Substrate Report for: Imidapril

Human arylacetamide deacetylase (AADAC) is an esterase responsible for the hydrolysis of some drugs, including flutamide, indiplon, phenacetin, and rifamycins. AADAC is highly expressed in the human liver, where carboxylesterase (CES) enzymes, namely, CES1 and CES2, are also expressed. It is generally recognized that CES1 prefers compounds with a large acyl moiety and a small alcohol or amine moiety as substrates, whereas CES2 prefers compounds with a small acyl moiety and a large alcohol or amine moiety. In a comparison of the chemical structures of known AADAC substrates, AADAC most likely prefers compounds with the same characteristics as does CES2. However, the substrate specificity of human AADAC has not been fully clarified. To expand the knowledge of substrates of human AADAC, we measured its hydrolase activities toward 13 compounds, including known human CES1 and CES2 substrates, using recombinant enzymes expressed in Sf21 cells. Recombinant AADAC catalyzed the hydrolysis of fluorescein diacetate, N-monoacetyldapsone, and propanil, which possess notably small acyl moieties, and these substrates were also hydrolyzed by CES2. However, AADAC could not hydrolyze another CES2 substrate, procaine, which possesses a moderately small acyl moiety. In addition, AADAC did not hydrolyze several known CES1 substrates, including clopidogrel and oseltamivir, which have large acyl moieties and small alcohol moieties. Collectively, these results suggest that AADAC prefers compounds with smaller acyl moieties than does CES2. The role of AADAC in the hydrolysis of drugs has been clarified. For this reason, AADAC should receive attention in ADMET studies during drug development.

Imidapril is an angiotensin-converting enzyme inhibitor that is widely used in treating hypertension, although the responses vary among individuals. We investigated whether a single nucleotide polymorphism at position -816 of the carboxylesterase 1 (CES1) gene, which activates imidapril in the liver, is involved in the responsiveness to imidapril medication. A total of 105 Japanese hypertensives with systolic/diastolic blood pressures (SBP/DBP) of 140/90 mmHg or higher were prescribed 5-10 mg/day of imidapril. At baseline, blood pressure levels were not different between patients with and those without the -816C allele (AA vs. AC+ CC groups). After 8 weeks of treatment, we classified the responders and non-responders based on the decline in their blood pressures, and found that the responder rate was significantly higher in the AC+CC group than in the AA group (p=0.0331). Also, the reduction in SBP was significantly greater in the AC+CC group than in the AA group (24.7+/-11.8 vs. 17.6+/-16.8 mmHg, p=0.0184). Furthermore, an in vitro reporter assay revealed that the -816C construct had significantly higher promoter activity (p<0.0001). These findings suggest that the A(-816)C polymorphism affects the transcriptional activity, and that this may account for the responsiveness to imidapril.

Carboxylesterases (EC 3.1.1.1) from human liver were purified using Q-Sepharose, Sephadex G-150, isoelectrofocusing and Con A-Sepharose. The calculated molecular mass of the pI 5.3 enzyme was 120 kDa and 61 kDa from the results of Sephadex G-150 gel filtration and SDS-polyacrylamide gel electrophoresis (PAGE), respectively, suggesting that this enzyme is a dimer. On the other hand, carboxylesterase pI 4.5, with a molecular mass of 64 kDa, was a monomer. The activities of both enzymes were inhibited by typical serine enzyme inhibitors. Amino acid sequence analysis of the purified enzymes pI 5.3 and 4.5 showed high homology with rabbit carboxylesterase form 1 and 2, respectively. The results also suggested that carboxylesterase pI 5.3 is identical to the deduced amino acid sequence from cDNA for HU1, and that carboxylesterase pI 4.5 is identical to the deduced amino acid sequence from the cDNA registered as human carboxylesterase (hCE-2) in GenBank. We first purified carboxylesterase pI 4.5 and investigated its hydrolytic activity upon various drugs. The two enzymes differed in substrate specificity. Prodrugs of angiotensin-converting enzyme inhibitors, such as delapril and imidapril, were converted to active metabolites by carboxylesterase pI 5.3, but not by carboxylesterase pI 4.5. The hydrolysis velocity of temocapril by carboxylesterase pI 5.3 was 12-fold faster than by carboxylesterase pI 4.5. In contrast, aspirin, oxybutynin and procaine were hydrolyzed by only carboxylesterase pI 4.5. We also found that an amide-linkage in drugs, except for that in aniracetam, was not a good substrate for the two enzymes. Consequently, carboxylesterases pI 5.3 and 4.5 may be involved in the metabolism of various drugs containing an ester-linkage.

