Endogenous Opioid Levels Do Not Correlate With Itch Intensity and Therapeutic Interventions in Hepatic Pruritus

Abstract

Background: Chronic pruritus affects up to 70% of patients with immune-mediated hepatobiliary disorders. Antagonists of the μ-opioid receptor (MOR) and agonists of the κ-opioid receptor (KOR) are used to treat hepatic itch, albeit with limited success. An imbalance between ligands of MOR and KOR receptors has recently been suggested as a potential mechanism of hepatic pruritus. In this study, we therefore investigated systemic levels of important endogenous opioids such as β-endorphin, dynorphin A, Leu- and Met-enkephalin in plasma of a large cohort of well-characterized patients with immune-mediated cholestatic disorders, including patients with liver cirrhosis, and during effective anti-pruritic therapy. Methods: Plasma samples and clinical data were prospectively collected from well-characterized patients with primary/secondary sclerosing cholangitis (PSC/SSC), primary biliary cholangitis (PBC) and overlap syndromes suffering from pruritus (n = 29) and age-, gender- and disease-matched controls without pruritus (n = 27) as well as healthy controls (n = 20). General laboratory testing for hepatobiliary and renal function was performed. Levels of β-endorphin, dynorphin A, Leu- and Met-enkephalin were quantified in plasma by ELISA. Intensity of pruritus over the last week was evaluated using a visual analog scale (VAS, 0-10). Results: PBC and PSC patients with or without pruritus did neither differ in disease entity, disease stage, nor in the presence of cirrhosis. While both dynorphin A and β-endorphin concentrations were lower in pruritic patients compared to those without pruritus and healthy controls, the MOR/KOR ligand ratio was unaltered. No significant differences were observed for Leu- and Met-enkephalin concentrations. Opioid levels correlated with neither itch intensity nor stage of disease. Cirrhotic patients displayed higher concentrations of MOR agonist Leu-enkephalin and KOR agonist dynorphin A. Endogenous opioid levels remained largely unchanged after successful treatment with the potent anti-pruritic drugs rifampicin and bezafibrate. Conclusions: Endogenous opioid levels and the MOR/KOR ligand ratio neither correlate with itch intensity nor differentiate pruritic from non-pruritic patients with immune-mediated liver diseases. Thus, endogenous opioids may modulate signaling pathways involved in hepatic pruritus, but are unlikely to represent the major pruritogens in liver disease.

Keywords: bezafibrate; cholestasis; dynorphin; endorphin; enkephalin; liver; rifampicin; therapy.

Figure 1

Endogenous opioid plasma levels in patients with cholestatic liver diseases and healthy controls. Concentrations [pg/ml] of β-endorphin, dynorphin A, Leu-enkephalin and Met-enkephalin of patients with cholestatic liver diseases (n = 56, dark gray diamonds) and age- and gender matched controls (n = 20, light gray diamonds) are shown as individual values in (A–D). Significantly lower levels of both β-endorphin (A) and dynorphin A (B) were observed in the patient group. The difference for Leu-enkephalin was not significant (C). Data were statistically analyzed by multi-way analysis of variance (ANOVA) with least significant difference (LSD) post hoc testing. Significant differences are indicated by asterisks: **p < 0.01, ***p < 0.001.

Figure 2

Endogenous opioid plasma levels in patients with cholestatic liver diseases with and without associated pruritus and healthy controls. Concentrations [pg/ml] of β-endorphin (A) and dynorphin A (B) in pruritic patients (n = 29), non-pruritic patients (n = 27) and healthy controls (n = 20) are presented in the respective subfigures. (C) presents the ratio of β-endorphin to dynorphin A levels for all groups (C). Met-enkephalin and Leu-enkephalin levels in patients with and without pruritus as well as healthy controls are shown in subfigures (D) and (E). Data were statistically analyzed by multi-way analysis of variance (ANOVA) with least significant difference (LSD) post hoc testing. Significant differences are indicated by asterisks: *p < 0.05, ***p < 0.001.

Figure 3

Endogenous opioid plasma levels in patients with cholestatic liver diseases with and without cirrhosis. Leu-enkephalin and dynorphin A concentrations [pg/ml] were significantly higher in patients with liver cirrhosis (A, B, n = 13) compared to non-cirrhotic patients (n = 43), but no significant differences were observed for β-endorphin and Met-enkephalin concentrations between those groups (C,D). Data were statistically analyzed by multi-way analysis of variance (ANOVA) with least significant difference (LSD) post hoc testing. Significant differences are indicated by asterisks: *p < 0.05, **p < 0.01.

Figure 4

No correlation of endogenous opioid plasma levels with mean pruritus VAS scores of patients with hepatic pruritus. Symbols (circles) represent the opioid concentration and VAS score of the individual patients, respectively. Correlations were statistically calculated using Spearman's rank. Endogenous opioid levels did not correlate with the reported itch intensity assessed as the mean itch VAS score over the last week [(A) β-endorphin: r_s = −0.15; (B) dynorphin A: r_s = 0.01; (C) β-endorphin/dynorphin A ratio: r_s = −0.19; (D) Leu-enkephalin: r_s = 0.31; (E) Met-enkephalin: r_s = −0.23].

Figure 5

Endogenous opioid plasma levels in patients with cholestatic liver diseases before and after anti-pruritic treatment. Endogenous opioid levels were assessed in 22 patients treated with the anti-pruritic drugs rifampicin (150–300 mg qd) or bezafibrate (400 mg qd). Black squares represent individual patient values before and after treatment. (A) and (B) indicate the mean change of VAS over the last week before and after treatment with the respective drugs. Levels of β-endorphin, dynorphin A, Leu- and Met-enkephalin did not significantly change during treatment with both drugs (C–J). Data were statistically analyzed by Wilcoxon test.