Decoding the transcriptional regulation of therapeutic responses
The body’s defense system is finely tuned to detect, destroy, or export endogenous or foreign toxic substances. While this system has obvious protective benefits, it can also contribute to treatment failure by decreasing drug efficacy and increasing drug resistance and toxicity. Our laboratory uses promiscuous xenobiotic nuclear receptors as a model to investigate the transcriptional regulation of response to therapeutics. Using small molecules to modulate these receptors, we are able to control their role in drug metabolism to help improve clinical intervention.
By using both hypothesis-driven and technology-enabled approaches, our laboratory develops and uses small molecules targeting pregnane X receptor (PXR) and constitutive androstane receptor (CAR), xenobiotic receptors that detect toxic compounds and essentially act as switches of the garbage disposal in cells. PXR and CAR promiscuously bind to many toxic compounds and transcriptionally elevate the levels of enzymes [such as cytochrome P450 (CYP) CYP3A4 and CYP3A5] and transporters that destroy and export these toxic compounds. The ligand promiscuity and structural flexibility of PXR and CAR have historically hampered the development of chemical modulators, the lack of which has been critical barrier to investigating their transcriptional regulation. Our research aims to employ small molecules to inhibit and control PXR, CAR and their downstream targets CYP3A4 and CYP3A5 and, in turn, increase drug efficacy, decrease drug resistance and prevent toxicity.
To tackle the challenging problem of controlling the transcription factor PXR, our laboratory integrates model systems and multidisciplinary approaches including cell biology, medicinal chemistry, structural biology and pharmacology as well as high-throughput technologies. These approaches led us to develop the first specific PXR antagonist/inverse agonist (SPA70), and its analogs and derivatives with varying cellular activities – activating, inhibiting, or blocking. These mechanistically characterized novel chemical modulators enable us to understand how to precisely control the transcriptional activity of PXR in regulating drug metabolism. SPA70 has been used to attenuate PXR-mediated liver injury in an animal model, supporting the feasibility of pharmacologic inhibition of PXR as a remedy for drug toxicity.
In addition, we have developed selective CAR inverse agonists (e.g., CINPA1), and discovered the first selective inhibitor of CYP3A5 (i.e., clobetasol propionate). CYP3A4 and CYP3A5 are highly homologous enzymes, but they also preferentially metabolize different drugs. Together they metabolize more than half of marketed drugs. The mechanism for selective inhibition of CYP3A5 is needed to distinguish its role from that of CYP3A4 and guide the development of potential therapeutics. Using clobetasol propionate as a tool compound, we discovered that the unique conformation of a surface loop of CYP3A5 enables the selective binding of the inhibitor. These efforts support the feasibility of selectively inhibiting the highly homologous CYP3A family members, and provide additional targets downstream of PXR/CAR to control therapeutic responses.
Taosheng Chen, PhD, PMP
Chemical Biology & Therapeutics
MS 1000, Room E9054
St. Jude Children’s Research Hospital