Quantitative Systems Pharmacology Approaches for Immuno-Oncology: Adding Virtual Patients to the Development Paradigm.

Drug development in oncology commonly exploits the tools of molecular biology to gain therapeutic benefit through reprograming of cellular responses. In immuno-oncology (IO) the aim is to direct the patient's own immune system to fight cancer. After remarkable successes of antibodies targeting PD1/PD-L1 and CTLA4 receptors in targeted patient populations, the focus of further development has shifted toward combination therapies. However, the current drug-development approach of exploiting a vast number of possible combination targets and dosing regimens has proven to be challenging and is arguably inefficient. In particular, the unprecedented number of clinical trials testing different combinations may no longer be sustainable by the population of available patients. Further development in IO requires a step change in selection and validation of candidate therapies to decrease development attrition rate and limit the number of clinical trials. Quantitative systems pharmacology (QSP) proposes to tackle this challenge through mechanistic modeling and simulation. Compounds' pharmacokinetics, target binding, and mechanisms of action as well as existing knowledge on the underlying tumor and immune system biology are described by quantitative, dynamic models aiming to predict clinical results for novel combinations. Here, we review the current QSP approaches, the legacy of mathematical models available to quantitative clinical pharmacologists describing interaction between tumor and immune system, and the recent development of IO QSP platform models. We argue that QSP and virtual patients can be integrated as a new tool in existing IO drug development approaches to increase the efficiency and effectiveness of the search for novel combination therapies.

Population Pharmacokinetics of Apixaban in Subjects With Nonvalvular Atrial Fibrillation.

This analysis describes the population pharmacokinetics (PPK) of apixaban in nonvalvular atrial fibrillation (NVAF) subjects, and quantifies the impact of intrinsic and extrinsic factors on exposure. The PPK model was developed using data from phase I-III studies. Apixaban exposure was characterized by a two-compartment PPK model with first-order absorption and elimination. Predictive covariates on apparent clearance included age, sex, Asian race, renal function, and concomitant strong/moderate cytochrome P450 (CYP)3A4/P-glycoprotein (P-gp) inhibitors. Individual covariate effects generally resulted in < 25% change in apixaban exposure vs. the reference NVAF subject (non-Asian, male, aged 65 years, weighing 70 kg without concomitant CYP3A4/P-gp inhibitors), except for severe renal impairment, which resulted in 55% higher exposure than the reference subject. The dose-reduction algorithm resulted in a ~27% lower median exposure, with a large overlap between the 2.5-mg and 5-mg groups. The impact of Asian race on apixaban exposure was < 15% and not considered clinically significant.

Physiological based pharmacokinetic modeling to estimate in vivo Ki of ketoconazole on renal P-gp using human drug-drug interaction study result of fesoterodine and ketoconazole.

This study was conducted to estimate in vivo inhibition constant (Ki) of ketoconazole on renal P-glycoprotein (P-gp) using human drug-drug interaction (DDI) study result of fesoterodine and ketoconazole. Fesoterodine is a prodrug which is extensively hydrolyzed by non-specific esterases to the active metabolite 5-hydroxymethyl tolterodine (5-HMT). 5-HMT is then further metabolized via Cytochrome P450 (CYP) 2D6 and CYP3A4. It is reported that 5-HMT is a substrate of P-gp whereas fesoterodine is not. Renal clearance of 5-HMT is approximately two-times greater than renal glomerular filtration rate. This suggests the possibility that renal clearance of 5-HMT involves secretion by P-gp. Utilizing the available pharmacokinetic characteristics of fesoterodine and 5-HMT, we estimated in vivo Ki of ketoconazole on P-gp at kidney based on DDI study data using physiologically-based pharmacokinetic approach. The estimated in vivo Ki of ketoconazole for hepatic CYP3A4 (6.64 ng/mL) was consistent with the reported values. The in vivo Ki of ketoconazole for renal P-gp was successfully estimated as 2.27 ng/mL, which was notably lower than reported in vitro 50% inhibitory concentration (IC50) values ranged 223-2440 ng/mL due to different condition between in vitro and in vivo.

Population Pharmacokinetics of Eplerenone in Japanese Patients With Chronic Heart Failure.

