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

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.

Inhibition of CYP2D6 activity by treatment with propranolol and the role of 4-hydroxy propranolol

1. The 4-hydroxylation of propranolol by rat and human liver microsomes is associated with formation of a chemically reactive species which binds irreversibly to cytochrome P4502D6 (CYP2D6) destroying its catalytic function. Therefore, the effect of propranolol treatment (80 mg twice daily) on debrisoquine phenotype was examined, to see if it resulted in phenocopying in vivo. The role of 4-hydroxypropranolol (4OHP) in the inhibition of CYP2D6 activity was also studied using microsomes from yeast expressing CYP2D6 and from human livers; metoprolol was used as the CYP2D6 substrate. 2. Although a significant effect on apparent oxidation phenotype was demonstrated, the absolute change in the urinary debrisoquine/4-hydroxydebrisoquine ratio (D/4HD) was small, such that no extensive metaboliser who received propranolol treatment was reclassified as a poor metaboliser. The in vitro studies indicated that 4OHP is a potent inhibitor of metoprolol metabolism (Ki approximately 1 microM). This inhibitory effect was enhanced when 4OHP was pre-incubated in the presence of a NADPH generating system and human liver microsomes. The effect was decreased significantly when reduced glutathione was added to the pre-incubation mixture. Metabolism of 4OHP occurred when incubated with human liver microsomes in the presence of a NADPH generating system and irrespective of CYP2D6 phenotype; yeast expressing CYP2D6 did not metabolise 4OHP. 3. We conclude that, although treatment with propranolol 80 mg twice daily significantly decreases the catalytic function of CYP2D6, the inhibition is insufficient to result in phenocopying. The reactive intermediate produced by further metabolism of 4OHP is probably scavenged effectively in vivo by glutathione and other nucleophiles.

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)

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.

Catalytic activities of human debrisoquine 4-hydroxylase cytochrome P450 (CYP2D6) expressed in yeast

A 1.57kb BamH1 fragment containing a full-length human debrisoquine 4-hydroxylase cytochrome P450 (CYP2D6) cDNA was inserted into the BglII site of the yeast expression plasmid pMA91 and the resulting recombinant plasmid, PELT1, introduced into Saccharomyces cerevisiae strain AH22. Microsomes prepared from AH22/pELT1 cells gave an absorption maximum at 448 nm and a P450 content of 67 +/- 31 pmol/mg of microsomal protein. No P450 was detectable in microsomes prepared from AH22/pMA91 control cells. A western blot of microsomes prepared from yeast transformed with pELT1 were probed with a monoclonal antibody to CYP2D6 and revealed a strong band with a molecular mass consistent with that of CYP2D6 from human liver microsomes. No corresponding band was observed with microsomes from control yeast transformed with pMA91 alone. Microsomes from AH22/pELT cells showed catalytic activity towards metoprolol (alpha-hydroxylation and O-demethylation, 0.17 and 0.78 nmol/mg protein/h, respectively); and towards sparteine (2- and 5-dehydrogenation, 1.82 and 0.59 nmol/mg protein/h, respectively). The inhibition of metoprolol metabolism by quinidine (Qd) was 200 times more potent than that of quinine (Qn), both for alpha-hydroxylation (Qd IC50 = 0.05 microM; Qn IC50 = 4 microM) and O-demethylation (Qd IC50 = 0.05 microM; Qn IC50 = 4 microM). Negligible metabolism of tolbutamide and S-mephenytoin, substrates of the 2C sub-family, and of p-nitrophenol, a substrate of CYP2E1, was detected, although a trace of the N-deethylated metabolite of lignocaine, thought to be metabolised by CYP3A4, was detected with microsomes from CYP2D6-expressing yeast cells. The results indicate that yeast cells containing human CYP2D6 cDNA express a functionally active form of the enzyme, the immunochemical and catalytic properties of which are consistent with those of human liver.

A possible mechanism of the impairment of hepatic microsomal monooxygenase activities after multiple administration of propranolol in rats

The mechanism of selective inhibition of propranolol hydroxylations after multiple administration of the drug was investigated by metabolic inhibition studies in rat liver microsomes. The time course of irreversible binding of a reactive metabolic intermediate(s) of propranolol to liver microsomal protein, which was proposed as the cause of the impairment of enzymatic activities, had a delayed phase followed by a rapid linear rise, while the unmetabolized propranolol remaining in the reaction mixture showed a rapid linear decrease immediately after the onset of incubation. Thus, it was conceivable that the reactive intermediate(s) was not always formed directly from the parent drug, propranolol. Among four primary metabolites of propranolol, 4-hydroxypropranolol was the most potent inhibitor of propranolol hydroxylase activities, and this inhibition was much enhanced by preincubation of 4-hydroxypropranolol with NADPH. The type of inhibition kinetics of propranolol 5- and 7-hydroxylase activities by 4-hydroxypropranolol was changed from a competitive type to a non-competitive type by the preincubation. These results suggest that a reactive metabolite(s) of propranolol which impaired propranolol hydroxylase activities is a further metabolite(s) of 4-hydroxypropranolol.

Activation of propranolol and irreversible binding to rat liver microsomes: strain differences and effects of inhibitors

In summary, strain difference and inhibition studies showed that an enzyme(s) converting propranolol to a reactive metabolite capable of irreversible binding to microsomal macromolecules appeared to be a P450 isozyme(s) which catalyses debrisoquine 4-hydroxylation in rats. It seems likely that cytochrome P450 isozymes responsible for debrisoquine 4-hydroxylation activate propranolol and may be impaired after chronic use of propranolol also in human subjects. The findings obtained in the present study provide a clue for the elucidation of the mechanism of propranolol-induced impairment of the drug metabolizing enzyme system. Further studies using purified debrisoquine 4-hydroxylase are required to identify a P450 isozyme(s) responsible for the metabolic activation of propranolol. We are now performing experiments along this line.

The role of lipophilicity in the inhibition of polymorphic cytochrome P450IID6 oxidation by beta-blocking agents in vitro

The importance of lipophilicity as a determinant of the affinity of beta-adrenoceptor blocking agents for a specific human hepatic monooxygenase--cytochrome P450IID6 (responsible for the debrisoquine-type of oxidation polymorphism)--was investigated in vitro by estimating the inhibition constants of a series of compounds in a microsomal system with monitoring of the kinetics of dextromethorphan O-demethylation. Lipophilicity is a key predictor of the affinity of beta-blocking drugs for cytochrome P450IID6 and of their potential to cause specific competitive drug interactions, but more complex structural factors appear to be important as well. A high lipophilicity is also a necessary, but not a sufficient condition for these compounds to be metabolized by cytochrome P450IID6.