There are two isozymes of acyl-CoA:cholesterol acyltransferase (ACAT), an ER membrane enzyme responsible for synthesizing cholesteryl esters; ACAT1 is ubiquitously expressed in most tissues and cells including macrophages, while ACAT2 is exclusively expressed in intestine and liver. The isozymes play a different role, and have been an attractive target for prevention and treatment of hypercholesterolaemia and atherosclerosis, but no inhibitors have been clinically developed. Recently, further development of avasimibe and pactimibe, which inhibit both ACAT1 and ACAT2 isozymes, was failed.
     Pyripyropenes were originally isolated from the culture broth of fungal Aspergillus fumigatus FO-1289 as ACAT inhibitors in an enzyme assay using rat liver microsomes. Pyripyropenes are the most potent among natural ACAT inhibitors. We reported that pyripyropene A was the only ACAT2-selective inhibitor. In this study, we evaluated the selectivity of pyripyropenes and 200 semisynthetic derivatives toward the isozymes with a cell-based assay using ACAT1- or ACAT2-CHO cells.
     Among natural pyripyropenes tested, pyripyropene A was found to be the most potent and selective ACAT2 inhibitor. Among semisynthetic derivatives, PR-71 (7-O-isocaproyl derivative) and PR-109 (1,11-benzylidene derivative) showed 10 times more potent, but their selectivity ACAT2 inhibition was worse than pyripyropene A. Three O-acyl groups in pyripyropene A were responsible for ACAT inhibition. Fundamentally, the results of ACAT2 inhibition by the derivatives in the cell-based assay are consistent with those of ACAT inhibition in rat liver microsomes. Structure-activity relationship will be also presented.