The disposition of [14C]-labelled benazepril HCl in normal adult volunteers after single and repeated oral dose

Simultaneously Predicting the Pharmacokinetics of CES1-Metabolized Drugs and Their Metabolites Using Physiologically Based Pharmacokinetic Model in Cirrhosis Subjects

Hepatic carboxylesterase 1 (CES1) metabolizes numerous prodrugs into active ingredients or direct-acting drugs into inactive metabolites. We aimed to develop a semi-physiologically based pharmacokinetic (semi-PBPK) model to simultaneously predict the pharmacokinetics of CES1 substrates and their active metabolites in liver cirrhosis (LC) patients. Six prodrugs (enalapril, benazepril, cilazapril, temocapril, perindopril and oseltamivir) and three direct-acting drugs (flumazenil, pethidine and remimazolam) were selected. Parameters such as organ blood flows, plasma-binding protein concentrations, functional liver volume, hepatic enzymatic activity, glomerular filtration rate (GFR) and gastrointestinal transit rate were integrated into the simulation. The pharmacokinetic profiles of these drugs and their active metabolites were simulated for 1000 virtual individuals. The developed semi-PBPK model, after validation in healthy individuals, was extrapolated to LC patients. Most of the observations fell within the 5th and 95th percentiles of simulations from 1000 virtual patients. The estimated AUC and Cmax were within 0.5-2-fold of the observed values. The sensitivity analysis showed that the decreased plasma exposure of active metabolites due to the decreased CES1 was partly attenuated by the decreased GFR. Conclusion: The developed PBPK model successfully predicted the pharmacokinetics of CES1 substrates and their metabolites in healthy individuals and LC patients, facilitating tailored dosing of CES1 substrates in LC patients.

Antihypertensive drugs metabolism: an update to pharmacokinetic profiles and computational approaches

Drug discovery and development is a high-risk enterprise that requires significant investments in capital, time and scientific expertise. The studies of xenobiotic metabolism remain as one of the main topics in the research and development of drugs, cosmetics and nutritional supplements.

Antihypertensive drugs are used for the treatment of high blood pressure, which is one the most frequent symptoms of the patients that undergo cardiovascular diseases such as myocardial infraction and strokes. In current cardiovascular disease pharmacology, four drug clusters - Angiotensin Converting Enzyme Inhibitors, Beta-Blockers, Calcium Channel Blockers and Diuretics - cover the major therapeutic characteristics of the most antihypertensive drugs. The pharmacokinetic and specifically the metabolic profile of the antihypertensive agents are intensively studied because of the broad inter-individual variability on plasma concentrations and the diversity on the efficacy response especially due to the P450 dependent metabolic status they present.

Several computational methods have been developed with the aim to: (i) model and better understand the human drug metabolism; and (ii) enhance the experimental investigation of the metabolism of small xenobiotic molecules. The main predictive tools these methods employ are rule-based approaches, quantitative structure metabolism/activity relationships and docking approaches.

This review paper provides detailed metabolic profiles of the major clusters of antihypertensive agents, including their metabolites and their metabolizing enzymes, and it also provides specific information concerning the computational approaches that have been used to predict the metabolic profile of several antihypertensive drugs.

Pharmacological and Clinical Profile of Moexipril: A Concise Review

Angiotensin-converting enzyme (ACE) inhibitors are effective and safe antihypertensive drugs, with the exception of the rare occasion of angioedema. These drugs have demonstrated additional cardiovascular protective effects to their blood pressure lowering, and their combination with the diuretic hydrochlorothiazide potentiates their antihypertensive effectiveness. Moexipril is a long-acting ACE inhibitor suitable for once-daily administration, and like some ACE inhibitors, moexipril is a prodrug and needs to be hydrolyzed in the liver into its active carboxylic metabolite, moexiprilat, to become effective. Moexipril alone and in combination with low-dose hydrochlorothiazide has been shown in clinical trials to be effective in lowering blood pressure and be well tolerated and safe given in single daily doses. In this review, the pharmacological profile of this drug and its clinical usefulness are discussed.

