The Journey of Losartan

Losartan is well absorbed from the GI tract; systemic bioavailability
is approximately 33% following oral administration due to extensive hepatic first-pass metabolism.
Peak plasma concentrations of losartan are reached within 1 hour, and peak concentrations of the
active metabolite E-3174 (EXP-3174) occur at 3-4 hours. Food slows absorption but does not reduce
overall bioavailability significantly. The oral formulation is available in 25 mg, 50 mg, and 100 mg
strengths. Losartan is a tetrazole-containing compound with pKa of approximately 5.0, existing
predominantly as the anionic form at physiological pH. First-pass extraction of approximately 65%
occurs in the liver, where much of the drug is immediately oxidized to E-3174 by CYP2C9 and
CYP3A4. The contribution of gut wall CYP3A4 to first-pass extraction adds to the overall presystemic
metabolism.

Both losartan and E-3174 are highly protein-bound (>98% and 99.8%
respectively), primarily to albumin. Volume of distribution of losartan is approximately 34 L and
of E-3174 approximately 12 L — both modest, suggesting limited extravascular distribution. Despite
high protein binding, free drug reaches the AT1 receptor on vascular smooth muscle and adrenal
cortex. Losartan does not cross the blood-brain barrier significantly at therapeutic concentrations,
though animal studies suggest limited CNS penetration. The drug does not distribute appreciably
into red blood cells. Fetal exposure via placental transfer is possible; losartan (like all
renin-angiotensin system blockers) is contraindicated in pregnancy due to teratogenicity
(oligohydramnios, renal tubular dysgenesis, neonatal hypotension).

Angiotensin II is the primary effector hormone of the renin-angiotensin-
aldosterone system (RAAS), generated from angiotensin I by angiotensin-converting enzyme (ACE) in the
lungs and locally in vessels and kidney. AT1 receptors (AGTR1), a Gq/G12-coupled GPCR on vascular
smooth muscle cells and adrenal glomerulosa cells, mediate the vasoconstrictor, aldosterone-stimulating,
sodium-retaining, and organ-damaging effects of angiotensin II. Losartan and its active metabolite
E-3174 are selective, competitive antagonists at AT1 receptors; E-3174 binds with insurmountable
antagonism (non-competitive kinetics) due to very slow receptor dissociation, providing sustained
blockade beyond its plasma half-life. AT2 receptors (vasodilatory, antiproliferative) are left
unblocked — in contrast to ACE inhibitors which prevent both AT1 and AT2 signaling. Losartan
also has a uricosuric effect (inhibits URAT1 uric acid transporter), uniquely reducing serum uric
acid among ARBs.

Losartan is converted by CYP2C9 (primary) and CYP3A4 (secondary)
to E-3174 via cytochrome-mediated oxidative activation at the butyl imidazole methyl group, forming
the carboxylic acid metabolite. Approximately 14% of an oral dose is converted to E-3174. E-3174
is pharmacologically active with 10-40-fold higher AT1 receptor affinity than losartan and
a longer half-life (6-9 hours vs. 2 hours for losartan). CYP2C9 polymorphisms significantly affect
the losartan-to-E-3174 conversion: CYP2C9 poor metabolizers (*2/*2 or *3/*3 genotypes) produce
substantially less E-3174, resulting in reduced antihypertensive efficacy. CYP2C9 inhibitors
(fluconazole, amiodarone, some NSAIDs) reduce E-3174 formation. Both compounds are further
metabolized to inactive glucuronide conjugates.

Losartan and E-3174 are excreted via biliary (approximately 60%)
and renal (approximately 35%) routes. Approximately 4% of the oral dose is recovered as unchanged
losartan in urine and 6% as E-3174. Elimination half-life of losartan is 2 hours and of E-3174 is
6-9 hours. The extended pharmacodynamic effect (24-hour blood pressure control) of once-daily
dosing is explained by E-3174's long duration of AT1 receptor occupancy — pharmacodynamic half-life
exceeds the pharmacokinetic half-life. In severe hepatic impairment, the losartan-to-E-3174
conversion is reduced and clearance of both is impaired, requiring dose reduction. Renal impairment
does not significantly alter losartan or E-3174 clearance; no dose adjustment is required
for renal impairment alone.

Losartan is approved for hypertension, diabetic nephropathy in type 2
diabetes with left ventricular hypertrophy, and post-myocardial infarction heart failure. The
RENAAL trial demonstrated reduced progression of diabetic nephropathy (proteinuria reduction, ESRD
delay) independent of blood pressure lowering. The LIFE trial showed superior reduction in stroke
versus atenolol in hypertensive patients with LVH. A major advantage over ACE inhibitors is the
absence of ACE inhibitor-associated cough (bradykinin accumulation), though angioedema still occurs
rarely with ARBs. The uricosuric effect of losartan (unique among ARBs) provides a mild uric acid-
lowering benefit relevant in hypertensive patients with gout. Combination with ACE inhibitors is
contraindicated (dual RAAS blockade increases harm without additional benefit).