Development of BODIPY FL vindoline as a novel and high-affinity pregnane X receptor fluorescent probe

Chemical manipulation of an activation/inhibition switch in the nuclear receptor PXR

Nuclear receptors are ligand-activated transcription factors that can often be useful drug targets. Unfortunately, ligand promiscuity leads to two-thirds of receptors remaining clinically untargeted. PXR is a nuclear receptor that can be activated by diverse compounds to elevate metabolism, negatively impacting drug efficacy and safety. This presents a barrier to drug development because compounds designed to target other proteins must avoid PXR activation while retaining potency for the desired target. This problem could be avoided by using PXR antagonists, but these compounds are rare, and their molecular mechanisms remain unknown. Here, we report structurally related PXR-selective agonists and antagonists and their corresponding co-crystal structures to describe mechanisms of antagonism and selectivity. Structural and computational approaches show that antagonists induce PXR conformational changes incompatible with transcriptional coactivator recruitment. These results guide the design of compounds with predictable agonist/antagonist activities and bolster efforts to generate antagonists to prevent PXR activation interfering with other drugs.

A bromodomain-independent mechanism of gene regulation by the BET inhibitor JQ1: direct activation of nuclear receptor PXR

Bromodomain and extraterminal (BET) proteins are extensively studied in multiple pathologies, including cancer. BET proteins modulate transcription of various genes, including those synonymous with cancer, such as MYC. Thus, BET inhibitors are a major area of drug development efforts. (+)-JQ1 (JQ1) is the prototype inhibitor and is a common tool to probe BET functions. While showing therapeutic promise, JQ1 is not clinically usable, partly due to metabolic instability. Here, we show that JQ1 and the BET-inactive (-)-JQ1 are agonists of pregnane X receptor (PXR), a nuclear receptor that transcriptionally regulates genes encoding drug-metabolizing enzymes such as CYP3A4, which was previously shown to oxidize JQ1. A PXR-JQ1 co-crystal structure identified JQ1's tert-butyl moiety as a PXR anchor and explains binding by (-)-JQ1. Analogs differing at the tert-butyl lost PXR binding, validating our structural findings. Evaluation in liver cell models revealed both PXR-dependent and PXR-independent modulation of CYP3A4 expression by BET inhibitors. We have characterized a non-BET JQ1 target, a mechanism of physiological JQ1 instability, a biological function of (-)-JQ1, and BET-dependent transcriptional regulation of drug metabolism genes.

Structure-guided approach to modulate small molecule binding to a promiscuous ligand-activated protein

Ligand-binding promiscuity in detoxification systems protects the body from toxicological harm but is a roadblock to drug development due to the difficulty in optimizing small molecules to both retain target potency and avoid metabolic events. Immense effort is invested in evaluating metabolism of molecules to develop safer, more effective treatments, but engineering specificity into or out of promiscuous proteins and their ligands is a challenging task. To better understand the promiscuous nature of detoxification networks, we have used X-ray crystallography to characterize a structural feature of pregnane X receptor (PXR), a nuclear receptor that is activated by diverse molecules (with different structures and sizes) to up-regulate transcription of drug metabolism genes. We found that large ligands expand PXR's ligand-binding pocket, and the ligand-induced expansion occurs through a specific unfavorable compound-protein clash that likely contributes to reduced binding affinity. Removing the clash by compound modification resulted in more favorable binding modes with significantly enhanced binding affinity. We then engineered the unfavorable ligand-protein clash into a potent, small PXR ligand, resulting in marked reduction in PXR binding and activation. Structural analysis showed that PXR is remodeled, and the modified ligands reposition in the binding pocket to avoid clashes, but the conformational changes result in less favorable binding modes. Thus, ligand-induced binding pocket expansion increases ligand-binding potential of PXR but is an unfavorable event; therefore, drug candidates can be engineered to expand PXR's ligand-binding pocket and reduce their safety liability due to PXR binding.

Design and Optimization of 1H-1,2,3-Triazole-4-carboxamides as Novel, Potent, and Selective Inverse Agonists and Antagonists of PXR

The pregnane X receptor (PXR) is a key regulator of drug metabolism. Many drugs bind to and activate PXR, causing adverse drug responses. This suggests that PXR inhibitors have therapeutic value, but potent PXR inhibitors have so far been lacking. Herein, we report the structural optimization of a series of 1H-1,2,3-triazole-4-carboxamides compounds that led to the discovery of compound 85 as a selective and the most potent inverse agonist and antagonist of PXR, with low nanomolar IC50 values for binding and cellular activity. Importantly, compound 89, a close analog of 85, is a selective and pure antagonist with low nanomolar IC50 values for binding and cellular activity. This study has provided novel, selective, and most potent PXR inhibitors (a dual inverse agonist/antagonist and a pure antagonist) for use in basic research and future clinical studies and also shed light on how to reduce the binding affinity of a compound to PXR.

