Abstract 2725: Circulating CD9-GFAP-survivin exosomes during active specific immunotherapy, a potential biomarker for glioma

Background: We evaluated circulating exosomes isolated from the serum of malignant glioma patients enrolled in a completed phase I clinical trial of an anti-survivin vaccine (SurVaxM). Exosomes are microvesicular bodies with potential mechanisms for cell-cell communication. Identifying circulating exosomes from cancer patients as potential indicators of disease status and response to therapy is of interest. The inhibitor of apoptosis protein (IAP) survivin (SVN) promotes cancer cell proliferation and resistance to chemotherapy. Survivin is expressed in many cancer types including malignant gliomas and is a potential target for active immunotherapy. In the above mentioned clinical trial, five patients experienced early tumor progression with a mean of 2.8 months (1.9-5.4 months) from study entry, while three patients had either late (20.5-22.5 months) or no tumor progression (no evident disease at 48 months in one patient). Methods: To determine whether changes in CD9+/GFAP+/SVN+ exosomes correlated with disease progression, patient serum was evaluated prior to treatment (study entry), 8-10 weeks after initial vaccination, and at the time of MRI-defined tumor progression. By employing ImageStream flow cytometry technology we were able to analyze multiple markers on individual exosomes, identifying highly specific populations of survivin+ exosomes which co-expressed the brain/tumor marker protein GFAP. The combination of these markers made it possible to differentiate patient exosomes from those of healthy individuals. Results: The levels of CD9+/GFAP+, CD9+/SVN+, and CD9+/GFAP+/SVN+ (SVN+) exosomes were significantly increased in patients. Serum from three patients who experienced the longest progression-free intervals, showed 98-99% decreases in SVN+ exosomes after treatment with SurVaxM, and maintained similarly low exosome levels over several months. Five early progressing patients experienced a detectable persistence of, or increase in SVN+ exosomes at 9 weeks following initial vaccination. One patient had rising SVN+ exosomes detectable 16 weeks prior to the discovery of tumor progression by brain MRI scan. One patient with no tumor progression sustained a 99% reduction in serum SVN+ exosomes which was sustained over 48+ months. Conclusion: This study demonstrates that increased numbers of CD9+/GFAP+/SVN+ specific exosomes appear to be associated with early tumor progression in recurrent malignant glioma patients. Larger studies are needed to determine whether the correlation between SVN+ exosome levels and disease progression is specific to SurVaxM treatment, and whether rises in SVN+ exosomes levels are predictive of generalized glioma progression. Our study suggests that levels of circulating SVN+ exosomes may be altered during anti-survivin immunotherapy and could serve as a potential biomarker for clinical trials and tumor monitoring. Citation Format: Michael J. Ciesielski, Phillip Galbo, Sheila Figel, Laura Wiltsie, Cheryl Frank, Jingxin Qiu, Robert A. Fenstermaker. Circulating CD9-GFAP-survivin exosomes during active specific immunotherapy, a potential biomarker for glioma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2725. doi:10.1158/1538-7445.AM2017-2725