Evaluation of 4β-Hydroxycholesterol and 25-Hydroxycholesterol as Endogenous Biomarkers of CYP3A4: Study with CYP3A-Humanized Mice

Induction of hepatic CYP3A4 expression by cholesterol and cholic acid: Alterations of gene expression, microsomal activity, and pharmacokinetics

Human cytochrome P450 3A4 (CYP3A4) is a drug-metabolizing enzyme that is abundantly expressed in the liver and intestine. It is an important issue whether compounds of interest affect the expression of CYP3A4 because more than 30% of commercially available drugs are metabolized by CYP3A4. In this study, we examined the effects of cholesterol and cholic acid on the expression level and activity of CYP3A4 in hCYP3A mice that have a human CYP3A gene cluster and show human-like regulation of the coding genes. A normal diet (ND, CE-2), CE-2 with 1% cholesterol and 0.5% cholic acid (HCD) or CE-2 with 0.5% cholic acid was given to the mice. The plasma concentrations of cholesterol, cholic acid and its metabolites in HCD mice were higher than those in ND mice. In this condition, the expression levels of hepatic CYP3A4 and the hydroxylation activities of triazolam, a typical CYP3A4 substrate, in liver microsomes of HCD mice were higher than those in liver microsomes of ND mice. Furthermore, plasma concentrations of triazolam in HCD mice were lower than those in ND mice. In conclusion, our study suggested that hepatic CYP3A4 expression and activity are influenced by the combination of cholesterol and cholic acid in vivo.

Evaluation of the usefulness of plasma 4β-hydroxycholesterol concentration normalized by 4α-hydroxycholesterol for accurate CYP3A phenotyping

Plasma 4β-hydroxycholesterol (OHC) has drawn attention as an endogenous substrate indicating CYP3A activity. Plasma 4β-OHC is produced by hydroxylation by CYP3A4 and CYP3A5 and by cholesterol autoxidation. Plasma 4α-OHC is produced by cholesterol autoxidation and not affected by CYP3A activity. This study aimed to evaluate the usefulness of plasma 4β-OHC concentration minus plasma 4α-OHC concentration (4β-OHC-4α-OHC) compared with plasma 4β-OHC concentration and 4β-OHC/total cholesterol (TC) ratio in cross-sectional evaluation of CYP3A activity. Four hundred sixteen general adults were divided into 191 CYP3A5*1 carriers and 225 non-carriers. Twenty-six patients with chronic kidney disease (CKD) with CYP3A5*1 allele were divided into 14 with CKD stage 3 and 12 with stage 4-5D. Area under the receiver operating characteristic curve (AUC) for the three indices were evaluated for predicting presence or absence of CYP3A5*1 allele in general adults, and for predicting CKD stage 3 or stage 4-5D in patients with CKD. There was no significant difference between AUC of 4β-OHC-4α-OHC and AUC of plasma 4β-OHC concentration in general adults and in patients with CKD. AUC of 4β-OHC-4α-OHC was significantly smaller than that of 4β-OHC/TC ratio in general adults (p = 0.025), but the two indices did not differ in patients with CKD. In conclusion, in the present cross-sectional evaluation of CYP3A activity in general adults and in patients with CKD with CYP3A5*1 allele, the usefulness of 4β-OHC-4α-OHC was not different from plasma 4β-OHC concentration or 4β-OHC/TC ratio. However, because of the limitations in study design and subject selection of this research, these findings require verification in further studies.

The transcriptome pattern of liver, spleen and hypothalamus provides insights into genetic and biological changes in roosters in response to castration

Chicken is widely accepted by consumers because of its delicate taste and abundant animal protein. The rooster after castration (capon) is believed to show better flavor, however, the molecular changes of the underpinned metabolism after castration is not yet understood. In this study, we aimed to figure out the alternation of meat quality and underpinned molecular mechanism via transcriptomic profiling of liver, spleen and hypothalamus as targeted organs in response to the castration. We identified differential expressed genes and their enriched functions and pathways in these organs between capon and rooster samples through RNA-seq analysis. In the liver, the lipid metabolism with targeted FABP1gene was found significantly enriched, which may be as one of the factors contributing to increased fat deposition and thus better meat flavor in capons than roosters, as predicted by the significantly lower shear force in capons than in roosters in meat quality experiments. However, the ability to xenobiotic detoxification and excretion, vitamin metabolism, and antioxidative effect of hemoglobin evidenced of the capon may be compromised by the alternation of SULT, AOX1, CYP3A5, HBA1, HBBA, and HBAD. Besides, in both the spleen and hypothalamus, PTAFR, HPX, CTLA4, LAG3, ANPEP, CD24, ITGA2B, ITGB3, CD2, CD7, and BLB2 may play an important role in the immune system including function of platelet and T cell, development of monocyte/macrophage and B cell in capons as compared to roosters. In conclusion, our study sheds lights into the possible molecular mechanism of better meat flavor, fatty deposit, oxidative detoxification and immune response difference between capons and roosters.

