Objective: The objective of the present investigation was to prepare gastro-resistant microspheres of esomeprazole magnesium trihydrate (EMT) to prevent its degradation in the acidic environment of the stomach and enhance its bioavailability via intestinal absorption.Methods: EMT loaded gastro-resistant microspheres were prepared using hypromellose acetate succinate (HPMCAS) as the gastro-resistant polymer by ‘non-aqueous solvent evaporation' technique. A 3-factor 3 level factorial design was used to optimise EMT: HPMCAS ratio, the concentration of Span 80 and stirring speed with respect to percent entrapment efficiency and particle size. Further characterization was carried out using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), In vitro release study and In vivo anti-ulcer activity.Results: Fourier transform infrared (FTIR) study indicated compatibility between drug and polymer. DSC study revealed that the drug was molecularly dispersed in the polymer. The optimised batch showed 49.63±1.23% drug entrapment and 170.12±3.36 μm particle size. SEM study showed that microspheres were spherical in shape. In vitro drug release study showed only 4.28±1.23% drug release in simulated gastric media in 2 hr and 93.46±1.20% release in simulated intestinal media after 1 hr from the optimised batch.Conclusion: Results of in vitro release studies indicated the gastro-resistant nature of the developed microspheres. In vivo anti-ulcer activity demonstrated that EMT loaded microspheres were able to significantly reduce ethanol-induced ulcer formation in rats' stomach as compared to the aqueous solution of EMT. So it can be concluded that the developed gastro-resistant microspheres of EMT prevented drug release in the stomach which would lead to a significant improvement in its bioavailability through enhanced intestinal absorptio
