PDE4A and Alzheimer disease: However, further research on PDE8 is still needed because the understanding of its safety is still limited due to the lack of PDE8 inhibitors,20 especially when compared to some subtype selective PDE4 inhibitors with low emetic potential, such as PDE4B inhibitor A33.50 But PDE8 inhibitors can be still considered as a new opportunity to be explored as valuable drug candidates for AD.