Determination of a quantitative relationship between hepatic CYP3A5*1/*3 and CYP3A4 expression for use in the prediction of metabolic clearance in virtual populations

Physiologically-based pharmacokinetic modelling to investigate the effect of CYP3A4/3A5 maturation on tacrolimus pharmacokinetics in paediatric HSCT patients

Tacrolimus (FK506) is a cornerstone of GVHD-prophylaxis treatment in paediatrics undergoing haematopoietic stem cell transplantation (HSCT). However, due to concerns about highly inter/intra-individual variability, precision dosing of FK506 is crucial. Cytochrome P450 (CYP) 3A4 and 3A5 are considered important sources of FK506 pharmacokinetic variability. Nevertheless, the impact of age-related maturation in hepatic and intestinal CYP3A4/3A5 enzymes remains unknown in paediatric HSCT patients. Physiologically-based pharmacokinetic (PBPK) models were developed and verified in adult volunteers and adult HSCT patients using GastroPlus™ (version 9.0), and then extrapolated to paediatric HSCT patients, taking into account the maturation of CYP3A4 and CYP3A5. Default CYP3A4 and CYP3A5 ontogeny profiles were updated based on the latest reports. The paediatric PBPK model was evaluated with independent data collected from Sun Yat-sen Memorial Hospital (86 paediatric HSCT patients, 1 to 16 -year-old). Simulations were performed to evaluate a reported FK506 dosing regimen in infants and children with different CYP3A5 genotypes. Extensive PBPK model validation indicated good predictability, with the predicted/observed (P/O) ratios within the range of 0.80-fold to 1.25-fold. Blood tacrolimus concentration-time curves were comparable between the real and virtual patients. Simulations showed that the higher levels of tacrolimus in 9-month-old to 3-year-old infants were mainly attributed to the CYP3A4/3A5 ontogeny profiles, which resulted in lower clearance and higher exposure relative to dose. The oral dosage of 0.1 mg/kg/day (q12 h) is considered appropriate for paediatric HSCT patients 9 months to 15 years of age with CYP3A5 *1/*1 genotypes. Lower doses were required for paediatric HSCT patients with CYP3A5 *1/*3 (0.08 mg/kg/day, q12h) or CYP3A5 *3/*3 genotypes (0.07 mg/kg/day, q12h), and analyses demonstrated 12.5-20 % decreases in ≤3-year-old patients. The study highlights the feasibility of PBPK modelling to explore age-related enzyme maturation in infants and children (≤3-year-old) undergoing HSCT and emphasizes the need to include hepatic and gut CYP3A4/3A5 maturation parameters.

Individual dose recommendations for drug interaction between tacrolimus and voriconazole in adult liver transplant recipients: A semiphysiologically based population pharmacokinetic modeling approach

The magnitude of drug-drug interaction between tacrolimus and voriconazole is highly variable, and individually tailoring the tacrolimus dose when concomitantly administered with voriconazole remains difficult. This study aimed to develop a semiphysiologically based population pharmacokinetic (semi-PBPK) model and a web-based dashboard to identify the dynamic inhibition of tacrolimus metabolism caused by voriconazole and provide individual tacrolimus regimens for Chinese adult liver transplant recipients. A total of 264 tacrolimus concentrations and 146 voriconazole concentrations were prospectively collected from 32 transplant recipients. A semi-PBPK model with physiological compartments including the gut wall, portal vein, and liver was developed using the nonlinear mixed-effects modeling software NONMEM (version 7.4). A web-based dashboard was established in R software (version 3.6.1) to recommend the individual tacrolimus regimens when concomitantly administered with voriconazole. The reversible inhibition of tacrolimus metabolism caused by voriconazole was investigated in both the liver and the gut wall. Moreover, voriconazole could highly inhibit the CYP3A activity in the gut wall more than in the liver. BMI and postoperative days were identified as significant covariates on intrinsic intestinal and hepatic clearance of tacrolimus, respectively. Age and postoperative days were identified as significant covariates on the volume of distribution of voriconazole. The individual tacrolimus regimens when concomitantly administered with voriconazole could be recommended in the dashboard (https://tac-vor-ddi.shinyapps.io/shinyapp3/). In conclusion, the semi-PBPK model successfully described the dynamic inhibition process between tacrolimus and voriconazole, and the web-based dashboard could provide individual tacrolimus regimens when concomitantly administered with voriconazole.