Studies in the antiviral molecular mechanisms of 25-hydroxycholesterol: Disturbing cholesterol homeostasis and post-translational modification of proteins

Efficient antiviral drug discovery has been a pressing issue of global public health concern since the outbreak of coronavirus disease 2019. In recent years, numerous in vitro and in vivo studies have shown that 25-hydroxycholesterol (25HC), a reactive oxysterol catalyzed by cholesterol-25-hydroxylase, exerts broad-spectrum antiviral activity with high efficiency and low toxicity. 25HC restricts viral internalization and disturbs the maturity of viral proteins using multiple mechanisms. First, 25HC reduces lipid rafts and cholesterol in the cytomembrane by inhibiting sterol-regulatory element binding proteins-2, stimulating liver X receptor, and activating Acyl-coenzyme A: cholesterol acyl-transferase. Second, 25HC impairs endosomal pathways by restricting the function of oxysterol-binding protein or Niemann-pick protein C1, causing the virus to fail to release nucleic acid. Third, 25HC disturbs the prenylation of viral proteins by suppressing the sterol-regulatory element binding protein pathway and glycosylation by increasing the sensitivity of glycans to endoglycosidase. This paper reviews previous studies on the antiviral activity of 25HC in order to fully understand its role in innate immunity and how it may contribute to the development of urgently needed broad-spectrum antiviral drugs.

Correlations between 4β-hydroxycholesterol and hepatic and intestinal CYP3A4: protein expression, microsomal ex vivo activity, and in vivo activity in patients with a wide body weight range

PURPOSE

Variability in cytochrome P450 3A4 (CYP3A4) metabolism is mainly caused by non-genetic factors, hence providing a need for accurate phenotype biomarkers. Although 4β-hydroxycholesterol (4βOHC) is a promising endogenous CYP3A4 biomarker, additional investigations are required to evaluate its ability to predict CYP3A4 activity. This study investigated the correlations between 4βOHC concentrations and hepatic and intestinal CYP3A4 protein expression and ex vivo microsomal activity in paired liver and jejunum samples, as well as in vivo CYP3A4 phenotyping (midazolam) in patients with a wide body weight range.

METHODS

The patients (n = 96; 78 with obesity and 18 normal or overweight individuals) were included from the COCKTAIL-study (NCT02386917). Plasma samples for analysis of 4βOHC and midazolam concentrations, and liver (n = 56) and jejunal (n = 38) biopsies were obtained. The biopsies for determination of CYP3A4 protein concentration and microsomal activity were obtained during gastric bypass or cholecystectomy. In vivo CYP3A4 phenotyping was performed using semi-simultaneous oral (1.5 mg) and intravenous (1.0 mg) midazolam.

RESULTS

4βOHC concentrations were positively correlated with hepatic microsomal CYP3A4 activity (ρ = 0.53, p < 0.001), and hepatic CYP3A4 concentrations (ρ = 0.30, p = 0.027), but not with intestinal CYP3A4 concentrations (ρ = 0.18, p = 0.28) or intestinal microsomal CYP3A4 activity (ρ = 0.15, p = 0.53). 4βOHC concentrations correlated weakly with midazolam absolute bioavailability (ρ =  - 0.23, p = 0.027) and apparent oral clearance (ρ = 0.28, p = 0.008), but not with systemic clearance (ρ =  - 0.03, p = 0.81).

CONCLUSION

These findings suggest that 4βOHC concentrations reflect hepatic, but not intestinal, CYP3A4 activity. Further studies should investigate the potential value of 4βOHC as an endogenous biomarker for individual dose requirements of intravenously administered CYP3A4 substrate drugs.

TRIAL REGISTRATION

Clinical.

TRIALS

gov identifier: NCT02386917.

Oxysterols profiles in zebrafish (Danio rerio) embryos exposed to bisphenol A

INTRODUCTION

Oxysterols are cholesterol oxidation products and bioactive lipids involved in developmental signaling pathways, embryonic and postembryonic tissue patterning and homeostasis. The embryonic period is a very sensitive window of exposure to bisphenol A (BPA), hence the role of BPA on the levels of oxysterols in the very early stages of zebrafish embryogenesis is a relevant novel field of investigation.

