Effect of pH on the Stability of Amoxycillin in Buffered Aqueous Solutions and in Gastric Juice

Abstract

Helicobacter pylori is associated with gastritis and peptic ulcer. The organism is susceptible to amoxycillin and many other antibiotics in-vitro. Unless administered with omeprazole, amoxycillin is ineffective in eradicating H. pylori infection. To investigate the effect of instability of amoxycillin in the failure to eradicate the organism in-vivo, the effect of pH on the stability of amoxycillin in buffered aqueous solutions and gastric juice was studied using HPLC analysis. The gastric juice was obtained from H. pylori-negative volunteers before and after they had received a seven-day course of omeprazole. Amoxycillin was most stable in aqueous solutions of pH 4 to 7. The t 1/2 in the aqueous solutions of pH 2 and 7 were 19·0 ± 0·2 and 153·1 ± 3·3 h, respectively. The t 1/2 in the gastric juice samples of pH 2 was 15·2 ± 0·3 h. At pH 7, the t 1/2 values of amoxycillin in the gastric juice samples collected from volunteers before and after the seven-day course of omeprazole were 167·0 ± 79·2 and 68·1 ± 42·3 h, respectively. The co-administration of omeprazole during treatment of H. pylori with amoxycillin is likely to increase the stability of amoxycillin in the stomach. However, amoxycillin is sufficiently stable at normal gastric pH to maintain its therapeutic efficacy in treating H. pylori infections when given alone.