A drug’s most potent target is not necessarily the source of its anti-cancer activity

The small-molecule drug ralimetinib was developed as an inhibitor of the kinase p38α, and it has advanced to phase 2 clinical trials in oncology. Here, we apply a multi-modal approach to demonstrate that ralimetinib’s anti-cancer activity occurs due to its ability to inhibit EGFR, rather than p38α. We find that cancer cell lines driven by EGFR mutations exhibit the greatest sensitivity to ralimetinib treatment, and ralimetinib phenocopies established EGFR inhibitors in pharmacogenomic profiling experiments. We further demonstrate that ralimetinib inhibits EGFR kinase activityin vitroandin cellulo, albeit at >30-fold higher concentrations than it inhibits p38α. Finally, while deletion of the gene encoding p38α has no effect on ralimetinib sensitivity, expression of the EGFR-T790M gatekeeper mutation confers resistance to ralimetinib treatment. These findings suggest that future clinical trials involving ralimetinib could incorporate EGFR mutation status as a biomarker to identify sensitive patients. Moreover, our results demonstrate that a compound’s anti-cancer effects should not necessarily be attributed to the protein that it inhibits most strongly, and instead, comprehensive cellular and genetic profiling is required to understand a drug’s mechanism-of-action.