Abstract Neoadjuvant drug administration provides a window of opportunity for developing novel therapies. Specifically, neoadjuvant pembrolizumab (PD1 checkpoint inhibitor) has the potential for improving understanding of immunomodulatory effects on the tumor microenvironment and for treating recurrent GBM. It is known that a functional immune deficit exists within the tumor T-lymphocyte compartment. As pembrolizumab promotes lymphocyte infiltration, it may contribute to antitumor effects in the neoadjuvant setting. Our previous randomized clinical trial (RCT) suggested that neoadjuvant pembrolizumab improved survival for recurrent GBM patients with 1-2 relapses. Here, we sought to extend our original findings and analyze the role of off-label neoadjuvant pembrolizumab in patients with multiple recurrences. We identified a retrospective 22-patient cohort at UCLA. Every patient had a GBM tissue diagnosis, received standard treatment, and had one to five recurrences. Each received off-label neoadjuvant pembrolizumab and surgical resection within 7-21 days. Progression-free (PFS) and overall survival (OS) for our cohort of neoadjuvant-treated patients were calculated as days from surgical resection and compared to historical control data from the original RCT (n = 15 neoadjuvant, 15 adjuvant). Results show that administration of neoadjuvant pembrolizumab in our cohort significantly increased median PFS compared to our historical adjuvant control (114 vs 75 days, p = 0.03). The OS of our neoadjuvant cohort was 294 days as compared to 192 days in the historical control. Both PFS and OS in our neoadjuvant cohort were not statistically different than the historical neoadjuvant cohort (PFSp = 0.08, OSp = 0.13). Since our cohort was not limited to initial recurrences, this data suggests that patients with more advanced recurrences could also benefit from neo-adjuvant therapy. Ongoing work will evaluate the microenvironmental genetic signature induced by pembrolizumab and assess pre-surgical tumor volume so that adequate comparisons can be made with other trials. We believe this promising finding offers novel therapeutic options for those patients with multiple recurrences.