The expression of human cytochrome P450IA1 in the yeast Saccharomyces cerevisiae

Data from animal studies suggest that cytochrome P450IA1 catalyses the metabolic activation of several procarcinogenic compounds. In the present study, we have expressed human cytochrome P450IA1 in yeast cells. A 1.70 kb BclI/BamHI fragment containing a full-length human cytochrome P450IA1 cDNA was inserted into the BglII expression site of the yeast expression plasmid pMA91 thereby allowing the ATG initiation codon to be located adjacent to the PGK (phosphoglycerate kinase) promoter. The resulting recombinant plasmid, pCK-1, was introduced into Saccharomyces cerevisiae strains ATCC 44773 and AH22. Microsomes prepared from yeast transformatants of strain ATCC 44773 contained undetectable levels of cytochrome P450. In contrast, microsomes from strain AH22 contained cytochrome P450 with a specific content of 33.3 +/- 10.8 pmol/mg of microsomal protein and showed a reduced carbon monoxide difference spectrum with a peak at 448 nm. Control yeast cells transformed with pMA91 showed no cytochrome P450. Western blots were carried out using an antibody that reacts against rat cytochrome P450IA1 and an antibody that reacts against a synthetic peptide representing a short sequence of human cytochrome P450IA1. A band with a molecular weight of 54 kD was observed in microsomes of yeast transformed with pCK-1, but not with pMA91. When microsomes from yeast transformed with pCK-1 were incubated with benzo(a)pyrene (10 min, 10-160 microM), an estimated Km value of 7 microM was obtained. The availability of yeast cells with functionally active human cytochrome P450IA1 will facilitate molecular structure-activity studies of procarcinogen and drug metabolism by this enzyme in man.

Plasma concentrations of bupivacaine after supraclavicular brachial plexus blockade in patients with chronic renal failure

The plasma concentrations of bupivacaine and the latency and duration of anaesthesia after supraclavicular block with 30 ml of 0.5% bupivacaine were measured in 10 patients with chronic renal failure and in 10 patients with normal renal function. No significant difference was found between the two groups in respect of pharmacokinetic parameters, or in block latency or duration.

Characterization of metronidazole metabolism by human liver microsomes

The metabolism of metronidazole was studied in microsomes isolated from livers of human kidney donors. The formation of the major in vivo metabolite, hydroxymetronidazole, proceeded according to biphasic kinetics, suggesting the involvement of at least two enzymatic sites. The affinity constant (Km) of the high affinity site ranged from 140 to 320 microM and metabolism at this site contributed more than 75% of the intrinsic clearance. Thus, at therapeutic doses of metronidazole most of the hydroxylation in vivo should be associated with this site. Antipyrine, cimetidine, alpha-naphthoflavone, caffeine, theophylline, mephenytoin, tolbutamide, quinidine, acetone and nifedipine were poor inhibitors of the formation of hydroxymetronidazole by human liver microsomes. Propranolol (500 microM) inhibited the hydroxylation rate by 70%. Phenacetin inhibited metronidazole hydroxylation with a competitive inhibition constant (Ki) of 4-5 microM. However, metronidazole did not inhibit the O-deethylation of phenacetin. It is concluded that cytochromes P450 IA2, IIC9, IIC10, IID6, IIE1 and IIIA3 do not contribute significantly to the high affinity hydroxylation of metronidazole in man.

