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Variants in the CES1 gene encoding carboxylesterase 1 may affect the metabolism of enalapril to the active metabolite enalaprilat. It was shown that the A allele of rs71647871 and the C allele of rs2244613 led to a decrease in plasma enalaprilat concentrations. This study aimed to estimate the effect of structural haplotypes of CES1 containing the pseudogene CES1P1, or a hybrid of the gene and the pseudogene CES1A2, on the pharmacokinetics of enalapril. We included 286 Caucasian patients with arterial hypertension treated with enalapril. Genotyping was performed using real-time PCR and long-range PCR. Peak and trough plasma enalaprilat concentrations were lower in carriers of CES1A2 . The studied haplotypes were in linkage disequilibrium with rs2244613: generally, the A allele was in the haplotype containing the CES1P1 , and the C allele was in the haplotype with the CES1A2 . Thus, carriers of CES1A2 have reduced CES1 activity against enalapril. Linkage disequilibrium of the haplotype containing the CES1P1 or CES1A2 with rs2244613 should be taken into account when genotyping the CES1 gene.

Pharmacokinetic and pharmacodynamic of IV enalapril at 0.50 mg/kg, PO placebo and PO enalapril at three different doses (0.50, 1.00 and 2.00 mg/kg) were analyzed in 7 healthy horses. Serum concentrations of enalapril and enalaprilat were determined for pharmacokinetic analysis. Angiotensin-converting enzyme (ACE) activity, serum ureic nitrogen (SUN), creatinine and electrolytes were measured, and blood pressure was monitored for pharmacodynamic analysis. The elimination half-lives of enalapril and enalaprilat were 0.67 and 2.76 h respectively after IV enalapril. Enalapril concentrations after PO administrations were below the limit of quantification (10 ng/ml) in all horses and enalaprilat concentrations were below the limit of quantification in 4 of the 7 horses. Maximum mean ACE inhibitions from baseline were 88.38, 3.24, 21.69, 26.11 and 30.19% for IV enalapril at 0.50 mg/kg, placebo and PO enalapril at 0.50, 1.00 and 2.00 mg/kg, respectively. Blood pressures, SUN, creatinine and electrolytes remained unchanged during the experiments., (Copyright © 2014. Published by Elsevier Ltd.)

The angiotensin converting enzyme-inhibitor enalapril is the prodrug of enalaprilat and used in the treatment pediatric hypertension and chronic heart failure. Pharmacokinetic data are lacking to provide adequate dosing and for pediatric pharmacotherapeutical trials it is imperative to minimize sample volume. Therefore an HPLC-tandem mass spectrometry (MS) method for the determination of enalapril and enalaprilat in 100 μL of human serum was developed and validated with benazepril as internal standard (IS). After solid-phase extraction, chromatography was performed on a Luna(®) RP-C(18) (2) column with methanol-water-formic acid (65:35:1, v/v/v) as mobile phase and a flow rate of 0.4 mL/min. The MS was set to positive-mode electrospray ionization and multiple reaction monitoring, analyzing the m/z transitions channels 377.3 → 234.2, 349.3 → 206.1 and 425.3 → 351.2 for enalapril, enalaprilat and IS. Calibration curves were linear in the range of 1.61-206 ng/mL (enalapril) and 1.84-236 ng/mL (enalaprilat) with coefficients of determination >0.99. Relative standard deviations of intra- and inter-run precisions were below 7% and relative errors were below 6 ± 7% for both analytes. Also stabilities were acceptable for both analytes. As an application example, concentrations of enalapril and enalaprilat in serum after oral administration of 20 mg enalapril maleat in a healthy volunteer were determined., (Copyright © 2011 John Wiley & Sons, Ltd.)

Unlabelled: Enalapril maleate, available on the market in a variety of different pharmaceutical formulations, is commonly used for the control of systemic arterial hypertension. Many therapeutical failures have been reported thus far in clinical practice with respect to switching between different pharmaceutical formulations of the same product during pharmacological therapy. In the present study, plasma concentrations of enalapril and enalaprilate were measured in hypertensive patients undergoing treatment with different pharmaceutical formulations., Materials and Methods: Pharmaceutical formulations studied included the reference brand product, a generic formulation, and a third drug product marketed as "similar"; plasma samples were obtained from 30 hypertensive volunteer patients. Drug was extracted from the plasma by solid phase extraction and determined by liquid chromatography-tandem mass spectrometry. The method was validated for the main analytical parameters., Results: The analytical method developed in this study, using liquid chromatography-tandem mass spectrometry, was confirmed as suitable for application in the determination of plasma concentrations in patients and subsequently revealed statistically significant differences in plasma concentrations between the 3 treatment groups., Conclusions: Such differences reinforce the hypothesis that the bioequivalence tests currently proposed by the regulatory authorities to promote interchangeability between pharmaceutical formulations may not in fact represent a definitive parameter for guaranteeing similar plasma concentrations.

