Synthetic and translational studies of antitumor and antimicrobial natural products

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PROJECT SUMMARY/ABSTRACT: Our research program seeks to elucidate the mechanism of action and structure–function relationships of bio- active natural products, toward treatments for drug-resistant bacterial infections and cancer. The development of syntheses that enable precise manipulation of each class of molecules, with correlated insights into mecha- nism, are the unifying goals of each project. A major effort involves discovering new pleuromutilin antibiotics to treat drug-resistant Gram-negative infections. Pleuromutilins are bivalent molecules with a glycolic ester resi- due and a tricyclic skeleton that bind the P and A sites of the bacterial ribosome, respectively. While the gly- colic acid ester has been extensively optimized, limitations in chemistry have historically prevented exploitation of the tricyclic core. We developed semisynthetic and fully synthetic approaches to core-modified pleuromu- tilins; we have thereby synthesized derivatives with potencies exceeding the natural product. We will further develop this chemistry toward agents to treat carbapenem-resistant Enterobacteriaceae, pathogens for which few existing antibiotics are effective. Resistance to pleuromutilins is slow to develop; a long-term goal involves understanding the molecular basis of this durability. A second area of focus is the pimarane diterpenes known as the myrocins. Myrocins exhibit antiproliferative effects in murine models of adenocarcinoma. The structure of the active form of myrocins, their biological target, and their mechanism of action, are unknown. We have developed a powerful annulation strategy that allows us to prepare myrocins in nine steps from commercial reagents. We will use this chemistry to elucidate their target and mechanism of action, and thereby access optimized derivatives for preclinical evaluation. In a third project we seek to complete a total chemical synthe- sis of lomaiviticin A, a glycosylated bacterial metabolite that inhibits cancer cell growth at pM concentrations. We established that the cytotoxicity of lomaiviticin derives from induction of double-strand breaks in DNA; elu- cidated its mode of DNA binding; and determined the mechanism of cleavage. We have recently neared com- pletion of a total chemical synthesis of lomaiviticin; using this synthesis as a springboard, we will prepare car- bohydrate-modified derivatives with increased DNA affinity, sequence selectivity, and increased stability. Two new reactions discovered en route to lomaiviticin – an interrupted Barton vinyl iodide synthesis and a method for the stereocontrolled construction of attached rings – will be further developed. Finally, we will advance our studies of gukulenin A, a pseudodimeric α-tropolone natural product with nM activity against colorectal cancer. We will complete the synthesis of gukulenin, elucidate its structure-function relationships, and identify its bio- logical target. Two new methods for the synthesis of highly-substituted α-tropolones will be developed during the funding period. We have established an extensive network of collaborators to advance the translational aspects of each project. These studies also advance basic chemistry through strategy and reaction develop- ment, and will deliver new preclinical candidates to treat cancer and drug-resistant infections.