Human arylacetamide deacetylase (AADAC) is an esterase responsible for the hydrolysis of some drugs, including flutamide, indiplon, phenacetin, and rifamycins. AADAC is highly expressed in the human liver, where carboxylesterase (CES) enzymes, namely, CES1 and CES2, are also expressed. It is generally recognized that CES1 prefers compounds with a large acyl moiety and a small alcohol or amine moiety as substrates, whereas CES2 prefers compounds with a small acyl moiety and a large alcohol or amine moiety. In a comparison of the chemical structures of known AADAC substrates, AADAC most likely prefers compounds with the same characteristics as does CES2. However, the substrate specificity of human AADAC has not been fully clarified. To expand the knowledge of substrates of human AADAC, we measured its hydrolase activities toward 13 compounds, including known human CES1 and CES2 substrates, using recombinant enzymes expressed in Sf21 cells. Recombinant AADAC catalyzed the hydrolysis of fluorescein diacetate, N-monoacetyldapsone, and propanil, which possess notably small acyl moieties, and these substrates were also hydrolyzed by CES2. However, AADAC could not hydrolyze another CES2 substrate, procaine, which possesses a moderately small acyl moiety. In addition, AADAC did not hydrolyze several known CES1 substrates, including clopidogrel and oseltamivir, which have large acyl moieties and small alcohol moieties. Collectively, these results suggest that AADAC prefers compounds with smaller acyl moieties than does CES2. The role of AADAC in the hydrolysis of drugs has been clarified. For this reason, AADAC should receive attention in ADMET studies during drug development.

Abstract Human carboxylesterase 1 (CES1) and carboxylesterase 2 (CES2) are serine esterases responsible for the hydrolysis of ester and amide bonds present in a number of pharmaceutical products. Several common genetic variants of the CES1 and CES2 genes have been shown to influence drug metabolism and clinical outcomes. Polymorphisms of the CES1 gene have been reported to affect the metabolism of dabigatran etexilate, methylphenidate, oseltamivir, imidapril, and clopidogrel, whereas variants of the CES2 gene have been found to affect aspirin and irinotecan. Although the findings of these studies may be preliminary, they demonstrate the potential clinical utility of CES polymorphisms; however, more research is required, especially with respect to CES2. In this review, we outline the functional, molecular, and genetic properties of CES1 and CES2, and highlight recent studies that have shown relations between CES1 and CES2 variants and contemporary pharmacotherapy.

Two major forms of human carboxylesterase (CES), CES1A and CES2, dominate the pharmacokinetics of most prodrugs such as imidapril and irinotecan (CPT-11). Excipients, largely used as insert vehicles in formulation, have been recently reported to affect drug enzyme activity. The influence of excipients on the activity of CES remains undefined. In this study, the inhibitory effects of 25 excipients on the activities of CES1A1 and CES2 were evaluated. Imidapril and CPT-11 were used as substrates and cultured with liver microsomes in vitro. Imidapril hydrolase activities of recombinant CES1A1 and human liver microsomes (HLM) were strongly inhibited by sodium lauryl sulphate (SLS) and polyoxyl 40 hydrogenated castor oil (RH40) [Inhibition constant (Ki) = 0.04+/-0.01 mug/ml and 0.20+/-0.09 mug/ml for CES1A1, and 0.12+/-0.03 mug/ml and 0.76+/-0.33 mug/ml, respectively, for HLM]. The enzyme hydrolase activity of recombinant CES2 was substantially inhibited by Tween 20 and polyoxyl 35 castor oil (EL35) (Ki = 0.93+/-0.36 mug/ml and 4.4+/-1.24 mug/ml, respectively). Thus, these results demonstrate that surfactants such as SLS, RH40, Tween 20 and EL35 may attenuate the CES activity; such inhibition should be taken into consideration during drug administration.

