John Joseph Bisaha

Mode of action of famoxadone

Famoxadone is a preventative and curative fungicide recently developed for plant disease control. The molecule and its oxazolidinone analogs (OADs) are potent inhibitors of mitochondrial electron transport, specifically inhibiting the function of the enzyme ubiquinol:cytochrome c oxidoreductase (cytochrome bc 1 ). Visible absorbance spectral studies on the purified enzyme suggested that famoxadone bound close to the low potential heme of cytochrome b. This binding mode was confirmed in competitive binding experiments by studying the displacement of a radiolabelled OAD from submitochondria. EPR studies on the binding of famoxadone to submitochondria and purified bc 1 suggested its binding mode was more like that of myxothiazol than that of stigmatellin (ligands known to bind near the low potential heme). Zoospores of Phytophthora infestans, when given low concentrations of famoxadone and other OADs, were observed to cease oxygen consumption and motility within seconds and later the cells disintegrated, releasing the cellular contents. Famoxadone was a potent inhibitor of the growth of Saccharomyces cerevisiae when grown on non-fermentable carbon sources and it was an approximately 50-fold less potent inhibitor of growth when the yeast was grown on a fermentable carbon source, glucose. Such physiological observations are consistent with the loss of mitochondrial function imposed by famoxadone and OADs. Single amino acid changes in the apocytochrome b of bakers yeast cytochrome b located near the low potential heme altered the inhibition constants for the inhibitors famoxadone, myxothiazol, azoxystrobin and kresoxim-methyl differentially, thus strongly suggesting different binding interactions of the protein with the inhibitors.

Discovery of oxathiapiprolin, a new oomycete fungicide that targets an oxysterol binding protein.

Oxathiapiprolin is the first member of a new class of piperidinyl thiazole isoxazoline fungicides with exceptional activity against plant diseases caused by oomycete pathogens. It acts via inhibition of a novel fungal target-an oxysterol binding protein-resulting in excellent preventative, curative and residual efficacy against key diseases of grapes, potatoes and vegetables. Oxathiapiprolin is being developed globally as DuPont™ Zorvec™ disease control with first registration and sales anticipated in 2015. The discovery, synthesis, optimization and biological efficacy are presented.

Design of scytalone dehydratase inhibitors as rice blast fungicides: (N- phenoxypropyl)-carboxamides

Insights gained from a crystal structure of scytalone dehydratase led to the design of carboxamide inhibitors with a phenoxypropyl group substituted on the nitrogen atom Potent enzyme inhibitors were synthesized around this motif, the best of which provided excellent control of rice blast disease in greenhouse assays and outdoor field trials.

Oxazolidinones: a new chemical class of fungicides and inhibitors of mitochondrial cytochrome bc1 function

Famoxadone is a preventative and curative fungicide recently commercialized for plant-disease control. The molecule and its oxazolidinone analogs are potent inhibitors of mitochondrial ubiquinol : cytochrome c oxidoreductase (cytochrome bc1) and they bind in the Q0 site of the enzyme near the low potential heme of cytochrome b. Inhibitor binding constants for five mutant cytochrome bc1 enzymes from Saccharomyces cerevisiae having single amino acid changes in their apocytochrome b located near the low potential heme were compared with their two parental wild-type enzymes. The five individual amino acid changes altered the inhibition constants for the inhibitors famoxadone, myxothiazol, azoxystrobin, and kresoxim-methyl in dissimilar fashion. The log scale differences in binding constants relative to those of their parentals provide fingerprints for the effects of the amino acid changes on binding of the individual inhibitors, thus reflecting the structural diversity of the inhibitors. ©1999 Society of Chemical Industry

Tubulin modulating antifungal and antiproliferative pyrazinone derivatives

A novel class of synthetic tubulin polymerization disruptors, based on a substituted pyrazin-2-one core, has been discovered. These molecules have proven to be potent broad spectrum fungicides, with activity on agriculturally important ascomycete and basidiomycete pathogens. They have also been found to be particularly potent against human rhabdomyosarcoma cells. Using an efficient synthetic route, the agricultural and medicinal activity was explored.

Potential New Sites for Fungicides

One method of combating pesticide resistant fungi is to develop areas of chemistry active at a novel site in the pathogen. This can be accomplished in two ways: 1) large-scale screening of a wide variety of different structural classes of chemistry, identification of a class of chemistry active on a specific pathogen and optimization of that activity by an analoging program; or 2) selection of a specific process believed to be essential to the pathogen, and through biochemical knowledge at the molecular, enzyme, cellular and plant level develop active areas of chemistry to inhibit that process. In reality, the discovery process may incorporate both mechanisms to varying degrees. Due to the somewhat serendipitous aspect of the first method of discovery, this presentation will focus on the latter. Specific topics will deal with target site selection and verification, establishment of an appropriate assay, synthesis concerns and correlation of assay/greenhouse results. In addition to the general process, specific examples will be presented from our groups work on the sterol biosynthesis pathway.