Structural alert/reactive metabolite concept as applied in medicinal chemistry to mitigate the risk of idiosyncratic drug toxicity: a perspective based on the critical examination of trends in the top 200 drugs marketed in the United States

Because of a preconceived notion that eliminating reactive metabolite (RM) formation with new drug candidates could mitigate the risk of idiosyncratic drug toxicity, the potential for RM formation is routinely examined as part of lead optimization efforts in drug discovery. Likewise, avoidance of "structural alerts" is almost a norm in drug design. However, there is a growing concern that the perceived safety hazards associated with structural alerts and/or RM screening tools as standalone predictors of toxicity risks may be over exaggerated. In addition, the multifactorial nature of idiosyncratic toxicity is now well recognized based upon observations that mechanisms other than RM formation (e.g., mitochondrial toxicity and inhibition of bile salt export pump (BSEP)) also can account for certain target organ toxicities. Hence, fundamental questions arise such as: When is a molecule that contains a structural alert (RM positive or negative) a cause for concern? Could the molecule in its parent form exert toxicity? Can a low dose drug candidate truly mitigate metabolism-dependent and -independent idiosyncratic toxicity risks? In an effort to address these questions, we have retrospectively examined 68 drugs (recalled or associated with a black box warning due to idiosyncratic toxicity) and the top 200 drugs (prescription and sales) in the United States in 2009 for trends in physiochemical characteristics, daily doses, presence of structural alerts, evidence for RM formation as well as toxicity mechanism(s) potentially mediated by parent drugs. Collectively, our analysis revealed that a significant proportion (∼78-86%) of drugs associated with toxicity contained structural alerts and evidence indicating that RM formation as a causative factor for toxicity has been presented in 62-69% of these molecules. In several cases, mitochondrial toxicity and BSEP inhibition mediated by parent drugs were also noted as potential causative factors. Most drugs were administered at daily doses exceeding several hundred milligrams. There was no obvious link between idiosyncratic toxicity and physicochemical properties such as molecular weight, lipophilicity, etc. Approximately half of the top 200 drugs for 2009 (prescription and sales) also contained one or more alerts in their chemical architecture, and many were found to be RM-positive. Several instances of BSEP and mitochondrial liabilities were also noted with agents in the top 200 category. However, with relatively few exceptions, the vast majority of these drugs are rarely associated with idiosyncratic toxicity, despite years of patient use. The major differentiating factor appeared to be the daily dose; most of the drugs in the top 200 list are administered at low daily doses. In addition, competing detoxication pathways and/or alternate nonmetabolic clearance routes provided suitable justifications for the safety records of RM-positive drugs in the top 200 category. Thus, while RM elimination may be a useful and pragmatic starting point in mitigating idiosyncratic toxicity risks, our analysis suggests a need for a more integrated screening paradigm for chemical hazard identification in drug discovery. Thus, in addition to a detailed assessment of RM formation potential (in relationship to the overall elimination mechanisms of the compound(s)) for lead compounds, effects on cellular health (e.g., cytotoxicity assays), BSEP inhibition, and mitochondrial toxicity are the recommended suite of assays to characterize compound liabilities. However, the prospective use of such data in compound selection will require further validation of the cellular assays using marketed agents. Until we gain a better understanding of the pathophysiological mechanisms associated with idiosyncratic toxicities, improving pharmacokinetics and intrinsic potency as means of decreasing the dose size and the associated "body burden" of the parent drug and its metabolites will remain an overarching goal in drug discovery.

Antihypertensive and organ-protective effects of benazepril

Benazepril is a nonsulfhydryl ACE inhibitor with favorable pharmacodynamic and pharmacokinetic properties, well-established antihypertensive effects and a good tolerability profile. Recent clinical studies have demonstrated that patients treated with benazepril alone or in combination with hydrochlorothiazide or amlodipine may achieve beneficial renal outcomes that extend beyond blood pressure control. Furthermore, the recent Avoiding Cardiovascular Events Through Combination Therapy in Patients Living with Systolic Hypertension (ACCOMPLISH) trial showed decreased cardiovascular morbidity and mortality with benazepril when administered as a cotreatment. An additional novel therapeutic area for benazepril is atrial fibrillation. Differences between combination therapies have implications for which patients may be best suited to particular interventions, and further studies are required to fully ascertain this potential.

