Oxygen supplementation restores theophylline clearance to normal in cirrhotic rats

Review article: time to revisit Child-Pugh score as the basis for predicting drug clearance in hepatic impairment

BACKGROUND

Prescription information for many drugs entering the market lacks dosage guidance for hepatic impairment. Dedicated studies for assessing the fate of drugs in hepatic impairment commonly stratify patients using Child-Pugh score. Child-Pugh is a prognostic clinical score with limitations in reflecting the liver's metabolic capacity.

AIMS

To demonstrate the need for better drug dosing approaches in hepatic impairment, summarise the current status, identify knowledge gaps related to drug kinetic parameters in hepatic impairment, propose solutions for predicting the liver disease impact on drug exposure and discuss barriers to dosing guidance in those patients.

METHODS

Relevant reports on dosage adjustment in hepatic impairment were analysed concerning the prediction of the impairment impact on drug kinetics using physiologically-based pharmacokinetic (PBPK) modelling.

RESULTS

PBPK models are suggested as a potential framework to understand drug clearance changes in hepatic impairment. Quantifying changes in abundance and activity of drug-metabolising enzymes and transporters, understanding the impact of shunting, and accounting for interindividual variations in drug absorption could help in extending the success of these models in hepatically-impaired populations. These variables might not correlate with Child-Pugh score as a whole. Therefore, new metabolic activity markers, imaging techniques and other scoring systems are proposed to either support or substitute Child-Pugh score.

CONCLUSIONS

Many physiological changes in hepatic impairment determining the fate of drugs do not necessarily correlate with Child-Pugh score. Quantifying these changes in individual patients is essential in future hepatic impairment studies. Further studies assessing Child-Pugh alternatives are recommended to allow better prediction of drug exposure.

The Endothelium as a Driver of Liver Fibrosis and Regeneration

Liver fibrosis is a common feature of sustained liver injury and represents a major public health problem worldwide. Fibrosis is an active research field and discoveries in the last years have contributed to the development of new antifibrotic drugs, although none of them have been approved yet. Liver sinusoidal endothelial cells (LSEC) are highly specialized endothelial cells localized at the interface between the blood and other liver cell types. They lack a basement membrane and display open channels (fenestrae), making them exceptionally permeable. LSEC are the first cells affected by any kind of liver injury orchestrating the liver response to damage. LSEC govern the regenerative process initiation, but aberrant LSEC activation in chronic liver injury induces fibrosis. LSEC are also main players in fibrosis resolution. They maintain liver homeostasis and keep hepatic stellate cell and Kupffer cell quiescence. After sustained hepatic injury, they lose their phenotype and protective properties, promoting angiogenesis and vasoconstriction and contributing to inflammation and fibrosis. Therefore, improving LSEC phenotype is a promising strategy to prevent liver injury progression and complications. This review focuses on changes occurring in LSEC after liver injury and their consequences on fibrosis progression, liver regeneration, and resolution. Finally, a synopsis of the available strategies for LSEC-specific targeting is provided.

Assessment of Hepatic Impairment and Implications for Pharmacokinetics of Substance Use Treatment

Although the liver is the primary site of metabolism and biliary excretion for many medications, data are limited on the liver's pharmacokinetic abilities in cirrhosis. Cirrhosis develops through collagen deposition, eventually culminating in end-stage liver disease that compromises hepatic drug metabolism. Consequently, the US Food and Drug Administration (FDA) recommends evaluating the pharmacokinetics of medications in subjects with hepatic impairment if hepatic metabolism constitutes more than 20% of their elimination or if they have a narrow therapeutic range. A variety of noninvasive indices and radiologic procedures can be employed to assess hepatic drug metabolism and excretion. The Child-Pugh score is the most commonly used scale for assessing hepatic impairment among drugs submitted for US FDA approval. The score, originally developed to guide operative mortality in patients undergoing hepatic resection, has not been modified since its inception 5 decades ago. Furthermore, the score was not originally intended to be a guide for potential dose modification in patients with hepatic impairment. These reasons, in combination with the availability of a variety of new imaging modalities and an enhanced understanding of hepatic biology, should foster the development of novel methods to assess the effect of hepatic impairment on liver drug metabolism.

