Synthesis, and evaluation of novel low nanomolar isoindigo-based RET kinase inhibitors

Abstract

Acquired resistance to selective kinase inhibitors remains a primary challenge in targeted cancer therapy. The clinical efficacy of potent rearranged during transfection (RET) inhibitors, for instance, is often attenuated by the emergence of on-target mutations, such as the V804L gatekeeper variant. To address this, a novel series of isoindigo-based compounds were designed, synthesized, and evaluated for RET kinase inhibitory activity. This effort led to the identification of compound 4c, which demonstrated potent, low-nanomolar inhibition of wild-type RET kinase (IC₅₀ = 13.7 nM) and retained, even enhanced, biochemical potency against the clinically relevant RET V804L gatekeeper mutant (IC₅₀ = 5.2 nM). Kinase selectivity profiling confirmed that 4c displays a favorable selectivity profile, with negligible activity against c-KIT, a common off-target associated with toxicity in less selective multi-kinase inhibitors. Molecular modelling provided a structural rationale for the enhanced potency against the V804L mutant. These findings identify compound 4c as an early lead with promising biochemical potency in this initial exploration of isoindigo-based RET inhibitors. However, it should be noted that these results are based on biochemical (enzymatic) assays only, and further cell-based validation is required to assess the translational potential of this compound.