9592 Background: MCC and cSCC have limited treatment options when refractory to immune checkpoint inhibitor (ICI) therapy. IFx-Hu2.0 (IFx) is a plasmid DNA encoding for an immunogenic bacterial protein, Emm55, formulated with a transfection agent for direct intra-tumoral (IT) injection. In this Phase 1b study, we evaluated the safety and efficacy of different schedules of IT IFx in patients (pts) with advanced MCC or cSCC. Methods: In the first trial stage (n=9), IT IFx was administered in up to 3 lesions on 3 schedules following a 3+3 exposure escalation schema; weekly x 1, 2, or 3. In the second trial expansion stage (n=11), IT IFx was administered weekly x 3. The primary objective of the study was to establish safety and feasibility of repeated IT administrations of the investigational agent. ≥80% completion of planned study therapy was predefined as a successful feasibility outcome. Given the proposed potential for immune priming effects of IFx, we performed an unplanned exploratory analysis of post-protocol treatment efficacy to evaluate response to ICI rechallenge. Results: We treated 22 pts (MCC, 13; cSCC, 9). All pts had received prior anti-PD(L)1 based treatment with disease progression being the reason for ICI discontinuation in all patients but one. Two pts did not complete planned study therapy due to rapid clinical progression and were replaced but included in the safety analysis. IT IFx was well tolerated at all dose schedules evaluated with 1 high-grade toxicity deemed possibly treatment related (G3 hepatitis in a patient recently treated with ICI). The study therefore met predefined study endpoints for safety and efficacy. After protocol specified IT therapy, 11 MCC pts and 6 cSCC pts were treated with anti-PD(L)1 based therapy as the immediate post-protocol treatment. Five of 9 (56%) evaluable MCC patients and 1 of 7 (14%) cSCC patients experienced an objective response to this ICI rechallenge, with duration of response ongoing in 4 patients (6+, 19+, 21+, 23+ months) and the two other responses lasting 23 and 33 months. The two remaining MCC patients were not evaluable for response from IO rechallenge due to radiation administered to the only measurable disease site(s), but both remain progression free at 11+ and 13+ months with previously progressive disease. Conclusions: IT IFx-Hu2.0 is safe and feasible to administer at weekly dosing repeated up to 3 weeks. An unplanned exploratory analysis revealed frequent (56%) and durable responses in advanced MCC to ICI re-challenge despite prior resistance to this class of drug, suggesting that IFx induced an immune priming effect. Preliminary biomarker analyses to investigate this phenomenon are ongoing and will be presented. Clinical trial information: NCT04160065 .