7. Project Summary/Abstract There remains an urgent, and largely unmet need for the advancement of highly selective and effective therapeutic agents for the treatment of metastatic and triple negative breast cancer. This proposal advances an innovative and promising strategy focused on the first time synthesis and evaluation of betabody-drug conjugates (BDCs), which feature engineered proteins (referred to as betabodies) that target phosphatidylserine (PS). PS is selectively exposed on the outer leaflet of the plasma membrane of cancer cells and endothelial cells in the tumor microenvironment, but is confined to the inner leaflet of the plasma membrane of normal cells. This provides an exquisite opportunity for selective targeting of potent small-molecule anticancer payloads (KGP18 and KGP156) to the tumor microenvironment. Betabodies are smaller allowing for better tumor penetration compared with previously evaluated PS-selective antibodies, which demonstrated poor distribution beyond the vasculature. BDCs are designed to selectively release KGP18 and KGP156 extracellularly by enzymes (cathepsin B, urokinase-type plasminogen activator, and plasmin) that are upregulated and/or demonstrate enhanced activity by breast cancer cells and associated tumor vasculature. KGP18 and KGP156 are potent inhibitors of tubulin polymerization and demonstrate dual mechanism of action functioning as remarkably active antiproliferative agents (cytotoxicity in low nM to pM range) and as profoundly effective vascular disrupting agents, which impart irreversible damage to tumor-associated vasculature ultimately leading to tumor necrosis. These synthetic benzosuberene-based payloads (KGP18 and KGP156) will be evaluated in comparative studies to monomethyl-auristatin E (MMAE), which is a payload of choice in many clinically relevant antibody-drug conjugates. It should be noted that VDAs are mechanistically distinct from the well-studied angiogenesis inhibiting agents. Two individual delivery strategies will be investigated. KGP18 and KGP156 will be synthetically functionalized with distinct, protease-selective peptide-based linkers rendering the drug-linker prodrug constructs biologically inert until cleaved by specific proteases that are present at high levels in the tumor microenvironment, thus selectively releasing the potent payload (KGP18 or KGP156). In a second strategy, BDCs will be prepared from the best drug-protease selective linker constructs. The hypothesis is that appropriately designed drug-linker constructs and their corresponding BDCs tethered to these highly potent payloads (KGP18 or KGP156) will demonstrate high selectivity for tumor vasculature and tumor cells. Pharmacokinetic studies will evaluate the release of the effector drugs from their corresponding constructs and conjugates. The efficacy and selectivity of these drug-linker constructs and BDCs will be evaluated using cell-based studies and established murine orthotopic breast cancer models (MDA...