Background: MTL-CEBPA is the first saRNA to enter clinical trials and targets the transcription factor C/EBPα, a master regulator of myeloid cell differentiation. We have previously reported a favourable safety profile of MTL-CEBPA given as a single agent QWx3 every 28 days, in patients with HCC (Sarker D et al, ASCO 2018). After discontinuation of MTL-CEBPA, three out of five patients treated with sorafenib off study have had maintained complete radiological response (CR) of 7-18 months duration; 2 patients demonstrated resolution of lung metastases > 1 year. Here we provide updated data on phase 1 patients treated with sorafenib off study as well as subsequent combination cohorts. Methods: MTL-CEBPA 130mg/m² QWx3 or BIW and sorafenib 400mg bd were administered to patients with HCC using combination or sequential dosing regimens, in cohorts either tyrosine kinase inhibitor naive or resistant. On treatment liver biopsies evaluated changes in M2 macrophages (CD163). Flow cytometry of peripheral blood determined changes in myeloid cell populations. Results: 12 patients have been treated with MTL-CEBPA co-administered with sorafenib and 14 patients treated with MTL-CEBPA followed by sorafenib (23M/3F, median age 65.5years, range 44-83, ECOG PS 0/1: 18/8). The most common treatment-related AEs (all grades/grade 3) in this group include facial flushing (4/0), raised AST (3/1) raised ALT (2/1), fatigue (5/0), raised ALP (2/0), and anaemia (2/2), diarrhoea (3/0), rash (2/0) and anorexia (1/0). 1 TKI naïve patient in the co-administration cohort has maintained CR at 7 months and two patients have SD (ongoing at 3 & 4 months both in sequential cohort). IHC in the patient with CR has demonstrated 95% reduction in M2 macrophages with significant decrease in frequency of immature CD10- neutrophils (-85.7%; p = 0.0078), PMN-MDSCs (-49.3%; p = 0.00145) and M-MDSCs (-18.4%; P = 0.0072). All responding patients have underlying HBV or HCV. Conclusions: MTL-CEBPA is a novel saRNA targeting myeloid cells which may result in a significantly enhanced oncological response in HCC of viral aetiology. Updated safety and efficacy data will be presented. Clinical trial information: NCT02716012