Human carboxylesterase (CES) 1A and CES2, two major forms of human CES, dominate the pharmacokinetics of most prodrugs such as imidapril and irinotecan (CPT-11). Antihypertensive drugs are often prescribed for clinical therapy concurrently with others. Moreover, two or more antihypertensive drugs are ubiquitously combined. The influences of antihypertensive drugs on the activity of CES remain undefined. In the present study, the inhibitory effects of 17 antihypertensive drugs on the CES1A1 and CES2 activities were evaluated. Imidapril and CPT-11 were used as substrates and cultured with liver microsomes in vitro. The imidapril hydrolase activities by recombinant CES1A1 and human liver microsomes (HLM) were intensely inhibited by telmisartan and nitrendipine (Ki = 0.49 +/- 0.09 and 1.12 +/- 0.39 microM for CES1A1, 1.69 +/- 0.17 microM and 1.24 +/- 0.27 microM for HLM, respectively). However, other drugs did not exert strong inhibition. The enzyme hydrolase activity of recombinant CES2 was substantially inhibited by diltiazem and verapamil (Ki = 0.25 +/- 0.02 and 3.84 +/- 0.99 microM, respectively). Hence, diltiazem, verapamil, nitrendipine and telmisartan may attenuate the drug efficacy of catalyzed prodrugs by changing the activities of CES1A1 and CES2.

Human carboxylesterase (CES) 1A is responsible for the biotransformation of angiotensin-converting enzyme (ACE) inhibitors such as imidapril and temocapril. Because antidiabetic or antihyperlipidemic drugs are often coadministered with ACE inhibitors in clinical pharmacotherapy, the inhibitory effect of these drugs on CES1A1 enzyme activity was investigated. In addition, the inhibitory effect on CES2 enzyme activity was evaluated to compare it with that on CES1A1. The inhibitory effects were evaluated with 11 antidiabetic and 12 antihyperlipidemic drugs. The imidapril hydrolase activity by recombinant CES1A1 was substantially inhibited by lactone ring-containing statins such as simvastatin and lovastatin and thiazolidinediones such as troglitazone and rosiglitazone. The activity in human liver microsomes was also strongly inhibited by simvastatin and troglitazone (K(i) = 0.8 +/- 0.1 and 5.6 +/- 0.2 muM, respectively). However, statins containing no lactone ring such as pravastatin and fluvastatin did not show strong inhibition. 7-Ethyl-10-[4-(1-piperidono)-1-piperidono]carbonyloxycamptothecin hydrolase activity by recombinant human CES2 was substantially inhibited by fenofibrate (K(i) = 0.04 +/- 0.01 muM) as well as by simvastatin (0.67 +/- 0.09 muM). Other fibrates such as clinofibrate and bezafibrate did not show strong inhibition. Thus, the inhibitory effects of the thiazolidinediones and fenofibrate on CES1A1 and CES2 were different. Some statins such as simvastatin and lovastatin, thiazolidinediones, and fenofibrate might attenuate the drug efficacy of prodrugs biotransformed by CES1A and CES2.

In vitro inhibition studies on drug-metabolizing enzyme activity are useful for understanding drug-drug interactions and for drug development. However, the profile of the inhibitory effects of carboxylesterase (CES) activity has not been fully investigated concerning species and tissue differences. In the present study, we measured the inhibitory effects of 15 drugs and 1 compound on CES activity using liver and jejunum microsomes and cytosol in human and rat. In addition, the inhibition constant (K(i) values) and patterns were determined for the compounds exhibiting strong inhibition. Hydrolysis of imidapril and irinotecan hydrochloride (CPT-11) is catalyzed mainly by CES1 and CES2, respectively. In the inhibition study, imidaprilat formation from imidapril in human liver was strongly inhibited by nordihydroguaiaretic acid (NDGA) and procainamide. The inhibition profile and pattern were similar in human liver and rat liver. The compounds showing potent inhibition were similar between liver and jejunum. The K(i) value of NDGA (K(i) = 13.3 +/- 1.5 microM) in human liver microsomes was 30-fold higher than that in rat liver microsomes (K(i) = 0.4 +/- 0.0 microM). On the other hand, 7-ethyl-10-hydroxycamptothecin (SN-38) formation from CPT-11 was not inhibited except by carvedilol, manidipine, and physostigmine. The K(i) value of physostigmine (K(i) = 0.3 +/- 0.0 microM) in human jejunum cytosol was 10-fold lower than that in rat jejunum cytosol (K(i) = 3.1 +/- 0.4 microM) and was similar to that for manidipine. The present study clarified the species differences in CES inhibition. These results are useful for the development of prodrugs.