Influence of debrisoquine oxidation phenotype on exercise tolerance and subjective fatigue after metoprolol and atenolol in healthy subjects

1. The effects of single doses of metoprolol 50 mg, metoprolol 100 mg and atenolol 100 mg on exercise tolerance were compared with placebo in a double-blind random cross-over study in 12 healthy subjects. Nine subjects were extensive metabolisers of debrisoquine, and three were poor metabolisers. 2. Three hours after dosing beta-adrenoceptor blocker treatments significantly reduced exercise heart rate, prolonged time to complete exercise, and increased subjective fatigue measured by visual analogue scale. 3. Scores for subjective fatigue did not correlate with reduction in exercise heart rate or prolongation of exercise time. Exercise time prolongation was weakly but not significantly correlated with exercise heart rate reduction. 4. When compared with placebo, prolongation of exercise time and increased fatigue with metoprolol were not significantly related to debrisoquine oxidation phenotype or to the debrisoquine/4-hydroxydebrisoquine (D/4OH-D) ratio. 5. When metoprolol responses were compared with those for atenolol, changes in exercise time and fatigue scores were significantly related to oxidation phenotype. For metoprolol 100 mg, poor metabolisers required 20.8 s longer to complete exercise (P less than 0.05) and had higher fatigue scores by 78% (P less than 0.05) as compared with extensive metabolisers.(ABSTRACT TRUNCATED AT 400 WORDS)

Indocyanine green elimination in patients with liver disease and in normal subjects

1. The validity of a two-compartment pharmacokinetic model for the estimation of the hepatic extraction ratio of Indocyanine Green was tested in six patients with cirrhosis of the liver. 2. No agreement was found between the value of the hepatic extraction ratio measured directly and that calculated using the two-compartment model. 3. To investigate the reasons for the failure of the model, an extended sampling period was used to define the time course of Indocyanine Green in plasma in six healthy subjects and in six patients with cirrhosis of the liver after a bolus injection of the dye. 4. Indocyanine Green was measurable in the plasma for up to 10 h after injection in healthy subjects, and up to 48 h after injection in the patients. The plasma elimination curve in both groups was best described by a triexponential function. 5. The clearance of Indocyanine Green calculated using data collected in the first 20 min after injection overestimated that calculated using data collected for as long as Indocyanine Green was measurable in the plasma. In the patients with cirrhosis the mean overestimate was 87%. 6. Thus, a two-compartment pharmacokinetic model was inappropriate for the description of the disposition of Indocyanine Green and estimates of the hepatic extraction ratio obtained using this model in patients with cirrhosis were inaccurate.

Backtracking booze with Bayes--the retrospective interpretation of blood alcohol data

1. A Bayesian method is described which allows the explicit estimation of errors produced in estimating drug concentrations at times for which samples are not available for analysis. 2. This method was applied to the problem of 'backtracking' alcohol concentrations for medico-legal purposes. 3. Computer simulation allowed the effect of continuing alcohol absorption on the position and range of estimates of alcohol concentrations to be studied.

Pharmacokinetic-pharmacogenetic modelling in the detection of polymorphisms in xenobiotic metabolism

Study of the genetic control of xenobiotic metabolism is hindered in the areas of detecting new polymorphisms and estimating the frequency and enzyme activity of each phenotype. Using computer simulation we have looked at pharmacogenetic-pharmacokinetic models based on two alleles under Hardy-Weinberg equilibrium. The distributions of the area under the concentration-time curve (AUC) or urinary ratios were modelled and the effects of incomplete urine collections, sequential, parallel and non-linear pathways investigated. The statistical methods for the detection of bimodality in these distributions were explored. The drug/metabolite ratio, which has a good theoretical basis, was confirmed to be most sensitive and robust to changes in bioavailability, urinary excretion, hepatic blood flow and variation in non-polymorphic enzyme activity but not parallel, sequential or non-linear routes of metabolism. Graphical methods, while able to illustrate deviations from normality, were not specific in detecting bimodality and the hypothesis testing methods were found to be heavily dependent upon their assumptions.

