Fortuitousin vitrocompound degradation produces a tractable hit againstMycobacterium tuberculosisdethiobiotin synthetase: a cautionary tale of what goes in, does not always come out

Abstract

AbstractWe previously reported potent ligands and inhibitors ofMycobacterium tuberculosisdethiobiotin synthetase (MtDTBS), a promising target for antituberculosis drug development (Schumannet al.,ACS Chem Biol. 2021, 16, 2339-2347); here the unconventional origin of the fragment compound they were derived from is described for the first time. Compound1(9b-hydroxy-6b,7,8,9,9a,9b-hexahydrocyclopenta[3,4]cyclobuta[1,2-c]chromen-6(6aH)-one), identified byin silicofragment screen, was subsequently shown by surface plasmon resonance to have dose-responsive binding (KD0.6 mM). Clear electron density was revealed in the DAPA substrate binding pocket, when1was soaked intoMtDTBS crystals, but the density was inconsistent with the structure of1. Here we show the lactone of1hydrolyses to carboxylic acid2under basic conditions, including those of the crystallography soak, with subsequent ring-opening of the component cyclobutane ring to form cyclopentylacetic acid3. Crystals soaked directly with authentic3produced electron density that matched that of crystals soaked with presumed1, confirming the identity of the bound ligand. The synthetic utility of fortuitously formed3enabled subsequent compound development into nanomolar inhibitors. Our findings represent an example of chemical modification within drug discovery assays and demonstrate the value of high-resolution structural data in the fragment hit validation process.SynopsisA molecule flagged in anin silicodocking screen againstMtDTBS, was inadvertently hydrolysed in the crystal conditions used for hit validation. The resulting fragment-sized molecule bound to the DAPA substrate binding pocket of the target enzyme (MtDTBS) with millimolar affinity, as measured by surface plasmon resonance, but was later modified to a highly potent (nanomolar) ligand and promising lead for the development of novel tuberculosis treatments.Graphical Abstract