S-309309 has poor stability under acidic conditions; it decomposes to form compound 8 (Table ). Therefore, it is important to minimize contact between S-309309 and gastric fluid to suppress the amount of compound 8 formed in the body. We tried to encapsulate S-309309 and evaluated the effect of encapsulation on the increase of compound 8 amount in acidic solution. To prevent the decomposition of drugs in the stomach, entericcoated capsules or tablets with enteric coating are usually prepared. However, when enteric-coated formulations are used, a decrease in bioavailability is often observed. Therefore, in this study, we used a rapidly dissolving capsule considering the gastric emptying rate in the fasting state , which has a low pH. The time-dependent formation of the compound 8 amount from S-309309 capsule or unencapsulated S-309309 in hydrochloric acid solutions was evaluated (Table ). When S-309309 was encapsulated, no compound 8 was observed for 20 min, and even after 30 min, the increase of compound 8 amount was suppressed compared with the case for unencapsulated S-309309. These results suggest that encapsulating S-309309 will suppress its decomposition in the stomach.