Selective 3-hydroxylation deficiency of lidocaine and its metabolite in Dark Agouti rats

Polymorphism of human cytochrome P450 enzymes and its clinical impact

Pharmacogenetics is the study of how interindividual variations in the DNA sequence of specific genes affect drug response. This article highlights current pharmacogenetic knowledge on important human drug-metabolizing cytochrome P450s (CYPs) to understand the large interindividual variability in drug clearance and responses in clinical practice. The human CYP superfamily contains 57 functional genes and 58 pseudogenes, with members of the 1, 2, and 3 families playing an important role in the metabolism of therapeutic drugs, other xenobiotics, and some endogenous compounds. Polymorphisms in the CYP family may have had the most impact on the fate of therapeutic drugs. CYP2D6, 2C19, and 2C9 polymorphisms account for the most frequent variations in phase I metabolism of drugs, since almost 80% of drugs in use today are metabolized by these enzymes. Approximately 5-14% of Caucasians, 0-5% Africans, and 0-1% of Asians lack CYP2D6 activity, and these individuals are known as poor metabolizers. CYP2C9 is another clinically significant enzyme that demonstrates multiple genetic variants with a potentially functional impact on the efficacy and adverse effects of drugs that are mainly eliminated by this enzyme. Studies into the CYP2C9 polymorphism have highlighted the importance of the CYP2C9*2 and *3 alleles. Extensive polymorphism also occurs in other CYP genes, such as CYP1A1, 2A6, 2A13, 2C8, 3A4, and 3A5. Since several of these CYPs (e.g., CYP1A1 and 1A2) play a role in the bioactivation of many procarcinogens, polymorphisms of these enzymes may contribute to the variable susceptibility to carcinogenesis. The distribution of the common variant alleles of CYP genes varies among different ethnic populations. Pharmacogenetics has the potential to achieve optimal quality use of medicines, and to improve the efficacy and safety of both prospective and currently available drugs. Further studies are warranted to explore the gene-dose, gene-concentration, and gene-response relationships for these important drug-metabolizing CYPs.

Application of substrate depletion assay for early prediction of nonlinear pharmacokinetics in drug discovery: Assessment of nonlinearity of metoprolol, timolol, and propranolol

The purpose of this study was to investigate the advantages of the substrate depletion assay for evaluating linearity of pharmacokinetics compared with the metabolite formation assay. For propranolol, metoprolol, and nisoldipine with multiple and/or sequential metabolisms, the Michaelis constant (Km) and maximum metabolic intrinsic clearance obtained from the depletion assay using rat and human liver microsomes showed a good correlation with relevant parameters with the formation assay. In vitro kinetics and in vivo pharmacokinetic profiles after oral administration of timolol, metoprolol, and propranolol, were investigated in rats using the depletion assay. The same rank order was found between nonlinearities based on dose-normalized areas under the plasma concentration curve (AUC/Dose) and Km values. Using the kinetic parameters of these compounds, AUC was predicted based on a physiological based pharmacokinetic model incorporated saturable metabolism. The AUCs predicted for propranolol and metoprolol had a good relationship with those observed in the in vivo studies, implying that the depletion assay could be useful for assessing linearity of pharmacokinetics.

Porcine FAD-containing monooxygenase metabolizes lidocaine, bupivacaine and propranolol in vitro

Lidocaine, bupivacaine and propranolol are amines that can be expected to act as substrates for FAD-containing monooxygensae (FMO) (EC 1. 14. 13. 8). We found that FMO metabolizes lidocaine, bupivacaine and propranolol. The Km and Vmax values of lidocaine, bupivacaine and propranolol for FMO are 143, 408 and 210 microM, and 145, 119 and 135 nmol/min/mg FMO protein, respectively. The lipophilicity of the drugs decreased in the following order: lidocaine>propranolol>bupivacaine, under our experimental conditions. Furthermore, the metabolic products of FMO were separated by high-performance liquid chromatography and analyzed by gas chromatography-mass spectrometry, and were found to be the N-oxides and N-hydroxylamines of the respective drugs. These findings suggest that lidocaine, bupivacaine and propranolol are substrates for FMO, and the enzymatic toward lidocaine or bupivacaine may be inhibited exclusively and competitively by propranolol.

