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Fully Synthetic Prodiginines as Probes of Protein-Protein Interactions Regulating

Patrick G Harran

1 Collaborator(s)

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National Cancer Institute (NIH)
Reestablishing apoptosis in cancer cells holds considerable promise for targeted, non-toxic therapy. We demonstrated this previously with the development of 'smac mimetics', small molecules that disrupt protein- protein interactions that suppress proper caspase activation. Smac mimetics from several organizations have since advanced to human clinical trials. These experiments remain ongoing. In this proposal we outline new chemistry aimed at a second set of protein-protein interactions that block apoptosis in cancer and contribute to drug resistance. Bcl-2 family proteins gate mitochondrial outer membrane permeability and regulate cytoplasmic release of factors essential for normal cell death. Over-expression of anti-apoptotic Bcl-2 proteins is common in chemotherapy and radiation resistant cancers. As a result, the Bcl-2 system has been intensely studied as a target for drug development. The most advanced of these efforts have reached clinic stages. However, despite tremendous promise for the mechanism, unanticipated off-target effects and, notably, Mcl-1 based resistance to agents targeting Bcl-XL are limiting progress. New compounds with superior profiles are needed. Ansa-bridged prodiginines are cytotoxic pigments produced by bacteria. One of these natural products, streptorubin B, was identified by Shore as a molecule that potentiates apoptosis in cell culture - reportedly through interactions with Bcl-2 proteins. This observation seeded development of obatoclax, a synthetic prodiginine now in clinical trials as therapy for lymphocytic leukemia. Obatoclax is described as a pan Bcl-2 antagonist that targets BH3 helix-binding surfaces, including the Noxa binding site on Mcl-1. Data is promising but tempered by selectivity. The tri-pyrrolic chromophore in obatoclax generates ROS in the presence of metal ions and can neutralize pH gradients via transmembrane ion transport. Both effects damage normal cells. We propose to study new pyrrolophane variants of streptorubin B that lack the structural liabilities of obatoclax. Based on our recently completed total synthesis f roseophilin, we outline new methods to synthesize marineosins A and B - newly isolated anti-leukemic prodiginines produced by marine bacteria. In collaboration with Gordon Shore at McGill University, we have discovered intermediates in our roseophilin synthesis that block t-Bid dependent Mcl-1 functions in vitro. We propose to build on these results by advancing our synthetic chemistry along several lines. Based on computational models, we believe the ansa- bridged heterocyclic cores of the natural products in question can scaffold a new class of α-helix peptidomimetics. Experiments to test this hypothesis are described. These include custom syntheses of ligands targeting the Mcl-1/Noxa, Bcl-XL /Bim and p53/mdm2 interactions selectively. Lastly, as means to identify non Bcl-2 based effects in cell culture and in vivo, we propose to search for additional targets of this structural class using affinity based biochemical methods and state-of-the-art functional genomic screens.

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