Receptor mediated endocytosis (RME) plays a major role in the disposition of therapeutic protein drugs in the body. It is suspected to be a major source of nonlinear pharmacokinetic behavior observed in clinical pharmacokinetic data. So far, mostly empirical or semi-mechanistic approaches have been used to represent RME. A thorough understanding of the impact of the properties of the drug and of the receptor system on the resulting nonlinear disposition is still missing, as is how to best represent RME in pharmacokinetic models. In this article, we present a detailed mechanistic model of RME that explicitly takes into account receptor binding and trafficking inside the cell and that is used to derive reduced models of RME which retain a mechanistic interpretation. We find that RME can be described by an extended Michaelis-Menten model that accounts for both the distribution and the elimination aspect of RME. If the amount of drug in the receptor system is negligible a standard Michaelis-Menten model is capable of describing the elimination by RME. Notably, a receptor system can efficiently eliminate drug from the extracellular space even if the total number of receptors is small. We find that drug elimination by RME can result in substantial nonlinear pharmacokinetics. The extent of nonlinearity is higher for drug/receptor systems with higher receptor availability at the membrane, or faster internalization and degradation of extracellular drug. Our approach is exemplified for the epidermal growth factor receptor system.
A population pharmacokinetic model based on data from three phase I studies was to be developed including a covariate analysis to describe the concentration -time profiles of matuzumab, a novel humanised monoclonal antibody. Matuzumab was administered as multiple 1 h i.v. infusions with 11 different dosing regimens ranging from 400 to 2000 mg, q1w -q3w. For analysis, 90 patients with 1256 serum concentration -time data were simultaneously fitted using the software NONMEMt. Data were best described using a two-compartment model with the parameters central (V 1 ) and peripheral distribution volume (V 2 ), intercompartmental (Q) and linear (CLL) clearance and an additional nonlinear elimination pathway (K m , V max ). Structural parameters were in agreement with immunoglobulin characteristics. In total, interindividual variability on V max , CLL, V 1 and V 2 and interoccasion variability on CLL was 22 -62% CV. A covariate analysis identified weight having an influence on V 1 ( þ 0.44% per kg) and CLL ( þ 0.87% per kg). All parameters were estimated with good precision (RSEo39%). A robust population pharmacokinetic model for matuzumab was developed, including a nonlinear pharmacokinetic process. In addition, relevant and plausible covariates were identified and incorporated into the model. When correlated to efficacy, this model could serve as a tool to guide dose selection for this 'targeted' cancer therapy.
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