Extrapolation of physiologically based pharmacokinetic model for tacrolimus from renal to liver transplant patients

Physiologically based pharmacokinetic (PBPK) modeling is useful for evaluating differences in drug exposure among special populations, but it has not yet been employed to evaluate the absorption process of tacrolimus. In this study, we developed a minimal PBPK model with a compartmental absorption and transit model for renal transplant patients using available data in the literature and clinical data from our hospital. The effective permeability value of tacrolimus absorption and parameters for the single adjusting compartment were optimized via sensitivity analyses, generating a PBPK model of tacrolimus for renal transplant patients with good predictability. Next, we extrapolated the pharmacokinetics of tacrolimus for liver transplant patients by changing the population demographic parameters of the model. When the physiological parameters of a population with normal liver function were changed to those of a population with impaired hepatic function (Child-Pugh class A) in the constructed renal transplant PBPK model, the predicted tacrolimus concentrations were consistent with the observed concentrations in liver transplant patients. In conclusion, the constructed tacrolimus PBPK model for renal transplant patients could predict the pharmacokinetics in liver transplant patients by slightly reducing the hepatic function, even at three weeks post-transplantation.

Investigation of the Impact of CYP3A5 Polymorphism on Drug-Drug Interaction between Tacrolimus and Schisantherin A/Schisandrin A Based on Physiologically-Based Pharmacokinetic Modeling

Wuzhi capsule (WZC) is commonly prescribed with tacrolimus in China to ease drug-induced hepatotoxicity. Two abundant active ingredients, schisantherin A (STA) and schisandrin A (SIA) are known to inhibit CYP3A enzymes and increase tacrolimus’s exposure. Our previous study has quantitatively demonstrated the contribution of STA and SIA to tacrolimus pharmacokinetics based on physiologically-based pharmacokinetic (PBPK) modeling. In the current work, we performed reversible inhibition (RI) and time-dependent inhibition (TDI) assays with CYP3A5 genotyped human liver microsomes (HLMs), and further integrated the acquired parameters into the PBPK model to predict the drug–drug interaction (DDI) in patients with different CYP3A5 alleles. The results indicated STA was a time-dependent and reversible inhibitor of CYP3A4 while only a reversible inhibitor of CYP3A5; SIA inhibited CYP3A4 and 3A5 in a time-dependent manner but also reversibly inhibited CYP3A5. The predicted fold-increases of tacrolimus exposure were 2.70 and 2.41, respectively, after the multidose simulations of STA. SIA also increased tacrolimus’s exposure but to a smaller extent compared to STA. An optimized physiologically-based pharmacokinetic (PBPK) model integrated with CYP3A5 polymorphism was successfully established, providing more insights regarding the long-term DDI between tacrolimus and Wuzhi capsules in patients with different CYP3A5 genotypes.

Abundance and Associated Variations of Cytochrome P450 Drug-Metabolizing Enzymes in the Liver of East Asian Adults: A Meta-Analysis

BACKGROUND

Cytochrome P450 (CYP) enzymes are one of the main sources of variability in drug metabolic clearance. Information on their abundance levels is therefore crucial to optimize scaling factors for in vitro-in vivo extrapolation (IVIVE) to predict metabolic clearance.

OBJECTIVE

This study aims to quantify the abundance data of hepatic drug-metabolizing CYP enzymes in East Asian subjects reported from various sources in the literature using meta-analysis.

METHOD

We conducted a meta-analysis on the abundance of drug-metabolizing CYP enzymes in the liver of East Asian adults. Eligible reports were identified based on predefined criteria-(1) individual liver microsomal samples, and (2) absolute protein abundance data from normal tissues of East Asian adult subjects. Subgroup and sensitivity analyses were also performed.

