Background: c-MYC is a master transcription factor (TF) critical for multiple cell physiologies. As a pleiotropic TF, MYC regulates the tumor microenvironment (TME) and impacts cancer cell initiation, growth, and survival. Although MYC expression is normally tightly controlled in normal cells, its activity is frequently dysregulated in cancer (e.g., HCC, NSCLC, Burkitt’s lymphoma). A direct MYC targeting anti-cancer agent has remained elusive, largely due the absence of a drug binding pocket and tight autoregulation. The MYC gene and its regulatory elements reside alone within an insulated genomic domain and represents a potential target for gene regulation via an epigenetic approach. Methods: We are developing programmable epigenetic mRNA medicines designed to controllably tune gene expression pre-transcriptionally with high specificity and durability. We have rationally designed MYC-targeted Omega Epigenomic (MYC-OEC: clinical candidate OTX-2002; MYC Lung OEC; mouse surrogate muMYC-OEC) to downregulate MYC expression, thereby selectively killing cancer cells while sparing normal cells. We investigated the role of MYC-OECs in the modulation of the TME and demonstrated in human and mouse tumor models that MYC-OECs enhanced the host immune response. Results: In HCC and NSCLC cell lines (Hep 3B, SKHEP1, H2009 and H460), treatment with MYC-OECs in vitro significantly inhibited PD-L1 surface expression. Using the muMYC-OEC in vivo, we demonstrated tumor growth inhibition (TGI) in Hepa1-6 liver cancer syngeneic tumor models with 70% of the mice exhibiting complete tumor response with good tolerability. Cancer cells used to rechallenge the cured mice failed to grow, demonstrating that muMYC-OEC monotherapy induced immune memory. In addition to inhibiting tumor growth with either muMYC-OEC or checkpoint blockade immunotherapy (CBI; anti-PD-1 or anti-PD-L1) treatment, combination therapy further enhanced TGI. Tumor-infiltrating lymphocyte profiling demonstrated an increase in activated T cells and a reduction in Tregs following combination treatment, resulting in an increased effector T cell to Treg ratio over monotherapy. Depletion of CD4⁺, CD8⁺ and NK cells in vivo prior to single agent or combination treatment demonstrated a requirement for both CD4⁺ and CD8⁺ T cell activity for TGI. NK cells were dispensable. Taken together, these data demonstrate that the improved CBI/MYC-OEC combination activity is mediated through the recruitment of activated T cells and reduction of immune suppressive Tregs. Conclusions: These efficacy and tolerability data support the combination of MYC-OEC and CBI treatment in patients with solid tumors. OTX-2002 is currently being evaluated in a Phase 1/2 clinical trial as a monotherapy and in combination with SoC (i.e., TKI and CBIs) for patients with HCC and other solid tumor types known for an association with MYC.