Biological evaluation and synthesis of calcitroic acid

We describe the synthesis and broad profiling of calcitroic acid (CTA) as vitamin D receptor (VDR) ligand. The x-ray co-crystal structure of the Danio Rerio VDR ligand binding domain in complex with CTA and peptide MED1 confirmed an agonistic conformation of the receptor. CTA adopted a similar conformation as 1,25(OH)₂D₃ in the binding pocket. A hydrogen bond with His333 and a water molecule were observed in the binding pocket, which was accommodated due to the shorter CTA side chain. In contrast, 1,25(OH)₂D₃ interacted with His423 and His333 due to its longer side chain. In vitro, the EC₅₀ values of CTA and CTA-ME for VDR-mediated transcription were 2.89 µM and 0.66 µM, respectively, confirming both compounds as VDR agonists. CTA was further evaluated for interaction with fourteen nuclear receptors demonstrating selective activation of VDR. VDR mediated gene regulation by CTA in intestinal cells was observed for the VDR target gene CYP24A1. CTA at 10 µM upregulated CYP24A1 with similar efficacy as 1,25(OH)₂D₃ at 20 nM and 100-fold stronger compared to lithocholic acid at 10 µM. CTA reduced the transcription of iNOS and IL-1β in interferon γ and lipopolysaccharide stimulated mouse macrophages resulting in a reduction of nitric oxide production and secretion of IL-1β. These observed anti-inflammatory properties of 20 µM CTA were similar to 20 nM 1,25(OH)₂D₃.

General Stepwise Approach to Optimize a TR-FRET Assay for Characterizing the BRD/PROTAC/CRBN Ternary Complex

Proteolysis-targeting chimeras (PROTACs) degrade target proteins by engaging the ubiquitin-proteasome system. Assays detecting target-PROTAC-E3 ligase ternary complexes are critical for PROTAC development. Both time-resolved fluorescence energy transfer (TR-FRET) assays and amplified luminescent proximity homogeneous assays can characterize ternary complexes and assess PROTAC efficacy; stepwise optimization protocols for these assays are lacking. To identify assay conditions that can be applied to various targets and PROTACs, we used a stepwise approach to optimize a TR-FRET assay of BRD2(BD1)/PROTACs/CRBN ternary complexes. This assay is sensitive and specific and responds to the bivalent PROTACs dBET1, PROTAC BET Degrader-1, and PROTAC BET Degrader-2 but not to non-PROTAC ligands of BRD2(BD1) or CRBN. The activity rank order of dBET1, PROTAC BET Degrader-1, and PROTAC BET Degrader-2 in the TR-FRET assay corresponded with previously reported cell growth inhibition assays, indicating the effectiveness of our assay for predicting PROTAC cellular activity. The TR-FRET ternary complex formation assay for BRD2(BD1)/PROTAC/CRBN can be configured to characterize the binding activities of BRD2(BD1) and CRBN ligands with the same compound activity rank order as that of previously reported binary binding assays for individual targets but with the advantage of simultaneously assessing the ligand activities for both targets. Our assay is modular in nature, as BRD2(BD1) can be replaced with other BRDs and successfully detect ternary complexes without modifying other assay conditions. Therefore, the TR-FRET ternary complex assay for BRDs provides a general assay protocol for establishing assays for other targets and bivalent molecules.

Building a Chemical Toolbox for Human Pregnane X Receptor Research: Discovery of Agonists, Inverse Agonists, and Antagonists Among Analogs Based on the Unique Chemical Scaffold of SPA70

Pregnane X receptor (PXR) plays roles in detoxification and other physiological processes. PXR activation may enhance drug metabolism (leading to adverse drug reactions) or inhibit inflammation. Therefore, PXR agonists, antagonists, and inverse agonists may serve as research tools and drug candidates. However, a specific PXR modulator with an associated structure-activity relationship is lacking. Based on the scaffold of specific human PXR (hPXR) antagonist SPA70 (10), we developed 81 SPA70 analogs and evaluated their receptor-binding and cellular activities. Interestingly, analogs with subtle structural differences displayed divergent cellular activities, including agonistic, dual inverse agonistic and antagonistic, antagonistic, and partial agonistic/partial antagonistic activities (as in compounds 111, 10, 97, and 42, respectively). We generated a pharmacophore model that represents 81 SPA70 analogs, and docking models that correlate strong interactions between the compounds and residues in the AF-2 helix with agonistic activity. These compounds are novel chemical tools for studying hPXR.

