Clinical cardiology

Sometimes, a drug’s metabolite is more potent than its parent molecule, yet loses one of its most interesting pharmacological properties. Losartan is a fascinating example. Following administration, losartan undergoes hepatic metabolism, primarily through CYP2C9 and CYP3A4, generating its active metabolite EXP3174. This oxidation converts a hydroxymethyl group into a carboxylic acid, producing a molecule with substantially greater affinity for the angiotensin II type 1 receptor (AT₁R). Consequently, EXP3174 contributes significantly to losartan’s sustained antihypertensive activity by preventing angiotensin II-mediated Gq/11 signalling, phospholipase C activation and intracellular Ca²⁺ mobilisation. But here comes the interesting part. The parent molecule possesses a pharmacological activity that its more potent metabolite largely lacks: the ability to promote uric acid excretion. In the kidney, URAT1 (SLC22A12), located on the apical membrane of proximal tubular epithelial cells, normally reabsorbs filtered urate through exchange with intracellular organic anions. Losartan directly inhibits URAT1, reducing urate reabsorption and increasing its urinary excretion. This uricosuric effect lowers circulating urate concentrations. Interestingly, the structural modification that enhances EXP3174's affinity for AT₁R markedly diminishes its ability to inhibit URAT1. This demonstrates that losartan's uricosuric activity is mechanistically independent of AT₁R blockade and is primarily attributable to the parent drug. Although other angiotensin receptor blockers may interact with urate transporters experimentally, losartan stands out for its consistently demonstrated clinical uricosuric activity. One drug, two molecular targets, two distinct pharmacological effects. And a reminder that greater potency at one receptor does not necessarily mean broader biological activity.