In vitro inhibition of UDP glucuronosyltransferases by atazanavir and other HIV protease inhibitors and the relationship of this property to in vivo bilirubin glucuronidation

Evaluating the impact of loperamide on irinotecan-induced adverse events: a disproportionality analysis of data from the World Health Organization pharmacovigilance database (VigiBase)

PURPOSE

SN-38, the active metabolite of irinotecan, may cause adverse events necessitating treatment discontinuation and management. Diarrhea, which is treated with loperamide, is one such event. However, loperamide may delay SN-38 elimination, causing more adverse events. Therefore, understanding the adverse events caused by symptomatic drugs is crucial for safe drug therapy. This study aimed to assess the association between loperamide and irinotecan-induced adverse events.

METHODS

We analyzed data up to December 2022 from VigiBase, the World Health Organization's adverse event database. The study used reporting odds ratios (RORs) to evaluate the associations between concomitant medications and irinotecan-induced adverse events. Fisher's exact probability test was used to analyze the adverse events. Logistic regression analysis was performed to identify associated adverse event signals.

RESULTS

Of the 32,520,983 cases analyzed, 57,454 involved the use of irinotecan. Among these, 1589 (2.8%) patients were co-treated with loperamide. Signals for neutropenia (ROR 1.37, 95% confidence interval (CI) 1.20-1.57, p < 0.001), anemia (ROR 1.81, 95% CI 1.43-2.30, p < 0.001), and alopecia (ROR 1.89, 95% CI 1.30-2.74, p < 0.01) were detected with concomitant loperamide. Multivariate logistic regression analysis confirmed that concomitant loperamide use was associated with signals for neutropenia, anemia, and alopecia.

CONCLUSION

Our results suggest that loperamide increases the risk of irinotecan-induced adverse events and enhances irinotecan toxicity. The study methodology may be useful for predicting adverse event risk when choosing symptomatic therapy drugs during irinotecan use.

UGT2B10 is the Major UDP-Glucuronosyltransferase 2B Isoform Involved in the Metabolism of Lamotrigine and is Implicated in the Drug-Drug Interaction with Valproic Acid

Lamotrigine is a phenyltriazine anticonvulsant that is primarily metabolized by phase II UDP-glucuronosyltransferases (UGT) to a quaternary N2-glucuronide, which accounts for ~ 90% of the excreted dose in humans. While there is consensus that UGT1A4 plays a predominant role in the formation of the N2-glucuronide, there is compelling evidence in the literature to suggest that the metabolism of lamotrigine is catalyzed by another UGT isoform. However, the exact identity of the UGT isoform that contribute to the formation of this glucuronide remains uncertain. In this study, we harnessed a robust reaction phenotyping strategy to delineate the identities and its associated fraction metabolized (fm) of the UGTs involved in lamotrigine N2-glucuronidation. Foremost, human recombinant UGT mapping experiments revealed that the N2-glucuronide is catalyzed by multiple UGT isoforms. (i.e., UGT1A1, 1A3, 1A4, 1A9, 2B4, 2B7, and 2B10). Thereafter, scaling the apparent intrinsic clearances obtained from the enzyme kinetic experiments with our in-house liver-derived relative expression factors (REF) and relative activity factors (RAF) revealed that, in addition to UGT1A4, UGT2B10 was involved in the N2-glucuronidation of lamotrigine. This was further confirmed via chemical inhibition in human liver microsomes with the UGT1A4-selective inhibitor hecogenin and the UGT2B10-selective inhibitor desloratadine. By integrating various orthogonal approaches (i.e., REF- and RAF-scaling, and chemical inhibition), we quantitatively determined that the fm for UGT1A4 and UGT2B10 ranged from 0.42 - 0.64 and 0.32 - 0.57, respectively. Finally, we also provided nascent evidence that the pharmacokinetic interaction between lamotrigine and valproic acid likely arose from the in vivo inhibition of its UGT2B10-mediated pathway.

Drug-drug interactions in HIV-infected patients receiving chemotherapy

INTRODUCTION

Coadministration of antiretrovirals and anti-cancer medications may present many complex clinical scenarios. This is characterized by the potential for drug-drug interactions (DDIs) and the challenges that arise in patient management. In this article, we investigate the potential for DDIs between antiretrovirals, including protease inhibitors (PIs), non-nucleoside reverse transcriptase inhibitors, nucleoside reverse transcriptase inhibitors (NRTIs), integrase strand transfer inhibitors (INSTIs), and anti-cancer medications.

AREAS COVERED

PubMed, Google Scholar, and Clinicaltrials.gov were searched for relevant articles in April 2024. Our review highlights PIs and NNRTIs as particularly prone to DDIs with anticancer agents, with implications for efficacy and toxicity of concomitant cancer therapy. We explain the mechanisms for interactions, emphasizing the significance of pharmacokinetic effects and enzyme induction or inhibition. We discuss clinical challenges encountered in the management of patients receiving combined ART and cancer therapy regimens.

EXPERT OPINION

Data are lacking for potential DDIs between antiretroviral and anti-cancer agents. While some interactions are documented, others are theoretical and based on the pharmacokinetic properties of the medications. Awareness of these interactions, inter-collaborative care between healthcare providers, and standardized treatment guidelines are all crucial for achieving optimal treatment outcomes and ensuring the well-being of patients with HIV/AIDS and cancer comorbidities.

Discovery of Unprecedented Human Stercobilin Conjugates

Two unique metabolites (M18 and M19) were detected in feces of human volunteers dosed orally with [14C]inavolisib with a molecular ion of parent plus 304 Da. They were generated in vitro by incubation with fecal homogenates and we have evidence that they are formed chemically and possibly enzymatically. Structural elucidation by high resolution mass spectrometry and nuclear magnetic resonance spectroscopy showed that the imidazole ring of inavolisib was covalently bound to partial structures derived from stercobilin, an end-product of heme catabolism produced by the gut microbiome. The structural difference between the two metabolites was the position of methyl and ethyl groups on the pyrrolidin-2-one moieties. We propose a mechanism of M18 and M19 generation from inavolisib and stercobilin whereby nucleophilic attack from the imidazole ring of inavolisib occurs to the bridging carbon of a stercobilin molecule. The proposed mechanism was supported by computational calculations of molecular orbitals and transition geometry. SIGNIFICANCE STATEMENT: We report the characterization of two previously undescribed conjugates of the phosphoinositide 3-kinase inhibitor inavolisib, generated by reaction with stercobilin, an end-product of heme catabolism produced by the gut microbiome. These conjugates were confirmed by generating them using in vitro fecal homogenate incubation via nonenzymatic and possibly enzymatic reactions. Given the unique nature of the conjugate, it is plausible that it may have been overlooked with other small molecule drugs in prior studies.

Pharmacokinetics and Safety of Twice-daily Ritonavir-boosted Atazanavir With Rifampicin

BACKGROUND

Critical drug-drug interactions (DDI) and hepatotoxicity complicate concurrent use of rifampicin and protease inhibitors. We investigated whether dose escalation of atazanavir/ritonavir could safely overcome the DDI with rifampicin.

METHODS

DERIVE (NCT04121195, EDCTP) was a dose-escalation trial in people with human immunodeficiency virus (HIV) on atazanavir/ritonavir-based antiretroviral therapy (ART) in Uganda. Four intensive pharmacokinetic (PK) visits were performed: PK1 300/100 mg OD (baseline); PK2 300/100 mg OD with rifampicin 600 mg; PK3 300/100 mg twice a day (BID) with rifampicin 600 mg OD; PK4 300/100 mg BID with rifampicin 1200 mg OD. Dolutegravir 50 mg BID throughout the study period ensured participants remained protected from subtherapeutic atazanavir concentrations. The data were interpreted with noncompartmental analysis. The target minimum concentration was atazanavir's protein-adjusted IC90 (PA-IC90), 0.014 mg/L.

RESULTS

We enrolled 26 participants (23 female) with median (range) age 44 (28-61) years and weight 67 (50-75) kg. Compared with PK1, atazanavir Ctau, and AUC were significantly reduced at PK2 by 96% and 85%, respectively. The escalation to BID dosing (PK3) reduced this difference in Ctau, and AUC24 to 18% lower and 8% higher, respectively. Comparable exposures were maintained with double doses of rifampicin. Lowest Ctau during PK1, PK3, and PK4 were 12.7-, 4.8-, and 8.6-fold higher than PA-IC90, respectively, whereas 65% of PK2 Ctau were below the limit of quantification (0.03 mg/L), hence likely below PA-IC90. No participant developed significant elevation of liver enzymes, reported a serious adverse event (SAE) or experienced rebound viraemia.

CONCLUSIONS

Twice daily atazanavir/ritonavir during rifampicin co-administration was well tolerated and achieved plasma concentrations above the target.

CLINICAL TRIALS REGISTRATION

NCT04121195. Registered on 09 October 2019, https://clinicaltrials.gov/ct2/show/NCT04121195.