RESULTS

Among the 11 CYP isoforms analyzed in East Asian subjects, CYP3A5 and CYP3A4 had the highest protein levels. In particular, the number of studies and the liver sample used to quantify the abundance of CYP3A4 were the largest. Of the isoforms involved, CYP2J2 and CYP2B6 had the lowest abundance level, i.e., <5 pmol/ mg of microsomal protein. For enzymes with abundance values available in both Chinese and Japanese subjects (CYP1A2, CYP2C9, CYP3A4, and CYP3A5), the abundance level of each CYP isoform appeared to be higher in Chinese than in Japanese subjects. The most distinct difference was observed in CYP3A5 abundance.

CONCLUSION

The current meta-analysis shows that the abundance levels of CYP enzymes appear to vary greatly among different East Asian individuals who have similar ethnic backgrounds and food habits. The pooled data of CYP abundance can be used as preliminary reference values along with the associated variations for the projections of pharmacokinetics through physiologically based pharmacokinetic (PBPK) approaches.

Translational approaches to treating dynamical diseases through in silico clinical trials

The primary goal of drug developers is to establish efficient and effective therapeutic protocols. Multifactorial pathologies, including dynamical diseases and complex disorders, can be difficult to treat, given the high degree of inter- and intra-patient variability and nonlinear physiological relationships. Quantitative approaches combining mechanistic disease modeling and computational strategies are increasingly leveraged to rationalize pre-clinical and clinical studies and to establish effective treatment strategies. The development of clinical trials has led to new computational methods that allow for large clinical data sets to be combined with pharmacokinetic and pharmacodynamic models of diseases. Here, we discuss recent progress using in silico clinical trials to explore treatments for a variety of complex diseases, ultimately demonstrating the immense utility of quantitative methods in drug development and medicine.

Quantitative Prediction of CYP3A4- and CYP3A5-Mediated Drug Interactions

We verified a physiologically-based pharmacokinetic (PBPK) model to predict cytochrome P450 3A4/5-mediated drug-drug interactions (DDIs). A midazolam (MDZ)-ketoconazole (KTZ) interaction study in 24 subjects selected by CYP3A5 genotype, and liquid chromatography and mass spectroscopy quantification of CYP3A4/5 abundance from independently acquired and genotyped human liver (n = 136) and small intestinal (N = 12) samples, were conducted. The observed CYP3A5 genetic effect on MDZ systemic and oral clearance was successfully replicated by a mechanistic framework incorporating the proteomics-informed CYP3A abundance and optimized small intestinal CYP3A4 abundance based on MDZ intestinal availability (F_G ) of 0.44. Furthermore, combined with a modified KTZ PBPK model, this framework recapitulated the observed geometric mean ratio of MDZ area under the curve (AUCR) following 200 or 400 mg KTZ, which was, respectively, 2.7-3.4 and 3.9-4.7-fold in intravenous administration and 11.4-13.4 and 17.0-19.7-fold in oral administration, with AUCR numerically lower (P > 0.05) in CYP3A5 expressers than nonexpressers. In conclusion, the developed mechanistic framework supports dynamic prediction of CYP3A-mediated DDIs in study planning by bridging DDIs between CYP3A5 expressers and nonexpressers.

Optimized Renal Transporter Quantification by Using Aquaporin 1 and Aquaporin 2 as Anatomical Markers: Application in Characterizing the Ontogeny of Renal Transporters and Its Correlation with Hepatic Transporters in Paired Human Samples

Renal transporters, which are primarily located in the proximal tubules, play an important role in secretion and nephrotoxicity of drugs. The goal of this study was to characterize the age-dependent protein abundance of human renal transporters. A total of 43 human kidneys, 26 of which were paired with livers from the same donors, were obtained and classified into three age groups: children (< 12 years), adolescents (12 to < 18 years), and adults (> 18 years). Protein abundance of kidney-specific anatomical markers, aquaporins 1 and 2 (markers of proximal and distal/collecting tubules, respectively), and 17 transporters was quantified by LC-MS/MS proteomics. Six out of 43 kidney samples were identified as outliers (Grubbs' test) that were significantly different from the others with relatively higher aquaporin 2 to aquaporin 1 ratio, indicating that these cortex samples were likely contaminated by medulla (representing distal/collecting tubules). No significant age-related changes (age > 1 year) were observed for renal transporter abundance, albeit OCT2 abundance was modestly higher (< 50%) in adolescents than that in adults. Higher protein-protein correlation between transporters was observed in the kidney but abundance of transporters between tissues was not correlated. The use of aquaporins 1 and 2 provides a method for identifying kidney cortex with significant contamination from medulla containing distal and collecting tubules. The abundance and protein-protein correlation data can be used in physiologically based pharmacokinetic (PBPK) modeling and simulation of renal drug disposition and clearance in pediatric populations.

