The role of phytochemicals in modulating breast cancer resistance protein at the blood-brain barrier and the blood-tumour barrier

Abstract

The blood-brain barrier (BBB) and the blood-tumour barrier (BTB) represent insidious obstacles for the delivery of anti-cancer agents to solid brain tumours, not only because of their morphological features, but also due to the presence of the drug efflux transporter breast cancer resistant protein (BCRP), localised at both the BBB and BTB. This efflux transporter restricts the permeation of anti-cancer agents across both barriers leading to suboptimal concentrations of drugs at the intended site of action. This work examined 13 naturally occurring phytochemicals, which were screened for their dual ability to modulate the efflux function of BCRP in addition to their anti-cancer properties in human LN229 glioblastoma cells, namely: (i) inhibition of cellular migration; (ii) activation of apoptosis; (iii) reactive oxygen species (ROS) production and (iv) activation of caspase pathways. Phytochemicals displayed minimal cytotoxicity, were able to modulate BCRP which led to enhancing the permeability of the fluorescent probe substrate H33342, in addition to inhibiting cellular migration. Hesperetin and baicalin displayed the optimal modulatory potential and demonstrated a similar ability to generate ROS and activate Caspase-3/7 when compared to the anti-cancer agents methotrexate and temozolomide. Subsequently, hesperetin was progressed as the optimal candidate, and its ability to permeate across the BBB was confirmed after conducting a permeability study using an in-vitro primary porcine brain microvascular endothelial cell (PBMEC) BBB model. We demonstrated that hesperetin was highly permeable across the BBB, can modulate the efflux function of BCRP and overall enhance the apparent permeability (Papp) of mitoxantrone and methotrexate. Thereafter, we assessed the impact of shear stress fashioned by laminal flow on the morphology of PBMEC using a Quasi Vivo 600® perfusion system. The results displayed a significant increase in Transepithelial Electrical Resistance (TEER) values, improved formation of zonula occludens-1 (ZO-1), and higher expression of efflux transporter proteins, suggesting the formation of a better in-vitro BBB model with hesperetin still being highly permeable across the barrier further confirming its ability to bypass the BBB and reach the BTB. This work highlights the anticancer and BCRP modulatory capabilities of phytochemicals as well as the ability of hesperetin to bypass the BBB.