Automation and Miniaturization of the Bioluminescent UGT-Glo Assay for Screening of UDP-Glucuronosyltransferase Inhibition by Various Compounds

The uridine 5'-diphospho-glucuronosyltransferase (UGT) family of enzymes is involved in the metabolism of various compounds. These enzymes transfer a hydrophilic glucuronic acid moiety to their substrates, rendering them more water soluble and amenable to excretion. The UGTs act on various endogenous substrates, such as bilirubin, 17β-estradiol, and testosterone, and drugs and other xenobiotics. The function of these enzymes is essential for the clearance of drugs and toxicants, and alteration of UGT activity is a potential cause of adverse drug—drug interactions in vivo. This has stimulated an increased interest in the study of UGT function and inhibition, and the desire to profile new drug entities against UGT enzymes, similar to CYP450 profiling. However, certain factors have hindered the development of a robust method for UGT profiling. Current methods for assessing UGT enzyme activity are laborious and involve protein precipitation and/or chromatographic separation steps, which are not amenable to rapid screening applications for UGT inhibitors or substrates. The approach presented here is a bioluminescent assay for measuring UGT enzyme activity and inhibition in vitro. Using flexible, robust instrumentation in a 384-well microplate format, this study highlights the quick and easy assay implementation for estimation of inhibition kinetics with a variety of known and suspected UGT substrates and inhibitors.

What is the Objective of the Mass Balance Study? A Retrospective Analysis of Data in Animal and Human Excretion Studies Employing Radiolabeled Drugs

Mass balance excretion studies in laboratory animals and humans using radiolabeled compounds represent a standard part of the development process for new drugs. From these studies, the total fate of drug-related material is obtained: mass balance, routes of excretion, and, with additional analyses, metabolic pathways. However, rarely does the mass balance in radiolabeled excretion studies truly achieve 100% recovery. Many definitions of cutoff criteria for mass balance that identify acceptable versus unacceptable recovery have been presented as ad hoc statements without a strong rationale. To address this, a retrospective analysis was undertaken to explore the overall performance of mass balance studies in both laboratory animal species and humans using data for 27 proprietary compounds within Pfizer and extensive review of published studies. The review has examined variation in recovery and the question of whether low recovery was a cause for concern in terms of drug safety. Overall, mean recovery was greater in rats and dogs than in humans. When the circulating half-life of total radioactivity is greater than 50 h, the recovery tends to be lower. Excretion data from the literature were queried as to whether drugs linked with toxicities associated with sequestration in tissues or covalent binding exhibit low mass balance. This was not the case, unless the sequestration led to a long elimination half-life of drug-related material. In the vast majority of cases, sequestration or concentration of drug-related material in an organ or tissue was without deleterious effect and, in some cases, was related to the pharmacological mechanism of action. Overall, from these data, recovery of radiolabel would normally be equal to or greater than 90%, 85%, and 80% in rat, dog, and human, respectively. Since several technical limitations can underlie a lack of mass balance and since mass balance data are not sensitive indicators of the potential for toxicity arising via tissue sequestration, absolute recovery in humans should not be used as a major decision criteria as to whether a radiolabeled study has met its objectives. Instead, the study should be seen as an integral part of drug development answering four principal questions: 1) Is the proposed clearance mechanism sufficiently supported by the identities of the drug-related materials in excreta, so as to provide a complete understanding of clearance and potential contributors to interpatient variability and drug-drug interactions? 2) What are the drug-related entities present in circulation that are the active principals contributing to primary and secondary pharmacology? 3) Are there findings (low extraction recovery of radiolabel from plasma, metabolite structures indicative of chemically reactive intermediates) that suggest potential safety issues requiring further risk assessment? 4) Do questions 2 and 3 have appropriate preclinical support in terms of pharmacology, safety pharmacology, and toxicology? Only if one or more of these four questions remain unanswered should additional mass balance studies be considered.

