Eplerenone

Selective mineralocorticoid receptor antagonist — blocks aldosterone's effects on the kidney and vasculature without spironolactone's androgen/progesterone receptor crosstalk. Two A-tier outcome trials in heart failure (EPHESUS post-MI HF, EMPHASIS-HF mild-mod HFrEF) anchor its real-world value. For men, the headline distinction vs spironolactone is "no gynecomastia, no libido suppression, no anti-androgen effect" — which also means no benefit for hormonal acne or female hirsutism. For Dylan: NOT-RELEVANT now (no HF, no HTN, no AAS use). The only exploratory hook is topical eplerenone for MPB if minoxidil underperforms — niche, weak evidence, WATCH-LIST.

The aldosterone story in plain English:

1. Adrenal glands release aldosterone in response to angiotensin II, hyperkalemia, or low renal perfusion.
2. Aldosterone binds the mineralocorticoid receptor (MR) in the kidney's distal nephron → Na+ and water reabsorption, K+ and H+ excretion. Result: BP up, K+ down.
3. MR is also expressed in the heart, blood vessels, and brain. Chronic MR activation in those tissues drives fibrosis (cardiac and vascular), sympathetic activation, endothelial dysfunction, and vascular stiffening. This is why MR blockers reduce mortality in heart failure beyond what their BP effect alone explains.

Eplerenone is a competitive antagonist at the MR. Block aldosterone's binding → renal Na+/water retention drops, K+ retention rises, vascular and cardiac fibrosis slows over months.

What makes eplerenone different from spironolactone (the older non-selective MRA):

• Receptor selectivity. Spironolactone binds MR, but it also binds the androgen receptor (AR) as an antagonist and the progesterone receptor (PR) as a partial agonist. In men this produces gynecomastia (5-10% at therapeutic doses, much higher at high dose), libido drop, and ejaculatory issues. In women: menstrual irregularities. Eplerenone has roughly 500x lower AR affinity and ~100x lower PR affinity than spironolactone — enough that gynecomastia rates fall to placebo levels in trials and libido is generally unaffected.
• Lower MR potency. Selectivity costs you affinity — eplerenone is ~50-75% as potent at MR as spironolactone, so doses run higher (25-100 mg/day vs spiro's 12.5-50 mg/day for similar effect).
• Shorter half-life (~4-6 hr). Spironolactone's active metabolites give it a much longer effective duration (~16+ hr); eplerenone is dosed once or twice daily depending on indication.
• CYP3A4-cleared (vs spiro's broader hepatic metabolism). This means eplerenone is sensitive to strong CYP3A4 inhibitors (ketoconazole, clarithromycin, ritonavir) — concomitant use is contraindicated because plasma levels can spike 5-10x.
• No anti-androgen effect. This is the double-edged differentiator. For male HF/HTN patients it's pure upside (no gynecomastia). For users seeking anti-androgen effects (acne, female hirsutism, transgender feminization, MPB by AR-blockade) it eliminates the major reason to pick spironolactone. Eplerenone simply doesn't do that work.

Topical eplerenone for MPB — the one place selectivity matters in reverse:

Topical spironolactone is sometimes used for male-pattern baldness because its AR-antagonism locally counters DHT at the follicle. Topical eplerenone has been proposed as an alternative when systemic anti-androgen exposure is undesirable — but the rationale is shaky: without AR blockade, the only mechanism left is local MR blockade in the dermal papilla, which has been hypothesized (some animal data on hair growth from MR blockade) but has minimal human evidence. So topical eplerenone is "if you wanted topical spiro but specifically don't want any anti-androgen effect" — which is a narrow niche.