A Minimal Physiologically-Based Pharmacokinetic Model for Tacrolimus in Living-Donor Liver Transplantation: Perspectives Related to Liver Regeneration and the cytochrome P450 3A5 (CYP3A5) Genotype

In adult patients after living‐donor liver transplantation, postoperative days and the cytochrome P450 3A5 (CYP3A5) genotype are known to affect tacrolimus pharmacokinetics. In this study, we constructed a physiologically‐based pharmacokinetic model adapted to the clinical data and evaluated the contribution of liver regeneration as well as hepatic and intestine CYP3A5 genotypes on tacrolimus pharmacokinetics. As a result, liver function recovered immediately and affected the total body clearance of tacrolimus only during a limited period after living‐donor liver transplantation. The clearance was about 1.35‐fold higher in the recipients who had a liver with the CYP3A5*1 allele than in those with the CYP3A5*3/*3 genotype, whereas bioavailability was ~0.7‐fold higher in the recipients who had intestines with the CYP3A5*1 allele than those with CYP3A5*3/*3. In conclusion, the constructed physiologically‐based pharmacokinetic model clarified that the oral clearance of tacrolimus was affected by the CYP3A5 genotypes in both the liver and intestine to the same extent.

A Theoretical Physiologically-Based Pharmacokinetic Approach to Ascertain Covariates Explaining the Large Interpatient Variability in Tacrolimus Disposition

Physiologically‐based pharmacokinetic (PBPK) modeling allows assessment of the covariates contributing to the large pharmacokinetic (PK) variability of tacrolimus; these include multiple physiological and biochemical differences among patients. A PBPK model of tacrolimus was developed, including a virtual population with physiological parameter distributions reflecting renal transplant patients. The ratios of predicted to observed dose‐normalized maximum plasma concentration (C_max), 0–12‐hour area under the concentration–time curve (AUC_0–12 hour), and trough plasma concentration (C_trough) ranged from 0.92‐fold to 1.15‐fold, indicating good predictive performance. The model quantitatively indicated the impact of cytochrome P450 (CYP)3A4 abundance, hematocrit, and serum albumin levels, in addition to CYP3A5 genotype status, on tacrolimus PK and associated variability. Age‐dependent change in tacrolimus trough concentration in pediatric patients was mainly attributed to the CYP3A ontogeny profile. This study demonstrates the utility of PBPK modeling as a tool for mechanistic and quantitative assessment of the impact of patient physiological differences on observed large PK variability.

Proteomic Quantification of Human Blood-Brain Barrier SLC and ABC Transporters in Healthy Individuals and Dementia Patients

The blood-brain barrier (BBB) maintains brain homeostasis by controlling traffic of molecules from the circulation into the brain. This function is predominantly dependent on proteins expressed at the BBB, especially transporters and tight junction proteins. Alterations to the level and function of BBB proteins can impact the susceptibility of the central nervous system to exposure to xenobiotics in the systemic circulation with potential consequent effects on brain function. In this study, expression profiles of drug transporters and solute carriers in the BBB were assessed in tissues from healthy individuals ( n = 12), Alzheimer's patients ( n = 5), and dementia with Lewy bodies patients ( n = 5), using targeted, accurate mass retention time (AMRT) and global proteomic methods. A total of 53 transporters were quantified, 19 for the first time in the BBB. A further 20 novel transporters were identified but not quantified. The global proteomic method identified another 3333 BBB proteins. Transporter abundances, taken together with the scaling factor, microvessel protein content per unit tissue (BMvPGB also measured here), can be used in quantitative systems pharmacology models predicting drug disposition in the brain and permitting dose adjustment (precision dosing) in special populations of patients, such as those with dementia. Even in this small study, we see differences in transporter profile between healthy and diseased brain tissue.

