Background: Delivery barriers due to intratumoral pressure and immunosuppression related to myeloid-derived suppressor cells (MDSC) in liver tumors have created challenges for immunotherapy. TLR9 agonists seem to improve response to ICI. PERIO-01 is a first-in-man trial of PEDD of SD-101 using hepatic arterial infusion (HAI) with ICI in MUM. Methods: PERIO-01 is an open-label phase 1 trial of SD-101 given by HAI in MUM (NCT04935229). The study consists of dose-escalation cohorts of SD-101 alone (Cohort A), with nivolumab (Cohort B), or nivolumab + ipilimumab (Cohort C). SD-101 is delivered over 2 cycles, with 3 weekly doses per cycle. Blood, liver metastasis (LM) and normal liver biopsies are collected for correlative studies. Results: At data cutoff, 33 patients were enrolled, 30 of whom received at least one dose of SD-101, 13 in Cohort A (2, 4, and 8 mg), 15 patients in Cohort B (2 mg and 4 mg) and 2 patients in Cohort C (2 mg). The median age was 63 years of equal gender. Only 3 patients were treatment-naïve and 2 were HLA-A*-02:01+ who received prior tebentafusp. Nine subjects had 4-7 LM and 4 had > 10. The median index LM size was 4.7 cm. Only one patient experienced a grade ≥3 adverse event (AE) related to SD-101, which was increased liver enzymes. All AEs related to cytokine release syndrome have been low-grade with the most common being fever (9), chills (5), and dizziness (4). Treatment resulted in high liver drug levels (median at 2 mg = 1540 ng/g and 8 mg = 2325 ng/g, p = 0.035), with only transient exposure in the periphery ( < 4 hours) and a maximum peak serum level of 554 mg/ml 30 minutes post-infusion. Increases in serum IL-18 and IFNγ were noted, with highest levels at 8 mg. Expansion of natural killer cells were detected in 9 of 10 patients from flow cytometry data peripherally. Monocytic MDSC levels were decreased in 5 of 5 patients on immunofluorescence, and NanoString analysis revealed decreases in ARG-1 and IDO-1 gene levels up to 100 days from initial treatment. Broad immunostimulatory gene expression changes were noted in tumor and normal liver following SD-101, including increases in IFNB1 and IL-9. Using the available samples from 13 patients, 7 of 10 had decrease in ctDNA, with complete clearance in 3, along with circulating tumor cell decreases in 6 of 13. In cohort B at 2 mg, 5 of 6 with available response data have stable disease with a median duration of disease control of 12 weeks (range = 7.5-24) at data cutoff. Conclusions: HAI of SD-101 has been well tolerated and associated with encouraging immunologic activity. Evidence of biologic effects at the lower doses of SD-101 with nivolumab is encouraging and enrollment with escalation continues in Cohorts B and C. Clinical trial information: NCT04935229.