Background: Recombinant interleukin-2 (rIL-2; aldesleukin) is an approved immunotherapy in melanoma and renal cell carcinoma based on complete durable remissions. The anti-neoplastic properties of IL-2 are mediated by interactions with the beta-gamma chain (IL-2Rβγ) on naïve CD8+ T cells, which lead to their expansion and differentiation into T effector and T memory cells directed against the tumor. However, the widespread use of IL-2 in oncology is limited by interaction with the high affinity IL-2 receptor alpha chain (IL-2R⍺) on regulatory CD4+ T cells (Tregs), which leads to immunosuppression, and on innate lymphoid cells in the vascular endothelium, which leads to eosinophilic recruitment and activation, and the sometimes fatal complication of vascular leak syndrome (VLS). A rIL-2 biased toward IL-2βγ affinity with no IL-2R⍺interaction could fill unmet needs in oncology. Methods: Using a synthetic biology platform, we have engineered THOR-707, a rIL-2 that contains a novel amino acid encoded in the IL-2 gene via a new DNA base pair (X-Y). The novel amino acid serves as a hook for site specific pegylation that extends half-life, blocks IL-2R⍺ engagement and binds to the IL-2Rβγ. Results: In non-human primates, THOR-707 can be dosed to maximize the level of cytotoxic CD8+ T lymphocytes without elevation of VLS-inducing eosinophils. In murine tumor models, THOR-707 induced the expansion of peripheral and intratumoral CD8+ T cells without expansion of suppressive Tregs. Single-agent dose-dependent anti-tumor efficacy was observed in two syngeneic mouse models. In combination with a PD-1 inhibitor, survival of tumor-bearing mice was longer than either agent as monotherapy. Efficacy in tumor models was durable, suggesting activation of CD8+ memory T cell populations. Conclusions: THOR-707 is a reprogrammed, site-directed, singly-pegylated rIL-2 that changes the pharmacologic profile of IL-2, potentially providing a favorable risk-benefit profile. First-in-human studies are expected to begin this year evaluating THOR-707 as monotherapy and in combination with a PD-1 inhibitor. Based on preclinical evidence to-date, THOR-707 may potentially address existing and emerging unmet needs across multiple solid tumors.