Background: BO-112 is a double stranded synthetic RNA formulated with polyethyleneimine (PEI), that acts as an agonist to toll-like receptor 3 and targets the cytosolic helicase melanoma differentiation-associated gene 5 and retinoic acid-inducible gene I. By mimicking the effect of a viral infection, it mobilizes the immune system, including activation of dendritic cells, CD8 T-cell infiltration, induction of interferons (IFNs), induction of apoptosis and enhancement of immunogenic cell death. Clinical data are available from the first-in-human study (NCT02828098) which evaluated single intratumoral (IT) BO-112 (Part 1; N = 16) and the combination of IT BO-112 with pembrolizumab or nivolumab (Part 2; N = 28). Part 2 showed an ORR of 11% and DCR of 46% in patients with multiple tumor types. Of them, 2 out of 10 (20%) patients with melanoma resistant to anti PD-1 achieved a partial response. Safety profile of BO-112, both as single agent and in combination with anti-PD-1, is manageable and currently characterized by Grade 1 fever and other flu-like symptoms. A phase 2 clinical study of IT BO-112 in combination with pembrolizumab in patients with liver metastases from colorectal or gastric/gastro-esophageal junction cancer patients is currently ongoing. Methods: Phase 2, single arm, open label study of IT BO-112 in combination with pembrolizumab in patients with advanced and/or metastatic melanoma that have progressed on anti-PD-1-containing treatment (NCT04570332). BO-112 will be administered once weekly (QW) in 1 to 8 tumor lesions, total dose 1-2 mg (depending on the number of injected lesions), for the first 7 weeks and then once every three weeks (Q3W); pembrolizumab 200 mg will be administered Q3W. Key eligibility criteria include histologically confirmed, unresectable cutaneous or mucosal melanoma with known BRAF status. Patients must have progressed on or after treatment with an anti-PD-1/L1 mAb. At least one lesion RECIST 1.1 measurable and amenable for weekly IT injection is needed. Primary efficacy variable is ORR by RECIST 1.1, assessed by independent central radiologist (by QUIBIM Precision platform). Secondary efficacy variables include clinical activity in terms of DCR, DOR, PFS, OS, iRECIST, safety and PKs. Exploratory objectives include itRECIST and evaluation of tumor microenvironment (by Pangaea laboratory). A 1-sided alpha of 4.19% and power of 81.8% are used. A total of 40 patients will be enrolled. If less than 8 patients out of 40 have ORR, the study will not meet the statistical bar. Study was approved on 14 December in Spain; enrollment is open; two sites are active as of 18 December 2020. Nineteen sites (12 in Spain and 7 in France) are planned to be activated. Citation Format: Iván Márquez-Rodas, Miguel Fernández de Sanmamed Gutiérrez, María González Cao, Ana M. Arance, Alfonso Berrocal, Eduardo Castañon, Sofía España, Pablo Cerezuela-Fuentes, Juan F. Rodríguez-Moreno, Pilar López Criado, Juana Oramas, Luis de la Cruz Merino, Stéphane Dalle, Caroline Dutriaux, Julie Charles, Caroline Robert, Brigitte Dréno, Henri Montaudié, Philippe Saiag, Javier Sánchez-López, María Rojas, Helena Escuin-Ordinas, Vanesa Pons Sanz, Sonia Maciá, Marisol Quintero. Phase 2 clinical study to evaluate the efficacy and safety of intratumoral BO-112 in combination with pembrolizumab in patients with advanced melanoma that have progressive disease on anti-PD-1-based therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr CT233.