A rapid, selective and sensitive high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method was developed to simultaneously determine enalapril and enalaprilat in human plasma. With benazepril as internal standard, sample pretreatment involved in a one-step protein precipitation (PPT) with methanol of 0.2 ml plasma. Analysis was performed on an Ultimate XB-C(18) column (50 mm x 2.1 mm, i.d., 3 microm) with mobile phase consisting of methanol-water-formic acid (62:38:0.2, v/v/v). The detection was performed on a triple quadrupole tandem mass spectrometer by multiple reaction-monitoring (MRM) mode via electrospray ionization (ESI) source. Each plasma sample was chromatographed within 2.5 min. The linear calibration curves for enalapril and enalaprilat were both obtained in the concentration range of 0.638-255 ng/ml (r(2) > or = 0.99) with the lower limit of quantification (LLOQ) of 0.638 ng/ml. The intra-day precision (R.S.D.) was below 7.2% and inter-day R.S.D. was less than 14%, while accuracy (relative error R.E.) was within +/-8.7 and +/-5.5%, determined from QC samples for enalapril and enalaprilat which corresponded to requirement of the guidance of FDA. The HPLC-MS/MS method herein described was fully validated and successfully applied to the pharmacokinetic study of enalapril maleate capsules in 20 healthy male volunteers after oral administration.

Background and Objectives: Most of the pharmacokinetic (PK) parameters for enalapril and enalaprilat were established following determination of the drug and its metabolite, using angiotensin converting enzyme (ACE) inhibition assays. In these methods, enalapril has to be hydrolysed to enalaprilat first and then assayed. The purpose of this study was to re-estimate the PK parameters of enalapril and enalaprilat in healthy volunteers using two specific enzyme immunoassays for enalapril and enalaprilat., Methods: The rate and extent of absorption of enalapril and enalaprilat from a 10-mg dose of two enalapril maleate commercial brands (Renetic and Enalapril) were estimated using a two-way-cross over design with 1-week washout period. Blood pressure was also measured at specified time intervals and correlated to enalaprilat plasma concentrations., Results: For enalapril, the AUC(o-->infinity) values (Mean+/-SD) were 450.0+/-199.5 and 479.6+/-215.6 ng h/mL, Cmax values were 313.5+/-139.6 and 310.1+/-186.6 ng/mL, Tmax values were 1.06+/-0.30 h and 1.13+/-0.22 h, and t1/2 ranged between 0.3 to 6.1 h (1.6+/-1.5) and 0.40 to 5.05 h (1.3+/-1.0), for the two brands. For enalaprilat, the AUC(o-->infinity) values were 266.9+/-122.7 and 255.9+/-121.8 ng h/ml, Cmax values were 54.8+/-29.5 and 57.2+/-29.0 ng/mL, Tmax values were 4.6+/-1.6 h and 4.3+/-1.45 h, and t1/2 ranged between 1.1 to 10.5 h (4.5+/-2.9) and 0.6 to 9.4 h (3.5+/-2.5) for the two brands., Conclusions: Cmax values for enalapril are about 10 times those published in the literature and the rate and extent of absorption of the two brands of enalapril and their deesterification to enalaprilat following the administration of either brand were bioequivalent. Secondly, enalaprilat concentrations at 12-24 h following a single oral dose of enalapril in healthy volunteers were lower than those reported in the literature. The values reported here correlated with the return of blood pressure to predose level. Thirdly, enzyme immunoassays for enalapril and enalaprilat are better than ACE inhibition assays and can be used in bioequivalence assessment of enalapril and enalaprilat and for therapeutic drug monitoring in a clinical laboratory setting.