Identification of Bromophenols' glucuronidation and its induction on UDP- glucuronosyltransferases isoforms

Bromophenols (BPs) are prominent environmental pollutants extensively utilized in aquaculture, pharmaceuticals, and chemical manufacturing. This study aims to identify UDP- glucuronosyltransferases (UGTs) isoforms involved in the metabolic elimination of BPs. Mono-glucuronides of BPs were detected in human liver microsomes (HLMs) incubated with the co-factor uridine-diphosphate glucuronic acid (UDPGA). The glucuronidation metabolism reactions catalyzed by HLMs followed Michaelis-Menten or substrate inhibition kinetics. Recombinant enzymes and inhibition experiments with chemical reagents were employed to phenotype the principal UGT isoforms participating in BP glucuronidation. UGT1A6 emerged as the major enzyme in the glucuronidation of 4-Bromophenol (4-BP), while UGT1A1, UGT1A6, and UGT1A8 were identified as the most essential isoforms for metabolizing 2,4-dibromophenol (2,4-DBP). UGT1A1, UGT1A8, and UGT2B4 were deemed the most critical isoforms in the catalysis of 2,4,6-tribromophenol (2,4,6-TBP) glucuronidation. Species differences were investigated using the liver microsomes of pig (PLM), rat (RLM), monkey (MyLM), and dog (DLM). Additionally, 2,4,6-TBP effects on the expression of UGT1A1 and UGT2B7 in HepG2 cells were evaluated. The results demonstrated potential induction of UGT1A1 and UGT2B7 upon exposure to 2,4,6-TBP at a concentration of 50 μM. Collectively, these findings contribute to elucidating the metabolic elimination and toxicity of BPs.

Inhibition of Uridine 5'-diphospho-glucuronosyltransferases A10 and B7 by vitamins: insights from in silico and in vitro studies

Uridine 5'-diphospho-glucuronosyltransferases (UGTs) have been considered as a family of enzymes responsible for the glucuronidation process, a crucial phase II detoxification reaction. Among the various UGT isoforms, UGTs A10 and B7 have garnered significant attention due to their broad substrate specificity and involvement in the metabolism of numerous compounds. Recent studies have suggested that certain vitamins may exert inhibitory effects on UGT activity, thereby influencing the metabolism of drugs, environmental toxins, and endogenous substances, ultimately impacting their biological activities. In the present study, the inhibition potential of vitamins (A, B1, B2, B3, B5, B6, B7, B9, D3, E, and C) on UGT1A10 and UGT2B7 was determined using in silico and in vitro approaches. A 3-dimensional model of UGT1A10 and UGT2B7 enzymes was built using Swiss Model, ITASSER, and ROSETTA and verified using Ramachandran plot and SAVES tools. Molecular docking studies revealed that vitamins interact with UGT1A10 and UGT2B7 enzymes by binding within the active site pocket and interacting with residues. Among all vitamins, the highest binding affinity predicted by molecular docking was - 8.61 kcal/mol with vitamin B1. The in vitro studies results demonstrated the inhibition of the glucuronidation activity of UGTs by vitamins A, B1, B2, B6, B9, C, D, and E, with IC50 values of 3.28 ± 1.07 µg/mL, 24.21 ± 1.11 µg/mL, 3.69 ± 1.02 µg/mL, 23.60 ± 1.08 µg/mL, 6.77 ± 1.08 µg/mL, 83.95 ± 1.09 µg/ml, 3.27 ± 1.13 µg/mL and 3.89 ± 1.12 µg/mL, respectively. These studies provided the valuable insights into the mechanisms underlying drug-vitamins interactions and have the potential to guide personalized medicine approaches, optimizing therapeutic outcomes, and ensuring patient safety. Indeed, further research in the area of UGT (UDP-glucuronosyltransferase) inhibition by vitamins is essential to fully understand the clinical relevance and implications of these interactions. UGTs play a crucial role in the metabolism and elimination of various drugs, toxins, and endogenous compounds in the body. Therefore, any factors that can modulate UGT activity, including vitamins, can have implications for drug metabolism, drug-drug interactions, and overall health.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40203-023-00182-0.

Establishment of UGT1A1-knockout human iPS-derived hepatic organoids for UGT1A1-specific kinetics and toxicity evaluation

Uridine diphosphate glucuronosyltransferases (UGTs) are highly expressed in the liver and are involved in the metabolism of many drugs. In particular, UGT1A1 has a genetic polymorphism that causes decreased activity, leading to drug-induced hepatotoxicity. Therefore, an in vitro evaluation system that accurately predicts the kinetics of drugs involving UGT1A1 is required. However, there is no such evaluation system because of the absence of the UGT1A1-selective inhibitor. Here, using human induced pluripotent stem (iPS) cells, genome editing technology, and organoid technology, we generated UGT1A1-knockout human iPS hepatocyte-derived liver organoids (UGT1A1-KO i-HOs) as a model for UGT1A1-specific kinetics and toxicity evaluation. i-HOs showed higher gene expression of many drug-metabolizing enzymes including UGT1A1 than human iPS cell-derived hepatocyte-like cells (iPS-HLCs), suggesting that hepatic organoid technology improves liver functions. Wild-type (WT) i-HOs showed similar levels of UGT1A1 activity to primary human (cryopreserved) hepatocytes, while UGT1A1-KO i-HOs completely lost the activity. Additionally, to evaluate whether this model can be used to predict drug-induced hepatotoxicity, UGT1A1-KO i-HOs were exposed to SN-38, the active metabolite of irinotecan, an anticancer drug, and acetaminophen and confirmed that these cells could predict UGT1A1-mediated toxicity. Thus, we succeeded in generating model cells that enable evaluation of UGT1A1-specific kinetics and toxicity.

Drug-drug interactions that alter the exposure of glucuronidated drugs: Scope, UDP-glucuronosyltransferase (UGT) enzyme selectivity, mechanisms (inhibition and induction), and clinical significance

Drug-drug interactions (DDIs) arising from the perturbation of drug metabolising enzyme activities represent both a clinical problem and a potential economic loss for the pharmaceutical industry. DDIs involving glucuronidated drugs have historically attracted little attention and there is a perception that interactions are of minor clinical relevance. This review critically examines the scope and aetiology of DDIs that result in altered exposure of glucuronidated drugs. Interaction mechanisms, namely inhibition and induction of UDP-glucuronosyltransferase (UGT) enzymes and the potential interplay with drug transporters, are reviewed in detail, as is the clinical significance of known DDIs. Altered victim drug exposure arising from modulation of UGT enzyme activities is relatively common and, notably, the incidence and importance of UGT induction as a DDI mechanism is greater than generally believed. Numerous DDIs are clinically relevant, resulting in either loss of efficacy or an increased risk of adverse effects, necessitating dose individualisation. Several generalisations relating to the likelihood of DDIs can be drawn from the known substrate and inhibitor selectivities of UGT enzymes, highlighting the importance of comprehensive reaction phenotyping studies at an early stage of drug development. Further, rigorous assessment of the DDI liability of new chemical entities that undergo glucuronidation to a significant extent has been recommended recently by regulatory guidance. Although evidence-based approaches exist for the in vitro characterisation of UGT enzyme inhibition and induction, the availability of drugs considered appropriate for use as 'probe' substrates in clinical DDI studies is limited and this should be research priority.

Glutamine Deficiency Promotes Immune and Endothelial Cell Dysfunction in COVID-19

The coronavirus disease 2019 (COVID-19) pandemic has caused the death of almost 7 million people worldwide. While vaccinations and new antiviral drugs have greatly reduced the number of COVID-19 cases, there remains a need for additional therapeutic strategies to combat this deadly disease. Accumulating clinical data have discovered a deficiency of circulating glutamine in patients with COVID-19 that associates with disease severity. Glutamine is a semi-essential amino acid that is metabolized to a plethora of metabolites that serve as central modulators of immune and endothelial cell function. A majority of glutamine is metabolized to glutamate and ammonia by the mitochondrial enzyme glutaminase (GLS). Notably, GLS activity is upregulated in COVID-19, favoring the catabolism of glutamine. This disturbance in glutamine metabolism may provoke immune and endothelial cell dysfunction that contributes to the development of severe infection, inflammation, oxidative stress, vasospasm, and coagulopathy, which leads to vascular occlusion, multi-organ failure, and death. Strategies that restore the plasma concentration of glutamine, its metabolites, and/or its downstream effectors, in conjunction with antiviral drugs, represent a promising therapeutic approach that may restore immune and endothelial cell function and prevent the development of occlusive vascular disease in patients stricken with COVID-19.

Therapeutic Drug Monitoring and Dosage Adjustments of Immunosuppressive Drugs When Combined With Nirmatrelvir/Ritonavir in Patients With COVID-19

ABSTRACT

Nirmatrelvir/ritonavir (Paxlovid) consists of a peptidomimetic inhibitor (nirmatrelvir) of the SARS-CoV-2 main protease and a pharmacokinetic enhancer (ritonavir). It is approved for the treatment of mild-to-moderate COVID-19. This combination of nirmatrelvir and ritonavir can mediate significant and complex drug-drug interactions (DDIs), primarily due to the ritonavir component. Indeed, ritonavir inhibits the metabolism of nirmatrelvir through cytochrome P450 3A (CYP3A) leading to higher plasma concentrations and a longer half-life of nirmatrelvir. Coadministration of nirmatrelvir/ritonavir with immunosuppressive drugs (ISDs) is particularly challenging given the major involvement of CYP3A in the metabolism of most of these drugs and their narrow therapeutic ranges. Exposure of ISDs will be drastically increased through the potent ritonavir-mediated inhibition of CYP3A, resulting in an increased risk of adverse drug reactions. Although a decrease in the dosage of ISDs can prevent toxicity, an inappropriate dosage regimen may also result in insufficient exposure and a risk of rejection. Here, we provide some general recommendations for therapeutic drug monitoring of ISDs and dosing recommendations when coadministered with nirmatrelvir/ritonavir. Particularly, tacrolimus should be discontinued, or patients should be given a microdose on day 1, whereas cyclosporine dosage should be reduced to 20% of the initial dosage during the antiviral treatment. Dosages of mammalian target of rapamycin inhibitors (m-TORis) should also be adjusted while dosages of mycophenolic acid and corticosteroids are expected to be less impacted.