OBJECTIVES

To compare the role of BPA on oxysterols levels in zebrafish embryos at 8 and 24 hours post fertilization (hpf) with cytochromes P450 (CYPs)-modulating chemicals (carbamazepine, ketoconazole, and hydrogen peroxide).

METHODS

Upon a dose range finding, zebrafish embryos were exposed to environmentally relevant (0.04μM) and toxicological (17.5 μM) BPA concentrations. Seven oxysterols were profiled by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS).

RESULTS

Similarly to the CYPs-modulating chemicals, BPA caused: i) no significant changes at 8 hpf and ii) a dose-dependent increase of total oxysterols at 24 hpf, with 27-hydroxycholesterol as the most regulated oxysterol.

DISCUSSION

In the first day post-fertilization of the zebrafish embryos, the role of BPA alike a CYPs-modulating chemical was confirmed by the similar oxysterol changes observed with the already known CYPs-modulating chemicals.

Identification cholesterol metabolites altered before the onset of nonalcoholic steatohepatitis by targeted metabolomics

Nonalcoholic steatohepatitis (NASH) is a disease with symptoms similar to those of alcoholic liver inflammation without alcohol intake. As an effective treatment strategy has not been established for this disease, a detailed understanding of the pathological progression mechanism is required. We focused on cholesterol metabolites, which are suspected to regulate NASH pathology, and investigated their relationship with the pathological progression in the early stages of NASH. First, the LC/MS/MS methods for bile acids and sterols were optimized and validated. Next, NASH model mice were established by feeding a choline-deficient, methionine-reduced high-fat diet, and the levels of hepatic cholesterol metabolites were measured. As a result, before the onset of NASH, desmosterol, 4β-hydroxycholesterol, campesterol, sitosterol, secondary bile acids such as taurodeoxycholic acid significantly decreased by up to 1/38 of NASH model group. Autoxidation-generated sterols significantly increased 2- to 5-fold, and various primary bile acids such as conjugated β-muricholic acids and cholic acids significantly increased 2- to 7-fold. In this study, the levels of cholesterol metabolites changed in the before the onset of NASH. These metabolic alterations involved in inflammation induction and detoxification for NASH may help the discovery of early diagnostic biomarkers in the future.

Variation in Expression of Cytochrome P450 3A Isoforms and Toxicological Effects: Endo- and Exogenous Substances as Regulatory Factors and Substrates

The CYP3A subfamily, which includes isoforms CYP3A4, CYP3A5, and CYP3A7 in humans, plays important roles in the metabolism of various endogenous and exogenous substances. Gene and protein expression of CYP3A4, CYP3A5, and CYP3A7 show large inter-individual differences, which are caused by many endogenous and exogenous factors. Inter-individual differences can cause negative outcomes, such as adverse drug events and disease development. Therefore, it is important to understand the variations in CYP3A expression caused by endo- and exogenous factors, as well as the variation in the metabolism and kinetics of endo- and exogenous substrates. In this review, we summarize the factors regulating CYP3A expression, such as bile acids, hormones, microRNA, inflammatory cytokines, drugs, environmental chemicals, and dietary factors. In addition, variations in CYP3A expression under pathological conditions, such as coronavirus disease 2019 and liver diseases, are described as examples of the physiological effects of endogenous factors. We also summarize endogenous and exogenous substrates metabolized by CYP3A isoforms, such as cholesterol, bile acids, hormones, arachidonic acid, vitamin D, and drugs. The relationship between the changes in the kinetics of these substrates and the toxicological effects in our bodies are discussed. The usefulness of these substrates and metabolites as endogenous biomarkers for CYP3A activity is also discussed. Notably, we focused on discrimination between CYP3A4, CYP3A5, and CYP3A7 to understand inter-individual differences in CYP3A expression and function.

