From Tracers to Kinetic Selectivity: Highlights from the Target Engagement in Chemical Biology Symposium

In April 2024, Promega hosted the “Target Engagement in Chemical Biology Symposium” at the Kornberg Center, a research and development hub on Promega’s campus in Madison, Wisconsin. The goal of the symposium was to gather interdisciplinary researchers interested in the field of small molecule target engagement to foster collaboration through knowledge sharing and innovation. The symposium featured a 1.5-day agenda packed with 23 speakers, 4 workshops, poster sessions and social events. Over 130 attendees gathered to participate in the multifaceted event, with participants from different geographic regions and in different research sectors from academia to government to industry.  

People gather in a large atrium with scientific posters and table displays.
Attendees gather for the poster session in Kornberg Atrium. Photo by Anna Bennett (Promega Corporation)

The symposium highlighted the collective commitment to overcoming the challenges in drug discovery by developing more targeted and efficacious treatments, driven by a shared determination to create innovative solutions that address unmet medical needs. While we cannot share all the exciting research presented at the symposium, we are thrilled to highlight a few talks that exemplify the novel work and collaborative spirit of this research community.  

Exploring Target Engagement at Protein Complexes with Matthew Robers

In a session focused on target engagement at protein complexes, Matthew Robers (Associate Director of R&D at Promega Corporation) gave an exciting talk about the application of NanoBRET® Target Engagement (TE) technology to understand compound binding at protein-protein, protein-DNA, and protein-metabolite complexes in live cells. He detailed several examples where this approach was applied toward synthetic lethality targets, including DNA Polymerase Theta and Protein Arginine Methyltransferase 5 (PRMT5).

Synthetic lethality is a genetics concept where mutations in two genes results in cell death, while mutation in only one gene allows the cell to survive. Researchers can leverage this dependency/weakness in the development of precision medicine where treatments are tailored to an individual’s disease. For example, in cancer, many mutations occur that can make a cancer dependent on another gene for survival. After identifying a gene that the cancer cell is dependent on, researchers can develop drugs that inhibit or eliminate the resulting protein product to cause cancer cell death.  

In the case of PRMT5, the Robers’ team used NanoBRET® TE technology to develop an assay that can measure the compound binding affinity at multiple binding sites on the protein. PRMT5 is an essential arginine methyltransferase that regulates a wide spectrum of cellular processes through the methylation of histone and non-histone substrates. In more than 10% of human cancers, a certain gene is deleted (MTAP), leading to accumulation of a cofactor (MTA) that binds PRMT5. The NanoBRET® TE method can measure the potency of compounds bound to the substrate pocket or the SAM/MTA pocket. Developing drugs for PRMT5 creates an opportunity for precision medicine targeting PRMT5 in MTAP-deficient cancer cells while sparing healthy cells.

Learn more about this work here.

Attendees gather in a breakout session. Photo by: Lindsay Walker (Promega Corporation)

Keeping Track of Tracers with Martin Schwalm

In another track, Martin Schwalm (PhD Student at the Goethe University, Frankfurt, Germany and Structural Genomics Consortium) presented a new tool called tracerDB. The goal of this crowd-sourced database is to serve as an open-access resource for researchers to deposit structural information and performance data about validated tracers. Many protein-ligand interaction assays rely on the use of labeled tracers—or fluorescent probes—that reversibly bind to the target protein. These can be used to determine the binding characteristics of an unlabeled compound through a displacement assay format. NanoBRET®TE assays rely on a robust, cell-permeable fluorescent tracers to determine cellular affinity of compounds for target proteins.

Validated tracers can be difficult to—well, trace. Although Promega offers a suite of validated fluorescent NanoBRET® TE tracers for hundreds of target proteins, not all tracers have this level of visibility or standardization. Results may be tucked away in publications. Also, results are likely not standardized— meaning assessment of performance can be a challenge. Because of this, Martin and his team built tracerDB with target engagement researchers in mind.

A key feature of tracerDB is a platform to ensure the standardization of the data provided for each tracer. In addition, tracerDB provides education by outlining specific criteria for high-quality tracers—emphasizing importance of the assay window (the measure of signal change between bound and unbound states) and the Z’ value (a score that quantifies assay performance, where values between 0.5 and 1 are considered “excellent”). Tracers that meet these criteria are classified as “robust”, while tracers with slightly lower values are marked as “expert assays,” requiring precise handling by experienced researchers. By differentiating between robust and expert assays, researchers are empowered to select the most appropriate tools for their specific experimental needs.

A Fresh Look at Selectivity with Dr. Peter Tonge

In the Kinetic Selectivity in Cells session, Dr. Peter Tonge (Distinguished Professor, Stony Brook University) gave an insightful talk on the importance of kinetic selectivity for drug discovery research. Selectivity refers to how well a drug can specifically target a particular protein or molecule in the body, while avoiding unintended interactions with other proteins (off-targets). Selectivity is an important measurement in drug discovery because off-target interactions can lead to side effects and reduce the therapeutic window (margin of safety) of the drug.

Traditionally, selectivity is defined by the relative affinity—or binding strength—of a drug for its intended target compared to its affinity for off-target proteins. This classical approach is known as thermodynamic selectivity. However, there’s another important but less commonly discussed aspect: kinetic selectivity. Kinetic selectivity considers how strongly a drug binds (affinity) and the rates at which it binds to (K on) and dissociates from (K off) its target and off-target proteins.

In his talk, Dr. Tonge covered the significance of kinetic selectivity and how this can impact target vulnerability. Kinases are enzymes that play critical roles in cell signaling and regulation and numerous diseases. Kinases belong in large families with similar structural properties, making it critical to target the intended kinase without affecting other closely related family members. In addition to understanding differential binding affinity, binding kinetics is another parameter that can be optimized during the drug development process, to design more effective and specific kinase inhibitors.

Read more about the importance of kinetics of drug binding in this work here.

To learn more about Kinase Target Engagement, check out this resource page.

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Anna Bennett

Anna Bennett

Anna earned her PhD in microbiology at the University of Minnesota in 2022 where she studied the microbial communities in hot springs. She joined Promega in 2023 as science writer within the Marketing Services department. When she's not writing, she enjoys being outdoors with her dog, Calvin.

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