A randomized, open-label trial of combined nitazoxanide and atazanavir/ritonavir for mild to moderate COVID-19

Background

The nitazoxanide plus atazanavir/ritonavir for COVID-19 (NACOVID) trial investigated the efficacy and safety of repurposed nitazoxanide combined with atazanavir/ritonavir for COVID-19.

Methods

This is a pilot, randomized, open-label multicenter trial conducted in Nigeria. Mild to moderate COVID-19 patients were randomly assigned to receive standard of care (SoC) or SoC plus a 14-day course of nitazoxanide (1,000 mg b.i.d.) and atazanavir/ritonavir (300/100 mg od) and followed through day 28. Study endpoints included time to clinical improvement, SARS-CoV-2 viral load change, and time to complete symptom resolution. Safety and pharmacokinetics were also evaluated (ClinicalTrials.gov ID: NCT04459286).

Results

There was no difference in time to clinical improvement between the SoC (n = 26) and SoC plus intervention arms (n = 31; Cox proportional hazards regression analysis adjusted hazard ratio, aHR = 0.898, 95% CI: 0.492-1.638, p = 0.725). No difference was observed in the pattern of saliva SARS-CoV-2 viral load changes from days 2-28 in the 35% of patients with detectable virus at baseline (20/57) (aHR = 0.948, 95% CI: 0.341-2.636, p = 0.919). There was no significant difference in time to complete symptom resolution (aHR = 0.535, 95% CI: 0.251-1.140, p = 0.105). Atazanavir/ritonavir increased tizoxanide plasma exposure by 68% and median trough plasma concentration was 1,546 ng/ml (95% CI: 797-2,557), above its putative EC90 in 54% of patients. Tizoxanide was undetectable in saliva.

Conclusion

Nitazoxanide co-administered with atazanavir/ritonavir was safe but not better than standard of care in treating COVID-19. These findings should be interpreted in the context of incomplete enrollment (64%) and the limited number of patients with detectable SARS-CoV-2 in saliva at baseline in this trial.

Clinical trial registration

[https://clinicaltrials.gov/ct2/show/NCT04459286], identifier [NCT04459286].

Improving Drug Delivery While Tailoring Prodrug Activation to Modulate Cmax and Cmin by Optimization of (Carbonyl)oxyalkyl Linker-Based Prodrugs of Atazanavir

Structure-property relationships associated with a series of (carbonyl)oxyalkyl amino acid ester prodrugs of the marketed HIV-1 protease inhibitor atazanavir (1), designed to enhance the systemic drug delivery, were examined. Compared to previously reported prodrugs, optimized candidates delivered significantly enhanced plasma exposure and trough concentration (C_min at 24 h) of 1 in rats while revealing differentiated PK paradigms based on the kinetics of prodrug activation and drug release. Prodrugs incorporating primary amine-containing amino acid promoieties offered the benefit of rapid bioactivation that translated into low circulating levels of the prodrug while delivering a high C_max value of 1. Interestingly, the kinetic profile of prodrug cleavage could be tailored for slower activation by structural modification of the amino terminus to either a tertiary amine or a dipeptide motif, which conferred a circulating depot of the prodrug that orchestrated a sustained release of 1 along with substantially reduced C_max and a further enhanced C_min.

UGT1A1 and UGT1A3 activity and inhibition in human liver and intestinal microsomes and a recombinant UGT system under similar assay conditions using selective substrates and inhibitors

In vitro enzyme kinetics and inhibition data was compared for UGT1A1 and UGT1A3 isoforms under similar assay conditions using human liver microsomes (HLM), human intestinal microsomes (HIM) and recombinant UGT (rUGT) enzyme systems.UGT1A1 catalysed β-estradiol 3-β-D-glucuronide formation showed allosteric sigmoidal kinetics in all enzyme systems; while UGT1A3 catalysed CDCA 24-acyl-β-D-glucuronide formation exhibited Michaelis-Menten kinetics in HLM, substrate inhibition kinetics in HIM and rUGT systems. Corresponding K_m or S₅₀ concentrations of β-estradiol and CDCA were employed in the respective UGT inhibition studies.Atazanavir inhibited the production of β-estradiol 3-β-D-glucuronide with IC50 values of 0.54 µM and 0.16 µM in HLM and rUGT1A1, respectively. But its inhibition potential was not observed in HIM, indicating potential cross-talk with other high-affinity intestinal UGT isozymes. On the other hand, zafirlukast, a pan UGT inhibitor, exhibited moderate inhibition in HIM with an IC50 value of 16.70 µM. Lithocholic acid, inhibited the production of CDCA 24-acyl-β-D-glucuronide with IC50 values of 1.68, 1.84, and 12.42 µM in HLM, rUGT1A3, and HIM, respectively.These results indicated that HLM, HIM, and rUGTs may be used as complementary in vitro systems to evaluate hepatic and intestinal UGT mediated DDIs at the screening stage.

PBPK Modeling as a Tool for Predicting and Understanding Intestinal Metabolism of Uridine 5'-Diphospho-glucuronosyltransferase Substrates

Uridine 5′-diphospho-glucuronosyltransferases (UGTs) are expressed in the small intestines, but prediction of first-pass extraction from the related metabolism is not well studied. This work assesses physiologically based pharmacokinetic (PBPK) modeling as a tool for predicting intestinal metabolism due to UGTs in the human gastrointestinal tract. Available data for intestinal UGT expression levels and in vitro approaches that can be used to predict intestinal metabolism of UGT substrates are reviewed. Human PBPK models for UGT substrates with varying extents of UGT-mediated intestinal metabolism (lorazepam, oxazepam, naloxone, zidovudine, cabotegravir, raltegravir, and dolutegravir) have demonstrated utility for predicting the extent of intestinal metabolism. Drug–drug interactions (DDIs) of UGT1A1 substrates dolutegravir and raltegravir with UGT1A1 inhibitor atazanavir have been simulated, and the role of intestinal metabolism in these clinical DDIs examined. Utility of an in silico tool for predicting substrate specificity for UGTs is discussed. Improved in vitro tools to study metabolism for UGT compounds, such as coculture models for low clearance compounds and better understanding of optimal conditions for in vitro studies, may provide an opportunity for improved in vitro–in vivo extrapolation (IVIVE) and prospective predictions. PBPK modeling shows promise as a useful tool for predicting intestinal metabolism for UGT substrates.

How Science Is Driving Regulatory Guidances

This chapter provides regulatory perspectives on how to translate in vitro drug metabolism findings into in vivo drug-drug interaction (DDI) predictions and how this affects the decision of conducting in vivo DDI evaluation. The chapter delineates rationale and analyses that have supported the recommendations in the U.S. Food and Drug Administration (FDA) DDI guidances in terms of in vitro-in vivo extrapolation of cytochrome P450 (CYP) inhibition-mediated DDI potential for investigational new drugs and their metabolites as substrates or inhibitors. The chapter also describes the framework and considerations to assess UDP-glucuronosyltransferase (UGT) inhibition-mediated DDI potential for drugs as substrates or inhibitors. The limitations of decision criteria and further improvements needed are also discussed. Case examples are provided throughout the chapter to illustrate how decision criteria have been utilized to evaluate in vivo DDI potential from in vitro data.

Case Study 1: Practical Considerations with Experimental Design and Interpretation

This chapter deals with practical considerations on key issues such as choosing an enzyme source, determining linear conditions, and choosing appropriate substrate and organic solvent concentrations. Practical solutions for working with limited resources and carrying out inhibition experiments are also addressed. Thus, after reading this chapter, the novice reader should have a better idea of how to design, develop, and interpret basic experiments using drug metabolism enzymes.

In vitro modulatory effects of ginsenoside compound K, 20(S)-protopanaxadiol and 20(S)-protopanaxatriol on uridine 5'-diphospho-glucuronosyltransferase activity and expression

We explored the inhibitory effect of ginsenoside compound K (CK), 20(S)-protopanaxadiol (PPD), and 20(S)-protopanaxatriol (PPT) on six uridine 5'-diphospho-glucuronosyltransferase (UGT) enzyme (UGT1A1, 1A3, 1A4, 1A6, 1A9, and 2B7) activities in human liver microsomes (HLMs) and 10 UGT enzyme (UGT1A1, 1A3, 1A4, 1A6, 1A9, 2B4, 2B7, 2B10, 2B15, and 2B17) activities in recombinant UGT isoforms.PPD was a potent inhibitor of UGT1A3 activity with half-maximal inhibitory concentration values of 5.62 and 3.38 μM in HLMs and recombinant UGT1A3, respectively. UGT1A3 inhibition by CK and PPD was competitive with inhibitory constant (K_i) values of 17.4 and 1.21 μM, respectively, and inhibition by PPT was non-competitive with a K_i value of 8.07 μM in HLMs. PPD exhibited more than 3.4-fold selectivity for UGT1A3 inhibition compared with other UGT isoforms inhibition, while CK and PPT showed more than 2.16- and 2.21-fold selectivity, respectively.PPD did not significantly increase the mRNA expression of UGT1A1, 1A3, 1A4, 1A9, and 2B7 in hepatocytes.Given the low plasma concentrations of PPD in healthy human subjects and the absence of induction potential on UGT isoforms, we conclude that PPD cause no pharmacokinetic interactions with other co-administered drugs metabolised by UGT1A3.