Involvement of liver carboxylesterases in the in vitro metabolism of lidocaine

Although lidocaine has been used clinically for more than half a century, the metabolism has still not been fully elucidated. In the present study we have addressed the involvement of hydroxylations, deethylations, and ester hydrolysis in the metabolism of lidocaine to 2,6-xylidine. Using microsomes isolated from male rat liver, we found that lidocaine is mainly metabolized by deethylation to N-(N-ethylglycyl)-2,6-xylidine, and N-(N-ethylglycyl)-2,6-xylidine is mainly metabolized to N-glycyl-2,6-xylidine, also by deethylation. However, 2,6-xylidine can be formed both from lidocaine and N-(N-ethylglycyl)-2,6-xylidine, but not from N-glycyl-2,6-xylidine, in an NADPH-independent reaction, suggesting that the amido bond in these compounds can be directly hydrolyzed by esterases. To test this hypothesis, we incubated lidocaine, N-(N-ethylglycyl)-2,6-xylidine, and N-glycyl-2,6-xylidine with purified liver carboxylesterases. Rat liver microsomal carboxylesterase ES-10, but not carboxylesterase ES-4, hydrolyzed lidocaine and N-(N-ethylglycyl)-2,6-xylidine to 2,6-xylidine, identifying this esterase as a candidate enzyme in the metabolism of lidocaine.

The validity of the MEGX test in correlation with histology after orthotopic rat liver transplantation

Lidocaine metabolism (MEGX test) as an indicator for liver function in the assessment of different degrees of liver disease and as a predictor for liver outcome after transplantation is well established. Since reduced liver function is associated with an alteration in parenchymal and non-parenchymal cells, we evaluated whether MEGX values correlate with histology in an in vivo model of orthotopic rat liver transplantation (ORLT) to assess histological damage without taking biopsy specimens. Livers from syngeneic Lewis rats were transplanted with rearterialization after 15-30 h of cold storage in UW solution and rinsing with Carolina Rinse Solution prior to implantation. Forty-eight hours after transplantation, the MEGX test was performed and metabolites were measured with a commercial kit as described elsewhere. Biopsy specimens were taken and graded three degrees of damage (mild, moderate, and severe) in a double blind fashion by a pathologist. MEGX values were assigned to the histological results. Statistical analyses were done with a Mann-Whitney test (n = 58) for mean values. The mean MEGX values attributed to histologies with a mild, moderate, severe degree of damage were 159.96, 78.46 and 44.42 ng/ml, respectively. When the histological groups were compared with the mean MEGX values, mild vs moderate, mild vs severe and moderate vs severe were significant (P - 0.0001). In conclusion, MEGX values correlate significantly with histological grading in a linear fashion after ORLT. The MEGX test may be of clinical value because it reflects the histological pattern of livers and may reduce the necessity to take biopsy specimens before and after transplantation.

Expression of four rat CYP2D isoforms in Saccharomyces cerevisiae and their catalytic specificity

We cloned four cDNAs belonging to the CYP2D subfamily to express these enzymes in yeast cells and to compare their catalytic activities simultaneously. Three are believed to be alleles of CYP2D1, 2D2, and 2D3, respectively, based on high nucleotide sequence similarity, while CYP2D4 had both sequences of CYP2D4 and CYP2D18. Expression plasmids carrying CYP2D cDNAs were transformed into Saccharomyces cerevisiae. Typical P450 CO-difference spectra with absorbance maximum at 448 nm were recorded with microsomal preparations from the yeast cells expressing the four CYP2D forms. A catalytic study of these CYP2D forms was done with debrisoquine, bufuralol, and lidocaine. CYP2D2 had the highest debrisoquine 4-hydroxylation (2.2 nmol/min/nmol P450) activity, similar to that (2.2 nmol/min/nmol) of human CYP2D6 expressed in yeast cells. CYP2D3 had high lidocaine N-deethylation (43 nmol/min/nmol P450) activity, and both CYP2D3 and 2D2 exhibited high lidocaine 3-hydroxylation (2.4 and 1.6 nmol/min/nmol P450, respectively) activity. Bufuralol 1'-hydroxylation catalytic capabilities were comparable among the four isoforms. The activity of CYP2D1 was relatively low toward the three substrates (debrisoquine, 0.091; bufuralol, 1.5; lidocaine 3-hydroxylation, 0.019; lidocaine N-deethylation, 2.8 nmol/min/nmol P450). These findings indicate that debrisoquine, a typical substrate for CYP2D forms, was mainly metabolized by CYP2D2 but not CYP2D1 in rat liver and that the CYP2D forms have different substrate specificity.