Propranolol oxidation by human liver microsomes--the use of cumene hydroperoxide to probe isoenzyme specificity and regio- and stereoselectivity

1. Three oxidations of the enantiomers of propranolol were studied in human liver microsomes under two reaction conditions. Previous in vitro studies had established that two of the livers were from poor metaboliser (PM) phenotypes for the debrisoquine 4-hydroxylase (cytochrome P-450IID6) and the remaining seven were from extensive metaboliser (EM) phenotypes. 2. In the presence of NADPH and oxygen 4- and 5-hydroxylation of propranolol occurred in microsomes from all nine livers, as did propranolol N-desisopropylation. R(+)-propranolol was oxidized preferentially along the three pathways, although enantioselectivity observed for N-desisopropylation may have arisen not only from stereoselectivity in formation rates, but also from stereoselectivity in subsequent microsomal metabolism, possibly by monoamine oxidase. The involvement of monoamine oxidase in the further microsomal metabolism of N-desisopropylpropranolol was indicated by inhibition of the metabolism of this compound when incubated with phenelzine. 3. Cumene hydroperoxide has been proposed to support only the activity of cytochrome P450IID6. This is consistent with the observations that a) propranolol 4- and 5-hydroxylation occurred in microsomes from the EM livers only and b) side-chain oxidation was not observed under these conditions in either PM or EM livers. 4. Using cumene hydroperoxide to support the reactions, the 4-hydroxylation of propranolol showed little enantioselectivity, whereas S(-)-propranolol was 5-hydroxylated about twice as fast as the R(+)-enantiomer. There were highly significant correlations between the rates of 4- and 5-hydroxylation of R(+)-propranolol (r = 0.96, P less than 0.001, n = 7 livers) and of S(-)-propranolol (r = 0.98, P less than 0.001). Both oxidations were described by single-site Michaelis-Menten kinetics. 5. The findings suggest that P450IID6 is involved in both the 4- and 5-hydroxylations of propranolol, but that these metabolites can also be formed by other P450 isoenzymes. It is confirmed that P450IID6 does not contribute to the N-desisopropylation of propranolol. Furthermore, the finding that mephenytoin did not inhibit the appearance of this metabolite is not consistent with the results of in vivo studies suggesting the involvement of the same enzyme in the side-chain oxidation of propranolol and the 4-hydroxylation of mephenytoin.

Plasma concentrations of the stereoisomers of prilocaine after administration of the racemate: implications for toxicity?

A chiral high pressure liquid chromatography method was developed to measure the separate isomers of prilocaine in plasma after administration of the racemate. The concentrations of the isomers in six patients were similar (S(+)/R(-) = 1.06 (SD 0.06)) after brachial plexus block with 1.5% (RS)-prilocaine hydrochloride 35 ml, suggesting that a higher systemic safety margin may not be achieved by substituting racemic prilocaine by one of its isomers. Much higher plasma concentrations of the S(+)- than the R(-)-form after oral administration of 300 mg of the racemate (n = 4) indicated a large difference in intrinsic metabolic clearance of the isomers on first pass through gut, liver or both organs.

Inhibition of metoprolol metabolism by chloroquine and other antimalarial drugs

The ability of a series of antimalarial drugs to impair the metabolism of metoprolol in rat and man has been examined. Chloroquine was a potent inhibitor in rat liver microsomes (Ki value for metoprolol alpha-hydroxylation = 0.18 microM and for O-demethylation = 0.36 microM). The other antimalarial drugs also inhibited metoprolol oxidation. Quinine was similar to chloroquine in potency, while quinidine, primaquine and mefloquine were slightly less potent. Chloroquine also inhibited metoprolol oxidation in human liver microsomes, although it was about two orders of magnitude less potent than in the rat and the extent of impairment varied greatly between individual livers. Intraperitoneal administration of chloroquine to anaesthetized rats decreased the clearance of metoprolol (40 mg tartrate salt kg-1 i.p.) to 54, 34, 20 and 26% of the control value at doses of 2.5, 4.0, 25 and 40 mg kg-1, respectively. We conclude that antimalarial treatment might have contributed to a previously reported difference in the metabolic pattern of metoprolol between Caucasians and Nigerians.

