Obtusaquinone: A Cysteine-Modifying Compound That Targets Keap1 for Degradation

We
have previously identified the natural product obtusaquinone
(OBT) as a potent antineoplastic agent with promising in vivo activity in glioblastoma and breast cancer through the activation
of oxidative stress; however, the molecular properties of this compound
remained elusive. We used a multidisciplinary approach comprising
medicinal chemistry, quantitative mass spectrometry-based proteomics,
functional studies in cancer cells, and pharmacokinetic analysis,
as well as mouse xenograft models to develop and validate novel OBT
analogs and characterize the molecular mechanism of action of OBT.
We show here that OBT binds to cysteine residues with a particular
affinity to cysteine-rich Keap1, a member of the CUL3 ubiquitin ligase
complex. This binding promotes an overall stress response and results
in ubiquitination and proteasomal degradation of Keap1 and downstream
activation of the Nrf2 pathway. Using positron emission tomography
(PET) imaging with the PET-tracer 2-[18F]­fluoro-2-deoxy-d-glucose (FDG), we confirm that OBT is able to penetrate the
brain and functionally target brain tumors. Finally, we show that
an OBT analog with improved pharmacological properties, including
enhanced potency, stability, and solubility, retains the antineoplastic
properties in a xenograft mouse model.