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J F Tjia

Publications and source records attributed to J F Tjia.

17 recordsLinked to original sources
PubMed1996

Theophylline metabolism in human liver microsomes: inhibition studies.

In this paper we describe the kinetics of formation of 1-methylxanthine (1-MX), 3-methylxanthine (3-MX) and 1,3-dimethyluric acid (1,3-DMU) from theophylline in human liver microsomal incubations and use the selective inhibitor approach to define the role of the individual cytochrome P450s (CYP) in each pathway. A biphasic model fitted the data best for the formation of each metabolite. The high-affinity site Km and Vmax values were: 1-MX, Km = 0.29 +/- 0.21 mM, Vmax = 5.92 +/- 3.74 pmol.mg(-1).min(-1) (mean +/- S.D.; n = 4); 3-MX, Km = 0.28 +/- 0.08 mM, Vmax = 3.32 +/- 2.19 pmol.mg(-1).min(-1); 1,3-DMU,Km = 0.31 +/- 0.14 mM, Vmax = 43.3 +/- 9.3 pmol.mg(-1).min(-1). The relative contribution of the high- and the low-affinity enzymes in 1,3-DMU formation was calculated based on the enzyme kinetic parameters. To characterize the high-affinity site, a range of CYP isozyme substrates and inhibitors were incubated with 100 microM theophylline. The CYP1A2 inhibitors furafylline, ellipticine and alpha-naphthoflavone were potent inhibitors of both 1-MX and 3-MX formation with more that 80% of N-demethylase activities inhibited below a concentration of 5 microM. These compounds also markedly inhibited 1,3-DMU formation. Enzyme kinetic and selective inhibition data indicated that about 80% of 1,3-DMU formation was catalyzed by the high-affinity isoform (CYP1A2) at a theophylline concentration of 100 microM. To investigate the role of other isoforms in 8-hydroxylation, experiments were performed involving incubation with a combination of inhibitors. It is evident that in addition to CYP1A2, CYP2E1 has a minor role om 8-hydroxylation. This based on the fact that 80% inhibition was seen on preincubation with furafylline and about 90% inhibition on preincubation with furafylline plus diethyldithiocarbamate. Low concentrations of ketoconazole (selective for CYP3A4) only produced marginal inhibition of 1,3-DMU and, therefore, CYP3A4 is only of minor significance in this reaction. Human B-lymphoblastoid cell lines expressing CYP1A2 catalyzed theophylline metabolism with formation of 1-MX, 3-MX and 1,3-MDU. CYP2E1 cells also catalyzed formation of 1,3-DMU. The CYP3A4 cell line did not catalyze theophylline metabolism.

Cells, Cultured↗
PubMed1992

The in-vitro mucosal conjugation of ethinyloestradiol and the bioavailability of oral contraceptive steroids in patients with treated and untreated coeliac disease.

The ethinyloestradiol (EO2) component of oral contraceptive steroids is extensively conjugated with sulphate by the gut wall. The ability of gastrointestinal mucosa to conjugate EO2 has been examined in vitro in samples of mucosa taken from normal women as well as from women with coeliac disease. The percentage conjugation per mg dry weight for normal tissue (n = 11) was 17.1 +/- 6.4 (mean +/- s.d.) while in untreated coeliac tissue (n = 6) the figure was 6.3 +/- 3.6% (P less than 0.01). In tissue from patients with treated coeliac disease (n = 5) the figure was 12.1 +/- 3.2%. Thus the ability of intestinal mucosa to conjugate ethinyloestradiol was significantly reduced in patients with coeliac disease, and restored towards normal following treatment. However, in patients with coeliac disease the pharmacokinetics of ethinyloestradiol were not significantly different from normal controls.

Administration, Oral↗
PubMed1992

Metabolism of 2',3'-dideoxyinosine (ddI) in human blood.

1. 2',3'-Dideoxyinosine (ddI) has potent activity against human immunodeficiency virus (HIV). It is converted within target cells to its active form dideoxyadenosine triphosphate(ddA-TP). 2. In addition to the intracellular formation of ddA-TP, ddI can be broken down to hypoxanthine, by purine nucleoside phosphorylase (PNP) and to uric acid, by xanthine oxidase. Since PNP is present in red blood cells we have examined the metabolism of [14C]-ddI by human blood. 3. When incubated with whole blood at 37 degrees C, ddI was extensively metabolised, principally to hypoxanthine (50.4 +/- 12.5% formed at 6 h; mean +/- s.d.; n = 16). Small amounts of uric acid were formed (3.8 +/- 2.4%). 4. ddI breakdown was temperature dependent, being virtually negligible at 4 degrees C. Metabolism to hypoxanthine occurred within red blood cells. 5. The short half-life of ddI in patients is probably the result of both hepatic and erythrocytic metabolism.

