Abstract
Monoamine oxidase-B (MAOB) is a crucial enzyme not only as a therapeutic target for Parkinson's disease but also as a binding target for PET tracers that image reactive astrogliosis. We have developed a PET tracer named [18F]SMBT-1 designed for in vivo MAOB imaging. This was achieved by altering the chemical structure of tau PET tracer THK5351, which had high affinity for MAOB. To better understand the interaction of MAOB with these PET tracers, we determined the atomic structure of MAOB-SMBT-1 and MAOB-THK5351 complexes by X-ray crystallography with soaking method. Initially, we confirmed the high binding affinity of [18F]SMBT-1 against purified MAOB (KD = 4.4, Bmax = 2.8 nmol/mg protein), which was consistent with previously determined values against commercially available microsomal MAOB (KD = 3.7 nM, Bmax = 0.11 nmol/mg protein). In vitro competitive binding and biochemical assays demonstrated that SMBT-1 and THK5351 possess higher binding affinity and enzymatic inhibitory activity against MAOB than clinically used MAOB inhibitors Rasagiline and Safinamide. The crystal structures of MAOB in complexes with SMBT-1 and THK5351 (determined at 2.25 - 2.6Å) showed that they bind in the active site cavity of the protein in front of the flavin adenine dinucleotide cofactor. The findings achieved in this study would provide a potentially practical guide to avoid off-target binding to MAOB or to develop higher potency radiopharmaceuticals.
