American patients with heart failure and a maintained ejection fraction engaged in the TOPCAT trial (Treatment of Preserved Cardiac Function Heart Failure With an Aldosterone Antagonist Trial) showed clinical improvements from spironolactone treatment. Spironolactone medication has been shown to improve survival in patients with heart failure with a preserved ejection fraction, but there has been no systematic evaluation of the biochemical pathways affected by this treatment. To determine which proteins and pathways are affected by spironolactone medication in heart failure with preserved ejection fraction, researchers performed a 5,284 aptamer-based to 4,928 proteomic analysis on plasma samples from TOPCAT participants in the Americas (n=164 people with paired samples at baseline and 1 year). The average percentage change from baseline was determined for every protein. Further, they analyzed the proteins affected by spironolactone by doing a pathway analysis. About 7 proteins showed statistically significant alterations in response to spironolactone treatment. Notable upregulations included caspase recruitment domain-containing protein 18 (CARD18), polycystin 2 (PKD2), and pregnancy-specific glycoprotein 2 (PSG2), while downregulations included hepatic growth factor (HGF), phospholipid transfer protein (PLTP), insulin-like growth factor 2 receptor (IGF2R), and switch-associated protein 70 (SWP70). The protein most significantly elevated by spironolactone was CARD18, an inhibitor of caspase-1f (-0.5% with placebo versus +66.5% with spironolactone, P<0.0001). Apolin signaling, stellate cell activation, glycoprotein 6 signaling, atherosclerosis signaling, liver X receptor activation, and farnesoid X receptor activation were the most strongly correlated canonical pathways. Collagens were a common thread among the most significant routes, rising in patients given the placebo but falling in those given spironolactone. The TOPCAT trial’s proteomic analysis showed that spironolactone affected several proteins and pathways, including the caspase inhibitor CARD18 and other collagen-related pathways. Further research into the potential antiapoptotic effects of spironolactone in heart failure with maintained ejection fraction is warranted based on the results of the trials.