Chromatography, High Pressure Liquid↗
PubMed1992

Azoles, allylamines and drug metabolism.

Four antifungal drugs, the azoles ketoconazole, itraconazole and fluconazole, and the allylamine terbinafine, were studied for their effects on the metabolism of cyclosporin A (CyA) and cortisol by human liver microsomes in vitro (n = 3). Ketoconazole produced marked inhibition of CyA hydroxylase (to metabolites M17 and M1) with IC50 and Ki values of 0.24 +/- 0.01 and 0.022 +/- 0.004 microM, respectively. On the basis of the IC50, itraconazole was 10 times less potent (IC50 of 2.2 +/- 0.2 microM), and fluconazole and terbinafine were each above 100 microM. No kinetic parameters were calculated for terbinafine because of the lack of inhibitory effects. Ketoconazole was the most potent inhibitor of cortisol metabolism (to 6 beta-hydroxycortisol, IC50 = 0.6 microM). Itraconazole produced marked inhibition of cortisol metabolism (IC50 = 2.4 microM), but fluconazole and terbinafine had little effect. These data confirm that ketoconazole is a potent inhibitor of cytochrome P-450-IIIA4, and this has clinical relevance. Although the inhibition with fluconazole was much less than with itraconazole at equimolar concentrations, it should be noted that in-vivo plasma concentrations of fluconazole are much greater than that of itraconazole. Clinical interactions of CyA with both fluconazole and itraconazole have been reported; in contrast to these azoles, terbinafine does not have the same interaction potential.

Antifungal Agents↗
PubMed1991

Effect of the progestogens, gestodene, 3-keto desogestrel, levonorgestrel, norethisterone and norgestimate on the oxidation of ethinyloestradiol and other substrates by human liver microsomes.

A number of different progestogens, levonorgestrel (LNG), norethisterone (NET), gestodene (GSD), desogestrel (DG) and norgestimate (NORG) are used in combination with the oestrogen ethinyloestradiol (EE2) in oral contraceptive steroid preparations. All the progestogens are acetylenic steroids and previous studies have indicated the potential of acetylenic steroids to cause mechanism-based or "suicide" inactivation of cytochrome P-450. We have compared the effects of the different progestogens on EE2 2-hydroxylation (a reaction catalyzed by enzymes from the P-450IIC, P-450IIIA and P-450IIE gene families) and also the oxidative metabolism of other drug substrates (cyclosporin, diazepam, tolbutamide) by human liver microsomes. On coincubation with EE2 as substrate, GSD, 3-keto desogestrel (3-KD, the active metabolite of desogestrel) and LNG produced some concentration-dependent inhibition of EE2 2-hydroxylation (maximum 32% inhibition at 100 microM 3-keto desogestrel). Ki values determined for GSD and 3-KD were 98.5 +/- 12.3 and 93.2 +/- 10.3 microM (mean +/- SD; n = 4), respectively. Preincubation of progestogens in a small volume (50 microliters) incubation for 30 min in the presence of an NADPH-generating system enhanced the inhibitory potential of all the steroids (at 100 microM, inhibition was for GSD 39%, 3-KD 46%, LNG 46%, NET 51% and NORG 43%). Inhibitory effects were therefore comparable and also similar to the macrolide antibiotic troleandomycin. The most marked inhibition seen was of diazepam N-demethylation and hydroxylation by GSD (71 and 57%, respectively) and 3-KD (62 and 50%, respectively). In preincubation studies involving cyclosporin as the substrate, the order of inhibitory potency was GSD greater than 3-KD greater than NET greater than LNG for production of both metabolite M17 and M21. The results of the study indicate that all the progestogens in common use have the propensity to inhibit a number of oxidative pathways but there is little evidence for one progestogen being more markedly inhibitory than others.

Contraceptives, Oral↗
PubMed1991

Comparative effects of the antimycotic drugs ketoconazole, fluconazole, itraconazole and terbinafine on the metabolism of cyclosporin by human liver microsomes.

