The echinocandins are a class of semisynthetic natural products that target ␤-1,3-glucan synthase (GS). Their proven clinical efficacy combined with minimal safety issues has made the echinocandins an important asset in the management of fungal infection in a variety of patient populations. However, the echinocandins are delivered only parenterally. A screen for antifungal bioactivities combined with mechanism-of-action studies identified a class of piperazinyl-pyridazinones that target GS. The compounds exhibited in vitro activity comparable, and in some cases superior, to that of the echinocandins. The compounds inhibit GS in vitro, and there was a strong correlation between enzyme inhibition and in vitro antifungal activity. In addition, like the echinocandins, the compounds caused a leakage of cytoplasmic contents from yeast and produced a morphological response in molds characteristic of GS inhibitors. Spontaneous mutants of Saccharomyces cerevisiae with reduced susceptibility to the piperazinyl-pyridazinones had substitutions in FKS1. The sites of these substitutions were distinct from those conferring resistance to echinocandins; likewise, echinocandin-resistant isolates remained susceptible to the test compounds. Finally, we present efficacy and pharmacokinetic data on an example of the piperazinyl-pyridazinone compounds that demonstrated efficacy in a murine model of Candida glabrata infection.

Optical difference spectroscopy was used to identify and quantify human adrenal microsomal and mitochondrial cytochrome P450 enzyme interactions with the histamine H₃ receptor antagonists thioperamide, clobenpropit and ciproxifan. Addition of these structurally diverse imidazole H₃ receptor antagonists to cytochrome-P450-containing human adrenal microsomal and mitochondrial preparations resulted in concentration-dependent type II optical difference spectra. Respective spectral dissociation constants (K_S) for the drug interactions with human adrenal microsomal and mitochondrial cytochrome P450 were 1.5 and 1.6 µmol/l for thioperamide, 3.1 and 0.28 µmol/l for clobenpropit and 0.10 and 0.11 µmol/l for ciproxifan. The three compounds demonstrated a similar activity profile in cytochrome-P450-containing bovine adrenal microsomal and mitochondrial preparations. Findings indicate direct coordination of these imidazole-containing H₃ receptor antagonists with the heme moiety of human adrenal cytochrome P450 isozymes.

Structure-activity relationship (SAR) studies by modification of the unsaturated side chain of potent anticancer marine natural product psymberin/irciniastatin A (1) suggest that substitution at C4 and C5 is important for the cytotoxicity of psymberin, but the terminal double bond is not essential for activity. An aryl group is a good replacement for the olefin. The total synthesis of structurally simplified C11-deoxypsymberin (29) was completed, and its activity is consistently more potent than the natural product which provides a unique opportunity for further SAR studies in the psymberin and pederin family. Preliminary mechanism studies suggest the mode of action of psymberin is through cell apoptosis.

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