The rise of targeted protein degradation (TPD) marks a transformative shift in drug discovery, from traditional occupancy-driven modes of action to event-driven pharmacology. Rather than inhibit protein function through sustained binding, degraders act catalytically by harnessing the cell’s ubiquitin-proteasome system to remove disease-causing proteins. This modality offers several clear advantages: catalytic efficiency and therefore lower doses of drug, potential for selectivity beyond binding affinity, and access to previously “undruggable” targets.
But with this new modality comes the challenge of characterizing efficacy through the lens of kinetics. Unlike conventional small molecule inhibitors, whose effect often correlates with the strength and duration of target binding, degraders operate through a cascade of dynamic steps (Figure 1). Understanding when and how fast a target is degraded and how the cell responds is critical to optimizing degrader efficacy, yet these kinetic dimensions are not always easily captured and can often be overlooked. To fully realize the potential of TPD, researchers need tools that can track degrader activity in real time in live cells.
Figure 1. While classic small molecule drug discovery equates efficacy with occupancy, degraders function through a catalytic series of events. Degrader efficiency depends on the timing of each of these steps.
CREDIT; SCIEX
A new pharmacological paradigm
Classical pharmacology has long equated efficacy with occupancy. The more a drug binds its target, the greater its effect in suppressing the target’s function. While this principle still informs many models of drug action, target engagement by degraders does not always correlate with potent degradation. Their function depends on a catalytic series of events: simultaneous binding to both a target protein and E3 ligase to form a ternary complex, triggering ubiquitination, and directing the target to the proteasome for degradation. Once this chain of events is initiated, the degrader can disengage and repeat the process.
The catalytic efficiency of a degrader often hinges on its ability to engage in rapid, transient interactions that support multiple rounds of degradation. Studies have shown that short-lived ternary complex formation can result in substantial and lasting biological effects, without high target occupancy or prolonged engagement.
What we learned from small molecule kinetics
The importance of kinetics in drug discovery is not new. For small molecule therapeutics, target binding kinetics, particularly the rates of association and dissociation, have proven critical for optimizing compounds for successful translation into clinical settings.
This has been exemplified with lower-affinity inhibitors that exert sustained biological effects through prolonged residence time on their targets, maintaining activity even when plasma concentrations fall below their binding thresholds. Furthermore, some kinase inhibitors have demonstrated kinetic selectivity, characterized by extended binding to the desired target and rapid dissociation from off-targets, thereby enhancing both efficacy and safety.
These lessons are even more relevant for TPD, where multiple dynamic protein-protein interactions must align to drive efficient degradation to reduce target levels faster than the cell can replenish them. Understanding these temporal dynamics is essential to optimizing degrader design.
Why degrader kinetics matter
Each stage of the TPD mechanism is inherently time-dependent. Binary binding, ternary complex formation, ubiquitination, and proteasomal engagement all have distinct kinetic profiles, and the rate limiting step may vary depending on the degrader, target, and cellular context.
Although the ternary complex is not always predictive of degradation, the lifetime of the ternary complex needed for successful degradation is thought to fall within a “Goldilocks zone”: long enough to enable ubiquitination, but not so long that the target and degrader become sequestered. If a ternary residence time is too short, it can lead to insufficient ubiquitination, but if it’s too long, it can impede target degradation and catalytic turnover.
Furthermore, degraders must contend with cellular mechanisms that counteract degradation such as efflux transporters that remove drugs, deubiquitinases that cleave ubiquitin, and cellular feedback loops that upregulate target synthesis. These competing processes add further layers of complexity that are often missed by static assays and can only be fully understood through real-time, kinetic measurement.
The power of live-cell kinetic assays
To fully capture the pharmacological behavior of degraders, assays must reflect the dynamic nature of the degradation process while preserving the native, endogenous cellular context. These assays should not only report on the overall efficiency of degradation but also enable dissection of the individual kinetic steps that influence that outcome. This includes the cellular kinetics of ternary complex formation and ubiquitination, which may serve as points of optimization during degrader development.
Live-cell kinetic degradation assays address this need by providing real-time insight into when degradation begins, how rapidly it proceeds, and how long the effect persists. They allow precise temporal resolution of key parameters such as the degradation rate, the time to maximal degradation, and the duration over which degradation is maintained. Importantly, when coupled with assays that monitor ternary complex formation or ubiquitination in real time, degradation kinetics can be interpreted in the context of upstream rate-limiting steps. For instance, delayed onset of degradation may reflect inefficient or unfavorable ternary complex formation, while shallow degradation could be indicative of limited ubiquitination efficiency or rapid protein resynthesis. By incorporating these mechanistic insights, researchers can more effectively diagnose suboptimal performance and identify strategies to improve degrader design.
These kinetic metrics are not only valuable for prioritizing leads and guiding structure-activity relationships but are also highly informative for translational modeling. They can help predict in vivo performance, inform dosing frequency, define therapeutic windows, and aid investigation of recovery dynamics following treatment removal.
Timing is everything
Targeted protein degraders act through a cascade of dynamic events. To fully evaluate their efficacy, we must move beyond static endpoints and measure how quickly, durably, and completely a target is degraded across relevant cellular contexts. Live-cell kinetic assays provide these insights, revealing features like degradation onset, duration, and recovery that static measurements often miss. Incorporating kinetic profiling early in degrader development helps identify compounds with the most efficient catalytic performance and strongest translational potential. In the end, the best degraders aren’t just effective, they’re fast.
This article was contributed by Kristin Riching, Senior Research Scientist at Promega Corporation.
