Overexpression of the serine/threonine polo-like kinase 1 (Plk1) is tightly associated with oncogenesis in several human cancers. Interference with Plk1 function induces apoptosis in tumor cells but not in normal cells. Accordingly, Plk1 is a potentially attractive anticancer chemotherapeutic target. Plk1 possesses a unique phosphopeptide-binding polo box domain (PBD), which functions by recognizing and binding to to phosphothreonine (pT)/phosphoserine (pS)-containing protein sequences. This recognition and binding is essential for intracellular localization and mitotic functions of Plk1. Unlike kinase domains, PBDs are only found among the Plks. Therefore, PBDs represent attractive targets for selectively down-regulating Plk function. Accordingly, we are engaged in efforts to develop Plk1 PBD-binding inhibitors. Starting from the 5-mer phosphopeptide PLHSpT (which specifically interacts with the Plk1 PBD, while failing to significantly interact with the PBDs of two closely-related kinases, Plk2 and Plk3), we recently identified three families of peptidic inhibitors that showed from 1000- to more than 10,000-fold improved PBD-binding affinity. In collaboration with Dr. Michael Yaffe (MIT), X-ray co-crystal structures of these peptides bound to Plk1 PBD indicated unanticipated modes of binding that take advantage of a cryptic binding channel that is not present in the non-liganded PBD or engaged by the parent pentamer phosphopeptide. Although critical elements in the high affinity recognition of peptides and proteins by PBD are derived from pT/pS-residues, the use of these residues in therapeutics is potentially limited by poor cellular uptake, in part due to high anionic charge of the phosphoryl moiety. We have recently discovered new synthetic transformations that reduce the overall peptide anionic charge by intramolecular charge masking, which provides peptides with enhanced efficacy in cellular assays. We have further modified these peptides by introducing bio-reversible prodrug protection of one phosphoryl acidic hydroxyl. This yielded neutral peptides that show even greater cellular efficacies. We have also explored the application of conformational constraint (in which binding entropy penalties are reduced by reducing ligand flexibility). We synthesized several families of macrocycles using methodologies that have not previously been reported for peptide macrocyclization. Efforts in this project are currently focused on attaining more drug-like compounds that are suitable for examination in rodent models of cancer. In further work we are developing proteins that merge properties of antibodies with biologically active small molecules. This work is being done in collaboration with Dr. Christoph Rader (Scripps Florida). Our approach employs monoclonal antibodies and antibody Fc fragments harboring a single C-terminal selenocysteine residue (Fc-Sec) as well as catalytic antibodies, which can be selectively covalently modified by azetidinone and beta-diketone-containing drug payloads. The resulting antibody drug conjugates (ADCs) are directed against a variety of targets by changing the peptide or small molecule to which they are conjugated. In one aspect of our work, we have employed a variety of chemistries to attach biologically-cleavable linkers that allow release of cargo once delivery to the target has been achieved. We have developed versatile hetero-bifunctional linkers incorporating biologically cleavable bonds that are compatible with multiple types of Cu-free Huisgen 1,3-dipolar cycloaddition reagents. These linkers contain both targeting functionality and drug payloads. In one aspect of our work involving the potently cytotoxic peptide, monomethyl auristatin F (MMAF), we are examining linkers that can be conjugated to the Fc-Sec protein by nucleophilic alkylation reactions. We are also examining the use of azetidinone and beta-diketone-containing drug payloads for loading onto catalytic antibodies.
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