Rational design and synthesis of a BODIPY-based probe selective for Tau tangles

Despite the numerous studies to elucidate the pathogenetic mechanisms underlying Alzheimer's disease (AD), to date there is still no effective therapy to treat AD or to significantly block the progression of symptoms.[1] Consequently, in order to develop a therapeutic strategy capable of preventing AD, the scientific research has focused on the study of accurate and specific diagnostic methods.[2] Recent studies have clearly demonstrated a strong correlation between the number of neurofibrillary tangles (NFTs) of tau protein and disease progression,[3] suggesting the NFTs as a biomarker of choice in the clinical diagnosis of AD.[4] Recently, the elucidation of the crystallographic structures of the PHF6 hexapeptide fragment by cryoEM microscopy, which plays a pivotal role in the propensity of tau protein to form self-assembled structures, have been reported.[5] In addiction, the construction of the computational model 6-mer of the PHF6 fragment of the most conserved channel have been evaluated for the realization of Tau-specific fluorophores.[6] In this regard, although several fluorescent probes for the identification of AD biomarkers have been described in literature and/or have been patented, only a limited number of these have been found to be highly selective in vivo for the tau tangles and, to date, tau-selective fluorophores are still not commercially available. Based on these findings, a small-size focused library of BODIPY probes, namely BT1-8, has been rationally designed by extending the conjugation of position 3 of the BODIPY core with a highly conjugated systems ending with an aliphatic amine. The distance between the electron donor portion and the acceptor relies in the range of 13-19Å. The interaction with the model 6-mer has been evaluated in terms of molecular docking and, among all, BT1 compound has been selected as the most promising selective P-tau probe in terms of in silico theoretical affinity, binding conformation and polarity. Accordingly, an efficient, versatile and cost-effective two-step synthetic strategy was developed. (Figure 1). The probe has been tested in vitro onto human iPSC-derived NGN2-induced neuronal cell cultures and has shown excellent photophysical properties and high selectivity for detecting NFTs on cell cultures of cortical neurons derived from human stem cells, confirming in silico studies. Accordingly, given the chemical versatility of the BODIPY scaffold, owning to their relative ease of substitution and generally highly fluorescent nature, our findings offer new directions into the structural optimization of specific compounds for different target proteins [7]. Acknowledges The authors wish to thank the Center for Life Nano and Neuro Science Imaging Facility, Istituto Italiano di Tecnologia. This research was funded by the CrestOptics- IIT JointLab for Advanced Microscopy (to S.D.A., A.B, and F.G.), and the MARBEL Life2020 grant (to S.D.A.and A.B). The work was financially supported by the Excellence Departments grant from MIUR (Art. 1, commi 314−337 Legge 232/2016) to the Department of Chemistry and Technology of Drugs. This work was partially supported by Sapienza University (to S.D.A. RM118163E0297F84; PH12017270934C3C) and Fondazione Istituto Italiano di Tecnologia. This research has been supported from Regione LAZIO, Lazio-Innova, POR FESR Lazio 2014-2020, European Union in the form of Grant A0375-2020-36549 CUP B85F20003340002 (www. europa. eu; www. lazio europa. it).