Targeting Xenobiotic Nuclear Receptors PXR and CAR to Prevent Cobicistat Hepatotoxicity

Liver-related diseases including drug-induced liver injury are becoming increasingly prominent in AIDS patients. Cobicistat (COBI) is the backbone of multiple regimens for antiretroviral therapy. The current work investigated the mechanisms of adverse drug-drug interactions associated with COBI that lead to liver damage. For individuals co-infected with HIV and tuberculosis (TB), the World Health Organization recommends the initiation of TB treatment followed by antiretroviral therapy. Rifampicin (RIF), a first line anti-TB drug, is a human specific activator of pregnane X receptor (PXR). Using PXR-humanized mice, we found that RIF-mediated PXR activation potentiates COBI hepatotoxicity. In contrast, rifabutin, a PXR-neutral analog of RIF, has no impact on COBI hepatotoxicity. Because of the crosstalk between PXR and the constitutive androstane receptor (CAR), the role of CAR in COBI hepatotoxicity was also investigated. Similar to PXR, ligand-dependent activation of CAR also potentiates COBI hepatotoxicity. Our further studies illustrated that PXR and CAR modulate COBI hepatotoxicity through the CYP3A4-dependent pathways. In summary, the current work determined PXR and CAR as key modulators of COBI hepatotoxicity. Given the fact that many prescription drugs and herbal supplements can activate PXR and CAR, these two receptors should be considered as targets to prevent COBI hepatotoxicity in the clinic.

ABCG2 Deficiency Does Not Alter Dolutegravir Metabolism and Pharmacokinetics

Dolutegravir (DTG) is a potent integrase inhibitor of human immunodeficiency virus. Because DTG is a substrate of the efflux transporter ABCG2 and ABCG2 is highly polymorphic, we asked whether dose adjustment of DTG is needed for ABCG2-deficient individuals. Using Abcg2-null mice, the current work investigated the impact of ABCG2 deficiency on DTG metabolism and pharmacokinetics. Compared with wild-type mice, no statistically significant difference was found in the systemic and tissue-specific (liver, kidney, and brain) pharmacokinetics of DTG in Abcg2-null mice. In addition, ABCG2 deficiency had no statistically significant impact on the production and excretion of DTG metabolites. In summary, this study demonstrated that deficiency of ABCG2 does not alter DTG metabolism and pharmacokinetics, suggesting that dose adjustment of DTG is not needed for individuals with ABCG2 deficiency. SIGNIFICANCE STATEMENT: The current work demonstrated that deficiency of ATP-binding cassette subfamily G member 2 (ABCG2) does not alter Dolutegravir (DTG) metabolism and pharmacokinetics, suggesting that dose adjustment of DTG is not needed for individuals with ABCG2 deficiency.

The essential role of the transporter ABCG2 in the pathophysiology of erythropoietic protoporphyria

Erythropoietic protoporphyria (EPP) is an inherited disease caused by loss-of-function mutations of ferrochelatase, an enzyme in the heme biosynthesis pathway that converts protoporphyrin IX (PPIX) into heme. PPIX accumulation in patients with EPP leads to phototoxicity and hepatotoxicity, and there is no cure. Here, we demonstrated that the PPIX efflux transporter ABCG2 (also called BCRP) determines EPP-associated phototoxicity and hepatotoxicity. We found that ABCG2 deficiency decreases PPIX distribution to the skin and therefore prevents EPP-associated phototoxicity. We also found that ABCG2 deficiency protects against EPP-associated hepatotoxicity by modulating PPIX distribution, metabolism, and excretion. In summary, our work has uncovered an essential role of ABCG2 in the pathophysiology of EPP, which suggests the potential for novel strategies in the development of therapy for EPP.

Pregnane X receptor activation potentiates ritonavir hepatotoxicity

Ritonavir (RTV) is on the World Health Organization's List of Essential Medicines for antiretroviral therapy, but can cause hepatotoxicity by unknown mechanisms. Multiple clinical studies found that hepatotoxicity occurred in 100% of participants who were pretreated with rifampicin or efavirenz followed by RTV-containing regimens. Both rifampicin and efavirenz are activators of the pregnane X receptor (PXR), a transcription factor with significant inter-species differences in ligand-dependent activation. Using PXR-humanized mouse models, we recapitulated the RTV hepatotoxicity observed in the clinic. PXR was found to modulate RTV hepatotoxicity through CYP3A4-dependent pathways involved in RTV bioactivation, oxidative stress, and endoplasmic reticulum stress. In summary, the current work demonstrated the essential roles of human PXR and CYP3A4 in RTV hepatotoxicity, which can be applied to guide the safe use of RTV-containing regimens in the clinic.

Identification of Novel Pathways in Idelalisib Metabolism and Bioactivation

Idelalisib (ILB) is a selective phosphatidylinositol-3-kinase delta inhibitor approved for the treatment of hematological malignancies. However, ILB frequently causes hepatotoxicity, and the exact mechanism remains unclear. The current study profiled the metabolites of ILB in mouse liver, urine, and feces. The major metabolites found in the liver were oxidized metabolite GS-563117 (M1) and ILB-glutathione (GSH) adduct (M2). These metabolic pathways were confirmed by analysis of urine and feces from mice treated with ILB. Identification of ILB-GSH adduct (M2) suggests the formation of reactive metabolites of ILB. We also found that M1 can produce reactive metabolites and form M1-GSH adducts. The GSH-conjugates identified in mouse liver were also found in the incubations of ILB and M1 with human liver microsomes. Furthermore, we illustrated that CYP3A4 and 2C9 are the key enzymes contributing to the bioactivation pathway of ILB and M1. In summary, our work revealed that both ILB and its major metabolite M1 can undergo bioactivation to produce reactive metabolites in the liver. Further studies are required to determine whether these metabolic pathways contribute to ILB hepatotoxicity.

