Eplerenone is an antimineralocorticoid , or an antagonist of the mineralocorticoid receptor (MR).  Eplerenone is also known chemically as 9,11α-epoxy-7α-methoxycarbonyl-3-oxo-17α-pregn-4-ene-21,17-carbolactone and "was derived from spironolactone by the introduction of a 9α,11α-epoxy bridge and by substitution of the 17α-thoacetyl group of spironolactone with a carbomethoxy group".  The drug controls high blood pressure by blocking the binding of aldosterone to the mineralocorticoid receptor (MR) in epithelial tissues, such as the kidney.  Blocking the action of aldosterone decreases blood volume and lowers blood pressure.  It has 10- to 20-fold lower affinity for the MR relative to spironolactone ,  and is less potent in vivo as an antimineralocorticoid.  However, in contrast to spironolactone, eplerenone has little affinity for the androgen , progesterone , and glucocorticoid receptors .   It also has more consistently observed non-genomic antimineralocorticoid effects relative to spironolactone (see membrane mineralocorticoid receptor ).  Eplerenone differs from spironolactone in its extensive metabolism, with a short half-life and inactive metabolites.