Rifamycins are a cornerstone of antimycobacterial therapy. However, their clinical use is limited by drug-drug interactions, arising from activation of the nuclear receptor pregnane X receptor (PXR). PXR activation induces expression of drug-metabolizing enzymes, including the cytochrome P450 3A4 isoform (CYP3A4), and accelerates clearance of coadministered medications. The structural understanding of rifamycin-PXR interactions remains limited. We designed a series of C25-modified rifabutin analogs and systematically evaluated their PXR binding, transcriptional activation, and pharmacological profile. Several analogs retained PXR binding affinity yet showed reduced CYP3A4 induction, exhibiting behavior consistent with antagonists or inverse agonists and revealing a disconnect between receptor binding and transcriptional activation. Molecular dynamics simulations indicated C25 modifications may disrupt positioning of the PXR α12 helix through steric interactions. These findings demonstrate conservative modifications of rifamycin can potentially convert PXR agonists into antagonists or inverse agonists and offer a structure-guided framework for developing rifamycins with attenuated CYP3A4 induction.

Spinal and bulbar muscular atrophy (SBMA) is an X-linked neuromuscular disease caused by a CAG-repeat expansion in the androgen receptor (AR) gene, translated into an elongated polyglutamine (polyQ) tract in the protein. Androgens trigger ARpolyQ toxicity, thus most potential therapeutic approaches involve androgen reduction or AR negative modulation, with severe endocrine side effects.
A start codon (I-AUG) controls AR translation, while a second one, (II-AUG) downstream to I-AUG and the CAG repeat, drives translation of the shorter AR isoform (AR-A) lacking the polyQ tract, but preserves all relevant AR functional domains. Here, we studied AR-A behaviour and its effect on ARpolyQ toxicity in SBMA.
We took advantage of cellular, mice and Drosophila melanogaster SBMA models to asses AR-A behaviour and effect on ARpolyQ aggregation and functions.
Comparative expression analyses of AR isoforms revealed that AR-A is predominantly located in the mouse brainstem and spinal cord of the central nervous system, but not in muscle. AR-A retains partial androgenic activity, but does not aggregate. Since ARpolyQ and AR-A can heterodimerize, we tested the effect of AR-A on ARpolyQ behaviour, showing that AR-A has a pro-solubilizing effect on ARpolyQ aggregates and that the heterodimers retain a partial transcriptional activity. Finally, we tested the effect of AR-A in a fly model of SBMA. Flies expressing AR-A alone showed no signs of external eye degeneration, in contrast to those expressing expanded ARpolyQ. Notably, co-expression of AR-A with ARpolyQ significantly reduced ARpolyQ aggregation and eye degeneration, supporting our hypothesis that AR-A enhances ARpolyQ solubility and mitigates its toxicity in vivo.
Altogether, our results demonstrate that an increased expression of AR-A may have a role in protecting against ARpolyQ aggregation and toxicity. These findings suggest that AR-A could represent a promising avenue for developing an alternative therapeutic strategy that warrants further investigation.
Copyright © 2026. Published by Elsevier B.V.

Nuclear receptor antagonists are used to treat various diseases, but the precise antagonist mechanisms differ among receptors and compounds. Understanding the interplay between ligand-receptor interactions and transcriptional outcomes is critical. The nuclear receptor pregnane X receptor (PXR) is activated by many medicinal compounds and upregulates drug metabolism genes in response, decreasing efficacy and/or increasing toxicity of drugs. Co-administered PXR antagonists could reduce these effects, but such compounds have only recently been identified, and molecular elements governing their actions remain largely unknown. Here, we show chemically similar PXR ligands with three distinct activities (agonist, antagonist, and inverse agonist) that are altered by PXR mutations. These diverging activities are linked to ligand-induced changes at the intersection of ligand, receptor ligand-binding pocket, and receptor surface where transcriptional coregulators are recruited. We also find that antagonists can act by multiple mechanisms regarding coregulator recruitment, highlighting the complexity of ligand-receptor interactions that influence transcriptional activity.
Copyright © 2025 The Author(s). Published by Elsevier Inc. All rights reserved.

Proteolysis-targeting chimeras (PROTACs) containing a target protein ligand linked to an E3 ubiquitin ligase ligand induce target protein degradation through E3 recruitment. Most PROTACs bind a surface cleft of the protein of interest rather than a buried pocket. Using the nuclear receptor PXR, we previously described the inherent difficulties of PROTAC targeting via a deep solvent-inaccessible ligand binding pocket. Here, we discover that the CRBN-dependent MDM2 PROTAC MD-224 is a potent PXR degrader that achieves its activity from binding adjacent to the ligand-binding pocket. Furthermore, because the proximal region is a structural feature common among nuclear receptors, MD-224 also targets additional receptors for proteasomal degradation. Using structure- and activity-guided medicinal chemistry, we ablated MDM2 degradation and generated MD-224 analogs with activities skewed toward different receptors. Thus, we describe (1) PROTAC repurposing as a potential route of degrader discovery and (2) nuclear receptor-targeted degradation through a noncanonical binding site.
© 2025. The Author(s).

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional molecules containing a ligand for a protein of interest linked to an E3 ubiquitin ligase ligand that induce protein degradation through E3 recruitment to the target protein. Small changes in PROTAC linkers can have drastic consequences, including loss of degradation activity, but the structural mechanisms governing such changes are unclear. To study this phenomenon, we screened PROTACs of diverse targeting modalities and identified dTAG-13 as an activator of the xenobiotic-sensing pregnane X receptor (PXR), which promiscuously binds various ligands. Characterization of dTAG-13 analogs and precursors revealed interplay between the PXR-binding moiety, linker, and E3 ligand that altered PXR activity without inducing degradation. A crystal structure of PXR ligand binding domain bound to a precursor ligand showed ligand-induced binding pocket distortions and a linker-punctured tunnel to the protein exterior at a region incompatible with E3 complex formation, highlighting the effects of linker environment on PROTAC activity.
Copyright © 2024 Elsevier Inc. All rights reserved.