Use of generally recognized as safe or dietary compounds to inhibit buprenorphine metabolism: potential to improve buprenorphine oral bioavailability

The present study evaluated the potential of five generally recognized as safe (GRAS) or dietary compounds (α-mangostin, chrysin, ginger extract, pterostilbene and silybin) to inhibit oxidative (CYP) and conjugative (UGT) metabolism using pooled human intestinal and liver microsomes. Buprenorphine was chosen as the model substrate as it is extensively metabolized by CYPs to norbuprenorphine and by UGTs to buprenorphine glucuronide. Chrysin, ginger extract, α-mangostin, pterostilbene and silybin were tested for their inhibition of the formation of norbuprenorphine or buprenorphine glucuronide in both intestinal and liver microsomes. Pterostilbene was the most potent inhibitor of norbuprenorphine formation in both intestinal and liver microsomes, with IC50 values of 1.3 and 0.8 μM, respectively, while α-mangostin and silybin most potently inhibited buprenorphine glucuronide formation. The equipotent combination of pterostilbene and ginger extract additively inhibited both pathways in intestinal microsomes. Since pterostilbene and ginger extract showed potent CYP and/or UGT inhibition of buprenorphine metabolism, their equipotent combination was tested to assess the presence of synergistic inhibition. However, because the combination showed additive inhibition, it was not used while performing IVIVE analysis. Based on quantitative in vitro-in vivo extrapolation, pterostilbene (21 mg oral dose) appeared to be most effective in improving the mean predicted Foral and AUC∞ PO of buprenorphine from 3 ± 2% and 340 ± 330 ng*min/ml to 75 ± 8% and 36,000 ± 25,000 ng*min/ml, respectively. At a 10-fold lower dose of pterostilbene, the predicted buprenorphine Foral approximated sublingual bioavailability (~35%) and showed a 2-4 fold reduction in the variability around the predicted AUC∞ PO of buprenorphine. These results demonstrate the feasibility of using various GRAS/dietary compounds to inhibit substantially the metabolism by CYP and UGT enzymes to achieve higher and less variable oral bioavailability. This inhibitor strategy may be useful for drugs suffering from low and variable oral bioavailability due to extensive presystemic oxidative and/or conjugative metabolism.

Implications of intercorrelation between hepatic CYP3A4-CYP2C8 enzymes for the evaluation of drug-drug interactions: a case study with repaglinide

AIMS

Statistically significant positive correlations are reported for the abundance of hepatic drug-metabolizing enzymes. We investigate, as an example, the impact of CYP3A4-CYP2C8 intercorrelation on the predicted interindividual variabilities of clearance and drug-drug interactions (DDIs) for repaglinide using physiologically based pharmacokinetic (PBPK) modelling.

METHODS

PBPK modelling and simulation were employed using Simcyp Simulator (v15.1). Virtual populations were generated assuming intercorrelations between hepatic CYP3A4-CYP2C8 abundances derived from observed values in 24 human livers. A repaglinide PBPK model was used to predict PK parameters in the presence and absence of gemfibrozil in virtual populations, and the results were compared with a clinical DDI study.

RESULTS

Coefficient of variation (CV) of oral clearance was 52.5% in the absence of intercorrelation between CYP3A4-CYP2C8 abundances, which increased to 54.2% when incorporating intercorrelation. In contrast, CV for predicted DDI (as measured by AUC ratio before and after inhibition) was reduced from 46.0% in the absence of intercorrelation between enzymes to 43.8% when incorporating intercorrelation: these CVs were associated with 5th/95th percentiles (2.48-11.29 vs. 2.49-9.69). The range of predicted DDI was larger in the absence of intercorrelation (1.55-77.06) than when incorporating intercorrelation (1.79-25.15), which was closer to clinical observations (2.6-12).

CONCLUSIONS

The present study demonstrates via a systematic investigation that population-based PBPK modelling incorporating intercorrelation led to more consistent estimation of extreme values than those observed in interindividual variabilities of clearance and DDI. As the intercorrelations more realistically reflect enzyme abundances, virtual population studies involving PBPK and DDI should avoid using Monte Carlo assignment of enzyme abundance.

