Introduction

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Much interest is currently focused on the early identification of the drug-metabolizing enzymes responsible for the biotransformations commonly encountered in drug development (Becquemont et al., 1998). Such information may help identify the key organs for clearance and explain or even predict the observed variability in pharmacokinetics with some substrates and prioritize drug-drug interaction studies. Because most (~60%) marketed compounds are cleared metabolically by cytochrome P450 (CYP)² enzymes, the major activity in this area has focused on this family of enzymes (Bertz and Granneman, 1997).

Traditionally, human liver microsomes (HLM) have been the in vitro tool for these studies and have provided both qualitative, e.g., identifying which CYP isoform(s) metabolize the compound of interest (Pichard et al., 1990; Andersson et al., 1993; Otton et al., 1990; Jacqz-Aigrain et al., 1993; Doecke et al., 1991; Wester et al., 2000; Yasumori et al., 1993; Kroemer et al., 1993) and quantitative information, e.g., predicted CL_int (Houston, 1994; Rodrigues, 1994; Carlile et al., 1999). Identifying the enzymology of metabolism by human CYPs has proved somewhat labor- and time-intensive, requiring comparative kinetics across a bank of characterized HLM, chemical, and/or antibody inhibition followed by the use of recombinant CYP isoforms (Rodrigues, 1999). The routine access to recombinant CYPs has facilitated direct identification of the isoform(s) responsible for the oxidative metabolism of the drug of interest, although their use in vitro has generally been to support HLM data (Aoyama et al., 1990; Tassaneeyakul et al., 1992; Kroemer et al., 1993; Rodrigues et al., 1994; Yamazaki et al., 1997; Von Moltke et al., 1998; Rodrigues, 1999).

With the advent of combinatorial chemistry and parallel synthesis techniques, there is an expectation to achieve both higher throughput and faster turnaround times in many biological assays. There is an increasing emphasis within drug metabolism in the pharmaceutical industry to develop enhanced throughput frontline in vitro models, including those to determine both the extent and route of the metabolism of new chemical entities (NCEs) and to screen for inducers and inhibitors of drug-metabolizing enzymes (Ayrton et al., 1998; Moody et al., 1999).

The ability to predict directly the human enzymology using enhanced throughput methods would represent a major breakthrough in this technology (Becquemont et al., 1998; Roy et al., 1999) in a similar manner to that adopted for CYP inhibition assays (Crespi et al., 1998; Moody et al., 1999). This laboratory has demonstrated that the five major human hepatic CYPs expressed in Escherichia coli (CYP1A2, -2C9, -2C19, -2D6, and -3A4) are faithful surrogates for their human liver counterparts with respect to their kinetic profiles and inhibition properties (McGinnity et al., 1999; Moody et al., 1999). In this study, the application of recombinant enzymes as a first line approach for identifying the CYP(s) responsible for metabolizing NCEs has been proposed. A fully automated assay has been developed using the major drug-metabolizing human hepatic cytochrome P450s (CYP1A2, -2C9, -2C19, -2D6, and -3A4) coexpressed functionally in E. coli with human NADPH-P450 reductase, to predict the CYP isoform(s) involved in the oxidative metabolism of NCEs.

	Materials and Methods

Top Abstract Introduction Materials and Methods Results Discussion References

Chemicals. All chemicals and reagents used were of the highest available commercial grade. Diltiazem, testosterone, dextromethorphan, (±)-propranolol, (±)-metoprolol, diazepam, tolbutamide, ibuprofen, and -nicotinamide adenine dinucleotide phosphate, reduced form (-NADPH) were purchased from Sigma Chemical Co. (Poole, UK). (±)-Verapamil was purchased from Aldrich Chemical Co. Ltd. (Gillingham, UK). Omeprazole was synthesized at AstraZeneca R&D Charnwood (Loughborough, UK).

