Background: Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) is a negative regulator of T-cell responses, also known as an immune checkpoint. The clinical relevance of CTLA-4 blockade was demonstrated by the approval of ipilimumab for the treatment of melanoma, both in the adjuvant as well as metastatic settings. ADU-1604 is a humanized hIgG1 CTLA-4 antagonist antibody. ADU-1604 binds a unique epitope on CTLA-4 demonstrating, in contrast to ipilimumab full blockade of both CD80 and CD86 interactions. In vitro and in vivo ADU-1604 demonstrates at least as potent efficacy as compared to ipilimumab, was well tolerated in non-human primates and demonstrated enhanced immunogenicity against hepatitis B vaccine in non-human primates. Methods: This is a phase 1, first-in-human (FIH), two-part, open-label clinical trial of intravenous (IV) administration of ADU-1604 given as monotherapy in subjects with advanced-stage, relapsed/refractory melanoma who relapsed or were refractory to a prior anti-PD-1/PD-L1 therapy. Main endpoints are safety of ADU-1604 monotherapy, pharmacokinetics, pharmacodynamics (upregulation of ICOS and Ki-67 on circulation CD4+ T cells and ALC, CD4+ and CD8+ T cells) as well as preliminary clinical efficacy. 20 subjects received escalating doses of ADU-1604 IV (25, 75, 225, 450 mg flat dose) Q3W. In the dose expansion part up to 20 additional patients will be treated with 225 mg dose to confirm recommended Phase 2 dose (RP2D). Safety and preliminary efficacy of ADU-1604 monotherapy in PD1 relapsed/refractory melanoma patients will be evaluated. The study was initiated in June 2022. Results: ADU-1604 was demonstrated to have a typical pharmacokinetic behavior for a human IgG1 antibody and similar exposure was observed as described for ipilimumab. Administration of ADU-1604 in cycle 1 and 2 showed a dose-dependent modulation of ICOS and Ki-67 on circulating CD4+ T cells. Similarly, in cycles 3 and 4 a dose-dependent increase of ALC and CD4+ and CD8+ T cells was detected. Notably, no DLTs across the 25, 75, 225 and 450 mg dose level have occurred. Of the patients treated at 225 and 450 mg dose levels (N=6 at 225 mg and N=6 at 450 mg), two Grade 2 severe adverse events (both immune-related enterocolitis) and two Grade 3 AESIs (ALT elevation and immune-related gastritis) suggesting that ADU-1604 has a relatively mild safety profile. Two patients at 225 mg and one patient at 450 mg demonstrated clinical efficacy. Conclusions: ADU-1604 Phase 1 dose-escalation data indicates clinical activity in line with what has been observed with other CTLA-4 blocking antibodies. Importantly, no DLTs and very limited safety signals were reported during the study. Dose expansion at 225 mg dose is ongoing in subjects with advanced-stage, relapsed/refractory melanoma who relapsed or were refractory to a prior anti-PD-1/PD-L1 therapy. Clinical trial information: 2021-002623-38.