Four antimycotic drugs, the azoles ketoconazole, itraconazole and fluconazole, and the allylamine terbinafine have been studied for their effect on the metabolism of cyclosporin by human liver microsomes (n = 3) in vitro. Ketoconazole caused marked inhibition of cyclosporin hydroxylase (to metabolites M17 and M1) with IC50 and Ki values of 0.24 +/- 0.01 and 0.022 +/- 0.004 microM, respectively. Based on IC50 values, itraconazole was ten times less potent (IC50 value of 2.2 +/- 0.2 microM) and both fluconazole and terbinafine had values above 100 microM. Ki values for itraconazole and fluconazole were 0.7 +/- 0.2 and 40 +/- 5.6 microM, respectively. No kinetic parameters were calculated for terbinafine because of the lack of inhibitory effects. Based on these data, ketoconazole is confirmed as being a potent inhibitor of cyclosporin metabolism and this has clinical relevance. Although inhibition by fluconazole was much less than that by itraconazole at equimolar concentrations, it should be noted that in patients plasma concentrations of fluconazole are much greater than those of itraconazole. Clinical interactions of cyclosporin with both fluconazole and itraconazole have been reported. In contrast to the azoles, terbinafine does not have the same potential for interaction.

Adult↗
PubMed1991

Cyclosporin metabolism by the gastrointestinal mucosa.

The intestinal mucosal metabolism of the immunosuppressant cyclosporin (CsA) has been studied in vitro using the Ussing chamber technique. Histologically normal colon was obtained from six patients undergoing resections. The mucosal sheets were mounted between two perspex chambers. Three hours after addition of [3H]-CsA (0.2 microCi; 10 microM) to the mucosal chamber, more than 90% of the radioactivity was present in that chamber. Metabolite analysis, by high performance liquid chromatography, indicated that 77.6 +/- 9.2% (mean +/- s.d.) of the drug present was CsA, 9.9 +/- 4.4% and 8.7 +/- 4.7% were the oxidative metabolites M17 and M21 respectively (metabolites identified by co-chromatography with authentic standards). Total metabolite production in tissues from the six individuals was variable (10.1-30.6% at 3 h) and increased over the time period of the study. A different pattern of metabolism was obtained from a single sample of gastric mucosa. More than 20% of CsA was metabolised although neither M17 nor M21 were detected. The results of this study suggest that the gut wall is involved in the first pass metabolism of CsA in vivo and that this could be a contributory factor to the poor systemic availability of CsA seen in some patients.

Chromatography, High Pressure Liquid↗
PubMed1989

Comparative effects of two antimycotic agents, ketoconazole and terbinafine on the metabolism of tolbutamide, ethinyloestradiol, cyclosporin and ethoxycoumarin by human liver microsomes in vitro.

Two antimycotic agents, the azole ketoconazole and the allylamine terbinafine, have been examined for their effects on the metabolism of tolbutamide, ethinyloestradiol, cyclosporin and ethoxycoumarin by human liver microsomes (n = 4) in vitro. Ketoconazole caused marked inhibition of all enzyme activities with mean IC50 values (concentration producing 50% inhibition) of 17.9 microM (tolbutamide hydroxylase), 1.9 microM (ethinyloestradiol 2-hydroxylase), 2.0 microM (cyclosporin N-demethylase), 2.1 microM (cyclosporin hydroxylase) and 25 microM (ethoxycoumarin O-deethylase). At 50 microM terbinafine concentration, inhibition was less than 5% for tolbutamide and ethoxycoumarin, approximately 12% for both cyclosporin pathways and 35% for ethinyloestradiol. Terbinafine does not have the same inhibitory potential for cytochrome P-450 isozymes as ketoconazole.

Antifungal Agents↗
PubMed1989

Calcium channel antagonists and cyclosporine metabolism: in vitro studies with human liver microsomes.

The effects of four Ca2+ channel antagonists on the metabolism of cyclosporine (CsA) by human liver microsomes (n = 4) in vitro have been examined. Nicardipine produced marked inhibition of both M17 and M21 (IC50 = 7.0 microM) formation. In contrast nifedipine produced less than 20% inhibition of M17 and M21 even at the highest concentration examined (50 microM). Diltiazem data were comparable to those for nifedipine. Verapamil (50 microM) produced 30 and 28% inhibition of M17 and M21 formation, respectively. These findings give a basis to the increase in CsA blood concentrations seen in transplant patients who are also given nicardipine.

Adult↗
PubMed1988

In vitro inhibition studies of tolbutamide hydroxylase activity of human liver microsomes by azoles, sulphonamides and quinolines.