The pharmacokinetics of enalapril (0.5 mg/kg i.v.) and the pharmacodynamics of enalapril (0.5 mg/kg PO) in 5 mares were investigated. After single i.v. dosing, concentrations of enalapril and enalaprilat, its active metabolite, were measured. Two weeks later, enalapril was administered by nasogastric tube. Potassium, creatinine, blood urea nitrogen (BUN), enalapril, and enalaprilat concentrations and angiotensin converting enzyme (ACE) activity were measured in serum. In addition, heart rate, blood pressure, digital venous blood gases, and lactate were measured. Two weeks later, enalapril was again administered by nasogastric tube. To mimic activation of the renin-angiotensin-aldosterone system, angiotensin I (0.5 microg/kg) was administered at fixed intervals, followed by blood-pressure and heart-rate measurement. The elimination half lives of enalapril and enalaprilat were 0.59 and 1.25 hours, respectively, after i.v. administration. After PO administration, enalapril and enalaprilat were not detectable in serum. There was a tendency (P = .0625) toward a decrease in ACE activity 45-120 minutes after enalapril administration, but ACE activity suppression was never > 16%. There was a tendency (P = .0625) toward a decrease in mean arterial pressure (MAP) 6-8 hours after enalapril administration. Serum concentrations of potassium, creatinine, and BUN and digital venous blood gases and lactate concentrations did not change. In response to angiotensin I, there was a tendency (P = .0625) toward a decrease in the MAP response 4-24 hours after enalapril administration. Single-dose enalapril at 0.5 mg/kg PO did not demonstrate significant availability, pharmacodynamic effect, or substantial suppression of ACE activity.

The pharmacokinetic and relative bioavailability studies of 20-mg enalapril tablets, the test product manufactured by Biolab, Thailand compared to the reference product (Merck Sharp & Dohme, USA) was conducted in 14 healthy Thai male volunteers following a single dose, two-period, crossover design. Each subject received 20-mg enalapril tablets of both formulations with a 1-week washout period. Plasma samples collected over a 24-h period after administration were analyzed by LC/MS/MS. Pharmacokinetic parameters were determined by using non-compartmental analysis. Regarding bioequivalence testing, the 90 per cent confidence intervals of Cmax and AUC(0-infinity) ratios (test/reference) of enalapril were 101.8-134.9 per cent and 105.9-121.4 per cent and those of enalaprilat were 104.2-122.3 per cent and 104.5-118.1 per cent. Based on the European bioequivalence guideline, the 90 per cent confidence intervals of Cmax and AUC(0-infinity) ratios of both parent and metabolite forms were within the acceptable ranges of 70-143 per cent and 80-125 per cent, respectively. It was concluded that the test formulation was bioequivalent to the reference formulation and both formulations can be used interchangeably in clinical practice.

Enalapril and enalaprilat concentrations were measured after enalapril maleate (0.05 to 0.3 mg/kg) was administered orally to 12 pediatric patients (age range, 10 days to 6 1/2 years) with congestive heart failure caused by congenital heart disease and compared with those obtained from seven normal adults (age range, 21 to 39 years). When normalized to the oral 1 mg/m2 dose of enalapril maleate, the mean +/- SD area under the serum concentration-time curve (AUC) of enalaprilat, a pharmacologically active angiotensin-converting enzyme inhibitor, did not differ significantly between the pediatric group aged > 20 days and adult group (83.1 +/- 47.0 versus 64.6 +/- 17.8 ng.hr/ml per 1 mg/m2). When normalized to the oral 0.1 mg/kg dose, the mean AUC was significantly (p < 0.05) smaller in this pediatric group than in the adult group (138.4 +/- 69.2 versus 245.7 +/- 61.8 ng.hr/ml per 0.1 mg/kg). The AUC observed in three younger (age < 20 days) subjects tended to be much greater compared with infants aged > 20 days. The mean AUC ratio of enalaprilat to enalapril was significantly (p < 0.05) lower in the older pediatric subgroup (2.0 +/- 1.0) than in the adult group (3.4 +/- 1.6), whereas the mean ratios were comparable between the two subdivided pediatric groups. The results suggest that the oral enalapril dose would be better determined on a body surface area rather than on a body weight basis in pediatric patients with congestive heart failure aged > 20 days. The oral dosage should be much reduced in infants with congestive heart failure aged < 20 days compared with those aged > 20 days.(ABSTRACT TRUNCATED AT 250 WORDS)

Twelve normal volunteers were given 10 mg enalapril maleate by single and 2 weeks multiple dose administration, and blood samples were collected for the determination of enalaprilat concentration and angiotensin converting enzyme (ACE) activity. The mean terminal elimination half-life following a single administration of 10 mg enalapril, was 27 hours. The inhibition of ACE activity paralleled enalaprilat concentrations following both single and multiple dosing and the time of maximum inhibition of ACE activity was associated on both occasions with maximum concentration of enalaprilat. Emax modelling of enalaprilat concentration and ACE activity gave comparable values of Emax for both methods of administration. An accumulation factor of 1.7 was calculated from the area under the concentration time curve (AUC) of enalaprilat within a dosing interval at steady-state and the total AUC following single administration of enalapril. There were no significant differences between males and females in the accumulation factor, half-life and AUCinf.

