A Target-Mediated Drug Disposition model to quantify the relationship between Otelixizumab in vitro concentration and TCR/CD3 engagement

Objectives: Otelixizumab is a monoclonal antibody (mAb) directed against CD3ε, a protein forming part of the CD3/T-cell receptor (TCR) complex on T lymphocytes [1]. An in vitro culture system was proposed to investigate Otelixizumab binding characteristics in a static situation. Specifically, the objective of this work was to quantify the relationship between Otelixizumab in vitro concentration and TCR/CD3 engagement. Within this framework, the Target Mediated Drug Disposition (TMDD) hypothesis [2] was investigated for Otelixizumab disposition. Methods: A wide range of doses was proposed in order to detect and properly quantify nonlinearities in Otelixizumab PK. For this purpose, peripheral blood mononuclear cells (PBMCs) from two donors were incubated with titrated Otelixizumab initial concentrations in the range 0.001-10 μg/ml. To investigate the kinetics of CD3/TCR re-expression, cells were washed on day 2 to remove exogenous Otelixizumab and thereby allow CD3/TCR complex re-expression to be monitored. At each time point free, bound and total TCR/CD3 expression on both CD4+ and CD8+ T cells and the amount of free antibody in the supernatant were measured. A TMDD model [3] accounting for Otelixizumab binding on both CD4+ and CD8+ lymphocytes was proposed with the intent to describe in vitro experimental data. Analyses were conducted using NONMEM version 7.2. Final models were selected based upon change in OFV, precision of estimates and diagnostic goodness-of-fit plots. Results: All parameters were estimated with reasonable precision (<40%). The proposed Target Mediated Drug Disposition model well captured the PK and PD profiles of Otelixizumab. No significant difference was found in the binding constants for binding on CD4+ or CD8+ lymphocytes (Kon=51.5/nM/day and Koff=4.64/day). The estimated values for binding parameters suggested high Otelixizumab affinity to TCR/CD3 receptor (KD = Koff/Kon = 90 pM). Estimated internalization rates (Kint4=1.26/day and Kint8=1.29/day) were 5 times higher than degradation rates (Kdeg4=0.273/day and Kdeg8=0.275/day). Conclusions: The TCR/CD3 receptor has been shown to have a major role in determining the non-linear PK of Otelixizumab. A TMDD model accounting for Otelixizumab binding to TCR/CD3 on both CD4+ and CD8+ lymphocytes successfully captured the PK and PD in vitro data, confirming that the assumptions of this model are reasonable for Otelixizumab. References: [1] Routledge EG, Lloyd I, Gorman SD, Clark M, Waldmann H. A humanized monovalent CD3 antibody which can activate homologous complement. Eur J Immunol. 21, 2717-2725 (1991) [2] Gerhard Levy. Pharmacologic target-mediated drug disposition. Clinical pharmacology and therapeutics, 56(3):248, 1994. [3] Donald E. Mager and William J. Jusko. General pharmacokinetic model for drugs exhibiting target-mediated drug disposition. Journal of Pharmacokinetics and Pharmacodynamics, 28(6):507-532, 2001.