Esterified drugs such as imidapril, derapril, and oxybutynin hydrolyzed by carboxylesterase 1 (CES1) are extensively used in clinical practice. The kinetics using the CES1 substrates have not fully clarified, especially concerning species and tissue differences. In the present study, we performed the kinetic analyses in humans and rats in order to clarify these differences. The imidaprilat formation from imidapril exhibited sigmoidal kinetics in human liver microsomes (HLM) and cytosol (HLC) but Michaelis-Menten kinetics in rat liver microsomes and cytosol. The 2-cyclohexyl-2-phenylglycolic acid (CPGA) formation from oxybutynin were not detected in enzyme sources from rats, although HLM showed high activity. The kinetics were clarified to be different among species, tissues, and preparations. In individual HLM and HLC, there was large interindividual variability in imidaprilat (31- and 24-fold) and CPGA formations (15- and 9-fold). Imidaprilat formations exhibited Michaelis-Menten kinetics in HLM and HLC with high activity but sigmoidal kinetics in those with low activity. CPGA formations showed sigmoidal kinetics in high activity HLM but Michaelis-Menten kinetics in HLM with low activity. We revealed that the kinetics were different between individuals. These results could be useful for understanding interindividual variability and for the development of oral prodrugs.

Carboxylesterase 1 (CES1) is involved in metabolic activation of a variety of prodrugs into active derivatives and plays an important role in pharmacokinetics. We previously reported that a single nucleotide polymorphism (SNP), -816A/C of the CES1A2 gene associates with the responsiveness to an angiotensin-converting enzyme (ACE) inhibitor, imidapril, whose activity is achieved by CES1. To identify relevant functional polymorphisms, we re-sequenced the CES1A2 promoter region ( approximately 1kb) in 100 Japanese hypertensive patients. Altogether 10 SNPs and one insertion/deletion (I/D) were identified, among which seven SNPs and one I/D residing between -62 and -32 were in almost complete linkage disequilibrium (D'=1.00, r2=0.97). They consisted a minor and a major haplotype, the allele frequencies of which were 22% and 74%, respectively. The minor haplotype possessed two putative Sp1 binding sites while the major haplotype did not have any Sp1 binding site. The minor haplotype had a higher transcription and Sp1 binding activities than the major haplotype, invitro. The original -816A/C was in high linkage disequilibrium with these haplotypes (D'=0.92, r2=0.85), and well agreed with the efficacy of imidapril medication. These results suggest that the Sp1 binding site variation in the CES1A2 promoter is functional, and are good candidates for the pharmacogenetic studies of CES1-activated drugs.