Comparison of Hepatotoxicity Associated With New BCR-ABL Tyrosine Kinase Inhibitors vs Imatinib Among Patients With Chronic Myeloid Leukemia: A Systematic Review and Meta-analysis

IMPORTANCE

Although BCR-ABL fusion oncoprotein tyrosine kinase inhibitors (BCR-ABL TKIs) can substantially improve the survival rate of chronic myeloid leukemia (CML), they are clinically accompanied by severe hepatotoxicity.

OBJECTIVE

To compare the relative risk (RR) of hepatotoxicity of new-generation BCR-ABL TKIs with that of imatinib, and to provide an overall assessment of the clinical benefit.

DATA SOURCES

PubMed, Embase, Cochrane library databases, and ClinicalTrials.gov were searched for clinical trials published between January 2000 and April 2020.

STUDY SELECTION

Study selection was conducted independently by 2 investigators according to the inclusion and exclusion criteria published previously in the protocol: only randomized phase 2 or phase 3 clinical trials that compared bosutinib, dasatinib, nilotinib, or ponatinib with imatinib were included. Among the 2666 records identified, 9 studies finally fulfilled the established criteria.

DATA EXTRACTION AND SYNTHESIS

Two investigators extracted study characteristics and data independently using a standardized data extraction form. Data were extracted according to Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) reporting guidelines. When substantial heterogeneity was observed, pooled estimates were calculated based on the random-effect model; otherwise, the fixed-effect model was used.

MAIN OUTCOMES AND MEASURES

Data extracted included study characteristics, baseline patient information, interventions and data on all-grade and high-grade (grades 3 and 4) elevation of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels, overall survival, and major molecular response (MMR). The RRs and 95% CIs were calculated using the inverse variance method.

RESULTS

Nine trials involving 3475 patients were analyzed; the median (range) age was 49 (18-91) years; 2059 (59.2%) were male patients. Increased risks were observed for each new-generation TKI except for dasatinib. Patients receiving new-generation TKIs were more likely to experience all grades of ALT elevation (pooled RR, 2.89; 95% CI, 1.78-4.69; P < .001) and grades 3 and 4 ALT elevation (pooled RR, 4.36; 95% CI, 2.00-9.50; P < .001) compared with those receiving imatinib. Patients receiving new-generation TKIs were also more likely to experience all grades of AST elevation (pooled RR, 2.20; 95% CI, 1.63-2.98; P < .001) and grades 3 and 4 AST elevation (pooled RR, 2.65; 95% CI, 1.59-4.42; P < .001) compared with those receiving imatinib. New-generation TKIs were associated with a significantly higher rate of MMR at 1 year compared with imatinib (pooled RR, 1.59; 95% CI, 1.44-1.75; P < .001). No statistical difference in overall survival at 1 year was found between new-generation TKIs and imatinib (pooled RR, 1.00; 95% CI, 1.00-1.01; P = .33).

CONCLUSIONS AND RELEVANCE

When compared to imatinib, bosutinib, nilotinib, and ponatinib had higher relative risks of hepatotoxicity. Treatment with new-generation TKIs was associated with a higher MMR rate at 1 year but not with 1-year overall survival.

Pharmacogenetic Associations Between Atazanavir/UGT1A1*28 and Efavirenz/rs3745274 (CYP2B6) Account for Specific Adverse Reactions in Chilean Patients Undergoing Antiretroviral Therapy

Background: Efavirenz (EFV), a non-nucleoside reverse transcriptase inhibitor, and atazanavir (ATV), a protease inhibitor, are drugs widely used in antiretroviral therapy (ART) for people living with HIV. These drugs have shown high interindividual variability in adverse drug reactions (ADRs). UGT1A1*28 and CYP2B6 c.516G>T have been proposed to be related with higher toxicity by ATV and EFV, respectively. Objective: To study the association between genetic polymorphisms and ADRs related to EFV or ATV in patients living with HIV treated at a public hospital in Chile. Methods: Epidemiologic, case-control, retrospective, observational study in 67 adult patients under EFV or ATV treatment was conducted, in the San Juan de Dios Hospital. Data were obtained from patients' medical records. Genotype analyses were performed using rtPCR for rs887829 (indirect identification of UGT1A1*28 allele) and rs3745274 (CYP2B6 c.516G>T), with TaqMan® probes. The association analyses were performed with univariate logistic regression between genetic variants using three inheritance models (codominant, recessive, and dominant). Results: In ATV-treated patients, hyperbilirubinemia (total bilirubin >1.2 mg/dl) had the main incidence (61.11%), and moderate and severe hyperbilirubinemia (total bilirubin >1.9 mg/dl) were statistically associated with UGT1A1*28 in recessive and codominant inheritance models (OR = 16.33, p = 0.028 and OR = 10.82, p = 0.036, respectively). On the other hand, in EFV-treated patients adverse reactions associated with CNS toxicity reached 34.21%. In this respect, nightmares showed significant association with CYP2B6 c.516G>T, in codominant and recessive inheritance models (OR = 12.00, p = 0.031 and OR = 7.14, p = 0.042, respectively). Grouped CNS ADRs (nightmares, insomnia, anxiety, and suicide attempt) also showed a statistically significant association with CYP2B6 c.516G > T in the codominant and recessive models (OR = 30.00, p = 0.011 and OR = 14.99, p = 0.021, respectively). Conclusion: Our findings suggest that after treatment with ATV or EFV, UGT1A1*28 and CYP2B6 c.516G>T influence the appearance of moderate-to-severe hyperbilirubinemia and CNS toxicity, respectively. However, larger prospective studies will be necessary to validate these associations in our population. Without a doubt, improving adherence in patients living with HIV is a critical issue to the success of therapy. Hence, validating and applying international pharmacogenetic recommendations in Latin American countries would improve the precision of ART: a fundamental aspect to achieve the 95-95-95 treatment target proposed by UNAIDS.

Evaluation of the Absorption, Metabolism, and Excretion of a Single Oral 1-mg Dose of Tropifexor in Healthy Male Subjects and the Concentration Dependence of Tropifexor Metabolism

Tropifexor (NVP-LJN452) is a highly potent, selective, nonsteroidal, non-bile acid farnesoid X receptor agonist for the treatment of nonalcoholic steatohepatitis. Its absorption, metabolism, and excretion were studied after a 1-mg oral dose of [¹⁴C]tropifexor was given to four healthy male subjects. Mass balance was achieved with ∼94% of the administered dose recovered in excreta through a 312-hour collection period. Fecal excretion of tropifexor-related radioactivity played a major role (∼65% of the total dose). Tropifexor reached a maximum blood concentration (C_max) of 33.5 ng/ml with a median time to reach C_max of 4 hours and was eliminated with a plasma elimination half-life of 13.5 hours. Unchanged tropifexor was the principal drug-related component found in plasma (∼92% of total radioactivity). Two minor oxidative metabolites, M11.6 and M22.4, were observed in circulation. Tropifexor was eliminated predominantly via metabolism with >68% of the dose recovered as metabolites in excreta. Oxidative metabolism appeared to be the major clearance pathway of tropifexor. Metabolites containing multiple oxidative modifications and combined oxidation and glucuronidation were also observed in human excreta. The involvement of direct glucuronidation could not be ruled out based on previous in vitro and nonclinical in vivo studies indicating its contribution to tropifexor clearance. The relative contribution of the oxidation and glucuronidation pathways appeared to be dose-dependent upon further in vitro investigation. Because of these complexities and the instability of glucuronide metabolites in the gastrointestinal tract, the contribution of glucuronidation remained undefined in this study. SIGNIFICANCE STATEMENT: Tropifexor was found to be primarily cleared from the human body via oxidative metabolism. In vitro metabolism experiments revealed that the relative contribution of oxidation and glucuronidation was concentration-dependent, with glucuronidation as the predominant pathway at higher concentrations and the oxidative process becoming more important at lower concentrations near clinical exposure range. The body of work demonstrated the importance of carefully designed in vivo and in vitro experiments for better understanding of disposition processes during drug development.

Nanoparticle delivery system, highly active antiretroviral therapy, and testicular morphology: The role of stereology

The conjugation of nanoparticles (NPs) with antiretroviral drugs is a drug delivery approach with great potential for managing HIV infections. Despite their promise, recent studies have highlighted the toxic effects of nanoparticles on testicular tissue and their impact on sperm morphology. This review explores the role of stereological techniques in assessing the testicular morphology in highly active antiretroviral therapy (HAART) when a nanoparticle drug delivery system is used. Also, NPs penetration and pharmacokinetics concerning the testicular tissue and blood-testis barrier form the vital part of this review. More so, various classes of NPs employed in biomedical and clinical research to deliver antiretroviral drugs were thoroughly discussed. In addition, considerations for minimizing nanoparticle-drugs toxicity, ensuring enhanced permeability of nanoparticles, maximizing drug efficacy, ensuring adequate bioavailability, and formulation of HAART-NPs fabrication are well discussed.

Investigation of CYP2B6, 3A4 and β-esterase interactions of Withania somnifera (L.) dunal in human liver microsomes and HepG2 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Withania somnifera (L.) Dunal (Solanaceae) is a traditional herb, used in African indigenous systems of medicine for the treatment of various diseases (including HIV/AIDS and tuberculosis). The relevance of clinically significant interactions of Withania with ARVs and anti-TB drugs needs to be investigated.