Source of Cytochrome P450. The LINK consortium, a collaboration between UK-based academia and industry, provided stocks of transformed cells with human CYP1A2, CYP2C9, CYP2D6, and CYP3A4 individually coexpressed with human NADPH-P450 reductase in E. coli as described previously (McGinnity et al., 1999). All experiments with CYP1A2, CYP2C9, CYP2D6, and CYP3A4 utilized the E. coli membrane source. All transformed cells were stored as glycerol stocks at 80°C. Expression of the recombinant proteins and preparation of the respective E. coli membranes were carried out as described previously (McGinnity et al., 1999).

Probe Substrates. Ten commercially available drugs were selected as probe substrates to establish the suitability of this approach. The compounds were selected from the literature as marketed drugs for which the relative CYP-dependent metabolic formation was known and their metabolism by the five CYPs was adequately represented: tolbutamide (Back et al., 1988; Bourrie et al., 1996; Jung et al., 1997; Wester et al., 2000); diazepam (Ono et al., 1996); metoprolol (Otton et al., 1988; Mautz et al., 1995); ibuprofen (Hamman et al., 1997); propranolol (Otton et al., 1990; Yoshimoto et al., 1995); dextromethorphan (Dayer et al., 1989; Broly et al., 1990; Jacqz-Aigrain et al., 1993; Kerry et al., 1994; Von Moltke et al., 1998); omeprazole (Andersson et al., 1993; Kobayashi et al., 1994; Yamazaki et al., 1997); diltiazem (Pichard et al., 1990; Sutton et al., 1997); testosterone (Waxman et al., 1988; Wang et al., 1997); and verapamil (Kroemer et al., 1993; Tracy et al., 1999).

Automated CYP CL_int Determination. CYP CL_int determination assays were fully automated and performed by a robotic sample processor (RSP) (Genesis RSP 150; Tecan, Reading, UK). Assays performed by the RSP were done using a program created by the user and not by a default program supplied with the hardware.³ The primary stock of all probe substrates was prepared manually in dimethyl sulfoxide or acetonitrile at 100-fold final incubation concentration. The final concentration of organic solvent in the incubation was 1% v/v. At this concentration dimethyl sulfoxide has been shown to reduce the activities of CYP2C9/19 (Chauret et al., 1998; Hickman et al., 1998), although this effect appears to be substrate-dependent. All substrates were incubated at 3 µM except tolbutamide (CL_int calculated by determining V_max and K_m), ibuprofen (10 µM), and testosterone (10 µM). The RSP was programmed to add chilled quenching solvent (100 µl of acetonitrile) to 96-well refrigerated blocks, and compound stocks were then prediluted in 100 mM potassium phosphate buffer, pH 7.4. E. coli membranes and microsomes prepared from baculovirus coexpressing individual CYPs and NADPH-reductase were added to incubation tubes (100 pmol of CYP · ml¹ final concentration) located in a 96-well heated block (37°C). A subaliquot was removed to produce a 0-min time point, and the assay was initiated via addition of NADPH (1 mM final concentration). Aliquots (50 µl) were removed at 5, 10, 15, and 20 min and quenched in acetonitrile. Samples were subsequently removed from the RSP, frozen for 1 h at 20°C, and then centrifuged at 3500 rpm for 20 min. The supernatants were removed and transferred into HPLC vials using the RSP.

Automated Human Liver Microsome CL_int Determination. HLM were diluted in 100 mM potassium phosphate buffer, pH 7.4 (1 mg · ml¹ final). Probe substrates were incubated at identical concentrations as the CYP CL_int assay, and incubations were carried out on the RSP as described above. Reactions were again initiated by addition of NADPH (1 mM final concentration), and several aliquots were taken over 45 min.

