Background: Neoantigens (NeoAg) are key targets for personalized immunotherapy but efficient methods for their systematic identification and therapeutic targeting remain elusive. We developed a methodology to reliably identify and verify somatic alteration-derived neoantigens based on natural T cell responses against them which formed the basis of an individualized therapeutic vaccine strategy. Methods: This is a phase Ib study to assess the immunogenicity, safety and early clinical activity of personalized synthetic long peptides (PSLP) cancer vaccines in combination with pembrolizumab for patients with treatment refractory metastatic solid tumors or PSLP vaccine alone as an adjuvant treatment with patients with no evidence of disease (NED) that incorporates patient-specific NeoAg identified by an HLA-agnostic, functional T-cell assay (see table). Results: At the time of data cutoff, a total of 5 patients had been treated on ARM-A, 5 patients on ARM-C and 2 patients on ARM-D. AES possibly attributed to personalized vaccine (PSLP), or pembrolizumab, or both include: Grade 1: Arthralgia (1); Diarrhea (1); Fever (4); Fatigue (7); Generalized muscle weakness (1); Headache (2); Nausea (1); Confusion (1); Injection site reaction (5); Rash maculo-papular (3); Flu like symptoms (5); Myalgia (1); and Grade 2: Diarrhea (1); Fatigue (1); Hyperhidrosis (1); Hypothyroidism (1); Injection site reaction (1); Proteinuria (1); Renal and Urinary – other (1); and Grade 3: Colitis (1). For the 9 patients with at least 1 radiographic assessment at the time of analysis 6 had a best response of stable disease (SD) and 3 had progressive disease (PD). Immune monitoring of peripheral blood specimens consistently demonstrated that NeoAg-specific T cell responses were enhanced following administration of PSLP vaccine. On-treatment biopsies demonstrated immune-editing with the variant allele frequency of targeted mutations decreasing following administration of the PSLP vaccine. Conclusions: Taken together, these data meet the trial primary endpoint by demonstrating PSLP vaccines targeting NeoAg identified using the HLA-agnostic pipeline augment effector T cell function against these targets. Clinical trial information: NCT02287428