Simultaneous and rapid quantitation of benazepril and benazeprilat in human plasma by high performance liquid chromatography with ultraviolet detection

A sensitive and accurate high-performance liquid chromatography (HPLC) method with ultraviolet (UV) detector was developed and validated for simultaneous determination of benazepril (BZL) and its active metabolite, benazeprilat (BZT), in human plasma. The plasma sample, after spiked with riluzole as an internal standard (IS), was subjected to a solid-phase extraction (SPE) prior to a HPLC analysis. Chromatographic separations were achieved on a Hypersil BDS C(18) (300 mm x 4.6mm, 5 microm). The mobile phase consisted of phosphate buffer (pH 2.6; 10mM) and acetonitrile mixture in a gradient mode. Detection was carried out at a wavelength of 237 nm. The retention times of BZL, BZT and IS were at about 6.2, 15.4 and 16.2 min, respectively. The calibration curve was linear in the range of 20-2000 ng/mL for both BZL and BZT (r(2)>0.997). At three quality control concentrations of 100, 500, and 1500 ng/mL, the intra-day and inter-day relative standard deviation ranged from 2.8 to 8.6% for BZL and from 2.2 to 8.5% for BZT, while the mean absolute percentage error ranged from -7.5 to 6.7% for BZL and from -6.0 to 3.2% for BZT. The limit of detection (LOD) was 10 ng/mL and the limit of quantification (LOQ) was 20 ng/mL for both BZL and BZT in human plasma. The method was successfully applied to bioequivalence evaluation of benazepril hydrochloride formulations in healthy Chinese.

Prodrug research: futile or fertile?

The objective of this Commentary is to help clarify and illustrate what prodrugs are, what they are not, which benefits they can offer, and what their limits are. To this end, a number of criteria of classification and evaluation are presented. This is followed by a discussion of the pharmaceutical, pharmacokinetic and pharmacodynamic objectives of prodrug research. Recent examples (e.g. oseltamivir, bambuterol, capecitabine, clopidogrel and tirapazamine) are discussed in a biochemical perspective to illustrate these objectives and to demonstrate some of the therapeutic benefits afforded by successful prodrugs. Attention is also called to the fact that the in vitro and in vivo behavior of prodrug candidates may differ from that of the parent drug in ways that go beyond the original pharmaceutical, pharmacokinetic or pharmacodynamic objective being pursued. We conclude that prodrugs offer a viable strategy to disentangle pharmacodynamic and pharmacokinetic optimization.

Mass spectral fragmentation reactions of angiotensin-converting enzyme (ACE) inhibitors

A variety of mass spectrometric techniques have been employed in the study of a series of structurally similar compounds used in the treatment of hypertension. The compounds, known collectively as angiotensin-converting enzyme (ACE) inhibitors, all share the amino acid residue proline or some variant thereof, as a common structural element. The gas phase fragmentation behavior of these compounds has been explored systematically using various instruments and techniques. An interesting dissociation process (rearrangement) unique to one of the compounds, lisinopril, has been investigated using isotopic labeling experiments and exact mass measurements. The general nature of the process has been probed through both the positive and negative ion analyses of fourteen related compounds exhibiting structural homology.

Pharmacological profile and clinical use of moexipril

Angiotensin-converting enzyme (ACE) inhibitors are effective and safe antihypertensive drugs with the exception of the rare occurrence of angioedema. These drugs have demonstrated additional cardiovascular protective effects to their blood pressure lowering and their combination with the diuretic hydrochlorothiazide potentiates their antihypertensive effectiveness. Moexipril (Univasc, Bayer) is a long-acting ACE inhibitor suitable for once-daily administration and, like enalapril, is a prodrug and needs to be hydrolyzed in the liver into its active carboxylic metabolite, moexiprilat, in order to become effective. Moexipril alone and in combination with low-dose hydrochlorothiazide has been shown in clinical trials to be effective in lowering blood pressure and to be well-tolerated and safe given in single daily doses. In this review, the pharmacological profile of this drug and its clinical usefulness will be discussed.

Evaluation of human intestinal absorption data and subsequent derivation of a quantitative structure–activity relationship (QSAR) with the Abraham descriptors

The human intestinal absorption of 241 drugs was evaluated. Three main methods were used to determine the human intestinal absorption: bioavailability, percentage of urinary excretion of drug-related material following oral administration, and the ratio of cumulative urinary excretion of drug-related material following oral and intravenous administration. The general solvation equation developed by Abraham's group was used to model the human intestinal absorption data of 169 drugs we considered to have reliable data. The model contains five Abraham descriptors calculated by the ABSOLV program. The results show that Abraham descriptors can successfully predict human intestinal absorption if the human absorption data is carefully classified based on solubility and administration dose to humans.

