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<h3>Background</h3> GEN2 is a replication-incompetent gene transfer vector, containing an mRNA genome encoding for an optimized prodrug activated enzyme [enhanced mutant herpes simplex thymidine kinase (HSV-eTK)] and an immunostimulatory cytokine (GM-CSF). In the presence of the oral prodrug, valganciclovir (VGCV), HSV-eTK triggers tumor cell killing by releasing the patient’s full complement of unique tumor antigens, including neoantigens, to directly stimulate immune activation facilitated and augmented by the locally secreted immunostimulatory cytokine. GEN2 is off-the shelf without the need for tumor biopsies and genomic sequencing in order to generate personalized neoantigens. <h3>Methods</h3> GEN2 in combination with VGCV was evaluated in a dose-escalating Phase 1 clinical trial in 61 adult patients with hepatocellular carcinoma (HCC) or advanced solid tumors metastatic to liver (NCT04313868). Three routes of administration were investigated (data from the hepatic artery infusion and intratumoral cohort will be presented elsewhere). For the intravenous cohort (IV; n=48), GEN2 was given on Days 1/2/3 every 3 weeks with VGCV Days 8–12. <h3>Results</h3> As of 16 April 2024, median age 59 years; 48% male, median KPS 90, 2 median prior therapies (range 0–9). The most common tumor type was HCC (n=19), followed by colon cancer (n=13) and breast cancer (BC; n=12). Drug exposure was a median of 3.5 cycles (range 1–28), with 13 cohorts tested over an 800-fold range (1.5 x10<sup>5</sup> – 1.2 x10<sup>8</sup> TU/kg). Two dose limiting toxicities were observed: drug hypersensitivity (HSR) in Cohort 3 and Grade 3 decreased appetite in Cohort 7. Common drug-related adverse events (AEs) included HSR (15%), abdominal pain, increased total bilirubin (both 10%), elevated AST, and decreased appetite (8%). No dose dependency for AEs was observed. Common Grade 3+ toxicities (all cause): anemia (13%), pneumonia (10%), elevated total bilirubin (10%) and elevated AST (6%). Plasma pharmacokinetics: rapid blood clearance with an elimination half-life of ~24 minutes. Linear relationship between dose:Cmax and dose:AUC<sub>0->inf</sub> was observed across the dosing range. Preliminary signs of clinical activity was observed in patients with HCC and hormone-receptor positive BC. <h3>Conclusions</h3> GEN2 was well tolerated at all dose levels administered. A maximum tolerated dose was not defined. Although HSRs were observed at lower doses, improved manufacturing processes (with removal of animal proteins) led to substantial reduction of this toxicity. Signs of clinical activity (HCC and HR+ BC) were observed. A second phase 1 trial is exploring IV administration on a different schedule (NCT06391918) with expansion cohorts planned at the Recommended Phase 2 Dose. <h3>Trial Registration</h3> NCT04313868. <h3>Ethics Approval</h3> The study protocol and any accompanying material provided to the patient was reviewed and approved in writing by the following institutional review board/independent ethics committee: • National Kidney Transplant Institute Research Ethics Committee • Makati Medical Center Institutional Review Board • The Medical City Institutional Review Board • St. Luke’s Medical Center Institutional Ethics Review Committee • Manila Doctors Hospital Institutional Review Board The IRB/IECs were appropriately constituted and operated in accordance with the Integrated Addendum to ICH E6(R1): Guideline for Good Clinical Practice ICH E6(R2). Patients provided written informed consent before any study-specific tests or procedures were performed.