John Wing Wong

Redesigning and characterizing the substrate specificity and activity of Vibrio fluvialis aminotransferase for the synthesis of imagabalin

Several protein engineering approaches were combined to optimize the selectivity and activity of Vibrio fluvialis aminotransferase (Vfat) for the synthesis of (3S,5R)-ethyl 3-amino-5-methyloctanoate; a key intermediate in the synthesis of imagabalin, an advanced candidate for the treatment of generalized anxiety disorder. Starting from wild-type Vfat, which had extremely low activity catalyzing the desired reaction, we engineered an improved enzyme with a 60-fold increase in initial reaction velocity for transamination of (R)-ethyl 5-methyl 3-oxooctanoate to (3S,5R)-ethyl 3-amino-5-methyloctanoate. To achieve this, <450 variants were screened, which allowed accurate assessment of enzyme performance using a low-throughput ultra performance liquid chromatography assay. During the course of this work, crystal structures of Vfat wild type and an improved variant (Vfat variant r414) were solved and they are reported here for the first time. This work also provides insight into the critical residues for substrate specificity for the transamination of (R)-ethyl 5-methyl 3-oxooctanoate and structurally related β-ketoesters.

Chemoenzymatic Asymmetric Synthesis of Pregabalin Precursors via Asymmetric Bioreduction of β-Cyanoacrylate Esters Using Ene-Reductases

The asymmetric bioreduction of a library of β-cyanoacrylate esters using ene-reductases was studied with the aim to provide a biocatalytic route to precursors for GABA analogues, such as pregabalin. The stereochemical outcome could be controlled by substrate-engineering through size-variation of the ester moiety and by employing stereochemically pure (E)- or (Z)-isomers, which allowed to access both enantiomers of each product in up to quantitative conversion in enantiomerically pure form. In addition, stereoselectivities and conversions could be improved by mutant variants of OPR1, and the utility of the system was demonstrated by preparative-scale applications.

Nitrile as Activating Group in the Asymmetric Bioreduction of β-Cyanoacrylic Acids Catalyzed by Ene-Reductases.

Asymmetric bioreduction of an (E)-β-cyano-2,4-dienoic acid derivative by ene-reductases allowed a shortened access to a precursor of pregabalin [(S)-3-(aminomethyl)-5-methylhexanoic acid] possessing the desired configuration in up to 94% conversion and >99% ee. Deuterium labelling studies showed that the nitrile moiety was the preferred activating/anchor group in the active site of the enzyme over the carboxylic acid or the corresponding methyl ester.

Metabolism of a serotonin-4 receptor partial agonist 4-{4-[4-tetrahydrofuran-3-yloxy)-benzo[d]isoxazol-3-yloxymethyl]-piperidin-1-ylmethyl}-tetrahydropyran-4-ol (TBPT): Identification of an unusual pharmacologically active cyclized oxazolidine metabolite in human

4-{4-[4-Tetrahydrofuran-3-yloxy)-benzo[d]isoxazol-3-yloxymethyl]-piperidin-1-ylmethyl}-tetrahydropyran-4-ol (PF-4995274, TBPT) is a new agent that is a partial agonist of the human serotonin-4 (5-HT4) receptor and is under investigation for neurological disorders. Metabolism of TBPT was examined in vitro in human liver microsomes and human hepatocytes. Metabolites were also identified in the plasma of healthy human subjects in a phase 1 clinical study. Human-derived metabolite profiles were compared with corresponding profiles obtained in laboratory animal species. There were two major routes of metabolism in vitro: N-dealkylation of the methyltetrahydropyran moiety (M1) and hydroxylation at the seven position of the benzisoxazole moiety (M4). These were also observed in human plasma; however, in that matrix, the major metabolite was an unusual cyclized oxazolidine entity (M2). M2 was proposed to be formed via generation of an intermediate 4° iminium ion on the piperidine ring followed by spontaneous cyclization by attack of the β-hydroxyl substituent of the tetrahydropyran ring to form a cyclized oxazolidine product. An authentic standard of the metabolite was generated using a methylene-blue-sensitized photochemical oxidation reaction as well as microbial transformation. Further investigation of this metabolite showed that it also possessed 5-HT4 agonism activity similar to the parent. The metabolite was 150-fold more highly protein bound in human plasma than TBPT, which is consistent with its presence as a major circulating metabolite while being only a minor metabolite in in vitro systems. Overall, this illustrates the importance of understanding the complex dispositional properties of a pharmacologically active metabolite.

Microbial hydroxylation of (Z)-2-benzylidene-1-azabicyclo[2.2.2]octan-3-one

Abstract Microbial hydroxylation of ( Z )-2-benzylidene-1-azabicyclo[2.2.2]octan-3-one 1 by various species of fungi and actinomycetes occurred regio- and stereoselectively at the 5 position of the quinuclidinone moiety. Most of the organisms produced α-(5 S )-( Z )-2-benzylidene-5-hydroxy-1-azabicyclo[2.2.2]octan-3-one 2 as the major product with enantiomeric excesses ranging from 30% to 84%. The (5β)-hydroxy epimer 3 and the (5α,7α)-dihydroxy derivative 4 were also produced by whole cell biotransformations of 1 .