Clinical Trial Evaluating DC/AML Fusion Cell Vaccination Alone and in Conjunction with PD-1 Blockade in AML Patients Who Achieve a Chemotherapy-Induced Remission

Abstract 948

Patients with acute myeloid leukemia (AML) achieve remission following chemotherapy; however, curative outcomes remain elusive due to relapse with chemotherapy-resistant disease. Allogeneic transplantation remains a potentially curative therapy for AML patients, but is associated with significant morbidity and mortality due to the lack of specificity of the alloreactive response. A promising area of investigation is the development of cancer vaccines that educate host immunity to more selectively target leukemia cells, including the stem cell compartment. Our group has developed a cancer vaccine model in which dendritic cells (DCs) are fused to autologous tumor cells, resulting in the presentation of multiple tumor antigens with the capacity to elicit a broad anti-tumor response. A fundamental challenge to developing a more effective tumor vaccine is overcoming the immunosuppressive milieu by which tumor cells evade host immunity. Key elements contributing to tumor-mediated immune suppression are the increased presence of regulatory T cells in patients with malignancy, and upregulation of the PD-1/PDL1 pathway. Tumor expression of PD-L1 promotes T cell tolerance by binding PD-1 on activated T cells and suppressing their capacity to secrete stimulatory cytokines. In addition, the PD-1/PDL-1 pathway has been shown to inhibit T cell-mediated lysis of tumor cells, potentially preventing a clinically meaningful immunologic response to vaccination. We are conducting a clinical trial in which AML patients who are in a first or second complete remission following chemotherapy receive three monthly doses of DC/AML fusion cells alone (Cohort 1) or in conjunction with anti-PD1 antibody, CT-011 (cohort 2). To date, 16 patients (9 males, 7 females; mean age 55 years) have been enrolled to the first cohort. All patients underwent successful tumor collection from either a bone marrow aspirate (N=12), collection of 20 cc of peripheral blood (N=3), or leukapheresis product (N=1) at the time of presentation with newly diagnosed AML (N=15) or first relapsed AML (N=1). The mean yield was 1.45×10⁸ cells, and the mean viability was 90%. Tumor cells were subjected to immunohistochemical analysis to identify antigens unique to the leukemia fusion partner. Those patients achieving complete remission following 1–2 cycles of induction chemotherapy underwent leukapheresis for dendritic cell generation. Adherent peripheral blood mononuclear cells were isolated, cultured in the presence of GM-CSF and IL-4 for 5–7 days, and then exposed to TNFα for 48–72 hours to generate mature DCs. Mean viability of the DC preparation was 92%. DCs strongly expressed the co-stimulatory molecule CD86 (mean 75% expression). One patient died during remission induction chemotherapy and 3 patients were removed from study after induction chemotherapy to undergo allogeneic transplantation. Vaccine was successfully generated in 9 patients at a dose of 5×10⁶ fusions cells, mean fusion efficiency of 30%, and viability of 87%. As a measure of their activity as antigen presenting cells, the capacity of the fusion cell preparation to stimulate allogeneic T cell proliferation ex vivo was quantified. In contrast to the leukemia preparation (mean stimulation index (SI) 3.7), the DC and fusion cell preparation were potent stimulators (mean SI 20.8 and 13.1, respectively). Vaccination with the DC/leukemia fusion vaccine was initiated within 12 weeks from count recovery following their final cycle of chemotherapy. 4 patients have completed vaccinations and are 2, 4, 5 and 6 months following the final vaccine. One patient was taken off study for disease progression one week after receiving his first vaccine. 4 patients experienced grade 1 vaccine site reactions. Biopsy of a vaccine site reaction demonstrated a dense T cell infiltrate. Additional vaccine related adverse events have included grade 1 ankle pain and edema. The remaining patients are undergoing chemotherapy, and when complete, will initiate vaccination. Peripheral blood samples are being collected prior to each vaccination and at 1, 3, and 6 months following completion of vaccination. Immune response targeting leukemia cells, leukemic stem cells, and leukemia associated antigens will be assessed. Levels of circulating regulatory T cells and T cell expression of PD1 will be measured. Time to disease progression will also be determined.