HPLC Methods. Aliquots (20 µl) were analyzed by HPLC-UV or HPLC-fluorescence for either parent loss or metabolite appearance using a model 1100 Chemstation (Hewlett-Packard, Palo Alto, CA) and a Hewlett-Packard 1046A fluorescence detector. A symmetry shield RP8 3.9- × 50-mm cartridge (Waters, Watford, UK) and a mobile phase of 0.025% (w/v) ammonium acetate (solvent 1A) and acetonitrile (solvent 1B) was used for the chromatography of most analytes. Testosterone required a mobile phase of 0.025% ammonium acetate:methanol (95:5, v/v) (solvent 2A) and acetonitrile:methanol (95:5, v/v) (solvent 2B). The flow rate for all methods was 1.5 ml · min¹. Diazepam, metoprolol, propranolol, omeprazole, diltiazem, and verapamil eluted using a 5-min linear gradient from 80% solvent 1A to 20% solvent 1A, tolbutamide 99% to 65% over 5 min, dextromethorphan 80% to 20% over 3.5 min, and ibuprofen 85% to 20% over 5 min. Testosterone was eluted using a linear gradient from 85% solvent 2A to 75% solvent 2A over 12 min, 75% to 20% over 3 min followed by isocratic conditions (20:80) for 2 min. UV detection was performed for omeprazole (302 nm), diltiazem (237 nm), ibuprofen (222 nm), and testosterone (254 nm) and for metabolites of diazepam (229 nm) and tolbutamide (230 nm). Fluorometric detection was performed for metoprolol (Excitation 222 nm and Emission 320 nm), dextromethorphan (270, 312 nm), propranolol (205, 340 nm) and verapamil (280 nm, 310 nm).

Metabolite Identification. HLM or recombinant CYPs were diluted in 100 mM potassium phosphate buffer, pH 7.4 (1 mg · ml¹ or 100 pmol · ml¹, respectively). Probe substrates were incubated at 30 µM, reactions were initiated by addition of NADPH (1 mM), and aliquots were quenched in 1:1 (v/v) methanol at 0 and 45 min. Aliquots (20 µl) were analyzed by liquid chromatography-mass spectrometry using the Hewlett-Packard 1100 Chemstation with a symmetry shield RP8 3.9- × 50-mm cartridge and a mobile phase of 0.025% (w/v) ammonium acetate (solvent 3A) and methanol (solvent 3B). Analytes were eluted using a gradient of 95% solvent 3A to 10% solvent 3A over 7 min. Metabolites were detected using a TSQ 7000 mass spectrophotometer (Finnigan MAT, San Diego, CA) with an atmospheric pressure chemical ionization ion source and a triple quadrupole mass analyzer in full scan mode. The molecular ion (either M + H⁺ or M  H⁺ depending on the orifice polarity) was detected for each metabolite.

Data Analysis. Throughout this study, several approaches were adopted for quantifying intrinsic clearance:

Metabolite appearancelow turnover compounds.

if S K_m (10%)
so

Parent loss. Because dose/C₀ gives a term for the volume of the incubation (expressed in ml · pmol of CYP¹) and the elimination rate constant k = 0.693/T_1/2, an equation expressing CL_int in terms of T_1/2 of parent loss can be derived:
The contribution of individual CYP to HLM CL_int was estimated as follows:
Table 2 shows the average percentage content of the five major isoforms in human hepatic microsomes.

View this table: [in this window] [in a new window]   TABLE 2 Estimated levels of the five major isoforms in HLM pools Mean total level = 320 pmol of CYP · mg1 of microsomal protein.

	Results

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Marker Substrates. CL_int values were obtained for each of the prototypic substrates, tolbutamide, diazepam, metoprolol, ibuprofen, propranolol, dextromethorphan, omeprazole, diltiazem, testosterone, and verapamil in three individual preparations of pooled HLM. Table 3 compares the CL_int for the three batches of HLM together with the mean and values obtained from the literature. For substrates with significant CYP3A4 metabolism, diltiazem, testosterone, and verapamil, CL_int was significantly higher in batch 1 compared with batches 2 and 3. The coefficient of variation was generally <15% for CL_int determined from one pool of HLM.

