3.3. Fexofenadine antacids interaction by HPLC
10 mL fexofenadine solution and 0.2 g antacid were taken in a conical flask and the flask was shaken for 2 h at 37 C, afterward the aliquots were filtered through a 0.45 lm Millipore filter paper. The mobile phase was prepared by mixing methanol and 6.8 g monobasic potassium phosphate in 1000-ml water and pH was adjusted to 7.4 with potassium hydroxide. The composition was methanol: phosphate buffer, 35:65 (v/v), respectively. The injection volume was 10 lL and the run time was 5.0 min. The mobile phase was filtered using a 0.45-lm membrane filter (Millipore) and degassed in ultrasonic bath. The mobile phase flow rate was 1.0 ml min1. Samples were injected through a 20 lL loop at ambient and the retention time for fexofenadine was 3.65–3.75 min (Arayne et al.,2009). Peak area and percentage of fexofenadine recovered have been discussed in Table 4. Peak areas were in contrast with standard fexofenadine solutions to evaluate the degree of interaction of fexofenadine with antacids. The chromatograms of adsorption studies carried out at HPLC have been shown in Figs. 7 and 8. In simulated gastric juice the percentage recovery of the fexofenadine increased in case of sodium bicarbonate (110.3%), magnesium carbonate (121.7%) and in all other antacids it decreased while in simulated intestinal juice the percent recovery of fexofenadine increased in case of sodium bicarbonate (115.03%), megaldrate (109.29%), Magnesium trisilicate (110.38%) and calcium hydroxide (106.01%). Fexofenadine (Fig. 6) has piperidine nitrogen and at tertiary carbon it has a carboxylic group and that is why it subsists as a zwitter ion in aqueous media at physiological pH. These two possible sites are responsible for complex formation and fexofenadine–antacid interaction takes place.