Metabolism of nafamostat, a clinically used serine protease inhibitor, was investigated with human blood and liver enzyme sources. All the enzyme sources examined (whole blood, erythrocytes, plasma and liver microsomes) showed nafamostat hydrolytic activity. V(max) and K(m) values for the nafamostat hydrolysis in erythrocytes were 278 nmol/min/mL blood fraction and 628 microM; those in plasma were 160 nmol/min/mL blood fraction and 8890 microM, respectively. Human liver microsomes exhibited a V(max) value of 26.9 nmol/min/mg protein and a K(m) value of 1790 microM. Hydrolytic activity of the erythrocytes and plasma was inhibited by 5, 5'-dithiobis(2-nitrobenzoic acid), an arylesterase inhibitor, in a concentration-dependent manner. In contrast, little or no suppression of these activities was seen with phenylmethylsulfonyl fluoride (PMSF), diisopropyl fluorophosphate (DFP), bis(p-nitrophenyl)phosphate (BNPP), BW284C51 and ethopropazine. The liver microsomal activity was markedly inhibited by PMSF, DFP and BNPP, indicating that carboxylesterase was involved in the nafamostat hydrolysis. Human carboxylesterase 2 expressed in COS-1 cells was capable of hydrolyzing nafamostat at 10 and 100 microM, whereas recombinant carboxylesterase 1 showed significant activity only at a higher substrate concentration (100 microM). The nafamostat hydrolysis in 18 human liver microsomes correlated with aspirin hydrolytic activity specific for carboxylesterase 2 (r=0.815, p<0.01) but not with imidapril hydrolysis catalyzed by carboxylesterase 1 (r=0.156, p=0.54). These results suggest that human arylesterases and carboxylesterase 2 may be predominantly responsible for the metabolism of nafamostat in the blood and liver, respectively.

Imidapril is an angiotensin-converting enzyme inhibitor that is widely used in treating hypertension, although the responses vary among individuals. We investigated whether a single nucleotide polymorphism at position -816 of the carboxylesterase 1 (CES1) gene, which activates imidapril in the liver, is involved in the responsiveness to imidapril medication. A total of 105 Japanese hypertensives with systolic/diastolic blood pressures (SBP/DBP) of 140/90 mmHg or higher were prescribed 5-10 mg/day of imidapril. At baseline, blood pressure levels were not different between patients with and those without the -816C allele (AA vs. AC+ CC groups). After 8 weeks of treatment, we classified the responders and non-responders based on the decline in their blood pressures, and found that the responder rate was significantly higher in the AC+CC group than in the AA group (p=0.0331). Also, the reduction in SBP was significantly greater in the AC+CC group than in the AA group (24.7+/-11.8 vs. 17.6+/-16.8 mmHg, p=0.0184). Furthermore, an in vitro reporter assay revealed that the -816C construct had significantly higher promoter activity (p<0.0001). These findings suggest that the A(-816)C polymorphism affects the transcriptional activity, and that this may account for the responsiveness to imidapril.

Carboxylesterases (EC 3.1.1.1) from human liver were purified using Q-Sepharose, Sephadex G-150, isoelectrofocusing and Con A-Sepharose. The calculated molecular mass of the pI 5.3 enzyme was 120 kDa and 61 kDa from the results of Sephadex G-150 gel filtration and SDS-polyacrylamide gel electrophoresis (PAGE), respectively, suggesting that this enzyme is a dimer. On the other hand, carboxylesterase pI 4.5, with a molecular mass of 64 kDa, was a monomer. The activities of both enzymes were inhibited by typical serine enzyme inhibitors. Amino acid sequence analysis of the purified enzymes pI 5.3 and 4.5 showed high homology with rabbit carboxylesterase form 1 and 2, respectively. The results also suggested that carboxylesterase pI 5.3 is identical to the deduced amino acid sequence from cDNA for HU1, and that carboxylesterase pI 4.5 is identical to the deduced amino acid sequence from the cDNA registered as human carboxylesterase (hCE-2) in GenBank. We first purified carboxylesterase pI 4.5 and investigated its hydrolytic activity upon various drugs. The two enzymes differed in substrate specificity. Prodrugs of angiotensin-converting enzyme inhibitors, such as delapril and imidapril, were converted to active metabolites by carboxylesterase pI 5.3, but not by carboxylesterase pI 4.5. The hydrolysis velocity of temocapril by carboxylesterase pI 5.3 was 12-fold faster than by carboxylesterase pI 4.5. In contrast, aspirin, oxybutynin and procaine were hydrolyzed by only carboxylesterase pI 4.5. We also found that an amide-linkage in drugs, except for that in aniracetam, was not a good substrate for the two enzymes. Consequently, carboxylesterases pI 5.3 and 4.5 may be involved in the metabolism of various drugs containing an ester-linkage.