Gd-EOB-DTPA-enhanced T1 relaxometry for assessment of liver function determined by real-time 13C-methacetin breath test

OBJECTIVES

To determine whether liver function as determined by intravenous administration of ¹³C-methacetin and continuous real-time breath analysis can be estimated quantitatively from gadoxetic acid (Gd-EOB-DTPA)-enhanced magnetic resonance (MR) relaxometry.

METHODS

Sixty-six patients underwent a ¹³C-methacetin breath test (¹³C-MBT) for evaluation of liver function and Gd-EOB-DTPA-enhanced T1-relaxometry at 3 T. A transverse 3D VIBE sequence with an inline T1 calculation based on variable flip angles was acquired prior to (T1 pre) and 20 min post-Gd-EOB-DTPA (T1 post) administration. The reduction rate of T1 relaxation time (rrT1) and T1 relaxation velocity index (∆R1) between pre- and post-contrast images was evaluated. ¹³C-MBT values were correlated with T1_post, ∆R1 and rrT1, providing an MRI-based estimated ¹³C-MBT value. The interobserver reliability was assessed by determining the intraclass correlation coefficient (ICC).

RESULTS

Stratified by three different categories of ¹³C-MBT readouts, there was a constant increase of T1 post with increasing progression of diminished liver function (p ≤ 0.030) and a constant significant decrease of ∆R1 (p ≤ 0.025) and rrT1 (p < 0.018) with progression of liver damage as assessed by ¹³C-methacetin breath analysis. ICC for all T1 relaxation values and indices was excellent (> 0.88). A simple regression model showed a log-linear correlation of ¹³C-MBT values with T1_post (r = 0.57; p < 0.001), ∆R1 (r = 0.59; p < 0.001) and rrT1 (r = 0.70; p < 0.001).

CONCLUSION

Liver function as determined using real-time ¹³C-methacetin breath analysis can be estimated quantitatively from Gd-EOB-DTPA-enhanced MR relaxometry.

KEY POINTS

• Gd-EOB-DTPA-enhanced T1 relaxometry quantifies liver function • Gd-EOB-DTPA-enhanced MR relaxometry may provide parameters for assessing liver function before surgery • Gd-EOB-DTPA-enhanced MR relaxometry may be useful for monitoring liver disease progression • Gd-EOB-DTPA-enhanced MR relaxometry has the potential to become a novel liver function index.

Two-year follow-up of splenic radiofrequency ablation in patients with cirrhotic hypersplenism: does increased hepatic arterial flow induce liver regeneration?

BACKGROUND

Hepatocyte hypoxia may be a mechanism determining abnormal tissue oxygenation and dysfunction of the cirrhotic liver. Since the introduction of radiofrequency ablation (RFA) for patients with cirrhotic hypersplenism, we observed a phenomenon of visible hepatic regeneration. This study aims to investigate the potential mechanism of RFA-induced liver regeneration, and the 2-year outcomes of splenic RFA.

METHODS

Forty patients who underwent splenic RFA for cirrhotic hypersplenism were followed for 24 months. Before and after RFA procedures, portal hemodynamics and liver and spleen volumes were measured by Doppler ultrasonography and computed tomography volumetry. Liver function tests and blood counts were also determined.

RESULTS

The splenic and portal venous flows decreased, but hepatic arterial flow (HAF) increased dramatically after the RFA procedure. Liver volumes at 3 month post-RFA increased compared to the baseline volumes (872 +/- 107 vs. 821 +/- 99 cm(3), P = .031). A correlation was found between maximum absolute values of liver volumes (triangle upliver volumes) and that of HAF (triangle upHAF) in Child-Pugh class A/B patients (r = 0.60; P < .001). Leukocyte and platelet counts, as well as liver function, improved substantially during the 2-year follow-up. Patients with > or = 40% of spleen volume ablated had better improvement of thrombocytopenia. No death or severe complications occurred.

CONCLUSIONS

RFA for cirrhotic hypersplenism is safe and efficacious. The increase in HAF as a result of splenic RFA may improve liver function and induce liver regeneration in cirrhotics, but further studies are necessary to clarify the underlying mechanisms.

