Transporters and Drug-Metabolizing Enzymes in Drug Toxicity. Albert P. Li
intermediate is believed to be responsible for felbamate hepatotoxicity as well as for time‐dependence of CYP3A4, the P450 isoform responsible for nefazodone metabolism [119]. Nefazodone and its metabolites are found to cause mitochondrial damage, leading to reactive oxygen species (ROS) formation as demonstrated by reduced GSH depletion [120]. An in vitro human hepatocyte assay for the identification of drugs associated with liver failures using ROS/cellular adenosine triphosphate (ATP) ratio as an endpoint routinely used nefazodone as a positive control representing DILI drugs [121].
3.6.2 Transporters and Toxicity
in vitro studies with human hepatocytes suggest that inhibition of the transporter ABCB11 (bile salt export pump [BSEP]), leading to intracellular accumulation of the cytotoxic bile salts, may be a mechanism of nefazodone hepatotoxicity [122, 123]. Nefazodone was reported to inhibit the efflux transporters MDR1 and MRP2, demonstrating that the drug and its metabolites may interact with efflux transporters in vivo [124]. While BSEP inhibition has been associated with DILI as a result of bile salt accumulation, the involvement of MDR1 and MRP2 in hepatotoxicity has not yet been clearly established. One possibility is the increased accumulation of the parent drug or its toxic metabolites which are efflux transporter substrates.
3.6.3 Risk Factors
The current knowledge on nefazodone metabolism suggest that increased metabolic activation (e.g. enhanced CYP3A4 activity due to environmental and genetic factors) as well as compromised detoxification pathways (e.g. decreased glutathione S‐transferase (GST) activity and depletion of GSH by environmental agents) are likely risk factors of nefazodone hepatotoxicity. As nefazodone hepatotoxicity may be related to bile salt accumulation via BSEP inhibition, environmental, and genetic factors leading to compromised BSEP functions may also be a risk factor for its hepatotoxicity.
3.7 Obeticholic Acid
Obeticholic acid (6a‐ethyl‐chenodeoxycholic acid (OCA) is a farnesoid X receptor (FXR) agonist approved by FDA in May 2016 under the accelerated approval program for the treatment of primary biliary cholangitis (PBC) [125]. PBC is a chronic autoimmune cholestatic liver disease that predominantly affects women in early to middle age. It is typically associated with autoantibodies to mitochondrial antigens and results in immune‐mediated destruction of small and medium‐sized intrahepatic bile ducts leading to cholestasis, hepatic fibrosis and may progress to cirrhosis or hepatic failure and, in some cases, hepatocellular carcinoma [126–129].
FXR is an orphan nuclear hormone receptor found in the nucleus of cells in the liver, intestine, kidney, and adrenal glands [130–132] with bile acids – chenodeoxycholic acid, lithocholic acid, and deoxycholic acid as ligands [131]. OCA activation of FXR leads to reduction of hepatic bile acids via two mechanisms: (i) Reduction of the synthesis of the bile acid precursor, cholesterol, via downregulation of CYP7A1, the key enzyme for cholesterol synthesis [132, 133], and (ii) Upregulation of the efflux transporters of bile acids, BSEP [134], OST‐a, and OST‐b [135, 136].
OCA is used either as a single therapy or in combination with ursodeoxycholic acid (UDCA) in adult patients, for the 40% of PBC patients with an inadequate response to UDCA, and for the 10% of PBC patients who is unable to tolerate UDCA. As an agonist of FXR, OCA treatment reduces bile acid synthesis. The clinical efficacy of OCA was clearly demonstrated in PBC patients using plasma biomarkers for liver, alkaline phosphatase, gamma‐glutamyl transpeptidase, and alanine aminotransferase as biomarkers of PBC progression [126–129,137–142].
OCA administration has been associated with liver damage [143]. As a result of the occurrence of 19 deaths associated with OCA treatment, the FDA released not only a Drug Safety Communication on 21 September 2017, warning of serious liver injury caused by OCA due to incorrect dosing but also a Boxed Warning. FDA is also requiring a Medication Guide for patients to inform them about this issue (see weblink: FDA Adds Black Box Warning to Intercept’s Liver Disease Medicine | 2018‐02‐06 | FDANews).