Development of BODIPY FL VH032 as a High-Affinity and Selective von Hippel-Lindau E3 Ligase Fluorescent Probe and Its Application in a Time-Resolved Fluorescence Resonance Energy-Transfer Assay

The von Hippel-Lindau (VHL) tumor suppressor associates with transcription factors elongin-C and elongin-B to form the VHL-elongin-C-elongin-B protein complex and carry out its functions, such as degradation of hypoxia-inducible factors. VHL ligands are used not only to modulate hypoxia-signaling pathways and potentially treat chronic anemia or ischemia but also to form bivalent ligands as proteolysis-targeting chimeras to degrade proteins for potential therapeutic applications. Sensitive and selective VHL-based binding assays are critical for identifying and characterizing VHL ligands with high-throughput screening approaches. VHL ligand-binding assays, such as isothermal titration calorimetry, surface plasmon resonance, and fluorescence polarization assays, are reported but with limitations. Isothermal titration calorimetry requires higher protein concentrations with a lower throughput than fluorescence-based assays do. Surface plasmon resonance requires protein immobilization, which introduces variation and is not suitable for testing a large number of ligands. Fluorescence polarization can be sensitive with high-throughput capability but is susceptible to assay interference, and small-molecule-based fluorescent probes are not available. We developed the first small-molecule-based high-affinity VHL fluorescent probe BODIPY FL VH032 (5), with a K _d of 3.01 nM, for a time-resolved fluorescence resonance energy-transfer assay. This new assay is sensitive, selective, resistant to assay interference, and capable of characterizing VHL ligands with a wide range of affinities. It is also suitable for VHL ligand identification and characterization with high-throughput screening.

Design and Application of Receptor-Targeted Fluorescent Probes Based on Small Molecular Fluorescent Dyes

In recent years, a variety of receptor-targeted fluorescent probes have been developed and widely used to realize the visualization of certain receptors, which facilitates the early diagnosis and treatment of diseases. In this Review, we focus on the recent achievements in design, chemical structure, imaging characterization, and potential applications of receptor-targeted fluorescent probes from the past 10 years. The development and application of receptor-targeted fluorescent probes will expand our knowledge of the distribution and function of disease-related receptors, shed light on the drug discovery for clinical diseases where receptors are implicated, and feed into the diagnosis and treatment of a plethora of diseases, including tumors.

Development of BODIPY FL Thalidomide As a High-Affinity Fluorescent Probe for Cereblon in a Time-Resolved Fluorescence Resonance Energy Transfer Assay

Ligands for cereblon, a component of a functional E3 ligase complex that targets proteins for proteolysis, are critical for developing molecular glues and proteolysis-targeting chimeras (PROTACs), which have therapeutic implications for various diseases. However, the lack of sensitivity of previously reported assays limits characterization of cereblon ligands. To address this shortcoming, we developed BODIPY FL thalidomide (10) as a high-affinity fluorescent probe for the human cereblon protein, with a K_d value of 3.6 nM. We then used BODIPY FL thalidomide (10) to develop a cereblon time-resolved fluorescence resonance energy transfer (TR-FRET) binding assay. The IC₅₀ values of the cereblon ligand pomalidomide (8) were 6.4 nM in our cereblon TR-FRET binding assay, 264.8 nM in a previously reported Cy5-conjugated thalidomide (7)-mediated fluorescence polarization (FP) assay, and 1.2 μM in a previously reported Cy5-conjugated cereblon modulator (compound 7) (9)-mediated TR-FRET assay, indicating that our cereblon TR-FRET binding assay is 41- and 187-fold more sensitive than these two previously published assays. With our cereblon TR-FRET binding assay, we detected binding of cereblon ligands but not binding of bromodomain-containing protein 4 or von Hippel-Lindau ligands, thereby demonstrating its selectivity. Our cereblon TR-FRET binding assay was very stable and detected changes in phthalimide activity due to thalidomide isomerization. Therefore, the BODIPY FL thalidomide (10)-mediated cereblon TR-FRET binding assay we designed is highly sensitive, selective, and stable and will aid the development and characterization of novel cereblon ligands.

