e14634 Background: Introducing amino acid sequence changes in highly expressed self-antigens for prostate cancer (PCa) patients (pts) might lead to avoidance of immune tolerance. We evaluated a DNA vaccine (INO-5150) including SynCon PSA and PSMA. Administration of INO-5150 to PCa pts along with plasmid encoded adjuvant IL-12 (INO-9012) via electroporation (EP) is postulated to break tolerance, resulting in antigen-specific immune responses which could lead to stabilization of disease progression. Methods: Phase I, open-label, multicenter study of PCa pts post-definitive therapy with a rising PSA ≥ 1.0 ng/ml after surgery, or ≥ 2.0 ng/ml above nadir after RT and PSADT > 3 months, testosterone > 150 ng/dL, no concomitant androgen deprivation therapy and no evidence of metastases within 12 months. We evaluated safety, tolerability and for efficacy signals. INO-5150 low (2 mg, arms A and C) or high (8.5 mg, arms B and D) dose with or without INO-9012 (1 mg) was administered IM followed by EP in total 4 dosing arms on Day 0 and at Wks 3, 12, and 24 in 60 planned pts (15/arm). Pts were followed for 72 Wks. Results: 62 pts, 16 each in arms A and D and 15 in B and C were enrolled. Median age: 69.5 yrs (range 55.4-87.7), Gleason score: 7 (5-10), time from initial diagnosis: 8.2 yrs (0.5-23.8) and ECOG PS: 0 (0-1). As of data cutoff of 23Jan17, 52 pts had EOT visit, 7 withdrawn from treatment and 6 (10%) reported disease progression, 3 biochemical and 3 radiographic. Median serum PSA at enrollment was 4.6 ng/mL (range 1.2, 113.7) and at EOT was 6.5 ng/mL (0.1, 73.6). Median PSADT at enrollment was 8.7 months (3.1, 218.1) and at EOT it was 3.1 months (-23.1, 100.0). Safety: no reports of Grade 4-5 SAEs. 6 Grade 3 SAEs in 5 pts: presyncope, cardiac disorder, fall, neoplasm, ALT and AST elevation. Grade 1-3 AEs reported in 51 (82%) pts: 12 (75%) in Arm A, 13 (87%) B, 13 (87%) C, and 13 (81%) in D. Common AEs were injection site pain (24/39%), swelling (14/23%), erythema (14/23%), all Grade 1-2. Conclusions: INO-5150 (+) and (-) INO-9012 was generally safe and well-tolerated at all 4 dose levels in this patient population. Preliminary data suggest PSA stabilization in some patients. Immune analyses are ongoing. (NCT02514213) Clinical trial information: NCT02514213.