Sipuleucel-T is designed to stimulate an anti-tumor immune response. It is prepared from autologous antigen presenting cells (APCs) that are incubated with a recombinant protein composed of prostatic acid phosphatase (PAP) linked to granulocyte-macrophage colony-stimulating factor (GM-CSF). PAP was identified as an attractive antigen target because it is expressed in prostatic tissue and the vast majority of prostate carcinomas, exhibits minimal or no expression in other tissues [2], and does not share a high degree of sequence homology with any other known protein. The GM-CSF moiety enhances

antigen uptake by APCs. In preclinical Modulators development, 3 treatments (at 14-day intervals) of APCs incubated with a recombinant

MEK inhibitor clinical trial fusion protein consisting of rat PAP and rat GM-CSF elicited lymphocytic infiltrates in rat prostate tissue [3] (Fig. 1B). The high tissue specificity of the treatment, with immune cell infiltration seen only in prostate tissue, indicated the breaking of tolerance to a self-antigen, and the effective engagement of the adaptive arm of the immune system. Of note, the treatment response was attenuated when either APCs or GM-CSF (Fig. 1A) were removed from the preparation, suggesting that all 3 treatment components (APCs, GM-CSF, and target antigen) were critical for producing Z-VAD-FMK a robust T cell response. Additional preclinical experiments demonstrated that when PAP-expressing tumor cells (MatLu cells) were co-cultured with splenocytes aminophylline from animals immunized with PAP-GM-CSF pulsed APCs,

tumor cell proliferation was inhibited [3]. In clinical development, sipuleucel-T was manufactured from autologous APC-containing peripheral blood mononuclear cells (PBMCs) of prostate cancer patients. PBMCs were obtained from a leukapheresis procedure that processes 1.5–2.0 times the blood volume of the subject. These cells were cultured for 36–44 h with PA2024, the recombinant fusion protein of human PAP-GM-CSF, prior to reinfusion. Of note, sipuleucel-T comprises multiple types of mononuclear cells including APCs, CD4 and CD8 T cells, NK cells, and B cells. Initial clinical studies demonstrated antigen-specific immune responses to the immunizing antigen, with no dose-limiting toxicities [4] and [5]. In the randomized, controlled, Phase 3 trials of sipuleucel-T (D9901, D9902A, and D9902B [IMPACT]), sipuleucel-T was manufactured from PBMCs isolated during 3 leukapheresis procedures at 2-week intervals (weeks 0, 2, and 4) [6], [7] and [8]. The median values for white blood cells, and absolute neutrophil, lymphocyte, and monocyte counts at weeks 6, 14, and 26 remained within normal ranges [9]. Control subjects received non-activated autologous cells; i.e., cells that were maintained in the absence of PA2024.