Fragments vs the E3 Ligase KLHL12

Key Takeaways

• 13,824 fragments screened, 35 hits identified
• Lead compound 7k achieved sub‑micromolar KLHL12 affinity
• 7k showed selective binding versus KEAP1
• NanoBRET confirmed cell‑permeable KLHL12 engagement
• KLHL12 overexpressed in cancers, absent in heart tissue

Pulse Analysis

Targeted protein degradation has reshaped drug discovery, but the success of PROTACs hinges on the availability of suitable E3 ligases. While CRBN and VHL dominate the landscape, their ubiquitous expression can limit therapeutic windows. KLHL12, a Kelch‑like protein highly expressed in tumor cells yet silent in cardiac tissue, offers a promising alternative for cancer‑selective degradation. By leveraging a high‑throughput NMR fragment screen, the Vanderbilt team identified a modest hit set, underscoring the difficulty of finding drug‑like binders for this class of ligases.

The optimization pathway focused on two chemical series: an aniline‑based scaffold and a benzimidazole core. Iterative structure‑activity relationship work, guided by crystal structures of both the initial hit and the more potent 7k analogue, pushed affinity into the sub‑micromolar range. Importantly, 7k retained selectivity over the closely related KEAP1 ligase, addressing a common concern given KLHL12’s 35% sequence identity with nine other Kelch proteins. Although many analogues suffered from flat SAR and reduced ligand efficiency, the high‑resolution structure of 7k illuminated unexplored pockets that could be exploited in future designs.

Cellular validation came via a nanoBRET assay, confirming that 7k permeates membranes and engages KLHL12 at low micromolar concentrations. This proof‑of‑concept suggests that fragment‑derived ligands can be transformed into functional degrader warheads, enabling the construction of KLHL12‑directed PROTACs with a built‑in safety profile. The study highlights both the promise and the challenges of expanding the E3 ligase toolbox, and it sets the stage for next‑generation, tissue‑specific degraders that could minimize off‑target toxicity in oncology applications.