PAGE. Abstracts of the Annual Meeting of the Population Approach Group in Europe.
PAGE 23 (2014) Abstr 3036 [www.page-meeting.org/?abstract=3036]
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Oral: Lewis Sheiner Student Session
Nikolaos Tsamandouras (1), Gemma Dickinson (2), Yingying Guo (2), Stephen Hall (2), Amin Rostami-Hodjegan (1,3), Aleksandra Galetin (1) and Leon Aarons (1)
(1) Centre for Applied Pharmacokinetic Research, University of Manchester, UK, (2) Eli Lilly and Company, Indianapolis, IN, USA, (3) Simcyp Ltd, Sheffield, UK
Objectives: Simvastatin (SV) is a prodrug with complex pharmacokinetics (PK) due to the interconversion between the parent drug and its main active metabolite, simvastatin acid (SVA) [1, 2]. Both SV and SVA are subject to drug-drug interactions (DDIs)  and are affected by genetic variation in several enzyme/transporter proteins relevant for their disposition [4-7]. The most serious adverse effect of SV is related to skeletal muscle and it can range from myalgia to myopathy and ultimately to the rare but potentially life-threatening rhabdomyolysis [4, 8]. Simvastatin-induced myotoxicity is a concentration-dependent adverse effect which is at least partly of a pharmacokinetic origin . However, our understanding regarding the complex SV/SVA PK and especially their variability in the population is very limited.
This work aims to develop a joint population SV/SVA pharmacokinetic model that incorporates the effects of multiple genetic polymorphisms and clinical/demographic characteristics. Besides that, it also aims to provide a practical example of a covariate analysis where several genetic polymorphisms are tested and the degree of correlation between them through linkage disequilibrium has to be considered [10, 11].
In parallel, this work aims to develop an alternative much more complex but mechanistic population SV/SVA pharmacokinetic model. Such a model has the advantage that it can provide predictions not only for concentrations in plasma, but also in the clinically relevant tissues of muscle (toxicity) and liver (efficacy), while also it allows extrapolation outside the studied conditions. We have recently discussed the various methodological issues related with parameter estimation in such complex models . Therefore, in this work we aim to also explore further methodological aspects such as: incorporation of population variability in system-related parameters which are subject to certain physiological constraints; parameter estimation with the aid of prior distributions; and application of different estimation algorithms.
Methods: SV/SVA plasma concentrations, demographic/clinical data and genotypes for 18 genetic variants were collected from 74 individuals and analyzed with NONMEM 7.2 in order to develop the population pharmacokinetic-pharmacogenetic model . Several structural empirical parent-metabolite pharmacokinetic models were evaluated. Covariate selection was performed using a stepwise forward inclusion – backward elimination process that also examines the degree of linkage disequilibrium between correlated SNPs upon inclusion in each step. Finally the developed model was also used to identify individuals harbouring genetic and other clinical/demographic risk factor combinations that can be associated with clinically important increases on SV/SVA plasma exposure.
For the development of the complex mechanistic pharmacokinetic model, rich SV/SVA plasma concentration data (34 healthy volunteers) were simultaneously analysed with NONMEM 7.2. Additional rich plasma concentration data (28 healthy volunteers) were used for external validation of the model . The implemented mechanistic model has a complex compartmental structure (presented in an earlier version of this work ) that allows interconversion between SV and SVA in different tissues. Prior information for model parameters and (when available) their variability was extracted from physiology, in vitro experiments and in silico methods in order to construct appropriate prior distributions. Variability in system related parameters was incorporated not only by linking them with several well-defined physiological covariates (e.g. body weight), but also by the use of random inter-individual variability terms. Therefore, a generalisation of the logit-normal distribution was applied to constrain some of the system parameters within their physiological range (e.g. gastric residence time) and a multivariate logistic normal distribution was applied on compositional parameters for which the physiological constraints apply on their joint distribution (e.g. fractions of cardiac output that reaches each tissue). The prior functionality in NONMEM [16, 17] was used to integrate prior information with the information from the clinical data and obtain maximum a posteriori (MAP) parameter estimates under the first order conditional estimation method with interaction (FOCE-I) and importance sampling assisted by mode a posteriori (IMPMAP) methods . The developed model was finally employed to simulate concentration profiles in plasma, liver and muscle and investigate the impact of polymorphisms and a range of DDIs (clarithromycin, diltiazem, erythromycin, itraconazole).
Results: The empirical model that best described the data included a two- and a one-compartment disposition model for SV and SVA respectively. Age, weight, Japanese ethnicity and 7 genetic polymorphisms, rs4149056 (SLCO1B1), rs776746 (CYP3A5), rs12422149 (SLCO2B1), rs2231142 (ABCG2), rs4148162 (ABCG2), rs4253728 (PPARA) and rs35599367 (CYP3A4), were identified to significantly affect model parameters. The present work highlighted specific characteristics associated with altered SV/SVA PK and subsequently myopathy cases which cannot be solely attributed to a single genotype. Extensive results of this pharmacogenetic analysis are reported in .
The complex mechanistic model provided a good fit to the plasma SV/SVA concentration data and their variability. The incorporation of priors allowed precise estimation of model parameters without neglecting the random variability in key system and drug related parameters and the uncertainty  on the results of in vitro experiments or in silico predictions that are used to inform them. The FOCE-I and IMPMAP estimates were comparable, with the latter method (parallelised) showing significantly reduced computation times for convergence, similarly to previous studies with complex mechanistic models [20-22]. Simulations with the developed model using reduced SVA hepatic uptake (mimicking a SLCO1B1 polymorphism) recovered the reported increase in exposure of SVA in plasma ; comparable increased fold-exposure was simulated in the muscle, consistent with the risk of toxicity in this tissue; whereas impact on liver exposure was minimal. In addition the developed model successfully predicted the effect of a range of DDIs on the PK of both SV and SVA.
Conclusions: The population based approaches developed in this work overall provide further insight into the PK of simvastatin and the related population variability. These approaches can be of significant clinical application due to the widespread use of simvastatin and the clinical burden of muscle toxicity. In addition the present work illustrates the feasibility of combining traditional PBPK with population approaches and information from clinical data in order to develop mechanistically sound population models with clinical relevance.