Development of a Target cell-Biologics-Effector cell (TBE) complex-based cell killing model to characterize target cell depletion by T cell redirecting bispecific agents

Abstract

T-cell redirecting bispecific antibodies (bsAbs) or antibody-derived agents that combine tumor antigen recognition with CD3-mediated T cell recruitment are highly potent tumor-killing molecules. Despite the tremendous progress achieved in the last decade, development of such bsAbs still faces many challenges. This work aimed to develop a mechanism-based pharmacokinetic/pharmacodynamic (PK/PD) modeling framework that can be used to assist the development of T-cell redirecting bsAbs. A Target cell-Biologics-Effector cell (TBE) complex-based cell killing model was developed using in vitro and in vivo data, which incorporates information on binding affinities of bsAbs to CD3 and target receptors, expression levels of CD3 and target receptors, concentrations of effector and target cells, as well as respective physiological parameters. This TBE model can simultaneously evaluate the effect of multiple system-specific and drug-specific factors on the T-cell redirecting bsAb exposure-response relationship on a physiological basis; it reasonably captured multiple reported in vitro cytotoxicity data, and successfully predicted the effect of some key factors on in vitro cytotoxicity assays and the efficacious dose of blinatumomab in humans. The mechanistic nature of this model uniquely positions it as a knowledge-based platform that can be readily expanded to guide target selection, drug design, candidate selection and clinical dosing regimen projection, and thus support the overall discovery and development of T-cell redirecting bsAbs.

Keywords: T cell redirecting; Target cell-Biologics-Effector cell (TBE) complex; bispecific antibodies (bsAbs); cell killing model; immune oncology; mechanism-based pharmacokinetic-pharmacodynamic modeling; tumor immunotherapy.

Figure 1.

The structure of the TBE complex-based cell killing model, describing T cell redirecting bispecific agent (bsAb)-induced redirection of effector cells (T cells) for the elimination of target cells.

Figure 2.

Development of the TBE model with blinatumomab in vitro cytotoxicity data on CD19+ Nalm-6 cells. Symbols and lines with corresponding color represent observed and model predicted Nalm-6 target cell depletion profiles by a human T cell-clone at different effector-to-target ratios in the presence of blinatumomab after 4 hours (a) and 24 hours (b) of incubation.¹⁹

Figure 3.

Evaluation by the TBE model on characterizing the impact of key determining factors. The impact of target receptor expression level was evaluated with cytotoxicity profiles of OCI-AML3 target cells with different CD33 target receptor expression levels (500 – 10,000 receptors per cell) by T cells from healthy donors at E:T ratios of 10: 1 (a) and 1:1 (b), in the presence of AMG 330 after 48 hours of incubation.³ The impact of binding affinity was evaluated with cytotoxicity profiles of FLT3+ REH cells by CD8 + T cells in the presence of FLT3xCD3 bsAbs with differentiated binding affinity to FLT3 after 8 hours of incubation (c),²⁵ and that of CD20 + B cells by CD3 + T cells in the presence of CD20xCD3 bsAbs with differentiated binding affinity to CD3 after 24 hours of incubation (d).²⁶ The impact of effector-to-target (E:T) ratio was evaluated with CD19+ Blin-1 cell cytotoxicity profiles by PBLs in the presence of blinatumomab after 4 hours of incubation (e),²⁷ and CD19+ Nalm-6 cell cytotoxicity profiles by purified human peripheral T cells in the presence blinatumomab after 24 hours of incubation (f).²³ Symbols represent observed in vitro cytotoxicity data; solid lines with corresponding color represent model estimation results; dashed lines with corresponding color represent model simulation results.

Figure 4.

Sensitivity analysis for the impact of BsAb Concentration and Target Binding Affinity on the formation of TBE complex per target cell, expressed as TBEPC_R (a–b), and relative target cell killing potency of T cells, expressed as k_el/k_max (c–f). The top panel (a, c, e) shows the scenario with high target receptor expression (20,000 receptors per target cell) and the bottom panel (b, d, f) shows the scenario of low target receptor expression (2,000 receptors per target cell). k_el/k_max was assessed using bsAbs with either high (EC_{50_A} = 0.1 TBEPC_R), (c-d) or low (EC_{50_B} = 5 TBEPC_R), (e-f) intrinsic engagement potencies. In (a) and (b), the curved solid and dashed lines stand for 0.1 and 5 TBEPC_R, respectively. The vertical dotted line corresponds to BsAb concentration (Conc.) at 15 nM, which equals the KDE to CD3 used for this simulation.

Figure 5.

Projection of exposure-response relationships of blinatumomab in blood (a) and bone marrow (b) for patients with acute lymphoblastic leukemia (ALL), using TBE model fitting results of in vitro cytotoxicity data from 4 different cell lines (Tables 1 and 2) and physiology & pathology information of ALL patients. Steady state plasma drug concentration information following the priming dose (9 µg/day light gray bar) and the full dose (28 µg/day, dark gray bar) was obtained from literature.³² Steady state bone marrow drug concentrations following the priming dose (9 µg/day light gray bar) and the full dose (28 µg/day, dark gray bar) were projected under the assumption that drug exposure in bone marrow was about 30% of that in plasma.⁴⁶