The Hepatic Sinusoid in Aging and Cirrhosis

The fenestrated sinusoidal endothelium ('liver sieve') and space of Disse in the healthy liver do not impede the transfer of most substrates, including drugs and oxygen, from the sinusoidal lumen to the hepatocyte. Plasma components transfer freely in both directions through the endothelial fenestrations and into the space of Disse. The endothelium is attenuated, there is no basement membrane and there is minimum collagen in the space of Disse, thus minimising any barriers to substrate diffusion. Both cirrhosis and aging are associated with marked structural changes in the sinusoidal endothelium and space of Disse that are likely to influence bulk plasma transfer into the space of Disse, and diffusion through the endothelium and space of Disse. These changes, termed capillarisation and pseudocapillarisation in cirrhosis and aging, respectively, impede the transfer of various substrates. Capillarisation is associated with exclusion of albumin, protein-bound drugs and macromolecules from the space of Disse, and the progressive transformation of flow-limited to barrier-limited distribution of some substrates. There is evidence that the sinusoidal changes in cirrhosis and aging contribute to hepatocyte hypoxia, thus providing a mechanism for the apparent differential reduction of oxygen-dependent phase I metabolic pathways in these conditions. Structural change and subsequent dysfunction of the liver sieve warrant consideration as a significant factor in the impairment of overall substrate handling and hepatic drug metabolism in cirrhosis and aging.

Hypoxia potentiates transforming growth factor-beta expression of hepatocyte during the cirrhotic condition in rat liver

BACKGROUND/AIMS

Many studies have reported that hypoxia might be associated with angiogenesis and fibrogenesis, and the level of transforming growth factor-beta1 (TGF-beta1) was increased in fibrotic liver and maximal at cirrhosis. Therefore, we examined the expression of TGF-beta1, phosphorylated-Smad2/3 (p-Smad2/3) of the TGF-beta immediate down stream signaling system and hypoxic status during hepatic fibrogenesis.

METHODS

Fibrosis of rats was induced by carbon tetrachloride. Collagens were detected with Azan stain. Immunohistochemistry and immunoblotting was used.

RESULTS

TGF-beta1 was mainly produced by hypoxic hepatocytes at cirrhosis although myofibroblasts (MFBs) and macrophages producing TGF-beta1 were decreased. Moreover, distribution of p-Smad2/3 in hepatocytes was consistent with those of hypoxic hepatocytes regardless of MFBs. Furthermore, in recovery, most MFBs disappeared, whereas positive reactions of p-Smad2/3 still existed in the hepatocytes of hypoxic areas. Therefore, TGF-beta1 expression in hepatocytes might have been associated with hypoxia.

CONCLUSIONS

We put forward the hypothesis that TGF-beta1 is mainly produced by MFBs and macrophages at early and middle stages of fibrotic processes, but it is predominantly released by hypoxic hepatocytes in the last fibrotic stage or cirrhosis.

Rat liver sinusoidal endothelial cell phenotype is maintained by paracrine and autocrine regulation

The phenotypic features of liver sinusoidal endothelial cells (SEC), open fenestrae in sieve plates and lack of a basement membrane, are lost with capillarization. The current study examines localization of CD31 as a marker for the dedifferentiated, nonfenestrated SEC and examines regulation of SEC phenotype in vitro. CD31 localization in SEC was examined by confocal microscopy and immunogold-scanning electron microscopy. SEC cultured for 1 day express CD31 in the cytoplasm, whereas after 3 days, CD31 is also expressed on cell-cell junctions. Immunogold-scanning electron microscopy confirmed the absence of CD31 surface expression on fenestrated SEC 1 day after isolation and demonstrated the appearance of CD31 surface expression on SEC that had lost fenestration after 3 days in culture. SEC isolated from fibrotic liver do show increased expression of CD31 on the cell surface. Coculture with either hepatocytes or stellate cells prevents CD31 surface expression, and this effect does not require heterotypic contact. The paracrine effect of hepatocytes or stellate cells on SEC phenotype is abolished with anti-VEGF antibody and is reproduced by addition of VEGF to SEC cultured alone. VEGF stimulates SEC production of nitric oxide. NG-nitro-L-arginine methyl ester blocked the paracrine effect of hepatocytes or stellate cells on SEC phenotype and blocked the ability of VEGF to preserve the phenotype of SEC cultured alone. In conclusion, surface expression of CD31 is a marker of a dedifferentiated, nonfenestrated SEC. The VEGF-mediated paracrine effect of hepatocytes or stellate cells on maintenance of SEC phenotype requires autocrine production of nitric oxide by SEC.

