Bioequivalence (BE) testing is usually assessed by measuring the rate and extent to which the drug product (Test) is absorbed into the blood stream in comparison with the innovator for the marketing approval of generic drugs. Locally acting compounds such as bile acid and phosphate binders gets released in the gastrointestinal environment and are not absorbed into the systemic circulation binds phosphate and bile acids to form insoluble complexes in the intestine and excreted through the feaces. Conventional BE studies does not allow to characterize the bioequivalence of locally acting drugs. Hence FDA has developed a set of In vitro BE guidelines for bile acid and phosphate binding drugs to compare the extent and rate of binding between Test and Reference formulations. Following experiments are suggested in FDA published guidelines.
Kinetic binding study:
To characterize the maximum (saturation) binding of bile acids or phosphate to the test and innovator with respect to the time. This study is a support to the pivotal equilibrium binding study.
Equilibrium binding study:
The equilibrium binding study is the pivotal bioequivalence (BE) study to characterize the Langmuir binding constants such as affinity (K1) and capacity constant (K2 for) for test and reference formulations. Bioequivalence is established between Test and Reference formulation based on K2 with an appropriate confidence interval.
In vitro BE study is a surrogate to conventional in vivo BE for the localized efficacy drug products to establish the bioequivalence between test and reference formulations.
Kinetic binding and equilibrium studies needs to be performed on the finished products (whole tablets) from a single manufacturing lot.
The sequential steps involved in designing and conduct of the studies are as follows.
- Method development and validation of bile acids/phosphate using appropriate instrumentation as per bioanalytical guidelines published by FDA.
- Kinetic binding study for test and reference formulations with appropriate bile acids/phosphate concentrations and pH variation resembling the gastrointestinal pH. Free bile acids/phosphate concentration is determined and bound concentration is computed. The study needs to be replicated 12 times at each specified conditions.
- Equilibrium binding study for test and reference formulations with appropriate bile acids/phosphate ranging 8 different concentrations and pH variation resembling the gastrointestinal pH. Free bile acids/phosphate concentration is determined and bound concentration is computed. The study needs to be replicated 12 times at each specified conditions.
- Langmuir binding affinity (K1) and capacity (K2) constants are computed and bioequivalence is determined based on the capacity constant (K2).