AIM OF THE STUDY

This study evaluated the effects of its roots on cytochromes P450 (CYPs) 2B6, 3A4, and rifampicin metabolism pathway, using methanol, ethanol, aqueous, and ethyl acetate solvent extractions.

MATERIALS AND METHODS

The extracts were tested on human liver microsomes (HLM) for CYP inhibition, mRNA expression in HepG2 cells for CYP induction. Biochemical qualitative tests and LC-MS/MS methodology were used to determine active phytoconstituents.

RESULTS

The methanolic and ethyl acetate extracts inhibited CYP2B6 with IC₅₀s 79.16 and 57.96 μg/ml respectively, while none of the extracts had any effect on rifampicin metabolism or showed time-dependant inhibition (TDI). All extracts were moderate inducers of CYP3A4; the aqueous extract exhibited 38%-fold shift induction of CYP3A4 compared to the control. The methanolic extract had the lowest CTC₅₀ (50% of cytotoxicity inhibition) (67.13 ± 0.83 μg/ml). LC-MS/MS-PDA full scans were consistent with the presence of flavone salvigenin (m/z 327), alkaloid isopelletierine (m/z 133), steroidal lactone 2,3-dihydrowithaferin-A (m/z 472), and other withanolides including withaperuvin I (m/z 533), withaferin derivative (m/z 567), some of these compounds likely being responsible for the observed CYP2B6 inhibition and CYP3A4 induction. The putative gastrointestinal tract (GIT) concentration for the active extracts was 1800 μg/ml and the hepatic circulation concentrations were estimated at about 220 μg/ml and 13.5 μg/ml for the methanolic and ethyl acetate extracts, respectively. The extrapolated in vivo percentage of inhibition was at 85% for the methanolic extract against CYP2B6.

CONCLUSIONS

The findings reported in this study suggest that W. somnifera extracts have the potential of causing clinically significant herb-drug interactions (HDI) as moderate inducer of CYP3A4 and inhibitor of CYP2B6 metabolism pathway (methanol and ethyl acetate extracts).

Case Study 6: Deconvoluting Hyperbilirubinemia-Differentiating Between Hepatotoxicity and Reversible Inhibition of UGT1A1, MRP2, or OATP1B1 in Drug Development

New molecular entities (NMEs) are evaluated using a rigorous set of in vitro and in vivo studies to assess their safety and suitability for testing in humans. Regulatory health authorities require that therapeutic and supratherapeutic doses be administered, by the intended route of administration, to two nonclinical species prior to human testing. The purpose of these studies is to identify potential target organ toxicity and to determine if the effects are reversible. Liver is a potential site for toxicity caused by orally administered NMEs due to high exposure during first pass after oral administration. A range of clinical chemistry analytes are routinely measured in both nonclinical and clinical studies to evaluate and monitor for hepatotoxicity. While bilirubin itself circulates within a wide range of concentrations in many animal species and humans, without causing adverse effects and possibly providing benefits, bilirubin is one of the few readily monitored circulating biomarkers that can provide insight into liver function. Therefore, any changes in plasma or urine bilirubin levels must be carefully evaluated. Changes in bilirubin may occur as a result of adaptive nontoxic changes or severe toxicity. Examples of adaptive nontoxic changes in liver function, which may elevate direct (conjugated) and/or indirect (unconjugated) bilirubin above baseline levels, include reversible inhibition of UGT1A1-mediated bilirubin metabolism and OATP1B1-, OATP1B3-, or MRP2-mediated transport. Alternatively, hepatocellular necrosis, hypoalbuminuria, or cholestasis may also lead to elevation of bilirubin; in some cases, these effects may be irreversible.This chapter aims to demonstrate application of enzyme kinetic principles in understanding the risk of bilirubin elevation through inhibition of multiple processes-involving both enzymes and transporters. In the sections that follow, we first provide a brief summary of bilirubin formation and disposition. Two case examples are then provided to illustrate the enzyme kinetic studies needed for risk assessment and for identifying the mechanisms of bilirubin elevation. Caveats of methods and data interpretation are discussed in these case studies. The data presented in this chapter is unpublished at the time of compilation of this book. It has been incorporated in this chapter to provide a sense of complexities in enzyme kinetics to the reader.

Differential effects of antiretroviral drug toxicity in male versus female children who are HIV-exposed but uninfected

: In recent years, widespread use of antiretroviral therapy (ART) during pregnancy has been increasingly effective in reducing risk of vertical transmission of HIV, with over 80% of pregnant women living with HIV now accessing ART, and a 41% reduction in new infections in children between 2010 and 2018. Despite these strides, the developmental toxicity of widely administered antiretroviral drugs (ARVs) remains poorly described and existing literature often fails to account for fetal and infant sex as a variable. Recent reports have identified associations between in-utero exposure to commonly used antiretroviral regimens and alteration in neurodevelopment, growth, and metabolism amongst children who are HIV-exposed but uninfected, with findings of sex differences in the prevalence and severity of ARV toxicity. These differences are potentially explained by variable exposure to ARV drugs in utero or exacerbation of existing sex-linked risk factors. Fetal ARV exposure is mediated by placental and fetal drug transporters and metabolic enzymes, which may contribute to the manifestation of sex differences. Existing evidence of sex differences in ARV toxicity in fetal development is concerning, and demands further research to guide optimal treatment options for maternal health and prevention of vertical HIV transmission.

Enzyme Kinetics of Uridine Diphosphate Glucuronosyltransferases (UGTs)

Glucuronidation, catalyzed by uridine diphosphate glucuronosyltransferases (UGTs), is an important process for the metabolism and clearance of many lipophilic chemicals, including drugs, environmental chemicals, and endogenous compounds. Glucuronidation is a bisubstrate reaction that requires the aglycone and the cofactor, UDP-GlcUA. Accumulating evidence suggests that the bisubstrate reaction follows a compulsory-order ternary mechanism. To simplify the kinetic modeling of glucuronidation reactions in vitro, UDP-GlcUA is usually added to incubations in large excess. Many factors have been shown to influence UGT activity and kinetics in vitro, and these must be accounted for during experimental design and data interpretation. While the assessment of drug-drug interactions resulting from UGT inhibition has been challenging in the past, the increasing availability of UGT enzyme-selective substrate and inhibitor "probes" provides the prospect for more reliable reaction phenotyping and assessment of drug-drug interaction potential. Although extrapolation of the in vitro intrinsic clearance of a glucuronidated drug often underpredicts in vivo clearance, careful selection of in vitro experimental conditions and inclusion of extrahepatic glucuronidation may improve the predictivity of in vitro-in vivo extrapolation. Physiologically based pharmacokinetic (PBPK) modeling has also shown to be of value for predicting PK of drugs eliminated by glucuronidation.

Drug-drug interaction of atazanavir on UGT1A1-mediated glucuronidation of molidustat in human

Molidustat is an oral inhibitor of hypoxia‐inducible factor (HIF) prolyl‐hydroxylase enhancing the erythropoietin (EPO) response to HIF; it is in clinical development for the treatment of anaemia related to chronic kidney disease. The predominant role of glucuronidation for molidustat clearance (formation of N‐glucuronide metabolite M1) and subsequent renal excretion was confirmed in a human mass balance study, with about 85% of the drug being excreted as M1 in urine. The inhibitory effects of 176 drugs and xenobiotics from various compound classes on the UGT‐mediated glucuronidation of molidustat in human liver microsomes (HLMs) were investigated. Based on preclinical findings, glucuronidation of molidustat was predominantly mediated by the 5'‐diphospho‐glucuronosyltransferase (UGT) isoform UGT1A1. Therefore, atazanavir, which is a potent inhibitor of UGT1A1, was chosen for the evaluation of pharmacokinetics and EPO release following a single oral dose of 25 mg molidustat. Molidustat exposure increased about twofold upon coadministration with atazanavir when considering area under plasma concentration‐time curve from zero to infinity (AUC) and maximum plasma concentration (C_max). Baseline‐corrected increase of EPO was 14% and 34% for C_max and AUC (calculated over 24 hours), respectively. Coadministration of molidustat and atazanavir was well tolerated.

Evidence-based strategies for the characterisation of human drug and chemical glucuronidation in vitro and UDP-glucuronosyltransferase reaction phenotyping

Enzymes of the UDP-glucuronosyltransferase (UGT) superfamily contribute to the elimination of drugs from almost all therapeutic classes. Awareness of the importance of glucuronidation as a drug clearance mechanism along with increased knowledge of the enzymology of drug and chemical metabolism has stimulated interest in the development and application of approaches for the characterisation of human drug glucuronidation in vitro, in particular reaction phenotyping (the fractional contribution of the individual UGT enzymes responsible for the glucuronidation of a given drug), assessment of metabolic stability, and UGT enzyme inhibition by drugs and other xenobiotics. In turn, this has permitted the implementation of in vitro - in vivo extrapolation approaches for the prediction of drug metabolic clearance, intestinal availability, and drug-drug interaction liability, all of which are of considerable importance in pre-clinical drug development. Indeed, regulatory agencies (FDA and EMA) require UGT reaction phenotyping for new chemical entities if glucuronidation accounts for ≥25% of total metabolism. In vitro studies are most commonly performed with recombinant UGT enzymes and human liver microsomes (HLM) as the enzyme sources. Despite the widespread use of in vitro approaches for the characterisation of drug and chemical glucuronidation by HLM and recombinant enzymes, evidence-based guidelines relating to experimental approaches are lacking. Here we present evidence-based strategies for the characterisation of drug and chemical glucuronidation in vitro, and for UGT reaction phenotyping. We anticipate that the strategies will inform practice, encourage development of standardised experimental procedures where feasible, and guide ongoing research in the field.