13C-Methacetin metabolism in patients with cirrhosis: relation to disease severity, haemoglobin content and oxygen supply

BACKGROUND

Hypoxia may contribute to impairment of liver function and thus interfere with results of liver tests. In patients with cirrhosis, cytochrome P-450 mediated metabolism of substrates is facilitated in the presence of supplemental oxygen. It has not been studied how this relates to liver function and haemoglobin content.

AIM

We questioned how oxygen supplementation would influence the hepatic microsomal function as assessed by the 13C-methacetin breath test in patients with cirrhosis of different severity and different degrees of anaemia.

METHODS

13C/12C ratios in exhaled breath assessed by non-dispersive infrared spectrometry were studied in 34 patients with cirrhosis (Child A/B/C 9/17/8) after administration of 75 mg 13C-methacetin p.o. with and without oxygen inhalation (4 L/min).

RESULTS

In patients breathing room air the total amount of 13C exhaled weakly correlated both with the Child-Pugh score (r = - 0.41, P < 0.02) and haemoglobin concentrations (r = 0.46; p = 0.006). Oxygen supplementation increased the total amount of 13C exhaled by 68 +/- 90% (P < 0.001). This effect was similar in Child-Pugh classes A (43 +/- 55%), B (83 +/- 80%) and C (66 +/- 123%) and not related to the Child-Pugh score.

CONCLUSIONS

Our results suggest that tests of microsomal liver function are independently influenced both by oxygen delivery and the Child-Pugh score.

A novel method for determining hepatic sinusoidal oxygen permeability in the isolated perfused rat liver using [15O]O2

Measurement of hepatic sinusoidal permeability of oxygen and other substrates may help elucidate the mechanisms responsible for impaired liver function in cirrhosis. However studies of sinusoidal oxygen permeability in normal liver and various disease states have been limited due to the considerable technical difficulties involved in the use of standard techniques. We have developed a new method for measuring sinusoidal oxygen permeability in the isolated perfused rat liver that overcomes the difficulties of previous methods by using [(15)O]O(2) and an in-line fluid monitor. This method uses data obtained from impulse response curves of radiolabelled red cells, albumin and oxygen that are fitted mathematically using the axial dispersion model to yield rate constants that describe oxygen transit through the liver. We have demonstrated the utility and reproducibility of this method by comparing multiple injections and permeability determinations in the same preparation. This approach could be used in isolated perfused organs to study oxygen permeability in a range of disease states.

Functional significance of hepatic arterial flow reserve in patients with cirrhosis

In cirrhosis, hepatic arterial vasodilatation occurs in response to reduced portal venous blood flow. However, although the hepatic arterial flow reserve is high in patients with cirrhosis, its impact on hepatic function is unknown. This study investigated the effect of adenosine-induced hepatic arterial vasodilatation on different markers of liver function. In 20 patients with cirrhosis (Child-Pugh class A/B/C: n = 2/7/11) adenosine (2-30 microg x min(-1) x kg body wt(-1)) was infused into the hepatic artery and hepatic arterial average peak flow velocities (APV), pulsatility indices (PI), and blood flow volumes (HABF) were measured using digital angiography and intravascular Doppler sonography. Indocyanine green (ICG), lidocaine, and galactose were administered intravenously in doses of 0.5, 1.0, and 500 mg/kg body weight in the presence of adenosine-induced hepatic arterial vasodilatation and, on a separate study day, without adenosine. ICG disappearance, galactose elimination capacity (GEC), and formation of the lidocaine metabolite monoethylglycinxylidide (MEGX) were assessed. Adenosine markedly increased APV and HABF and markedly decreased PI. Serum MEGX concentrations were 63.7 +/- 18.2 (median, 62; range, 36-107) and 99.0 +/- 46.3 (82.5; 49-198) ng/mL in the absence and presence of adenosine infusion, respectively (P =.001). Adenosine-induced changes in MEGX concentrations were correlated inversely to changes in APV (r = -0.5, P =.02) and PI (r = -0.55, P =.01) and were more marked in Child-Pugh class C compared with Child-Pugh class A patients (57.4 +/- 49.9 [44; -14 to 140] vs. 8.4 +/- 16.5 [13; -11 to 35] ng/mL, P <.01). In conclusion, hepatic arterial vasodilatation provides substantial functional benefit in patients with cirrhosis. The effect does not depend directly on hepatic arterial macroperfusion and is observed preferentially in patients with decompensated disease.