Single-pass and recirculating rat liver perfusion studies were conducted with [14C]enalapril and [3H]enalaprilat, a precursor-product pair, and the data were modeled according to a physiological model to compare the different biliary clearances for the solely formed metabolite, [14C]enalaprilat, with that of preformed [3H]enalaprilat. With single-pass perfusion, the apparent extraction ratio (or biliary clearance) of formed [14C]enalaprilat was 15-fold the extraction ratio of preformed [3H]enalaprilat, an observation attributed to the presence of a barrier for cellular entry of the metabolite. Upon recirculation of bolus doses of [14C]enalapril and [3H]enalaprilat, the biliary clearance, estimated conventionally as metabolite excretion rate/midtime metabolite concentration, for formed [14C]enalaprilat was again 10- to 15-fold higher than the biliary clearance for preformed [3H]enalaprilat, but this decayed with perfusion time and gradually approached values for preformed [3H]enalaprilat. The decreasing biliary clearance of formed enalaprilat with recirculation was explained by the dual contribution of the circulating and intrahepatic metabolite (formed from circulating drug) to excretion. Physiological modeling predicted (i) an influx barrier (from blood to cell) at the sinusoidal membrane as the rate-limiting process in the overall removal of enalaprilat, (ii) a 15-fold greater extraction ratio or biliary clearance for formed [14C]enalaprilat over [3H]enalaprilat during single-pass perfusion, and (iii) the time-dependent and declining behaviour of the biliary clearance for formed [14C]enalaprilat during recirculation of the medium. In the absence of a direct knowledge of eliminating organs in vivo, this variable pattern for excretory clearance of the formed metabolite within the organ is indicative of a metabolite formation organ.

Combined recirculation of the rat liver (L) and kidney (IPK) at 10 ml min-1 per organ (LK) was developed to examine the hepatorenal handling of the precursor-metabolite pair: [14C]-enalapril and [3H]enalaprilat. Loading doses followed by constant infusion of [14C]enalapril and preformed [3H]enalaprilat to the reservoirs of the IPK or the LK preparation was used to achieve steady state conditions. In both organs, enalapril was mostly metabolized to its dicarboxylic acid metabolite, enalaprilat, which was excreted unchanged. At steady state, the fractional excretion for [14C]enalapril (FE = 0.45 to 0.48) and preformed [3H]enalaprilat (FE[pmi] = 1.1) were constant and similar for both the IPK and LK. The additivity of clearance was demonstrated in the LK preparation, namely, the total clearance of enalapril was the sum of its hepatic and renal clearances. However, the apparent fractional excretion for formed [14C]enalaprilat, FE(mi) and the apparent urinary clearance were time-dependent and higher than the corresponding values for preformed [3H]enalaprilat in both the IPK and LK. The FE(mi) and urinary clearance values further differed between the IPK and LK. Biliary clearance of formed vs. preformed enalaprilat displayed the same discrepant trends as observed for FE(mi) vs. FE(pmi) for the LK. These observations on the time-dependent and variable excretory clearance (urinary or biliary) of the formed metabolite vs. the constant, and much reduced, excretory clearance of the preformed metabolite are due to dual contributions to formed metabolite excretion: the nascently formed, intracellular metabolite which immediately underwent excretion and the formed metabolite which reentered the circulation, behaved as a preformed species. When data for the IPK and LK preparations were modeled with a physiological model with parameters previously reported for the L and IPK, all data, including metabolite excretory clearances, were well predicted. Model simulations revealed that the apparent FE(mi) differed between the LK and IPK preparations when the liver was present as an additional metabolite formation organ; the apparent excretory (urinary or biliary) clearance of the formed metabolite was further modulated by the volume of distribution of the metabolite, which altered levels of the formed, circulating metabolite.

1. Eight healthy male volunteers received oral enalapril, 10 mg, in the presence and absence of pretreatment with captopril, 50 mg, twice daily for 5 days. 2. Enalaprilat pharmacokinetics were characterised after both doses of enalapril to investigate the effect of saturating ACE binding sites by pretreatment with captopril. 3. The pharmacokinetics of enalaprilat were best described by a one compartment model with zero order input incorporating saturable binding to plasma and tissue ACE. 4. Values of AUC (0.72 h) for enalaprilat were 419 +/- 97 and 450 +/- 87 ng ml-1 h in the presence and absence of captopril, respectively. The difference was not statistically significant nor were there any other differences in model parameters. 5. Induction of ACE by captopril resulting in an increase in the number of ACE binding sites, may have obscured any effect of captopril on the occupancy of ACE binding sites by enalapril.