To characterize eplerenone pharmacokinetics (PK) in Japanese chronic heart failure (CHF) patients and to estimate the impact of factors that may affect eplerenone PK, population pharmacokinetic (PPK) analysis was conducted. In addition, PK of Japanese CHF and Western CHF patients from a previous clinical pharmacology study were compared in the analysis. Eplerenone PK was characterized by a 1-compartment PPK model with first-order absorption and lag time in Japanese CHF patients. The population mean of apparent oral clearance (CL/F) in Japanese CHF patients was estimated as 5.31 L/h, which was similar to the mean CL/F for Western CHF patients. In the full model approach, creatinine clearance (CLcr) on CL/F and body weight on apparent central volume of distribution (Vc/F) were selected as factors that may affect PK. The effect of CLcr on CL/F predicted that CL/F would be decreased by 25% when CLcr was decreased from 80 mL/min to 50 mL/min. The effect of body weight on Vc/F predicted that Vc/F would be decreased by 18% when body weight was decreased from 80 kg to 60 kg. Distribution of individual CL/F estimates for Japanese CHF patients overlapped CL/F observed values for Western CHF patients, and CL/F values for Western CHF patients were contained within the distribution of CL/F estimates for Japanese CHF patients. No obvious difference between Japanese and Western subjects was detected even in the updated model by adding the data obtained from Western CHF patients and Western healthy adults to the model constructed with data from Japanese CHF patients.

Population pharmacokinetics of the 5-hydroxymethyl metabolite of tolterodine after administration of fesoterodine sustained release tablet in Western and East Asian populations.

This analysis was conducted to investigate factors that affect 5-hydroxymethyl tolterodine (5-HMT) pharmacokinetics after administration of fesoterodine sustained release tablets to Westerners and East Asians. Ten pharmacokinetic studies and three efficacy/safety studies in overactive bladder (OAB) patients were pooled for the population pharmacokinetic analysis. The plasma 5-HMT concentration data were described by a 1-compartment model with first order absorption and a lag time. Creatinine clearance (CLCR), hepatic impairment, CYP2D6 genotype, and concomitant medication with CYP3A inhibitor/inducer were identified as influential covariates. It was estimated that decreasing of CLCR from 80 to 15 mL/min resulted in a 39.5% reduction in 5-HMT apparent oral clearance (CL/F). Hepatic impairment, CYP2D6 poor metabolizer, and CYP3A inhibitor were estimated to reduce CL/F by about 60%, 40%, and 50%, respectively. CYP3A inducer resulted in about fourfold increase in CL/F. Although sex and Japanese ethnicity were selected as covariates on CL/F, each resulted in only about 10% decrease and increase of CL/F, respectively. Of the influential covariates of 5-HMT CL/F, CLCR, hepatic impairment, CYP2D6 genotype, and concomitant medication with CYP3A inhibitor/inducer were of significance, whereas sex and Japanese ethnicity covariates were considered not to have clinically significant impact on exposures of 5-HMT.

A Comparison of the Pharmacokinetics and Drug Safety Among East Asian Populations.

Global clinical studies conducted in various countries and regions are increasing. Race and extrinsic ethnic factors are key covariates that may affect the pharmacokinetics (PK), efficacy, and safety of the drug. Genetic similarity among East Asian populations has been confirmed; thus, PK, efficacy, and safety in these populations are expected to be similar, but this has not been confirmed. This study presents a comparison of PK and safety among East Asians from clinical studies sponsored by Pfizer. Four compounds with different characteristics, including mechanism of actions and PK profiles, were selected, and retrospective PK and safety comparisons in East Asians were conducted. No distinct differences were observed in PK and safety across the 4 compounds. These results are consistent with previous reports on PK comparisons and meet the expectations based on genetic similarity among East Asians. Extrapolation of these findings to other compounds should be done with caution, but these results should support the consideration of mutual use of clinical data among East Asian countries.

Different truncation methods of AUC between Japan and the EU for bioequivalence assessment: influence on the regulatory judgment.