Liver metabolomics in a mouse model of erythropoietic protoporphyria

Erythropoietic protoporphyria (EPP) is a genetic disease that results from the defective mutation in the gene encoding ferrochelatase (FECH), the enzyme that converts protoporphyrin IX (PPIX) to heme. Liver injury and even liver failure can occur in EPP patients because of PPIX accumulation in the liver. The current study profiled the liver metabolome in an EPP mouse model caused by a Fech mutation (Fech-mut). As expected, we observed the accumulation of PPIX in the liver of Fech-mut mice. In addition, our metabolomic analysis revealed the accumulation of bile acids and ceramide (Cer) in the liver of Fech-mut mice. High levels of bile acids and Cer are toxic to the liver. Furthermore, we found that the major phosphatidylcholines (PC) in the liver and the ratio of total PC to PPIX in the bile were decreased in Fech-mut mice compared to wild type mice. A decrease of the ratio of PC to PPIX in the bile can potentiate the accumulation of PPIX in the liver because PC increases PPIX solubility and excretion. These metabolomic findings suggest that the accumulation of PPIX, together with the disruption of the homeostasis of bile acids, Cer, and PC, contributes to EPP-associated liver injury.

Deficiency of N-acetyltransferase increases the interactions of isoniazid with endobiotics in mouse liver

Acetylation is the major metabolic pathway of isoniazid (INH) mediated by N-acetyltransferases (NATs). Previous reports suggest that slow acetylators have higher risks of INH hepatotoxicity than rapid acetylators, but the detailed mechanisms remain elusive. The current study used Nat1/2(-/-) mice to mimic NAT slow metabolizers and to investigate INH metabolism in the liver. We found that INH acetylation is abolished in the liver of Nat1/2(-/-) mice, suggesting that INH acetylation is fully dependent on NAT1/2. In addition to the acetylation pathway, INH can be hydrolyzed to form hydrazine (Hz) and isonicotinic acid (INA). We found that INA level was not altered in the liver of Nat1/2(-/-) mice, indicating that deficiency of NAT1/2 has no effect on INH hydrolysis. Because INH acetylation was abolished and INH hydrolysis was not altered in Nat1/2(-/-) mice, we expected an extremely high level of INH in the liver. However, we only observed a modest accumulation of INH in the liver of Nat1/2(-/-) mice, suggesting that there are alternative pathways in INH metabolism in NAT1/2 deficient condition. Our further studies revealed that the conjugated metabolites of INH with endobiotics, including fatty acids and vitamin B6, were significantly increased in the liver of Nat1/2(-/-) mice. In summary, this study illustrated that deficiency of NAT1/2 decreases INH acetylation, but increases the interactions of INH with endobiotics in the liver. These findings can be used to guide future studies on the mechanisms of INH hepatotoxicity in NAT slow metabolizers.

Biotransformation of Cobicistat: Metabolic Pathways and Enzymes

BACKGROUND

Cobicistat (COBI) is a pharmacoenhancer for antiretroviral therapy.

OBJECTIVE

The current study was designed to profile the metabolic pathways of COBI and to determine the enzymes that contribute to COBI metabolism.

METHOD

We screened COBI metabolites in mice and human liver microsomes. We also used cDNAexpressed human cytochromes P450 (CYPs) to explore the role of human enzymes in COBI metabolism.

RESULTS

Twenty new and three known metabolites of COBI were identified in mouse urine and feces. These new metabolic pathways of COBI include glycine conjugation, N-acetyl cysteine conjugation, morpholine ring-opening, and thiazole ring-opening. Twelve of COBI metabolites were further confirmed in mouse and human liver microsomes, including nine new metabolites. Consistent with the previous report, CYP3A4 and CYP2D6 were determined as the major enzymes that contribute to COBI metabolism.

CONCLUSION

This study provided a full map of COBI metabolism. These results can be used to manage CYP-mediated drug-drug interactions and adverse drug reactions that are associated with COBI-containing regimens in human.

The pregnane X receptor in tuberculosis therapeutics

INTRODUCTION

Among the infectious diseases, tuberculosis (TB) remains the second most common cause of death after HIV. TB treatment requires the combination of multiple drugs including the rifamycin class. However, rifamycins are activators of human pregnane X receptor (PXR), a transcription factor that regulates drug metabolism, drug resistance, energy metabolism and immune response. Rifamycin-mediated PXR activation may affect the outcome of TB therapy.

AREAS COVERED

This review describes the role of PXR in modulating metabolism, efficacy, toxicity and resistance to anti-TB drugs; as well as polymorphisms of PXR that potentially affect TB susceptibility.

EXPERT OPINION

The wide range of PXR functions that mediate drug metabolism and toxicity in TB therapy are often underappreciated and thus understudied. Further studies are needed to determine the overall impact of PXR activation on the outcome of TB therapy.