COMPARATIVE PHARMACOKINETIC-PHARMACODYNAMIC STUDY OF PROTON PUMP INHIBITORS, OMEPRAZOLE AND LANSOPRAZOLE IN RATS

The relationship between plasma concentrations and inhibitory effects on gastric acid secretion by omeprazole (OPZ) and lansoprazole (LPZ), which are used as antiulcer drugs in the clinical stage, was analyzed using the pharmaco kinetic/pharmacodynamic (PK/PD) model in rats. Afte r intravenous administration of OPZ and LPZ (1 mg/kg), OPZ was elimina ted 1-exponentially and LPZ was eliminated 2-exponentially from plasma . Elimination was rapid with total body clearance of 57.6 ml/min/kg fo r OPZ and 58.6 ml/min/kg for LPZ. The volumes of distribution at stead y-state were 0.66 liter/kg for OPZ and 1.04 liter/kg for LPZ, and the plasma unbound fractions were 0.105 and 0.069. The dose at which 50% o f the maximum effect is elicited for the suppression of gastric acid s ecretion stimulated by histamine was 0.28 +/- 0.13 mg/kg (estimated va lue +/- SD) for OPZ and 0.18 +/- 0.03 mg/kg for LPZ. Second-order rate constants for association of OPZ or LPZ and H+,K+-ATPase based on a P K/PD model were 72.5 +/- 30.0 (estimated value +/- SD) and 124 +/- 58 ml/mu g/hr, respectively. Apparent turnover rate of H+,K+-ATPase was 8 .8 hr as half-life, assuming the same value for both drugs. We conclud ed that pharmacokinetic elimination patterns of OPZ and LPZ were diffe rent, whereas the pharmacodynamic characteristics of both drugs are ne arly the same in rats.