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View larger version (14K): [in this window] [in a new window]   Fig. 1.   CYP1A2, -2C9, -2C19, -2D6, and -3A4-dependent clearance of propranolol. The determination of the CYP-dependent propranolol metabolism using the automated assay is as described under Materials and Methods. Aliquots were taken at 0, 5, 10, 15, and 20 min, and the amount of propranolol remaining in the incubation media is reflected by the peak area after HPLC-fluorescence detection. The data represent propranolol clearance by E. coli membranes expressing CYP1A2 (), CYP2C9 (), CYP2D6 (), CYP3A4 (), and baculovirus expressing CYP2C19 (), as described under Materials and Methods. The solid lines indicate linear regression of the data.

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View larger version (16K): [in this window] [in a new window]   Fig. 2.   Comparison of summed CYP CLint with HLM CLint for several probe substrates. Both the summed CYP and HLM CLint determinations were carried out as described under Materials and Methods. The data points represent the mean CLint determinations, and the error bars reflect the standard deviation from the mean as shown in Table 6. The dotted boxes illustrate HLM CLints of <8 µl · min1mg1 (low clearance) and >65 µl · min1mg1 (high clearance). The solid line depicts a linear regression analysis of the data (r2 = 0.8, P < .001) with the equation logCYP CLint = 0.91 × logHLM CLint + 0.3. The dashed line indicates line of unity.

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View larger version (11K): [in this window] [in a new window]   Fig. 3.   Determination of propranolol CLint at different concentrations of HLM. The HLM CLint determinations were carried out as described under Materials and Methods. The histograms reflect a mean CLint, and the error bars give the standard deviation from the mean. Experiments were carried out in duplicate a minimum of three times.

View larger version (27K): [in this window] [in a new window]   Fig. 4.   Metabolite profile for the CYP-dependent clearance of dextromethorphan. The identification of CYP-dependent dextromethorphan metabolite formation is as described under Materials and Methods. The data from this laboratory, where dextromethorphan was incubated at 30 µM, are compared with that of Von Moltke et al. (1998) where the data reflect CLint.

	Discussion

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To generate confidence that recombinant CYPs may be used to predict HLM CL_int data, the in vitro kinetics for commonly used CYP probes were determined previously in this laboratory in both pooled HLM and CYP coexpressed with NADPH-reductase in E. coli cells (McGinnity et al., 1999). The kinetic parameters (including CL_int) of these recombinant enzymes were similar to their human liver counterparts for the enzyme substrate pairs that were directly comparable, and thus they would appear to be faithful surrogates. Indeed, Eddershaw and Dickins (1999) reported an excellent comparison between the rates of metabolism of several compounds determined from HLM and microsomes containing a mixture of the major recombinant CYPs. However, this "artificial HLM" approach gives little information as to the enzymology of metabolism.

To demonstrate the potential for predicting both the extent and route of oxidative metabolic clearance for NCEs by recombinant human CYPs, several marketed drugs were selected in which metabolism via CYP pathways was well established. Of the marketed drugs that are primarily cleared by human hepatic CYP-mediated metabolism, the vast majority were metabolized by one or more of the five isoforms, CYP1A2, -2C9, -2C19, -2D6, and -3A4 (Bertz and Granneman, 1997), and, for that reason, only these isoforms were employed in this initial study. There are limited available data on the relative levels of the five major isoforms in human hepatic microsomes (Table 2), and we have relied heavily on the seminal study by Shimada et al. (1994), which is widely cited for this purpose. The marker compounds tolbutamide, diazepam, metoprolol, ibuprofen, propranolol, dextromethorphan, omeprazole, diltiazem, testosterone, and verapamil were chosen so that metabolism by each of the five CYPs was adequately represented. The choice of DMSO as a solvent was based on its value for compounds with relatively low solubility (often encountered in early drug discovery programs) and its implementation as the solvent of choice in many liquid banks. Any inhibitory effects should not affect the comparison between recombinant CYPs and HLM.