Mutation of a single amino acid of pregnane X receptor switches an antagonist to agonist by altering AF-2 helix positioning

Pregnane X receptor (PXR) is activated by chemicals to transcriptionally regulate drug disposition and possibly decrease drug efficacy and increase resistance, suggesting therapeutic value for PXR antagonists. We previously reported the antagonist SPA70 and its analog SJB7, which unexpectedly is an agonist. Here, we describe another unexpected observation: mutating a single residue (W299A) within the PXR ligand-binding domain converts SPA70 to an agonist. After characterizing wild-type and W299A PXR activity profiles, we used molecular dynamics simulations to reveal that in wild-type PXR, agonists stabilize the activation function 2 (AF-2) helix in an "inward" position, but SPA70 displaces the AF-2. In W299A, however, SPA70 stabilizes the AF-2 "inward", like agonists. We validated our model by predicting the antagonist SJC2 to be a W299A agonist, which was confirmed experimentally. Our work correlates previously unobserved ligand-induced conformational changes to PXR cellular activity and, for the first time, reveals how PXR antagonists work.

CITCO Directly Binds to and Activates Human Pregnane X Receptor

The xenobiotic receptors pregnane X receptor (PXR) and constitutive androstane receptor (CAR) are activated by structurally diverse chemicals to regulate the expression of target genes, and they have overlapping regulation in terms of ligands and target genes. Receptor-selective agonists are, therefore, critical for studying the overlapping function of PXR and CAR. An early effort identified 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime (CITCO) as a selective human CAR (hCAR) agonist, and this has since been widely used to distinguish the function of hCAR from that of human PXR (hPXR). The selectivity was demonstrated in a green monkey kidney cell line, CV-1, in which CITCO displayed >100-fold selectivity for hCAR over hPXR. However, whether the selectivity observed in CV-1 cells also represented CITCO activity in liver cell models was not hitherto investigated. In this study, we showed that CITCO: 1) binds directly to hPXR; 2) activates hPXR in HepG2 cells, with activation being blocked by an hPXR-specific antagonist, SPA70; 3) does not activate mouse PXR; 4) depends on tryptophan-299 to activate hPXR; 5) recruits steroid receptor coactivator 1 to hPXR; 6) activates hPXR in HepaRG cell lines even when hCAR is knocked out; and 7) activates hPXR in primary human hepatocytes. Together, these data indicate that CITCO binds directly to the hPXR ligand-binding domain to activate hPXR. As CITCO has been widely used, its confirmation as a dual agonist for hCAR and hPXR is important for appropriately interpreting existing data and designing future experiments to understand the regulation of hPXR and hCAR. SIGNIFICANCE STATEMENT: The results of this study demonstrate that 6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde-O-(3,4-dichlorobenzyl)oxime (CITCO) is a dual agonist for human constitutive androstane receptor (hCAR) and human pregnane X receptor (hPXR). As CITCO has been widely used to activate hCAR, and hPXR and hCAR have distinct and overlapping biological functions, these results highlight the value of receptor-selective agonists and the importance of appropriately interpreting data in the context of receptor selectivity of such agonists.

Synthesis of Small Fluorescent Molecules and Evaluation of Photophysical Properties

A series of aniline-based fluorophores were newly synthesized. To increase their fluorescence quantum yields, it was particularly important to substitute 3,3,3-trifluoroprop-1-enyl (TFPE) groups next to the amino group to benefit from an extended π-electron delocalization. Among these, 5-CN-2-TFPE-aniline was found to behave as an excellent fluorophore with a reasonable fluorescence quantum yield of 0.89 even in aqueous solution. l-Alanine peptide, a nonfluorescent analogue of 5-CN-2-TFPE-aniline, was synthesized and successfully employed as an enzyme probe to detect aminopeptidase N activity.

