Inhibition of leukotriene omega-oxidation by isonicotinic acid hydrazide (isoniazid)

Pharmacokinetic Interactions Between Isoniazid and Theophylline in Rats

Pharmacokinetic interactions between isoniazid and theophylline were studied in male Wistar rats, 206±17 g. Concomitant oral administration of 2 × 5 mg kg−1 isoniazid accelerated slightly the disposition of theophylline (10 mg kg−1, i.v.) whereas 2 × 25 mg kg−1 isoniazid slowed it marginally. The differences in distribution volume, systemic clearance and area under the concentration-time curve (AUC) between the high and the low dose, however, were statistically significant. One week pretreatment with 10 mg kg−1 isoniazid tended towards inhibition (significant decrease of systemic clearance, increase of AUC) and 50 mg kg−1 to acceleration (decrease of half-life, mean residence time and AUC, increase of systemic clearance) of theophylline disposition. After oral pretreatment with 20 mg kg−1 theophylline, neither the kinetics of free isoniazid (50 mg kg−1, i.v.) and the amount acetylated nor the acetylation indices differed from the controls. There was no evidence that concomitant or subacute administration of different doses of isoniazid affects major metabolic pathways of theophylline or that prolonged theophylline treatment interacts with the N-acetylation capacity.

Inhibition of cytochrome P450 (CYP450) isoforms by isoniazid: potent inhibition of CYP2C19 and CYP3A

Isoniazid (INH) remains the most safe and cost-effective drug for the treatment and prophylaxis of tuberculosis. The use of INH has increased over the past years, largely as a result of the coepidemic of human immunodeficiency virus infection. It is frequently given chronically to critically ill patients who are coprescribed multiple medications. The ability of INH to elevate the concentrations in plasma and/or toxicity of coadministered drugs, including those of narrow therapeutic range (e.g., phenytoin), has been documented in humans, but the mechanisms involved are not well understood. Using human liver microsomes (HLMs), we tested the inhibitory effect of INH on the activity of common drug-metabolizing human cytochrome P450 (CYP450) isoforms using isoform-specific substrate probe reactions. Incubation experiments were performed at a single concentration of each substrate probe at its K(m) value with a range of INH concentrations. CYP2C19 and CYP3A were inhibited potently by INH in a concentration-dependent manner. At 50 microM INH (approximately 6.86 microg/ml), the activities of these isoforms decreased by approximately 40%. INH did not show significant inhibition (<10% at 50 microM) of other isoforms (CYP2C9, CYP1A2, and CYP2D6). To accurately estimate the inhibition constants (K(i) values) for each isoform, four concentrations of INH were incubated across a range of five concentrations of specific substrate probes. The mean K(i) values (+/- standard deviation) for the inhibition of CYP2C19 by INH in HLMs and recombinant human CYP2C19 were 25.4 +/- 6.2 and 13 +/- 2.4 microM, respectively. INH showed potent noncompetitive inhibition of CYP3A (K(i) = 51.8 +/- 2.5 to 75.9 +/- 7.8 microM, depending on the substrate used). INH was a weak noncompetitive inhibitor of CYP2E1 (K(i) = 110 +/- 33 microM) and a competitive inhibitor of CYP2D6 (K(i) = 126 +/- 23 microM), but the mean K(i) values for the inhibition of CYP2C9 and CYP1A2 were above 500 microM. Inhibition of one or both CYP2C19 and CYP3A isoforms is the likely mechanism by which INH slows the elimination of coadministered drugs, including phenytoin, carbamazepine, diazepam, triazolam, and primidone. Slow acetylators of INH may be at greater risk for adverse drug interactions, as the degree of inhibition was concentration dependent. These data provide a rational basis for understanding drug interaction with INH and predict that other drugs metabolized by these two enzymes may also interact.

Effect of 3'-methoxy-4'-nitroflavone on benzo[a]pyrene toxicity. Aryl hydrocarbon receptor-dependent and -independent mechanisms

This laboratory has studied a number of flavone derivatives for aryl hydrocarbon receptor (AhR) agonist and antagonist potential using cell-free and cell culture systems. The current report extends these investigations by testing the potent AhR antagonist 3'-methoxy-4'-nitroflavone (3'M4'NF) for in vivo activity. Wild-type C57Bl/6 male mice were treated with solvent, benzo[a]pyrene (B[a]P; 150 mg/kg), or concurrently with B[a]P and 3'M4'NF (60 mg/kg; delivered as a split dose). Since B[a]P is bioactivated to genotoxic metabolites by AhR-regulated enzymes, we measured B[a]P-induced chromosomal damage in peripheral blood (i.e. micronuclei) to characterize the antagonistic potential of 3'M4'NF in vivo. The influence of AhR signal transduction was investigated further by challenging wild-type and Ahr null allele mice with B[a]P with and without a 3'M4'NF co-treatment. The micronucleus data obtained from these experiments indicated that 3'M4'NF can attenuate the genotoxicity of B[a]P significantly. Since 3'M4'NF also protected Ahr null allele mice from B[a]P-induced genetic damage, it was apparent that AhR-independent mechanisms contribute to the effects observed. However, as opposed to the protective effects observed with the micronucleus endpoint, histological observations and lethality data indicated that some B[a]P effects are enhanced by 3'M4'NF. Potentiated B[a]P toxicity may be explained by inhibition of basal and induced CYP1A1/2 activities. Both in vitro and in vivo data presented herein support this hypothesis.

Hepatic uptake and metabolic disposition of leukotriene B4 in rats

1. In isolated perfused rat liver and in vivo, up to 25% of [3H]leukotriene B4 was eliminated from the circulation via hepatic uptake and biliary excretion within 1 h. Total body recovery of 3H amounted to about 60% of infused [3H]leukotriene B4. 2. Hepatobiliary excretion of leukotriene B4 and its metabolites exceeded renal elimination by about 4-fold and depended, in contrast with excretion of cysteinyl leukotriene E4, upon continuous taurocholate supply. 3. Analyses of bile, liver and recirculated perfusate using h.p.l.c. indicated that the liver metabolized leukotriene B4 extensively to omega-carboxyleukotriene B4 and its beta-oxidized derivatives, and no unmetabolized leukotriene B4 appeared in bile. These results substantiate the important contribution of the hepatobiliary system with respect to the metabolic fate of leukotriene B4.