Abstract Background: Combination therapy of anti-PD-1 agents with other class of chemotherapeutic agents to improve treatment outcome is being attempted. Anthracycline-derived chemotherapeutic agents are known to activate immunogenic cell-death mechanism. Previously, we developed caspase-3 specific prodrug, MPD-1 containing doxorubicin. It is linked to peptide moiety cleavable by caspase-3 produced during apoptosis induced by radiation therapy (RT). We hypothesized that anti-PD-1 alone may activate MPD-1 by producing caspase-3 and reinforce antitumor efficacy of MPD-1 and anti-PD-1 effect. This study was designed to evaluate the antitumor effects of combined treatment of anti-PD-1 and MPD-1 in murine squamous cell carcinoma models. Methods: Murine SCC VII (squamous cell carcinoma) cells were inoculated to flank of mice, female C3H/J. We divided 6 treatment groups; control, RT, anti-PD-1, anti-PD-1 with MPD-1, RT with MPD-1, and triple therapy (anti-PD-1 + RT + MPD-1) (n = 5/each group). After tumor grew up to 100-200 mm3, treatment was initiated. Tumor size and mice weight were measured. Expression of caspase-3 was evaluated in cell lines and tumor tissues after each treatment. Distribution of CD8+, CD4+, and FoxP3+ cells was analyzed. Results: RT with MPD-1, anti-PD-1 with MPD-1, and triple treatment enhanced the synergistic antitumor activity compared to single treatment in SCC VII bearing mice. Anti-PD-1 with MPD-1 presented slightly better antitumor effect than RT with MPD-1. Caspase-3 was produced in tumor tissue of anti-PD-1 treatment group, which enhanced antitumor effect of activated MPD-1, while cell after anti-PD-1 did not enhance caspase-3 expression. Triple treatment suppressed tumor growth mostly, which was sustained more than 2 weeks even after cessation of treatment. Proportion of CD45+CD8+ T-cells in triple treatment group was increased mostly among tumor-infiltrating lymphocytes. Body weight of each treatment group was not significantly different. Conclusion: Systemic anti-PD-1 initiates apoptosis and produces caspase-3. This process activates MPD-1 and induces sequential mediated by tumor-infiltrating lymphocytes. Combined anti-PD-1 and MPD-1 treatment will be effective for the distant metastatic lesions. Citation Format: Yoon Se Lee, Minsu Kwon, In Kyung Son, Youngro Byun, Sang Yoon Kim. Robust antitumor effect of doxorubicin prodrug combined with anti-PD-1 in murine squamous cell cancer model [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B19.