Pharmacokinetics and metabolism of formestane in breast cancer patients

Formestane (Lentaron(R), 4-hydroxyandrostenedione) is a steroidal aromatase inhibitor used for treatment of advanced breast cancer. Clinically, it is administered as a depot form once fortnightly by intramuscular (i.m.) injection. To investigate the pharmacokinetics, bioavailability and metabolism of the drug, seven patients received single 250 mg i.m. doses of commercial formestane on Days 0, 21, 35, 49 and 63 of this trial. On Day 63, three of the patients received an additional single intravenous (i.v.) pulse dose of 1 mg of 14C-labelled formestane. The plasma kinetics after i.m. dosing confirmed a sustained release of formestane from the site of injection. Within 24-48 h of the first dose, the circulating drug reached a C(max) of 48.0+/-20.9 nmol/l (mean+/-S.D.; N=7). At the end of the dosing interval, after 14 days, the plasma concentration was still at 2.3+/-1.8 nmol/l. The kinetic variables did not significantly change during prolonged treatment. Intramuscular doses appear to be fully bioavailable. Following i.v. injection of 14C-formestane, the unchanged drug disappeared rapidly from plasma, the terminal elimination half-life being 18+/-2 min (N=3). Plasma clearance, CL was 4.2+/-1.3 l/(h kg) and the terminal distribution volume V(z) was 1.8+/-0.5 l/kg. The drug is mainly eliminated by metabolism, renal excretion of metabolites accounting for 95% of dose. The excretory balance of 14C-compounds in urine and faeces totals up to 98.9+/-0.8% of the i.v. dose after 168 h. The 14C-compounds in plasma and urine were separated by HPLC, and three major metabolites were submitted to structural analysis by MS, NMR and UV spectroscopy. One of the metabolites is the direct 4-O-glucuronide of formestane. The other two represent 3-O-sulfates of the exocons 3beta,4beta-dihydroxy-5alpha-androstane-17-one and 3alpha,4beta-dihydroxy-5alpha-androstane-17-one, their ratio being 7:3. These exocons are formed by stereoselective 3-keto reduction, accompanied by reduction of the 4,5-enol function. The exocons do not inhibit human placental aromatase activity in vitro.

A validated method for the determination and purity evaluation of benazepril hydrochloride in bulk and in pharmaceutical dosage forms by liquid chromatography

A gradient liquid chromatographic (LC) method has been developed for the determination and purity evaluation of benazepril hydrochloride in bulk and pharmaceutical dosage forms. The method is simple, rapid and selective. 5-Methyl-2-nitro phenol has been used as internal standard. The method is linear in the range of 50-800 microg. The precision for inter and intra-day assay variation of benazepril hydrochloride is below 1.6% RSD. The accuracy determined as relative mean error (RME) for the intra-day assay is within +/- 2.0%. The method is stability indicating, and is useful in the quality control of bulk manufacturing and also in pharmaceutical formulations.

Pharmacokinetics, disposition and biotransformation of [14C]-radiolabelled valsartan in healthy male volunteers after a single oral dose

1. The disposition of valsartan, a potent angiotensin II receptor antagonist, was investigated in six healthy male volunteers. They each received a single oral dose of 80 mg of a 14C-labelled preparation as a neutral buffered solution. 2. Peak concentrations of radioactivity and valsartan in plasma measured 1 h after dosing showed rapid onset of absorption. The results of this study combined with other available data indicate that at least 51% of the dose was absorbed. 3. Valsartan was the predominant radioactive compound in plasma. Elimination of valsartan and radioactivity was fast and multiexponential. beta-Half-lives of 6 +/- 1 h were observed. In a terminal elimination phase, low radioactivity levels decreased with a half-life of 81 +/- 33 h. A minor, pharmacologically inactive metabolite (valeryl-4-hydroxy-valsartan; M1) was detected in the plasma at time points later than 2 h after dosing, representing approximately 11% of the AUC(24 h) of plasma radioactivity. 4. The bulk of the dose was excreted within 4 days. The total excretion within 7 days amounted to 99 +/- 1% of dose. Faecal excretion was predominant (86 +/- 5% of dose). Valsartan was largely excreted unchanged (81 +/- 5% of the dose in the excreta). The predominant clearance mechanism appeared to be direct elimination via bile. 5. An inactive metabolite, M1, was formed by oxidative biotransformation and accounted for 9 +/- 3% of the dose in the excreta.

