Selectivity in Cells May Vary

Key Takeaways

• DiscoverX assay often overestimates kinase inhibitor potency.
• NanoBRET reveals cell-specific potency differences.
• Cabozantinib potently engages PIP4K2C in cells.
• Approved drugs unexpectedly inhibit tumor suppressor STK11.
• New selective probes proposed for BRSK1/2 and DDR1/2.

Summary

The Chemical Probes Portal’s >30‑fold selectivity rule, originally based on cell‑free assays, is challenged by a new open‑access J. Med. Chem. study comparing DiscoverX kinase panels with NanoBRET cellular profiling. Researchers found that most inhibitors appear less potent in living cells, due to factors such as ATP competition, membrane permeability, and full‑length protein context, though a few kinases show heightened cellular sensitivity. Notably, cabozantinib engages the immuno‑oncology target PIP4K2C at mid‑nanomolar levels, while several approved drugs unexpectedly bind the tumor suppressor STK11. The work also proposes new selective probes for understudied kinases BRSK1/2 and DDR1/2.

Pulse Analysis

Selectivity remains a cornerstone of chemical probe design, yet the industry has long relied on cell‑free platforms such as the Eurofins DiscoverX panel to set the >30‑fold benchmark mandated by the Chemical Probes Portal. While these assays provide high‑throughput coverage of hundreds of kinases, they omit critical cellular variables—ATP concentrations, membrane barriers, and full‑length protein architecture—that can dramatically reshape binding profiles. As the kinase family expands beyond 500 members, the need for context‑aware validation grows, especially for drug candidates that must demonstrate on‑target engagement in vivo.

The recent J. Med. Chem. paper leverages NanoBRET, a luminescence‑based resonance energy transfer method, to directly monitor kinase‑inhibitor interactions inside living cells. By juxtaposing NanoBRET data with DiscoverX dissociation constants for four promiscuous inhibitors, the authors reveal a systematic loss of potency in the cellular environment, attributing it to ATP competition, limited permeability, and the use of truncated versus full‑length kinases. Intriguingly, a subset of kinases—including PIP4K2C—exhibited greater cellular inhibition, highlighting opportunities for repurposing drugs like cabozantinib in immuno‑oncology. Conversely, the unexpected engagement of the tumor suppressor STK11 by several approved agents flags a potential safety concern that would be invisible in a purely biochemical screen.

For the broader drug discovery community, these insights underscore the importance of integrating cell‑based selectivity assays early in the pipeline. The identification of highly selective probes for BRSK1/2 and DDR1/2 demonstrates that compounds deemed moderately selective in vitro can achieve true specificity in vivo, expanding the toolbox for target validation. Companies should therefore complement DiscoverX profiling with NanoBRET or comparable cellular platforms to mitigate off‑target risks, refine therapeutic windows, and accelerate the translation of kinase inhibitors from bench to bedside.