Targeting Heme Oxygenase-1 in the Arterial Response to Injury and Disease

Heme oxygenase-1 (HO-1) catalyzes the degradation of heme into carbon monoxide (CO), iron, and biliverdin, which is rapidly metabolized to bilirubin. The activation of vascular smooth muscle cells (SMCs) plays a critical role in mediating the aberrant arterial response to injury and a number of vascular diseases. Pharmacological induction or gene transfer of HO-1 improves arterial remodeling in animal models of post-angioplasty restenosis, vascular access failure, atherosclerosis, transplant arteriosclerosis, vein grafting, and pulmonary arterial hypertension, whereas genetic loss of HO-1 exacerbates the remodeling response. The vasoprotection evoked by HO-1 is largely ascribed to the generation of CO and/or the bile pigments, biliverdin and bilirubin, which exert potent antioxidant and anti-inflammatory effects. In addition, these molecules inhibit vascular SMC proliferation, migration, apoptosis, and phenotypic switching. Several therapeutic strategies are currently being pursued that may allow for the targeting of HO-1 in arterial remodeling in various pathologies, including the use of gene delivery approaches, the development of novel inducers of the enzyme, and the administration of unique formulations of CO and bilirubin.

HIV-1 Sanctuary Sites-the Role of Membrane-Associated Drug Transporters and Drug Metabolic Enzymes

Despite significant advances in the treatment of human immunodeficiency virus-1 (HIV) infection with highly active antiretroviral drug therapy, the persistence of the virus in cellular and anatomic reservoirs is a major obstacle preventing total HIV eradication. Viral persistence could result from a variety of contributing factors including, but not limited to, non-adherence to treatment and adverse drug reactions, latently infected cells carrying replication-competent virus, drug-drug interactions, and inadequate antiretroviral drug (ARV) concentrations reached in several anatomic sites such as the brain, testis, and gut-associated lymphoid tissues. The distribution of ARVs at specific sites of infection is primarily dependent on drug physicochemical properties and drug plasma protein binding, as well as drug efflux, influx, and metabolic processes. A thorough understanding of the functional roles of drug transporters and metabolic enzymes in the disposition of ARVs in immune cell types and tissues that are characterized as HIV reservoirs and sanctuaries is critical to overcome the challenge of suboptimal drug distribution at sites of persistent HIV infection. This review summarizes the current knowledge related to the expression and function of drug transporters and metabolic enzymes in HIV cellular and anatomic reservoirs, and their potential contribution to drug-drug interactions and insufficient drug concentration at these sites.

Main contribution of UGT1A1 and CYP2C9 in the metabolism of UR-1102, a novel agent for the treatment of gout

UR-1102, a novel uricosuric agent for treating gout, has been confirmed to exhibit a pharmacological effect in patients. We clarified its metabolic pathway, estimated the contribution of each metabolic enzyme, and assessed the impact of genetic polymorphisms using human in vitro materials. Glucuronide, sulfate and oxidative metabolites of UR-1102 were detected in human hepatocytes. The intrinsic clearance by glucuronidation or oxidation in human liver microsomes was comparable, but sulfation in the cytosol was much lower, indicating that the rank order of contribution was glucuronidation ≥ oxidation > sulfation. Recombinant UGT1A1 and UGT1A3 showed high glucuronidation of UR-1102. We took advantage of a difference in the inhibitory sensitivity of atazanavir to the UGT isoforms and estimated the fraction metabolised (fm) with UGT1A1 to be 70%. Studies using recombinant CYPs and CYP isoform-specific inhibitors showed that oxidation was mediated exclusively by CYP2C9. The effect of UGT1A1 and CYP2C9 inhibitors on UR-1102 metabolism in hepatocytes did not differ markedly between the wild type and variants.

In Vitro Assessment of the Interaction Potential of Ocimum basilicum (L.) Extracts on CYP2B6, 3A4, and Rifampicin Metabolism

Ocimum basilicum L. or basilicum is a common culinary herb, used as a traditional medicine for various medical conditions including HIV/AIDS and tuberculosis, in Africa. The objective of this study was to evaluate the effect of methanol, ethanol, aqueous and ethyl acetate extracts of the dried leaves and inflorescence of O. basilicum, on the activity of cytochrome P450 enzymes (CYPs) CYP2B6 and 3A4, as well as esterase-mediated metabolism of rifampicin to 25-O-desacetyl rifampicin (25ODESRIF). Human liver microsomes (HLM) were used to evaluate inhibition and CYP2B6/3A4 mRNA expression HepG2 assays were used to measure induction. Furthermore, the phytoconstituents likely involved in causing the observed effect were analyzed using biochemical tests and LC-MS. The aqueous and methanolic extracts showed reversible and time-dependent inhibition (TDI) of CYP2B6 with TDI-IC₅₀s 33.35 μg/ml (IC₅₀ shift-fold >1.5) and 4.93 μg/ml (IC₅₀ shift-fold >7) respectively, while the methanolic and ethanolic extracts inhibited 25ODESRIF formation (IC₅₀s 31 μg/ml, 8.94 μg/ml). In HepG2 assays, the methanolic and ethanolic extracts moderately induced CYP2B6, 3A4 mRNA with 38%-, 28%-fold shift, and 22%-, 44%-fold shift respectively. LC-MS full scans identified phenols rosmarinic acid [m/z 359 (M-H)^-, approximately 2298 mg/L in aqueous extract] and caftaric acid along with flavones salvigenin [m/z 329 (M+H)⁺, approximately 1855 mg/L in ethanolic extract], eupatorin [m/z 345 (M+H)⁺, 668.772 mg/L in ethanolic extract], rutin [m/z 609 (M-H)^-] and isoquercetin [m/z 463 (M-H)^-] and other compounds—linalool [m/z 153 (M-H)^-], hydroxyjasmonic acid [m/z 225 (M-H)^-], eucommiol [m/z 187 (M-H)^-] and trihydroxy octadecenoic acid [m/z 329 (M-H)^-, 530 mg/L in ethanolic extract]. The putative gastrointestinal tract (GIT) concentration for all extracts was calculated as 2,400 μg/ml and hepatic circulation concentrations were estimated at 805.68 μg/ml for the aqueous extract, and 226.56 μg/ml for methanolic extract. Based on the putative GIT concentration, estimated hepatic circulation concentration [I] and inhibition constant K_i, the predicted percentile of inhibition in vivo was highest for the aqueous extract on CYP2B6 (96.7%). The observations indicated that O. basilicum extracts may have the potential to cause clinically relevant herb-drug interactions (HDI) with CYP2B6 and rifampicin metabolism in vivo, if sufficient hepatic concentrations are reached in humans.

Association Between Bilirubin, Atazanavir, and Cardiovascular Disease Events Among People Living With HIV Across the United States

OBJECTIVE

Bilirubin is an antioxidant that may suppress lipid oxidation. Elevated bilirubin is associated with decreased cardiovascular events in HIV-uninfected populations. We examined these associations in people living with HIV (PLWH).

METHODS

Potential myocardial infarctions (MIs) and strokes were centrally adjudicated. We examined MI types: type 1 MI (T1MI) from atherosclerotic plaque instability and type 2 MI (T2MI) in the setting of oxygen demand/supply mismatch such as sepsis. We used multivariable Cox regression analyses to determine associations between total bilirubin levels and outcomes adjusting for traditional and HIV-specific risk factors. To minimize confounding by hepatobiliary disease, we conducted analyses limited to bilirubin values <2.1 mg/dL; among those with fibrosis-4 values <3.25; and among everyone. We repeated analyses stratified by hepatitis C status and time-updated atazanavir use.

RESULTS

Among 25,816 PLWH, there were 392 T1MI and 356 T2MI during follow-up. Adjusted hazard ratios for the association of higher bilirubin levels with T1MI were not significant. Higher bilirubin levels were associated with T2MI. By contrast, among PLWH on atazanavir, higher bilirubin levels were associated with fewer T2MI (hazard ratio 0.56:0.33-1.00). Higher bilirubin levels among those on atazanavir were associated with fewer T1MI combined with ischemic stroke.

LIMITATIONS

Analyses were conducted with total rather than unconjugated bilirubin.

CONCLUSIONS

Among PLWH, higher bilirubin levels were associated with T2MI among some subgroups. However, among those on atazanavir, there was a protective association between bilirubin and T2MI. These findings demonstrate different associations between outcomes and elevated bilirubin due to diverse causes and the importance of distinguishing MI types.

Key Potentially Inappropriate Drugs in Pediatrics: The KIDs List

OBJECTIVES

The safe use of medications in pediatric patients requires practitioners to consider the unique pharmacokinetics and pharmacodynamics of drugs prescribed in this age group. In an effort to create a standard of care for the safe use of medications in this population, a list of drugs that are potentially inappropriate for use in pediatric patients has been developed and titled the "KIDs List."

METHODS

A panel of 7 pediatric pharmacists from the Pediatric Pharmacy Association were recruited to evaluate primary, secondary, and tertiary literature; FDA Pediatric Safety Communications; the Lexicomp electronic database; and product information for drugs that should be considered potentially inappropriate for use in pediatric patients. Information was rated using predefined criteria. A PubMed search was conducted using the following terms: adverse drug events OR adverse drug reactions. The search was limited to humans; age <18 years; case reports, observational studies, or clinical trials; and English language. No date range was used. Results were used to create an evidence-based list of candidate drugs that was then peer-reviewed and subjected to a 30-day public comment period prior to being finalized.

