Optimal Structure Requirements for Pluronic Block Copolymers in Modifying P-glycoprotein Drug Efflux Transporter Activity in Bovine Brain Microvessel Endothelial Cells

Abstract

Pluronic block copolymer P85 was shown to inhibit the Pglycoprotein (Pgp) drug efflux system and to increase the permeability of a broad spectrum of drugs in the blood-brain barrier (BBB). However, there is an entire series of Pluronics varying in lengths of propylene oxide and ethylene oxide and overall lipophilicity. This study identifies those structural characteristics of Pluronics required for maximal impact on drug efflux transporter activity in bovine brain microvessel endothelial cells (BBMECs). Using a wide range of block copolymers, differing in hydrophilic-lipophilic balance (HLB), this study shows that lipophilic Pluronics with intermediate length of propylene oxide block (from 30 to 60 units) and HLB Ͻ20 are the most effective at inhibiting Pgp efflux in BBMECs. The methods used included 1) cellular accumulation studies with the Pgp substrate rhodamine 123 in BBMECs to assess Pgp activity; 2) luciferin/luciferase ATP assay to evaluate changes in cellular ATP; 3) 1,6-diphenyl-1,3,5-hexatriene membrane microviscosity studies to determine alterations in membrane fluidity; and 4) Pgp ATPase assays using human Pgp-expressing membranes. Pluronics with intermediate lipophilic properties showed the strongest fluidization effect on the cell membranes along with the most efficient reduction of intracellular ATP synthesis in BBMEC monolayers. The relationship between the structure of Pluronic block copolymers and their biological response-modifying effects in BBMECs are useful for determining formulations with maximal efficacy for increasing BBB permeability.

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“…Some representative examples are shown in Table 1. Maximal activity was observed with copolymers having intermediate lengths of PPG blocks and relatively hydrophobic such as Pluronic ® P85 or L61 [302,303]. Mechanism of inhibition is very different compared to conventional low molecular weight inhibitors [303,304].…”

“…Maximal activity was observed with copolymers having intermediate lengths of PPG blocks and relatively hydrophobic such as Pluronic ® P85 or L61 [302,303]. Mechanism of inhibition is very different compared to conventional low molecular weight inhibitors [303,304]. Pluronic ® PPG chains immerse into cell membrane, resulting in local alterations of membrane structure and decrease in net membrane microviscosity ("membrane fluidization") [304].…”

New Technologies for Drug Delivery Across the Blood Brain Barrier

“…Hydrophilic block copolymers, which have an extended EO block, do not incorporate into lipid bilayers and practically do not transport into the cells. As a result, they have little effect on either Pgp ATPase activity or ATP levels, which explains their negligible effect on Pgp efflux pump in BBMEC (29). Very lipophilic block copolymers with long PO blocks anchor in the plasma membranes and remain there for an extended period of time.…”

“…Recent findings suggest that effects of Pluronic ® on drug efflux transport proteins involve interactions of the block copolymers with the cell membranes (24,29). The hydrophobic PO chains of Pluronic ® immerse into the membrane hydrophobic areas, resulting in alterations of the membrane structure, and decrease of its microviscosity ("membrane fluidization") At relatively low concentrations (e.g.…”

“…It was demonstrated that a fine balance between hydrophilic (EO) and lipophilic (PO) components in the Pluronic ® molecule should be accomplished to potent enable inhibition of the drug efflux systems (29). Overall, the most efficacious block copolymers are those with intermediate lengths of PO block and relatively hydrophobic structure (HLB < 20), such as Pluronic ® P85 or L61 (23).…”