There is a reasonable agreement between the CL_int of the probe substrates determined in HLM to available literature values (Table 3), although the comparison is somewhat compromised due to the large spread of the literature data. There is agreement as to whether a compound demonstrates a low, intermediate, or high CL_int. The limitations of such an interlaboratory comparison and the inherent variability of such an exercise are well established (Boobis et al., 1998). Literature CL_int values have been obtained from a variety of sources, including V_max/K_m calculations, microsomes (prepared from individual as well as pooled livers), and hepatocytes [data converted to µl · min¹mg¹ assuming 2.67 × 10⁶ cells/mg of microsomal protein (Carlile et al., 1999)]. Variability will also result from the fact that isoform levels and activities may vary significantly between the different metabolizing sources (Boobis et al., 1998). However, the agreement is excellent where a direct comparison between two laboratories determining HLM CL_int for several compounds can be made (Obach, 1999).

Without exception, our data and the prevailing literature assign the same isoform to be the predominant CYP responsible for the metabolism of each marker compound. Diazepam at low micromolar concentrations was metabolized by CYP2C19, which agrees with Jung et al. (1997), Yasumori et al. (1993), and Andersson et al. (1994). Indeed, detailed HLM kinetics of diazepam metabolism (not shown) suggests the involvement of multiple CYPs (e.g., CYP2C9/18, -2B6, and -3A4), but the data indicate that the high affinity component of diazepam N-demethylation in vivo may be CYP2C19. Metoprolol and dextromethorphan are primarily CYP2D6 substrates (Otton et al., 1988; Dayer et al., 1989; Jacqz-Aigrain et al., 1993; Kerry et al., 1994; Von Moltke et al., 1998). Diltiazem, testosterone, and verapamil are predominantly metabolized by CYP3A4 (Waxman et al., 1988; Pichard et al., 1990; Kroemer et al., 1993; Sutton et al., 1997; Tracy et al., 1999).

In addition, for propranolol, omeprazole, tolbutamide, and ibuprofen, there was excellent concordance between our data and the literature on the relative contribution of several isoforms in the metabolism of the respective compounds. Tolbutamide is metabolized by both CYP2C9 (70%) and CYP2C19 (30%), which agrees with Inoue et al. (1997), Wester et al. (2000), Venkatakrishnan et al. (1998), and Lasker et al. (1998). Similarly, ibuprofen is metabolized by CYP2C9 (90%) and CYP2C19 (10%) (Leemann et al., 1993; Hamman et al., 1997) and omeprazole by CYP2C19 (68%) and CYP3A4 (32%) (Andersson et al., 1993; Karam et al., 1996; Yamazaki et al., 1997; Lasker et al., 1998). Indeed, when recombinant CYP2C19 and CYP3A4 were mixed at a ratio similar to that found in HLM, the metabolism of omeprazole resembled that of HLM (Yamazaki et al., 1997). The assignment of CYP2D6 (59%)-, CYP1A2 (26%)-, and CYP2C19 (15%)-dependent metabolism for propranolol agrees with several sources (Lennard et al., 1984; Otton et al., 1990; Yoshimoto et al., 1995), which implicate these three isoforms. In addition, the appropriate metabolites from each isoform were identified by mass spectrometry analysis.

A method based on the rate of enzyme activity (relative activity factor) of recombinant CYPs and HLM has proven useful in assigning the contribution of individual CYPs to several biotransformations (Rodrigues, 1999; Roy et al., 1999). Recently, it has been suggested that a complementary approach using the ratio of intrinsic clearance as a relative activity factor may be more predictive, where the kinetics for recombinant CYPs and HLM are equivalent (Nakajima et al., 1999). The correlation observed in this study between the sum of CL_int from the different CYP isoforms and HLM CL_int for the compounds tested confirms this concept. This study has additionally provided a more thorough evaluation of these recombinant proteins expressed in E. coli.