Development of an Adult Physiologically Based Pharmacokinetic Model of Solithromycin in Plasma and Epithelial Lining Fluid

Solithromycin is a fluoroketolide antibiotic under investigation for community-acquired bacterial pneumonia (CABP). We developed a whole-body physiologically based pharmacokinetic (PBPK) model for solithromycin in adults using PK-Sim and MoBi version 6.2, which incorporated time-dependent CYP3A4 auto-inhibition. The model was developed and evaluated using plasma and epithelial lining fluid (ELF) concentration data from 100 healthy subjects and 22 patients with CABP (1,966 plasma, 30 ELF samples). We performed population simulations and calculated the number of observations falling outside the 90% prediction interval. For the oral regimen (800 mg on day 1 and 400 mg daily on days 2-5) that was evaluated in phase III studies, 11% and 23% of observations from healthy adults fell outside the 90% prediction interval for plasma and ELF, respectively. This regimen should be effective because ≥97% of simulated adults achieved area under the concentration vs. time curve (AUC) to minimum inhibitory concentration ratios associated with a log₁₀ colony forming unit reduction in ELF.

Evaluating optimal therapy robustness by virtual expansion of a sample population, with a case study in cancer immunotherapy

Cancer is a highly heterogeneous disease, exhibiting spatial and temporal variations that pose challenges for designing robust therapies. Here, we propose the VEPART (Virtual Expansion of Populations for Analyzing Robustness of Therapies) technique as a platform that integrates experimental data, mathematical modeling, and statistical analyses for identifying robust optimal treatment protocols. VEPART begins with time course experimental data for a sample population, and a mathematical model fit to aggregate data from that sample population. Using nonparametric statistics, the sample population is amplified and used to create a large number of virtual populations. At the final step of VEPART, robustness is assessed by identifying and analyzing the optimal therapy (perhaps restricted to a set of clinically realizable protocols) across each virtual population. As proof of concept, we have applied the VEPART method to study the robustness of treatment response in a mouse model of melanoma subject to treatment with immunostimulatory oncolytic viruses and dendritic cell vaccines. Our analysis (i) showed that every scheduling variant of the experimentally used treatment protocol is fragile (nonrobust) and (ii) discovered an alternative region of dosing space (lower oncolytic virus dose, higher dendritic cell dose) for which a robust optimal protocol exists.

The impact of CYP3A5*3 polymorphism on sirolimus pharmacokinetics: insights from predictions with a physiologically-based pharmacokinetic model

AIMS

Sirolimus is an mTOR inhibitor metabolized by CYP3A4 and CYP3A5. Reported effects of CYP3A5 polymorphisms on sirolimus pharmacokinetics (PK) have shown unexplained discrepancies across studies. We quantitatively assessed the effect of CYP3A5*3 status on sirolimus PK by in vitro assessment and simulation using a physiologically-based PK (PBPK) model. In addition, we explored designs for an adequately powered pharmacogenetic association study.

METHOD

In vitro metabolism studies were conducted to confirm individual CYP contribution to sirolimus metabolism. PK profiles were simulated in CYP3A5 expressers and non-expressers with a PBPK model. The pre-dose concentration predictions were used as the outcome parameter to estimate the required sample size for a pharmacogenetic association study.

RESULTS

Sirolimus metabolism was inhibited by over 90% by ketoconazole, a CYP3A specific inhibitor. The PBPK model developed based on CL(int) of recombinant CYP3A4, CYP3A5 and CYP2C8 predicted a small CYP3A5*3 effect on simulated sirolimus PK profiles. A subsequent power analysis based on these findings indicated that at least 80 subjects in an enrichment design, 40 CYP3A5 expressers and 40 non-expressers, would be required to detect a significant difference in the predicted trough concentrations at 1 month of therapy (P < 0.05, 80% power).

CONCLUSIONS

This study suggests that CYP3A5 contribution to sirolimus metabolism is much smaller than that of CYP3A4. Observed discrepancies across studies could be explained as the result of inadequate sample size. PBPK model simulations allowed mechanism-based evaluation of the effects of CYP3A5 genotype on sirolimus PK and provided preliminary data for the design of a future prospective study.