Disposition kinetics of ropivacaine in humans

The pharmacokinetic characteristics of a new local anesthetic drug, ropivacaine, were determined after intravenous infusion of 50 mg of the hydrochloride salt into six healthy male volunteers. Results showed that the disposition of ropivacaine can be described by a biexponential function. Its blood clearance (0.72 +/- 0.16 L/min) is intermediate between that of mepivacaine and bupivacaine. Plasma binding averaged 94% +/- 1% and the volume of distribution at steady state based on blood drug concentration was 59 +/- 7 L. The terminal elimination half-life was 111 +/- 62 min.

Testing for bimodality in frequency distributions of data suggesting polymorphisms of drug metabolism--histograms and probit plots

1. The shape of histograms used to illustrate density distributions of indices of polymorphic drug metabolism was shown to be sensitive to the position of the cell divisions. 2. Non-linearity of the probit plot was shown not to indicate bimodality of the original density distribution. Computer simulation was used to generate examples of unimodal density distributions with curvilinear probit plots. 3. Using the same technique probit plots for bimodal density distributions were constructed. Some were shown to differ less from the probit plots of certain unimodal distributions than did the original density distributions. 4. The position of the antimode was shown not to coincide with inflections seen in the probit plots. 5. A new method for determining the linearity of probit plots is suggested.

Testing for bimodality in frequency distributions of data suggesting polymorphisms of drug metabolism--hypothesis testing

1. The theory of methods of hypothesis testing in relation to the detection of bimodality in density distributions is discussed. 2. Practical problems arising from these methods are outlined. 3. The power of three methods of hypothesis testing was compared using simulated data from bimodal distributions with varying separation between components. None of the methods could determine bimodality until the separation between components was 2 standard deviation units and could only do so reliably (greater than 90%) when the separation was as great as 4-6 standard deviation units. 4. The robustness of a parametric and a non-parametric method of hypothesis testing was compared using simulated unimodal distributions known to deviate markedly from normality. Both methods had a high frequency of falsely indicating bimodality with distributions where the components had markedly differing variances. 5. A further test of robustness using power transformation of data from a normal distribution showed that the algorithms could accurately determine unimodality only when the skew of the distribution was in the range 0-1.45.

Inhibition of lignocaine metabolism by beta-adrenoceptor antagonists in rat and human liver microsomes

1. The inhibition of lignocaine metabolism by beta-adrenoceptor antagonists (beta-blockers) was investigated in rat and human liver microsomes. 2. Thirteen beta-blockers (concn. 50 microM) incubated with substrate (4.27 microM) and rat liver microsomes, showed a strong linear correlation between percentage inhibition of lignocaine metabolism and the distribution coefficients of the beta-blockers (r2 = 0.842, P less than 0.001). Similar results for four beta-blockers were obtained using human liver microsomes. 3. In rat liver, which metabolizes lignocaine by aromatic hydroxylation and N-dealkylation, inhibition was selective for the former route. Human liver microsomes metabolize the drug mainly by N-dealkylation and inhibition of this pathway was observed. 4. Liver microsomes from rats treated orally with beta-blockers (0.34 nmol kg per day for 5 days) showed impaired metabolism of lignocaine and impaired formation of 3-hydroxy-lignocaine, despite the absence of significant residues of beta-blocker. 5. 14C-Propanolol was bound irreversibly to rat liver microsomal protein; binding accounted for 4.1 +/- 0.3% (n = 4) dose after 30 min incubation. Exclusion of co-factors and addition of glutathione (GSH, 1 mM) lowered binding by 96% and 70%, respectively. Propanolol inhibited lignocaine metabolism to the same extent in the presence or absence of GSH. The 14C-propanolol bound to liver microsomes from propranolol-treated rats decreased in parallel with inhibition of lignocaine metabolism at 18 to 48 h after pretreatment. 6. These studies indicate at least two mechanisms for the inhibition of lignocaine metabolism by beta-blockers, namely, a 'lipid solubility hypothesis', where the effects may be related to the unchanged drug and a 'metabolite hypothesis', with the possible involvement of an irreversibly bound species.

