BackgroundPatients with advanced melanoma who progress after treatment with immune checkpoint-inhibitors (ICI) and BRAF-/MEK-inhibitors (ifBRAF^V600mutated) have no remaining effective treatment options. The presence of CD1c (BDCA-1)⁺and CD141 (BDCA-3)⁺myeloid dendritic cells (myDC) in the tumor microenvironment correlates with pre-existing immune recognition and responsiveness to immune checkpoint blockade. The synthetic saponin-based immune adjuvant AS01_Benhances adaptive immunity through the involvement of myDC.MethodsIn this first-in-human phase I clinical trial, patients with metastatic melanoma refractory to ICI and BRAF-/MEK inhibitors (when indicated) were recruited. Patients received an intravenous administration of low-dose nivolumab (10 mg, every 2 weeks) plus an intratumoral (IT) administration of 10 mg ipilimumab and 50 µg (0.5 mL) AS01_B(every 2 weeks). All myDC, isolated from blood, were injected on day 2 into the same metastatic lesion. Tumor biopsies and blood samples were collected at baseline and repeatedly on treatment. Multiplex immunohistochemistry (mIHC) was performed on biopsy sections to characterize and quantify the IT and peritumoral immune cell composition.ResultsStudy treatment was feasible and well tolerated without the occurrence of unexpected adverse events in all eight patients. Four patients (50%) obtained a complete response (CR) in the injected lesions. Of these, two patients obtained an overall CR, and one patient a partial response. All responses are ongoing after more than 1 year of follow-up. One additional patient had a stable disease as best response. The disease control rate was 50%. Median progression-free survival and overall survival were 24.1 and 41.9 weeks, respectively. Baseline tumor biopsies from patients who responded to treatment had features of T-cell exclusion. During treatment, there was an increased T-cell infiltration, with a reduced mean distance between T cells and tumor cells. Peripheral blood immune cell composition did not significantly change during study treatment.ConclusionsCombining an intratumoral injection of CD1c (BDCA-1)⁺and CD141 (BDCA-3)⁺myDC with repeated IT administration of ipilimumab and AS01_Band systemic low-dose nivolumab is safe, feasible with promising early results, worthy of further clinical investigation.Trial registration numberClinicalTrials.gov identifierNCT03707808.