Development of a tumour growth inhibition model to elucidate the effect of ritonavir on intratumoural metabolism and anti-tumour effect of docetaxel in a mouse model for hereditary breast cancer
Huixin Yu (1), Jeroen J. M. A. Hendrikx (1), Alfred H. Schinkel (2), Sven Rottenberg (2), Jan H. M. Schellens (3,4,5), Jos H. Beijnen (1,4,5), Alwin D. R. Huitema (1,4)
(1) Department of Pharmacy & Pharmacology, Netherlands Cancer Institute-Antoni van Leeuwenhoek, Amsterdam, The Netherlands; (2) Department of Molecular Oncology, Netherlands Cancer Institute-Antoni van Leeuwenhoek, Amsterdam, The Netherlands; (3) Department of Medical Oncology, Netherlands Cancer Institute-Antoni van Leeuwenhoek, Amsterdam, The Netherlands; (4) Department of Clinical Pharmacology, Netherlands Cancer Institute-Antoni van Leeuwenhoek, Amsterdam, The Netherlands; (5) Utrecht Institute of Pharmaceutical Sciences, Utrecht University, Utrecht, The Netherlands.
Docetaxel, administered intravenously, is widely used as anti-cancer agent . An oral formulation of docetaxel has been successfully developed in our group . One major limitation for oral administration of docetaxel is its low bioavailability due to its affinity for P-glycoprotein and Cytochrome P450 (CYP) 3A [3-4]. Ritonavir strongly inhibits CYP3A4 and thereby proved to be a potent booster of docetaxel exposure in both mice and human [5-6]. Phase I trials with oral docetaxel and ritonavir showed the feasibility of this oral treatment [7-8].
Although ritonavir is primarily used as exposure booster, it has also been suggested that ritonavir may have an anti-tumour effect itself [9-13]. In order to explore the effect of ritonavir on docetaxel metabolism as well as anti-tumour effects of ritonavir, we performed an allograft study in mice . The results suggested that co-treatment of docetaxel and ritonavir significantly prolonged survival and substantially reduced average tumour size compared to that of docetaxel only-treated mice. However, from the empirical data analysis, it could not be definitively concluded whether this was only the consequence of decreased docetaxel metabolism in tumours or additional factors, e.g. direct anti-tumour effect of ritonavir.
The aims of the current study were: (1) to develop a pharmacokinetic (PK)- pharmacodynamic (PD) model based on central and tumour exposure of docetaxel and tumour size measurements from this previous study; (2) to further evaluate and quantify the effects of ritonavir on systemic and intratumoral exposure and anti-tumour effects of docetaxel when combined.
70 host mice lacking Cyp3a (Cyp3a-/-) with implanted tumour tissue presenting inherent Cyp3a expression were divided randomly over 4 groups . Group I was not treated and used as control for tumour growth (control group, n=20). Group II was treated with 12.5 mg/kg oral ritonavir (ritonavir only-treated group, n=20), group III was treated with 20 mg/kg intravenous docetaxel (docetaxel only-treated group, n=30), and group IV was treated with both 20 mg/kg intravenous docetaxel and 12.5 mg/kg oral ritonavir (co-treated group, n=30). Docetaxel and ritonavir PK were measured in both plasma and tumour. PK samples (n=5 per time point) were collected on day 2, 9 and 16 at approximately 24 hours after docetaxel administration. Tumour volumes were measured daily in all groups.
A previously established two-compartment model was used for docetaxel plasma PK . Although Cyp3a-/- mice were studied, it was hypothesized that ritonavir might still influence docetaxel plasma PK. Docetaxel tumour concentrations were described with a separate compartment with a first-order absorption rate from the system, and a first-order elimination rate from the tumour. Ritonavir systemic PK was fixed with a one-compartment model similar as used in a previous study . Measured ritonavir tumour concentrations were modelled analogously to docetaxel. In the co-treated group, the inhibition of docetaxel tumour metabolism by ritonavir was explored with ritonavir tumour concentrations by an Emax-type inhibition model. The ritonavir tumour concentration that inhibits half of Cyp3a enzymes (IC50RTV) was fixed according to the literature as 2.5 ng/g .
Non-perturbed tumour growth in untreated mice was described by an exponential net tumour growth rate. In docetaxel treated mice, a progression factor that represents the increase in tumour growth rate over time was considered. Docetaxel anti-tumour effect was described by a delayed effect model using transition of cells from the proliferative pool to an apoptotic pool. This transition of tumour cells was described with a first-order rate constant and was dependent on the docetaxel tumour concentration using an Emax-type model. The cells in the apoptotic compartment were eliminated by a first-order rate constant.
Firstly, docetaxel effect parameters were estimated based on the docetaxel only-treated group. Secondly, these parameter estimates were used to predict tumour size profiles in the combined treatment group taking only the increased docetaxel tumour concentrations as a result of ritonavir co-administration into account. Subsequently, it was explored whether ritonavir had an additional anti-tumour effect independent from the increased tumour docetaxel concentrations.
A sensitivity analysis was conducted to explore whether the assumed value for IC50RTV influenced the final outcome (±10% difference from fixed IC50RTV parameter).
The final PK model adequately described the observed data. In the Cyp3a-/- host, ritonavir slightly decreased docetaxel systemic clearance by 8% (relatively standard error (RSE) 0.4%) in the co-treated group. As expected, docetaxel tumour exposure was increased with mean area under the concentration-time curve 2.5-fold higher when co-treated with ritonavir.
PK/PD model & Hypothesis tests
Firstly, effect parameters in the docetaxel only-treated group were successfully estimated. A model improvement with a drop of objective function value (OFV) of 30 points (p <0.001) was found when an increased tumour growth rate over time was considered.
Secondly, these effect parameters from docetaxel only-treated group together with increased docetaxel tumour concentration were used to predict the tumour growth profiles in the co-treated group. This resulted in a slight underestimation of the time to tumour re-growth in the co-treated group. Also in early phases of treatment the anti-tumour effect was underestimated. This indicated that the observed enhanced anti-cancer effect in the co-treated group, compared to the docetaxel only-treated group, could not be fully explained by the increased docetaxel tumour concentrations alone.
Subsequently, a potential ritonavir anti-cancer effect was modelled analogously to that of docetaxel. Inclusion of this effect of ritonavir resulted in drop of OFV of 59 points (p <0.001). Bias in the model predictions of the co-treated group disappeared by inclusion of this effect.
In the final PK/PD model, non-perturbed tumour growth was estimated with a net growth rate of 1.32 week-1. This rate exponentially accelerated with 0.06 week-1 in treated groups. Docetaxel tumour concentration with 50% of maximum anti-tumour effect was estimated as 307 ng/g.
Difference of ±10% on fixation of IC50RTV suggested no influence on the final model parameters.
A PK/PD model has been successfully built describing the complex interaction between docetaxel and ritonavir when co-administered in a mouse model for hereditary breast cancer. We showed that the increased tumour growth inhibition in co-treatment of docetaxel with ritonavir is mainly caused by boosting the tumour exposure to docetaxel and to a minor extent by a direct tumour growth inhibitory effect of ritonavir.
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