Characterization of the oxidation reactions catalyzed by CYP2D enzyme in rat renal microsomes

Monooxygenase activities in rat renal microsomes were determined with the substrates of hepatic CYP2D enzymes. Seven kinds of CYP2D-mediated monooxygenase activities and immunochemically determined CYP2D contents in kidneys corresponded to approximately 3% of those in livers. Debrisoquine 4-hydroxylase and bunitrolol 4-hydroxylase in renal microsomes were inhibited almost completely by the antibody against a CYP2D enzyme purified from rat liver. A marked strain difference (Wistar > Dark Agouti) in these activities was observed in kidney like in liver. The two hydroxylases were inhibited stereoselectively by quinine and quinidine both in renal and hepatic microsomes. Substrate stereoselectivity in (+)- and (-)-bunitrolol 4-hydroxylase activities in kidneys was also consistent with that in livers. These results suggested that the CYP2D enzyme(s) was expressed in the kidney at levels much less than in the liver but had similar functions to those in the liver.

Lidocaine elimination and monoethylglycinexylidide formation in the dehydrated camel

The elimination kinetics and the formation of the monoethylglycinexylidide (MEGX), a major metabolite of lidocaine, were studied in camels deprived of water for 14 days. The study was conducted on four camels in a crossover design. Lidocaine was administered intravenously at a dose of 1 mg/kg to adult female camels when water was given ad libitum (stage 1) and to the same camels after 14 days of dehydration. Blood samples were taken up to 6 h after dosing. Serum lidocaine and MEGX levels were analysed by polarization fluorescence immunoassay. The elimination profiles of lidocaine and the formation of the metabolite MEGX in the two phases of the study were essentially identical. No difference in any pharmacokinetic parameter was noticed between normally hydrated and water-deprived camels. It is thus concluded that dehydration does not affect the cytochrome P450 isozymes involved in degradation of lidocaine to MEGX nor does it affect the hepatic blood flow, which is a major determinant in the clearance of lidocaine. The very low clearance of lidocaine in the camel in comparison with other ruminant or monogastric mammals may be associated with the camel's ability to survive drought in the desert.

An evaluation of cytochrome P450 isoform activities in the female dark agouti (DA) rat: relevance to its use as a model of the CYP2D6 poor metaboliser phenotype

The female dark agouti (DA) rat lacks CYP2D1, the equivalent enzyme in the rat to human CYP2D6 (debrisoquine hydroxylase), and shows impaired metabolism of a number of CYP2D6 substrates. However, from the data available in the literature it is not entirely clear whether the enzyme deficiency in the DA rat is restricted to CYP2D1, and whether factors such as age and substrate concentration are important determinants of interstrain differences in the activity of this enzyme. Given that the female DA rat is used as a model of the human CYP2D6 poor metaboliser phenotype, there is a need for a systematic evaluation of the P450 activities in the DA rat, and of its suitability as a model of the PM phenotype. In the present study metoprolol was used as a probe substrate to investigate CYP2D1 activity since both the alpha-hydroxylation and O-demethylation of this drug are catalysed by CYP2D6 in man. Formation of alpha-hydroxymetoprolol (AHM) and O-demethylmetoprolol (ODM) was 10- and 2.5-fold lower in liver microsomes from female DA rats compared with microsomes from age-matched female Wistar rats, the latter representing the extensive metaboliser strain. Kinetic analysis suggested that in both strains of rat both the alpha-hydroxylation and O-demethylation of metoprolol were catalysed by more than one enzyme. By using quinine as a specific inhibitor of the enzyme, CYP2D1 was identified as an intermediate affinity site in the Wistar strain and was shown to have impaired activity in the DA strain. The activities of lower and higher affinity sites were similar in the two strains. Thus, the only difference between the two strains with respect to both routes of metoprolol metabolism appeared to be in the activity of CYP2D1. Interstrain differences were found to be highly dependent on the choice of substrate concentration, being more marked at lower concentrations. We have also investigated the metabolism of a number of probe compounds for some of the other P450 isoforms commonly involved in drug metabolism to determine the selectivity of the deficiency in the DA strain. p-Nitrophenol hydroxylation and erythromycin N-demethylation were catalysed at higher rates by DA than by Wistar liver microsomes, indicating higher levels of activity of CYP2E1 and CYP3A in the former strain. Felodipine oxidation, tolbutamide hydroxylation and both the hydroxylation and N-demethylation of S-mephenytoin were catalysed at similar rates by microsomes from the two strains, indicating similar activities of enzymes in the CYP2C and CYP3A families.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of two P-450 isozymes placed in the rat CYP2D subfamily