Development of a Physiologically-Based Pharmacokinetic Model for Sirolimus: Predicting Bioavailability Based on Intestinal CYP3A Content

Sirolimus is an inhibitor of mammalian target of rapamycin (mTOR) and is increasingly being used in transplantation and cancer therapies. Sirolimus has low oral bioavailability and exhibits large pharmacokinetic variability. The underlying mechanisms for this variability have not been explored to a large extent. Sirolimus metabolism was characterized by in vitro intrinsic clearance estimation. Pathway contribution ranked from CYP3A4 > CYP3A5 > CYP2C8. With the well stirred and Q_gut models sirolimus bioavailability was predicted at 15%. Interindividual differences in bioavailability could be attributed to variable intestinal CYP3A expression. The physiologically-based pharmacokinetics (PBPK) model developed in Simcyp predicted a high distribution of sirolimus into adipose tissue and another elimination pathway in addition to CYP-mediated metabolism. PBPK model predictive performance was acceptable with C_max and area under the curve (AUC) estimates within 20% of observed data in a dose escalation study. The model also showed potential to assess the impact of hepatic impairment and drug–drug interaction (DDI) on sirolimus pharmacokinetics.

From gut to kidney: transporting and metabolizing calcineurin-inhibitors in solid organ transplantation

Since their introduction circa 35 years ago, calcineurin-inhibitors (CNI) have become the cornerstone of immunosuppressive therapy in solid organ transplantation. However, CNI's possess a narrow therapeutic index with potential severe consequences of drug under- or overexposure. This demands a meticulous policy of Therapeutic Drug Monitoring (TDM) to optimize outcome. In clinical practice optimal dosing is difficult to achieve due to important inter- and intraindividual variation in CNI pharmacokinetics. A complex and often interdependent set of factors appears relevant in determining drug exposure. These include recipient characteristics such as age, race, body composition, organ function, and food intake, but also graft-related characteristics such as: size, donor-age, and time after transplantation can be important. Fundamental (in vitro) and clinical studies have pointed out the intrinsic relation between the aforementioned variables and the functional capacity of enzymes and transporters involved in CNI metabolism, primarily located in intestine, liver and kidney. Commonly occurring polymorphisms in genes responsible for CNI metabolism (CYP3A4, CYP3A5, CYP3A7, PXR, POR, ABCB1 (P-gp) and possibly UGT) are able to explain an important part of interindividual variability. In particular, a highly prevalent SNP in CYP3A5 has proven to be an important determinant of CNI dose requirements and drug-dose-interactions. In addition, a discrepancy in genotype between graft and receptor has to be taken into account. Furthermore, common phenomena in solid organ transplantation such as inflammation, ischemia- reperfusion injury, graft function, co-medication, altered food intake and intestinal motility can have a differential effect on the expression enzymes and transporters involved in CNI metabolism. Notwithstanding the built-up knowledge, predicting individual CNI pharmacokinetics and dose requirements on the basis of current clinical and experimental data remains a challenge.

Bottom-up modeling and simulation of tacrolimus clearance: prospective investigation of blood cell distribution, sex and CYP3A5 expression as covariates and assessment of study power

The objectives were to investigate the ability of population-based in vitro-in vivo extrapolation (IVIVE) to reproduce the influence of haematocrit on the clearance of tacrolimus, observed previously, and to assess the power of clinical studies to detect the effects of covariates on the clearance of tacrolimus. A population-based pharmacokinetic simulator (Simcyp) was used to simulate tacrolimus clearance from in vitro metabolism data and demographic characteristics of Japanese liver transplant patients (JLTs). The relationship between haematocrit and dose-to-concentration (D/C) ratio was validated using seven JLTs, whose highly variable haematocrit and D/C ratio were previously analysed. This validation was used as a surrogate for establishing 'interindividual' variability and to assess the power of clinical studies to discern the effect of haematocrit, sex and CYP3A5 genotype on tacrolimus clearance in a virtual JLT population. The relationship between haematocrit and D/C ratio was reproducible by Simcyp and corresponded well to those observed in seven JLTs. The number of JLTs required to detect the influence of CYP3A5 genotype and sex were estimated to be about 50 and > 600, respectively, which was consistent with the results of previous population pharmacokinetic studies for tacrolimus. In conclusion, population-based IVIVE is considered to be a useful approach to assess the influence of covariates a priori before conducting clinical studies. This is also helpful with study design and assessment of the statistical power of clinical studies involving population-based pharmacokinetics to detect the effects of covariates.