In each step (lowest dose, middle dose, highest dose), subjects are randomly assigned in a double blinded manner to receive either ASP7991 (n=9) or Placebo (n=3) and are administered repeatedly. Once daily, the subjects are administered ASP7991 or placebo orally 30 minutes after breakfast with 150 mL water for 7 days. After the each step the sponsor evaluates the safety in the blinded manner for deciding an escalation to the step.