Histamine inhibition of mixed function oxidase activity in rat and human liver microsomes and in the isolated perfused rat liver

The imidazole ring is a common structural feature of some xenobiotics that inhibit cytochrome P-450-catalysed reactions. Histamine is a 4-substituted imidazole and a preliminary study has shown it to be an inhibitor of rat liver microsomal drug oxidation. This work has now been extended. Histamine appears to be a competitive inhibitor of the alpha-hydroxylation (HM) (Ki = 164 microM; IC50 at 20 microM = 308 microM) and O-demethylation (ODM) (Ki = 243 microns; IC50 at 20 microM = 400 microM) of metoprolol in rat liver microsomes. Of the metabolites of histamine only N-acetylhistamine showed comparable inhibitory potency to that of the parent compound. Histamine impaired the disappearance of lignocaine when incubated with rat liver microsomes. This was accompanied by a corresponding inhibition of 3-hydroxy-lignocaine appearance. Histamine produced a type II spectral interaction with rat liver microsomes (lambda max = 432 nm, lambda min = 408 nm; Ks = 0.11 mM). When histamine was incubated alone with rat liver microsomes no loss of substrate was observed. The oxidation of metoprolol by human liver microsomes was impaired by histamine (IC50 values for ODM appearance at 25 microM: liver HL1 greater than 10, HL3 = 3.8 and HL4 = 3.7 mM). In comparison, cimetidine had an IC50 value of 1.5 mM using microsomes from liver HL3. Addition of histamine impaired the elimination of metoprolol by the isolated perfused rat liver in a dose-dependent manner (P less than 0.001, one-way analysis of variance). These data demonstrate that histamine can enter hepatocytes, interact with cytochrome P-450 and inhibit some drug oxidation reactions. The physiological relevance of inhibition of drug metabolism by histamine remains to be determined.

Direct and indirect measurement of the hepatic extraction ratio of indocyanine green in the rat

1. In order to estimate liver blood flow in the rat, the extraction ratio of Indocyanine Green was determined using a two-compartment model fitted to the plasma concentration time data after a single intravenous bolus dose and compared with values obtained directly by transhepatic sampling, both in the intact rat and in an isolated perfused rat liver preparation. 2. There was no agreement between estimates of the extraction ratio obtained by using the kinetic model and the directly measured values. 3. Elimination curves for Indocyanine Green were simulated to yield varied clearance values. Despite a 250% variation in clearance, extraction ratios derived using the two-compartment model were all greater than 0.9 and varied by less than 6%. 4. Estimates of liver blood flow obtained by deriving a value of the extraction ratio of Indocyanine Green using the two-compartment model are inaccurate.

Stereoselective metabolism of metoprolol in Caucasians and Nigerians--relationship to debrisoquine oxidation phenotype

The relationship between debrisoquine oxidation phenotype and the stereoselective metabolism of metoprolol was investigated in populations of British Caucasians (n = 139) and Nigerian subjects (n = 117). The 0-8 h urinary S/R-metoprolol (S/R-M) ratio was related to the ability to metabolise metoprolol and debrisoquine in both ethnic groups. The median S/R-M ratio was significantly higher in Caucasians (1.27) than in Nigerians (1.10). In the Caucasian population poor metabolisers of debrisoquine had significantly lower S/R-M ratio (median = 0.84) than extensive metabolisers (median = 1.28). Bimodality in the frequency distribution of the S/R-M ratio was not apparent in either ethnic group.

