PROTAC, innovative strategy to trigger the xenoreceptor nuclear receptor PXR (NR1I2) to proteolysis – PROTACsPXR

The goal of this project is to develop an innovative strategy to target the xenoreceptor PXR to degradation for multiple medical benefits. The nuclear receptor PXR plays a central role in the metabolism of endo- and xenobiotics, i.e. toxins and drugs. When activated, PXR coordinates expression of drug metabolizing enzymes and transporters. As a consequence, PXR is responsible for clinically important drug-drug interactions, drug resistance and also for metabolic disorders. In addition, there is a growing consensus that PXR activation is an important mechanism of chemotherapy resistance and it even accelerates tumor aggressiveness. Thus, PXR inactivation opens an avenue to counteract these multiple adverse effects. However, while nuclear receptors usually respond to a specific set of high affinity ligands, PXR is unusual in being activated by a broad spectrum of low-affinity compounds. Crystallographic studies revealed a ligand binding domain (LBD) with a large and malleable binding pocket that makes it difficult to identify PXR antagonists using in silico or conventional medicinal chemistry approaches. Accordingly, despite intensive research pursued in academic and pharmaceutical laboratories, a few and unspecific PXR antagonists (L-sulforaphane or ketoconazole) have been identified and characterized. However, these compounds fall short of clinical utility as they are non-selective and/or highly-toxic. We therefore need selective and less-toxic PXR inhibitors to overcome PXR-mediated adverse drug interactions and chemoresistance..
The originality of this research program is in its multidisciplinary approach at the crossroad between structure-function, peptido-mimetics chemistry, biochemistry and cell biology. We will design innovative specific PROteolysis-TArgeting Chimeras (PROTACs) that trigger PXR for proteasomal degradation. The PROTAC strategy can turn essentially any protein ligand into a specific target degrader. These molecules are bifunctional compounds of which one end binds to the protein of interest while the other end hijacks cellular quality control mechanisms to induce the degradation of the target protein. Unlike conventional chemical inhibitors, PROTACs do not simply inhibit protein functions, they actually destroy targeted proteins. PROTACs follow an event-driven rather than an occupancy-driven pharmacological paradigm and act like enzymes to degrade super-stoichiometric amounts of the target protein. The recent development of small non-peptide E3 substrates has demonstrated that PROTACs are not just useful chemical tools to degrade a given protein but are also viable therapeutic candidates. This new generation of PROTACs are thus seen, and proposed, as a unique strategy to degrade vast swathes of the currently “undruggable proteome.
We will develop small molecule PROTACs that will simultaneously bind PXR and recruit E3 ligases, based on i) our expertise in PXR biology, synthetic and peptidomimetics chemistries, ii) our previous works on the identification of highly potent (sub-nanomolar) PXR ligands and iii) our knowledge of the crystal structures of the human PXR LBD. We will further characterize and solve the crystal structures of the interactions between the PXR LBD and our PROTACs to rationally increase their affinity and specificity for PXR. We will validate the ability of these molecules to specifically knockdown PXR protein expression in primary hepatocyte cultures. Finally, we will assess their pharmacological properties and in vivo efficacy, to select lead compounds for efficient drug development. As a proof of concept that PROTACs could degrade PXR protein, we have already designed and synthetized active PXR PROTACs.