31P and 1H NMR spectroscopic studies of liver extracts of carbon tetrachloride-treated rats

NMR spectroscopy was used to examine hepatic metabolism in cirrhosis with a particular focus on markers of functional cellular hypoxia. (31)P and (1)H NMR spectra were obtained from liver extracts from control rats and from rats with carbon tetrachloride-induced cirrhosis. A decrease of 34% in total phosphorus content was observed in cirrhotic rats, parallelling a reduction of 40% in hepatocyte mass as determined by morphometric analysis. Hypoxia appeared to be present in cirrhotic rats, as evidenced by increased inorganic phosphate levels, decreased ATP levels, decreased ATP:ADP ratios (1.72 +/- 0.40 vs 2.48 +/- 0.50, p < 0.01), and increased inorganic phosphate:ATP ratios (2.77 +/- 0.48 vs 1.62 +/- 0.24, p < 0.00001). When expressed as a percentage of the total phosphorus content, higher levels of phosphoethanolamine and lower levels of NAD and glycerophosphoethanolamine were detected in cirrhotic rats. Cirrhotic rats also had increased phosphomonoester:phosphodiester ratios (5.73 +/- 2.88 vs 2.53 +/- 0.52, p < 0.01). These findings are indicative of extensive changes in cellular metabolism in the cirrhotic liver, with many findings attributable to the presence of intracellular hypoxia.

Comparative effects of oxygen supplementation on theophylline and acetaminophen clearance in human cirrhosis

BACKGROUND & AIMS

Sinusoidal capillarization in cirrhosis may impair the transfer of oxygen into hepatocytes; this may contribute to impaired oxidative drug metabolism. The aim of this study was to test this hypothesis by comparing the effects of oxygen supplementation in cirrhotic patients on the clearance of theophylline, which is dependent on hepatic oxidative metabolism, with its effect on the clearance of acetaminophen, which is reliant on hepatic conjugation reactions.

METHODS

Ten cirrhotic patients awaiting liver transplant and 5 control subjects were studied. Oral acetaminophen (1000 mg) and intravenous theophylline (3 mg/kg) were administered simultaneously on two separate occasions, 7 days apart. Subjects were randomized to breathe either room air or oxygen via face mask at 12 L/min for 9 hours of blood sampling.

RESULTS

Theophylline and acetaminophen clearances were significantly reduced by a mean of 54% and 50%, respectively, in cirrhotic patients compared with controls. Oxygen supplementation improved plasma theophylline clearance in cirrhotic patients by a mean of 34% (P = 0. 001), whereas acetaminophen clearance remained unchanged.

CONCLUSIONS

These findings indicate that, in cirrhosis, impaired hepatocyte oxygenation contributes to reduced oxidative drug metabolism and that oxidative drug metabolism can be improved by oxygen supplementation.

Effect of cirrhosis on sulphation by the isolated perfused rat liver

BACKGROUND/AIMS

There is evidence to suggest that not all pathways of drug metabolism are similarly affected in cirrhosis. The effect of cirrhosis on drug oxidation and glucuronidation has been extensively investigated but little is known of the effect of cirrhosis on drug sulphation. The aim of this study was to investigate the effect of cirrhosis on sulphation.

METHODS

We investigated the effect of cirrhosis on p-nitrophenol sulphation and compared this with the effect of cirrhosis on p-nitrophenol glucuronidation as well as on d-propranolol oxidation simultaneously in the single-pass isolated perfused rat liver. The perfusate contained added inorganic sulphate to maximise production of p-nitrophenol sulphate.

