examined the mechanisms whereby CDX-modified liposomes cross the BBB in vivo using the brain efflux-index method. Transport of CDX and its modified liposomes was dominantly mediated via the lipid raft/caveolae endocytic pathway. Both the endoplasmic reticulum (ER) and Golgi complex participated in delivering CDX-modified liposomes to the plasma membrane (PM). CDX-modified liposomes also participated in the endosome/lysosome pathway (with high-efficiency BBB crossing observed in vitro), while competing for the ER/Golgi/PM pathway. In addition, nAChR α7 did not promote the transportation of CDX-modified liposomes in vivo or in vitro, as assessed with α7-knockout mice and by performing α-bungarotoxin (α-Bgt) binding-competition experiments. P-glycoprotein (P-gp) was identified as the main efflux transporter across the BBB, in vivo and in vitro. Using a xenograft nude mouse model of human glioblastoma multiforme, blocking the efflux function of P-gp with verapamil enhanced the therapeutic efficiency of CDX-modified liposomes that were formulated with doxorubicin against glioblastoma. The findings of this study reveal novel mechanisms underlying crossing of the BBB by CDX-modified liposomes, suggesting that CDX-modified liposomes can potentially serve as a powerful therapeutic tool for treating glioma.