Timolol metabolism and debrisoquine oxidation polymorphism: a population study

1. The metabolism of orally administered timolol (T) to its ring cleavage ethanolamine (TE) and glycine (TG) products was studied in 108 unrelated hypertensive patients. 2. Statistically significant correlations between the 0-8 h urinary debrisoquine/4-hydroxy-debrisoquine ratio and the T/TE (rs = 0.74, P less than 0.001), T/TG (rs = 0.42, P less than 0.001) and T/TE + TG (rs = 0.49, P less than 0.001) ratios were found. 3. The log10 T/TE, T/TG and T/TE + TG ratios from poor metabolisers of debrisoquine (PMs) were grouped at the upper end of a unimodal distribution. 4. These results indicate that timolol metabolism is partly under monogenic control of the debrisoquine-type. 5. The mean +/- s.d. plasma timolol concentration in PMs (82 +/- 43 ng ml-1) was double that in extensive metabolisers (45 +/- 19 ng ml-1) (P = 0.011). The clinical significance of this observation remains to be established.

Use of quinidine inhibition to define the role of the sparteine/debrisoquine cytochrome P450 in metoprolol oxidation by human liver microsomes

The oxidation of the beta adrenoceptor antagonist metoprolol exhibits genetic polymorphism of the sparteine/debrisoquine (SP/DB) type. The alpha-hydroxylation of metoprolol is absent in poor metabolizers, whereas metoprolol O-demethylation is only partially impaired, suggesting that an enzyme or enzymes other than cytochrome P450-SP/DB contribute to the latter reaction. Using inhibition by the quinidine/quinine isomer pair as a marker for the activity of cytochrome P450-SP/DB, the role of this enzyme in the in vitro oxidation of the enantiomers of metoprolol by human liver microsomes was examined. Unlike alpha-hydroxylation, only a portion of metoprolol O-demethylation showed the stereoselective inhibition by quinidine and quinine characteristic of in vitro reactions catalyzed by cytochrome P450-SP/DB. Furthermore, the kinetics of metoprolol O-demethylation were biphasic, the two components of O-demethylase activity being distinguishable by their enantioselectivity and sensitivity to inhibition by quinidine. Microsomes from one liver formed no detectable alpha-hydroxymetoprolol, and O-demethylation by these microsomes corresponded to the low affinity site observed in eight other livers. The rate of metoprolol O-demethylation by the quinidine-inhibitable high affinity component was directly proportional to the rate of alpha-hydroxylation. These findings support the hypothesis that cytochrome P450-SP/DB catalyzes the formation of alpha-hydroxymetoprolol, but is only partially responsible for metoprolol O-demethylation. Such a mechanism could explain the previously reported inability to detect polymorphism in the O-demethylation pathway in vivo.

The effect of combined therapy on the pharmacokinetics and pharmacodynamics of verapamil and propranolol in patients with angina pectoris

1. The pharmacokinetics and pharmacodynamics of oral verapamil and propranolol were studied in patients with stable angina pectoris during chronic mono- and dual therapy. 2. The peak plasma concentrations (Cmax) and areas under the plasma concentration-time curves (AUC) of verapamil were similar during combined treatment with propranolol (mean +/- s.d.: Cmax = 491 +/- 397 ng ml-1; AUC = 2075 +/- 1524 ng ml-1 h) or atenolol (mean +/- s.d.: Cmax = 372 +/- 320 ng ml-1; AUC = 1985 +/- 1660 ng ml-1 h). 3. No differences in Cmax and AUC were observed during verapamil monotherapy (mean +/- s.d.: Cmax = 287 +/- 105 ng ml-1; AUC = 1375 +/- 455 ng ml-1 h) vs combined treatment with propranolol (mean +/- s.d.: Cmax = 312 +/- 55 ng ml-1; AUC = 1566 +/- 486 ng ml-1 h). 4. Treatment with verapamil increased the Cmax (mean +/- s.d.: 227 +/- 117 vs 116 +/- 62 ng ml-1, P less than 0.05) and AUC (1389 +/- 617 vs 837 +/- 316 ng ml-1 h, P = 0.0625) of propranolol in all subjects. 5. Transient atrioventricular dissociation occurred in two patients 2 h after dosing with verapamil and propranolol or atenolol. 6. Close observation of patients is essential when beta-adrenoceptor antagonists and verapamil are used together.