Concentrations and pharmacokinetic parameters of MRI and SPECT hepatobiliary agents in rat liver compartments

Background In hepatobiliary imaging, systems detect the total amount of agents originating from extracellular space, bile canaliculi, and hepatocytes. They add in situ concentration of each compartment corrected by its respective volume ratio to provide liver concentrations. In vivo contribution of each compartment to liver concentration is inaccessible. Our aim was to quantify the compartmental distribution of two hepatobiliary agents in an ex vivo model and determine how their liver extraction ratios and cholestasis (livers lacking canalicular transporters) might modify it. Methods We perfused labelled gadobenate dimeglumine (Bopta, 200 μM, 7% liver extraction ratio) and mebrofenin (Meb, 64 μM, 94% liver extraction ratio) in normal (n = 18) and cholestatic (n = 6) rat livers. We quantified liver concentrations with a gamma counter placed over livers. Concentrations in hepatocytes and bile canaliculi were calculated. Mann-Whitney and Kruskal-Wallis tests were used. Results Hepatocyte concentrations were 2,043 ± 333 μM (Meb) versus 360 ± 69 μM (Bopta, p < 0.001). Meb extracellular concentrations did not contribute to liver concentrations (1.3 ± 0.3%). The contribution of Bopta extracellular concentration was 12.4 ± 1.9% (p < 0.001 versus Meb). Contribution of canaliculi was similar for both agents (16%). Cholestatic livers had no Bopta in canaliculi but their hepatocyte concentrations increased in comparison to normal livers. Conclusion Hepatocyte concentrations are correlated to liver extraction ratios of hepatobiliary agents. When Bopta is not present in canaliculi of cholestatic livers, hepatocyte concentrations increase in comparison to normal livers. This new understanding extends the interpretation of clinical liver images.

Interactions of anti-COVID-19 drug candidates with hepatic transporters may cause liver toxicity and affect pharmacokinetics

Transporters in the human liver play a major role in the clearance of endo- and xenobiotics. Apical (canalicular) transporters extrude compounds to the bile, while basolateral hepatocyte transporters promote the uptake of, or expel, various compounds from/into the venous blood stream. In the present work we have examined the in vitro interactions of some key repurposed drugs advocated to treat COVID-19 (lopinavir, ritonavir, ivermectin, remdesivir and favipiravir), with the key drug transporters of hepatocytes. These transporters included ABCB11/BSEP, ABCC2/MRP2, and SLC47A1/MATE1 in the canalicular membrane, as well as ABCC3/MRP3, ABCC4/MRP4, SLC22A1/OCT1, SLCO1B1/OATP1B1, SLCO1B3/OATP1B3, and SLC10A1/NTCP, residing in the basolateral membrane. Lopinavir and ritonavir in low micromolar concentrations inhibited BSEP and MATE1 exporters, as well as OATP1B1/1B3 uptake transporters. Ritonavir had a similar inhibitory pattern, also inhibiting OCT1. Remdesivir strongly inhibited MRP4, OATP1B1/1B3, MATE1 and OCT1. Favipiravir had no significant effect on any of these transporters. Since both general drug metabolism and drug-induced liver toxicity are strongly dependent on the functioning of these transporters, the various interactions reported here may have important clinical relevance in the drug treatment of this viral disease and the existing co-morbidities.

Interactions of Anti-COVID-19 Drug Candidates With Hepatic Transporters May Cause Liver Toxicity and Affect Drug Metabolism

Transporters in the human liver play a major role in the metabolism of endo-and xenobiotics. Apical (canalicular) transporters extrude compounds to the bile, while basolateral hepatocyte transporters promote the uptake or expel various compounds into the venous blood stream. In the present work we have examined the in vitro interactions of some key repurposed drugs advocated to treat COVID-19 (lopinavir, ritonavir, ivermectin, remdesivir and favipiravir), with the relevant key transporters in the hepatocytes. These transporters included the ABCB11/BSEP, ABCC2/MRP2, and MATE1 in the canalicular membrane, as well as ABCC3/MRP3, ABCC4/MRP4, OCT1, OATP1B1, OATP1B3, and NTCP, residing in the basolateral membrane. Lopinavir and ritonavir in low micromolar concentrations inhibited the ABCB11/BSEP and MATE1 exporters, as well as the OATP1B1/1B3 uptake transporters. Ritonavir had a similar inhibitory pattern, also inhibiting OCT1. Remdesivir strongly inhibited ABCC4/MRP4, OATP1B1/1B3, MATE1 and OCT1. Thus, these agents may cause severe drug-drug interactions and drug-induced liver injury. Favipiravir had no significant effect on any of these transporters. Since both general drug metabolism and drug-induced liver toxicity are strongly dependent on the functioning of these transporters, the variable interactions reported here may have important clinical relevance in the drug treatment of this viral disease and the existing co-morbidities.

