Man-Wai Lo

Pharmacokinetics of losartan, an angiotensin II receptor antagonist, and its active metabolite EXP3174 in humans.

The pharmacokinetics of the angiotensin II receptor antagonist losartan potassium and its active carboxylic acid metabolite EXP3174 were characterized in 18 healthy male subjects after administration of intravenous losartan, intravenous EXP3174, and oral losartan. In these subjects, the average plasma clearance of losartan was 610 ml/min, and the volume of distribution was 34 L. Renal clearance (70 ml/min) accounted for 12% of plasma clearance. Terminal half‐life was 2.1 hours. In contrast, the average plasma clearance of EXP3174 was 47 ml/min, and its volume of distribution was 10 L. Renal clearance was 26 ml/min, which accounted for 55% of plasma clearance; terminal half‐life was 6.3 hours. After oral administration of losartan, peak concentrations of losartan were reached in 1 hour. Peak concentrations of EXP3174 were reached in 312 hours. The area under the plasma concentration‐time curve of EXP3174 was about four times that of losartan. The oral bioavailability of losartan tablets was 33%. The low bioavailability was mainly attributable to first‐pass metabolism. After intravenous or oral administration of losartan the conversion of losartan to the metabolite EXP3174 was 14%.

Biochemical Effects of Losartan, a Nonpeptide Angiotensin II Receptor Antagonist, on the Renin-Angiotensin-Aldosterone System in Hypertensive Patients

We investigated the effects of angiotensin II (Ang II) type 1 receptor blockade with losartan on the renin-angiotensin-aldosterone system in hypertensive patients (supine diastolic blood pressure, 95 to 110 mm Hg). Qualifying patients (n = 51) were allocated to placebo, 25 or 100 mg losartan, or 20 mg enalapril. Blood pressure, plasma drug concentrations, and renin-angiotensin-aldosterone system mediators were measured on 4 inpatient days: end of placebo run-in, after first dose, and 2 and 6 weeks of treatment. Plasma drug concentrations were similar after the first and last doses of losartan. At 6 weeks, 100 mg losartan and 20 mg enalapril showed comparable antihypertensive activity. Four hours after dosing, compared with the run-in day, 100 mg losartan increased plasma renin activity 1.7-fold and Ang II 2.5-fold, whereas enalapril increased plasma renin activity 2.8-fold and decreased Ang II 77%. Both drugs decreased plasma aldosterone concentration. For losartan, plasma renin activity and Ang II increases were greater at 2 than at 6 weeks. Effects of losartan were dose related. After the last dose of losartan, plasma renin activity and Ang II changes were similar to placebo changes by 36 hours. These results indicate that long-term blockade of the feedback Ang II receptor in hypertensive patients produces modest increases of plasma renin activity and Ang II that do not appear to affect the antihypertensive response to the antagonist.

Phenotypic and genotypic investigations of a healthy volunteer deficient in the conversion of losartan to its active metabolite E‐3174

Losartan is a potent angiotensin II–receptor antagonist with an active carboxylic acid metabolite, E-3174. This metabolite is a more potent angiotensin II antagonist than losartan and has a longer half-life. 1 In clinical pharmacokinetic studies, the extent of conversion of losartan to E-3174 after oral and intravenous administration was found to be 14.3% ± 3.9%. 1 However, minimal conversion of losartan to the active metabolite was observed in 4 subjects (<1% of individuals studied), suggesting a rare deficit in drug metabolism because more than 500 subjects in pharmacokinetic studies conducted (up to 1993) demonstrated adequate levels of E-3174 after the administration of losartan. 2,3

Simultaneous determination of losartan and EXP3174 in human plasma and urine utilizing liquid chromatography/tandem mass spectrometry

