Abstract Standard treatment of primary glioblastoma (GBM) is associated with poor survival. Adjunctive therapy with patient-specific vaccines may improve outcomes by inducing or enhancing each patient's anti-GBM immune response. A multi-institutional phase II clinical trial was designed with a primary objective of 75% survival 15 months after intent-to-treat enrollment. Key eligibility criteria were: (1) primary GBM diagnosis, (2) age < 70 years at time of tumor resection, (3) successful cell culture of GBM cells in serum-free media, (4) successful monocyte collection by leukapheresis, and (5) KPS > 70 after recovery from surgery. IL-4 and GM-CSF were used to generate dendritic cells (DC) from monocytes. DC were incubated with autologous tumor antigens (ATA) from the lysate of cultured GBM cells to produce each patient-specific DC-ATA vaccine. Each dose was admixed with GM-CSF at the time of subcutaneous injections at weeks 1, 2, 3, 8, 12, 16, 20 and 24. At the time of this analysis, cell line success rate is 61/63 (97%); monocyte collection success rate is 53/55 (96%), but 10 patients required a second apheresis. 50 of a planned 55 patients have enrolled after recovery from surgery, just prior to starting standard concurrent radiation therapy (RT) and temozolomide (TMZ), with intent-to-treat after recovery from RT/TMZ. The 50 patients include 36 men and 14 women; median age is 58 years with a range of 27 to 70. Average KPS is 82.8. MGMT methylation has been documented in 22% of patients and IDH mutation in 14%. 37 patients have started treatment and received 231 doses. 16 have completed all 8 doses, 7 received fewer than 8 doses, and 14 are currently undergoing treatment. No patient has discontinued treatment because of toxicity, but 20 patients have experienced 35 treatment-emergent serious adverse events including hospitalizations for falls and/or increased focal weakness (12 episodes), seizures (10 episodes), or severe headaches or visual changes (3 episodes). 6-month actual survival rate is 96% for the 28 patients at risk 6 months or longer from enrollment. Serologic analyses show that 12 of 16 patients (75%) had an increase in markers associated with Th1/NK, Th2/immunoglobulins, and Th2 hypersensitivity (eotaxins, IgE and IL17F) by week-3; 9 of 15 (60%) had a decrease in angiogenesis factors, growth factors, and tumor markers by week-8. On serial MRI scans, 7 of 13 patients (54%) have exhibited an increase in T2 (tumor) and flare (edema) for several weeks after starting DC-ATA, followed by a slow gradual decrease in both. This patient-specific DC-ATA immunotherapy approach is feasible, is associated with changes in serologic markers, and may be increasing intratumor inflammation that may be associated with on-target toxicity and efficacy. Longer follow up is needed before the survival objective can be assessed. [Clinicaltrials.gov NCT03400917] Citation Format: Daniella A. Bota, Christopher M. Duma, Santosh Kesari, David E. Piccioni, Renato V. LaRocca, Robert T. O'Donnell, Robert D. Aiken, Jose A. Carrillo, Candace Hsieh, Chris J. Langford, Krystal Carta, Adrienne M. Wang, James A. Langford, Thomas H. Taylor, Gabriel I. Nistor, Robert O. Dillman. Adjunctive treatment of primary glioblastoma with patient-specific dendritic cell vaccines pulsed with antigens from self-renewing autologous tumor cells: A phase II trial [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr CT182.