The utility of endogenous glycochenodeoxycholate-3-sulfate and 4β-hydroxycholesterol to evaluate the hepatic disposition of atorvastatin in rats

The liver is an important organ for drugs disposition, and thus how to accurately evaluate hepatic clearance is essential for proper drug dosing. However, there are many limitations in drug dosage adjustment based on liver function and pharmacogenomic testing. In this study, we evaluated the ability of endogenous glycochenodeoxycholate-3-sulfate (GCDCA-S) and 4β-hydroxycholesterol (4β-HC) plasma levels to evaluate organic anion-transporting polypeptide (Oatps)-mediated hepatic uptake and Cyp3a-meidated metabolism of atorvastatin (ATV) in rats. The concentration of ATV and its metabolites, 2-OH ATV and 4-OH ATV, was markedly increased after a single injection of rifampicin (RIF), an inhibitor of Oatps. Concurrently, plasma GCDCA-S levels were also elevated. After a single injection of the Cyp3a inhibitor ketoconazole (KTZ), plasma ATV concentrations were significantly increased and 2-OH ATV concentrations were decreased, consistent with the metabolism of ATV by Cyp3a. However, plasma 4β-HC was not affected by KTZ treatment despite it being a Cyp3a metabolite of cholesterol. After repeated oral administration of RIF, plasma concentrations of ATV, 2-OH ATV and 4-OH ATV were markedly increased and the hepatic uptake ratio of ATV and GCDCA-S was decreased. KTZ did not affect plasma concentrations of ATV, 2-OH ATV and 4-OH ATV, but significantly decreased the metabolic ratio of total and 4-OH ATV. However, the plasma level and hepatic metabolism of 4β-HC were not changed by KTZ. The inhibition of hepatic uptake of GCDCA-S by RIF was fully reversed after a 7-d washout of RIF. Plasma concentration and hepatic uptake ratio of GCDCA-S were correlated with the plasma level and hepatic uptake of ATV in rats with ANIT-induced liver injury, respectively. These results demonstrate that plasma GCDCA-S is a sensitive probe for the assessment of Oatps-mediated hepatic uptake of ATV. However, Cyp3a-mediated metabolism of ATV was not predicted by plasma 4β-HC levels in rats.

The detoxification effect of cytochrome P450 3A4 on gelsemine-induced toxicity

Gelsemine (GA), the principal alkaloid in Gelsemium elegans Benth, exhibits potent and specific antinociception in chronic pain without the induction of apparent tolerance. However, GA also exerts neurotoxicity and hepatotoxicity when overdosed, and potential detoxification pathways are urgently needed. Cytochrome P450 enzymes (CYPs) are important phase I enzymes involved in the detoxification of xenobiotic compounds. The study aimed to investigate the role of CYPs-mediated metabolism in GA-induced toxicity. Microsomes, chemical special inhibitors and human recombinant CYPs indicated that GA was mainly metabolized by CYP3A4/5. The major metabolite of GA was isolated and identified as 4-N-demethyl-GA by high-resolution mass spectrometry and nuclear magnetic resonance technology. The CYP3A4 inhibitor ketoconazole significantly inhibited the metabolism of GA. This drastically increased GA toxicity which is caused by increasing the level of malondialdehyde and decreasing the level of the superoxide dismutase in mice. In contrast, the CYP3A4 inducer dexamethasone significantly increased GA metabolism and markedly decreased GA toxicity in mice. Notably, in CYP3A4-humanized mice, the toxicity of GA was significantly reduced compared to normal mice. These findings demonstrated that CYP3A4-mediated metabolism is a robust detoxification pathway for GA-induced toxicity.

In vivo evaluation of intestinal human CYP3A inhibition by macrolide antibiotics in CYP3A-humanised mice

It is important to predict drug-drug interactions via inhibition of intestinal cytochrome P450 3A (CYP3A) which is a determinant of bioavailability of orally administered CYP3A substrates. However, inhibitory effects of macrolide antibiotics on CYP3A-mediated metabolism are not entirely identical between humans and rodents.We investigated the effects of macrolide antibiotics, clarithromycin and erythromycin, on in vitro and in vivo metabolism of triazolam, a CYP3A substrate, in CYP3A-humanised mice generated by using a mouse artificial chromosome vector carrying a human CYP3A gene.Metabolic activities of triazolam were inhibited by macrolide antibiotics in liver and intestine microsomes of CYP3A-humanised mice.The area under the plasma concentration-time curve ratios of 4-hydroxytriazolam to triazolam after oral dosing of triazolam were significantly decreased by multiple administration of macrolide antibiotics. The plasma concentrations ratios of α-hydroxytriazolam and 4-hydroxytriazolam to triazolam in portal blood were significantly decreased by multiple administration of clarithromycin in CYP3A-humanised mice.These results suggest that intestinal CYP3A activity was inhibited by macrolide antibiotics in CYP3A-humanised mice in vitro and in vivo. The plasma concentrations of triazolam and its metabolites in the portal blood of CYP3A-humanised mice would be useful for direct evaluation of intestinal CYP3A-mediated drug-drug interactions.