RESULTS

A PubMed search yielded 4049 unique titles, of which 210 were deemed relevant for full review. Practitioner recommendations highlighted an additional 77 drugs. FDA Pediatric Safety Communications and the Lexicomp database yielded 22 and 619 drugs, respectively. After critical analysis, peer review, and public review the final KIDs List contains 67 drugs and/or drug classes and 10 excipients.

CONCLUSIONS

This extensive effort led to compilation of the first list of drugs that are potentially inappropriate for prescribing in all or in a select subgroup of pediatric patients. If avoidance is not clinically possible, the drug should be used with caution and accompanied by appropriate monitoring.

Robustness testing and optimization of an adverse outcome pathway on cholestatic liver injury

Adverse outcome pathways (AOPs) have been recently introduced as tools to map the mechanisms underlying toxic events relevant for chemical risk assessment. AOPs particularly depict the linkage between a molecular initiating event and an adverse outcome through a number of intermediate key events. An AOP has been previously introduced for cholestatic liver injury. The objective of this study was to test the robustness of this AOP for different types of cholestatic insult and the in vitro to in vivo extrapolation. For this purpose, in vitro samples from human hepatoma HepaRG cell cultures were exposed to cholestatic drugs (i.e. intrahepatic cholestasis), while in vivo samples were obtained from livers of cholestatic mice (i.e. extrahepatic cholestasis). The occurrence of cholestasis in vitro was confirmed through analysis of bile transporter functionality and bile acid analysis. Transcriptomic analysis revealed inflammation and oxidative stress as key events in both types of cholestatic liver injury. Major transcriptional differences between intrahepatic and extrahepatic cholestatic liver insults were observed at the level of cell death and metabolism. Novel key events identified by pathway analysis included endoplasmic reticulum stress in intrahepatic cholestasis, and autophagy and necroptosis in both intrahepatic as extrahepatic cholestasis. This study demonstrates that AOPs constitute dynamic tools that should be frequently updated with new input information.

Glucuronidation and its effect on the bioactivity of amentoflavone, a biflavonoid from Ginkgo biloba leaves

OBJECTIVES

Ginkgo biloba leaves contain amentoflavone (AMF), a dietary flavonoid that possesses antioxidant and anticancer activity. Flavonoids are extensively subjected to glucuronidation. This study aimed to determine the metabolic profile of AMF and the effect of glucuronidation on AMF bioactivity.

METHODS

A pharmacokinetic study was conducted to determine the plasma concentrations of AMF and its metabolites. The metabolic profile of AMF was elucidated using different species of microsomes. The antioxidant activity of AMF metabolites was determined using DPPH/ABTS radical and nitric oxide assays. The anticancer activity of AMF metabolites was evaluated in U87MG/U251 cells.

KEY FINDINGS

Pharmacokinetic studies indicated that the oral bioavailability of AMF was 0.06 ± 0.04%, and the area under the curve of the glucuronidated AMF metabolites (410.938 ± 62.219 ng/ml h) was significantly higher than that of AMF (194.509 ± 16.915 ng/ml h). UGT1A1 and UGT1A3 greatly metabolized AMF. No significant difference was observed in the antioxidant activity between AMF and its metabolites. The anticancer activity of AMF metabolites significantly decreased.

CONCLUSIONS

A low AMF bioavailability was due to extensive glucuronidation, which was mediated by UGT1A1 and UGT1A3. Glucuronidated AMF metabolites had the same antioxidant but had a lower anticancer activity than that of AMF.

Perspectives from the Innovation and Quality Consortium Induction Working Group on Factors Impacting Clinical Drug-Drug Interactions Resulting from Induction: Focus on Cytochrome 3A Substrates

A recent publication from the Innovation and Quality Consortium Induction Working Group collated a large clinical data set with the goal of evaluating the accuracy of drug-drug interaction (DDI) prediction from in vitro data. Somewhat surprisingly, comparison across studies of the mean- or median-reported area under the curve ratio showed appreciable variability in the magnitude of outcome. This commentary explores the possible drivers of this range of outcomes observed in clinical induction studies. While recommendations on clinical study design are not being proposed, some key observations were informative during the aggregate analysis of clinical data. Although DDI data are often presented using median data, individual data would enable evaluation of how differences in study design, baseline expression, and the number of subjects contribute. Since variability in perpetrator pharmacokinetics (PK) could impact the overall DDI interpretation, should this be routinely captured? Maximal induction was typically observed after 5-7 days of dosing. Thus, when the half-life of the inducer is less than 30 hours, are there benefits to a more standardized study design? A large proportion of CYP3A4 inducers were also CYP3A4 inhibitors and/or inactivators based on in vitro data. In these cases, using CYP3A selective substrates has limitations. More intensive monitoring of changes in area under the curve over time is warranted. With selective CYP3A substrates, the net effect was often inhibition, whereas less selective substrates could discern induction through mechanisms not susceptible to inhibition. The latter included oral contraceptives, which raise concerns of reduced efficacy following induction. Alternative approaches for modeling induction, such as applying biomarkers and physiologically based pharmacokinetic modeling (PBPK), are also considered. SIGNIFICANCE STATEMENT: The goal of this commentary is to stimulate discussion on whether there are opportunities to optimize clinical drug-drug interaction study design. The overall aim is to reduce, understand and contextualize the variability observed in the magnitude of induction across reported clinical studies. A large clinical CYP3A induction dataset was collected and further analyzed to identify trends and gaps. Reporting individual victim PK data, characterizing perpetrator PK and including additional PK assessments for mixed-mechanism perpetrators may provide insights into how these factors impact differences observed in clinical outcomes. The potential utility of biomarkers and PBPK modeling are discussed in considering future directions.

Association between the UGT1A1*28 allele and hyperbilirubinemia in HIV-positive patients receiving atazanavir: a meta-analysis

Objectives The uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1)*28 allele in HIV-positive patients receiving atazanavir (ATV) might be associated with the risk of hyperbilirubinemia. Owing to mixed and inconclusive results, a meta-analysis was conducted to systematically summarize and clarify this association.

Methods Based on a comprehensive search of PubMed, Embase and Web of Science databases, studies investigating the association between UGT1A1 alleles and hyperbilirubinemia was retrieved. We evaluated the strength of this relationship using odds ratios (ORs) with 95% confidence intervals (CIs). Sensitivity analysis was performed by removing each study one at a time and calculating the pooled ORs of the remaining studies to test the robustness of the meta-analysis results. The Q statistic and the I² index statistic were used to assess heterogeneity. Publication bias was evaluated using Orwin’s fail-safe N test.

Results A total of six individual studies were included in this meta-analysis. A significantly increased risk of hyperbilirubinemia was observed in HIV-positive patients receiving ATV with the UGT1A1*1/*28 or UGT1A1*28/*28 genotype, and the risk was higher with the UGT1A1*28/*28 genotype than with the UGT1A1*1/*28 genotype. (UGT1A1*28/*28 versus UGT1A1*1/*28: OR = 3.69, 95%CI = 1.82–7.49; UGT1A1*1/*28 versus UGT1A1*1/*1: OR = 3.50, 95%CI = 1.35–9.08; UGT1A1*28/*28 versus UGT1A1*1/*1: OR = 10.07, 95%CI = 4.39–23.10). All of the pooled ORs were not significantly affected by the remaining studies and different modeling methods, indicating robust results.

Conclusions This meta-analysis suggests that the UGT1A1*28 allele represents a biomarker for an increased risk of hyperbilirubinemia in HIV-positive patients receiving ATV.

Doravirine and the Potential for CYP3A-Mediated Drug-Drug Interactions

Identifying and understanding potential drug-drug interactions (DDIs) are vital for the treatment of human immunodeficiency virus type 1 (HIV-1) infection. This article discusses DDIs between doravirine, a nonnucleoside reverse transcriptase inhibitor (NNRTI), and cytochrome P450 3A (CYP3A) substrates and drugs that modulate CYP3A activity. Consistent with previously published in vitro data and DDI trials with the CYP3A substrates midazolam and atorvastatin, doravirine did not have any meaningful impact on the pharmacokinetics of the CYP3A substrates ethinyl estradiol and levonorgestrel. Coadministration of doravirine with CYP3A inhibitors (ritonavir or ketoconazole) increased doravirine exposure approximately 3-fold. However, these increases were not considered clinically meaningful. Conversely, previously published trials showed that coadministered CYP3A inducers (rifampin and rifabutin) decreased doravirine exposure by 88% and 50%, respectively (K. L. Yee, S. G. Khalilieh, R. I. Sanchez, R. Liu, et al., Clin Drug Investig 37:659-667, 2017 [https://doi.org/10.1007/s40261-017-0513-4]; S. G. Khalilieh, K. L. Yee, R. I. Sanchez, R. Liu, et al., J Clin Pharmacol 58:1044-1052, 2018 [https://doi.org/10.1002/jcph.1103]), while doravirine exposure following prior efavirenz administration led to an initial reduction in doravirine exposure of 62%, but the reduction became less pronounced with time (K. L. Yee, R. I. Sanchez, P. Auger, R. Liu, et al., Antimicrob Agents Chemother 61:e01757-16, 2017 [https://doi.org/10.1128/AAC.01757-16]). Overall, the coadministration of doravirine with CYP3A inhibitors and substrates is, therefore, supported by these data together with efficacy and safety data from clinical trials, while coadministration with strong CYP3A inducers, such as rifampin, cannot be recommended. Concomitant dosing with rifabutin (a CYP3A inducer less potent than rifampin) is acceptable if doravirine dosing is adjusted from once to twice daily; however, the effect of other moderate inducers on doravirine pharmacokinetics is unknown.