Drug discovery technologies to identify and characterize modulators of the pregnane X receptor and the constitutive androstane receptor

The pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) are ligand-activated nuclear receptors (NRs) that are notorious for their role in drug metabolism, causing unintended drug-drug interactions and decreasing drug efficacy. They control the xenobiotic detoxification system by regulating the expression of an array of drug-metabolizing enzymes and transporters that excrete exogenous chemicals and maintain homeostasis of endogenous metabolites. Much effort has been invested in recognizing potential drugs for clinical use that can activate PXR and CAR to enhance the expression of their target genes, and in identifying PXR and CAR inhibitors that can be used as co-therapeutics to prevent adverse effects. Here, we present current technologies and assays used in the quest to characterize PXR and CAR modulators, which range from biochemical to cell-based and animal models.

HNRNPH1 is required for rhabdomyosarcoma cell growth and survival

Rhabdomyosarcoma (RMS) is an aggressive and difficult to treat cancer characterized by a muscle-like phenotype. Although the average 5-y survival rate is 65% for newly diagnosed RMS, the treatment options for metastatic disease are limited in efficacy, with the 5-y survival rate plummeting to 30%. Heterogenous nuclear ribonucleoprotein H1 (HNRNPH1) is an RNA-binding protein that is highly expressed in many cancers, including RMS. To determine the role HNRNPH1 plays in RMS tumorigenesis, we investigated its expression and effect on growth in three cellular models of RMS: RD, RH30, and RH41 cells. Upon knockdown of HNRNPH1, growth of all cell lines was reduced, most likely through a combination of apoptosis and cell cycle arrest. We then recapitulated this finding by performing in vivo xenograft studies, in which knockdown of HNRNPH1 resulted in a reduction of tumor formation and growth. We used RNA sequencing to identify changes in gene expression after HNRNPH1 knockdown and found altered splicing of some oncogenes. Our data contribute to understanding the role of HNRNPH1 in RMS development.

High-Throughput Screening Identifies 1,4,5-Substituted 1,2,3-Triazole Analogs as Potent and Specific Antagonists of Pregnane X Receptor

Human pregnane X receptor (hPXR) is a nuclear receptor that regulates the expression of phase I and phase II drug-metabolism enzymes, as well as that of drug transporters. hPXR is a "xenobiotics sensor" and can be activated by structurally diverse compounds. The activation of hPXR by its agonists increases the clearance of xenobiotics by increasing the expression of drug-metabolism enzymes and drug transporters, possibly leading to drug toxicity, drug resistance, and other adverse drug reactions. Therefore, hPXR antagonists might attenuate agonist-mediated activation of hPXR and reduce the risk of adverse drug reactions. Several hPXR antagonists have been reported, but none of them is specific for hPXR. In this study, we present the first large-scale, unbiased, cell-based high-throughput screen to identify specific hPXR antagonists. Among the 132,975 compounds screened, we identified the 1,4,5-substituted 1,2,3-triazole analogs as potent and specific hPXR antagonists by sequentially performing primary screening, retesting, and dose-response analysis using cell-based hPXR gene reporter and receptor binding assays, as well as receptor and promoter specificity assays. The compound SJ000076745-1 is the most potent and specific hPXR antagonist in the 1,4,5-substituted 1,2,3-triazole chemical class, having a cell-based hPXR antagonist 50% inhibitory concentration (IC₅₀) value of 377 ± 16 nM and an hPXR binding inhibitory IC₅₀ value of 563 ± 40 nM.

SPA70 is a potent antagonist of human pregnane X receptor

Many drugs bind to and activate human pregnane X receptor (hPXR) to upregulate drug-metabolizing enzymes, resulting in decreased drug efficacy and increased resistance. This suggests that hPXR antagonists have therapeutic value. Here we report that SPA70 is a potent and selective hPXR antagonist. SPA70 inhibits hPXR in human hepatocytes and humanized mouse models and enhances the chemosensitivity of cancer cells, consistent with the role of hPXR in drug resistance. Unexpectedly, SJB7, a close analog of SPA70, is an hPXR agonist. X-ray crystallography reveals that SJB7 resides in the ligand-binding domain (LBD) of hPXR, interacting with the AF-2 helix to stabilize the LBD for coactivator binding. Differential hydrogen/deuterium exchange analysis demonstrates that SPA70 and SJB7 interact with the hPXR LBD. Docking studies suggest that the lack of the para-methoxy group in SPA70 compromises its interaction with the AF-2, thus explaining its antagonism. SPA70 is an hPXR antagonist and promising therapeutic tool.The xenobiotic-activated human pregnane X receptor (hPXR) regulates drug metabolism. Here the authors develop hPXR modulators, which are of potential therapeutic interest and functionally and structurally characterize the antagonist SPA70 and the structurally related agonist SJB7.