Serum 4β-hydroxycholesterol increases during fluconazole treatment

PURPOSE

The antifungal drugs ketoconazole and itraconazole reduce serum concentrations of 4β-hydroxycholesterol, which is a validated marker for hepatic cytochrome P450 (CYP) 3A4 activity. We tested the effect of another antifungal triazole agent, fluconazole, on serum concentrations of different sterols and oxysterols within the cholesterol metabolism to see if this inhibitory reaction is a general side effect of azole antifungal agents.

METHODS

In a prospective, double-blind, placebo-controlled, two-way crossover design, we studied 17 healthy subjects (nine men, eight women) who received 400 mg fluconazole or placebo daily for 8 days. On day 1 before treatment and on day 8 after the last dose, fasting blood samples were collected. Serum cholesterol precursors and oxysterols were measured by gas chromatography-mass spectrometry-selected ion monitoring and expressed as the ratio to cholesterol (R_sterol).

RESULTS

Under fluconazole treatment, serum R_lanosterol and R_24,25-dihydrolanosterol increased significantly without affecting serum cholesterol or metabolic downstream markers of hepatic cholesterol synthesis. Serum R_4β-, R_24S-, and R_27-hydroxycholesterol increased significantly.

CONCLUSION

Fluconazole inhibits the 14α-demethylation of lanosterol and 24,25-dihydrolanosterol, regulated by CYP51A1, without reduction of total cholesterol synthesis. The increased serum level of R_4β-hydroxycholesterol under fluconazole treatment is in contrast to the reductions observed under ketoconazole and itraconazole treatments. The question, whether this increase is caused by induction of CYP3A4 or by inhibition of the catabolism of 4β-hydroxycholesterol, must be answered by mechanistic in vitro and in vivo studies comparing effects of various azole antifungal agents on hepatic CYP3A4 activity.

2019 White Paper on Recent Issues in Bioanalysis: Chromatographic Assays (Part 1 - Innovation in Small Molecules and Oligonucleotides & Mass Spectrometric Method Development Strategies for Large Molecule Bioanalysis)

The 2019 13^th Workshop on Recent Issues in Bioanalysis (WRIB) took place in New Orleans, LA, USA on April 1-5, 2019 with an attendance of over 1000 representatives from pharmaceutical/biopharmaceutical companies, biotechnology companies, contract research organizations and regulatory agencies worldwide. WRIB was once again a 5-day, week-long event - a full immersion week of bioanalysis, biomarkers, immunogenicity and gene therapy. As usual, it was specifically designed to facilitate sharing, reviewing, discussing and agreeing on approaches to address the most current issues of interest including both small- and large-molecule bioanalysis involving LCMS, hybrid LBA/LCMS, LBA cell-based/flow cytometry assays and qPCR approaches. This 2019 White Paper encompasses recommendations emerging from the extensive discussions held during the workshop, and is aimed to provide the bioanalytical community with key information and practical solutions on topics and issues addressed, in an effort to enable advances in scientific excellence, improved quality and better regulatory compliance. Due to its length, the 2019 edition of this comprehensive White Paper has been divided into three parts for editorial reasons. This publication (Part 1) covers the recommendations on Innovation in Small Molecules and Oligonucleotides & Mass Spec Method Development Strategies for Large Molecules Bioanalysis. Part 2 (2018 FDA BMV Guidance, 2019 ICH M10 BMV Draft Guideline and regulatory agencies' input on bioanalysis, biomarkers, immunogenicity and gene therapy) and Part 3 (New Insights in Biomarkers Assays Validation, Current & Effective Strategies for Critical Reagent Management, Flow Cytometry Validation in drug discovery & development & CLSI H62, Interpretation of the 2019 FDA Immunogenicity Guidance and The Gene Therapy Bioanalytical Challenges) are published in volume 11 of Bioanalysis, issues 23 and 24 (2019), respectively.

