Abstract Lessons Learned The novel therapeutic vaccine hVEGF26–104/RFASE was found to be safe and well tolerated in patients with cancer. hVEGF26–104/RFASE failed to induce seroconversion against native hVEGF165 and, accordingly, neither a decrease in circulating vascular endothelial growth factor (VEGF) levels nor clinical benefit was observed. Remarkably, hVEGF26–104/RFASE induced VEGF165-neutralizing antibodies in a nonhuman primate model. The absence of seroconversion in human calls for caution in the interpretation of efficacy of human vaccines in nonhuman primates. Background Targeting vascular endothelial growth factor-A (VEGF) is a well-established anticancer therapy. We designed a first-in-human clinical trial to investigate the safety and immunogenicity of the novel vaccine hVEGF26–104/RFASE. Methods Patients with advanced solid malignancies with no standard treatment options available were eligible for this phase I study with a 3+3 dose-escalation design. On days 0, 14, and 28, patients received intramuscular hVEGF26–104, a truncated synthetic three-dimensional (3D)-structured peptide mimic covering the amino acids 26–104 of the human VEGF165 isoform, emulsified in the novel adjuvant Raffinose Fatty Acid Sulphate Ester (RFASE), a sulpholipopolysaccharide. Objectives were to determine safety, induction of VEGF-neutralizing antibodies, and the maximum tolerated dose. Blood was sampled to measure VEGF levels and antibody titers. Results Eighteen of 27 enrolled patients received three immunizations in six different dose-levels up to 1,000 μg hVEGF26–104 and 40 mg RFASE. No dose-limiting toxicity was observed. Although in four patients an antibody titer against hVEGF26–104 was induced (highest titer: 2.77 10log), neither a reduction in VEGF levels nor neutralizing antibodies against native VEGF165 were detected. Conclusion Despite having an attractive safety profile, hVEGF26–104/RFASE was not able to elicit seroconversions against native VEGF165 and, consequently, did not decrease circulating VEGF levels. Deficient RFASE adjuvant activity, as well as dominant immunoreactivity toward neoepitopes, may have impeded hVEGF26–104/RFASE's efficacy in humans.