3.7.1 Drug Metabolism and Toxicity
As a relatively new drug, there are limited numbers of publications on OCA metabolism. Its metabolism is likely to be similar to that known for bile acids. Bile acids are subjected to glucuronidation by the UDP‐glucuronosyltransferases (UGTs) 1A1, 2B4, and 2B7 and sulfation by sulfotransferases SULT2A1 and SULT2A8. The conjugated bile acids are secreted into bile via the canalicular bile salt export pump (BSEP), into the gastrointestinal tract. Some bile acids are passively absorbed in the upper intestine, but most are reabsorbed in the ileum as free bile acids upon deconjugation by the intestinal flora [144–146].
Five clinical drug–drug interaction studies have been reported on the effects of oral daily doses of 10 or 25 mg OCA on single‐dose plasma pharmacokinetics of specific probe substrates for enzymes CYP1A2 (caffeine, R‐warfarin), CYP3A (midazolam, R‐warfarin), CYP2C9 (S‐warfarin), CYP2D6 (dextromethorphan), CYP2C19 (omeprazole), and drug transporters, BCRP/OATP1B1/OATP1B3 (rosuvastatin), and P‐gp (digoxin). OCA was found to have no interactions with S‐warfarin, digoxin, and dextromethorphan, and weak interactions with caffeine, omeprazole, rosuvastatin, and midazolam [147]. The clinical findings therefore suggest that OCA does not significantly inhibit CY2C9, CYP2D6, CYP2C19, and CYP3A4.
The roles of OCA metabolism with its association of liver failures have not yet been defined. Primary cultured human hepatocytes were found to reproduce OCA activation of FXR leading to CYP7A suppression, but without associated cytotoxicity upon treatment for 72 hours with 0.1–100 μM of OCA, thereby suggesting that the clinical hepatotoxicity of OCA may not be a result of its direct cytotoxic effects [148, 149].
3.7.2 Transporters and Toxicity
FXR regulates transporter‐mediated efflux via upregulation of BSEP [134], OST‐a, and OST‐b [135, 136]. OCA, as an agonist of FXR, has been shown to increase the mRNA levels of the bile salt export pump (BSEP), and the basolateral efflux heterodimer transporters, organic solute transporter alpha (OSTalpha) and OSTbeta in primary cultured human hepatocytes [148]. However, as of this writing, there the roles of transporters in OCA toxicity, if any, is not yet defined.
3.7.3 Risk Factors
As the mechanism of OCA toxicity is not yet elucidated, risk factors are not yet defined. Based on clinical findings, patients with preexisting liver cirrhosis may have a higher risk toward OCA hepatotoxicity. This “risk factor”, however, is yet to be confirmed (https://livertox.nih.gov/ObeticholicAcid.htm).
3.8 Sitaxentan
Sitaxentan is a benzodioxole drug marketed for the treatment of pulmonary arterial hypertension (PAH) [150, 151]. It also has been found to be an effective treatment of digital ulcers, easing pain, and preventing the formation of new ulcers [152]. It is a competitive antagonist of endothelin‐1 which has been found to be elevated in PAH patients [153]. Binding of endothelin‐1 to its receptors, endothelin‐A (ET‐A) and endothelin‐B (ET‐B), leads to pulmonary vasoconstriction [153]. Inhibition of endothelin‐1 binding to its receptors by Sitaxentan thereby inhibits the vasoconstriction effects of endothelin, leading to decreases in pulmonary vascular resistance [150, 151]. Sitaxentan was found to be associated with idiosyncratic acute liver failures [154–157], resulting in deaths and a need for liver transplantation. Sitaxentan was withdrawn from the market worldwide in 2010.
3.8.1 Drug Metabolism and Toxicity
As for most marketed drugs that are found to be associated with idiosyncratic liver toxicity, Sitaxentan has been extensively evaluated in preclinical safety tests in multiple animal species, including single‐ and multiple‐dose studies in mice, rats, and dogs. Preclinical findings were non‐remarkable. While signs of liver toxicity including liver hypertrophy were observed,