Losartan is an orally active angiotensin II receptor antagonist indicated for the treatment of hypertension. EXP3174 is an active metabolite, which contributes to the overall activity of losartan. Analytical methods for the simultaneous determination of losartan and its active metabolite EXP3174 in human plasma and urine with limited plasma sample size have been developed and validated to support a pediatric clinical program. In both methods, analytes are extracted from the matrixes by liquid-liquid extraction and separated using reverse phase high-performance liquid chromatography (HPLC). A tandem mass spectrometer (MS/MS) with a Turbo ionspray (TIS) interface in multiple-reaction-monitoring (MRM) mode is used for detection of the analytes in both methods. The plasma method has a lower limit of quantitation (LOQ) of 1 ng/ml with a linearity range of 1-500 ng/ml for losartan and EXP3174 using 100 microl of plasma. For the urine method, the LOQ for both losartan and EXP3174 is 2 ng/ml using 0.5 ml of urine, and the linearity range for both analytes is 2-1000 ng/ml. Validation procedures have proven that both methods are robust, accurate, and reproducible. Both methods have been used to assay clinical samples and provided satisfactory results.

Absence of a Pharmacokinetic Interaction between Losartan and Hydrochlorothiazide

To support the use of a combination of losartan, a highly specific and selective AT1 angiotensin II receptor antagonist, and hydrochlorothiazide for treatment of hypertension, a pharmacokinetic drug interaction study was conducted. In this open‐label, randomized, three‐period, crossover study, patients with mild to moderate hypertension received a 12.5‐mg tablet of hydrochlorothiazide, a 50‐mg losartan tablet, or a combination tablet of 12.5 mg of hydrochlorothiazide and 50 mg of losartan for 7 days. Twelve patients (age range, 35–55 years; mean age, 44 years) were allocated to treatment. Drug interactions were evaluated by comparing the 24‐hour area under the concentration‐time curve (AUC24) for losartan and its active metabolite, E‐3174, when losartan (50 mg) was given alone or in combination with 12.5 mg hydrochlorothiazide. The urinary recovery over the 24‐hour period of hydrochlorothiazide was compared for hydrochlorothiazide (12.5 mg) given alone or in combination with 50 mg losartan. A clinically significant interaction was defined as a treatment difference of more than 35%. There was no evidence of a clinically significant effect of hydrochlorothiazide on the pharmacokinetics of losartan or E‐3174, as the geometric mean AUC24 ratio (90% confidence interval [CI]) was 1.02 (0.95, 1.09) for losartan and 1.02 (0.96, 1.09) for E‐3174. Based on urinary recovery over a 24‐hour period of hydrochlorothiazide, losartan did not affect the pharmacokinetics of hydrochlorothiazide, as the geometric mean ratio of urinary hydrochlorothiazide recovery (90% CI) was 0.898 (0.79, 1.20). There was a minor (17%) decrease in the AUC24 of hydrochlorothiazide after administration of the combination tablet. Coadministration of hydrochlorothiazide and losartan was well tolerated.

Ketoconazole does not effect the systemic conversion of losartan to E‐3174

Clinical Pharmacology & Therapeutics (1996) 59, 169–169; doi: 10.1038/sj.clpt.1996.177

Pharmacokinetics of Intravenous and Oral Losartan in Patients with Heart Failure

The pharmacokinetics of a selective AT1‐subtype, nonpeptide, orally active, angiotensin II receptor antagonist, losartan, were characterized in 11 patients with heart failure (New York Heart Association class II, n = 6; class III, n = 4; class IV, n = 1) after oral and intravenous administration. In these patients, average plasma clearance of losartan was 566 mL/min, volume of distribution at steady‐state was 34 L, and terminal plasma half‐life was 1.5 hours. Average bioavailability was 36%. No clinically significant accumulation of losartan or its active metabolite, EXP3174, occurred after multiple‐dose oral administration for 7 to 8 days. Terminal plasma half‐life of EXP3174 after oral administration of losartan was 7.6 hours. The pharmacokinetics of losartan in patients in this study appear to be similar to those in healthy subjects studied previously.

