Abstract 2861: TriTAC-XR: An extended-release T cell engager platform designed to minimize cytokine release syndrome

CD3-targeted T cell engagers are potent anti-tumor therapies, but their development often requires management of cytokine release syndrome (CRS). One strategy to reduce CRS is subcutaneous dosing, which is hypothesized to mitigate CRS by reducing the maximum drug concentration (Cmax) and preserve efficacy by maintaining the same minimum drug concentration (Cmin) as intravenous dosing. Although promising for mitigating CRS, this approach is limited by its increased immunogenicity risks. A T cell engager designed to be dosed intravenously but engineered to mimic the PK properties of subcutaneous dosing could alleviate CRS without increasing immunogenicity. Here we describe TriTAC-XR, a platform of T cell engager prodrugs designed to become slowly activated in systemic circulation. This extended-release mechanism results in a slow build-up of circulating active drug and minimizes the Cmax/Cmin ratio, similar to subcutaneous dosing. TriTAC-XR prodrugs were engineered by adding a peptide mask and protease-cleavable linker to the N-terminus of a TriTAC, a constitutively active and half-life extended T cell engager. The mask binds to the anti-CD3ε domain and inhibits T cell binding. Upon cleavage by systemic proteases, active T cell engager is released. In vitro, TriTAC-XR has markedly reduced binding to recombinant CD3ε protein and to primary T cells as well as reduced potency in functional T cell-dependent cellular cytotoxicity (TDCC) assays compared to its unmasked active drug. In cynomolgus monkeys, TriTAC-XRs targeting multiple tumor antigens resulted in a gradual build-up of active drug during the first week post dose and significantly lower Cmax/Cmin ratios than comparable constitutively active TriTACs. Modeling based on these PK data predicts that TriTAC-XR dosed intravenously will result in a slower build-up of active drug and smaller Cmax/Cmin ratios than TriTAC dosed intravenously or subcutaneously. Cytokine release and target cell depletion in cynomolgus monkeys were used to compare the therapeutic index of TriTAC-XR to TriTAC. A single dose of FLT3-targeting TriTAC-XR resulted in 100-fold protection in cytokine release and similar FLT3 expressing cell depletion when compared to an equivalent FLT3-targeting TriTAC. Similarly, repeat dosing of a TriTAC-XR targeting B cells resulted in complete B cell depletion with substantially lower cytokines than a comparable TriTAC. TriTAC-XR is expected to improve the safety of T cell engagers by reducing CRS and