Frizzled Fissure to Improve Central Nervous System Drug Delivery?

Abstract

Delivery of therapeutics to the brain is challenging because of efflux pumps located in the vascular endothelium. A detailed analysis of Wnt signaling in a human brain endothelial cell line indicates that expression and function of P-glycoprotein, a major efflux transporter, is controlled by non-canonical Wnt signaling. Inhibition of this pathway leads to downregulation of P-glycoprotein and increased transcellular drug transport and reveals a potential strategy for improving drug delivery for treatment of neurologic diseases.

At a recent brain tumor conference, an accomplished neuro-oncologist remarked, ‘We have lots of drugs that would probably work against brain tumors, but the problem is how do we get them to the target’. Herein lies the challenge that applies to therapy delivery for nearly all central nervous system (CNS) diseases.

Primary CNS tumors and brain metastases lie outside the vascular wall that consists of endothelial cells forming a cellular interface between the blood and tissue. A unique and distinctive feature of neurovascular endothelial cells is the expression of specialized membrane proteins that form a bead-like linear array of adhesion contacts between adjacent cells. This effectively eliminates gaps or spaces that in other vascular beds allow diffusion of blood-borne molecules, such as antitumor agents, into the extravascular and interstitial space to reach their target. Thus, CNS delivery of clinically relevant drugs via paracellular diffusion is prevented and not an option.

A logical and appealing alternative is direct transfer or diffusion through the endothelial cellular membrane and is a strategy largely based on size, structure and physicochemical properties of the candidate entity (drug). However, research progress often reveals unexpected obstacles and new challenges. This is the twentieth anniversary year of the landmark paper by Schinkel et al ¹ who advanced our understanding of blood–brain drug transport when they discovered that P-glycoprotein, a member of the adenosine triphosphate (ATP)-binding cassette family, is highly expressed in brain endothelial cells and confers neuroprotection by pumping xenobiotics back into the bloodstream using the energy of ATP hydrolysis. The broad specificity of P-glycoprotein assures that many drugs with potential CNS activity are rejected and excluded from entering the brain and reaching their target. ²

In this issue Pinzòn-Daza et al ³ report findings that may reveal an exploitable fissure in the brain efflux mechanism that normally blocks drug entry into the brain. Using a carefully designed and executed study, the authors examine Wnt signal transduction pathways that regulate P-glycoprotein expression in hCMEC/D3 cells, a surrogate of human brain endothelial cells in vivo. Ligand binding to Frizzled receptors activates both the Wnt/GSK3 canonical and the Wnt/RhoA/RhoA kinase noncanonical pathways.^4,5 The new evidence indicates that both pathways cooperate in regulating P-glycoprotein expression. Furthermore, reduction of the noncanonical pathway by blocking RhoA expression with siRNA or by inhibiting RhoA kinase with a specific inhibitor (Y27632) significantly reduces P-glycoprotein expression and also increases doxorubicin transport through an endothelial cell monolayer sufficient to become cytotoxic to cocultured glioblastoma cells. The significance of this is that specific inhibitors that target the noncanonical pathway reduce P-glycoprotein expression and may enable greater drug penetration into the brain for the treatment of tumors, epilepsy, and other conditions. Moreover, fasudil, an analog of Y27632, is approved for human subjects and may soon be evaluated in the clinic for improving brain drug delivery.

The effectiveness of targeting the noncanonical Wnt pathway may depend on the relative importance of related drug-resistance proteins present in the neurovascular unit including BCRP, MRP1, MRP2, MRP4, and MRP5.^6,7 New experimentation will reveal if the fissure noted here becomes a passageway and whether patients will be benefitted.