Toward Mechanistic Design of Surrogate Buffers for Dissolution Testing of pH-Dependent Drug Delivery Systems

The in vivo dissolution of enteric-coated (EC) products is often overestimated by compendial in vitro dissolution experiments. It is of great interest to mimic the in vivo conditions as closely as possible in vitro in order to predict the in vivo behavior of EC dosage forms. The reason behind this is the overly high buffering capacity of the common compendial buffers compared to the intestinal bicarbonate buffer. However, a bicarbonate-based buffer is technically difficult to handle due to the need for continuous sparging of the media with CO2 to maintain the desired buffer pH. Therefore, bicarbonate buffers are not commonly used in routine practice and a non-volatile alternative is of interest. A mathematical mass transport modelling approach was previously found to enable accurate calculation of surrogate buffer molarities for small molecule compounds; however, the additional complexity of polymeric materials makes this difficult to achieve for an enteric coat. In this work, an approach was developed allowing relatively rapid screening of potential surrogate buffers for enteric coating. It was found that the effective buffering pKa of bicarbonate at the surface of a dissolving enteric polymer tended to be around 5.5, becoming higher when the dissolving enteric polymer formed a gel of greater firmness/viscosity and vice versa. Using succinate (pKa 5.2 under physiological ionic strength) and/or citrate (pKa 5.7 under physiological ionic strength) at conjugate base molarities corresponding to bicarbonate molarities in the intestinal segments of interest as an initial "guess" can minimize the number of experimental iterations necessary to design an appropriate surrogate.