Dynamic Localization of Hepatocellular Transporters: Role in Biliary Excretion and Impairment in Cholestasis

Bile flow generation is driven by the vectorial transfer of osmotically active compounds from sinusoidal blood into a confined space, the bile canaliculus. Hence, localization of hepatocellular transporters relevant to bile formation is crucial for bile secretion. Hepatocellular transporters are localized either in the plasma membrane or in recycling endosomes, from where they can be relocated to the plasma membrane on demand, or endocytosed when the demand decreases. The balance between endocytic internalization/ exocytic targeting to/from this recycling compartment is therefore the main determinant of the hepatic capability to generate bile, and to dispose endo- and xenobiotics. Furthermore, the exacerbated endocytic internalization is a common pathomechanisms in both experimental and human cholestasis; this results in bile secretory failure and, eventually, posttranslational transporter downregulation by increased degradation. This review summarizes the proposed structural mechanisms accounting for this pathological condition (e.g., alteration of function, localization or expression of F-actin or F-actin/transporter cross-linking proteins, and switch to membrane microdomains where they can be readily endocytosed), and the mediators implicated (e.g., triggering of “cholestatic” signaling transduction pathways). Lastly, we discussed the efficacy to counteract the cholestatic failure induced by transporter internalization of a number of therapeutic experimental approaches based upon the use of compounds that trigger exocytic targetting of canalicular transporters (e.g., cAMP, tauroursodeoxycholate). This therapeutics may complement treatments aimed to transcriptionally improve transporter expression, by affording proper localization and membrane stability to the de novo synthesized transporters.

Heme oxygenase-1 induction by hemin prevents oxidative stress-induced acute cholestasis in the rat

We previously demonstrated in in vitro and ex vivo models that physiological concentrations of unconjugated bilirubin (BR) prevent oxidative stress (OS)-induced hepatocanalicular dysfunction and cholestasis. Here, we aimed to ascertain, in the whole rat, whether a similar cholestatic OS injury can be counteracted by heme oxygenase-1 (HO-1) induction that consequently elevates endogenous BR levels. This was achieved through the administration of hemin, an inducer of HO-1, the rate-limiting step in BR generation. We found that BR peaked between 6 and 8 h after hemin administration. During this time period, HO-1 induction fully prevented the pro-oxidant tert-butylhydroperoxide (tBuOOH)-induced drop in bile flow, and in the biliary excretion of bile salts and glutathione, the two main driving forces of bile flow; this was associated with preservation of the membrane localization of their respective canalicular transporters, bile salt export pump (Bsep) and multidrug resistance-associated protein 2 (Mrp2), which are otherwise endocytosed by OS. HO-1 induction counteracted the oxidation of intracellular proteins and membrane lipids induced by tBuOOH, and fully prevented the increase in the oxidized-to-total glutathione (GSHt) ratio, a sensitive parameter of hepatocellular OS. Compensatory elevations of the activity of the antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD) were also prevented. We conclude that in vivo HO-1 induction protects the liver from acute oxidative injury, thus preventing consequent cholestasis. This reveals an important role for the induction of HO-1 and the consequently elevated levels of BR in preserving biliary secretory function under OS conditions, thus representing a novel therapeutic tool to limit the cholestatic injury that bears an oxidative background.

HOW DOES THE IN VIVO BILIARY ELIMINATION OF DRUGS CHANGE WITH AGE? EVIDENCE FROM CLINICAL DATA USING A SYSTEMS PHARMACOLOGY APPROACH

There is little information on the development of biliary drug elimination (BE) with age. The aims of this study were to collate literature data on the pharmacokinetics of biliary excreted drugs used in paediatrics and to apply a Physiologically Based Pharmacokinetic model to predict their systemic clearance (CLiv) across this age range.Drug parameters for azithromycin, ceftriaxone and digoxin were collated from the literature and validated against adult clinical data in Simcyp (V14R1). The change in CLiv with age was simulated in the paediatric model and compared to the observed data; the ontogeny function associated with BE was optimised in order to recover the age-related CLiv.For azithromycin (79% BE) a fraction of adult biliary excretion activity of 15% had to be assumed to be able to predict accurately the CL of the drug in neonates (24 to 28 weeks GA) whilst 100% activity was apparent by 7 months. For ceftriaxone (51% BE) full biliary excretion activity appeared to be present at full term birth. Finally, for digoxin (25% BE), a fraction of adult biliary excretion activity of 10% had to be assumed to predict the CL of the drug at birth whilst 100% activity was present by The ontogeny of BE for all three drugs appears to be rapid and reach adult levels at birth or in the first few months of postnatal age. More research is required in this area particularly on the ontogeny of specific canalicular transporters in humans.