Absorption of ACE inhibitors from small intestine and colon

The intestinal absorption of two ACE inhibitors was studied to determine the potential for colonic delivery of small peptides. In addition, studies were also performed to assess intestinal tissue uptake and evaluate a canine intestinal-access-port model as techniques for screening absorption. To evaluate the impact of differences in the contributions of passive permeation and carrier-mediated peptide transport on in vitro uptake and in vivo absorption, an esterified prodrug, benazepril, and a free diacid non-prodrug, CGS 16617, were selected for study. Potential colonic absorption enhancement utilizing coadministration of Intralipid was also investigated. Studies in rat everted intestinal rings verified that jejunal benazepril uptake included a carrier-mediated component while that of the diacid did not. Uptake of both drugs was purely passive in colonic rings. Equilibrium uptake and uptake rate of the more lipophilic prodrug was 2-fold greater than the diacid. Benazepril and CGS 16617 jejunal uptake rate at 0.01 mM was 3.5 and 2.5 times higher, respectively, than from colonic rings. Following jejunal administration in dogs, maximum benazepril plasma levels (Cmax) and area under the plasma level versus time curve (AUC) were 5.5 and 3.0 times higher, respectively, than following colonic administration. Maximum benazepril plasma levels following colonic administration in dogs was 2-fold greater than for CGS 16617, consistent with in vitro results. Colonic coadministration of the poorly-absorbed CGS 16617 with 2 mL of Intralipid (within dietary range for fecal fat content) enhanced Cmax and AUC 2.5- and 3.5-fold, respectively, in the dog and AUC 1.5-fold in the rat.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics and pharmacodynamics of benazepril hydrochloride in patients with major proteinuria

We have investigated whether the pharmacokinetics and pharmacodynamics of the ACE inhibitor benazepril hydrochloride are altered with proteinuria by studying 8 patients with major proteinuria of different causes who were given a single dose of 10 mg p.o. The maximum plasma concentration of benazepril was found between 0.5 and 2 h after dosing (median 1 h). Its elimination was almost complete within 6 h. Peak plasma levels of benazeprilat, the active metabolite of benazepril, were observed between 1 and 6 h (median 2.5 h). The elimination of benazeprilat from plasma was biphasic, with mean initial and terminal half-lives of 3.0 and 17.3 h, respectively. On average, the pharmacokinetic parameters of benazepril and benazeprilat in the patients did not differ from those in a historical control group of healthy volunteers, but intersubject variability in the AUC and half-lives of benazeprilat was greater in the patients. Plasma ACE was completely inhibited from 1.5 to 6 h after dosing, and at 48 h the mean inhibition was still 42%. Plasma renin showed substantial intersubject variation. Mean supine blood pressure (systolic/diastolic) was reduced from baseline by a maximum of 18/13 mm Hg at 6 h. Proteinuria was diminished after benazepril in 7 patients. In conclusion, the results of this study suggest that proteinuria in the nephrotic range does not require a change in benazepril dosage.

A novel statistical approach for the comparison of analytical methods to measure angiotensin converting enzyme activity, enalaprilat, and benazeprilat

Two different analytical techniques were used to measure angiotensin converting enzyme (ACE) activity, and three different methods were used to measure each of the ACE inhibitors enalaprilat and benazeprilat. All measurements were made in human plasma. The groups of methods were compared by two different statistical approaches. First, the means of the methods were compared by the paired t test or analysis of variance, depending on whether two or three different methods were under comparison. Second, the squared coefficients of variation of the methods were compared by the Jackknife technique. The dual statistical approach employed enabled both the accuracy and the variability of the analytical methods to be compared and is a superior approach to the inappropriate use of correlation coefficients that are commonly used to compare analytical techniques. No statistically significant difference was found between the two assays used to measure ACE activity. Differences were found between the three methods to measure enalaprilat, although no obvious reason could be found for this phenomenon. Significant differences were also found between the three methods used to measure benazeprilat and were attributed to the presence of metabolites interfering in the nonspecific assay methods.

The effect of saturation of ACE binding sites on the pharmacokinetics of enalaprilat in man

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.