GTS-21 was developed as a novel agent in the treatment of Alzheimer's disease. Until date, 87 healthy male volunteers were enrolled in four Phase I studies that assessed the safety, tolerability, pharmacokinetics, and effects on cognitive function of oral administration of GTS-21 of which 77 subjects received GTS-21 and 10 subjects received placebo. GTS-21 was found to be well tolerated up to a dose of 150 mg three times daily (450 mg/day) in healthy male subjects. The most common adverse event was headache, which occurred in 27% of subjects in the GTS-21 group compared to 21% of subjects in the placebo group. There were no serious adverse events, or severe adverse events reported during these studies. In one patient in the GTS-21 group transient mild elevation of liver enzymes was detected, without signs of hepatic dysfunction.