Mechanistic Evaluation of Siplizumab (MEDI-507) Activity on Normal and Malignant T-Lymphocytes.

Siplizumab is a humanized monoclonal antibody that selectively binds human CD2 (huCD2), which is expressed on virtually all cells of the T- or NK-cell lineage. Also, known as MEDI-507, siplizumab is being developed as a potential new treatment for lymphoid malignancies involving T- or NK-cells. Our present study was designed to increase further the mechanistic understanding of siplizumab activity. Preclinical studies demonstrated that siplizumab mediated efficient lysis of malignant T-cells in a concentration-dependent and effector-to-target cell ratio-dependent manner. ADCC contributed prominently to tumor cell clearance whereas complement-mediated killing (CDC) did not. Siplizumab treatment alone was sufficient to trigger limited internalization of its target antigen (CD2) from the surface of tumor cells. Moreover, the efficiency of CD2 internalization was increased when such studies were carried out in the presence of an activated monocyte cell line (THP-1) bearing Fcg receptors and incapable of mediating ADCC, and suggested that CD2 clearance from the cell surface can be distinguished from host effector mechanisms. CD2 has been long known to confer both inhibitory and stimulatory signals and we reasoned that target internalization might disrupt survival signals independent of host effector activity. Consistent with this concept, siplizumab treatment in the absence of effector cells transduced apoptotic signals as indicated by multiple standard markers of tumor cell apoptosis , including activation of caspase 3 activity and an increase in tumor cells with a sub-G₁ DNA content. In addition, the use of transgenic mouse models demonstrated that intravenous administration of siplizumab caused a rapid (≤ 24 hours) but transient reduction in the number of peripheral huCD2-positive T-cells. The number of T-cells gradually recovered to normal levels within 30 days of treatment. In contrast, siplizumab treatment did not alter the number of CD19-positive B-cells. These results indicate that siplizumab can utilize host effector mechanisms (ADCC) to facilitate malignant T-cell elimination. Our present findings also suggest that siplizumab can mediate changes in CD2 expression or function that result in apoptosis of CD2-positive tumor cells. In light of this potent anti-tumor activity, two Phase I dose escalation trials (MI-CP099 and MI-CP107) are ongoing to assess the maximum tolerated dose and safety of siplizumab in patients with CD2-positive T- and NK-cell malignancies.