Two P-450s with debrisoquine 4-hydroxylation activity, designated P-450 UT-7 and UT-7b, were purified and partially purified, respectively, from hepatic microsomes of untreated male rats. Both purified P-450s with an apparent molecular weight of 49,000, were associated with another protein with an apparent molecular weight of 29,000 which was designated 29 k-protein. The CO-reduced spectra of both P-450 UT-7 and UT-7b showed a peak at 448 nm. The NH2-terminal amino acid sequences of P-450 UT-7 and UT-7b were the same as the amino acid sequences of CYP2D1 and CYP2D2 deduced from the cDNA, respectively, except for the lack of a terminal methionine for P-450 UT-7b. In a reconstituted systems, P-450 UT-7 and UT-7b catalyzed lidocaine 3-hydroxylation and N-deethylation in the presence of the 29 k-protein. The Km and Vmax values for lidocaine 3-hydroxylation were 3.6 microM and 0.50 nmol/min/nmol of P-450 for P-450 UT-7, and 3.6 microM and 0.93 nmol/min/nmol of P-450 for P-450 UT-7b, respectively. Antibody against P-450 UT-7, which also cross-reacted with P-450 UT-7b, inhibited lidocaine 3-hydroxylation in liver microsomes from untreated male rats, but had little effect on lidocaine N-deethylation. These findings suggested that lidocaine 3-hydroxylation in hepatic microsomes from untreated male rats was catalyzed by P-450 UT-7 and/or UT-7b.P-450 UT-7 not containing 29 k-protein was obtained as the non-absorbed fraction from hydroxylapatite HPLC. The activities of debrisoquine 4-hydroxylation as well as lidocaine 3-hydroxylation and N-deethylation in a reconstituted system with P-450 UT-7 without 29 k-protein were one-fifth of those of P-450 UT-7 containing 29 k-protein at the same substrate concentration. These findings suggested that the 29 k-protein was essential to express the maximal metabolic activities. However, the lidocaine metabolic activity in a reconstituted system with P-450 UT-7 containing 29 k-protein and in hepatic microsomes were not inhibited by 29 k-protein antibody.

Participation of the CYP2D subfamily in lidocaine 3-hydroxylation and formation of a reactive metabolite covalently bound to liver microsomal protein in rats

Lidocaine metabolism was investigated in rat liver microsomes and in a reconstituted system containing P450BTL, a cytochrome (P450) isozyme belonging to the CYP2D subfamily (Suzuki et al., Drug Metab Dispos 20: 367-373, 1992). P450BTL biotransformed lidocaine into 3-hydroxylidocaine (3-OH-LID) but not monoethylglycinexylidide and 2-methylhydroxylidocaine, in the reconstituted system including NADPH-P450 reductase and dilauroylphosphatidylcholine. An antibody against P450BTL inhibited microsomal lidocaine 3-hydroxylase activity by 97%. Thus, P450BTL and/or its immunorelated P450 isozyme(s) belonging to the CYP2D subfamily appear to be involved in lidocaine 3-hydroxylation. Furthermore, the antibody also suppressed the amounts of a lidocaine metabolite(s) bound to microsomal protein. These results suggest that the CYP2D subfamily biotransformed lidocaine into 3-OH-LID via an epoxy intermediate, which binds to microsomal macromolecules.

