Abstract 6459: Development of FL115, a novel IL-15 superagonist, as subcutaneous injection for cancer immunotherapy

Abstract Background: FL115 is an engineered IL-15/IL15Rα-Fbody fusion protein, in which Fbody is a single-chain Fc designed to eliminate classical Fc effects including ADCC/CDC/ADCP while retaining FcRn engagement. In Phase I clinical studies in patients with advanced solid tumors, FL115 administered as weekly IV infusion (dose range: 3-90 μg/kg) demonstrated favorable safety profile and preliminary clinical efficacy including 2 confirmed PR. Dose-proportional pharmacokinetics was observed, with on-target pharmacodynamic activities including sustained significant expansion of NK and CD8+ T cells. Strong transient induction of IFN-γ in plasma was observed, with peak concentration at over 3000 pg/ml compared to IL-6 at ∼1 pg/ml at 3 μg/kg dose. Prior IL-15 therapeutic agents have shown significant profile enhancement when administered as subcutaneous injection compared to IV infusion. Here we present the preclinical data supporting further development of FL115 as subcutaneous injection. Methods: In an SD rat study, a single dose of FL115 was injected subcutaneously at 0.1,0.3 or 1.0 mg/kg or via IV at 0.3 mg/kg, and pharmacokinetic parameters were assessed. In a Cynomolgus monkey study, a single dose of FL115 was injected via IV at 0.23,0.43 or 1.02 mg/kg or subcutaneously at 0.43 mg/kg, and pharmacokinetic parameters were assessed. In a subcutaneous irritation study in New Zealand white rabbits, FL115 was injected subcutaneously at 0.1 ml/kg (FL115 concentration: 21.1mg/ml) per injection site at Day 1 and Day 7, and local injection site reaction was assessed. Results: In the Cynomolgus monkey study, FL115 of 0.43 mg/kg via IV injection resulted in serum Cmax of 9350-9360 ng/ml and Tmax of 0.08 hours, while 0.43 mg/kg via subcutaneous injection resulted in serum Cmax of 350-436 ng/ml and Tmax of 4 hours as well as bioavailability of 54.7-65.8%. In the SD rat study, FL115 of 0.3 mg/kg via IV injection resulted in serum Cmax of 4370 ng/ml and Tmax of 0.08 hours, while 0.3 mg/kg via subcutaneous injection resulted in serum Cmax of 223 ng/ml and Tmax of 6 hours as well as bioavailability of 67%. A single dose subcutaneous injection at 0.1,0.3 or 1.0 mg/kg in rat also showed good linearity. In the rabbit study, from Day 1 to 24, skin irritation at gross observation was at Grade 0 (no abnormality). At Day 10, histopathological assessment showed mild inflammatory cell infiltration under the skin and in muscle, which was fully resolved at Day 24. Conclusion: Compared to IV injection, subcutaneous FL115 in rats and monkeys showed a significant delay (up to 4-6 hours) and drastic lowering of Cmax (up to 26.7×) with bioavailability at ∼60% as well as good linearity in PK file. FL115 at approximately 9 mg per injection site in rabbits showed no significant gross skin irritation. These results support continuing development of FL115 as subcutaneous injection for cancer immunotherapy. Citation Format: Qinghua Wu, Quanxiao Li, Dong Wei. Development of FL115, a novel IL-15 superagonist, as subcutaneous injection for cancer immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6459.