Glycosylation Switches & PROTACs – More Precise Targeted Protein Degradation Therapies
On September 12, 2025, a team led by Christina M. Woo of Harvard University published an article titled "Enzyme-Activated Sugar-Coated Bifunctional Degraders" in the journal JACS. This study utilized O-GlcNAc glycosylation as a metabolic switch to design a sugar-coated PROTAC. This molecule activates its degradation function only after deglycosylation of the overactive O-GlcNAcase (OGA) in cancer cells, thereby precisely unleashing its potency in the lesion environment and significantly improving safety and selectivity.
Targeted protein degradation directs pathogenic proteins to the E3 ligase-ubiquitin-proteasome system for clearance. However, currently used E3 ligases (such as Cereblon, CRBN) are ubiquitously expressed in vivo and may result in non-selective toxicity. To metabolically gate PROTAC activity, the researchers developed an enzyme activation strategy: introducing an O-GlcNAc Glycosylation onto the CRBN ligand cyclimid (derived from the natural CRBN recognition motif). This sugar-coated PROTAC (SCP) was designed based on structural analysis to target the target protein BRD4. Results revealed that glycosylated cyclimid significantly reduced binding to CRBN and complex formation with BRD4, with activity restored only after O-GlcNAc cleavage by O-GlcNAcase (OGA). This OGA-dependent enzyme activation strategy was validated through in vitro binding assays, cell degradation assays, and cell viability assays. O-GlcNAc thus serves as an effective metabolic switch, providing a new, selective regulatory approach for targeted protein degradation and inspiring similar strategies based on other protein modifications.
Over the past decade, Targeted Protein Degradation (TPD) has become a hot topic in drug development. Unlike traditional inhibitors that simply latch onto target proteins, PROTACs, representative of TPD, can directly consign target proteins to the "trash bin"—that is, by simultaneously binding to the target protein and an E3 ligase, inducing ubiquitination and subsequent proteasomal degradation. This means that the target is not only inactivated but also completely eliminated. PROTACs have demonstrated great potential in treating diseases such as cancer, immunity, and neurology, with several candidate molecules entering clinical trials. However, they also present a potential concern: currently used E3 ligase ligands (such as Cereblon/CRBN and VHL) are ubiquitously expressed throughout the body. This acts like a "universal key" and can easily induce nonspecific toxicity.
Thus, improving PROTAC selectivity, ensuring their effectiveness is limited to the lesion site rather than indiscriminately targeting the entire body, remains a core challenge in the field.
To address this issue, researchers have proposed several conditional activation strategies: light-activated PROTACs, which trigger degradation with light; enzyme-sensitive PROTACs, which activate in the presence of specific enzymes; galactose-coated PROTACs, which leverage cellular metabolic signatures for selective release; and folic acid-coated PROTACs, which target specific cell types via ligands.
These designs attempt to answer the question: how can the PROTAC "key" be unlocked only in the presence of the appropriate lock? However, numerous practical challenges remain:
Therefore, scientists have been searching for an endogenous metabolically dependent switch, preferably one unique to tumor cells, that could automatically activate PROTACs in a diseased environment.
Fig. 1 Design and mechanism of O-GlcNAc sugar-coated PROTACs (SCPs) for conditional BRD4 degradation. (Zhu, et al. 2025)
O-GlcNAc modification is a common Post-Translational Modification, widely present on nuclear and cytoplasmic proteins.
It is added by the enzyme OGT (transferase) and removed by the enzyme OGA (glycosidase). Crucially, OGA is overexpressed in various cancers (such as pancreatic, breast, and colorectal cancers) and is closely associated with abnormal tumor metabolism. In other words, OGA activity is higher in tumor cells, providing a natural entry point for selective activation of PROTACs.
The researchers used the novel CRBN ligand cyclimid as a carrier and attached O-GlcNAc to a BRD4-targeting degrader (JQ1-ScQ).
This design acts like a "sugar lock" on the PROTAC, unlocked only by overactive OGA in cancer cells, thereby initiating degradation within the lesion.
This work demonstrates a metabolism-dependent PROTAC activation strategy. Through O-GlcNAc modification, CRBN recruitment is structurally blocked; OGA deglycosylation is relied upon to restore activity in the tumor environment, resulting in a "sugar-switch" degrader (SCP).
It exhibits enhanced selectivity, with higher activity in cancer cells than in normal cells; requires no exogenous stimulation, relying entirely on endogenous metabolic characteristics; and is highly controllable, even with the potential for artificial regulation using OGA inhibitors. This "sugar encapsulation" concept is not only applicable to PROTACs but may also be widely used in the design of conditionally activated drugs in the future. It represents a new trend in leveraging Disease-specific Metabolic Characteristics to build more precise and safer targeted therapies.
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