Metabolic interactions between acetaminophen (paracetamol) and two flavonoids, luteolin and quercetin, through in-vitro inhibition studies

Abstract

Objectives: Excessive exposure to acetaminophen (APAP, paracetamol) can cause liver injury through formation of a reactive metabolite that depletes hepatic glutathione and causes hepatocellular oxidative stress and damage. Generation of this metabolite is mediated by Cytochrome-P450 (CYP) isoforms, mainly CYP2E1. A number of naturally occurring flavonoids can mitigate APAP-induced hepatotoxicity in experimental animal models. Our objective was to determine the mechanism of these protective effects and to evaluate possible human applicability.

Methods: Two flavonoids, luteolin and quercetin, were evaluated as potential inhibitors of eight human CYP isoforms, of six UDP-glucuronosyltransferase (UGT) isoforms and of APAP glucuronidation and sulfation. The experimental model was based on in-vitro metabolism by human liver microsomes, using isoform-specific substrates.

Key findings: Luteolin and quercetin inhibited human CYP isoforms to varying degrees, with greatest potency towards CYP1A2 and CYP2C8. However, 50% inhibitory concentrations (IC₅₀ values) were generally in the micromolar range. UGT isoforms were minimally inhibited. Both luteolin and quercetin inhibited APAP sulfation but not glucuronidation.

Conclusions: Inhibition of human CYP activity by luteolin and quercetin occurred with IC₅₀ values exceeding customary in-vivo human exposure with tolerable supplemental doses of these compounds. The findings indicate that luteolin and quercetin are not likely to be of clinical value for preventing or treating APAP-induced hepatotoxicity.

Keywords: CYPs; UDP-glucuronosyltransferase; acetaminophen; drug interactions; flavonoids.

Figure 1

In vitro inhibitory effects of luteolin on (a) CYP1A2, (b) CYP3A, (c) CYP2B6, (d) CYP2C8, (e) CYP2C9, (f) CYP2C19, (g) CYP2D6 and (h) CYP2E1. Data points represent the means ± standard errors (SEM) of each concentration that was tested in duplicate. The dashed and solid lines represent the incubations without and with preincubation. IC₅₀ values were determined by nonlinear regression and summarized in Table 1.

Figure 2

In vitro inhibitory effects of quercetin on (a) CYP1A2, (b) CYP3A, (c) CYP2B6, (d) CYP2C8, (e) CYP2C9, (f) CYP2C19, (g) CYP2D6 and (h) CYP2E1. Data points represent the means ± standard errors (SEM) of each concentration that was tested in duplicate. The dashed and solid lines represent the incubations without and with preincubation. IC₅₀ values were determined by nonlinear regression and summarized in Table 1.

Figure 3

In vitro inhibitory effects of luteolin and quercetin on (a and b) UGT1A1 or (c and d) UGT1A4. Data points represent the means ± standard errors (SEM) of each concentration that was tested in duplicate. The dashed and solid lines represent the incubations without and with preincubation. IC₅₀ values were determined by nonlinear regression and summarized in Table 1. UGT, UDP-glucuronosyltransferase.

Figure 4

In vitro inhibitory effects of (a) luteolin, (b) quercetin and (c) probenecid on APAP glucuronidation with the pooled HLMs. The concentrations of probenecid are 0, 0.1, 0.25, 0.5, 1, 2.5 and 5 mm. Data points represent the means ± standard errors (SEM) of each drug concentration that was tested in duplicate. The dashed and solid lines represent the incubations without and with preincubation. IC₅₀ values were determined by nonlinear regression and summarized in Table 1. HLM, human liver microsome.

Figure 5

In vitro inhibitory effects of (a) luteolin and (b) quercetin on acetaminophen sulfation. Data points represent the means ± standard errors (SEM) of each drug concentration that was tested in duplicate. IC₅₀ values were determined by nonlinear regression and summarized in Table 1.

Figure 6

In vitro inhibition of CYP2E1 by (a) methanol and by (b) DMSO; and inhibition of CYP2C9 by (c) DMSO. Data points represent the means ± standard errors (SEM) of each solvent concentration that was tested in duplicate. IC₅₀ values were determined by nonlinear regression and summarized in Table 1. DMSO, dimethyl sulfoxide.