Abstract 4449: Plinabulin, a microtubule destabilizing agent, improves tumor control by enhancing dendritic cell maturation and CD8 T cell infiltration in combination with immunoradiotherapy

Abstract There are theoretical benefits of combining immune checkpoint blockade with radiotherapy, however, there are few indications that such combinations have translated to significant clinical benefit to patients. There is an unmet need to further enhance the effectiveness of these “immunoradiotherapy” combinations. We hypothesize that enhancing antigen presentation through augmenting dendritic cell (DC) activation could potentially improve the effectiveness of immunoradiotherapy. Towards this end, we have investigated the potential for combining the microtubule destabilizing first-in-class agent Plinabulin as the candidate DC activator, with radiotherapy and anti-PD1 immunotherapy (αPD1). First, we investigated how DCs could be affected by Plinabulin and irradiation in in vitro assays. We irradiated SP37A3 and XS106 DC lines with 10Gy and added Plinabulin to those cells. With the addition of Plinabulin, the phosphorylation of JNK and NFκB was upregulated and DC maturation was promoted with high MHC-II, CD86, CD80 and CD40 surface expressions (p<0.05, Dunnett's test). Combining Plinabulin and radiotherapy further augmented the response, particularly in sequence with plinabulin but not in combination. In in vivo models. TSA syngeneic breast cancer cells were implanted to the flank of BALB/c mice, irradiated with 24Gy/3fr, and treated with αPD1 and Plinabulin. There were no anti-tumoral effects with αPD1 alone, Plinabulin alone, and αPD1+Plinabulin groups. However, triple therapy showed much stronger tumor regression than the double therapy of irradiation and αPD1. Tumors were collected from the mice at 2.5-3 weeks after the irradiation and dissociated tumor cells were analyzed for DC activation and immune repertoire using flow cytometry. The percentage of CD8 T cells and CD86+ DCs in the tumor were significantly increased in the triple therapy group that were not seen for the monotherapy or bimodal therapy groups (p<0.05, Dunnett's test). Our findings may serve as a rationale for establishing clinical trial designs of synergistic combinational approaches to enhance immunoradiotherapy. Citation Format: Shinya Neri. Plinabulin, a microtubule destabilizing agent, improves tumor control by enhancing dendritic cell maturation and CD8 T cell infiltration in combination with immunoradiotherapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4449.