Losartan

Losartan is a synthetic small molecule of the angiotensin II receptor blocker (ARB) class, with the formula C22H23ClN6O and a tetrazole ring that stands in for a carboxylic acid group. Developed by Merck and patented in the mid-1980s, it was approved in the United States in 1995 as the first ARB to reach clinical use. After absorption the liver converts part of each dose into an active metabolite known as EXP3174, which binds the AT1 receptor far more tightly and acts for longer, so much of the drug's effect comes from this metabolite rather than the parent compound.

Its core uses are lowering blood pressure and slowing kidney damage. In the LIFE trial, losartan-based treatment reduced cardiovascular events, and stroke in particular, more than a beta-blocker regimen in patients with high blood pressure and an enlarged left ventricle, with benefits that appeared to go beyond blood pressure reduction alone ^[1]. In the RENAAL study of people with type 2 diabetes and nephropathy, losartan lowered the risk of a doubling of serum creatinine and of progression to end-stage kidney disease, and it cut protein leakage into the urine ^[2]. Head-to-head work found it about as effective as valsartan at lowering blood pressure, while also modestly reducing serum uric acid ^[3]; in patients with diabetes, a diuretic combination lowered blood pressure somewhat more than losartan alone ^[4].

A practical advantage over ACE inhibitors is that losartan acts downstream of angiotensin-converting enzyme and does not raise bradykinin, so the persistent dry cough common with that older class is rare. It also has a mild uric-acid-lowering (uricosuric) action, unusual among blood pressure drugs ^[3].

Losartan is supplied as an oral tablet, either on its own or in a fixed combination with the diuretic hydrochlorothiazide. It is prescription-only in most countries and is contraindicated in pregnancy because of fetal harm. Between 2018 and 2019 several manufacturers recalled batches over nitrosamine impurities, a manufacturing issue rather than a property of the drug itself.

Losartan is prescription only and generally well tolerated. The most important warning is that it must not be used in pregnancy because it can harm the fetus. It can raise blood potassium, especially alongside potassium supplements, spironolactone, or ACE inhibitors. It can cause dizziness from low blood pressure, particularly at the first dose or in people who are dehydrated. Kidney function and potassium should be checked after starting or increasing the dose. Combining it with NSAIDs can reduce its effect and stress the kidneys.

Losartan combined with ACE inhibitors (such as enalapril or lisinopril) significantly increases serum potassium and carries risk of hyperkalemia and metabolic acidosis. A retrospective study in 31 pediatric kidney transplant recipients receiving losartan 50 mg daily concurrent with enalapril showed a mean increase in serum potassium from 4.0 to 5.7 mmol/L (P<0.001) and a decrease in pH from 7.35 to 7.23 (P<0.001) over 1 to 6 months of combined therapy; patients on tacrolimus immunosuppression developed hyperkalemia earlier than those on cyclosporine ^[6]. Dual renin-angiotensin-aldosterone system (RAAS) inhibition with both losartan and lisinopril requires close monitoring of serum potassium and renal function, particularly in patients with renal impairment, diabetes, or concurrent use of other potassium-sparing agents ^[7].

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Losartan holds a distinctive place in cardiovascular medicine as the first angiotensin II receptor blocker to reach the market. It emerged from a research collaboration between DuPont and Merck, growing out of early imidazole leads that were refined into a potent, orally active, non-peptide antagonist of the AT1 receptor. The United States Food and Drug Administration approved it in 1995 under the brand name Cozaar, opening a new therapeutic class that offered the benefits of renin-angiotensin blockade without the cough associated with ACE inhibitors.

Two landmark trials broadened its reputation; the RENAAL study reported in 2001 that losartan slowed the progression of kidney disease in people with type 2 diabetes, and the LIFE study in 2002 showed advantages over the beta-blocker atenolol in hypertensive patients with left ventricular enlargement. Now long off patent, losartan is a low-cost generic and appears on the World Health Organization's List of Essential Medicines.

Losartan is widely regarded as a reliable, well-tolerated cornerstone of blood pressure treatment, and it earned that standing by being the pioneer of its class. Because it acts downstream of the angiotensin-converting enzyme, it avoids the persistent dry cough that leads some people to abandon ACE inhibitors, which makes it a favored alternative.

Beyond lowering blood pressure it has evidence for protecting the kidneys in diabetes and for reducing cardiovascular risk in specific populations, and a real-world claims analysis of tens of thousands of patients confirmed favorable clinical outcomes among common renin-angiotensin agents. It also carries a mild uric-acid-lowering property that is uncommon in its class. Its effects are gentle and predictable rather than dramatic, and like all drugs acting on this system it must be avoided in pregnancy and used with attention to potassium and kidney function; within those bounds it remains a trusted, inexpensive choice.