DuP 532, an angiotensin II receptor antagonist: first administration and comparison with losartan.

We investigated the tolerability and angiotensin II antagonist activity of oral DuP 532 in healthy male subjects. DuP 532 (1 to 200 mg) was well tolerated, with no effect on blood pressure or heart rate. Compared with losartan (100 mg), DuP 532 (200 mg) was a weak antagonist of pressor responses to intravenous angiotensin II. Maximum inhibition of diastolic pressor response was 86% (95% confidence interval [CI], 84%, 88%) approximately 4.6 hours after losartan and 48% (95% CI, 38%, 56%) 8.7 hours after DuP 532. Twenty‐four hours after dosing, inhibition by losartan and DuP 532 was similar (40% to 45%). DUP 532 is extensively bound in human plasma, with an in vitro free fraction of 0.06. Although DuP 532 and EXP3174 (losartans active metabolite) have similar AT1‐receptor potency, and plasma concentrations of DuP 532 were much greater than losartan/EXP3174, the level of antagonism was much less for DuP 532. These results indicate that multiple factors determine the in vivo potency of angiotensin II antagonists, including affinity for and distribution to the receptor as modulated by plasma binding.

Simultaneous determination of a novel thrombin inhibitor and its two metabolites in human plasma by liquid chromatography/tandem mass spectrometry.

I, 3-(2-phenethylamino)-6-methyl-1-(2-amino-6-methyl-5-methylene-c arboxamidomethylpyridinyl)-pyrazinone dihydrochloride monohydrate, is a potent, orally active thrombin inhibitor. The compound also has two metabolites which have been shown to have thrombin inhibitory activity: benzylic alcohol M3 metabolite (II) and hydroxymethylpyrazinone M7 metabolite (III). A liquid chromatography/tandem mass spectrometry (LC/MS/MS) method for the simultaneous determination of I and its two metabolites in human plasma has successfully been developed. The method consists of treating 0.5 ml plasma with 2 N NaOH and extracting I, II, III and internal standard (IV) with ethyl acetate:methyl-t-butyl ether (1:3, v/v). The analytes were then back-extracted into 2% formic acid. The analytes were chromatographed by high performance liquid chromatography (HPLC) and detected by MS/MS. Positive ionization was used on a heated nebulizer probe monitoring precursor --> product ion combinations in multiple reaction monitoring mode. The linear range is 0.5-1000 nM for I and III and 0.75-1000 nM for II. Recoveries were 88, 74, 78 and 102.1% for I, II and III and the internal standard, respectively in human plasma. Intraday variation using this method was < or = 7.9% for I, < or = 9.0%, for II and < or = 9.5% for III across the standard curve range. This method exhibits good linearity and reproducibility for the three analytes.

Simultaneous determination of a novel M3 muscarinic receptor antagonist and its active 5-OH metabolite in human plasma using liquid chromatography/tandem mass spectrometry

A sensitive, specific, and robust liquid chromatography (LC)/mass spectrometry (MS)/MS method has been developed and validated for a novel M(3) muscarinic receptor antagonist (I) and its active 5-OH metabolite (II) in human plasma. The assay involves a two-step liquid-liquid extraction of the compounds from human plasma, high performance liquid chromatography (HPLC) separation, and MS/MS for the detection of the analytes. The method provides a linear response from a quantitation limit of 0.05-20 ng/ml for I and 0.1-20 ng/ml for II using 1 ml of plasma. The mean absolute recovery was 85.4% for I and 80.8% for II, respectively. The intra-assay accuracy of I and II averaged from 95.0 to 105.3% with coefficient of variation (CV) values <or=6.5% over the standard curve range. The stability study showed that I and II are stable in the plasma matrix over a period of 11 months at -70 degrees C. The accuracy, ruggedness, and reproducibility of this method were demonstrated by analyzing over 5000 plasma samples in clinical pharmacokinetics studies over a 6-month period.