Kinetic analysis of mutual metabolic inhibition of lidocaine and propranolol in rat liver microsomes

The metabolic interaction between lidocaine (LD) and propranolol (PL) was analysed kinetically in rat liver microsomes. Employing a very short incubation time of 30 sec, we demonstrated that PL competitively inhibited liver microsomal 3-hydroxylation of LD, but did not affect either the formation of monoethylglycinexylidide or methylhydroxylidocaine from LD in PL concentrations up to 1 microM. On the other hand, LD competitively inhibited PL 4-, 5- and 7-hydroxylations, but the inhibition type of LD for PL N-desisopropylation could not be clarified. Comparison of the kinetic data for liver microsomes from Wistar and Dark Agouti rats indicated that among the primary metabolic pathways of LD, the Vmax value for 3-hydroxylation was markedly less in female Dark Agouti rats. The results suggest that LD 3-hydroxylation and PL ring hydroxylations are mediated by the same isozyme(s) belonging to the CYP2D subfamily.

Primary and secondary oxidative metabolism of dextromethorphan. In vitro studies with female Sprague-Dawley and Dark Agouti rat liver microsomes

The O-demethylation of dextromethorphan (DM) to dextrorphan (DR) is catalysed by the polymorphic CYP2D6 (cytochrome P4502D6) isozyme in man. DM is commonly used as a probe for phenotyping subjects as either poor or extensive metabolizers for the debrisoquine/sparteine oxidative polymorphism via CYP2D6. The enzyme kinetics of DM O- and N-demethylation, and the N- and O-demethylations of the primary metabolites DR and 3-methoxymorphinan (3MM), respectively, were studied in liver microsomes from female Dark Agouti (DA) rats, the poor metabolizer counterpart, and female Sprague-Dawley (SD) rats, the extensive metabolizer counterpart. The formation of metabolites was quantified by HPLC with fluorescence detection and kinetic parameters were calculated. The intrinsic clearance (Vmax/Km) of the O-demethylation of 3MM to 3-hydroxymorphinan (3OHM) was 180-fold lower in DA rats (0.11 vs 20.77 mL/hr/mg) due to a 60-fold higher Km (108.7 vs 1.76 microM) and 3-fold lower Vmax (11.5 vs 35.95 nmol/mg/hr). The kinetics for DR N-demethylation to 3OHM did not differ between rat strains. The Michaelis-Menten constant (Km) for DM N-demethylation to 3MM was similar between SD and DA rats (85.04 vs 68.99 microM); however, SD rats displayed a 2-fold higher Vmax (83.37 vs 35.49 nmol/mg/hr) and intrinsic clearance (0.96 vs 0.51 mL/hr/mg). The O-demethylation of DM to DR in SD rats showed a high and low affinity enzyme component, with the high affinity intrinsic clearance contributing 98% of the total intrinsic clearance in these rats. DM O-demethylation in DA rats was characterized by a single enzyme system. The high affinity O-demethylating enzyme in SD rats showed a 20-fold lower Km (2.5 vs 55.6 microM) and a three-fold higher Vmax (51.04 vs 16.84 nmol/mg/hr) resulting in a 66-fold higher intrinsic clearance (20.04 vs 0.31 mL/hr/mg) compared to DA rats. Quinine, dextropropoxyphene, (+/-)methadone and (+/-)propafenone were shown to be potent inhibitors of 3MM and DM O-demethylation but did not inhibit DR or DM N-demethylation at similar concentrations. SD and DA rats showed a clear strain difference in 3MM O-demethylation and DM O-demethylation. In contrast, DR N-demethylation and DM N-demethylation do not appear to be under genetic control in the female SD-DA rat model. Kinetic parameters and inhibition studies suggest that 3MM and DM O-demethylation pathways in the rat may be mediated by the same cytochrome P450 isozyme.

