Background: LTC004, a novel IL-2Rβ/γ cytokine agonist, designed to minimize toxicity with improved potency. It has selective affinity for the receptor IL-2Rβγ subunits, preferentially stimulates CD8⁺ effector T cells and natural killer cells which are associated with tumor killing, while minimizing the activation of immunosuppressive regulatory T cells. LTC004 has shown significant in vitro and in vivo anti-tumor activity in multiple cell lines and murine models respectively. Methods: This is a first-in-human, multicenter, open-label, dose-escalation and dose-expansion phase I study of LTC004 in advanced solid tumors. Dose escalation part assessed safety and tolerability of intravenous LTC004 with doses ranging from 3.0 to 360 µg/kg. The dose escalation schedule utilized an accelerated titration and Bayesian Optimal Interval (BOIN) design. Eligible pts with advanced solid tumor were required to have received prior standard therapy. LTC004 is administrated intravenously once every 3 weeks. Results: The study completed dose-escalation part (3.0 to 360 µg/kg) as of cutoff date (Jan 10, 2024). 17 pts of multiple tumor types (including non-small cell lung, cervical, colorectal, sarcoma, melanoma, parotid adenocarcinoma, thymic adenocarcinoma, gastric and ovarian cancers) were enrolled and received ≥1 dose of LTC004. At baseline, 12 pts (70.6%) had received ≥2 prior lines therapy, 11 pts (64.7%) had received prior targeted therapy, 10 pts (58.8%) had received prior immune checkpoint inhibitor (ICI) therapy. LTC004 generally well tolerated and no dose-limiting toxicities (DLTs) up to and including 360 µg/kg was observed. The most common TEAEs overall in ≥30% of pts were fever, white blood cell decreased, anemia, AST/ALT increased, neutrophil count decreased, GGT increased, nausea and hypotension. Most of the reported AEs were G1 and G2, all drug related events reversible and responsive to supportive care therapy. Fever can be resolved with standard anti-pyretic treatment or steroids, transient hypotension can be preventable with prophylactic fluid infusion. Of 17 pts enrolled evaluable for efficacy per RECIST v1.1, the ORR were 5.9% (1/17), DCR were 58.8% (10/17). A confirmed PR was achieved in a pMMR/MSS CRC patient with prior therapies including fluoropyrimidines, irinotecan, oxaliplatin, targeted therapy and TKI. The starting dose was 45µg/kg, titration to 90µg/kg after 4 consecutive doses at 45µg/kg Q3W. As of cutoff date, the patient sustained PR for 4.8 months with follow-up ongoing. Conclusions: LTC004 demonstrated encouraging anti-tumor efficacy including cold tumor, and well tolerated in patients with advanced or metastatic solid tumors. The further dose expansion in selected tumor types is ongoing, and evaluations in combination with other agents are being planned. Clinical trial information: NCT05666635.