Localisation of oxysterols at the sub-cellular level and in biological fluids

Oxysterols are oxidized derivatives of cholesterol that are formed enzymatically or via reactive oxygen species or both. Cholesterol or oxysterols ingested as food are absorbed and packed into lipoproteins that are taken up by hepatic cells. Within hepatic cells, excess cholesterol is metabolised to form bile acids. The endoplasmic reticulum acts as the main organelle in the bile acid synthesis pathway. Metabolised sterols originating from this pathway are distributed within other organelles and in the cell membrane. The alterations to membrane oxysterol:sterol ratio affects the integrity of the cell membrane. The presence of oxysterols changes membrane fluidity and receptor orientation. It is well documented that hydroxylase enzymes located in mitochondria facilitate oxysterol production via an acidic pathway. More recently, the presence of oxysterols was also reported in lysosomes. Peroxisomal deficiencies favour intracellular oxysterols accumulation. Despite the low abundance of oxysterols compared to cholesterol, the biological actions of oxysterols are numerous and important. Oxysterol levels are implicated in the pathogenesis of multiple diseases ranging from chronic inflammatory diseases (atherosclerosis, Alzheimer's disease and bowel disease), cancer and numerous neurodegenerative diseases. In this article, we review the distribution of oxysterols in sub-cellular organelles and in biological fluids.

Effect of Grapefruit Juice Intake on Serum Level of the Endogenous CYP3A4 Metabolite 4β-Hydroxycholesterol-an Interaction Study in Healthy Volunteers

4β-Hydroxycholesterol (4βOHC) is an endogenous CYP3A4 metabolite. However, it is unclear whether circulating levels of 4βOHC may reflect hepatic CYP3A4 activity or both hepatic and intestinal enzyme activity. The aim of this study was to investigate the effect of grapefruit juice, regarded to be a selective intestinal CYP3A4 inhibitor, on serum 4βOHC levels in healthy volunteers. The participants (n = 22) consumed grapefruit juice twice daily for 3 weeks followed by a 2-week washout period. Blood samples for measurements of 4βOHC and the non-CYP3A4-derived oxysterols 24-hydroxycholesterol (24OHC) and 27-hydroxycholesterol (27OHC), as well as lathosterol and total cholesterol, were drawn on days 0, 7, 21, and 35. Median individual changes (ratios) in cholesterol-corrected 4βOHC levels from baseline to weeks 1, 3, and 5 were 0.94 (P = 0.2), 0.98 (P = 0.3), and 0.97 (P = 0.9), respectively. In comparison, median changes (ratios) in cholesterol-corrected levels of 24OHC at the same points were 1.01 (P = 0.6), 0.98 (P = 0.3), and 0.99 (P = 0.5), and of 27OHC 1.01 (P = 0.8), 0.97 (P = 0.5), and 0.99 (P = 0.2). Surprisingly, serum concentration of cholesterol was significantly reduced by approximately 5% after 1 week (P = 0.03), while median cholesterol-corrected levels of lathosterol increased significantly and persistently by approximately 15% during the whole 5-week period (P < 0.04). In conclusion, the present findings suggest that intestinal CYP3A4 is not relevant for the overall formation of 4βOHC in healthy volunteers. The fact that grapefruit juice altered cholesterol homeostasis should be further investigated.

Elevated 4β-hydroxycholesterol/cholesterol ratio in anorexia nervosa patients

Recent studies have shown that the cytochrome P450 (CYP) 3A phenotype marker 4β-hydroxycholesterol/cholesterol (4βOHC/C) ratio is negatively correlated with body weight in healthy volunteers, and that obese patients have lower 4βOHC levels than healthy controls. However, 4βOHC/C ratio in underweight patients has yet to be reported. The aim of this study was to examine potential differences in CYP3A activity between underweight patients with anorexia nervosa and normal-weight volunteers by measuring plasma 4βOHC/C ratio. Furthermore, we wished to describe any association between body mass index (BMI) and 4βOHC/C ratio in underweight patients. A total of 20 underweight patients and 16 normal-weight volunteers were included in the study, all females. Underweight patients had a median 4βOHC/C ratio (molar ratio × 10-5) of 2.52 (range, 0.90-11.3) compared to 1.29 (0.56-2.09) in normal-weight subjects (Mann-Whitney P = 0.0005). 4βOHC/C ratio was negatively correlated with BMI in underweight patients (r = -0.56, P = 0.011), and in the whole study population (r = -0.67, P < 0.0001). This suggests that the negative correlation between 4βOHC/C and BMI, which has previously been reported between 4βOHC/C and body weight in healthy volunteers, extends to underweight patients. The findings indicate that CYP3A activity increases with decreasing BMI, resulting in higher CYP3A activity in underweight patients compared to normal-weight subjects. The potential clinical relevance of this needs to be studied further by comparing pharmacokinetics of drugs subjected to CYP3A-mediated metabolism in underweight vs. normal-weight individuals.