Enteric Polymer Based on pH-Responsive Aliphatic Polycarbonate Functionalized with Vitamin E To Facilitate Oral Delivery of Tacrolimus

To improve the bioavailability of orally administered drugs, we synthesized a pH-sensitive polymer (poly(ethylene glycol)–poly(2-methyl-2-carboxyl-propylene carbonate)–vitamin E, mPEG–PCC–VE) attempting to integrate the advantages of enteric coating and P-glycoprotein (P-gp) inhibition. The aliphatic polycarbonate chain was functionalized with carboxyl groups and vitamin E via postpolymerization modification. Optimized by comparison and central composite design, mPEG113–PCC32–VE4 exhibited low critical micelle concentration of 1.7 × 10–6 mg/mL and high drug loading ability for tacrolimus (21.2% ± 2.7%, w/w). The pH-responsive profile was demonstrated by pH-dependent swelling and in vitro drug release. Less than 4.0% tacrolimus was released under simulated gastric fluid after 2.5 h, whereas an immediate release was observed under simulated intestinal fluid. The mPEG113–PCC32–VE4 micelles significantly increased the absorption of P-gp substrate tacrolimus in the whole intestine. The oral bioavailability of tacrolimus micelles was 6-fold higher than that of tacrolimus solution in rats. This enteric polymer therefore has the potential to become a useful nanoscale carrier for oral delivery of drugs.