T Cell Responses during Long-Term Continuous Infusion of MT103 (MEDI-538; Anti-CD19 BiTE) in Patients with Relapsed B-NHL: Data from Dose-Escalation Study MT103-104.

MT103 (MEDI-538) is a bispecific single-chain antibody construct directed at CD3 on human T cells and CD19 on human B lymphoma and normal B cells. Transient linkage of B and T cells by MT103 provides T cells with a T cell receptor (TCR)-like signal leading to redirected lysis of B cell targets without apparent need of costimulation and inducing T cells to proliferate, secrete cytokines and upregulate surface activation markers. TCR-like signalling by MT103 is strictly dependent on the presence of target cells. Redirected lysis of CD19-positive cells by MT103 is seen at low picomolar concentrations and at low effector-to-target ratios. The in-vivo half-life of MT103 is approximately two hours. In the ongoing dose escalation study MT103-104, patients with relapsed B-NHL have so far received continuous infusion of MT103 at maintenance flow-rates of 0.5, 1.5, 5 and 15 μg/m²/24h for 4 or 8 weeks following a 3+3 dose escalation design. Serum concentrations of MT103 remained constant over the entire treatment period at a level depending on the respective maintenance flow-rate. Depletion of circulating B (lymphoma) cells could be observed more frequently with increasing dose levels (DL) from DL1 to DL3, and in all evaluable patients at DL4. Three of six evaluable patients at DL4 showed clinical responses (2 PR, 1 CR) according to standardized Cheson criteria, but no patient of DL1-3. The time courses of absolute CD4 and CD8 T cell counts in peripheral blood were determined by flow cytometry. CD8 T lymphocytes were further subdivided for analysis into naïve T cells, TCM (central memory T cells), TEM (effector memory T cells) and TEMRA (non-proliferating terminally differentiated CTL), and CD4 T lymphocytes into naïve T cells, TCM and TEM. Activation of CD4 and CD8 T cell subsets was determined by measuring upregulation of CD69, CD25 and HLA-DR. Serum levels of cytokines were determined as additional biomarkers for T cell activation. In 50% of patients at DL1 to DL3, CD4 and CD8 T cell counts increased during the course of treatment - over pre-treatment levels. The TEM subset from both CD4 and CD8 T cells accounted for most of the observed increases, while the naïve T cell subsets showed no increase but also no signs of apoptosis. The non-proliferative TEMRA subset of CD8 T cells also remained unchanged in most patients. This indicated that the selective increase of proliferation-competent TEM subsets was attributed to MT103-induced T cell proliferation. At DL4, all evaluable patients showed signs of T cell expansion after 2 weeks of MT103 infusion, which was most pronounced in those who developed a partial or complete remission. The increase of CD8 T cell counts was more pronounced than that of CD4 T cells. T cell expansion was accompanied by upregulation of T cell activation markers as well as by increases in serum concentrations of cytokines like IFN-γ. T cell expansion and activation reverted in all cases when the infusion of MT103 was stopped. In summary, MT103 induced a reversible secondary T cell response involving T cell activation and proliferation as well as T cell cytotoxicity against circulating B cells and lymphoma tissue. The dose-dependent T cell expansion observed during long-term infusion of MT103, particularly within the cytotoxic TEM subset of CD8 T cells, appears to play a key role for clinical activity.