Indinavir Increases Midazolam N-Glucuronidation in Humans: Identification of an Alternate CYP3A Inhibitor Using an In Vitro to In Vivo Approach

Midazolam is a widely used index substrate for assessing effects of xenobiotics on CYP3A activity. A previous study involving human hepatocytes showed the primary route of midazolam metabolism, 1'-hydroxylation, shifted to N-glucuronidation in the presence of the CYP3A inhibitor ketoconazole, which may lead to an overprediction of the magnitude of a xenobiotic-midazolam interaction. Because ketoconazole is no longer recommended as a clinical CYP3A inhibitor, indinavir was selected as an alternate CYP3A inhibitor to evaluate the contribution of the N-glucuronidation pathway to midazolam metabolism. The effects of indinavir on midazolam 1'-hydroxylation and N-glucuronidation were first characterized in human-derived in vitro systems. Compared with vehicle, indinavir (10 μM) inhibited midazolam 1'-hydroxylation by recombinant CYP3A4, human liver microsomes, and high-CYP3A activity cryopreserved human hepatocytes by ≥70%; the IC₅₀ obtained with hepatocytes (2.7 μM) was within reported human unbound indinavir C_max (≤5 μM). Midazolam N-glucuronidation in hepatocytes increased in the presence of indinavir in both a concentration-dependent (1-33 μM) and time-dependent (0-4 hours) manner (by up to 2.5-fold), prompting assessment in human volunteers (n = 8). As predicted by these in vitro data, indinavir was a strong inhibitor of the 1'-hydroxylation pathway, decreasing the 1'-hydroxymidazolam/midazolam area under the plasma concentration versus time curve (AUC)_0-12h ratio by 80%. Although not statistically significant, the midazolam N-glucuronide/midazolam AUC_0-12h ratio increased by 40%, suggesting a shift to the N-glucuronidation pathway. The amount of midazolam N-glucuronide recovered in urine increased 4-fold but remained <10% of the oral midazolam dose (2.5 mg). A powered clinical study would clarify whether N-glucuronidation should be considered when assessing the magnitude of a xenobiotic-midazolam interaction.

Recent progress and challenges in screening and characterization of UGT1A1 inhibitors

Uridine-diphosphate glucuronosyltransferase 1A1 (UGT1A1) is an important conjugative enzyme in mammals that is responsible for the conjugation and detoxification of both endogenous and xenobiotic compounds. Strong inhibition of UGT1A1 may trigger adverse drug/herb-drug interactions, or result in metabolic disorders of endobiotic metabolism. Therefore, both the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA) have recommended assaying the inhibitory potential of drugs under development on the human UGT1A1 prior to approval. This review focuses on the significance, progress and challenges in discovery and characterization of UGT1A1 inhibitors. Recent advances in the development of UGT1A1 probes and their application for screening UGT1A1 inhibitors are summarized and discussed in this review for the first time. Furthermore, a long list of UGT1A1 inhibitors, including information on their inhibition potency, inhibition mode, and affinity, has been prepared and analyzed. Challenges and future directions in this field are highlighted in the final section. The information and knowledge that are presented in this review provide guidance for rational use of drugs/herbs in order to avoid the occurrence of adverse effects via UGT1A1 inhibition, as well as presenting methods for rapid screening and characterization of UGT1A1 inhibitors and for facilitating investigations on UGT1A1-ligand interactions.

Isolated Silymarin Flavonoids Increase Systemic and Hepatic Bilirubin Concentrations and Lower Lipoperoxidation in Mice

Bilirubin is considered to be one of the most potent endogenous antioxidants in humans. Its serum concentrations are predominantly affected by the activity of hepatic bilirubin UDP-glucuronosyl transferase (UGT1A1). Our objective was to analyze the potential bilirubin-modulating effects of natural polyphenols from milk thistle (Silybum marianum), a hepatoprotective herb. Human hepatoblastoma HepG2 cells were exposed to major polyphenolic compounds isolated from milk thistle. Based on in vitro studies, 2,3-dehydrosilybins A and B were selected as the most efficient compounds and applied either intraperitoneally or orally for seven days to C57BL/6 mice. After, UGT1A1 mRNA expression, serum, intrahepatic bilirubin concentrations, and lipoperoxidation in the liver tissue were analyzed. All natural polyphenols used increased intracellular concentration of bilirubin in HepG2 cells to a similar extent as atazanavir, a known bilirubinemia-enhancing agent. Intraperitoneal application of 2,3-dehydrosilybins A and B (the most efficient flavonoids from in vitro studies) to mice (50 mg/kg) led to a significant downregulation of UGT1A1 mRNA expression (46 ± 3% of controls, p < 0.005) in the liver and also to a significant increase of the intracellular bilirubin concentration (0.98 ± 0.03vs.1.21 ± 0.02 nmol/mg, p < 0.05). Simultaneously, a significant decrease of lipoperoxidation (61 ± 2% of controls, p < 0.005) was detected in the liver tissue of treated animals, and similar results were also observed after oral treatment. Importantly, both application routes also led to a significant elevation of serum bilirubin concentrations (125 ± 3% and 160 ± 22% of the controls after intraperitoneal and oral administration, respectively, p < 0.005 in both cases). In conclusion, polyphenolic compounds contained in silymarin, in particular 2,3-dehydrosilybins A and B, affect hepatic and serum bilirubin concentrations, as well as lipoperoxidation in the liver. This phenomenon might contribute to the hepatoprotective effects of silymarin.

Efforts in redesigning the antileukemic drug 6-thiopurine: decreasing toxic side effects while maintaining efficacy

6-Thiopurine (6TP) is a currently prescribed drug in the treatment of diseases ranging from Crohn's disease to acute lymphocytic leukemia. While its potent mode of action is through incorporation into DNA as a thiol mimic of deoxyguanosine, severe toxicities are associated with its administration which hinder the potential therapeutic application. We have previously reported in vitro that the oxidative metabolites of 6TP, specifically 6-thiouric acid (6TU, K _i 7 μM), are potent inhibitors of UDP-glucose dehydrogenase (UDPGDH), an enzyme that is responsible for the formation of UDP-glucuronic acid (UDPGA), an essential substrate that is used in detoxification processes in the liver. An in vivo investigation was undertaken to probe if 6TU inhibits UDPGDH in rat hepatocytes, and it was observed that 6TU does greatly suppress the conjugation of bilirubin with UDPGA. The failed excretion of bilirubin is linked to a majority of the reported toxicities associated with 6TP administration. Efforts were undertaken for the construction of 6TP analogs, substituted at the C8 position, to reduce inhibition of UDPGDH while retaining therapeutic efficacy. Three new 6TP analogs bearing a halogen (Br, Cl, and F) at the C8 position have been achieved over five-synthetic steps in overall yields of 16 to 32%. Each of these analogs were shown to have reduced inhibition towards UDPGDH, with K _i values of 192, 163, 215 μM, respectively. In addition, the bromine, chlorine, and fluorine analogs were shown to possess cytotoxicity towards the REH cell line (acute lymphocytic leukemia) having IC₅₀ values of 9.54 μM (±0.97), 3.95 μM (±1.94), and 4.71 μM (±1.40), respectively. These three new 6TP analogs represent the first steps in the redesign of this potent anticancer agent into a better drug that possesses reduced toxic side effects while retaining therapeutic potency.

Influence of Stereochemistry on the Bioactivation and Glucuronidation of 4-Ipomeanol

A potential CYP4B1 suicide gene application in engineered T-cell treatment of blood cancers has revived interest in the use of 4-ipomeanol (IPO) in gene-directed enzyme prodrug therapy, in which disposition of the administered compound may be critical. IPO contains one chiral center at the carbon bearing a secondary alcohol group; it was of interest to determine the effect of stereochemistry on 1) CYP4B1-mediated bioactivation and 2) (UGT)-mediated glucuronidation. First, (R)-IPO and (S)-IPO were synthesized and used to assess cytotoxicity in HepG2 cells expressing rabbit CYP4B1 and re-engineered human CYP4B1, where the enantiomers were found to be equipotent. Next, a sensitive UPLC-MS/MS assay was developed to measure the IPO-glucuronide diastereomers and product stereoselectivity in human tissue microsomes. Human liver and kidney microsomes generated (R)- and (S)-IPO-glucuronide diastereomers in ratios of 57:43 and 79:21, respectively. In a panel of 13 recombinantly expressed UGTs, UGT1A9 and UGT2B7 were the major isoforms responsible for IPO glucuronidation. (R)-IPO-glucuronide diastereoselectivity was apparent with each recombinant UGT, except UGT2B15 and UGT2B17, which favored the formation of (S)-IPO-glucuronide. Incubations with IPO and the UGT1A9-specific chemical inhibitor niflumic acid significantly decreased glucuronidation in human kidney, but only marginally in human liver microsomes, consistent with known tissue expression patterns of UGTs. We conclude that IPO glucuronidation in human kidney is mediated by UGT1A9 and UGT2B7. In human liver, it is mediated primarily by UGT2B7 and, to a lesser extent, UGT1A9 and UGT2B15. Overall, the lack of pronounced stereoselectivity for IPO's bioactivation in CYP4B1-transfected HepG2 cells, or for hepatic glucuronidation, suggests the racemate is an appropriate choice for use in suicide gene therapies.