Using TR-FRET to Investigate Protein-Protein Interactions: A Case Study of PXR-Coregulator Interaction

Time-resolved fluorescence resonance energy transfer (TR-FRET) protein-protein interaction assays, especially in the format of receptor coregulator (coactivator and corepressor) recruitment/repression assays, have been widely used in nuclear receptor research to characterize the modes of action, efficacies, and binding affinities of ligands (including their properties as agonists, antagonists, and inverse agonists). However, there has been only limited progress in using this assay format for pregnane X receptor (PXR). In this chapter, we discuss TR-FRET protein-protein interaction assays and focus on a novel PXR TR-FRET coactivator interaction assay that we have developed based on a PXR coactivator cocrystal study. This new PXR TR-FRET coactivator interaction assay can characterize the binding affinities of PXR ligands and also differentiate antagonists from agonists. This assay is very robust, with the signal remaining stable over a long incubation time (up to 300min has been tested). It can tolerate high concentrations of DMSO (up to 5%) and has a high signal-to-noise ratio (six under typical assay conditions). This newly developed PXR TR-FRET coactivator interaction assay has potential application in high-throughput screening to identify and characterize novel PXR agonists and antagonists.

Glucose-dependent regulation of pregnane X receptor is modulated by AMP-activated protein kinase

Pregnane X receptor (PXR) is a xenobiotic receptor that regulates the detoxification and clearance of drugs and foreign compounds from the liver. There has been mounting evidence of crosstalk between the drug metabolism pathway and the energy metabolism pathway, but little is known about this cross-regulation. To further delineate the energy metabolism and drug metabolism crosstalk in this study, we exposed HepG2 cells to varying glucose concentrations. We observed that PXR activity was induced under high-glucose conditions. This finding is consistent with previous clinical reports of increased drug clearance in patients with untreated diabetes. We demonstrated that AMP-activated protein kinase (AMPK) modulates PXR transcriptional activity and that pharmacologically manipulated AMPK activation exhibits an inverse relation to PXR activity. Activation of AMPK was shown to downregulate PXR activity and, consistent with that, potentiate the response of cells to the drug. Taken together, our results delineate a hitherto unreported axis of regulation that involves the energy status of the cell, PXR regulation, and drug sensitivity.

Parallel fluorescent probe synthesis based on the large-scale preparation of BODIPY FL propionic acid

We describe a methodology for quick development of fluorescent probes with the desired potency for the target of interest by using a method of parallel synthesis, termed as Parallel Fluorescent Probe Synthesis (Parallel-FPS). BODIPY FL propionic acid 1 is a widely used fluorophore, but it is difficult to prepare a large amount of 1, which hinders its use in parallel synthesis. Optimization of a synthetic scheme enabled us to obtain 50g of 1 in one batch. With this large quantity of 1 in hand, we performed Parallel-FPS of BODIPY FL-labeled ligands for estrogen related receptor-α (ERRα). An initial trial of the parallel synthesis with various linkers provided a potent ligand for ERRα (Reporter IC50=80nM), demonstrating the usefulness of Parallel-FPS.

Marine invertebrate xenobiotic-activated nuclear receptors: their application as sensor elements in high-throughput bioassays for marine bioactive compounds

Developing high-throughput assays to screen marine extracts for bioactive compounds presents both conceptual and technical challenges. One major challenge is to develop assays that have well-grounded ecological and evolutionary rationales. In this review we propose that a specific group of ligand-activated transcription factors are particularly well-suited to act as sensors in such bioassays. More specifically, xenobiotic-activated nuclear receptors (XANRs) regulate transcription of genes involved in xenobiotic detoxification. XANR ligand-binding domains (LBDs) may adaptively evolve to bind those bioactive, and potentially toxic, compounds to which organisms are normally exposed to through their specific diets. A brief overview of the function and taxonomic distribution of both vertebrate and invertebrate XANRs is first provided. Proof-of-concept experiments are then described which confirm that a filter-feeding marine invertebrate XANR LBD is activated by marine bioactive compounds. We speculate that increasing access to marine invertebrate genome sequence data, in combination with the expression of functional recombinant marine invertebrate XANR LBDs, will facilitate the generation of high-throughput bioassays/biosensors of widely differing specificities, but all based on activation of XANR LBDs. Such assays may find application in screening marine extracts for bioactive compounds that could act as drug lead compounds.