Speculations on the substrate structure-activity relationship (SSAR) of cytochrome P450 enzymes

This brief review attempts to define the SSAR of two families of cytochrome P450. With P4502D catalytic competence is achieved by tight ionic binding which gives the enzyme high regioselectivity. In contrast P4503A achieves catalytic competence by a flexible binding site relying on hydrophobic forces that allow chemically vulnerable sites to be the principal sites of metabolism. In general, the different binding mechanism should be reflected in the enzyme, such that substrates of P4502D should have lower Km values than substrates of P4503A. Thus, routes of metabolism catalysed by P4502D may be saturated at substrate concentrations lower than routes catalysed by P4503A. The apparent differences between P4502D and P4503A in terms of substrate specificity bring into question what relationships govern other families of cytochrome P450. Our analysis of data suggests that the other principal form involved, generally, in the metabolism of pharmaceuticals in humans is P4502C9 (possibly 2C8 and 2C10). The enzyme is responsible for the metabolism of phenytoin, tolbutamide, tienilic acid [4], naproxen, ibuprofen, diclofenac [38], the 7-hydroxylation of S-warfarin [39] and the 7-hydroxylation of delta 1-tetrahydrocannabinol [40]. These compounds all have areas of strong hydrogen bond [4] forming potential (Fig. 8), all distanced 5-10A from the site of metabolism. Moreover the carboxylic acid function of naproxen, ibuprofen and diclofenac (pKa 4.5) and the sulfonylurea of tolbutamide (pKa 5.4) render the compounds ionized at physiological pH. The ionised group is positioned 7-11A from the site of metabolism. It is likely, therefore, that hydrogen bonding and possibly ion-pair interactions play a major role in determining the SSAR of the P4502C isoenzymes. These interactions would suggest that the P4502C enzymes are analogous to P4502D rather than P4503A. In this regard it is noteworthy that P4502C9 is selectively and potently inhibited by sulfaphenazole (IC50 of 0.6 microM), a compound that is structurally related (Fig. 8) to the substrates in terms of potential hydrogen bonding regions [4, 41]. Simplistically we suggest that the SSAR of the various P450 enzymes ranges from the highly selective enzymes dealing with endogenous substrates, through the enzymes metabolising exogenous substrates with narrow substrate structure requirements such as P4502D to P4503A with its broad substrate structure range. It would seem logical that animals and humans would evolve such combinations of isoenzymes to deal with the vast array of exogenous xenobiotics.

Metabolic activation of lidocaine and covalent binding to rat liver microsomal protein

Incubation of [14C]lidocaine with rat liver microsomes in the presence of an NADPH-generating system resulted in covalent bindings of a 14C-labelled material to microsomal protein. The covalent binding of radioactivity needed NADPH and atmospheric oxygen, and was diminished by purging of carbon monoxide and the addition of SKF-525A. Hence the covalent binding of a 14C-labelled material resulting from a reactive metabolite of lidocaine formed by cytochrome P450-dependent monooxygenation. The covalent binding measured at various concentrations of lidocaine (2.5-30 microM) followed Michaelis-Menten kinetics, and the Km value (4.52 microM) of the activation reaction was close to the Km value (1.78 microM) of lidocaine 3-hydroxylation. The metabolism-dependent covalent binding of lidocaine to microsomal protein as well as lidocaine 3-hydroxylase activity was much lower in the Dark Agouti strain rat, which is known as a poor-metabolizer animal model of debrisoquine 4-hydroxylation, than in the Wistar rat for the corresponding sexes. The covalent binding in male rats was greater than that in females of both strains, but the extent of the sex difference in the binding was smaller than that of the lidocaine N-deethylase activity in Wistar rats. Propranolol and quinidine, specific inhibitors of debrisoquine 4-hydroxylase, markedly inhibited lidocaine 3-hydroxylase activity of Wistar male rats, but not N-deethylase activity. These compounds also inhibited the metabolism-dependent covalent binding of lidocaine to microsomal protein. These strain difference and inhibition studies showed that the reaction converting lidocaine to a reactive metabolite capable of binding covalently to microsomal protein was related to lidocaine 3-hydroxylation, and may be catalysed by cytochrome P450 isozyme(s) belonging to the CYP2D subfamily. The covalent binding of radioactivity to rat liver microsomal protein was diminished by nucleophiles, reduced glutathione and cysteine, indicating that the reactive metabolic intermediate of lidocaine is an electrophilic metabolite such as an arene oxide.