Inhibition of P-glycoprotein pumps by PEO-PPO amphiphiles: branched versus linear derivatives

Inhibition of the activity of efflux transporters may relevantly improve the chemotherapy of cancer and infectious diseases. Aim: To explore the ability of poloxamines (Tetronic (R), X-shaped structure with a central ethylendiamine group and four branches of poly[ethylene oxide] poly[propylene oxide] [PEO-PPO]) to inhibit the activity of P-glycoprotein (P-gp) on Caco-2 cell monolayers and to elucidate the incidence of the molecular architecture of PEO-PPO block copolymers on the intracellular accumulation of a relevant substrate, doxorubicin, by comparison with poloxamers (Pluronic (R), linear triblock copolymers), well-known inhibitors of this efflux transporter. Methods: Both pristine and N-methylated poloxamines displaying a wide range of molecular weights and EO/PO ratios were tested regarding cytocompatibility and accumulation of doxorubicin in Caco-2 monolayers. Verapamil was used as a control. Results: The most active anti-P-gp poloxamines (which enhanced two- to three-fold doxorubicin accumulation compared with verapamil) resulted to be pristine medium-to-high hydrophobic T304, T904, T1301, T901 and T150R1. A notable dependence of the anti-P-gp activity on the copolymer concentration was found. A joint diagram of the inhibitory activity of poloxamers and poloxamines as a function of the effective length of the PPO block is proposed. Conclusion: The anti-P-gp activity is maxima for block copolymers possessing a low-to-medium hydrophilic lipophilic balance and an 'effective number' of PO units ranging from 30 to 50.