Coatings from blends of Eudragit® RL and L55: a novel approach in pH-controlled drug release. 
The aim of the present study was to investigate the drug release from theophylline pellets coated with blends of quaternary polymethacrylate and methacrylic acid-ethyl acrylate copolymers.  Pellets were coated with blends of Eudragit(®) RL PO (RL) and Eudragit(®) L 100-55 (L55) in either organic solution or aqueous dispersion at various copolymer ratios.  Generally, the coatings were less permeable for theophylline in phosphate buffer pH 6.8 than they were in hydrochloric acid pH 1.2.  Further dissolution experiments revealed that the differences in drug release are caused by the different pH values.  A design of experiments for historical data was performed on drug release data of pellets with different coating levels and blend ratios of RL and L55.  Drug release in hydrochloric acid was predominantly affected by the coating level, whereas for drug release in phosphate buffer pH 6.8 the blend ratio was the determining factor.  As expected, dissolution experiments at different pH values showed that drug release depends on the ratio of dissociated L55 to RL because ionization is a requirement for the functional groups to interact.  With the dissolution test for delayed-release solid dosage forms (Ph.  Eur.)  it was demonstrated that the unique release behavior in neutral media is preserved after the exposition to hydrochloric acid.  These findings indicate that the combination of RL and L55 in coatings prepared from solutions is a promising approach for controlled drug release.