In Part 1, in Period 1, a single dose of entinostat will be administered. In Period 2, multiple doses of omeprazole will be administered for 5 days with a single dose of entinostat co-administered. If preliminary results in Period 2 of Part 1 indicate that omeprazole exhibits clinically significant drug-drug interaction (DDI) with entinostat, then Parts 2 and 3 will be conducted. If no DDI interaction is concluded between entinostat and omeprazole in Part 1, then neither Part 2 nor 3 will be conducted.