Pyrazoloadenine Inhibitors of the RET Lung Cancer Oncoprotein Discovered by a Fragment Optimization Approach

A fragment-based drug-discovery approach was used on a pyrazoloadenine fragment library to uncover new molecules that target the RET (REarranged during Transfection) oncoprotein, which is a driver oncoprotein in ∼2 % of non-small-cell lung cancers. The fragment library was screened against the RET kinase and LC-2/ad (RET-driven), KM-12 (TRKA-driven matched control) and A549 (cytotoxic control) cells to identify selective scaffolds that could inhibit RET-driven growth. An unsubstituted pyrazoloadenine fragment was found to be active on RET in a biochemical assay, but reduced cell viability in non-RET-driven cell lines (EC₅₀=1 and 3 μM, respectively). To increase selectivity for RET, the pyrazoloadenine was modeled in the RET active site, and two domains were identified that were probed with pyrazoloadenine fragment derivatives to improve RET affinity. Scaffolds at each domain were merged to generate a novel lead compound, 8 p , which exhibited improved activity and selectivity for the RET oncoprotein (A549 EC₅₀=5.92 μM, LC-2/ad EC₅₀=0.016 μM, RET IC₅₀=0.000326 μM).