The mechanism of increased biliary lipid secretion in mice with genetic inactivation of bile salt export pump

Human bile salt export pump ( BSEP) mutations underlie progressive familial intrahepatic cholestasis type 2 (PFIC2). In the PFIC2 animal model, Bsep−/−mice, biliary secretion of bile salts (BS) is decreased, but that of phospholipids (PL) and cholesterol (CH) is increased. Under physiological conditions, the biliary secretion of PL and CH is positively related (“coupled”) to that of BS. We aimed to elucidate the mechanism of increased biliary lipid secretion in Bsep−/−mice. The secretion of the BS tauro-β-muricholic acid (TβMCA) is relatively preserved in Bsep−/−mice. We infused Bsep−/−and Bsep+/+(control) mice with TβMCA in stepwise increasing dosages (150–600 nmol/min) and determined biliary bile flow, BS, PL, and CH secretion. mRNA and protein expression of relevant canalicular transporters was analyzed in livers from noninfused Bsep−/−and control mice. TβMCA infusion increased BS secretion in both Bsep−/−and control mice. The secreted PL or CH amount per BS, i.e., the “coupling,” was continuously two- to threefold higher in Bsep−/−mice ( P < 0.05). Hepatic mRNA expression of canalicular lipid transporters Mdr2, Abcg5, and Abcg8 was 45–55% higher in Bsep−/−mice (Abcg5; P < 0.05), as was canalicular Mdr2 and Abcg5 protein expression. Potential other explanations for the increased coupling of the biliary secretion of PL and CH to that of BS in Bsep−/−mice could be excluded. We conclude that the mechanism of increased biliary lipid secretion in Bsep−/−mice is based on increased expression of the responsible canalicular transporter proteins.

Altered expression and function of canalicular transporters during early development of cholestatic liver injury in Abcb4-deficient mice

Deficiency of ABCB4 is associated with several forms of cholestasis in humans. Abcb4−/− mice also develop cholestasis, but it remains uncertain what role other canalicular transporters play in the development of this disease. We examined the expression of these transporters in Abcb4−/− mice compared with their wild-type littermate controls at ages of 10 days and 3, 6, and 12 wk. Elevated plasma bile acid levels were already detected at 10 days and at all ages thereafter in Abcb4−/− mice. The expression of Bsep, Mrp2, Atp8b1, Abcg5, and Abcg8 liver proteins did not change at 10 days, but Bsep, Mrp2, and Atp8b1 were reduced, whereas Abcg5 and Abcg8 expression were increased in Abcb4−/− mice at all later ages. Lower bile acid concentrations were also detected in the bile of 6-wk-old Abcb4−/− mice. Immunofluorescence labeling revealed distorted canalicular architecture in the liver tissue by 12 wk in Abcb4−/− mice. Whereas Bsep and Mrp2 remained associated with the apical membrane, Atp8b1 was now localized in discrete punctuate structures adjacent to the canalicular membrane in these mice. Expression of Bsep mRNA was increased in the livers of 10-day-old Abcb4−/− mice, whereas Ost-α was decreased. By 12 wk, Bsep, Mrp2, and Abcg5 mRNA were all increased, whereas Ost-α and Ntcp were reduced. These findings indicate that canalicular transporters that determine the formation of bile are altered early in the development of cholestasis in Abcb4−/− mice and may contribute to the pathogenesis of cholestasis in this disorder.

Effects of In Vivo Hepatic Ischemia-Reperfusion Injury on the Hepatobiliary Disposition of Rhodamine 123 and its Metabolites in Isolated Perfused Rat Livers

Purpose. A few studies have shown that normothermic hepatic ischemia-reperfusion (IR) injury may affect the mRNA and/or protein levels of canalicular transporters P-glycoprotein (P-gp) and multidrug resistance-associated protein 2 (Mrp2). However, the effects of the injury on the functions of these canalicular transporters with respect to the biliary excretion of drugs remain largely unknown. Therefore, the purpose of this study was to investigate the effects of warm hepatic IR on the hepatobiliary disposition of rhodamine 123 (RH-123), a P-gp substrate, and its glucuronidated metabolite (RH-Glu), an Mrp2 substrate, in rats. Methods. Twenty four or 72 h following a 60-min partial ischemia or sham operation in rats, livers were isolated and perfused ex vivo with a constant concentration (~100 ng/mL) of RH-123. The concentration of RH-123 and its glucuronidated (RH-Glu) and deacylated (RH-110) metabolites were determined in the outlet perfusate, bile, and the liver tissue using HPLC, and relevant pharmacokinetic parameters were estimated. Results. Twenty-four-h IR caused a significant reduction in the hepatic extraction ratio of RH-123 (IR: 0.857 ± 0.078; Sham: 0.980 ± 0.017) and the biliary recovery of the parent drug and RH-Glu by 43% and 44%, respectively. The reductions in the biliary recovery were associated with significant reductions in the apparent biliary clearance of RH-123 and RH-Glu. Mass balance data showed that the formation of the glucuronidated or deacylated metabolite was not significantly affected by the 24-h IR injury. In contrast to the 24-h IR, the injury did not have any effect on the hepatobiliary disposition of RH-123 or its metabolites following 72 h of reperfusion. Conclusions. It is concluded that the pharmacokinetics of drugs that are subject to biliary excretion by the canalicular P-gp and Mrp2 transporters may be altered shortly after hepatic IR injury. This article is open to POST-PUBLICATION REVIEW. Registered readers (see “For Readers”) may comment by clicking on ABSTRACT on the issue’s contents page.