1. A number of compounds have been examined for their ability to inhibit tolbutamide hydroxylase activity in human liver microsomes (control value at a substrate concentration of 150 microM being 0.27 +/- 0.12 nmol min-1 mg-1 protein; mean +/- s.d.; n = 7). 2. IC50 (concentration of inhibitor producing 50% inhibition) values were determined for a range of sulphonamides, imidazoles and aminoquinoline compounds. The most potent inhibition was evident with the 1-substituted imidazole antimycotic drugs ketoconazole, clotrimazole and miconazole and the sulphonamide sulphaphenazole (IC50 values of 16.5, 2.5, 0.85 and 0.5 microM respectively). A number of compounds showed little or no inhibition of tolbutamide hydroxylase as judged by an IC50 of greater than or equal to 500 microM. 3. The Km value for tolbutamide hydroxylase was 125 microM and Vmax, 0.44 nmol min-1 mg-1 protein. All the substituted imidazoles examined in kinetic studies 1v vs 1s, Line-weaver-Burk plots) produced either non-competitive or mixed inhibition. The sulphonamides exhibited competitive inhibition, the Ki for sulphaphenazole being 0.22 microM. Primaquine showed mixed inhibition. Dixon plots confirmed the type of inhibition produced. 4. Although the competitive inhibition between some sulphonamides and tolbutamide is consistent with metabolism by the same isozyme of cytochrome P-450 it does not prove it and further studies with purified enzymes will be necessary to confirm this.

Adolescent↗
PubMed1987

Single dose primaquine has no effect on paracetamol clearance.

The kinetics of paracetamol and the formation of metabolites were evaluated in 6 healthy volunteers before and during concomitant administration of a single dose (45 mg) of primaquine. There was no effect of the antimalarial drug on either conjugation (to paracetamol glucuronide and paracetamol sulphate) or oxidation (as judged by the presence of paracetamol cysteine and paracetamol mercapturate) pathways. Although primaquine inhibits certain oxidative metabolism (e.g. of antipyrine) it has no effect, in therapeutic doses, on paracetamol metabolism.

Acetaminophen↗
PubMed1985

Inhibition of tolbutamide metabolism by substituted imidazole drugs in vivo: evidence for a structure-activity relationship.

Tolbutamide has been used as a model drug for an examination of the effects of eleven substituted imidazole compounds on hepatic metabolism in vivo. The 1-substituted compounds 1-methylimidazole, miconazole, clotrimazole and ketoconazole produced marked alterations in tolbutamide kinetics (increased half-life, decreased clearance). However, if there was substitution in the 2- position, irrespective of a substituent on N-1, then the compound did not appear to inhibit metabolism (e.g. 2-methylimidazole, 1,2-dimethylimidazole, methimazole, metronidazole). The 4- substituted compounds, 4-methylimidazole and cimetidine were inhibitors. A structure-activity relationship for the inhibitory actions of the substituted imidazoles is thus evident in vivo.

Animals↗
PubMed1985

The effect of some benzimidazoles on the disposition of antipyrine and tolbutamide from the rat isolated perfused liver.

The anthelmintic benzimidazoles, mebendazole, albendazole and flubendazole have been screened for any propensity to alter the disposition of antipyrine and tolbutamide in the rat isolated perfused liver preparation. The benzimidazoles were added as a 2.5 mg bolus dose into the perfusate reservoir 5 min before the administration of either antipyrine or tolbutamide. Neither mebendazole or albendazole produced any significant effect on the pharmacokinetics of either of the substrate drugs. In contrast, flubendazole significantly decreased the clearance of antipyrine (by 40%) indicating inhibition of mixed function oxidase activity. However, flubendazole did not alter the disposition of tolbutamide. The results suggest that not all benzimidazoles inhibit hepatic drug metabolizing enzymes and that different forms of cytochrome P-450 are involved in the metabolism of antipyrine and tolbutamide.

Animals↗
PubMed1984

Tolbutamide as a model drug for the study of enzyme induction and enzyme inhibition in the rat.

The effects of various drugs on the pharmacokinetics of tolbutamide have been examined in the rat. Phenobarbitone pretreatment caused a significant decrease in half life and area under the curve (AUC) and a significant increase in clearance and volume of distribution (Vd). Acute administration of primaquine significantly increased half life and AUC and decreased clearance. In contrast, the related animoquinolone chloroquine, was without effect. Acute administration of cimetidine produced similar changes to primaquine but of lesser magnitude. Formation of the major metabolite hydroxytolbutamide, was markedly enhanced by phenobarbitone and reduced by primaquine and cimetidine. We conclude that due to its single pathway of metabolism, tolbutamide is a good substrate to use when examining pharmacokinetic interactions involving hepatic enzyme induction and inhibition.

Animals↗