Design and in vitro evaluation of a novel polymeric P-glycoprotein (P-gp) inhibitor.

The aim of the present study was to improve the inhibitory properties of poly(ethylene glycol) (PEG) as excipient in drug delivery systems by covalent attachment of thiol moieties. This was achieved by grafting PEG to polyethylenimine (PEI) and finally thiolation with γ-thiobutyrolatone. Furthermore, the potential of this novel thiolated PEG-g-PEI co-polymer on the transport of rhodamine-123 (Rho-123) as P-gp substrate across freshly excised rat intestinal mucosa was evaluated in Ussing-type chambers. Apparent permeability coefficients (P(app)) were calculated and compared with values gained from experiments with the well-established P-gp inhibitors verapamil, reduced GSH, 6-mercatopurine and vitamin E-TPGS and the structurally similar compounds, myrj 52 and brij 35. The thiolated co-polymer displayed 145.07 ± 1.64 μmol/g of remaining primary amino groups, 84.30 ± 5.43 μmol/g of immobilized thiol groups and 12.74 ± 1.57 μmol/g of disulfide bonds. The approximate molecular mass of the thiolated co-polymer was 16,000 Da. The (1)H-NMR spectrum of PEG-g-PEI co-polymer was characterized by the presence of signal groups of PEG, hexamethylene diisocyanate (HMDI) and PEI substructures. Studies with Caco-2 cells revealed that the thiolated co-polymer shows 6.69 ± 0.27% of cytotoxicity by LDH assay and 93.33 ± 0.07% of cell viability by MTT assay. The thiolated co-polymer in a concentration of 0.5% (w/v) displayed a more pronounced effect on the absorptive transport of Rho-123 (P(app)=15.2 ± 1.0 × 10(-)(6)cm/s) in comparison to reduced GSH, 6-mercatopurine, vitamin E-TPGS, myrj 52 and brij 35. The thiolated co-polymer increased the transport of Rho-123 up to 3.3-fold in comparison to Rho-123 without any inhibitor used as control (P(app)=4.7 ± 0.1 × 10(-)(6)cm/s). The thiolated co-polymer applied in a concentration of 0.1%, 0.25% and 0. 5% (w/v) did not only enhance the absorption but also decreased the secretory transport of Rho-123 resulting in efflux ratios (secretory P(app)/absorptive P(app)) of 1.0, 1.4 and 2.0, respectively. Because of these features the novel thiolated PEG-g-PEI co-polymer seems to exhibit promising properties as novel P-gp inhibitor.