850 HM16390, a long-acting IL-2 analog with enhanced IL-2Rβ binding and optimal IL-2Rα binding, selectively expands tumor-specific T cells, resulting in potent anti-tumor immunity in murine tumor models

Background Upon antigen stimulation, CD8 + T cells upregulate CD25 (IL-2Rα), which is considered a key marker of activated tumor antigen-specific T cells (TSTs). Accordingly, engagement of CD25 therefore presents a strategic opportunity to preferentially activate and expand TSTs while minimizing off-target immune activation, improving both efficacy and safety. HM16390 is a long-acting IL-2 analog engineered for enhanced CD122 (IL-2Rβ) binding and optimized CD25 binding, designed to significantly and safely expand cytotoxic effector T cells. To elucidate how CD25 engagement contributes to its anti-tumor efficacy, we further analyzed T cell subsets.Methods C57BL/6 mice bearing MC38 tumors were treated once weekly via subcutaneous injection with either HM16390 or a non-alpha variant. Tumor growth was monitored, and immune profiling of peripheral blood and tumor-infiltrating lymphocytes was evaluated on days 3 and 7 post-treatment. To evaluate the combination efficacy with immune checkpoint inhibitor (CPI), HM16390 was administered once weekly either as monotherapy or in combination with a biweekly anti-PD-1 antibody for 4 cycles in the murine 4T1 TNBC model, representative of an immunologically cold tumor.Results In the MC38 model, 25 mg/kg of HM16390 achieved 101.2% tumor growth inhibition (TGI) at 14 days and induced a 100% complete response (CR), without treatment-related toxicity. In contrast, the non-alpha variant, administered at its maximum tolerated dose (2.0 mg/kg), showed 91.4% TGI and 40% CR. While both variants increased CD8 + T cell infiltration into tumors, only HM16390 robustly expanded TSTs, as indicated by pentamer+CD8+ cells (14.8% vs 0.1%). These TSTs exhibited an activated phenotype, characterized by co-expression of PD-1 and CD25 (9.2%), suggesting their potential to synergize with PD-1 blockade. The anti-tumor efficacy observed in the 4T1 model further supported this combinatorial potential. Treatment with anti-PD-1 alone or its combination with paclitaxel, a standard-of-care for TNBC, resulted in only marginal tumor control (<40% TGI) without achieving CR. HM16390 elicited an improved, yet still insufficient, anti-tumor activity (TGI, 62.9%). Notably, the combination of HM16390 with anti-PD-1 achieved 102.5% TGI and induced CR in 57% of treated animals, demonstrating a robust synergistic effect.Conclusions Optimized CD25 engagement of HM16390 plays a pivotal role in its robust anti-tumor activity by selectively expanding PD-1 +CD25+ tumor-specific CD8+ T cells. Since these TSTs are key mediators of PD-1 blockade response, combining HM16390 with anti-PD-1 is expected to elicit a synergistic anti-tumor effect. These findings support the potential of HM16390 both as a monotherapy or combined with CPIs, particularly for immunotherapy-resistant tumors.