RESULTS

About 77% and 59% of p-nitrophenol was eliminated as the sulphate conjugate by the healthy (n=6) and cirrhotic (n=7) livers, respectively. Mean total p-nitrophenol clearance was decreased in cirrhosis (healthy: 18.5+/-0.2 vs. cirrhotic 15.3+/-4.0 ml/min; p<0.05). The decrease in total clearance of p-nitrophenol was due solely to the decrease in sulphate formation clearance, which was significantly decreased (healthy: 14.1+/-1.9 vs. cirrhotic: 9.27+/-3.33 ml/min; p<0.05). Mean glucuronide formation clearance (healthy: 5.11+/-0.94 vs cirrhotic: 5.79+/-0.85 ml/ min; p>0.05) was not significantly altered. Mean total propranolol clearance was decreased in cirrhosis (healthy: 19.9+/-0.1 vs. cirrhotics: 18.0+/-1.5 ml/min; p<0.05).

CONCLUSIONS

We have shown that in cirrhosis there is significant impairment of drug oxidation and sulphation, whilst glucuronidation is spared. The decreased sulphation of p-nitrophenol was most likely due to a decrease in phenol sulphotransferase and/or decrease in cofactor synthesis.

Biotransformation of theophylline in cirrhotic rats induced by biliary obstruction

The object of this work was to study the pharmacokinetic differences and the cause of these differences in cirrhotic rats induced by biliary obstruction when aminophylline (8 mg/kg as theophylline, i.v.) was administered. The concentrations of theophylline and its major metabolite (1,3-dimethyluric acid) in plasma were determined by HPLC. In addition, formation of 1,3-dimethyluric acid from theophylline in microsomes and the changes in the activity of drug metabolizing enzymes, which are suggested to be involved in theophylline metabolism, were determined. In cirrhotic rats, the systemic clearance of theophylline was reduced to 30% of the control value while AUC (area under the plasma concentration-time curve) and (t1/2)beta were increased 1.3 fold and 3.5 fold, respectively. The formation of 1,3-dimethyluric acid was decreased to 30% of the control value in microsomes of cirrhotic rat liver. In cirrhotic rat liver, activities of aniline hydroxylase (CYP2E1 related), erythromycin-N-demethylase (CYP3A related), and methoxyresorufin-O-demethylase (CYP1A2 related), which were reported to be related with theophylline metabolism, were decreased to 67%, 53%, and 76% that of normal rat liver, respectively. From the results, it can be concluded that in cirrhotic rats induced by biliary obstruction, the total body clearance of theophylline is markedly reduced and it may be due to decreased activity of drug metabolizing enzymes in liver.

The aging liver. Drug clearance and an oxygen diffusion barrier hypothesis

A change in drug clearance with age is considered an important factor in determining the high prevalence of adverse drug reactions associated with prescribing medications for the elderly. Despite this, no general principles have been available to guide drug administration in the elderly, although a substantial body of clearance and metabolism data has been generated in humans and experimental animals. A review of age-related change in drug clearances established that patterns of change are not simply explained in terms of hepatic blood flow, hepatic mass and protein binding changes. In particular, the maintained clearance of drugs subject to conjugation processes while oxygen-dependent metabolism declines, and all in vitro tests of enzyme function have been normal, requires new explanations. Reduction in hepatic oxygen diffusion as part of a general change in hepatocyte surface membrane permeability and conformation does provide one explanation for the paradoxical patterns of drug metabolism, and increased hepatocyte volume would also modify oxygen diffusion path lengths (the 'oxygen diffusion barrier' hypothesis). The reduction in clearances of high extraction drugs does correlate with observed reduction in hepatic perfusion. Dosage guidelines emerge from these considerations. The dosage of high clearance drugs should be reduced by approximately 40% in the elderly while the dosage of low clearance drugs should be reduced by approximately 30%, unless the compound is principally subject to conjugation mechanisms. If the hepatocyte diffusion barrier hypothesis is substantiated, this concept may lead to therapeutic (preventative and/or restorative) approaches to increased hepatocyte oxygenation in the elderly. This may lead to approaches for modification of the aging process in the liver.