The summed CYP CL_int correctly predicted a low HLM CL_int (<8 µl · min¹mg¹) for tolbutamide, diazepam, and metoprolol; an intermediate HLM CL_int (8-65 µl · min¹mg¹) for ibuprofen, propranolol, dextromethorphan, diltiazem, and testosterone; and a high HLM CL_int (>65 µl · min¹mg¹) for verapamil. However, the summed CYP CL_int of omeprazole and propranolol did overpredict somewhat HLM CL_int. One possible explanation for this is an increase in "futile" binding with increased protein concentration for some compounds. For propranolol there is 50% free at 0.4 mg · ml¹ and 25% at 2 mg · ml¹ (Obach, 1997), which results in a 2-fold decrease of propranolol CL_int (Fig. 4). Typical assay conditions used 0.2 to 0.4 mg of protein/ml¹ of CYPs (exact amount depended on the CYP expression level, because all incubations contain 100 pmol of CYP/ml¹) and 1 mg · ml¹ HLM. The HLM CL_int of propranolol at 0.4 mg · ml¹ was determined to be 22 ± 4 µl · min¹mg¹, which compares more favorably with the summed CYP CL_int at the same protein level (55 ± 15 µl · min¹mg¹).

Generally, lower protein levels in the recombinant CYP assay may allow a more accurate reflection of unbound CL_int and provide a greater dynamic CL_int range when discriminating between large numbers of compounds. There is likely to be no significant differences between the extent of futile binding for HLM and recombinant CYPs at the same total protein concentration (Venkatakrishnan et al., 2000). Differential protein binding between in vitro matrices for predicting in vivo Cl_met is currently under investigation.

In our experience, an accurate determination of a wide range of CL_int is achieved at an incubation concentration for recombinant CYP of 100 pmol of CYP/ml¹, which may be subsequently optimized. The molar ratio of NADPH-P450 reductase to recombinant CYP has been manipulated for the E. coli expression constructs to produce optimal reaction kinetics for probe substrates (McGinnity et al., 1999). For example, optimal CYP2C19-mediated diazepam N-demethylation can be achieved, in the absence of cytochrome b₅, by increasing the molar ratio of NADPH-P450 reductase:CYP2C19 to approximately 20:1 (McGinnity et al., 1999). Indeed, to optimize CYP expression systems, further elucidation of the role and importance of ancillary electron transporters such as b₅ in the metabolism of xenobiotics is required (Yamazaki et al., 1999).

A correlation between HLM and CYP CL_int allows compounds to be ranked with respect to metabolic stability, should expedite knowledge of the pharmacophore of individual CYP isoforms, and may facilitate more rational compound synthesis to achieve greater metabolic stability. Furthermore, an underprediction of HLM CL_int from the five major human hepatic isoforms should prompt an investigation into possible metabolism by the more minor human hepatic CYPs, i.e., CYP2A6, -2B6, -2C8, or -2E1 (Houston, 1994).

This automated assay is being used early in drug discovery at AstraZeneca R&D Charnwood, a strategy distinct from the comprehensive isoform profiling of a drug later in the development process by other groups (Machinist et al., 1998; Fischer et al., 1999; Nakajima et al., 1999; Roy et al., 1999). The early identification of the major CYP isoforms involved in the metabolism of a drug candidate is useful for several purposes, including understanding ligand-enzyme structure-activity relationships, expanding the database for substrates of the polymorphic isoforms, assessing the potential intersubject variability, and predicting the drug-drug interactions and, ultimately, the direction of clinical trials.

These data indicate that recombinant CYPs may be used to predict HLM CL_int. Furthermore, it may prove feasible to scale human CL_int data to the fractional metabolic clearance encountered clinically (Iwatsubo et al., 1997; Obach, 1999). Therefore, although very much in its infancy, data in this report demonstrate that E. coli-expressed CYPs may be useful as an early approach for the prediction of the enzymology of human CYP metabolism. Further efforts to examine the differential nonspecific binding between the separate in vitro models and the effects on CL_int are underway.