Losartan pharmacokinetics
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Losartan Pharmacokinetics: Absorption, Distribution, Metabolism, and Excretion
Absorption and Oral Bioavailability of Losartan
Losartan is rapidly absorbed after oral administration, with peak plasma concentrations typically reached within 1 hour. However, its oral bioavailability is relatively low, around 33%, mainly due to significant first-pass metabolism in the liver Lo1995Ohtawa1993. The absorption profile can show multiple peaks, which are influenced by gastric emptying rates. Changes in gastric emptying can alter the concentration-time profiles of both losartan and its active metabolite, EXP3174 .
Distribution and Volume of Distribution
After absorption, losartan has a moderate volume of distribution, averaging about 34 liters in healthy adults, indicating distribution into body tissues beyond the blood plasma . The apparent volume of distribution can be affected by co-administered substances, such as herbal medicines, which may decrease it .
Metabolism: Conversion to Active Metabolite EXP3174
Losartan undergoes extensive hepatic metabolism, primarily via cytochrome P450 enzymes (CYP2C9 and CYP3A4), to form its active metabolite, EXP3174. Only about 14% of an administered dose is converted to EXP3174 . The pharmacokinetics of losartan and EXP3174 are dose-proportional, and the metabolite generally reaches higher plasma concentrations and has a longer half-life than the parent drug .
Elimination: Clearance and Half-Life
Losartan is cleared from plasma at an average rate of 610 ml/min, with a terminal half-life of about 2.1 hours. Renal clearance accounts for a small portion (about 12%) of total plasma clearance, with most of the drug eliminated via hepatic metabolism . EXP3174, the active metabolite, has a much lower plasma clearance (47 ml/min) and a longer half-life (about 6.3 hours), with renal clearance accounting for a larger proportion of its elimination Lo1995Ohtawa1993.
Pharmacokinetics in Special Populations: Renal Insufficiency
In patients with varying degrees of renal impairment, the renal clearance of both losartan and EXP3174 decreases significantly. However, since only a small fraction of losartan is eliminated by the kidneys, the overall exposure (area under the curve, AUC) to losartan and EXP3174 does not change significantly. Therefore, dose adjustments are generally not necessary in patients with renal insufficiency .
Genetic and Drug Interactions Affecting Losartan Pharmacokinetics
Genetic Polymorphisms
CYP2C9 genetic variants (*2 or *3 alleles) can significantly affect losartan metabolism. Carriers of these variants have higher losartan exposure (AUC), lower EXP3174 exposure, and longer half-lives for both compounds compared to individuals with the wild-type genotype . Variants in the ABCB1 gene, which encodes the P-glycoprotein drug transporter, can also increase early-phase absorption of losartan, though they do not affect total absorption .
Drug and Herbal Interactions
Co-administration with certain herbal medicines can alter losartan pharmacokinetics. For example, Xuesaitong and salvianolic acid B (from Danshen) can enhance losartan metabolism and elimination, reducing its half-life and plasma concentration, while Ginkgo leaf tablets and tanshinone IIA can inhibit metabolism, increasing losartan exposure and prolonging its half-life Weina2019Wang2016Dong2018. These interactions are mainly due to modulation of CYP3A4 and CYP2C9 enzyme activity.
Dose Considerations
Clinical evidence suggests that a 50 mg dose of losartan may be suboptimal for some patients, and higher doses (such as 100 mg/day) may be more effective for achieving therapeutic goals .
Conclusion
Losartan is characterized by rapid absorption, moderate distribution, extensive hepatic metabolism to an active metabolite, and elimination primarily via the liver. Its pharmacokinetics can be influenced by genetic factors, renal function, and interactions with other drugs or herbal supplements. Understanding these factors is important for optimizing losartan therapy and ensuring effective blood pressure control.
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