In regulatory guidelines for bioequivalence (BE) assessment, the definitions of AUC for primary assessment are different in ICH countries, i.e., AUC from zero to the last sampling point (AUCall) in Japan, AUC from zero to infinity (AUCinf) or AUC from zero to the last measurable point (AUClast) in the US, and AUClast in the EU. To assure sufficient accuracy of truncated AUC for BE assessment, the ratio of truncated AUC (AUCall or AUClast) to AUCinf should be more than 80% both in Japanese and EU guidelines. We investigated how the difference in the definition of truncated AUC affects BE assessment of sustained release (SR) formulation. Our simulation result demonstrated that AUCall/AUCinf could be ≥80% despite AUClast/AUCinf being <80% and AUCall failed to detect formulation difference. In Japanese package inserts of generic drugs in SR formulation, there were products for which AUCall/AUCinf was ≥80% though AUClast/AUCinf was <80%. In conclusion, it was confirmed that the difference in definition of truncated AUC affected the judgment of validity of truncated AUC for BE assessment, and AUCall could fail to detect the substantially different in vivo dissolution profile of generic drugs with SR formulation from the original drug.

Pharmacokinetics of tolterodine in Japanese and Koreans: physiological and stochastic assessment of ethnic differences.

Tolterodine is known as a drug which exhibits ethnic differences in pharmacokinetics between Japanese and Koreans despite genetic similarities among the populations of East Asian countries. Tolterodine is mainly metabolized by CYP2D6 to a 5-hydroxymethyl metabolite (5-HM), and 5-HM is also metabolized by CYP2D6. The reduced-function allele CYP2D6*10 is frequently observed in Asian populations. We investigated differences in the pharmacokinetics of tolterodine between small Japanese and Korean study populations by physiological and stochastic approaches with consideration of the CYP2D6 genotype. The genotype frequencies of CYP2D6*10/*10 and CYP2D6*5/*10 were found to be higher in Koreans than in Japanese, which suggested that this frequency difference occurred incidentally. The effects of CYP2D6 genotype and ethnicity on the intrinsic clearance of tolterodine by CYP2D6 were tested and only genotype was found to be a significant factor by ANCOVA. A simulation was conducted to confirm whether the observed differences in tolterodine exposure could be explained by the differences in genotype frequency found in this study. It was confirmed that the variability of intrinsic clearance could be responsible for the incidental exposure differences. In conclusion, apparent differences in exposure were found between small Japanese and Korean study populations because of the variability of intrinsic clearances and genotype frequencies.

Effect of the CYP2D6*10 genotype on tolterodine pharmacokinetics.

This study was conducted to investigate the effect of the reduced function allele CYP2D6*10, which can be the cause of an intermediate metabolizer (IM), on tolterodine pharmacokinetics. Tolterodine is mainly metabolized to an active 5-hydroxymethyl metabolite (5-HM) by CYP2D6, and 5-HM is also metabolized by CYP2D6. Asian and white healthy volunteers (n = 108) received once daily multiple doses of tolterodine, and the serum concentrations of tolterodine and 5-HM were measured. All subjects were genotyped for CYP2D6. Tolterodine exposures [area under the curve (AUC)] increased in order of CYP2D6*1/*1 [extensive metabolizer (EM)] < CYP2D6*1/*10 < CYP2D6*10/*10 < CYP2D6*5/*10. It was expected that the order of 5-HM exposure would be reversed. However, the 5-HM AUC increased in the same order as that of tolterodine. This phenomenon was explained by considering CYP2D6 mediation of both production and elimination of 5-HM. The tolterodine and 5-HM exposures in CYP2D6*10/*10 were statistically higher than those for CYP2D6*1/*1 (3- and 1.5-fold, respectively). In CYP2D6*4/*4 [poor metabolizer (PM)], 5-HM was not produced and tolterodine exposure was 20-fold higher than that in CYP2D6*1/*1. With consideration for higher protein binding of tolterodine than 5-HM, the exposure as a sum of the unbound fraction of tolterodine and 5-HM (active moiety) in CYP2D6*10/*10 was 1.8-fold higher than that in CYP2D6*1/*1 and was also higher than that in CYP2D6*4/*4. Simulation using the values of EM and PM demonstrated that the maximum possible active moiety exposure was around the observed values of CYP2D6*5/*10, which were 1.9-fold higher than those for CYP2D6*1/*1. This is the first report to provide an example in which the IM shows higher exposure to pharmacological active moiety than the EM and PM.