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2542 Background: Cytotoxic T Lymphocyte-associated Antigen 4 (CTLA4) is an activation-induced T lymphocyte negative costimulatory receptor which down-regulates cellular immune responses. CTLA4 blockade may break peripheral immunological tolerance, leading to an effective immune response to cancer. Tools for assessing the effects of such a blockade are limited, as CTLA4 is not constitutively expressed on circulating T cells, and because activated lymphocytes are difficult to access in vivo. Therefore, we have employed ex vivo blood stimulation assays to define pharmacodynamic properties of the anti-CTLA4 antibody, ticilimumab. Methods: Ex vivo blood stimulation assays employed staphylococcal enterotoxin A (SEA) to stimulate isolated peripheral blood mononuclear cells (PBMC) or whole blood. Stimulation was monitored by production of interleukin 2 (IL-2). This assay was used preclinically to predict in vivo responses in animal models and in samples from cancer patients, as a batch release assay for production runs of ticilimumab, and clinically as a pharmacodynamic measurement in clinical trials of ticilimumab. Results: Screening experiments using the SEA assay allowed us to identify the lead candidate mAb with optimal CTLA4 blockade activity, ticilimumab. Dose-dependent increases in IL-2 production were observed in PBMC and whole blood samples up to an in vitro concentration of 100 ug/mL of ticilimumab, with 10 ug/mL identified as the minimum predicted efficacious concentration (Ceff). This functional potency assay was adapted for qualification of production lots of ticilimumab. Whole blood taken from cynomolgus monkeys dosed with ticilimumab demonstrated significant enhancement of IL-2 production at the same magnitude observed in in vitro experiments. Additionally, longitudinal samples taken from healthy volunteers and cancer patients suggested that an enhancement of 2.8 fold would be indicative of a pharmacodynamic effect of ticilimumab. Conclusions: The SEA assay provides a functional assessment of ticilimumab activity and can be used to guide the clinical development of this agent. Our data suggest that T cell reactivity is enhanced in the presence of ticilimumab in vitro, in primate models and in humans. Table: see text
Millham et al. (Tue,) studied this question.