Feasibility assessment of PBPK modelling for Endogenous Compounds: Baseline levels of endogenous compounds in the Caucasian population S. Neuhoff, H.E. Humphries, H. K. Crewe, Z. E. Barter, and K. Rowland-Yeo Simcyp Ltd (a Certara company), Blades Enterprise Centre, Sheffield, S2 4SU [email protected] BACKGROUND RESULTS (cont) RESULTS (cont) • There is increasing interest in the • Bilirubin and glucuronides 27-Hydroxycholesterol potential utility of Endogenous Cholesterol 1g/day A schematic of the formation and Compounds (EC’s) as biomarkers for CYP7A1 (liver) 500 mg/day CYP27A1 (many tissues) breakdown of bilirubin is shown in assessment of drug-induced enzyme 19 mg/day Figure 4. Total serum bilirubin (bilirubin 7α-Hydroxycholesterol level/activity changes of (a) CYP3A or CYP3A4 + bilirubin glucuronide), conjugated and Bile Acids (b) UGT1A1 following chronic dosing of 4-Hydroxycholesterol unconjugated plasma levels reflect the drug of interest. CYP27A1 (liver and extrahepatic) 24S-Hydroxycholesterol protein changes differently. • In July 2013, Consortium Members Pregnenolone were asked to identify a series of EC’s 4,27-Dihydroxycholesterol Macrophage CYP27A1 • Biliverdin reductase reduces Heme Hemeoxygenase for further evaluation, with a view to 4,24-Dihydroxycholesterol Biliverdin to unconjugated, water- Biliverdin 4,7α-Dihydroxycholesterol insoluble Bilirubin, which is Biliverdin reductase implementation within the Simcyp Bilirubin carried in blood bound to serum Unconjugated bilirubin Simulator. albumin complexed with albumin 3 ,4-Dihydroxy-5-cholestenoic acid • The two top ranked compounds and • Bilirubin is conjugated to OATP1B1 Liver Figure 1 – Cholesterol metabolism B their proposed utility as biomarkers are glucoronic acids by UGT1A1 uridinediphosphateglucoronyl BG MRP2 given in Table 1. transferase (UGT1A1) and Gut Baseline Population values conjugated bilirubin (water- BG soluble) is transported into bile bacteria Kidneys Urobilin Table 1: The top 2 ranked endogenous compounds and • The mean plasma level of 4bOH-C in canaliculi by MRP2 (Stercobilin) their utility Caucasians is 34 ng/mL (CV = 51%, • Intestinal bacteria deconjugate Biomarker Utility Urine Faeces n=374), which is higher than that and breakdown bilirubin into 4-β-OH Cholesterol (4bOH-C) Induction of CYP3A enzyme colourless urobilinogens, which Major pathway Minor pathway Bilirubin; -conjugates Induction and inhibition of reported for a Korean population are primarily excreted in faeces UGT1A1, OATP2B1 and MRP2 (Figure 2). Figure 4 – Bilirubin metabolism and AIMS elimination To evaluate the feasibility of developing the Caucasian Korean most popular EC’s (Table 1). • Elevated conjugated, unconjugated and METHODS: total serum bilirubin level (TSBL) are a marker for intrahepatic jaundice. • A systematic literature search using • In prehepatic jaundice the conjugated representative key words in PubMed and bilirubin and urinary bilirubin are absent. the internet for the two EC’s was In post-hepatic jaundice, conjugated and performed. total bilirubin levels are elevated. • Discussions were held with Consortium • Baseline TSBL are given in Figure 5. st Members (e.g., webinar 1 August 2013) on their experiences with these Figure 2 – Baseline population levels for biomarkers and potential data sharing plasma [4bOH-C] (Mean ± SD) in Healthy EM PM Volunteers . opportunities. • Key areas identified for investigation x = 0.94 were baseline plasma levels of each • It should be noted that there is a high x = 0.49 biomarker in a Healthy Volunteer degree of inter-individual variability (Figure 3), and therefore, it may be population and routes of biotransformation. more appropriate to represent [4bOH-C] levels relative to [cholesterol] levels. RESULTS These ratios in the Caucasian Figure 5 – Baseline levels for total serum • 4--Hydroxycholesterol population range between 0.06 and bilirubin in healthy Caucasians for UGT1A1 A biomarker for CYP3A4 as decreased 0.26, n= 74 (2 references). extensive and poor metabolisers (not levels could indicate need for a clinical DDI separated by OATP1B1 phenotypes). study - a mitigation strategy. N = 267 subjects Challenges 6 references Challenges • More information on the impact of • Complex metabolism. Contribution of OATP1B1 on levels of conjugated and CYP7A1 (CYP27A1, CYP46A1, and unconjugated bilirubin is required. CYP11A1) to the metabolism of • An induction model for UGT1A1 is Cholesterol and 4bOH-C (Figure 1). The needed, therefore turnover numbers for half-life before and after induction of UGT1A1 are required. CYP3A is reported to be 60 h and 400 h, Dear Consortium Member, respectively. Initial baseline levels and the routes of • [4bOH-C] reflects average changes in biotransformation for the top 2 hepatic CYP3A levels, thus only endogenous compounds have been clinically relevant following chronic collated. We would like to collate further dosing. Figure 3 – Frequency of the plasma [4bOH- C] in Caucasians. Each colour represents data. • Will not reflect gut CYP3A levels one reference. Please contact us, if you would like to • Transporter involvement is unknown contribute and discuss. .