Mechanistic PBPK as an aid in identifying the size of covariate effects and design of POPPK studies: An example focusing on haematocrit as a determinant of tacrolimus clearance Hisakazu Ohtani *(1), Zoe Barter (2, 3), Mohsen Aarabi (3), Tsuyoshi Minematsu (4), Masatoshi Makuuchi (5), Yasufumi Sawada (1), Geoffrey Tucker (2, 3), Amin Rostami-Hodjegan (2, 3) * Correspondence: (1) Department of Drug Informatics, Graduate School of Pharmaceutical Sciences, The University of Tokyo, Tokyo, Japan; (2) Academic Unit of Clinical Pharmacology, School of Medicine, University of Sheffield, Sheffield, UK; (3) Simcyp Ltd., Sheffield, UK; (4) Astellas Pharm Inc. Tokyo, Japan; (5) Japanese Red Cross Medical Center INTRODUCTION Non-linear mixed effect modeling is often used for detecting the patient covariates which significantly influence the kinetics or dynamics of candidate drugs under development. The power of these studies to detect the covariate effects depends on a number of variables such as the effect size and inherent variation of the measured entity. Assessing these, using mechanistic physiologically based pharmacokinetic (PBPK) models prior to carrying out population pharmacokinetic (POPPK) studies, would be of appreciable benefit in optimizing study design. Previously we have reported haematocrit (HC) as a co-variate for the dose-to-concentration (D/C) ratio of tacrolimus (TAC) in Japanese liver transplant (LT) subjects [Ref. 1]. AIMS Table 1. The demographic data of Japanese subjects after liver transplantation used in the present analysis (during p.o. administration until day 70 post operation) [Ref. 1] SubjectAgeBWHeightGendern *2 DoseHC (yrs)(kg)(cm) *1 (mg/12 hr)Gmean(CV) 16055(164)M (18.24%) 24648(168)M (17.18%) 35040(154)F (18.78%) 4 *3 5146(154)F (14.62%) 56140(151)F ( 8.63%) 63048(157)F ( 6.77%) 75345(153)F ( 9.99%) *1; estimated value *2: number of blood sampling point *3; excluded from the analysis in our previous report [Ref. 1] due to the her extreme kinetics 1.To investigate the ability to simulate the effects of HC on D/C of TAC in a LT patients using a mechanistic model of clearance implemented within population-based pharmacokinetic simulator, Simcyp V To use clinical trial simulation in the assessment of statistical power of in vivo studies to detect the effects of cytochrome (CYP) 3A5 genotype and gender on the D/C ratio of TAC in a Japanese LT population. METHODS In vitro pharmacokinetic parameters of TAC were obtained from literature and entered into Simcyp ® Clinical Trial Simulation software V7.11 ( The D/C ratios of TAC in blood and plasma were obtained from long term evaluation reported on six Japanese LT subjects (Table 1) and were used for investigating the effect of study design. Six patients incorporated in our previous report were assumed to be poor metabolizer (PM) for CYP3A5, while one subject, who was excluded from our previous analysis due an unusually high D/C value, was assumed to be an extensive metabolizer (EM) for CYP3A5 Liver volume for Japanese LT subjects was assumed to be 88% that of normal Japanese individuals [Ref. 2]. Regarding intraindividual variation of CYP3A4 and CYP3A5 enzyme abundance, a coefficient of variation (CV) of 9.75% was applied based on the intraindividual variation of midazolam clearance reported by Kashuba et al. [Ref. 3]. The statistical power to detect the effects of CYP3A5 genotype and gender was determined from 20 trials using a virtual Japanese LT population selected from the Simcyp Simulator population database. For the gender study, PM frequency for CYP3A5 of 58% was assumed [Ref. 4]. CYP3A4 abundance in female Japanese was assumed to be 45.3% higher than that in male Japanese, based on the abundance ratio in Caucasian. To assess the power to detect differences due to gender, steady-state concentrations of TAC in blood and plasma were simulated in individual Japanese virtual liver transplant subjects in studies of different sizes (n = 6 – 600). For CYP3A5 genotype study, studies containing 6– 500 virtual subjects from the same population with EM:PM ratio of 1:1 were simulated. The power of the study was measured by the rate of detection of statistically significant differences in D/C ratio with the P value of less than 0.05 (two tail t-test).

Figure 1. Comparison of the observed (1, 2) and simulated (3, 4) relationship between haematocrit and D/C values of TAC in plasma (1, 3) or blood (2, 4). Observed values were cited from [Ref. 1]. Right panels show the plot excluding Pt #4. (1) Observed, Plasma (2) Observed, Blood (3) Simulated, Plasma (4) Simulated, Blood r = r = r = r = Figure 2. Influence of the number of subjects on the power of simulated studies of TAC in blood and plasma to detect (A) the gender difference in CYP3A4 content and (B) effects of CYP3A5 genotype using Japanese liver transplant (LT) patients. The content of CYP3A4 in female was assumed to be higher by 45% than that in male. The simulated relationship between D/C values (in both blood and plasma) and HC corresponded well to those observed in vivo, i.e., the correlation coefficient between blood D/C ratio and HC was larger than that between plasma D/C ratio and HC (Figure 1; r=-0.51 vs ). They were in good accordance with the observed values (r=-0.53 vs ) [Ref. 1]. The simulated D/C values were considerably, but less than twofold, higher than the observed values (Figure 1). The unusually high D/C values observed for one patient (Pt #4) were, at least in part, explained by CYP3A5 genotype. From the power studies, at a male:female ratio of 1:2, 600 or more subjects were required to detect statistically significant differences in the D/C value on TAC between male and female (p < 0.05; two tail t- test) with 90% probability (Figure 2 (A)). For the detection of the effects of CYP3A5 genotype, 30 or more subjects were required to detect statistically significant differences in the D/C value of TAC with 90% probability (Figure 2(B)). RESULTS (A) Gender difference in CYP3A4(B) Effect of CYP3A5 genotype CONCLUSIONS Using population-based pharmacokinetic simulations, the relationship between D/C value of TAC and HC value was successfully reproduced. Therefore, mechanistic approach based on the in vitro-in vivo extrapolation [IVIVE] using Simcyp ® is considered a useful tool for assessment of the influences of covariates on the pharmacokinetics. This study implies the importance of investigating adequate numbers of individuals in the detection of the effect of covariates such as CYP3A5 genotype or gender on D/C ratio. The LT patients with the large variation in HC, that leads to increasing variability in D/C ratio, may not be an appropriate target population to analyze the influence of covariates on TAC pharmacokinetics. REFERENCES 1.Minematsu T et al., Transplant. Proc. 36: , Akamatsu N et al., Surgery. 139:765-72, Kashuba AD et al., Clin. Pharmacol. Ther. 64(3):269-77, Saeki M et al., Hum. Mutation. 21: 653, 2003