Abstract LB113: Immunogenicity of Atezolizumab plus personalized neoantigen vaccination (PGV-001) in patients with urothelial cancer in adjuvant vs metastatic setting

Abstract Background: Urothelial cancer (UC) of the genitourinary tract, most commonly arising from the bladder, has historically been a challenging area for the development of new therapeutics. Immune checkpoint blockade inhibitors (CBIs) induce durable responses only in a subset of patients with UC, potentially due to lack of neoantigen primed T cells. To build on this hypothesis, we designed a study to evaluate safety and immunogenicity of a combination treatment with atezolizumab and PGV-001, a personalized genomic neoantigen vaccine. Having previously reported clinical outcomes, here we describe analysis of immunogenicity induced by the treatment. Methods: In this phase I trial (NCT03359239) UC patients were enrolled to receive PGV-001 with atezolizumab in the adjuvant (undergone radical cystectomy or nephroureterectomy with a high risk of recurrence) or metastatic setting. Each PGV001 vaccine included up to 10 neoantigen (predicted using OpenVax pipeline) synthetic long peptides (SLPs). Each subject received 10 PGV-001 vaccines and 1200 mg i.v. atezolizumab every 3 weeks for up to 12 months (adjuvant) or 24 months (metastatic). To investigate T cell immunity, ex vivo IFNγ ELISPOT and T cell expansion assays were performed using 15mer overlapping peptide (OLP) pools corresponding to each neoantigen SLP. Results: Samples from 9 subjects (3 adjuvant and 6 metastatic) were evaluable for ex vivo ELISPOT analysis. In 100% of the subjects, at least 1 neoantigen was found to elicit an immune response. Overall, out of 87 evaluated neoantigens, 47% were immunogenic, with the immunogenic peptides inducing an average of 16.7 fold increase in ex vivo ELISPOT response over baseline. While 39% (11/28) of the peptides in adjuvant cohort were immunogenic vs 51% (39/50) in metastatic cohort, it is not possible to determine statistically significant difference in response between metastatic and adjuvant cohort. The T cell expansion assay was performed for 6 subjects, 3 each from the adjuvant and metastatic cohorts. Among 57 evaluated neoantigens, 46% and 54% of the peptides elicited a CD8+ and CD4+ T cell targeted immune response, respectively. A greater proportion of neoantigens induced both CD8+ and CD4+ T cell responses in subjects treated in the metastatic setting vs adjuvant setting. Upon epitope mapping, for 5 evaluable subjects, 141 OLPs were analyzed and a similar number of OLPs were found to be immunogenic for CD8+ (31.2%, 44/141) and CD4+ response (30%, 43/141). Conclusions: These data indicate that vaccination with personalized neoantigens in combination with systemic anti-PD-L1 can induce neoantigen-focused T cell responses in both adjuvant and metastatic disease settings. However, the single arm nature of the trial, and sample size, preclude definitive assessment of the contribution of the components of the combination therapy. Future studies will investigate the efficacy of this approach on a larger scale with pre- and on-treatment analyses of treatment-driven immunomodulation. Citation Format: Mansi Saxena, Jonathan Anker, Julia Kodysh, Timothy O'Donnell, Marcia Meseck, Olivia Hapanowicz, Scot Niglio, Hardik Shah, Yayoi Kinoshita, Rachel Brody, Alex Rubinsteyn, Robert Sebra, Nina Bhardwaj, Mathew Galsky. Immunogenicity of Atezolizumab plus personalized neoantigen vaccination (PGV-001) in patients with urothelial cancer in adjuvant vs metastatic setting [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB113.