A physiologically based kidney model was developed to describe the metabolism of enalapril and explain the observed discrepancies between generated and preformed enalaprilat (metabolite) elimination in the constant flow single-pass and recirculating isolated perfused rat kidney preparations (IPKs) as a result of the differing points of origin of the metabolite within the kidney, subsequent to the simultaneous delivery of 14C-enalapril and 3H-enalaprilat. The model incorporated clearances for diffusion/transport of drug and metabolite across the basolateral and luminal membranes of the renal cells, an intrinsic clearance for renal drug metabolism, in addition to physiological variables such as perfusate flow rate, glomerular filtration rate, and urine flow rate. Nonlinear curve fitting of single-pass and recirculating data was performed to estimate the rate-limiting step in the renal elimination of enalaprilat. Through fitting and simulation procedures, we were able to predict metabolic and excretory events for enalapril (renal extraction ratio approximately equal to 0.25-0.3; fractional excretion, FE, was less than unity) and the relatively constant pattern of urinary excretion of preformed enalaprilat (extraction ratio approximately equal to 0.07; FE approximately equal to 1). The extraction ratio of the intrarenally formed enalaprilat in single-pass IPK was about twofold that for the preformed metabolite, whereas the FEs of generated enalaprilat in recirculating IPKs were greater than 1, and tended to increase, then decrease with perfusion time. These observations were explained by the optimized parameters which indicated that efflux from cell to lumen was rate-controlling in the excretion of enalaprilat, and another small transport barrier also existed at the basolateral membrane; the lower extraction ratio of preformed enalaprilat was due to its poor transmembrane clearance at the basolateral membrane. The variable FEs for generated enalaprilat vs. the relatively constant FE for preformed metabolite in the recirculating IPK was explained by the changing contributions of both circulating and intrarenal metabolite to metabolite excretion.

Enalapril is an effective angiotensin-converting enzyme (ACE) inhibitor which produces salutary, beneficial haemodynamic effects in patients with congestive heart failure. Enalapril also produces beneficial neurohumoral changes, as well as improving abnormal coronary haemodynamics and myocardial energetics that are frequently encountered in these patients. Administration of enalapril once or twice daily appears to be quite effective during maintenance therapy in patients with chronic congestive heart failure.

An isolated recirculating or single pass red cell-perfused rat kidney preparation (IPK) was used to examine the differential handling of renal metabolites. In single pass experiments, enalapril was primarily metabolized to its polar, dicarboxylic acid metabolite, enalaprilat, and its fractional excretion (FE) was less than unity, suggesting net reabsorption. Its steady-state extraction ratio decreased from 0.3 to 0.2 at concentrations of 1.06 to 12.7 microM, due to a saturation of enzymes for esterolysis. Enalaprilat administered to the IPK was excreted into urine in a concentration-independent (0.41-35.3 microM) fashion, with FE values approximating unity, suggesting net filtration. Differences in handling were observed for enalaprilat, as a metabolite formed from enalapril and as an administered (preformed) species in the single pass IPK, when tracer concentrations of [14C]enalapril and [3H]enalaprilat were given simultaneously. A comparison made between steady-state extraction ratio Ess[mi] [generated metabolite]/glomerular filtration rate (GFR) and Ess[pmi] [preformed metabolite]/GFR, respectively, revealed a 2-fold difference. The finding suggests the presence of a barrier for entry of enalaprilat into the kidney. Or else, in absence of the barrier, the opposite would be observed, that is, Ess [pmi]/GFR greater than Ess [mi]/GFR because preformed enalaprilat, in contrast to generated enalaprilat, undergoes filtration and utilizes facilitative transport carriers at the basolateral membrane. In recirculating IPKs which received simultaneously a tracer bolus dose of [14C]enalapril and [3H]enalaprilat, the FE values for generated [14C]enalaprilat were high and variable, decreasing with perfusion time and exceeding those for preformed [3H]enalaprilat, which approached unity with perfusion time. The variable FE values for [14C]enalaprilat are due to time-dependent contributions of circulating enalaprilat (which behaves identically to preformed enalaprilat) and the intrarenally generated enalaprilat. Hence, with renal drug metabolism, the conventional method of estimating urinary clearance (or Fe[mi]) for the metabolite [(total) excretion rate/midpoint plasma FE[mi] metabolite concentration] results in a greater metabolite clearance than that predicted from the administration of preformed metabolite.